Luminescence Properties of Zno Nanostructures and Their Implementation As White Light Emitting Diodes (Leds)

Linköping Studies in Science and Technology

Dissertation No. 1378 Luminescence Properties of ZnO Nanostructures and Their Implementation as White Light Emitting Diodes (LEDs)

Naveed ul Hassan Alvi

Physical Electronics and Nanotechnology Division Department of Science and Technology (ITN) Campus Norrköping, Linköping University SE-60174 Norrköping Sweden

Linköping 2011

[email protected] [email protected]

ISBN: 978-91-7393-139-7

ISSN 0345-7524

Printed by LiU-Tryck, Linköping University, Linköping, Sweden June, 2011

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Luminescence Properties of ZnO Nanostructures and Their Implementation as White Light Emitting Diodes (LEDs) Naveed ul Hassan Alvi

Department of Science and Technology, Linköping University Sweden, 2011

Abstract: In the past decade the global research interest in wide band gap semiconductors has been focused on zinc oxide (ZnO) due to its excellent and unique properties as a semiconductor material. The high electron mobility, high thermal conductivity, good transparency, wide and direct band gap (3.37 eV), large exciton binding energy (60 meV) at room temperature and easiness of growing it in the nanostructure form, has made it suitable for wide range of applications in optoelectronics, piezoelectric devices, transparent and spin electronics, lasing and chemical sensing. In this thesis, luminescence properties of ZnO nanostructures (nanorods, nanotubes, nanowalls and nanoflowers) are investigated by different approaches for possible future application of these nanostructures as white light emitting diodes. ZnO nanostructures were grown by different growth techniques on different p-type substrates. Still it is a challenge for the researchers to produce a stable and reproducible high quality p-type ZnO and this seriously hinders the progress of ZnO homojunction LEDs. Therefore the excellent properties of ZnO can be utilized by constructing heterojunction with other p-type materials. The first part of the thesis includes paper I-IV. In this part, the luminescence properties of ZnO nanorods grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) grown on the same substrate were investigated. The effect of the post-growth annealing of ZnO nanorods and nanotubes on the deep level emissions and color rendering properties were also investigated. In paper I, ZnO nanorods were grown on p-type GaN and 4H-SiC substrates by low temperature aqueous chemical growth (ACG) method. The luminescence properties of the fabricated LEDs were investigated at room temperature by electroluminescence (EL) and photoluminescence (PL) measurements and consistency was found between both the measurements. The LEDs showed very bright emission that was a combination of three emission peaks in the violet-blue, green and orange-red regions in the visible spectrum. In paper II, different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) were grown on p-GaN and the luminescence properties of these nanostructures based LEDs were comparatively investigated by EL and PL measurements. The nanowalls structures were found to be emitting the highest emission in the visible region, while the nanorods have the highest emissions in the UV region due to its good crystal quality. It was also estimated that the ZnO nanowalls structures have strong white light with the highest color rendering index (CRI) of 95 with correlated color temperature (CCT) of 6518 K. In paper III, we have investigated the origin of the red emissions in ZnO by using post-growth annealing. The ZnO nanotubes were achieved on p-GaN and then annealed in different ambients (argon, air, oxygen and nitrogen) at 600 oC for 30 min. By comparative investigations of EL spectra of the LEDs it was found that more than one deep level defects

V are involved in the red emission from ZnO nanotubes/p-GaN LEDs. It was concluded that the red emission in ZnO can be attributed to oxygen interstitials (Oi) and oxygen vacancies (Vo) in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) and 690 nm (1.79 eV) to 750 nm (1.65 eV), respectively. In paper IV, we have investigated the effect of post-growth annealing on the color rendering properties of ZnO nanorods based LEDs. ZnO nanorods were grown on p- GaN by using ACG method. The as grown nanorods were annealed in nitrogen, oxygen, argon, and air ambients at 600 oC for 30 min. The color rendering indices (CRIs) and correlated color temperatures (CCTs) were estimated from the spectra emitted by the LEDs. It was found that the annealing ambients especially air, oxygen, and nitrogen were found to be very effective. The LEDs based on nanorods annealed in nitrogen ambient, have excellent color rendering properties with CRIs and CCTs of 97 and 2363 K in the forward bias and 98 and 3157 K in the reverse bias. In the 2nd part of the thesis, the junction temperature of n-ZnO nanorods based LEDs at the built-in potential was modeled and experiments were performed to validate the model. The LEDs were fabricated by ZnO nanorods grown on different p-type substrates (4H- SiC, GaN, and Si) by the ACG method. The model and experimental values of the temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared. It was found that the series resistance has the main contribution in the junction temperature of the fabricated devices.

In the 3rd part of the thesis, the influence of helium (He+) ion irradiation bombardment on luminescence properties of ZnO nanorods based LEDs were investigated. ZnO nanorods were grown by the vapor-liquid-solid (VLS) growth method. The fabricated LEDs were irradiated by using 2 MeV He+ ions with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2. It was observed that the He+ ions irradiation affects the near band edge emissions as well as the deep level emissions in ZnO. A blue shift about 0.0347 eV and 0.082 eV was observed in the PL spectra in the near band emission and green emission, respectively. EL measurements also showed a blue shift of 0.125 eV in the broad green emission after irradiation. He+ ion irradiation affects the color rendering properties and decreases the color rendering indices from 92 to 89.

List of publications included in the thesis

Paper I Fabrication and characterization of high brightness light emitting diodes based on n- ZnO nanorods grown by low temperature chemical method on p-4H-SiC and p-GaN N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander Semiconductor Science and Technology 25, 065004 (2010) Paper II Fabrication and comparative optical characterization of n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN Light emitting diodes N. H. Alvi, Syed M. Usman Ali, S. Hussain, O. Nur, and M. Willander Scripta Materiala 64, 697 (2011) Paper III The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes N. H. Alvi, K. ul Hasan, O. Nur, and M. Willander Nanoscale Research Letters 6, 130 (2011) Paper IV The effect of the post-growth annealing on the color rendering properties of n-ZnO nanorods /p-GaN light emitting diodes N. H. Alvi, M. Willander, and O. Nur (Accepted in Lighting Research and Technology) DOI: 10.1177/1477153511398025 Paper V Junction temperature in n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si) heterojunction light emitting diode N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander Solid State Electronics 54, 536 (2010) Paper VI

Influence of helium-ion bombardment on optical properties of ZnO nanorods/p-GaN light emitting diodes

N. H. Alvi, S. Hussain, J. Jensen, O. Nur, and M. Willander (Submitted to Nanoscale Research Letters)

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List of publications not included in the thesis Paper I Buckling and elastic stability of vertical ZnO nanotubes and nanorods M. Riaz, A. Fulati, G. Amin, N. H. Alvi, O. Nur, and M. Willander J. Appl. Phys. 106, 034309(2009) Paper II A potentiometric intracellular glucose biosensor based on the immobilization of glucose oxidize on the ZnO nanoflakes A. Fulati, Syed M. Usman Ali, M. H. Asif, N. H. Alvi, M. Willander, Cecilia Brännmark, Peter Strålfors, Sara I. Börjesson, and Fredrik Elinder Sensor and Actuators B 150, 673 (2010) Paper III The effect of the post-growth annealing on the electroluminescence properties of n-ZnO nanorods/p-GaN light emitting diodes N. H. Alvi, M. Willander, and O. Nur Superlattices and Microstructures 47, 754 (2010) Paper IV The impact of ion irradiation on piezoelectric power generation from ZnO nanorods array N. H. Alvi, S. Hussain, O. Nur, and M. Willander (manuscript) Paper V A comparative study of the electrodeposition and the aqueous chemical growth techniques for the utilization of ZnO nanorods on p-GaN for white light emitting diodes S. Kishwar, K. ul Hasan, N. H. Alvi, P. Klason, O. Nur, and M. Willander Superlattices and Microstructures 49, 32 (2011) Paper VI Selective potentiometric determination of Uric Acid with functionalized Uricase on ZnO nanowires Syed M. Usman Ali, N.H. Alvi, Omer Nur, Magnus Willander, and Bengt Danielsson Sensor and Actuators B 152, 241 (2011) Paper VII Single Nanowire-based UV photo detectors for fast switching K. ul Hasan, N. H. Alvi, Jun Lu, O. Nur, and Magnus Willander Nanoscale Research Letters 6, 348 (2011) Paper VIII Rectifying characteristics and electrical transport behavior of ZnO nanorods/4H-SiC heterojunction LEDs N. H. Alvi, G. Amin, O. Nur, and M. Willander (Manuscript) Paper IX Single ZnO nanowire biosensor for detection of glucose interactions K. ul Hasan, N. H. Alvi, U. A. Shah, Jun Lu, O. Nur, and M. Willander

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Paper X Comparative optical characterization of n-ZnO nanorods grown by different growth methods N. H. Alvi, S. Hussain, O. Nur, and M. Willander (Manuscript) Paper XI Fabrication and characterization of white light emitting diode based on ZnO nanorods on p-Si

M. M. Rahman, P. Klason, H.A. Naveed, M. Willander

2008 8th IEEE Conference on Nanotechnology (2008: Arlington TX United States) p. 51 – 54 Proceedings IEEE-NANO. (2008)

Paper XII

Photonic nano-devices and coherent phenomena in some low dimensional systems

Magnus Willander, Y.E. Lozovik , S.P. Merkulova , Omer Nour , A. Wadeasa , P. Klason , B. Nargis , N.H. Alvi , S. Kishwar

214th Electrochemcial Society Meeting, Abstract #2034 Honolulu, Hawaii, USA (2008)

Paper XIII

Intrinsic white-light emission from zinc oxide nanorods heterojunctions on large-area substrates

M. Willander, O. Nur, S. Zaman, A. Zainelabdin, G. Amin, J. R. Sadaf, M. Q. Israr, N. Bano, I. Hussain and N. H. Alvi

(Proceedings of SPIE Volume 7940) DOI: 10.1117/12.879327.

Mediterranean Conference on Innovative Materials and Applications held in Beirut, Lebanon in March, 2011

Paper XVI

ZnO as an energy efficient material for white LEDs and UV LEDs

M. Willander, O. Nur, N. Bano, I. Hussain, A. Zainelabdin, S. Zaman, M. Q. Israr, and N. H. Alvi

(Manuscript)

ACKNOWLEDGEMENT

On this memorable night in my life when I am going to finish the writing of this thesis, first of all I bestowed the thanks before Allah Almighty who vigorate me with capability to complete this research work. In the way to the completion of this thesis, my family, teachers, colleagues, and friends all contributed in different ways. At this moment I am very thankful to all of them.

I would like to express my deep sense of gratitude to my supervisor Prof. Magnus Willander for his useful and valuable suggestions, inspiring guidance and consistent encouragement without which this thesis could have never been materialized. He always taught me how to face hard moments in research and life.

I also wish to record my sincere thanks to my co-supervisor Associate Prof. Omer Nour for his kind cooperation, valuable contribution, patience and guidance during my study and research work.

I am also thankful to the ex-research administrator Lise-Lotte Lönndahl Ragnar and our group research administrator Ann-Christin Norén for their administrative help during my studies and research work.

I am also thankful to Dr. Peter Klason, Dr. Georgios Tzamalis, Dr. Alim Fulati, Dr. Muhammad Riaz, Dr. Lili Yang, Dr. Pranciškus Vitta, Prof. Artūras Žukauskas, and Dr. Jens Jensen for their endless cooperation in my research work and nice company.

I am also thankful to all my teachers in my academic career who gave me light of knowledge, every possible help, and guidance which enabled me to be a PhD researcher which was my dream in life.

Words are lacking to express my obligations to Higher Education Commission (HEC) government of Pakistan for partial financial help in my research work. I am also very thankful to Dr. Atta ur Rehman (Ex-Chairman HEC), Dr. Javeed Laghari (Chairman HEC), Muhammad Ashfaq, project manager (HEC), Dr. Sohail Naqvi, and Dr. Yasir Jameel for their cooperation and good wishes.

I offer my sincerest wishes and warmest thanks to all my group members. Many thanks for your cooperation and nice company. It was always my pleasure to work with you all.

I am also thankful to my friend Zia ullah Khan and his family. They always treated me like a brother. Thank you for help, guidance and nice company.

My gratitude will remain incomplete if I do not mention that great contribution of my caring brothers Waseed ul Hassan Alvi, Waheed ul Hassan Alvi, Ameed ul Hassan Alvi and my sisters Aisha Bano Alvi, Fatima Alvi, Wahida Alvi, and Saima Alvi during whole my studies. I wish to express thanks for their love and affection for me in every aspect of life.

I would like to express my profound admiration and salute to my affectionate Father Dr. Khursheed Ahmad Alvi and my sweet mother who taught me the first word to speak, the first alphabet to write and the first step to take. Thanks for your prayers, encouragement and unforgettable sacrifices with patience throughout my career. I am also thankful to all my family, especially my uncles Moulana Nazir Ahmad Alvi and Moulana Abdur Rasheed Alvi for their prayers and encouragement.

I am also very thankful to my loving wife Rahat Alvi (Gul Bano). Thank you for all your help with reference and figure corrections and also for taking care of the house and for cooking delicious foods during my studies and research. I love you 

Finally I would like to thank you for reading this thesis and I hope you will enjoy the reading.

Naveed ul Hassan Alvi

Norrköping, Friday, 13th May 2011

Table of Contents:

Chapter 1 ...... 1 Introduction and motivation ...... 1 Chapter 2 ...... 7 Properties of ZnO ...... 7 2.1 Basic properties of ZnO...... 7 2.2 The basic physical parameters for ZnO ...... 9 2.5 Defects and luminescence in ZnO ...... 14 2.6 Electrical properties of ZnO ...... 17 2.7 Mechanical properties ...... 19 Chapter 3 ...... 27 Device applications of ZnO nanostructures ...... 27 3.1 Light emitting diodes ...... 27 3.2 ZnO based UV photodetectors ...... 35 3.3 ZnO based biosensors ...... 36 Chapter 4 ...... 43 Synthesis of ZnO nanostructures and fabrication of LEDs ...... 43 4.1 Substrate preparation ...... 43 4.2 Growth of ZnO nanostructures ...... 45 4.2.1 The synthesis of nanorods by vapor-liquid-solid method...... 45 4.2.2 Synthesis of nanostructures by the aqueous chemical growth (ACG) method...... 50 4.2.2.1 Synthesis of ZnO nanorods ...... 50 4.2.2.2 Growth of ZnO nanotubes ...... 52 4.2.2.3 Growth of ZnO nanowalls ...... 52 4.2.2.4 Growth of ZnO nanoflowers ...... 52 4.3 Bottom contacts deposition ...... 55 4.4 Spin coating of photo resist and plasma etching ...... 56 4.5 Top contacts deposition ...... 56 Chapter 5 ...... 61 Experimental and characterization techniques ...... 61 5.1 Scanning electron microscopy ...... 61 5.2 Atomic force microscope ...... 64 5.3 X-ray diffraction ...... 64

5.4 Photoluminescence ...... 66 5.5 Electroluminescence ...... 69 5.6 Electrical characterization ...... 70 5.7 Annealing studies ...... 70 5.8 Color rendering extraction ...... 71 5.9 Irradiation studies ...... 72 References ...... 73 Chapter 6 ...... 75 Results ...... 75 6.1 Luminescence properties of ZnO nanostructures ...... 75 6.2 Junction temperature of n-ZnO nanorods based LEDs ...... 92 6.3 The influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods based LEDs ...... 98 Chapter 7 ...... 107 Conclusion and outlook ...... 107

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List of Figures

Figure 2.1: The hexagonal wurtzite structure of ZnO. One unit cell of the crystal is out lined for clarity. 8 Figure 2.2: The zincblende (left) and rock salt (right) phases of ZnO. Only one unit cell is illustrated for clarity. 8 Figure 2.3: The band structure of bulk wurtzite ZnO calculated by the LDA method (a) and self-interaction corrected pseudopotential (SIC-PP) method (b). Reprinted with permission from ref [22]. 12 Figure 2.4: The band structure and symmetries of hexagonal ZnO with splitting of the valance and the conduction bands in the vicinity of the fundamental band-gap. 13 Figure 2.5: Shows the PL spectrum of ZnO nanoflowers and EL spectrum of ZnO nanorods based LED at room temperature [49]. 14 Figure 2.6: Schematic band diagram of some deep level emissions (DLE) in ZnO based on the full potential linear muffin-tin orbital method and other reported data [84, 92]. 18 Figure 2. 7: A typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs [49]. 18 Figure 2.8: (Color online) Load vs displacement curve of the as grown ZnO nanorods from indentation experiment, (a) buckled ZnO nanorods, and (b) bending flexibility of ZnO nanorods curve [106]. 20 Figure 2.9: (Color online) Load vs displacement curve of the etched ZnO nanotubes from the nanoindentation experiment, (a) buckled ZnO nanotubes, and (b) bending flexibility of ZnO nanotubes curve [106]. 20 Figure 3.1: The electroluminescence (EL) of the as grown ZnO nanorods on p-GaN substrates [15]. 31 Figure 3.2: The photoluminescence (PL) of the as grown ZnO nanorods on p-GaN substrates [15]. 31 Figure3.3: Anderson model energy band diagram of the n-ZnO/p-GaN heterojunction structure. 32 Figure 3.4: Shows typical I-V characteristic for different ZnO (nanostructures) /p-GaN LEDs [16]. 34 Figure 3.5: Shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [16]. 34 Figure 3.6: Photo-response of a single ZnO nanowire under pulsed illumination by a 365 nm wavelength UV light with (a) Schottky contact on one side, and (b) ohmic contacts on both sides [66]. 36 Figure 3.7: Reproducibility and stability of the glucose-sensing microelectrodes [30]. 37 Figure 3.8: Time response of the ZnO nanowires/Uricase sensor electrodes in 100 µM uric acid solution (a) without membrane, and (b) with membrane. The calibration curves for the uric acid sensor (c) with membrane, and (d) without membrane [31]. 38 Figure 4.1: Schematic illustraion of the device fabrication. 45 Figure 4.2: A schematic diagram of the vapor-liquid-solid technique. 46 Figure 4.3: The VLS growth presentation of ZnO nanorods. 46 Figure 4.4: SEM images for ZnO nanorods grown on different substrates, (a, b) p-GaN, (c-e) 4H-SiC, and (f-h) Si under different growth parameters. 49 Figure 4.5: Shows the SEM images of ZnO nanorods with different diameters, (a) ~440 nm, (b) ~200 nm, (c) ~150 nm, and (d) ~35 nm. 51 Figure 4.6: SEM images of (a) ZnO nanotubes, (b) ZnO nanowalls, and (d) ZnO nanoflowers. 53 Figure 4.7: SEM image of grown ZnO nanorods by the sol-gel method. 54 Figure 4.8: The SEM image of grown ZnO nanorods by Electro-Chemical deposition (ECD) method. 55 Figure 5.1: Schematic diagram of the scanning electron microscopy. 62 Figure 5.2: Typical SEM images of different ZnO nanostructures (a) nanorods, (b) nanoflowers, (c) nanotubes, and (d) nanowalls. 63 Figure 5.3: A 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN. 64 Figure 5.4: Shows a schematic diagram of Bragg reflection from crystalline lattice planes having interplan distace “d” between two lattice plane. 65 Figure 5.5: Display the θ-2θ XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes grown on p-GaN substrates, respectively. 66 Figure 5.6: A schematic diagram of the PL setup. 68

