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Exploring MBE Growth of Quantum Dots andrew john vassilios griffiths EXPLORINGMBEGROWTHOFQUANTUMDOTS:LOWDENSITYGROWTH FORQUANTUMINFORMATIONDEVICES. university of sheffield EXPLORINGMBEGROWTHOFQUANTUMDOTS:LOW DENSITYGROWTHFORQUANTUMINFORMATIONDEVICES. andrew john vassilios griffiths for the award of doctor of philosophy (phd). department of electronic and electrical engineering, university of sheffield. december 2014 Andrew John Vassilios Griffiths: Exploring MBE Growth of Quantum Dots: Low Density Growth for Quantum Information Devices., PhD Thesis, © December 2014 4Love suffereth long, and is kind; love envieth not; love vaunteth not itself, is not puffed up, 5doth not behave itself unseemly, seeketh not its own, is not provoked, taketh not account of evil; 6rejoiceth not in unrighteousness, but rejoiceth with the truth; 7beareth all things, believeth all things, hopeth all things, endureth all things. 8Love never faileth: but whether there be prophecies, they shall be done away; whether there be tongues, they shall cease; whether there be knowledge, it shall be done away. — 1 Corinthians 13: 4-8 (Revised Version) Dedicated to my dear grandmother, Judith Harrison. 1933–2013 ABSTRACT This thesis concerns the growth and subsequent application of InAs/GaAs quantum dots for novel research applications including entangled photon sources, and coupled quantum well/quantum dot devices. This thesis gives an overview of quantum dot growth and tunable para- meters of quantum dots (QDs), showing a review of prior work detailing their composition and properties. Special attention is given to wavelength tuning, and optimisation of the linewidth for a target application, including analysis of post-growth annealing. Subsequent discussion focuses on low-density quantum dot samples, both in terms of growth and application. In conjunction with Cambridge Univer- sity, micro-photoluminescence results of a short-wavelength QD sample are demonstrated, showing successful use of the rotation stop growth method, with further analysis and comparisons made with photoluminescence maps and atomic force microscopy. Analysis of long- wavelength (>1300nm at room temperature) QDs is also shown, with single dot work being per- formed on rotation-stop growth samples in analysis of Fine Structure Split- ting (FSS) of individual QDs by Heriot Watt University. Results show an unexpectedly low FSS value for samples grown at the University of Shef- field, with potential for long wavelength entangled photon emitters. Growth optimisation of both the long- and short-wavelength structures is described, with optimisation required for the longer-wavelength samples, due to a com- parative lack of cross-wafer QD density variation. A novel adjustment to photoluminescence excitation is also discussed, with polarisation analysis being performed on QD samples designed for op- tical emission. Results indicate that QD properties have little to no effect on the spin retention in GaAs-capped QDs: instead surrounding material has a vii larger effect, indicating that spin loss happens primarily between excitation and capture. viii ACKNOWLEDGEMENTS First and foremost, thanks must go to my family for their incredible support throughout everything I’ve done, a special mention to my grandmother who sadly is unable to see the completion of this degree; something she wanted to do. Hope this makes her proud! My parents, Lis and Neil Griffiths, have provided me with unwavering support and belief where I lacked it, giving me the strength to overcome any hurdle. I’d like to thank the EPSRC for the student ship and the grant that enabled me to perform my work, and gain such valuable experience in the National Centre for III-V Technologies. Thanks also to Gavin Bell at the University of Warwick for aiding with AFM analysis; to Richard Phillips’ group at Cambridge for their micro-PL analysis, and to Brian Gerardot and group at Heriot Watt (with a special mention to Luca Sapienza) for your FSS work. Without the indispensable Matthew Taylor and Peter Spencer from Imperial College, I would not have the fifth chapter of this thesis; a massive thank you to them and all at Imperial College for accommodating for a breakthrough two weeks. A huge thank you to the people that I worked with- Edmund Clarke, without you I wouldn’t have a degree, MBE experience, or in-depth know- ledge of random football teams and bands! Thank you doesn’t quite cover your invaluable insight and help. Thanks also to Peter Houston for your sup- port academically and with the nitty-gritty of the PhD, your knowledge and insight have been indispensable. Richard Hogg deserves a special mention and huge thank you for 8 years of putting up with me and my pitfalls, both academically and alcoholically! Trips to Revolucion de Cuba and Menzels wouldn’t have been quite so fun. A similar thanks to Dave Childs, Kenneth Kennedy, Ben