MID-INFRARED ANTIMONIDE BASED TYPE II QUANTUM DOT LASERS FOR USE IN GAS SENSING By Qi Lu A thesis submitted in fulfilment for the degree of Doctor of Philosophy at the University of Lancaster No part of this thesis has been submitted to any other university or other academic institution February 2015 Mid-Infrared Antimonide Based Type II Quantum Dot Lasers for Use in Gas Sensing By Qi Lu Abstract Type II InSb/InAs quantum dots (QDs) emitting in the 3-4 µm range are promising candidate as the gain medium for semiconductor laser diodes. The molecular beam epitaxy (MBE) growth of the QDs on GaAs and InP substrates can largely cut down the costs for future devices and massively broaden its application possibilities using the more mature material platforms. Different metamorphic growth techniques including inter-facial misfit (IMF) arrays were experimented for the integration of the InSb QDs on GaAs substrates. The density of threading dislocations and the quality of the QDs were investigated using cross-sectional transmission electron microscopy (TEM) images, high resolution X-ray diffraction (XRD) and photo-luminescence (PL). The 4 K PL intensity and linewidth of InSb QDs grown onto a 3 μm thick InAs buffer layer directly deposited onto GaAs proved to be superior to that from QDs grown on 0.5 μm thick InAs buffer layers using either AlSb or GaSb interlayers with IMF technique. Even though the dislocation densities are still high in all the 3 samples (~109 cm-2), they all achieved comparable PL intensity as the QDs grown on InAs substrates. Electro-luminescence (EL) from the QDs on GaAs substrates were obtained up to 180 K, I which was the first step towards making mid-infrared InSb QD light sources on GaAs. From the study of PL temperature quenching, thermal excitation of holes out of the QDs was identified as one of the major reasons for weaker PL/EL signals at higher temperature range. To compensate the thermal leakage problem, the QDs integrated on InP substrates were grown between InGaAs barriers, which can provide a larger valence band offset compared with InAs. The QD PL peak moved to shorter wavelength (~2.7 μm) partly due to the stronger confinement, and the PL quenching was significantly slower for T > 100 K. From microscopy images, PL characteristics and calculations, the size and composition of the QDs were estimated. The InSb QD laser structures on InAs substrates emitting at around 3.1 µm were improved by using liquid phase epitaxy (LPE) grown InAsSbP p-cladding layers and two step InAs n- cladding layers. The maximum working temperature was increased from 60 K to 120 K. The gain was determined to be 2.9 cm-1 per QD layer and the waveguide loss was around 15 cm-1 at 4 K. The emission wavelength of these lasers showed first a blue shift followed by a red shift with increasing temperature, identical with the PL characteristics. Multimodal spectra were measured using Fourier transform infrared spectroscopy (FTIR). Spontaneous emission measurements below threshold revealed a blue shift of the peak wavelength with increasing current, which was caused by the charging effect in the QDs. The characteristic temperature, T0 = 101K below 50 K, but decreased to 48K at higher temperatures. Current leakage from the active region into the cladding layers was possibly the main reason for the increase of threshold current and decay of T0 with rising temperature. II Acknowledgements First of all, I would like to thank my supervisor Professor Anthony Krier for all the support, guidance and discussions throughout my PhD study. Secondly, special thanks need to be given to my vice supervisor Dr. Qiandong Zhuang for growing most of the samples for the project and the training I received on molecular beam epitaxy. I need to thank Dr Andrew Marshall for his help and advice on clean room techniques, thank Dr. Manoj Keraria for the useful discussions on the material characterizations, thank Hiromi Fujita and Dr. Michael Thompson for the discussions on fabrication and measurement techniques, and thank Dr. Min Yin for the help and training on LPE growth and laser fabrication. I would like to thank all the colleagues in my office for providing a pleasant working environment, including Ezekiel Anyebe, Kylie O’shea, Juanita James, Jonathan Hayton, Aiyeshah Alhodaib, Samual Harrison, Clair Tinker and Hayfaa Alradhi. For all the excellent technical support in the physics department, I would like to thank Dr. Atif Aziz, Stephen Holden, Shonah Ion, Tim Clough, Stephen Holt, Peter Livesley, Robin Lewsey, Martin Ward and John Windsor. I also need to give my acknowledgements to our external collaborators: Professor Abderrahim Ramdane, Dr Anthony Martinez and Konstantinos Papatryfonos in Laboratory in Photonics and Nanostructures, CNRS, France, for the trainings I received on laser fabrications and useful discussions with them, Dr Richard Beanland in Warwick University for TEM images, Dr. Krzysztof Klos in Procal for gas sensing training. I’d like to thank all the members in the Marie Curie initial training network PROPHET. We had very useful discussions and had a great time during conferences and summer schools. At last, special acknowledgements need to be given to the EU Marie Curie program PROPHET for providing the funding for my work and training. III Publications 1. Q. Lu, Q. Zhuang, J. Hayton, M. Yin and A. Krier, Gain and tuning characteristics of mid-infrared InSb quantum dot diode lasers, Appl. Phys. Lett. 105, 031115 (2014). 