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Figure 5.7: A typical I-V characteristics for different ZnO (nanostructures)/p-GaN LEDs [3]. 68 Figure 6.1: (a) Room temperature photoluminescence spectrum for the ZnO nanorods on p-4H-SiC substrate, and in (b) the room temperature photoluminescence spectrum for the ZnO nanorods on p-GaN substrate [1]. 77 Figure 6.2: (a)Electroluminescence spectrum for ZnO nanorods/p-4H-SiC LED, and in (b) electroluminescence spectrum for the ZnO nanorods/p-GaN LED [1]. 77 Figure 6.3: Room temperature photoluminescence spectrum for ZnO nanostructures (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes on p-GaN and (e) shows the combine PL spectra of all the four nanostructures [4]. 80 Figure 6.4: Displays the electroluminescence spectra for n-ZnO (nanostructures)/p-GaN LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) shows the EL spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [4]. 82 Figure 6.5: Typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs as indicated in the figure [4]. 82 Figure 6.6: Schematic band diagram of the DLE emissions in ZnO based on the full potential linear muffin- tin orbital method and the reported data and references in [6]. 85 Figure 6.7: Electroluminescence spectra of different LEDs at an injection current of 3 mA, under forward bias of 25 V and it shows the shift in emission peak after annealing in different ambients [6]. 85 Figure 6.8: The CIE 1931 x, y chromaticity space of ZnO nanotubes annealed in different ambients [6]. 88 Figure 6.9: Normalized spectral power distributions for the LEDs based on the as-grown n-ZnO nanorods and after annealing in air, oxygen, and nitrogen ambients [11]. 88 Figure 6.10: The chromaticity coordinates of different LEDs (a) under forward bias and (b) under reverse bias plotted on the CIE (1931) x,y, chromaticity diagram [11]. 91 Figure 6.11: The energy band diagram of the n-ZnO nanorods/p-4H-SiC heterostructure [13]. 93 Figure 6.12: (a), (b), and (c) illustrate the measured forward voltage versus temperature for the n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterostructures at different values of the current respectively and (d), (e), (f) show the measured series resistance versus the junction temperature for n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si), respectively [13]. 96 Figure 6.13: (a, b) show the measured I-V characteristics of the n-ZnO nanorods/ p-4H-SiC, p-GaN at different temperatures, (c) shows the measured I-V characteristics of the n-ZnO nanorods/ p-Si LEDs and (d, e) shows the electroluminescence spectrum for ZnO (NRs)/p-4H-SiC and p-GaN LEDs [13]. 96 Figure 6.14: Room temperature photoluminescence spectra for ZnO nanorods (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the PL spectra of all the samples together for comparison [18] 99 Figure 6.15: Display the electroluminescence spectra for n-ZnO nanorods/p-GaN LEDs, (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the EL spectra of all the LEDs together for comparison [18]. 99 Figure 6.16: Display the CIE 1931 x, y chromaticity space, showing the chromaticity coordinates of LEDs under forward bias for ZnO NRs/p-GaN LEDs, (a) as grown ZnO NRs, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) all together for comparison [18]. 102

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List of Tables

Table 2.1: Basic physical properties of ZnO at room temperature [1, 9-13] 10 Table 4.1: Different ohmic contacts schemes for p-type GaN 56 Table 4.2: Different ohmic contacts schemes for n-type ZnO 57

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Chapter 1 Introduction and motivation Nanotechnology has developed a bridge among all the fields of science and technology. Materials and structures with low dimensions have excellent properties which enable them to play a crucial role in the rapid progress of the fields of science. With these amazing properties, one dimensional nanostructures have become the back bone of research in all the fields of natural sciences.

In the past decade, the global research interest in wide band gap semiconductors has been significantly focused to zinc oxide (ZnO) due to its excellent properties as a semiconductor material. The high electron mobility, high thermal conductivity, good transparency, wide and direct band gap (3.37 eV), large exciton binding energy and easiness of growing it in the nanostructure form by many different methods make ZnO suitable for wide range of uses in optoelectronics, transparent electronics, lasing and sensing applications [1-4]. In last decade, the number of publications on ZnO has increased annually and in 2007 ZnO has become the second most popular semiconductor after Si and its popularity is still increasing with time [5].

Obtaining controllable, reliable, reproducible and high conductive p-type doping in ZnO has proved to be very difficult task [6-9], due to the low formation energies for intrinsic donor defects such as zinc interstitials (Zni) and oxygen vacancies (VO) which can compensate the accepters. The efficiency of light emitting diodes can be limited by the low carrier concentration and mobility of holes therefore the excellent properties of ZnO might be best utilized by constructing heterojunctions with other semiconductors. Therefore the growth of n-type ZnO on other p-type materials could provide an alternative way to realize ZnO based p-n heterojunctions. In this way, the emission properties of LEDs can still be determined by the excellent optical properties of ZnO. Various heterojunctions of ZnO thin films have been achieved using various p-type materials like, GaN, AlGaN, Si, CdTe, GaAs, and diamond [10-15]. The p-GaN is the best among the candidates for developing heterojunction based LEDs with n-ZnO because it has many advantages over other p-type materials. Both ZnO and GaN have the same wurtzite crystal structure, the same lattice parameters (lattice mismatch is only 1.8%) and have almost the same band gap of 3.37 eV and 3.4 eV, respectively at room temperature.

There are also reports on the fabrication of more complex device structures. To modify the device structure, some insulating or undoped layers were introduced between n- ZnO nanorods and p-GaN but insertion of such layers in the device structures has changed the emission spectra as compared to simple n-ZnO/p-GaN LEDs [16-21].

The n-ZnO/p-GaN LEDs have great potential to be a possible candidate for white light source as they emits emission covering the whole visible spectrum with no need of light conversion. There is a large variety of results that have been reported in the literature on the emission spectra and heterojunction investigations for ZnO nanostructures/p-GaN LEDs. The comprehensive investigations of the properties of n-ZnO/p-GaN LEDs are still great of interest. The low cost grown ZnO nanostructures/p-GaN thin films LEDs are of special interest. The ZnO nanorods and nanotubes based LEDs are more interesting as they have the potential to improve light extraction [22].

In this research work, n-ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) were grown by low cost aqueous chemical growth (ACG) and vapor liquid solid (VLS) growth techniques on p type GaN, 4H-SiC and Si substrates to construct p-n heterojunction LEDs. The luminescence properties and color quality of the fabricated LEDs were investigated. A relation for junction temperature of the n-ZnO nanorods/p-GaN, p- 4HSiC, p-Si LEDs was also modeled and experiments were performed to validate the model. The influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods/p-GaN LEDs was also investigated for nuclear and space application.

Now the world is seeking to replace the high energy consumption conventional light bulbs with low energy consumption LEDs, and in this way there will be a decrease in the energy consumption by around 20%. According to the recent analysis by the U.S. Department of Energy (DOE), the estimated cumulative energy saving for replacing lighting with LEDs for period spanning 2010-2030 is $ 120 billion at today’s energy prices and it will also reduce the emission of carbon in the environment by 246 million metric ton [23].

However, the GaN is still leading the commercial LEDs in the market but ZnO has also much potential to compete and over head the GaN based LEDs. But the problem is to understand and control the origin of visible emissions which is still controversial after investigations for decades. This thesis is mainly devoted to investigations about the luminescence properties of ZnO nanostructures and the implementation of these nanostructures as white LEDs. Annealing studies of ZnO nanotubes were also performed to

2 investigate the origin of different emissions in ZnO by using photoluminescence (PL) and electroluminescence (EL) experiments at room temperature. The luminescence comparison study of ZnO nanorods grown on different p-type substrates and different nanostructures on the same substrate were also used to investigate the emissions in ZnO.

This thesis has been organized in this way; chapter 2 focuses on some basic properties of ZnO and gives a brief discussion about luminescence, electrical and mechanical properties of ZnO. Chapter 3 focuses on LEDs, UV detectors, and biosensing applications of ZnO nanostructures. Chapter 4 describes the synthesis of different ZnO nanostructures by using different growth techniques and fabrication of LEDs. Chapter 5 focuses on the experimental and characterization techniques used in this research work. Chapter 6 describes the results and finally in chapter 7 the thesis ends with concluding remarks.

References:

[1] A. Janotti and C. G. Van de Walle, Rep. Prog. Phys. 72, 126501 (2009) [2] M. Willander et al., Nanotechnology, 20, 332001 (2009) [3] Z. L. Wang, Materials Today 7, 26 (2004) [4] U. Ozgur et al., J. Appl. Phys. 98, 1 (2005) [5] Peter Klason, Zinc oxide bulk and nanorods, A study of optical and mechanical properties, PhD thesis, University of Gothenburg, (2008) [6] S. B. Ogale, Thin films and Heterostructures for Oxide Elelectronics (New York: Springer) (2005) [7] N. H. Nickel, and E. Terukov, (ed) Zinc Oxide—A Material for Micro- and Optoelectronic Applications (Netherlands:Springer) (2005) [8] U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 (2005) [9] C. Jagadish and S. J. Pearton (ed) Zinic Oxide Bulk, Thin Films, and Nanostructures (New York: Elsevier) (2006) [10] C. X. Wang, G. W. Yang, H. W. Y. H. Han. J. F. Luo, C. X. Gao, and G. T. Zou, Appl. Phys. Lett. 84, 2427 (2004) [11] X. D. Chen, C.C. Ling, S. Fung, C. D. Beling, Y. F. Mei, R. K. Y. Fu, G. G. Siu, and P. K. Chu, Appl. Phys. Lett. 88, 132104 (2006) [12] Y. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, Appl. Phys. Lett. 83, 4719 (2003) [13] J. A. Aranovich, D. Golmyo, A. L. Fahrebruch, and R. H. Bube, J. Appl. Phys. 51, 4260 (1980). [14] Q. Qin, L. W. Guo, Z. Zhou, H. Chen, X. I. Du, Z. X. Mel, J. F. Jia, Q. K. Xue, and J. M. Zhou, Chin. Phys. Lett. 22, 2298 (2005) [15] Y. Alivov, J. E. V. Nostrand, D. C. Look, B. M. Ataev, and A. K. Omaev, Appl. Phys. Lett. 83, 2943 (2003) [16] R. W. Chuang, R. X. Wu, L. W. Lai, and C. T. Lee, Appl. Phys. Lett. 91, 231113 (2007) [17] C. P. Chen, M. Y. Ke, C. C. Liu, Y. J. Chang, F. H. Yang, and J. J. Huang, Appl. Phys. Lett. 91, 091107 (2007) [18] J. W. Sun, Y. M. Lu, Y. C. Liu, D. Z. Shen, Z. Z. Zhang, B. H. Li, J. Y. Zhang, B. Yao, D. X. Zhao, and X. W. Fan, J. Phys. D: Appl. Phys. 41, 155103 (2008)

[19] L. Zhao, C. S. Xu, Y. X. Liu, C. L. Shao, X. H. Li, and Y. C. Liu, Appl. Phys. B 92, 185 (2008) [20] M. K. Wu, Y. T. Shih, W. C. Li, H. C. Chen, M. J. Chen, H. Kuan, J. R. Yang, and M. Shiojiri, IEEE Photon. Technol. Lett. 20, 1772 (2008) [21] S. J. An, and G. C. Yi, Appl. Phys. Lett. 91, 123109 (2007) [22] A. M. C. Ng, Y. Y. Xi, Y. F. Hsu, A. B. Djurisic, W. K. Chan, S. G. wo, H. L. Tam, K. W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, Nanotechnology 20, 445201 (2009) [23] http://www1.eere.energy.gov/buildings/ssl/news_detail.html?news_id=15806

Chapter 2 Properties of ZnO In the past decade global research interest in wide band gap semiconductors has been attracted towards zinc oxide (ZnO) due to its excellent properties as a s emiconductor material. The high electron mobility, high thermal conductivity, good transparency, wide direct band gap (3.37 eV), large exciton binding energy and easiness of growing it in the nanostructure form make ZnO suitable for optoelectronics, transparent electronics, lasing, sensing, and wide range of applications [1-4].

2.1 Basic properties of ZnO ZnO crystallize preferentially in the stable hexagonal wurtzite structure at room temperature and normal atmospheric pressure as shown in figure 2.1. It has lattice parameters a= 3.296 nm, c = 0.520 nm with a density of 5.60 g cm-3. The electronegativity values of O-2 and Zn+2 are 3.44 and 1.65, respectively resulting in very strong ionic bonding between Zn+2 and O-2. Its wurtzite structure is very simple to explain, where each oxygen ion is surrounded tetrahedrally by four zinc ions, and vice versa, stacked alternatively along the c-axis. It is clear that this kind of tetrahedral arrangement o f O -2 and Zn+2 in ZnO will form a non central s ymmetric structure composed of two interpenetrating hexagonally closed packed sub-lattices of zinc and oxygen that are displaced with respect to each other by an amount of 0.375 along the hexagonal axis. This is responsible for the piezoelectricity observed in ZnO. It also plays a vital role in crystal growth, defect generation and etching.

Other basic characteristics of ZnO are the polar surfaces that are formed by oppositely charged ions produced by positively charged Zn+ (0001) and negatively charged O- (000ī) polar surfaces. It is responsible for the spontaneous polarization observed in ZnO. The polar surfaces of ZnO have non-transferable and non-flowable ionic charges. The interaction among the polar charges at the surface depends on their distribution, therefore the structure is arranged in such a way to minimize the electrostatic energy, which is the main driving force for growing polar surface dominated nanostructures. This effect results in a growth of various ZnO nanostructures such as nanowires, nanosprings, nanocages, nanobelts, nanocombs, nanorings, and nanohelices [1].

Figure 2.1: The hexagonal wurtzite structure of ZnO. One unit cell of the crystal is out lined for clarity.

Figure 2.2: The zincblende (left) and rock salt (right) phases of ZnO. Only one unit cell is illustrated for clarity.

The wurtzite ZnO has four common face terminations and these are polar Zn terminated (0001), and the O terminated (000ī) along c-axis. The non-polar faces are (1120) and (1010). The non-polar surfaces contain equal number of Zn and O atoms. The polar faces are stable and have different chemical and physical properties. The O terminated face (000ī) has slightly different electronic structure than the other three faces [2]. Due to lack of center of inversion in the wurtzite ZnO, the grown ZnO nanorods and nanotubes have two different polar surfaces on the opposite sides of the crystal. These different polar surfaces are formed due to the sudden termination of the Zn terminated (0001) surface with Zn cations outermost and the O terminated (000ī) surface with O anion outermost [1-3]. In addition to the wurtzite structure, ZnO can also crystallize in the cubic zinc-blende and the rock-salt (NaCl) structures which are illustrated in figure 2.2 [1].

The growth of the zincblende ZnO is a challenge as zincblende ZnO is stable only by growth in cubic structures [4-5]. The cubic rock salt structure exists only at high presser (10 GPa) and cannot be epitaxially stabilized [6]. In the rock salt structures each Zn or O atom has six nearest neighbor atoms but in the wurtzite and zincblende structure each Zn and O atom has only four nearest neighbors. The zincblende has lower ionicity as compared to the wurtzite structure and leads to lower carrier scattering and high doping efficiencies [7]. Theoretical calculations indicated that a fourth phase cubic cesium chloride, may be possible at extremely high temperatures, however, this phase has not yet been experimentally observed [8].

2.2 The basic physical parameters for ZnO The basic physical parameters of ZnO at room temperature are shown in table 2.1 [1, 9-13]. There is still uncertainty in the values of the thermal conductivity due to the influence of defects in the material. A stable and reproducible p-type ZnO is still a challenge and cannot be achieved and the hole mobility and its effective mass are still doubtful. The values of the carrier mobility can surely be increased after achieving good control on the defects in the material [14].

Properties of wurtzite ZnO

Lattice parameters at 300 K

a0 0.32495 nm

c0 0.52069 nm

a0/c0 1.602 (ideal hexagonal structure shows 1.633) Density 5.606 g/cm3 Stable phase at 300 K Wurtzite Melting point 1975 oC Thermal conductivity 0.6, 0.13 1–1.2 o -6 -6 Linear expansion coefficient (/ C) a0: 6.5 × 10 , c0: 3.0 × 10 Static dielectric constant 8.656 Refractive index 2.008, 2.029 Energy gap 3.4 eV, direct <106 cm-3 (max n-type doping>1020 cm-3 electrons; Intrinsic carrier concentration max p-type doping<1017 cm-3 Exciton binding energy 60 meV Electron effective mass 0.24 Electron Hall mobility at 300 K for 200 cm2/V s low n-type conductivity Hole effective mass 0.59 Hole Hall mobility at 300 K for low 5–50 cm2/Vs p-type conductivity Bulk Young’s modulus E (GPa) 111.2 ± 4.7 Bulk hardness, H (GPa) 5.0 ± 0.1

Table 2.1: Basic physical properties of ZnO at room temperature [1, 9-13]

2.3 Electronic band structure of ZnO

The electronic band structure of a semiconductor is very important to be understood for its utility in devices and for further improving the performance of these devices [12, 15]. The electronic band structure gives understanding of the electron/hole states. ZnO is a direct band gap semiconductor. Several theoretical approaches such as the Local

Density Approximation, the Green’s functional method and the first principles were used to calculate the energy band diagram of wurtzite as well as zincblende and rocksalt polytypes of ZnO [15-31]. In parallel to the theoretical efforts, a number of experimental techniques such as X-ray induced photo absorption, photoemission spectroscopy, angle resolved photo- electron spectroscopy, and low energy electron diffraction have been commonly employed to understand the electronic states of wurtzite ZnO [32-45].

There was a good agreement between the theoretical and the experimental results qualitatively for the wide spread of the valance band but quantitatively there was a disagreement and the prediction about the Zn 3d states remained a challenge for the research community. Recently it was found that by including the Zn 3d level effects in the calculations, the researchers have achieved good agreements with experimental data [17, 21-22]. The first theoretical calculation of the energy band of ZnO was reported in 1969 by U. Rössler and later on many approaches have been introduced and the process of theoretical improvement continued [15-31]. In 1995 D. Vogel et al. [22] have calculated the electronic band structure in which they included the Zn 3d electrons effects, their results are shown in figure 2.3. It shows that the lowest conduction band minima and highest valance band maxima are at the Γ point k = 0, which clearly indicates that ZnO is a direct band gap material. In figure 2.3 (b), there are ten bands at ~ -9 eV in the bottom while these bands are absent in figure 2.3 (a). These bands in the bottom are due to Zn 3d levels, as the effect of these levels were included in the calculation of the figure 2.3 (b), while in the figure 2.3 (a) these effects were not included. In figure 2.3 (b), six bands between -5 eV-0 eV are representing the O 2p bonding states. The bandgap measured in the figure 2.3 (a) is about 3 eV. The bandgap shrinks here due to the fact that to simplify the calculations in the standard LDA method, the Zn 3d states have been taken as core levels. While in the SIC-PP calculation in figure 2.3 (b), the bands are considerably shifted down in energy and in response, the bandgap opened drastically. The band gap calculated by this method was 3.37 eV, which is very close to the experimental value that is 3.34 eV.

(b)

Figure 2.3: The band structure of bulk wurtzite ZnO calculated by the LDA method (a) and self-interaction corrected pseudopotential (SIC-PP) method (b). Reprinted with permission from ref [22].

