Thesis Report Investigation of Structural and Optical Properties of Nanocrystalline ZnO Sajjad Hussain LiTH-IFM-A-EX-08/1928-SE Linköpings Universitet INSTITUTE OF TECHNOLOGY The Department of Physics, Chemistry and Biology Linköpings University SE-58183 Linköping, Sweden Avdelning, institution Datum Division, Department Date 24/04/2008 Chemistry Department of Physics, Chemistry and Biology Linköping University Språk Rapporttyp ISBN Language Report category Svenska/Swedish Licentiatavhandling ISRN: LITH-IFM-A-EX-08/1928-SE Engelska/English Examensarbete _________________________________________________________________ C-uppsats D-uppsats Serietitel och serienummer ISSN ________________ Övrig rapport Title of series, numbering LITH-IFM-A-EX-08/1928-SE _____________ URL för elektronisk version Titel Title Investigation of Structural and Optional Properties of Nanpcrystalline ZnO Författare Author Sajjad Hussain Abstract The structural quality of material (concentration and nature of defects) and optical properties (intensity and spectral emission range) of semiconductor materials are usually closely correlated. The idea of this work was to carry out a basic characterization of the structural (by X-ray diffraction technique and scanning electron microscopy) and optical (by micro photoluminescence measurements) properties of nanocrystalline ZnO samples and find a correlation. A number of ZnO samples prepared by atmospheric pressure metalorganic chemical vapor deposition at different regimes and on different substrates were investigated. According to the aim of the work the most important results can be summarized as following. The analysis of ZnO nanocrystalline structures deposited on Si (100) substrates have displayed a dependence of structural quality, morphology and microstructure as well as the optical spectral purity on the deposition temperature. The deposition at 500 ºС resulted in the massive of 1D ZnO nanopillars that demonstrated the best optical properties: a mono-emission in the ultraviolet spectral range was observed. Moreover, the results of microstructure investigation give a suggestion to the explanation of the ZnO nanopillars growth. The results obtained from ZnO on sapphire substrates revealed a moderate influence of the oxygen content during deposition on the structural quality of zinc oxide. However, a strong correlation between the oxygen content and deep-level emission intensity from ZnO nanostructures has been observed, which confirms the determinative role of oxygen for the defect emission from ZnO. It was shown that during the deposition of ZnO on specially prepared homoepitaxial template the substrate surface has not the major effect on the morphology of depositing ZnO structures. SiC was revealed to be the most appropriate substrate for hetero-deposition of textured ZnO nanostructures: the growth results in the massive of epitaxially related ZnO hexagons on the SiC (0001) plane. A number of factors - p-type conductivity of the substrate used, regular and uniform epitaxial growth of ZnO nanostructure, their excellent mono-spectral emission in short wavelength range of spectra, provides a strong background for further investigation of the electroluminescence properties of the obtained heterostructures and are of great importance for the progress of optoelectronics towards low-scaled elements. Nyckelord Keyword XRD (X-Ray Diffraction), PL (Photoluminescence), SEM (Scanning Electron Microscopy), NPs (Nanopillers), ZnO (Zinc Oxide), AFM (Atomic Force Microscopy) Investigation of Structural and Optical Properties of Nanocrystalline ZnO Abstract The structural quality of material (concentration and nature of defects) and optical properties (intensity and spectral emission range) of semiconductor materials are usually closely correlated. The idea of this work was to carry out a basic characterization of the structural (by X-ray diffraction technique and scanning electron microscopy) and optical (by micro photoluminescence measurements) properties of nanocrystalline ZnO samples and find a correlation. A number of ZnO samples prepared by atmospheric pressure metalorganic chemical vapor deposition at different regimes and on different substrates were investigated. According to the aim of the work the most important results can be summarized as following. The analysis of ZnO nanocrystalline structures deposited on Si (100) substrates have displayed a dependence of structural quality, morphology and microstructure as well as the optical spectral purity on the deposition temperature. The deposition at 500 ºС resulted in the massive of 1D ZnO nanopillars that demonstrated the best optical properties: a mono-emission in the ultraviolet spectral range was observed. Moreover, the results of microstructure investigation give a suggestion to the explanation of the ZnO nanopillars growth. The results obtained from ZnO on sapphire substrates revealed a moderate influence of the oxygen content during deposition on the structural quality of zinc oxide. However, a strong correlation between the oxygen content and deep-level emission intensity from ZnO nanostructures has been observed, which confirms the determinative role of oxygen for the defect emission from ZnO. It was shown that during the deposition of ZnO on specially prepared homoepitaxial template the substrate surface has not the major effect on the morphology of depositing ZnO structures. SiC was revealed to be the most appropriate substrate for hetero-deposition of textured ZnO nanostructures: the growth results in the massive of epitaxially related ZnO hexagons on the SiC (0001) plane. A number of factors - p-type conductivity of the substrate used, regular and uniform epitaxial growth of ZnO nanostructure, their excellent mono-spectral emission in short wavelength range of spectra, provides a strong background for further investigation of the electroluminescence properties of the obtained heterostructures and are of great importance for the progress of optoelectronics towards low-scaled elements. i Investigation of Structural and Optical Properties of Nanocrystalline ZnO All praises for Allah who is entire source of knowledge and wisdom endowed to mankind and all respect for Holy Prophet (PBUH) who is forever a torch of guidance. ii Investigation of Structural and Optical Properties of Nanocrystalline ZnO Acknowledgements I express my sense of gratitude to my supervisor and examiner Professor Rositza Yakimova for all knowledge and confidence she has given me and for her patience and kindness that she showed toward me. I would definitely like to thank my co-supervisor Post Doc Volodymyr Khranovskyy for providing me with the samples for investigation and fruitful ideas for explanation of the obtained results. I greatly appreciate his support and helpful suggestions through all the stages of my work. Here I must acknowledge Reza Yazdi for helping me in the XRD measurements during his busy time. I would like to say thank Arvid Larsson (Doktorand) for his helping me during PL measurements. I appreciate all the help and concern from Professor Leif Johansson. I would like to express my gratitude to all the members of the Materials Science Division. It is great honor for me to have completed my diploma work in this group in such a good environment. I would also like to express my heart-felt appreciation for Asad Abbas; nothing was possible without his sincerity, support and love. I would also like to say thanks to all Pakistani friends for being helping, friendly and cooperative. I would like to say thank to all my classmates for being so friendly and helpful. It was impossible to achieve this task without the prayers of my parents, sisters and without the love and encouragement of my wife. iii Investigation of Structural and Optical Properties of Nanocrystalline ZnO Table of contents 1. Introduction................................................................................................ 1 1.1 Brief overview of ZnO characteristics .................................................................. 1 1.2 Comparison of different semiconductors ............................................................. 3 1.3 Motivation and aim of the work ............................................................................ 6 2. Fundamental properties of ZnO................................................................ 8 2.1 Crystal structure..................................................................................................... 8 2.2 Lattice parameters................................................................................................ 10 2.3 Electronic band structure .................................................................................... 10 2.3.1 Band gap engineering .................................................................................................. 12 2.4 Properties of wurtzite ZnO .................................................................................. 14 2.4.1 Optical Properties ........................................................................................................15 2.4.2 Thermal Properties....................................................................................................... 16 2.4.3 Electrical Properties.....................................................................................................18 2.5 Doping and defects in ZnO .................................................................................. 19 3. Characterization techniques...................................................................