2. Q. Lu, Q. Zhuang, A. Marshall, M. Kesaria, R. Beanland and A. Krier, InSb quantum dots for the mid-infrared spectral range grown on GaAs substrates using metamorphic InAs buffer layers, Semicond. Sci. Technol. 29, 075011 (2014). Conference presentations 1. Q. Lu, Q. Zhuang, J. Hayton and A. Krier, Gain and tuning characteristics of mid- infrared InSb quantum dot diode lasers, Mid-infrared optoelectronics materials and devices conference (MIOMD) 16, Montpellier, France, 5th -9th, October, 2014 (talk) 2. Q. Lu, Q. Zhuang, J. Hayton, M. Yin and A. Krier, Type II InSb/InAs quantum dot lasers emitting at 3.1 μm, Quantum dot day conference, Sheffield, UK, 10th, January, 2014 (poster) 3. Q. Lu, Q. Zhuang, A. Marshall and A. Krier, Luminescence study of type II InSb/InAs quantum dots on GaAs substrates. iNOW summer school, Cargese, France, 19th – 30th, August, 2013 (poster) 4. Q. Lu, Q. Zhuang, A. Krier, K. Papatryfonos, A. Martinez and A. Ramdane, Mid- infrared photo-luminescence and electro-luminescence from type II InSb/InAs quantum dots grown on GaAs substrates, 16th International conference on narrow gap semiconductors (NGS-16), Hangzhou, China, 2nd - 6th, August, 2013 (talk) 5. Q. Zhuang, M. Yin, Q. Lu, A. Marshall and A. Krier, Thermophotovoltaic cells based on pseudomorphic InAs on GaAs, 16th International conference on narrow gap semiconductors (NGS-16), Hangzhou, China, 2nd - 6th, August, 2013(talk) IV 6. Q. Lu, Q. Zhuang and A. Krier, Type II InSb/InAs quantum dots grown on GaAs substrates and design of laser structures, COST MP1024 summer school, Cortona, Italy, 20th – 23rd, May, 2013 (poster) 7. Q. Lu, P. Carrington, Q. Zhuang and A. Krier, Growth and Characterization of Type II InSb Quantum Dots for Mid-infrared Diode Lasers on GaAs Substrates, SIOE, Cardiff, 9th -11th, April, 2013 (talk) 8. Q. Lu, P. Carrington, Q. Zhuang and A. Krier, Antimonide based quantum dot lasers for use in gas sensing, UK semiconductors annual conference, Sheffield, UK, 3rd – 4th, July 2012 (poster) V Contents 1. Introduction ............................................................................................................................ 1 2. Theory and fundamental concepts ......................................................................................... 6 2.1 Energy bands in semiconductors ...................................................................................... 6 2.1.1 Direct and indirect band semiconductors .................................................................. 7 2.1.2 Electronic states ........................................................................................................ 8 2.1.3 Density of states ........................................................................................................ 9 2.1.4 Level occupation ..................................................................................................... 10 2.1.5 Band alignments ...................................................................................................... 11 2.1.6 Band bending in p-n junctions ................................................................................ 13 2.2 III-V semiconductors and alloys .................................................................................... 16 2.2.1 Band gaps ................................................................................................................ 16 2.2.2 Refractive indices .................................................................................................... 18 2.3 Recombination mechanisms in semiconductors ............................................................ 19 2.3.1 Radiative recombination ......................................................................................... 19 2.3.2 Non-radiative recombination .................................................................................. 23 2.3.3 Competition between radiative and non-radiative recombination .......................... 28 2.4 Semiconductor quantum dots ........................................................................................