The band structure and symmetry of the hexagonal ZnO structure are shown in figure 2.4. Schematically it shows how the ZnO valance band splits experimentally to three subbands A, B, and C in the vicinity of the bandgap of the ZnO due to the interaction of the crystal field and spin orbit. There is Γ7 symmetry in the A, C subbands while B subband has

Γ9 symmetry. The transitions from A and B subbands to the conduction band are dipole spin flip and can be allowed only when E ┴ c and from C subband to conduction band can be only when E║ c [46-47].

Figure 2.4: The band structure and symmetries of hexagonal ZnO with splitting of the valance and the conduction bands in the vicinity of the fundamental band-gap.

2.4 Optical properties of ZnO

The optical properties of a s emiconductor are dependent on both the intrinsic and the extrinsic defects in the crystal structure. The investigations of the optical properties of ZnO has a long history that started in the 1960s [48] and recently it has become very attractive among wide band gap materials due to its direct wide band gap (3.37 eV) with large exciton energy (60 meV) at room temperature. The efficient radiative recombinations has made ZnO promising for its applications in optoelectronics. The optical properties of ZnO, bulk and nanostructures have been investigated extensively by luminescence techniques at low and room temperatures. The photoluminescence (PL) spectra of ZnO nanoflowers and electroluminescence (EL) spectra of ZnO nanorods/p-GaN LED at room temperature are shown in figure 2.5 (a, b). The ultra-violet (UV) emission band and a broad emission band are observed. The UV emission band is commonly attributed to transition recombinations of free excitons in the near band-edge of ZnO. An exciton is a pair of excited electron and hole that are bound together by their Coulomb attraction. There are two classes of excitons. The

13 excitons can be free to move through the crystal or can be bound to neutral or charged donors and accepters [49]. The broad emission band in the visible region (420 nm - 750 nm) is attributed to deep level defects in ZnO. There are many different deep level defects in the crystal structure of ZnO and they affect the optical and electrical properties of ZnO. Their details are explained in the next section.

540 nm 376 nm 25000 12000 (PL spectra) 450 nm (EL spectra)

10000 20000

8000 15000 6000 10000 4000 525 nm IntensityEL (a.u.) 400 nm PL IntensityPL (a.u.) 2000 5000

0 0 300 400 500 600 700 400 500 600 700 800 900 Wavelength (nm) Wavelength (nm)

Figure 2.5: Shows the PL spectrum of ZnO nanoflowers and EL spectrum of ZnO nanorods based LED at room temperature [49]. 2.5 Defects and luminescence in ZnO

For any luminescent material, it is very important to investigate the origin of its luminescent centers and it is a key topic in optoelectronics. The electrical and optical properties of a semiconductor material can be controlled and modified by controlling the quantity and nature of the defects in it. These defects can be introduced during the growth process or by post-growth treatments such as annealing or ion implantation. It is very crucial to understand the behavior of these defects in ZnO. Both extrinsic and intrinsic luminescence properties of ZnO based on e xtrinsic and intrinsic defects are under debate since 1960s. Especially the origin of intrinsic luminescence in ZnO is still controversial due to native point defects. The ZnO has donor and accepter energy levels below and above the conduction and valance bands, respectively and these are responsible for the near-band edge emissions. ZnO has also deep energy levels in the band gap with different energies and these deep levels are responsible for the emissions in the whole visible region from 400-750 nm. The origin of

14 these deep level emissions have a considerable interest and is still under debate and many people suggested different origins of these deep level emissions [1, 12, 50-66].

There are three common types of defects such as, line defects, point defects and complex defects. Line defects belong to the rows of atoms such as dislocations, while point defects belong to the isolated atoms in localized regions and the composition of more than one point defect form the complex defects. There are intrinsic and extrinsic point defects and both contribute to the luminescence properties in ZnO [12-1]. If foreign atoms such as impurities are involved in the defects then these defects are called extrinsic point defects. If the defects only consist of host atom, then these defects are called intrinsic atoms. Intrinsic optical recombinations take place between the electrons in the conduction band and holes in the valance band [12]. The deep level emission (DLE) band in ZnO has been previously attributed to different intrinsic defects in the crystal structure of ZnO such as oxygen vacancies (VO) [4-

8], oxygen interstitial (Oi) [56-59], zinc vacancies (VZn) [60-63], zinc interstitial (Zni) [64-65] and oxygen anti-site (OZn) and zinc anti-site (ZnO) [66]. The extrinsic defects such as substitution Cu and Li [67, 58] are also suggested to be involved in deep level emissions.

The vacancy defects are formed when a host atom C is missing in the crystal and it is denoted by VC. In ZnO, oxygen vacancy (VO) and zinc vacancies (VZn) are the two most common defects. Vacancy is an intrinsic defect. The single ionized oxygen vacancies in ZnO are responsible for the green emission in ZnO. The oxygen vacancy has lower formation energy than the zinc interstitial and dominates in zinc rich growth conditions. The red luminescence from ZnO is attributed to doubly ionized oxygen vacancies [68]. The origin of the green emission in ZnO is the most controversial and many hypotheses have been proposed for this emission [50, 69-76]. Zinc vacancies were thoroughly investigated and suggested by many researchers to be the source of the green emission appeared at 2.4 - 2.6 eV below the conduction band in ZnO [77-78, 63]. Many researchers also suggested oxygen vacancies as the source of green emission in ZnO [79-80, 87, 55]. There are also reports that oxygen interstitials and extrinsic deep levels such as Cu are sources of the green emission in ZnO [81, 46, 12]. Recently it has been investigated that more than one deep levels are involved in the green emission in ZnO. It is found that VO and VZn both contribute to the green emission [69, 82-83]. The blue emission in ZnO belongs to zinc vacancies. The blue emission is attributed to the recombination between zinc interstitial (Zni) energy level to VZn energy level and it is approximately 2.84 eV (436 nm). It can be explained by the full potential linear muffin-tin orbital method, which explains that the position of the VZn level is approximately located at

3.06 eV below the conduction band and the position of the Zni level is theoretically located at 0.22 eV below the conduction band [84].

The interstitial defects are formed when an excess atom D occupying an interstitial site between the normal sites in the crystal structure and it is denoted by Di. In

ZnO, oxygen interstitial (Oi) and zinc interstitial (Zni) are the two common defects. These are also intrinsic defects. The zinc interstitial defects are normally located at 0.22 eV below the conduction band and play vital role in the visible emissions in ZnO by recombination between

Zni and different defects in the deep levels such as oxygen and zinc vacancies, oxygen interstitials and produce green, red and blue emissions in ZnO [84]. Oxygen interstitials defects are normally located at 2.28 eV below the conduction band and are responsible for the orange-red emissions in ZnO [75-87].

The yellow emission in ZnO has also been attributed to oxygen interstitials [62, 87]. Recently the yellow emission was observed in ZnO nanorods grown at low temperature by the aqueous chemical growth method and it was attributed to Oi and the Li impurities in the growth material [58]. The yellow emission in the chemically grown ZnO nanorods was also attributed to the presence of Zn(OH)2 that is attached to the surface of the nanorods.

Anti-site defects are formed when atoms occupy wrong lattice position. In ZnO, the oxygen and zinc anti-site defects are formed when zinc occupies oxygen position or oxygen occupies zinc position in the lattice. These defects can be introduced in ZnO by irradiation or ion implantation treatments. The transitions at 1.52 eV and 1.77 eV above the valance band are attributed to OZn related deep levels [66].

Some cluster defects are also present in ZnO that are formed by combination of more than one point defect. The cluster defects can also be formed by a combination of point and extrinsic defects such as VOZni cluster, which is formed by oxygen vacancy and zinc interstitial and it was reported that it is situated at 2.16 eV below the conduction band [66].

Extrinsic defects also play a vital role in the luminescence from ZnO. The ultra- violet (UV) emissions in ZnO at 3.35 eV are commonly related to the excitons bound to the extrinsic defects such as Li, and Na accepters in ZnO [12]. The emission at 2.85 eV is due to copper impurities in ZnO [67]. The yellow emission at 2.2 eV was observed in Li doped ZnO thin film and Li related defects and this is located at 2.4 eV below the conduction band [89, 90]. Mn, Cu, Li, Fe, and OH are common extrinsic defects in ZnO and these defects are

16 involved in luminescence from ZnO. Defects with different energies produce the same color, for example ZnO:Cu and ZnO:Co have different energies but both emit green color [46]. Hydrogen has also an interesting role in luminescence from ZnO. It is not a deep level defect and lays at 0.03-0.05 eV below the conduction band [91]. Hydrogen is always positive in ZnO and plays an important role as a donor and has low ionization energy, more details on this can be found in [46, 12]. The luminescence defects and their possible transitions in ZnO indicate that ZnO has a potential to emit excellent luminescence covering the whole visible region and it has the potential to be used in the fabrication and development of white light emitting diodes.

2.6 Electrical properties of ZnO

It is very crucial to understand the electrical properties of ZnO for applications in nanoelectronics. The electrical behavior of undoped ZnO nanostructures is n-type and it is widely believed that it is due to native defects such as oxygen vacancies and zinc interstitials [93]. The electron mobility in undoped ZnO nanostructures is not constant and it depends on growth method and it is approximately 120-440 cm2 V-1 s-1 at room temperature [12].

By doping, the highest reported carrier concentration is ~ 1020cm-3 and ~1019 cm- 3 for electron and holes, respectively [94]. However, such high levels of p-conductivity are not stable or reproducible. The doping affects the carrier mobility in ZnO and doped ZnO has lower mobility as compared to undoped ZnO. It is due to carrier scattering mechanism which includes ionized impurity, non-ionized impurity, polar optical-phonon and acoustic phonon scatterings. [12]. At room temperature the mobility of electrons is 200 cm2 V-1 s-1 and the hole 2 -1 -1 mobility is 5-50 cm V s . The effective mass of electrons is 0.24m0 and the effective mass of the holes is 0.59m0 and due to this difference in effective mass, the holes have very less mobility as compared to electrons [11].

We have fabricated n-ZnO (nanostructures)/p-GaN LEDs [37]. The current voltage (I-V) characterization of these fabricated LEDs is shown in figure 2.7 All the LEDs show rectifying behavior as expected. Reasonable p-n heterojunctions are achieved and the turn-on voltage of these LEDs was around 4 V . ZnO nanotubes based LED shows higher current as compared to other nanostructures based LEDs and it is due to the large surface area of the nanotubes as compared to other nanostructures.

Figure 2.6: Schematic band diagram of some deep level emissions (DLE) in ZnO based on the full potential linear muffin-tin orbital method and other reported data [84, 92].

nanotubes 1.0 nanorods

0.9 nanoflowers nanowalls

0.8 1 0.7

0.6 0.1 0.5

0.4 Current (mA) 0.01

Current (mA) 0.3 1E-3 0.2 -8 -6 -4 -2 0 2 4 6 8 Voltage (V) 0.1 0.0

-0.1 -8 -6 -4 -2 0 2 4 6 8 Voltage (V)

Figure 2. 7: A typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs [49].

2.7 Mechanical properties

ZnO has lack of center of symmetry and it gives rise to piezoelectric effect in ZnO. By using this piezoelectric effect in ZnO, mechanical stress or strain can be converted into electrical power. At the bottom scale, self powered energy is highly desired to operate many devices. It is one of the main targets of nanotechnology to develop ultra small, self powered nanosystems. Recently Z. L. Wang et al. have investigated ZnO nanowires for producing electric power [95]. ZnO nanostructures based nanogenerators have potential to fill the demand of ultra small self powered nanosystems. Therefore, it is very important to investigate the mechanical characteristics of ZnO nanostructures. There are many reports on the investigations of ZnO nanowires for mechanical buckling properties [96-105]. We have investigated the buckling and elastic stability of vertical ZnO nanorods and nanotubes quantitatively by nano-indentation technique [106]. Euler buckling model and shell cylindrical model were used for the analysis of the mechanical properties. The critical load, modulus of elasticity, and flexibility were measured. The critical load of nanorods was found to be five times larger than that of nanotubes and nanotubes were found to be five time more flexible than nanorods. The critical load, critical stress, strain, and Young modulus of elasticity were measured.

Figure 2.8: (Color online) Load vs displacement curve of the as grown ZnO nanorods from indentation experiment, (a) buckled ZnO nanorods, and (b) bending flexibility of ZnO nanorods curve [106].

Figure 2.9: (Color online) Load vs displacement curve of the etched ZnO nanotubes from the nanoindentation experiment, (a) buckled ZnO nanotubes, and (b) bending flexibility of ZnO nanotubes curve [106].

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Chapter 3 Device applications of ZnO nanostructures ZnO is a large band gap semiconductor with excellent, optical, electrical, chemical, piezoelectric and mechanical properties. It is a very attractive material for applications in electronics, photonics, sensing and acoustics. ZnO is considered to be an alternative to GaN for device applications due to many reasons such as low production cost and excellent optical properties. Its high exciton binding energy is one of the properties that make it superior over other semiconductors. It is an n-type material by un-intentional growth. The achievement of stable and reproducible p-type ZnO is still a challenge for the research community and it is the main obstacle in the fabrication and development of p-n homojunction ZnO based optical devices. ZnO is also very attractive for transparent electronics due to high transmittivity (for visible light). It is very attractive for acoustic wave resonators, biosensors, gas sensors and solar cell device applications. ZnO has a rich variety of nanostructures and these nanostructures are used in a variety of devices such as, LEDs [1- 23], bio-sensors [24-31], UV detectors [32-38], gas sensors [39-41] and nano-generators [42- 43] and bulk acoustic wave resonators (BAW) [44-47]. 3.1 Light emitting diodes The artificial lighting has become the soul of modern human society. In USA 20% of the total generated electricity is used for general lighting. It is an ever-growing desire of the society to develop efficient and environment friendly white light sources. Light emitting diodes (LEDs) based light sources have potential to decrease electricity consumption for light sources by 50 % and decrease the environmental pollution threats. The conventional light sources are responsible for environmental pollution and add 1900 million tons of CO2 in the environment each year. LEDs based light sources are more reliable than conventional light sources. The lifetime of incandescent and fluorescent lamps is 1000 h and 10000 h, respectively. The fluorescent lamps use phosphors for white light generation which decreases the output power and increases the absorption within the lamp. The theoretical expected life time for LEDs is 50000 h and they are strong candidates for future light sources [48-51]. ZnO is very attractive material for optical devices and has a large potential to be used in light emitting diodes. ZnO has an important advantage in optoelectronic over other semiconductors like GaN which is due to its high exciton binding energy (60 meV) at room temperature. It is attractive for LEDs in the ultra-violet region due to large exciton binding

27 energy (60 meV) at room temperature and in the visible region due to deep level defects. These deep level defects are responsible for emissions in the visible region from 420 nm to 750 nm. The achievement of ZnO p-n homojunction based LEDs is still very attractive but it is dependent on the achievement of stable p-type ZnO. A stable and reproducible p-type ZnO with acceptable electrical and optical properties is not achieved yet, that’s why p-n homojunction LED is still a challenge. However some laboratories reported the fabrication of ZnO based p-n homojunction LEDs but the electroluminescence from these fabricated LEDs was too week [1-3], and the results were not reproduced by any other research group. This problem motivated the research community to develop hybrid LEDs. The excellent optical properties of ZnO can be utilized in the best way by constructing ZnO heterojunctions with other p-type materials such as Si, SiC, GaN, AlGaN, Cu2O, GaAs, diamond, ZnTe, CdTe and NiO or p-type organic materials [21, 52]. There are different designs for n-ZnO based hybrid single and double heterostructure LEDs which have been reported such as: (1) n-ZnO/p-GaN single heterostructure LED (2) Inverted p-ZnO/n-GaN single heterostructure LED (3) n-ZnO/p-SiC single heterostructure LED (4) n-ZnO/p-AlGaN single heterostructure LED (5) GaN/ZnO/GaN double-heterostructure LED (6) MgZnO/ZnO/AlGaN/GaN double heterostructure LED (7) Double heterostructure LED with CdZnO active layer More details about these designs of ZnO based LEDs can be found in [20]. It is reported that in both single and double heterostructure LEDs interfacial charges due to spontaneous electric polarization have significant impact on the operation of these LEDs. Due to type II band alignment at the ZnO/III-N interface it is estimated that near the heterostructure interface the polarization charge may provide an effective carrier confinement. This may give tunneling radiative recombination of electrons and holes on opposite sides of the interface of the LED and reduce the performance of the LEDs [20]. ZnO based LEDs are facing the following problems that can affect their performance a lot [20]. (1) There is a considerable unbalance between the electrons and hole partial currents that results in additional carrier losses on the contacts. (2) These LEDs can have high series resistance that is not necessarily related to low conductivity of the p-doped layers.

(3) Poor control on the emission spectra of these devices. Some of the p-type materials are not suitable for constructing heterojunctions with ZnO as the hetero-interface contains a large lattice mismatch and can strongly affect the performance of the devices. Mostly, people prefer ZnO heterojunction with p-GaN or p- AlGaN as these have similar physical properties to ZnO [9-14]. P-type GaN is a good choice for constructing heterojunction with ZnO because the industry is already using GaN for the production of blue light emitting diodes and lasers diodes. P-type GaN is preferred over other p-type materials due to the following similarities with ZnO, (1) Both have the same wurtzite crystal structure. (2) Almost both have the same lattice parameters and the lattice mismatch is only 1.8%. (3) At room temperature both ZnO and GaN have almost the same band gap of 3.37 eV and 3.4 eV, respectively. There are also few reports that show that there can be imbalance between the partial electron and hole currents in n-ZnO/p-GaN LEDs heterojunction. The imbalance in electron and hole mobilities can be due to the difference in electron and hole mobilities and difference in the heights of the potential barriers for electrons and holes in the space charge regions at the heterojunction interface of the n-ZnO/p-GaN LEDs [20]. Some people reported the fabrication of more complex device structures. Some insulating or undoped layers were introduced between n-ZnO nanorods and p-GaN to modify the device structure but insertion of such layers in the device structures have changed the emission spectra as compared to simple n-ZnO/p-GaN LEDs [53-58]. The n-ZnO/p-GaN LEDs have still great potential to be a possible candidate for white light source as it emits emission covering the whole visible spectrum with no need of light conversion. There is a large variety of results that have been reported in the literature on the emission spectra and heterojunction investigations for ZnO nanostructures/p-GaN LEDs. The comprehensive investigations of the properties of n-ZnO/p-GaN LEDs have still large of interest. The low cost grown ZnO nanostructure/p-GaN thin films LEDs have special interest. The ZnO nanorods and nanotubes based LEDs are more interesting as they have the possibility to improve light extraction [59]. We have fabricated ZnO nanostructures/p-GaN and ZnO nanorods/p-4H-SiC LEDs [15]. Figure 3.1 shows the electroluminescence (EL) of ZnO nanorods/p-GaN LEDs. It was measured by photomultiplier detector at room temperature in the forward bias at a current of 4 mA. The EL emission was so clear that it can be easily seen by the naked eye. The EL

29 spectrum of the fabricated LEDs shows three peaks that are approximately centered at 457 nm (violet-blue emission), 531 nm (green emission), and 665 nm (orange-red emission). These emissions can be attributed to different radiative recombinations due to deep level defects in ZnO and in the Mg-doped p-type GaN substrate. At lower injection currents <2 mA the EL emission cannot be detected which may be related to the presence of non-radiative recombinations centers in the space charge region and provide a shunt path for the current [60]. At higher injection currents >2 mA, these non-radiative recombination centers are saturated and the injected carrier recombine radiatively and lead to the emission of photons. The challenge in the improvement of the performance of fabricated devices is to reduce the interface effects and to control the crystalline defects in ZnO. Further investigations are still underway to control the defects concentration at the interface of the heterojunction [60]. Figure 3.2 shows the photoluminescence (PL) of the as grown ZnO nanorods on p-GaN substrates [15]. Photoluminescence measurements were performed at room temperature. Laser lines with a wavelength of 266 nm from a laser diode pumped resonant frequency doubling unit (MBD266) was used as an excitation source. The peaks are observed at around 383 nm (band edge emission), 553 nm (green emission peak), and 693 nm (orange- red emission), respectively. The observation of the band edge emissions at 383 nm are attributed to free exciton near band edges and the presence of this peak show that our as grown ZnO nanorods have good crystalline quality. By comparing the EL and the PL spectra it is found that the PL spectrum is consistent with the EL spectrum and it is also clear that in the EL spectra the violet-blue peak centered at 457 nm does not appear in the PL spectrum. It might be suggested that it is from the substrate or from the heterojunction. Most probably it is from the Mg-doped p-GaN substrate and is attributed to transitions from the conduction band or shallow donors to deep Mg accepters.