Stevens, Nasser Babadazeh, Omar Ghazal, Saurabh Kumar and Richard Taylor for their support/sarcastic put downs throughout! A ix huge thank you to Richard Frith for enduring the constant barrage of break- ages that I put him through, and also the random food packages that would mysteriously appear on my desk. Last, but not least, a huge thank you to all of my friends that have kept me (almost) sane over the years. Alix Dietzel, you’ve been an inspiration and perpetual source of help, our meals, runs and random excursions have always been a pleasure. Charis Lestrange, without you I wouldn’t have been able to return to Sheffield, and since I did, you’ve been my rock, particularly when it came to caffeine consumption! Thanks also to Emily Thew, Nicola Laing and Kate Geller who have helped keep me sane over lunch/coffee breaks! This thanks section would be far far too long if I was to list everyone that has been there for me over the years, but although your name isn’t here in print, your contribution to my life has still been invaluable to me. A special mention to SURLFC, who have kept me in a permanent state of injury for the past 6 years. You’ve made those years incredible. Back yourself. x CONTENTS i thesis1 1 introduction3 1.1 Background 3 1.2 Thesis Overview 6 1.3 References 8 2 practical work 11 2.1 Molecular Beam Epitaxy (MBE) 11 2.1.1 Growth Apparatus 12 2.1.2 Pumping 13 2.1.3 Pressure Measurement 15 2.1.4 Substrate Heating and Temperature Measurement 16 2.1.5 Material Sources 17 2.1.6 The Growth Process 23 2.1.7 Growth Modes 29 2.1.8 RHEED Oscillations 30 2.1.9 Altering the Electrical Properties of a Grown Film 31 2.2 Semiconductor Characterisation 32 2.2.1 Photoluminescence Spectroscopy (PL) 32 2.2.2 Photoluminescence Excitation Spectroscopy (PLE) 34 2.2.3 Atomic Force Microscopy (AFM) 34 2.2.4 Doping Measurement 36 2.2.5 X-Ray Diffraction (XRD) 41 2.3 References 48 3 an introduction to quantum dot growth 51 3.1 Light Emission from Quantum Dots 53 3.1.1 Bimodality of quantum dot ensembles. 61 3.2 Quantum Dot Growth 64 xi xii contents 3.2.1 Wavelength Control: Indium Coverage Control 64 3.2.2 Wavelength Control: Indium Flush 68 3.2.3 Wavelength Control: DWELL/InGaAs Capping 70 3.2.4 Wavelength Control: Post-Growth Annealing 71 3.3 Conclusion 83 3.4 References 84 4 low density quantum dot growth 91 4.1 Rotation-Stop Growth Method 97 4.1.1 Short-Wavelength Rotation-Stop Quantum Dots 97 4.1.2 Long-Wavelength Rotation-Stop Quantum Dots 103 4.2 Applications for Low-Density Quantum Dots 105 4.2.1 Entangled Photon Sources at Telecoms Wavelengths 105 4.2.2 Fine Structure Splitting (FSS) 107 4.2.3 Minimising FSS at Telecom Wavelengths 109 4.3 Conclusion 113 4.4 References 114 5 spin polarised ple analysis of quantum dot optical samples 117 5.1 Experimental Setup 118 5.2 Theory 120 5.2.1 Selection Rules 120 5.2.2 Spin Loss Mechanisms 122 5.2.3 Analysis of the Spin PLE Plot 124 5.3 Practical Results 127 5.3.1 “Standard” QD Samples 128 5.3.2 Indium Coverage Samples 131 5.3.3 In-Flush QD Samples 136 5.3.4 Coupled QD/QW Samples 140 5.4 Conclusion 148 5.5 References 151 6 conclusion 159 6.1 Further Work 161 ii appendix 163 a appendix: polarisation of in-flush qds 165 contents xiii a.1 Conclusion 168 LISTOFPUBLICATIONS 1. A J V Griffiths, E M Clarke, J S Ng, C H Tan, Y L Goh, J P R David, P A Houston. Growth and optimisation of 1300nm GaInNAs Avalanche Pho- todetectors. COST Training School: Novel Gain Materials and Devices Based on III-V-N Compounds, April 2012. 2. A J V Griffiths, Z Zhang, E M Clarke, M Hugues, P A Houston Growth and characterisation of quantum dots for single QD applications from 950- 1300 nm. UK Semiconductor Conference, July 2012. 3. A J V Griffiths. Low-Density Quantum Dot Growth for Telecom-Wavelength Applications. MBE Workers Party Meeting, March 2013. 4. A J V Griffiths, E M Clarke, M Hugues, P A Houston, R Parras, P Spencer, M Taylor, R Murray. Spin dynamics during carrier capture in InAs/GaAs Quantum Dots and Coupled Nanostructures. UK Semicon- ductor Conference, July 2013 5. L Sapienza, R N E Malein, C E Kuklewicz, P E Kremer, K Srinivasan, A J V Griffiths, E M Clarke, M Gong, R J Warburton, and B D Gerardot. Exciton fine-structure splitting of telecom-wavelength single quantum dots: Statistics and external strain tuning. Physical Review B, 88(15):155330, October 2013. 6. A J V Griffiths. Spin dynamics during carrier capture in InAs/GaAs Quantum Dots and Coupled Nanostructures. Yet to be published. xv Part I THESIS 1 INTRODUCTION 1.1 Background III-V compound semiconductors have been an area of intense research since the first GaAs laser diodes were demonstrated in 1962[8–10, 18], with growth techniques paving the way for theoretical devices becoming a reality: Kroe- mer [16] theorised a number of solutions to inherent problems found in the first devices (namely extremely large threshold current density).
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