Figure 3.1: The electroluminescence (EL) of the as grown ZnO nanorods on p-GaN substrates [15].

Figure 3.2: The photoluminescence (PL) of the as grown ZnO nanorods on p-GaN substrates [15].

It can be concluded that the EL emission from the fabricated LEDs is the combination of emissions from the n-ZnO nanorods and the p-GaN substrate. It can also be

31 explained by the energy band diagram which was developed by using the Anderson model [61] as shown in figure 3.3. For the band diagram construction, the ZnO and GaN electron affinities (χ) and band gap energies (Eg) were assumed to be (4.35 eV, 4.2 e V) and (3.37 eV, 3.4 eV), respectively. From the band diagram it can be seen clearly that the energetic barrier for electrons is:

ΔEC = χ (ZnO) – χ (GaN) = 4.35 – 4.2 = 0.15 eV and the energetic barrier for holes is:

ΔEV = Eg (ZnO) + ΔEC – Eg (GaN) = 3.37 + 0.15 – 3.4 = 0.12 eV

This means that the energy barrier for electrons and holes are approximately the same [60].

Figure3.3: Anderson model energy band diagram of the n-ZnO/p-GaN heterojunction structure.

Recently it has been reported that the performance of the ZnO film/p-GaN LEDs is decreased due to the formation of nonradiative centers at the interface [9, 11, and 13] and to improve the performance of the LEDs, a wide band gap MgO has been introduced into ZnO film/p-GaN LEDs [9]. It is also reported that intermediate layer of AlN is inserted into ZnO film/p-GaN LEDs to improve the performance of fabricated LEDs. AlN has the largest direct

32 band gap of 6.2 eV among the III-nitride semiconductors with excellent physical and chemical properties [62]. N-ZnO (nanostructures) /p-GaN based LEDs showed excellent current voltage (I-V) characterization [16]. The I-V curves of the fabricated n-ZnO nanostructures/p-GaN LEDs are shown in figure 3.4. All the LEDs showed rectifying behavior as expected. The I-V curves indicate that reasonable p-n heterojunctions were achieved between the n-ZnO nanostructures and the p-GaN substrate. The turn-on voltage of these fabricated heterojunctions LEDs is around 4 V. It can be seen that ZnO nanotubes based LEDs have higher current when compared to other nanostructures based LEDs and this is due to the large surface area of the nanotubes as compared to the other nanostructures [16]. The CIE 1931 color space chromaticity diagram in the (x, y) coordinates system for the n-ZnO nanostructures/p-GaN LEDs is shown in figure 3.5 [ 16]. The chromaticity coordinates for n-ZnO nanowalls and nanoflowers are very close to Planckian locus (about > 1 MacAdam ellipse away). It can definitely be considered white light LEDs according to the US standard ANSI_ANSLG C78, 377-2008 for solid state light sources which states that light sources having chromaticity coordinates within three MacAdam ellipses from the Planckian locus produce white light. The LEDs based on nanorods and nanotubes are more than three MacAdam ellipses away from the Planckian locus and cannot be considered white light but are very close to white light.

1.0 0.9 0.8 0.7 nanotubes 0.6 nanorods 0.5 nanoflowers nanowalls 0.4

Current (mA) 0.3 0.2 0.1 0.0 -0.1 -8 -6 -4 -2 0 2 4 6 8 Voltage (V)

Figure 3.4: Shows typical I-V characteristic for different ZnO (nanostructures) /p-GaN LEDs [16].

Figure 3.5: Shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [16].

3.2 ZnO based UV photodetectors Recently ZnO based ultra-violet (UV) photo detection has attracted a great attention due to the unique excellent structural, electrical and optical properties of ZnO. Due to these properties it is very suitable material for the production and development of UV detectors having high performance with high speed and high-detectivity. These detectors can also be low cost, radiation resistant, chemically stable, and have the potential to be used in civil, military, space, and nuclear applications [63]. In 1940, the first ultra-violet photo-response in ZnO thin films was observed [64]. In 1980s, research on ZnO based photo detectors flourished gradually and in the beginning, the properties of the fabricated UV detectors were not so good [65]. Different techniques have been adopted to improve the efficiency of ZnO thin film based UV detectors. Highly efficient ZnO based p-n and p-i-n junction and Schottky junction photo-detectors have already been reported [63]. There are many reports on ZnO photo-detectors including photoconductors, metal-semiconductor-metal (MSM) photo-detectors, Schottky photodiodes and p-n junction photodiodes [63]. Recently we have fabricated single nanowires-based UV photo-detectors for fast switching in our research group [66]. The Schottky photodiodes have many advantages over the photoconductors and metal-semiconductor-metal photodiodes in many ways as Schottky photodiodes have high quantum efficiency, high response speed, low dark current, high UV/visible contrast, and have the possibility to operate at zero bias [67-68]. The first ZnO based Schottky diode was fabricated in 1986 [65]. Many research groups have reported ZnO based Schottky photodiodes by different methods [32-34]. We have fabricated Schottky diodes by using single long (30 μm) ZnO nanowires by applying Au and Pt electrodes across the single ZnO nanowires [66]. We also compared these diodes with diode having ohmic contacts. The fabricated Schottky diodes showed rectifying behavior with no reverse breakdown up to -5 V and under forward biasing it showed high current. The fabricated schottky diode is stable up to 12 V and shows more sensitivity, lower dark current and much faster switching under pulsed UV illumination as shown in figure 3.6 (a, b) [66].

Figure 3.6: Photo-response of a single ZnO nanowire under pulsed illumination by a 365 nm wavelength UV light with (a) Schottky contact on one side, and (b) ohmic contacts on both sides [66].

3.3 ZnO based biosensors ZnO has become very attractive in the development and the application of biosensors. Researchers working in the biosensing field have put their attention to ZnO due to its excellent properties such as the bio-safety and non-toxicity. It is chemically stable and electro-chemically active and it can easily be grown in the nanostructure forms at very low cost. These promising properties have made ZnO favorite for detecting different biological molecules. The use of ZnO nanostructures as biosensors is at an earlier stage and few reports are available [24-31]. We have fabricated an improved intracellular glucose sensor based on flake material which shows better sensitivity even for very small glucose concentrations as compared to the earlier reported glucose sensors [30] and the sensitivity of the fabricated nanoflakes based glucose sensors is 1.8 times higher than ZnO nanorods based glucose sensors at the same conditions. ZnO nanoflakes grown on the tip of a borosilicate glass capillary were used as a selective intracellular glucose sensor to measure the glucose concentrations in human adipocytes and frog oocytes [30]. We have prepared glucose solutions with different glucose concentrations from 500 nM to 10 mM in a buffer (PBS pH 7.4) for the calibration of the extracellular potentiometric response of the sensor electrode. The response time of the sensor electrode became very fast when glucose was added. In 4 s it reached 95% of the steady state voltage. The response of the sensor electrode in 50 μM has a linear relationship between the steady state voltage and the glucose dependent electrochemical potential difference over the complete glucose concentration [30]. For a single human adipocyte and for a single frog oocyte, the measured intracellular glucose concentrations were approximately 60 ± 15 μM and 110 ± 20 μM,

36 respectively and these values were consistent with the reported data in the literature. The reproducibility of the fabricated sensors is shown in figure 3.7 [30]. We have also fabricated sensitive, selective, stable and reproducible potentiometric uric acid biosensor by immobilization of uricase on ZnO nanowires [31]. The potentiometric response of the fabricated biosensor vs Ag/AgCl reference electrode was linear over a relatively wide logarithmic concentration range (1 µM to 650 µM). The time response of the fabricated uric acid biosensor in 100 μM is shown in figure 3.8 (a, b) and the calibration curves for the uric acid sensor with membrane and without membrane are shown in figure 3.8 (c, d). It was found that after applying a nafion membrane on the sensor the linear range increased from 1µM to 1000 µM and the response time also increased from 6.25 s to > 9 s. The durability of the sensor also increased after applying the membrane. It was also investigated that the normal concentrations of common interferents such as ascorbic acid glucose have no effect on the sensor response [31].

Figure 3.7: Reproducibility and stability of the glucose-sensing microelectrodes [30].

Response time of Sensor in 100 uM Response time of sensor in 100uM 0.14 uric acid solution without membrane 0.14 uric acid solution with Membrane 0.12 0.12 0.10 0.10 0.08 (a) 0.08 (b) 0.06 0.06 EMF [V] EMF [V] 0.04 0.04 0.02 0.02 0.00 0.00 0 5 10 15 20 25 30 35 40 45 50 55 60 65 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Time [s ] Time [s] 0.18 0.20 0.16 (c) 0.18 (d) 0.16 0.14 0.14 0.12 EMF [V] 0.12 EMF [V] 0.10 0.10 0.08 0.08 0.06 0.06 0.04 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 Log [concentration]M Log [Concentration]M

Figure 3.8: Time response of the ZnO nanowires/Uricase sensor electrodes in 100 µM uric acid solution (a) without membrane, and (b) with membrane. The calibration curves for the uric acid sensor (c) with membrane, and (d) without membrane [31].

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Chapter 4

Synthesis of ZnO nanostructures and fabrication of LEDs Zinc oxide is a unique material with versatile variety of nanostructures and can be grown easily in many nanostructure forms such as nanorods, nanotubes, nanowalls, nanoflowers, nanowires, nanobelts, nanorings, nanocages nanosprings, and nanohelixes by using different approaches such as aqueous chemical growth (ACG), vapor-liquid-solid (VLS), electro-chemical deposition (ECD), chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), physical vapor deposition (PVD), and chemical vapor transport and condensation (CVTC) [1-21]. ZnO nanostructures used in this research work were grown on the surface of different p-type substrates (4H-SiC, GaN, Si) by different low and high temperature growth techniques. The device fabrication can be divided into five steps as shown in figure 4.1, (1) substrate preparation, (2) growth of nanostructures, (3) bottom contacts deposition, (4) spinning of photo resist and plasma etching, and finally (5) top contacts deposition. Details of these fabrication steps are given below.

4.1 Substrate preparation

First of all the substrate preparation is very crucial for achieving high quality and vertical aligned growth. Commercially purchased substrates from different companies were cleaned to remove dirty particles, chemicals, and oxide layers from the surface. The cleaning process follows the following steps; at first the samples (especially Si) were immersed in DI-H2O: HF (9:1) solution for three minutes to remove native oxide layers from the surface of the substrates. Then the substrates were immersed in acetone for sonication bath for 5 minutes at 40 oC and this process was repeated in isopropanol and ethanol as well. Between these steps the samples were washed with deionised water. After this process the samples were cleaned with deionised water many time and then dried with nitrogen.

For the growth of ZnO nanorods on Si substrate by the vapor-liquid-solid (VLS) method, the substrate needs special treatment. In the VLS growth method, a thin film of pure Au (99.99%) with thickness of 2.0 nm-5.0 nm was deposited on the substrate in a low vacuum metallization chamber. Au is used as a catalyst metal for growth of ZnO nanostructures and it has high diffusion rate into the Si at high temperatures. At the start of the VLS growth process, as the temperature increases to few hundred centigrades, the Au film diffuses into the

Si substrate and there leaves no more gold to be used as a catalyst for the growth of ZnO nanostructures. This problem can be solved by introducing a thin diffusion barrier of SiO2 between the substrate and the Au film. To introduce thin film of native oxide of silicon on the surface of silicon, at first the Si substrate was immersed for15 min in a preheated solution (at 70 oC) of hydrogen peroxide: ammonium hydroxide: deionized water with ratio of 1:1:5 to remove the insoluble organic residues that are based on the oxidation desorption and complexing with the solution. After removing the substrate from the solution, it was cleaned with deionized water several times. There is a possibility of organic contamination during the process and it can be removed by immersing the substrate in DI-H2O: HF (100:2) solution. In nd o the 2 step the substrate was placed in DI-H2O: HCL: H2O2 solution at 70 C for ten minutes to remove metal ions and heavy metal ions. This process produces a high quality layer of native oxide on the surface of silicon substrate. After that the substrates were cleaned with the process described earlier [22].

Figure 4.1: Schematic illustraion of the device fabrication. 4.2 Growth of ZnO nanostructures

We grow ZnO nanostructures by using the following methods, (1) The vapor- liquid-solid (VLS) method, (2) The aqueous chemical growth method (ACG), (3) sol gel method, and (4) Electro-chemical deposition (ECD).

4.2.1 The synthesis of nanorods by vapor-liquid-solid method Recently, the vapor-liquid-solid technique has become very well known technique for the growth of several types of ZnO nanostructures. In 1964, the theory of this growth mechanism was proposed and established by R.S. Wagner and W.C. Ellis at the Bell telephone laboratories [23]. For the growth of ZnO nanorods, using this process a thin film (2 nm - 8 nm) of Au was plated on the substrate in a high vacuum metallization chamber. After that we used two methods to grow ZnO nanorods with different types of source materials. In the first method, the source material was prepared by mixing graphite (99.99%) with ZnO (99.99%) powder in 1:1 ratio.

Figure 4.2: A schematic diagram of the vapor-liquid-solid technique.

Figure 4.3: The VLS growth presentation of ZnO nanorods.

Graphite acts as a catalyst and it reduces the vaporization temperature of the source material from ~1300 oC to ~800 oC. The source material was placed into a ceramic boat and the substrate was placed just above the source material on the boat with face downward to the source material. Argon gas was used as a carrier gas with a flow rate of 50- 80 sccm (standard cubic centimeters per minute). The growth temperature was 860 oC - 950 oC. The heating system takes 10-12 minutes to reach the growth temperature from room temperature, but for cooling from growth temperature to room temperature takes about 5 hours. The growth time was from 10 minutes to 30 minutes.

In the 2nd procedure, the source material was pure zinc powder (99.99%). The zinc powder was heated up under pure oxygen flow. The substrate was placed in the downstream side on the boat with face down towards the bottom of the boat. It was 1-2 cm away from the source material. Mixture of argon and oxygen gases with a ratio of 8:1 was introduced in the quartz tube. The oxygen pressure affects the morphology of the nanostructures significantly and it should be chosen carefully. Argon was applied as a carrier gas and oxygen as reactant gas [20, 24-25]. The growth temperature was 600 oC - 750 oC.

The growth process was conducted in a horizontal quartz tube furnace system as shown in figure 4.2. At first the boat with the source material and the substrate was placed in the middle of the tube. The quartz tube ends were sealed with rubber rings system and cooling water passed through the ends of the tubes to cool them during the growth process. Argon (carrier) and oxygen (reactant) gasses enter from one end and exist from the other end of the tube. The flow of the gasses in the tube was controlled by gas flow meters.

Around the quartz tube, there was a cylindrical heating chamber with six heating elements to maintain the temperature constant along the tube in the heating chamber. The temperature of the tube in the heating chamber was controlled by three heating sensors; one controls the temperature of the central part and the other two for the ends of the tube in the heating chamber. The heating sensors are controlled by a three digital display systems. The growth mechanism of ZnO nanorods by this method is described schematically in figure 4.3.

At high temperature Zn, CO, CO2 gas vapors are produced and the carrying gas transferred these to the Au droplet surface under the following reaction [26].

o ZnO (s) + C(S) → Zn (vapor) + CO (g) at ~ 890 C

o CO (g)+ ZnO(S) → CO2 (g) + Zn (g) at ~ 890 C

The zinc atoms have higher adsorption than CO, CO2 on the surface of Au droplet. The possible adsorption of CO/CO2 molecules is the substrate- liquid interface [27, 28]. More details of the VLS growth mechanism can be found in [29, 30].

The growth of the nanostructures can be affected by different parameters such as, growth temperature, flow rate of carrier gas and other gasses, growth time, substrates used, the distance between source material and substrate, and catalyst metal (Au) thickness. The ZnO nanorods with different diameters and lengths were grown, as shown in figure 4.4.

The growth temperature affects the density of the ZnO nanowires. At low growth temperature ~ 800 oC the density of ZnO nanorods is less as compared to higher growth temperature ~ 950 oC. It may be due to the less concentration of free Zn+2 and O+ at lower temperature (~ 800 oC) as compared to higher temperature (~ 950 oC). Due to higher concentration of free Zn+2 and O+ at higher temperatures, the super saturation of Au occurs earlier and results in a dense ZnO nanorods growth [31]. The length of the nanorods increases with growth time. We have grown ZnO nanorods from 1.2 µm to 40 µm in length.

Two things are very important for the substrates used for the growth of ZnO nanostructures by the VLS method. First the substrate must be stable at the growth temperature and the second is that the used substrate must have less lattice mismatch with ZnO. GaN has a wurtzite crystal structure which has almost the same structure of ZnO, and has very less lattice mismatch (1.9 %) [32]. Therefore growth of ZnO nanorods on GaN produces very good alignment as shown in the scanning electron microscopy (SEM) images in figure 4.4 (a). 4H-SiC has also wurtzite crystal structure and has a lattices mismatch of (5.5 %) [33] with ZnO, therefore ZnO nanorods have good alignment on 4H-SiC substrate, as shown in figure 4.4 (b-d). The Si has diamond crystal structure and do not have a good lattice mismatch (18.6%) [34] with ZnO, therefore the grown ZnO nanorods don’t show good alignment and grow randomly as shown in figure 4.4 (f, h).

(a) (b)

Figure 4.4: SEM images for ZnO nanorods grown on different substrates, (a, b) p-GaN, (c-e) 4H-SiC, and (f-h) Si under different growth parameters.

4.2.2 Synthesis of nanostructures by the aqueous chemical growth (ACG) method

4.2.2.1 Synthesis of ZnO nanorods The aqueous chemical growth (ACG) is the most common and simple method for the growth of ZnO nanorods and it was described by Vayssieres et al. [35]. It is a low o temperature (<100 C) growth technique. In this method zinc nitrate (Zn(NO3)2 6H2O) is mixed with hexa-methylene-tetramine (HMT, C6H12N4). An equimolar concentration of HMT and zinc nitrate (0.010- 0.075 mM) is usually used for growth. The substrate is placed in the solution with the growth face of the substrate down towards the bottom of the solution container and the solution container is placed in an oven at 50- 95 ºC for 2-5 hours. The growth of ZnO nanorods proceeds through the following reactions [36]. At first the HMT reacts with water and produces ammonia according to the following reaction:

(CH2)6 N4 + 6H2O → 6HCHO + 4NH3

The ammonia produced in the above reaction reacts with water and disassociates into ammonium and hydroxide ions under the following reaction:

+ - NH3 + H2O → NH4 + OH

The hydroxide ions produced in the above reaction react with zinc ions to grow solid ZnO nanorods on the substrate according to the following reaction:

- +2 2OH + Zn → ZnO (s) + H2O

After the growth, the samples were cleaned with deionized water and dried in air. The ZnO nanorods grown by the above method have less density and alignment. To improve the quality of the grown ZnO nanostructures, the growth method is combined with substrate preparation technique developed by Greene et al. [37]. In this method, ZnO seed layer is usually spun coated on substrates to form a thin and uniform seed layer. The seed solution can be prepared by two methods. In the first method 5 mM zinc acetate dihydrate is diluted in pure ethanol. The ethanol needs to be at least 99% pure otherwise growth will not lead to well aligned nanorods. The solution is shaken well until all of the zinc acetate crystals are dissolved. The solution should look transparent; if it is not transparent then it needs more pure ethanol. The second seed solution is introduced by Womelsdof et al. [38]. Using this method, we mix 0.01M zinc acetate dihydrate in methanol. The solution should also be

50 transparent. This solution is heated to 60 oC under continuous stirring and we mix 0.03 M KOH in methanol. We shake the solution until it looks transparent. We add the KOH solution drop wise to the zinc acetate solution, still under stirring. The resulting solution should be kept at 60 oC for 2h under stirring. The seed solution is applied to the substrate by spin coating. In this way we cover the sample with the seed solution and spin it at 5000 rpm. This process was repeated at least three times to get a uniform layer. Then the substrate is heated in air at 250 ºC for 20 minutes. By using this treatment, well aligned ZnO nanorods can be grown. The growth temperature, molar concentration of the HMT and the zinc nitrate, and growth time affect the length, diameter and density of the grown ZnO nanorods. ZnO nanorods with different diameters were grown by changing some growth parameters. SEM images of ZnO nanorods with different diameters ~ 440, 200, 150 and 35 nm are shown in figure 4.5 (a-d), respectively.

(a) (b)

Figure 4.5: Shows the SEM images of ZnO nanorods with different diameters, (a) ~440 nm, (b) ~200 nm, (c) ~150 nm, and (d) ~35 nm.

4.2.2.2 Growth of ZnO nanotubes ZnO nano tubes were achieved from ZnO nanorods. First the ZnO nanorods were grown by aqueous chemical growth method as described in section 4.2.2.1. The as grown ZnO nanorods were etched along the growth direction by placing the samples in 5-7.5 molar KCl (potassium chloride) solution for 5-10 hours at 95 ºC. Figure 4.6 (a) shows the SEM image of the ZnO nanotubes achieved by etching the ZnO nanorods. From the SEM images, the mean inner and outer diameters of the obtained ZnO nanotubes were approximately 360 nm and 400 nm, respectively. The Cl-1 ions in the solution of KCL can easily be adsorbed on the top surface (0001) as compared to the most stable lateral walls surface (1010) of ZnO. As the Cl-1 ions adsorb on (0001) surface, it decreases the positive charge density of the surface and makes it less stable to be etched along the c-axis [39].

4.2.2.3 Growth of ZnO nanowalls The ZnO nanowalls structures were grown on p-GaN substrate. Aluminum with a thickness of 10 nm was uniformly deposited on the substrate in a high vacuum evaporation chamber then the substrate was spin coated with a ZnO seed solution three times. The spin speed was 2000 rpm and the spinning time was 45s and the substrate was then annealed at 200 °C for 15 minutes. After that the substrate was inserted in the nutrient solution which contained equimolar concentration (0.03 M) of zinc nitride hexa-hydrate [(Zn(NO3)26H2O)] and hexa methylene tetramine (HMT) [(C6H12N4)]. The substrates are kept in the solution with growth face downward to the bottom of the solution container. The solution was heated at 90 °C for 2-5 hours to form layered hydroxide zinc acetate (LHZA),

Zn5(OH)8(CH3COO)2.2H2O, as a self template and then after that the LHZA films were transformed into nanocrystalline ZnO nanowalls without morphological deformation by heating at 150 °C in air [40-41]. The SEM image of the grown ZnO nanowalls is shown in figure 4.6 (b).The thickness of the walls was estimated from the SEM images and it was found to be around 50 nm.

4.2.2.4 Growth of ZnO nanoflowers ZnO nanoflowers were grown on p-GaN substrates by placing the substrates in an aqueous solution of 0.025 M zinc nitrate and ammonium solution (NH4OH). The ammonium solution was used to control the pH of the solution. The pH of the solution to grow the nanoflowers was around 10.5. The substrate is placed in the solution with the growth surface downward to the bottom of the solution container, and heated at 90 ºC for 3-6 hours

[42]. Figure 4.6 (c) shows the SEM image of the grown ZnO nanoflowers. The length of the leaves of the flowers is about 2.2 μm and the diameter is about 500 nm.

(a) (b)

(c)

Figure 4.6: SEM images of (a) ZnO nanotubes, (b) ZnO nanowalls, and (d) ZnO nanoflowers.

4.2.3 Growth of ZnO nanorods by sol-gel method Sol-gel method is also very common technique for the growth of well aligned ZnO nanorods at low temperature (<100 oC) [43]. In this method a sol-gel seed layers solution was prepared by mixing zinc acetate (Zn(CH3COO)2 2H2O; 0.7 M) solution in the mixture solution of 2-methoxyethanol and monoethanolamine (MEA) with equimolar ratio. Then the resultant solution was stirred at 60 oC for 30 min to yield a clear and homogeneous solution. Sol-gel solution was spun coated on the surface of the substrate with a spin speed of 3000 rpm for one minute. This process was repeated three times. The samples were then annealed at 500 oC in air for 1 h [44]. After the annealing, the substrate was put in zinc nitrate

(Zn(NO3)26H2O) and hexa-methylene-tetramine (HMT, C6H12N4) solution prepared with

53 equimolar concentration for 4-8 hrs. The SEM image of resulting ZnO nanowires grown through sol gel route is shown in figure (4.7).

Figure 4.7: SEM image of grown ZnO nanorods by the sol-gel method.

4.2.4 Synthesis of ZnO nanorods by the electro-chemical deposition (ECD)

Using this growth method, ZnO nanorods were grown on p-GaN substrate. The substrate was spun coated with ZnO seed solution three times at a spin speed of 2000 rpm for 45s and then annealed at 200 °C for 15 minutes. Then it was immersed in equimolar solution of zinc nitrate (Zn(NO3)26H2O) and hexa-methylene-tetramine (HMT, C6H12N4). The solution was then saturated with pure oxygen by bubbling it for 1 hour before starting the growth process. The substrate was used as working electrode while a platinum wire was used as a counter electrode. A constant bias of 1.1 V was applied between the electrodes for 2 hours at 90 °C. More details can be found in [45]. SEM image of grown ZnO nanorods is shown in figure (4.8).

Figure 4.8: The SEM image of grown ZnO nanorods by Electro-Chemical deposition (ECD) method.

4.3 Bottom contacts deposition

The reliability and durability of ZnO based devices depend on the development of metallization schemes with low specific contact resistance. The contacts with low resistance between a semiconductor and a metal play a vital role in the electro-optical and electrical characteristics and affects the life time of the devices. High contact resistance can influence the performance of the device [46]. Ohmic contacts serve to inject and exit the electrical current into the device and should ideally have symmetric and linear I-V relation. In this research work different metal alloys were used to form the bottom contacts to the p type GaN, 4-H-SiC, Si substrates. For the GaN, Pt/Ni/Au alloy with thickness of Pt, Ni, and Au layers of 20 nm, 30 nm, and 80 nm, respectively were used. The sample was annealed at 350

ºC for 1 min in a flowing nitrogen (N2/Ar) gas atmosphere. This alloy gives a minimum specific contacts resistance of 5.1×10-4 Ω-cm-2 [52]. Different ohmic contacts schemes for the p-GaN are shown in table 4.1. For 4H-SiC, Ni/Al alloys with thickness of Ni and Al layer of 50 nm and 400 nm were used to form ohmic contacts. The samples were annealed in nitrogen

(N2/Ar) gas at 500 ºC for 2 minutes. This contact gives minimum specific contact resistance of 7.9× 10-6 Ω-cm-2 [47]. Al layer of thickness 150 nm was used to form ohmic contact to the p-Si substrates. Different ohmic contacts schemes for p-GaN are shown in table 4.1.

Table 4.1: Different ohmic contacts schemes for p-type GaN

-2 Metallization Annealing temperature Lowest ρc (Ω- cm ) Ref Pd/Au 500 oC 4.3×10-4 48 Pt/Au 750 oC 1.5×10-3 49 Ni/Pd/Au 550 oC 4.5×10-6 50 Ni/Au 750 oC 3.3×10-2 49 Pd/Pt/Au 600 oC 5.5×10-4 51 Pt/Ni/Au 350 oC 5.1×10-4 52 Pd/Ni/Au 450 oC 5.1×10-4 53 Ni/Au 500 oC (annealed under 1.0×10-4 54 partial O2 ambient to form NiO)

4.4 Spin coating of photo resist and plasma etching

To fill the gaps between the nanorods and to isolate electrical contacts on the ZnO nanorods from reaching the p-type substrate, an insulating photo-resist layer was spun coated on the ZnO nanorods. It also helps to prevent the carrier cross talk among the nanorods. After the deposition of the insulating photo-resist, the top of the ZnO nanorods were exposed by using oxygen plasma ion etching.

4.5 Top contacts deposition

Non-alloyed Pt/Al metal system was used to form the ohmic contacts to the ZnO nanostructures. The thickness of the Pt and the Al layers were 50 nm and 60 nm, respectively. This contact gives a minimum specific contact resistance of 1.2×10-5 Ω -cm-2 [61]. Different ohmic contacts schemes for n-ZnO are shown in table 4.2.

Table 4.2: Different ohmic contacts schemes for n-type ZnO

-2 Metallization scheme Annealing Lowest ρc (Ω- cm ) Ref temperature Ti/Al/Pt/Au 200 oC 3.9×10-7 55, 56 Ti/Au 300 oC 2.0×10-4 57, 58 Ti/Au 600 oC 3.0×10-3 59 Ti/Au 500 oC 1.0×10-3 57, 58 Ti/Au None 4.3×10-5 60 Al/Pt None 2.0×10-6 61,62 Ti/Al/Pt/Au None 8.0×10-7 55, 56 Al None 4.0×10-4 61 Zn/Au 500 oC 2.36 ×10-5 63 In None 7 ×10-1 64 Re/Ti/Au 700 oC 1.7 ×10-7 65 Ru 700 3.2 ×10-5 66 Pt-Ga None 3.1 ×10-4 67

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Chapter 5 Experimental and characterization techniques Different experimental and characterization techniques were used to investigate the morphologycal, structural, optical, electrical and electro-optical properties of different ZnO nanostructures and light emitting diodes (LEDs). We have used the following characterization and experimental techniques:

(1) Scanning electron miscropy (SEM), (2) Atomic force microscopy (AFM), (3) X-ray diffraction, (4) Photoluminescence (PL), (5) Electroluminescence (EL), (6) Electrical characterization, (7) Annealing study, (8) Color rendering index extraction, and (9) Irradiation study.

5.1 Scanning electron microscopy

Scanning electron microscopy (SEM) is an important tool to investigate the surface and morophology of nanostructures. By using it we can estimate the diameter, length, thickness, density, shape and orientation of the nanostructures. Figure 5.1 s hows the schematic diagram of the SEM apparatus. An electron gun made of tungsten or LaB6 filament is used to emmit electrons by heating and these emitted electrons are directed and focused on the sample by using an anode and various electromagnetic lenses between the electron gun and sample. These projected electrons eject secondery and back scattered electrons after hitting the sample. These ejected secondary and back scattered electrons from the sample are detected by detectors and these detectors tranfer these detected electrons into electronic signal which is sent to a computer to display the image.

Figure 5.1: Schematic diagram of the scanning electron microscopy.

JEOL JSM-6301F and LEO 1550 scanning electron miscropes were used in this research work. During the measurements the vaccuum pressure in the chamber was about 10-6 mbar and the electron gun voltage was 12 kV. The maximum resolution of the SEM was up to 10 nm. Figure 5.2 (a-d) shows the SEM images of ZnO nanostructures on p-GaN substrate grown by low temperature techniques. The SEM image shows that well aligned hexagonal vertical ZnO nanorods and nanotubes were grown with high density. The grown nanorods and nanotubes cover the whole substrate uniformly. The grown nanorods and nanotubes had a uniaxial orientation of (0001) with respect to the substrate. From the SEM images we estimated that the mean diameters of our as grown ZnO nanorods was approximately 175 nm and the length of the both nanorods and nanotubes is same (1.2 μm ). The nanotubes were achieved by chemical etching of the nanorods. The average thickness of the nanowalls was estimated to be around 50 nm and length of the nanoflower leaf was about 2.2 μm.

Figure 5.2: Typical SEM images of different ZnO nanostructures (a) nanorods, (b) nanoflowers, (c) nanotubes, and (d) nanowalls.

5.2 Atomic force microscope

Atomic force microscope (AFM) is a high resolution scanning probe microscopy. It is a useful tool used by the research community in material science to investigate the surface profiles in three dimensions at the nanoscale level and it is also used in manuplating atoms. AFM can not only scan the surface of a material at the nanoscale level but it can also measure the force at nano-newton scale [1-2]. AFM is superior to SEM in many aspects, It provides three dimensional image, in AFM the samples need no preparation to improve the conductivity of the surface as it is needed for the SEM, therefore it is very useful in the study of biological surfaces.

Figure 5.3 shows a 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN. It shows that the ZnO nanorods are dense and grown uniformly on the substrate. The avearge height of the nanorods is about 1µm and the average diameter is about 150 nm.

Figure 5.3: A 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN.

5.3 X-ray diffraction

The interatomic distance in solids is approximately one angstram (Å) and the energy coresponds to this distance can be calculated as

E= hc/λ for λ= 1 Å

E ≈ 12×103 eV

If the structure of the solid is to be probed by electromagnetic waves then these waves should have energy at least equal to the interatomic distance energies. X-rays have approximately same energy (keV) as the interatomic distance energies of solids. X-ray diffraction (XRD) is the most common characterization technique to investigate the composition, quality, lattice parameters, orientation, defects, stress and strain in semiconductor materials. After the discovery of X-rays in 1895, a new way was possible to probe the crystalline materials at the atomic scale. In 1913 W.H and W.L. Bragg found that the X-rays gave characteristics patterens after reflection from crystalline materials and these patterens were different from those that were obtained from liquids. Later on the x-ray diffraction patterens became an important tool for the material science research.

Figure 5.4 shows a schematic diagram of Bragg refelection from crystalline planes having interplan distace d. The incident and reflected x-rays from the two planes are also shown.

Bragg concluded that the path difference between the two x-rays diffracted from two consective lattice planes is 2dsinθ and it leads to Bragg’s law, which states that the condition for diffraction of X-rays for a crystalline material is:

nλ= 2dsinθ

Here θ is the angle of incidence and λ is the wavelength of the x-rays, n is an integer and it is the order of refelection, and d is the distance between the lattice planes.

Figure 5.4: Shows a schematic diagram of Bragg reflection from crystalline lattice planes having interplan distace “d” between two lattice plane.

Figure 5.5 (a-d) show the XRD patterns for different ZnO structures (nanowalls, nanorods, nanoflowers and nanotubes), respectively. From the XRD spectrum, a strong (002) peaks and weak (004) peaks indicate clearly that a hexagonal-shaped ZnO nanowalls, nanotubes, nanorods and nanowalls were obtained along the c-axis. The (002) peak has a strong and sharp intensity and narrower spectral width as compared to the other peaks obtained in the XRD pattern. This indicate that the grown ZnO naostructures have a high- quality wurtzite hexagonal ZnO phase [3-4].

ZnO (002) 700000 ZnO (002) (a) 2500 (b) 600000 2000 500000

400000 1500

300000 Intensity (a.u.) Intensity

Intensity (a.u.) Intensity 1000 200000 500 100000 ZnO (004) ZnO (004) 0 0 30 40 50 60 70 80 30 40 50 60 70 80 6000 2 Theta (deg.) 7000 2 Theta (deg.) ZnO (002) ZnO (002) 5000 (c) 6000 (d) 5000 4000 4000 3000 3000 Intensity (a.u.) Intensity

Intensity (a.u.) Intensity 2000 2000 ZnO (004) 1000 1000 ZnO (002) 0 0 30 40 50 60 70 80 30 40 50 60 70 80 2 Theta (deg.) 2 Theta (deg.)

Figure 5.5: Display the θ-2θ XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes grown on p-GaN substrates, respectively.

5.4 Photoluminescence

Photoluminescene (PL) is a non-destructive technique used for the investigation of extrinsic and intrinsic properties of semiconductors. In this technique the semiconductor under investigation is excited optically and then the PL spectrum of the spontanious emission from radiative recombinations in the semiconductor band gap is obtained. During the PL process both radiative and nonradiative recombinations happen but in this research work we have studied the radiative recombinations only. The PL thechnique can be used to dtermine the bandgap, impurity levels, defects dectection and recombination process in the semiconductor materials. More details can be found in [5]. In this research work, PL measurements were performed at room temperature by using laser lines with a wavelength of

266 nm from a diode laser pumped resonant frequency doubling unit (MBD266) as an excitation source.

A schematic diagram of the PL setup used in this research work is shown in figure 5.6. The PL measurement has three main steps,

(1) In the first step, the semiconductor is optically excitated to create electron-hole pairs. Different kinds of lasers such as He-Cd laser and Ar+ laser with wave lengths of 325 nm and 316 nm are comonnly used for the excitations in ZnO. The laser beam is then projected on the semiconductor sample with the help of a setup as shown in the schematic diagram. (2) In the second step, the excited electron-hole pairs recombine radiatively and emit light. (3) In the final step, the emitted light is detected and dispersed by a double grating monochromator and photomultiplier detectors. The final spectrum is collected and analyzed in a computer.

Figure 5.6: A schematic diagram of the PL setup.

nanotubes 1.0 nanorods 0.9 nanoflowers nanowalls 0.8 1 0.7 0.6 0.1 0.5

0.4 Current (mA) 0.01

Current (mA) 0.3 1E-3 0.2 -8 -6 -4 -2 0 2 4 6 8 Voltage (V) 0.1 0.0 -0.1 -8 -6 -4 -2 0 2 4 6 8 Voltage (V)

Figure 5.7: A typical I-V characteristics for different ZnO (nanostructures)/p-GaN LEDs [3].

During recombination, the electron-hole pairs in the semiconductor can recombine radiatively in many ways. It can be band-to-band transition or can be donor- accepter recombinations in the band gap. The direct band gap semiconductors have high possibility of band-to-band electron-hole recombinations. Therefore direct band-gap semiconductors are attractive for LEDs. The band-to-band recombination rate is high at high temperatures because of all shallow defects ionized at high temperatures, and at low temperatures free-to-bound recombination rate dominates because at low temperature free carriers bound to the defects. Both donors and accepters can also be found in many semiconductors. In these semiconductors, some accepters capture electrons from the donors and in consequence it contains ionized accepters and donors. These ionized donors and accepters trap the carriers and introduce neutral donor and accepter excitons in the band gap. There are free and bound excitons and both increase the transitions. In the donor-accepter pair transitions, the electrons recombine with the holes in neutral hole centers. The free exciton PL peaks in ZnO are due to recombinations of electrons and holes. Its transition energy is slightly less than the band gap energy. The emission in bound excitons is due to recombination of electrons and holes in the excitons bound to donors or accepters. Its transition energy is slightly less than the free exciton recombination energy [6].

The excitons are very stable in ZnO at room temperature and give very strong near band edge emission. In the radiative recombinations in ZnO, the excitonic emission dominates the band-to-band transitions and the exciton binding energy is high compared to other direct band gap semiconductors such as GaN. In the PL spectra at room temperature, free excitons emission dominates but at low temperature bound excitons emission dominates the excitonic emissions in ZnO. The free exciton PL peak is oftenly centered approximately around 376-387 nm in ZnO. The broad emission peaks in the visible region from 420-750 nm in ZnO belongs to deep level defects and these defects are present in the band gap at different energies.

5.5 Electroluminescence

The electroluminescence (EL) is an electro-optical phenomenon with electrical input and optical out put. In this phenomenon, an electrical current input is applied across the material and it emits light. In this phenomenon, the emission spectrum is the result of electron-hole radiative recombinations in the semiconductor. The electroluminescence is a figure of merit for the LEDs. By improving the EL emission of the LEDs the efficiency can

69 be improved. Different key factors such as charge injection, charge transport and charge recombination contribute to the efficiency of the LEDs.

In this research work the electroluminescence of n-ZnO nanostructures/p-GaN, p-4H-SiC LEDs is investigated for a possible future application of ZnO nanostructures in white LEDs based lighting. The EL measurements were performed at room temperature. A photomultiplier detector was used to detect the emitted light from the fabricated LEDs. The fabricated LEDs were illuminated by applying a forward biasing injection current across the electrodes by using a kiethely 2400 apparatus with the help of the metal probes. The spectra were measured from the top contacts of the LEDs by detecting the light escaping from the edge of the top contact electrode.

5.6 Electrical characterization

Semiconductor parameter analyzer (SPA) is a useful instrument for the electrical characterization of different semiconductors based devices such as, LEDs, transisters, Schotkey diode, logic gates, and so on. In this research work, it was used for electrical characterization of the fabricated ZnO nanostructures based LEDs. The current versus voltage (I vs V) measurements were performed for the fabricated LEDs. The I-V curves provide different information about the LEDs such as its turn-on voltage, forward and reverse currents, different current regims, break-down voltage, ideality factor, parallel and series resistance of the fabricated LEDs.

Figure 5.7 [3] shows the I-V curve of the different n-ZnO nanostructures/p-GaN LEDs. The I-V curves show rectifying behavior as expected from these LEDs. It indicates clearly that reasonable p-n heterojunctions were achieved. The turn-on voltage of these heterojunctions LEDs is around 4 V.

5.7 Annealing studies

Annealing is an important approach used by material researchers comunity to change the material properties and to modify the defects in the material. In this research work, we used post-growth annealing of ZnO nanrods and nanotubes. In this annealing study, ZnO nanorods and nanotubes were annealed at 600 oC and then cooled down to room temperature. The samples were kept at annealing temperature for 30 minutes. The post-growth annealing was done in different ambients introduced in the annealing chamber. These ambients were

70 argon, air, oxygen and nitrogen. These annealing ambients introduce a control on the intrinsic defects and consequently the luminescence properties can be controlled.

In paper III, the origin of the red emission in ZnO was investigated by using the post-growth annealing of ZnO nanotubes in different ambients and in paper IV, the post- growth annealing technique was used to investigate the effect of different post-growth annealing ambients on the color rendering properties of ZnO nanorods based LEDs.

5.8 Color rendering extraction

One of the most important requirement when using LEDs for indoor and outdoor lighting applications is that the color rendering indices (CRIs) of these LEDs should be high. Color rendering is the ability of a light source to render the true colors of the objects in the enviorment. The Sun or the black body radiator are used as a reference and these have maximum value of color rendering inex that is 100.

Color rendering indices (CRIs), correlated color temperature (CCTs), chromaticity coordinates in CIE 1931 color space, and values of the set of fourteen test-color samples were extracted from the emitted spectra of the fabricated LEDs by using a computer software. These color parameters can be extracted from EL spectra or from the power spectra (spectral power distributions of the radiation emitted by the LEDs). A photomultiplier detector was used to measure the EL spectra of the the emitted light from the fabricated LEDs. The fabricated LEDs were illuminated by applying a forward biasing injection current across the electrodes by using the kiethely 2400 apparatus with the help of the metal probes. The spectra were measured from the top contacts of the LEDs by detecting the light escaping from the edge of the top contact electrode. In order to measure the spectral power distributions of the fabricated LEDs, the electroluminescence was measured by using fiber coupled CCD spectrometer Hamamatsu C7374. For the calibration of the spectrometer a calibration lamp Bentham CL2 was used. Light can be precisely characterized by the power in each wavelength in the visible spectrum. The measured spectral power distribution (SPD) spectrum contains all the basic physical data necessary for the quantitative analysis of the color. The chromaticity coordinates, color rendering indices and correlated color temperatures were calculated from the spectra emitted by the LEDs [7].

5.9 Irradiation studies

ZnO has unique and very attractive material properties for applications in variety of devices such as, LEDs, bio-sensors, UV detectors, gas sensors and nano-generators and so on. It is very interesting to investigate the effect of internal and external phenomenon on the performance and reliability of these ZnO based devices. In this research work, influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods was investigated. When energetic particles (electrons or ions) hit a material they change the properties of the targeted material and change the density of defects in the material [8]. The purpose of this study was to investigate the reliability of ZnO nanorods based LEDs in space and nuclear applications. The ZnO nanorods and LEDs based on these nanorods were irradiated by using 2.0 MeV He+ ions with fluences of ~ 2×1013 ions cm-2 and ~ 4×1013 ions cm-2. All the samples were irradiated at room temperature under normal incidence at the Tandem Laboratory, Uppsala University Sweden. The ion beam flux was about 6×1010 ions s-1 cm-2 and the beam spot was roughly 1 cm2. The projected ion range was calculated by the SRIM2008 code [9] to be 4.9 μm assuming a density for ZnO of 5.6 g/cm3, so the major part of the whole ZnO nanorods were influenced by the beam.

References [1] http://en.wikipedia.org/wiki/Atomic_microscopy [2] http://www.chembio.uoguelph.ca/educmat/chm729/afm/introdn.htm [3] N. H. Alvi, S. M. Usman Ali, S. Hussain, O. Nur, and M. Willander, Scripta Materiala 64, 697 (2011) [4] L. B. Freund, and S. Suresh (Eds.), Thin Film Materials, Cambridge University Press, Cambridge, (2003) [5] G. D. Gilliland, Mater. Sci. Eng. R18, 99 (1997) [6] Peter Klason, Zinc oxide bulk and nanorods, A study of optical and mechanical properties, PhD thesis, University of Gothenburg., (2008) [7] N. H. Alvi, K. ul Hasan, M. Willander, and O. Nur, (Accepted in Lighting Research and Technology) DOI: 10.1177/1477153511398025 [8] A. V. Krasheninnikov, and K. Nordlund, J. Appl. Phys. 107, 071301 (2010) [9] SRIM (Stopping and Range of Ion in Matter) Software: http://www.srim.org.

Chapter 6 Results Some of the results achieved in my research work are presented in this chapter. The results are divided into three sections; in the first section, results from papers I-IV are included. In this section a discussion about luminescence properties of ZnO nanorods grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers and nanowalls) on the same substrates are included. The effect of post-growth annealing on deep level emissions and color rendering index properties of ZnO nanorods and nanotubes based LEDs are also investigated in this section.

In the second section, the junction temperature of n-ZnO nanorods based LEDs at the built-in potential was modeled and experiments were performed to validate the model. In the third section, the influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods based LEDs is presented.

6.1 Luminescence properties of ZnO nanostructures

In paper 1, the luminescence properties of ZnO nanorods grown on two different substrates (p-4H-SiC, p-GaN) were investigated [1]. The purpose of the growth of ZnO nanorods on two different substrates was to compare the luminescence from these ZnO nanorods. The main purpose of this comparison was to know the contribution of the substrate, the nanorods and the p-n junction in the luminescence spectra (especially EL spectra) of the fabricated LEDs.

ZnO nanorods were grown by the aqueous chemical growth (ACG) technique. This growth method has been described already in details in chapter 4. The luminescence properties were carried out by photoluminescence (PL) measurements of the grown ZnO nanorods and the electroluminescence (EL) measurements of the fabricated LEDs based on these grown nanorods. Both measurements were performed at room temperature. Figure 6.1 (a, b) shows the PL spectra of ZnO nanorods grown on p-4H-SiC and p-GaN substrates, respectively. Three emission peaks are observed. The band edge emissions are approximately centered at 387 nm (3.20 eV), 383 nm (3.23 eV). A broad green and a orange-red deep level emission peaks are approximately centered at 550 nm (2.25 eV) and 693 nm (1.78 eV),

75 respectively [1]. The band edge emissions at 387 nm (3.20 eV) and 383 nm (3.23 eV) in ZnO nanorods show that ZnO nanorods are of good crystalline quality [2].

The EL spectra of the fabricated ZnO nanorods/p-4H-SiC, p-GaN LEDs at room temperature is shown in figure 6.2 (a, b) [1]. A forward bias with injection current of 6 mA and 4 mA was applied to the ZnO nanorods/p-4H-SiC, p-GaN LEDs, respectively. The EL spectrum of the n-ZnO nanorods/p-4H-SiC showed violet-blue, green and orange-red emissions that are approximately centered at 430 nm (2.88 eV), 531 nm (2.33 eV) and 665 nm (1.86 eV), respectively. The EL spectrum for the ZnO nanorods/p-GaN LED showed violet- blue, green, and orange-red, emissions that are approximately centered at 457 nm (2.71 eV), 531 nm (2.33 eV) and 665 nm (1.86 eV), respectively [1].

The comparison study of the EL and the PL spectra of these nanorods based LEDs on different substrates helps to understand the luminescence properties of ZnO. It was found that there is a consistency between the PL and the EL spectra of both LEDs but it can be observed that the violet-blue peak in the EL spectrum is absent from the PL spectra. It indicates that the violet-blue emission is not originating from the ZnO nanorods. It was also found that the violet-blue emission in both LEDs has different position, the peak position for n-ZnO nanorods/p-4H-SiC was approximately centered at 430 nm (2.88 eV) and for n-ZnO nanorods/p-GaN LEDs it was centered at 457 nm (2.71 eV). This difference in peak position is may be due to the effect of the substrate. Moreover p-GaN is magnesium doped and deep magnesium accepters are commonly considered responsible for blue emission in GaN. ZnO nanorods/p-4H-SiC LED also emits violet-blue emission and it suggests that this peak is originating from the heterojunctions or from the substrate. Zhang et al. [3] also suggested that this peak is originating from the heterojunction of ZnO nanorods/p-GaN LEDs. It can also be seen clearly that violet-blue emission in ZnO nanorods/p-GaN LED has high EL intensity as compared to n-ZnO nanorods/p-4H-SiC LED. This is also suggested that it is due to difference of the substrate. In ZnO nanorods/p-GaN LEDs, p-GaN substrate also contributes to the blue emission in the EL spectrum. The peak positions of the green and the orange-red emissions are approximately centered at the same position in both LEDs. This suggests that these emissions are from the ZnO nanorods and the substrates have no contribution or have very little contribution to these emissions. The EL spectra of both LEDs showed very bright white light as observed by the naked eyes during EL measurements. The measured luminescence spectra is the result of the combination of three emission lines which are the violet-blue, the green, and the orange-red peaks observed from both LEDs [1].

Figure 6.1: (a) Room temperature photoluminescence spectrum for the ZnO nanorods on p- 4H-SiC substrate, and in (b) the room temperature photoluminescence spectrum for the ZnO nanorods on p-GaN substrate [1].

Figure 6.2: (a) Electroluminescence spectrum for ZnO nanorods/p-4H-SiC LED, and in (b) electroluminescence spectrum for the ZnO nanorods/p-GaN LED [1].

In Paper II, the luminescence properties of different ZnO nanostructures grown on the same (p-GaN) substrate were investigated [4]. The purpose of this study was to investigate the difference in luminescence properties of different ZnO nanostructures grown on the same substrate and was to know which ZnO nanostructure has the best luminescence properties to be used as possible candidate for ZnO based white LEDs. The different ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes) were grown by the aqueous chemical growth (ACG) technique. The electrical, optical and electro-optical characteristics of these nanostructures were comparatively investigated. The luminescence properties were investigated by measuring the PL and the EL spectra at room temperature [4].

Figure 6.3 (a) [4] shows the PL spectra for ZnO nanowalls structure. The PL emission peaks are centered approximately at 361 nm (3.43 eV), 378 nm (3.29 eV), and 490 nm (2.53 eV). The observation of the band-edge emission peak at 378 nm (3.29 eV) showed that the as grown ZnO nanowalls have good crystalline quality [2]. The deep level emission approximately centered at 490 nm (2.53 eV) can be attributed to oxygen vacancies (Vo) related defects [5]. Figure 6.3 (b-d) [4] shows the PL spectrum for the ZnO nanoflowers, nanorods, and nanotubes. The PL intensity peaks approximately centered at 376 nm (3.29 eV) and a broad peak (covering the visible region from 400 nm to 700 nm) were observed in all three nanostructures. The emission at 361 nm (3.43 eV) in the nanowalls structures can be attributed to the band edge emissions in p-GaN substrate and the emission of this peak shows that the commercially purchased p-GaN substrates were of high optical quality. It was detected only in ZnO nanowalls as the nanowalls structures are not so dense on the substrate therefore during the PL measurement the excitation laser beam reach the substrate easily [4].

From the comparison of the PL spectra, as shown in figure 6.3 (e) it was found that among all grown ZnO nanostructures, ZnO nanorods have the highest excitonic band edge PL emissions at 3.29 eV which indicates that ZnO nanorods have the best crystal quality as compared to all other nanostructures. But in the visible region the nanowalls structures have the higher PL intensity as compared to other nanostructures. This indicates that the nanowalls structure have more density of deep level defects as compared to the other three nanostructures [4].

Figure 6.4 (a-d) [4] shows the room temperature EL spectra for LEDs based on the four different nanostructures. During the EL measurements, the LEDs were under forward biasing of 25 V with injection current of 4 mA. Figure 6.4 (a) [4] shows the EL spectra of the n-ZnO nanowalls/p-GaN LEDs and a violet, a violet-blue and a broad EL peaks covering the visible region from 480 nm to 700 nm were observed. The EL peaks were approximately centered at 420 nm (2.95 eV), 450 nm (2.75 eV) and a broad peak covering EL emissions from 480 nm to 700 nm. The broad peak includes the green, yellow, orange and red emissions. Figure 6.4 (b) [4] shows the EL spectrum for n-ZnO nanoflowers/p-GaN LED and the violet, violet-blue and the broad peaks were observed. These observed peaks were centered approximately at 400 nm (3.09 eV), 450 nm (2.75 eV) and a broad peak covering the EL emission from 480nm to 700 nm. Figure 6.4 (c, d) [4] shows the EL spectra for the LEDs based on n-ZnO nanorods, nanotubes, respectively. Both LEDs have the same spectra because the ZnO nanotubes were achieved from ZnO nanorods by chemical etching. There is only

78 difference of the charge injection surface area and the ZnO nanotubes have more surface area as compared to the ZnO nanorods. The electrons can also be injected from the inner side of the hallow nanotube and this increases the emission intensity. A violet, a violet-blue and a green EL peaks were observed in both LEDs and these peaks were centered approximately at 400 nm (3.09 eV), 450 nm (2.75 eV) and 540 nm (2.29 eV), respectively [4].

From the comparison of the PL spectra of all the LEDs it was found that the nanorods have the highest PL intensity peak in the ultra-violet (UV) region when it was compared to the PL intensity of the all other nanostructures. It means that ZnO nanorods have good crystal quality and can be attractive for UV diodes. ZnO nanowalls structures have the highest PL intensity emission in visible region when compared to other nanostructures. In ZnO the deep levels defects are responsible for emissions in the visible range. This means that ZnO nanowalls have the highest density of deep level defects as compared to all other nanostructures [4].

5000 361 nm 378 nm (a) 25000 (b) 376 nm 490 nm 4000 20000

3000 15000

2000 10000 PL IntensityPL (a.u.) PL IntensityPL (a.u.) 1000 5000 0 0 300 400 500 600 700 800 300 400 500 600 700 Wavelength (nm) 7000 Wavelength (nm) 12000 376 nm (c) 6000 (d) 378 nm 10000 5000 8000 4000 6000 3000

4000 IntensityPL (a.u.) 2000 518 nm 518 nm PL IntensityPL (a.u.) 2000 1000 0 0 300 400 500 600 700 300 400 500 600 700 Wavelength (nm) Wavelength (nm)

25000 (e) Nanowalls 20000 Nanorods Nanoflowers 15000 Nanotubes

10000 PL IntensityPL (a.u.) 5000

0 300 400 500 600 700 Wavelength (nm)

Figure 6.3: Room temperature photoluminescence spectrum for ZnO nanostructures (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes on p -GaN and (e) shows the combine PL spectra of all the four nanostructures [4].

From the comparison of the EL spectra of the LEDs based on t hese different nanostructures it was found that the nanorods and the nanotubes have identical EL spectra. The nanotubes have the highest EL intensity in the visible region as expected because nanotubes have more charge injection surface area under the contact as compared to other nanostructures. From the comparison of the PL spectra it was found that the nano-walls

80 structures have the highest defect related emissions but have small charge injection surface area under the contact as compared to other nanostructures. ZnO nanotubes are denser under the contact as compared to nanowalls. It can also be seen from the I-V curves for all the LEDs as shown in figure 6.5. The LEDs based on ZnO nanotubes showed the highest current as compared to all other LEDs. The same results were obtained when we investigated the effect of the post-growth annealing on the color rendering properties of ZnO nanorods and nanotube based LEDs and ZnO nanotubes based LEDs showed higher EL emission intensity as compared to ZnO nanorods based LEDs under the same injection of current [4].

450 nm 450 nm 5000 (a) 8000 7000 4000 (b) 6000 3000 5000 4000 2000 420 nm 3000 EL IntensityEL (a.u.) 400 nm EL IntensityEL (a.u.) 1000 2000 1000 0 0 400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) 540 nm 540 nm 30000 25000 450 nm 450 nm (c) 25000 (d) 20000 20000 15000 15000

10000 IntensityEL (a.u.) 10000 EL IntensityEL (a.u.) 400 nm

5000 400 nm 5000

0 0 400 500 600 700 800 900 400 500 600 700 800 900 Wavelength (nm) Wavelength (nm) nanotubes 30000 nanorods (e) nanoflowers (f) 25000 nanowalls 20000

15000

10000

EL intensityEL (a.u.) 5000

350 400 450 500 550 600 650 700 750 Wavelength (nm)

Figure 6.4: Displays the electroluminescence spectra for n-ZnO (nanostructures)/p-GaN LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) shows the EL spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [4].

1.0 0.9 0.8 0.7 nanotubes 0.6 nanorods 0.5 nanoflowers nanowalls 0.4

Current (mA) 0.3 0.2 0.1 0.0 -0.1 -8 -6 -4 -2 0 2 4 6 8 Voltage (V)

Figure 6.5: Typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs as indicated in the figure [4].

By comparing the EL spectra of ZnO nanorods/p-GaN LEDs in paper I and paper II it was found that the dominating EL peak in paper I is the orange-red peak which is approximately centered at 665 nm (1.86 eV) while in paper II the dominating EL peak is the green peak which is approximately centered at 540 nm (2.29). We have investigated the cause of this difference and it was found that during the fabrication of the LEDs in paper 1, the ZnO nanorods were annealed in nitrogen ambient after the growth for the deposition of the bottom contacts. The samples were annealed at a temperature of 350 oC in nitrogen ambient for 2 minutes. For annealing the bottom contacts, we use a vapor-liquid-solid (VLS) growth chamber and it takes 4 minutes to reach 350 oC but it takes a minimum of three hours to cool down. Later on from annealing studies we found that nitrogen ambient play an important role in shifting the dominating peak from the green region towards the red region [6]. It will be described in details in the annealing studies in the coming section. The EL spectra in paper V also support this argument where ZnO nanorods/p-4H-SiC, p-GaN LEDs were annealed in different ambients and due to this difference both LEDs show different dominating peak, The ZnO nanorods/p-4H-SiC LED was annealed in nitrogen ambient and it gave a dominating EL peak centered approximately at 674 nm and the ZnO nanorods/p-GaN LED was annealed in argon ambient and it gave a dominating EL peak approximately centered at 550 nm.

Another difference in the EL spectrum of ZnO nanorods based LEDs is that the violet emission at 400 nm is absent in paper I. It is also due to the annealing effect as we investigated in post-growth annealing studies that after annealing the ZnO nanotubes in nitrogen ambient the violet emission at 400 nm almost disappear as compared to the as grown ZnO nanotubes based LEDs [6].

The color rendering indices (CRIs) and correlated color temperatures (CCTs) for all the LEDs were also extracted and it was found from the comparison that the nanowalls based LEDs have the highest color rendering index (CRI) with a value of 95 with a correlated color temperature of 6518 K while the nanorods have the lowest CRI reaching 87 with a correlated color temperature of 4807 K. Figure 6.3 (f) shows the CIE 1931 color space chromaticity diagram in the (x, y) coordinates system. The chromaticity coordinates for LEDs based on ZnO nanowalls, nanorods, nanoflowers, and nanotubes are (0.3131, 0.3245), (0.3332, 3470), (0.3558, 0.3970), and (0.3555, 0.3935), respectively. The chromaticity coordinates for the ZnO nanowalls and nanoflowers are very close to the Planckian locus, actually about less than one MacAdam ellipses away and can be called white light according to the US standard ANSI_ANSLG C78, 377-2008 for solid state lighting which indicates that

83 the light sources with chromaticity coordinates less than three MacAdam ellipses from the Planckian locus can be considered as white light. The chromaticity coordinates for the ZnO nanorods and nanotubes are about 3 Mac-Adam ellipses away and are very close to white light. It is clear from the chromaticity coordinates that the fabricated LEDs based on different nanostructures are white light emitting diodes [4].

In paper III we have investigated the origin of the red emission in ZnO nanotubes [6]. The origin of different deep level defects related emissions in ZnO is still controversial and cannot be fully understood and a considerable interest exists in investigation of the origin of deep level emissions in ZnO in general and particularly in ZnO nanostructures as nanostructures have great potential to be used in opto-electronics. In this paper we have investigated the origin of the red emission in ZnO by comparing the EL spectra from ZnO nanotubes (annealed in different ambients)/p-GaN LEDs. Aqueous chemically achieved ZnO nanotubes were annealed in argon, oxygen, air, and nitrogen ambients at 600 oC for 30 minutes [6]. For comparison, the ZnO nanotubes (as grown)/p-GaN LED was used as a reference. Figure 6.7 shows the EL spectra for the LEDs based on ZnO nanotubes (as grown and annealed in argon, air, oxygen, and nitrogen ambients). The EL spectra were measured by applying a forward biasing of 25 V across the electrodes. The EL spectra of all the LEDs show EL peaks in the violet, violet-blue and broad dominating peaks centered in green, orange, orange-red and red regions. The violet and violet-blue peaks are centered approximately at 400 nm (3.1 eV) and 452 nm (2.74 eV) in all LEDs, respectively. The broad dominating green EL emission peak centered approximately at 536 nm (2.31 eV), was observed in the as grown and annealed in argon ZnO nanotubes based LEDs. The broad dominating orange and orange-red EL emission peaks approximately centered at 597 nm (2.07 eV) and 618 nm (2.00 eV) were observed in ZnO nanotubes (annealed in air and oxygen ambients) based LEDs, respectively. The broad dominating red EL emission peak centered approximately at 705 nm (1.75 eV) was observed in the ZnO nanotubes (annealed in nitrogen ambient) based LEDs [6].

Figure 6.6: Schematic band diagram of the DLE emissions in ZnO based on the full potential linear muffin-tin orbital method and the reported data and references in [6].

618 nm as grown annealed in argon 597 nm 705 nm 50000 annealed in air 45000 536 nm annealed in oxygen 40000 annealed in nitrogen 35000 452 nm 30000 25000 20000 EL intensityEL (a.u.) 15000 10000 400 nm 5000 0 300 400 500 600 700 800 900 Wavelength (nm)

Figure 6.7: Electroluminescence spectra of different LEDs at an injection current of 3 mA, under forward bias of 25 V and it shows the shift in emission peak after annealing in different ambients [6].

From the comparison of the EL spectra of all the LEDs it was investigated that air, oxygen and nitrogen annealing ambients have strongly affected deep levels emission bands in ZnO. It was observed from the electroluminescence investigation that more than one deep level defects are involved in the red emission appearing between 620-750 nm in the visible region [6].

It was concluded that the red emission in ZnO can be attributed to oxygen interstitials (Oi) appearing in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV). The orange- red peaks in the EL spectra of the LEDs based on ZnO nanotubes annealed in air and oxygen are centered at 597 nm (2.07 eV) and 618 nm (2.00 eV), respectively [6]. These emissions are attributed to oxygen interstitials (Oi) and it is believed that it is due to band transition from zinc interstitial (Zni) to Oi defect levels in ZnO [7]. The position of the Oi level is located approximately at 2.28 eV below the conduction band and the position of the Zni level is theoretically located at 0.22 eV below the conduction band. It is expected that the band transition from Zni to Oi level is approximately 2.06 eV [7]. This agrees well with the orange- red peaks centered approximately at 2.00 eV and 2.07 eV [6].

The red emission in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV) in ZnO is attributed to oxygen vacancies (VO). The red emission in the ZnO nanotube (annealed in nitrogen)/p-GaN LED is centered at 705 nm (1.75 eV) and it can be attributed to oxygen vacancies (VO). When the ZnO nanotubes are annealed in nitrogen ambient, the following process of oxygen desorption may occur:

ZnO → VO + ZnZn + 1/2O2

In this process the zinc vacancies are filled with zinc when ZnO nanotubes were annealed in the nitrogen ambient. Then the majority of the defects in the deep level of ZnO are oxygen vacancies, created by the evaporation of oxygen [8]. The red emission in the ZnO nanotubes (annealed in nitrogen ambient)/p-GaN LED centered at 706 nm (1.75 eV) may be attributed to electron-hole recombinations from VO to the top of the valance band in ZnO. It can be calculated by using the full-potential linear muffin-tin orbital method that the energy level of the VO in ZnO is 1.62 e V below the conduction band [9]. Therefore the energy difference from the VO energy level to the top of the valence band is approximately 1.75 eV. This agrees well with the energy of the red emission centered at 1.75 eV [6].

By comparing the EL spectra of the ZnO nanotubes (annealed in oxygen and nitrogen)/p-GaN LEDs, it was observed that the total red emission in ZnO from 620 nm (1.99 eV) to 750 nm (1.65 eV) is not from the same defect and it is a combination of the red emission from Oi and VO deep level defects in ZnO. The red emission in the range of 620 nm

(1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi deep level defects in ZnO. It can be explained by comparing the EL spectra of the LEDs based on ZnO nanotubes annealed in oxygen ambient and the LEDs based on the as grown ZnO nanotubes. It can be observed that in the EL spectra for the LED based on ZnO nanotubes annealed in oxygen ambient, the red emission intensity in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) is enhanced significantly as compared to the red EL emission intensity in the LED based on the as grown nanotube [6].

Similarly the red emission in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV) can be attributed to VO deep level defects in ZnO. It can be explained by comparing the EL spectra of the LEDs based on Z nO nanotubes annealed in nitrogen ambient and the LEDs based on a s grown ZnO nanotubes. It can be observed that in the EL spectra for the LED based on the ZnO nanotubes annealed in nitrogen ambient, the red emission intensity in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV) is enhanced significantly as compared to the EL intensity of red emission in the LED based on the as grown ZnO nanotubes. It can be seen clearly that in the EL spectra of ZnO nanotubes annealed in nitrogen ambient, the EL intensity of the green, yellow, orange, and the red emission (from 620 nm to 690 nm) are decreased but the EL intensity of the red emission (from 690 nm to 750 nm) has increased significantly as compared to the as grown ZnO nanotubes based LEDs. So it is clear that the red emission from 620 nm to 690 nm and from 690 nm to 750nm has different origins. The red emission in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi and in the range of

690 nm (1.79 eV) to 750 nm (1.65 eV) can be attributed to VO [6].

The color rendering properties of ZnO nanotubes annealed in different ambients are similar to the color rendering properties of ZnO nanorods annealed in different ambients. The annealing ambients (argon, air, oxygen and nitrogen) showed the same effect on color rendering properties for ZnO nanotubes and nanorods. Figure 6.8 shows the CIE 1931 color space chromaticity diagram in the (x, y) coordinates system of the LEDs based on ZnO nanotubes annealed in different ambients. The chromaticity coordinates are (0.3559, 0.3970), (0.3557, 3934), (0.4300, 0.4348), (0.4800, 0.4486), and (0.4602, 0.3963) with correlated color temperatures (CCTs) of 4802 K, 4795 K, 3353 K, 2713 K, and 2583 K for the LEDs based on

ZnO nanotubes as grown, annealed in argon, air, oxygen and nitrogen, in the forward bias, respectively [6].

Figure 6.8: The CIE 1931 x, y chromaticity space of ZnO nanotubes annealed in different ambients [6].

Figure 6.9: Normalized spectral power distributions for the LEDs based on the as-grown n- ZnO nanorods and after annealing in air, oxygen, and nitrogen ambients [11].

In paper IV we have investigated the effect of post-growth annealing on the color quality of light emitted from different n-ZnO nanorods/p-GaN LEDs. The post-growth annealing is an effective tool to enhance the luminescence properties. The ZnO nanorods were grown by the ACG method. It is a low temperature growth technique and it is expected that the grown ZnO nanorods have lattice and surface defects and their crystallinity and optical properties can be altered by post-growth annealing. [10].

The as grown ZnO nanorods were annealed in nitrogen, oxygen, argon, and air ambient at 600 oC for 30 minutes. After annealing the ZnO nanorods in different ambients, the LEDs were fabricated from these post-growth annealed ZnO nanorods. After the fabrication of the LEDs the color rendering indices (CRIs) and correlated color temperatures (CCTs) were extracted from the spectra emitted by these fabricated LEDs [11].

It was investigated that the post-growth annealing ambients have an affective role in modifying the light color quality and especially the nitrogen ambient is very effective for altering the CRIs and the CCTs of the fabricated LEDs. The LEDs based on ZnO nanorods annealed in nitrogen ambient showed excellent color rendering properties with CRIs of 97 in the forward bias and 98 in the reverse bias and have CCTs of 2363 K in the forward bias and 3157 K in the reverse bias [11].

Figure 6.9 show [11] the normalized spectral power distributions (SPD) of the light emitted by the LEDs based on ZnO nanorods annealed in different ambients. A forward bias was applied across the electrodes with injection current of 7 mA. The LED based on the as grown ZnO nanorods was used as a reference for normalization. The reference LED had a spectral radiant flux of ~ 0.447 mW [11].

We can analyze and characterize light from any source precisely by measuring the power contained by each wavelength in the visible spectrum. The measured spectral power distribution (SPD) contains all the basic physical information that is necessary for the quantitative analyses of the light. Color parameters such as chromaticity coordinates, color rendering indices, and correlated color temperatures of the light emitted by the different LEDs were extracted from the spectra emitted by these LEDs [11].

Figure 6.10 (a) [11] shows the CIE 1931 color space chromaticity diagram in the (x, y) coordinates system for all the LEDs when operated in the forward bias. The color rendering indices and correlated color temperatures were extracted from the

89 electroluminescence (EL) spectra of the LEDs and we have discussed the EL spectra in details in [12]. The chromaticity coordinates are (0.3559, 0.3970), (0.3870,4213), (0.4220, 0.4209), (0.4885, 0.4296) and (0.4819,0.4029) with the correlated color temperatures (CCTs) of 4802 K, 4124 K, 3398 K, 2480 K, and 2363 K for LEDs based on the as grown n-ZnO NRs/p-GaN and those annealed in argon, air, oxygen, and nitrogen ambient, respectively [11].

It was investigated that the chromaticity coordinates of the LEDs based on ZnO nanorods annealed in oxygen and nitrogen are very close to the Planckian locus (~ 1 MacAdam ellipse away) and these LEDs can be called white LEDs and the LED based on nanorods annealed in air is about three MacAdam ellipses away and it can also be considered as white light. The LEDs based on the ZnO nanorods as grown and annealed in argon are more than 3 MacAdam ellipses away from the Planckian locus. The US standard ANSI_ANSLG C78, 377-2008 for solid state light sources explains that the light sources having chromaticity coordinates within three MacAdam ellipses from the Planckian locus can be considered to produce white light. If the distance between two colors in the CIE (1931) x, y chromaticity diagram is less than three MacAdam ellipses then the human vision does not notice much difference between them [11].

Figure 6.10: The chromaticity coordinates of different LEDs (a) under forward bias and (b) under reverse bias plotted on the CIE (1931) x,y, chromaticity diagram [11].

Figure 6.10 (b) [11] shows the chromaticity coordinates on the CIE (1931) x, y chromaticity diagram for the light emitted from different LEDs in reverse bias. The chromaticity coordinates for the LEDs based on ZnO nanorods annealed in air, oxygen, and nitrogen are very close to the Planckian locus and are about 1 MacAdam ellipse away. These LEDs can be definitely called white LEDs according to the US standard ANSI_ANSLG C78, 377-2008. The LEDs based on the as-grown n-ZnO nanorods/p-GaN and annealed in argon are more than three MacAdam ellipses away from the Planckian locus [11].

From the comparison of color measurements from all the LEDs it was found that the annealing ambient is very altering the color rendering properties of the fabricated LEDs. The CIE chromaticity coordinates of the LEDs based on the as grown ZnO nanorods were away and could not be considered as white LEDs according to light standards but after annealing the CIE chromaticity coordinates are shifted close to the Planckian locus and become white LEDs according to light standards. The same behavior was observed in the CIE color rendering (CRIs). The CRIs of the LEDs also increase after annealing. The CRIs are dependent on the broadness and the intensity of the emission peaks in the visible region. After annealing the ZnO nanorods in air, oxygen and nitrogen ambients, the intensities of the emission peaks increase and this results in an increase of their CRIs. After annealing the ZnO nanorods in argon ambient, the light emission intensities decrease but the broadness of the light emission peak increases and this increases the CRI. The CRIs also increase after annealing the ZnO nanorods in air, oxygen and nitrogen ambients. It is due to the fact that after annealing in these ambients the deep level defects responsible for the orange and red emission become more active as compared to the defects responsible for green emissions in the deep levels of ZnO and as a result there is a red shift in the dominating peak, it has been discussed in details in paper III. Due to this the emission, the spectra shifted close to the black body radiator spectra and the black body spectra are the standard reference for white light with CRIs of 100 [11].

6.2 Junction temperature of n-ZnO nanorods based LEDs

In paper V, The junction temperature of n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction light emitting diode (LED) at the built-in potential was modeled and experiments were performed at various temperatures (15 ºC - 65 ºC) to validate the model [13]. The junction temperature of the LEDs is a cr itical parameter that affects the internal efficiency, the maximum output power and the reliability of LEDs. It also influences the

92 chromaticity and the color-rendering properties of LEDs [14]. As all the LEDs operate near the built-in potential therefore it is interesting to investigate the temperature coefficient of the forward voltage near the built-in potential (~Vo). The modeled and experimental values of the temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared [13].

First the derivation of the model will be explained. A relationship for the temperature dependence of the forward voltage was modeled and then it was generalized for n-ZnO nanorods/(p-GaN and p-Si) LEDs. The theoretical and experimental results at the built-in potential (Vo) are compared and discussed [13].

Figure 6.11: The energy band diagram of the n-ZnO nanorods/p-4H-SiC heterostructure [13].

The relation for the temperature dependent forward voltage was derived from the energy band diagram of n-ZnO nanorods/p-4H-SiC heterostructure as shown in Figure 6.11. The relation for the temperature dependent forward voltage was derived for n-ZnO nanorods/p-4H-SiC heterojunction and then it was generalized for n-ZnO nanorods/(p-GaN and n-ZnO nanorods/p-Si heterojunction LEDs [13].

Here it is assumed that there is an abrupt interface and any effects due to the interface states are neglected [15]. From the energy band diagram the equilibrium energy can be given as,

eVo ()()()E χ pgp { f VP CN EEEE +−+−−+= χ nf } (1)

The band gap energies of ZnO and 4H-SiC were taken as 3.37 eV and 3.25 eV, respectively. Where, Vo is built-in potential, Ef is Fermi energy level, Egp, χP, EVP, are the energy band gap, the electron affinity and effective densities of states in the valence band of p-4H-SiC, and χN, ECN are electron affinity and effective densities of states in the conduction band of n-ZnO. The difference in energy between the Fermi level and the band edges can be taken from Boltzmann’s statistics assuming that all dopants are fully ionized [15]. Now equation 1 can be written as,

Egp ∆Ec kT NN AD VO += + ln (2) e e e VP NN CN

Let’s assume that all the LEDs are operated in the forward direction close to the built-in voltage.

So Vf =Vo and we differentiate equation (2) with respect to temperature (T) and assume

dVf/dT= dVo/dT

we get,

   dV f ∆Ed C 11 dEgp d kT  NN AD  =  ++  ln  (3a) edT edT dT dT  e  VP NN CN 

Where,

  d kT  NN AD  k  NN AD  kT VP NN CN d NN AD  ln  = ln  + (3b) dT  e  VP NN CN  e  VP NN CN  e NN AD dT VP NN CN

As all dopants are fully ionized ND and NA are independent of temperature, Eq 3(b) can be written as,

  d kT  NN AD  k  NN AD  3k  ln  = ln  − (4) dT  e  VP NN CN  e  VP NN CN  e

Van Vechten et al. [16] derived a formula for the temperature dependence of the band offsets for semiconductor heterojunctions as;

∆ C ()/ BAEd  ∆ CV ()AEd   ∆ CV ()BEd  ()±= 005.077.1   −   (5) dT  dT   dT 

The temperature dependence of the energy band gap of a semiconductor can be given by Varshni formula [17];

By differentiating the Varshni equation with respect to T, we get

1 dEg ()TT + 2βα −= (6) e dT ()Te + β 2

By substituting equation (4), (5) and (6) in equation (3a), finally we get,

  dV f  ()TT + 2βα   ()TT + 2βα  k  NN  3k −= 77.1   + 77.0   + ln AD  −   2   2    (7) dT  ()Te + β ()Te + β e NN e     n   p  VP CN  

Equation (7) is a useful relation for the determination of the temperature dependence of the forward voltage close to the built-in potential without including the 18 -3 16 -3 16 -3 resistance of the device. By taking ND =10 cm , 10 cm and 10 cm for p-4H-SiC, p- 17 -3 GaN, and p-Si respectively and NA = 10 cm for ZnO. The calculated value of Vo is about 3.37 V for n-ZnO nanorods/p-4H-SiC heterojunction light emitting diode (LED). In eqn. (7) the first and the second terms are the temperature dependence of the band gap energy. The third term is the temperature dependence of the intrinsic carrier concentration and the fourth term is the temperature dependence of the density of states (DOS) [13].

(a) (b) (c) 19 6.6 18 6.4 4.5 17 1mA 6.2 6.0 4.0 1mA 16 2mA 5.8 2mA 15 3mA 5.6 0.1mA 3.5 3mA 14 4mA 5.4 0.2mA 13 5.2 0.3mA 4mA 12 5mA 5.0 3.0 6mA 4.8 0.4mA 11 4.6 2.5 10 7mA 4.4 9 8mA 4.2 2.0 8 9mA

Forward voltage (v) 4.0 7 10mA 3.8 Forward voltage (V) 1.5 6 3.6 Forward voltage (V) 5 3.4 4 3.2 1.0 10 20 30 40 50 60 70 20 30 40 50 60 70 20 30 40 50 60 70 Temperature (ºC) Temperature (ºC) Temperature (ºC)

(f) 1800 (d) 0.55 1.6 (e) 1700 1.4 0.50 1600 1.2 0.45 1.0 0.40 1500 0.8

Resistance (K-ohm) 0.35 1400 0.6 Resistance (K - ohm) Resistance (K - ohm) 0.30 1300 0.4 20 30 40 50 60 70 20 30 40 50 60 70 20 30 40 50 60 70 Temperature (ºC) Temperature (ºC) Temperature (ºC)

Figure 6.12: (a), (b), and (c) illustrate the measured forward voltage versus temperature for the n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterostructures at different values of the current respectively and (d), (e), (f) show the measured series resistance versus the junction temperature for n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si), respectively [13].

0.009 (a) 2.0 (b) 0.18 (c) 0.008 1.8 0.16 15ºC 0.007 25ºC 1.6 0.14 0.006 35ºC 1.4 15ºC 0.12 45ºC 1.2 30ºC 0.005 55ºC 0.10 1.0 45ºC 0.004 65ºC 60ºC 0.08 0.8 Current (A)

0.003 Current (mA) Current (mA) 0.6 0.06 0.002 0.4 0.04 0.001 0.2 0.02 0.000 0.0 0.00 -0.001 -0.2 -0.02 -5 0 5 10 15 20 -6 -4 -2 0 2 4 6 8 10 12 -2 -1 0 1 2 Voltage (V) Voltage (V) Wavelength (nm) (d) 20000 (e) 8000 15000 6000

10000 4000 EL IntensityEL (a.u.) EL IntensityEL (a.u.) 5000 2000

0 0

400 500 600 700 800 900 1000 300 400 500 600 700 800 900 Wavelength (nm) Wavelength (nm)

Figure 6.13: (a, b) show the measured I-V characteristics of the n-ZnO nanorods/ p-4H-SiC, p-GaN at different temperatures, (c) shows the measured I-V characteristics of the n-ZnO nanorods/ p-Si LEDs and (d, e) shows the electroluminescence spectrum for ZnO (NRs)/p- 4H-SiC and p-GaN LEDs [13].

To validate the reliability of the model of the junction temperature of n-ZnO nanorods/p-4H-SiC heterojunction it was crucial to measure the junction temperature experimentally. Figure 6.12 (a-c) shows the experimental results of the fabricated LEDs for the forward voltage (Vf) versus temperature (T) at different constant currents. It was extracted that the experimental value of the temperature coefficient for the forward voltage from 1mA to 10 mA was -12mV/K to -49mV/K, respectively. Using equation (7) we calculated the forward voltage dependence temperature dVf/dT and it was -0.950 mV/K, -0.66 mV/K and - 1.29 mV/K for n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively. It was found that there was a difference between the experimental and the theoretical results of the temperature coefficient of the forward voltage. This difference can be due to the series resistance effect and this effect was not included in the model. But after including the series resistance term in the model we found good agreement between the theoretical and the experimental results. It was found that the series resistance has an important role in the heterojunction temperature of LEDs. Therefore the theoretical model for the temperature coefficient of the forward voltage after including the series resistance effect will be [13],

  dV f  ()TT + 2βα   ()TT + 2βα  k  NN  3k  dR  −= 77.1   + 77.0   + ln AD  − AJ S  (8)  2   2     f  dT  ()Te + β ()Te + β e NN e  dT     n   p  VP CN  

The temperature coefficient of the forward voltage from eqn. (8) was calculated and it gives a values of -11.27mV/K, -5.66 mV/K and -6.54 mV/K for n-ZnO nanorods/(p- 4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively. These values are very close to the experimental values (-12 mV/K, -6 mV/K and -6.95 mV/K) of the temperature coefficient of the forward voltage at 1mA. There is very less difference between the experimental and theoretical values and it was less than 7 %. At higher currents the difference increases, which indicates that the theoretical model is suitable for the lower range near to the built-in potential. Figure 6.12 (d-f) shows the temperature dependence of the resistance of the n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively. The dependence of series resistance on temperature was measured by increasing temperature from 25 ºC to 65 ºC.

The series resistance dependence temperature term S dTdR was calculated from the slope of dR the graph, and the value of I S was calculated to be -10.32mV/K, -5 mV/K and -5.25 dT mV/K for n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively.

During the resistance measurements, n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs show current of 0.1 μA, 0.2 mA, and 1mA, respectively. The n-ZnO nanorods/p-4H-SiC LEDs have very small current due to high resistance of the device. The cause of this high resistance needs more investigations [13].

Figure 6.13 (a, b) [13] shows the current (I) versus voltage (V) of the n-ZnO nanorods/p-4H-SiC and p-GaN LEDs at different constant temperature. I-V curves showed rectifying behavior as expected from these LEDs. It is observed that the slope of the I-V curve is increasing with increasing the temperature. As the slope is inversely proportional to the series resistance, this shows that the series resistance is decreasing with increasing the temperature. Figure 6.13 (c) shows the I-V curve of the n-ZnO nanorods/p-Si LEDs. The I-V curves clearly show a nonlinear increase of the current under the forward bias, which indicates that reasonable pn heterojunction characteristics are achieved [13].

Figure 6.13(d) [13] shows the EL spectrum for n-ZnO nanorods/p-4H-SiC LEDs. Three peaks approximately centered at 430 nm (violet-blue emission), 520 nm (green emission) and 674 nm (orange-red emission) are observed. Figure 6.13 (e) shows the EL spectrum for n-ZnO nanorods/p-GaN LEDs. The observed peaks are violet, violet-blue, and green, respectively. At room temperature the peaks are approximately centered at 400 nm (violet emission), 455 nm (blue) and 550 nm (green). The n-ZnO nanorods/p-Si LEDs showed no EL emission [13].

6.3 The influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods based LEDs

In paper VI [18], the influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods based LEDs was investigated by PL and EL measurements. When the energetic particles (electrons or ions) hit a material they change the properties of the targeted material and can change the density of defects in the material [19]. The purpose of this study is to investigate the reliability of the ZnO nanorods based LEDs in space and nuclear applications. ZnO nanorods were grown by the vapor-liquid-solid (VLS) growth method and the details of this growth method can be found in chapter 4. The LEDs based on these catalytically grown ZnO nanorods were irradiated by using 2 MeV He+ ions with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2 [18].

380 nm 375 nm 5000 8000 (a) (b) 4000 512 nm 650 nm 6000 650 nm 3000 4000 530 nm 2000

2000 IntensityPL (a.u.) PL IntensityPL (a.u.) 1000 0 0 300 400 500 600 700 300 400 500 600 700 Wavelength (nm) Wavelength (nm) 380 nm 380 nm 375 nm (d) 375 nm 8000 (c) (a) 6000 375 nm (b)

370 380 390 (c) 6000 Wavelength (nm) 512 nm 650 nm 4000 650 nm 512 nm530 nm 4000

2000 PL IntensityPL (a.u.)

PL IntensityPL (a.u.) 2000

0 0 300 400 500 600 700 300 400 500 600 700 Wavelength (nm) Wavelength (nm)

Figure 6.14: Room temperature photoluminescence spectra for ZnO nanorods (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the PL spectra of all the samples together for comparison [18]

456 nm 560 nm 560 nm 4000 (a) 4000 (b) 3500 3500 456 nm 3000 3000 2500 2500 2000 2000 1500 1500 EL IntensityEL (a.u.)

1000 397 nm IntensityEL (a.u.) 1000 500 500 0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 Wavelength (nm) Wavelength530 nm (nm) 5000 5000 530 nm 560 nm (a) 456 nm (b) (c) 456 nm 4000 (d) 4000 (c)

3000 3000 2000 2000

EL IntensityEL (a.u.) 397 nm EL IntensityEL (a.u.) 1000 1000 0 0 300 400 500 600 700 800 900 300 400 500 600 700 800 900 Wavelength (nm) Wavelength (nm)

Figure 6.15: Display the electroluminescence spectra for n-ZnO nanorods/p-GaN LEDs, (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the EL spectra of all the LEDs together for comparison [18].

Figure 6.14 [18] (a-c) display the photoluminescence spectra from the as grown and irradiated ZnO nanorods. Figure 6.14 (a) displays the PL spectrum for the as grown ZnO nanorods. Three PL emission intensity peaks are observed. The band edge emission (UV) and the deep level emission (green and orange-red) peaks are observed and these peaks are centered approximately at 380 nm (3.26 eV), and 530 nm (2.33 eV) and 650 ( 1.90 eV), respectively. Figure 6.14 (b-c) displays the PL spectra for irradiated ZnO nanorods with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2, respectively. Three PL intensity peaks are observed. The edge emission (UV) and the deep level (green and orange-red) emission peaks are observed and these are centered approximately at 375 nm (3.30 eV), and 512 nm (2.42 eV) and 650 nm (190 eV), respectively [18].

Comparison of the PL spectra of the as grown ZnO nanorods and the He+ ion irradiated is shown in figure 6.14 (d). From the comparison it can be observed that the PL emission intensity in the UV region decreases and the PL intensity in the visible region increases after the He+ ion irradiation. The UV emission decreases after irradiation due to degradation in the crystalline quality and the enhancement in the green emission is due to the increase of radiative defects such as oxygen vacancies [20] while the orange- red emission is less affected by the irradiation.

From the comparison of the PL spectra of the as grown and the He+ ion irradiated ZnO nanorods it is also observed that after the He+ ion irradiation there is a blue shift in the UV peak and it is about 0.0347 eV. This blue shift in the excitonic UV emission peak can be attributed to the presence of homogeneous compressive strain in the ZnO nanorod and it was produced by the irradiation of the ZnO nanorods by He+ ions. This compressive strain increases the band gap and affects the optical properties of the material [21].

From the comparison it is also observed that there is also a blue shift in the green region centered peak after He+ ion irradiation of the ZnO nanorods and this blue shift is about 0.082 e V. This blue shift can be attributed to the fact that the He+ ion irradiation produced more oxygen vacancy related defects as compared to oxygen interstitials defects in the ZnO. The green emission in ZnO is the combination of the green emission due to oxygen vacancies and oxygen interstitials. It is commonly observed that the as grown undoped ZnO nanorods introduce green emission approximately centered at 2.33 eV and it is very close to the energy of oxygen interstitials below the conduction band but after He+ irradiation oxygen vacancies defects in the deep levels increase and irradiated ZnO nanorods have green

100 emission that is centered at approximately 2.42 eV and it is very close to the energy of oxygen vacancies below the conduction band. There is no shift in the orange-red PL emission peak centered at 665 nm (1.90 eV). It is due to the fact that the orange-red emission can be attributed to the transition from zinc interstitial (Zni) to oxygen interstitial (Oi) defect levels in ZnO [22]. As He+ ion irradiation affect the oxygen vacancies and oxygen interstitials are not affected therefore orange-red emission remains unaffected. The details, why He+ ion irradiation is not affecting the orange-red emission needs more investigation.

Figure 6.15 (a) [18] displays the EL spectra of LEDs based on the as grown ZnO nanorods. Three EL emission peaks in the violet, violet-blue and green regions are observed. These peaks are centered approximately at 397 nm (3.12 eV), 456 nm (2.71 eV) and 560 nm (2.21 eV), respectively. Figure 6.15 (b, c) [18] show the EL spectra of the LEDs based on He+ ions irradiated ZnO nanorods. The EL spectra shows the violet emission peak that was observed in the EL spectra of the LEDs based on the as grown nanorods and it almost disappeared after the He+ ion irradiation. It disappears due to the poor crystalline quality after the He+ ion irradiation. The centre of the green emission EL peak is blue shifted by about 0.125 eV as compared to the as grown nanorods based LED. The blue EL emission peak looks stable and there is no change in its position as it is believed that it is from the p-GaN substrate. It was also observed that the position of the green emission peak in the EL spectra is red shifted as compared to the PL spectra and this red shift can be attributed to the heating effect of the device [18].

101

(a) (b)

(b)

(c)

Figure 6.16: Display the CIE 1931 x, y chromaticity space, showing the chromaticity coordinates of LEDs under forward bias for ZnO NRs/p-GaN LEDs, (a) as grown ZnO NRs, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 13 2 4×10 ions/cm , and (d) all together for comparison [18].

Figure 6.16 (a-c) [18] displays the CIE 1931 color space chromaticity diagram in the (x, y) coordinates system. The chromaticity coordinates of the non-radiated and He+ ion radiated ZnO nanorods with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2 are (0.3557, 0.3805), (0.3735, 4020), and (03594, 0.3983) with correlated color temperatures (CCTs) of 4745 K, 4345 K, and 4708 K, respectively. It was observed that the chromaticity coordinates for non-radiated LED are close to Planckian locus (about 2 Mac-Adam ellipses away) and can be called white light according to the US standard ANSI_ANSLG C78, 377- 2008. The chromaticity coordinates of the He+ ion radiated LEDs are shifted slightly away from the Planckian Locus and are very close to white light. They are about 4 M ac-Adam ellipses away from the Planckian Locus. The color rendering indices are 92, 90, and 89 for the non-radiated and He+ ion radiated f LEDs, respectively [18].

From the comparison of the PL and EL investigations it was observed that the irradiation of ZnO nanorods with He+ ions has influenced the high energy defects in the ZnO

102 nanorods such as the defects which are responsible for ultra-violet (UV), violet and green emission in ZnO. It was concluded that after irradiation the crystallinity in ZnO nanorods decreases and as a result the PL intensity of UV emission decreases. The blue shift in UV and green emission was also observed. The EL spectra show the same blue shift in the green emission as observed from the PL spectra. The He+ ion irradiation also affects the color rendering properties of the LEDs and decreases the color rendering indices from 92 to 89 [18].

103

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Chapter 7 Conclusion and outlook

This research work is mainly concentrated on the luminescence properties of ZnO nanostructures and color quality of the light emitted from ZnO nanostructures based LEDs. The investigation of the luminescence from ZnO is very crucial to understand the origin of deep level emissions in ZnO. It is very important for the future possible application of ZnO nanostructures in the development of white LEDs to replace conventional light sources. Low cost production is a main attraction of any product and it is believed that ZnO has much potential to introduce low cost LEDs in the market. In this research work different ZnO nanostructures were grown by different low cost growth methods on different substrates.

In the first part of the thesis, the nanostructures were grown by low temperature (<100 oC) aqueous chemical growth (ACG). The luminescence from ZnO nanorods/p-GaN, p- 4H-SiC heterojunction LEDs was investigated. The same ZnO nanostructure (nanorods) were grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers and nanowalls) were grown on same p-GaN substrate. The luminescence spectra (EL and PL) from these different ZnO nanostructures and different p- type substrate based LEDs were compared to understand the luminescence from these LEDs.

Radiative recombinations in ZnO give near band edge ultra-violet emission due to the presence of free electron-hole exciton in ZnO and visible emission covering the whole visible region. Zinc vacancies (VZn), oxygen vacancies (VO), zinc interstitials (Zni), oxygen interstitials (Oi) related deep level defects were said to be involved in the deep level emissions in ZnO. ZnO nanotubes were annealed in different ambients (argon, air, oxygen and nitrogen). The EL spectra of the LEDs based on these annealed ZnO nanotubes were compared and investigated to understand the origin of the red emission in ZnO and it was found that more than one deep level (VO, Oi) are involved in red emission. It was also found that the quality of light from ZnO based LEDs also improved after annealing ZnO nanorods and nanotubes in different ambients, especially nitrogen ambient was found to be very effective.

It was found that different ZnO nanostructures have different density of deep level defects that are responsible for light emission in the whole visible region. The density of the structures and their surface area are also important, different nanostructures have different

107 surface area under the contact. The ZnO nanowalls have more density of deep level defects but ZnO nanotubes have more charge injection surface area under the contact. The light from ZnO nanotubes based LEDs is more intense as compared to nanowalls based LEDs.

In the second part of the thesis a relation for junction temperature of the ZnO nanorods/p-GaN, p-4HSiC, p-Si LEDs was modeled and experiments were performed to validate the model. It was found that the series resistance has the main contribution in the junction temperature. There was a large difference between the theoretical and the experimental values when series resistance was not included but after including the series resistance, there was good agreement between the theoretical and the experimental values.

In third part of the thesis, influence of helium (He+) irradiation on luminescence properties of ZnO nanorods/p-GaN LEDs were investigated. The ZnO nanorods were grown by high temperature (~700 oC) vapor-liquid-solid (VLS) method. The EL and PL luminescence spectra of as grown and irradiated samples were compared. It was found that He+ ions irradiation influence the UV emission (near band edge) emissions as well as it also affects the deep level emissions in ZnO. A blue shift in the PL peaks in the near band emission and green emission was observed. The EL measurements also showed a blue shift in the broad green emission after irradiation. The irradiation also affects the color rendering indices and decreases these from 92 to 89.

ZnO based devices especially LEDs have much potential to become popular and attractive in the market. Now the community is wishing to replace conventional light sources with white LEDs, LEDs based on ZnO especially ZnO nanostructures have potential to become the choice of the world community due to the white light quality and the light colors emitted from the ZnO based LEDs can be easily controlled by post-growth annealing ambients. ZnO based LEDs can also be attractive for the market due to low cost production. The results in this thesis concerning the luminescence and color quality investigation of ZnO nanostructures based LEDs can be useful for the fabrication and development of ZnO based white LEDs for lighting.

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