Compound Semiconductor Materials and Processing Technologies for Photonic Devices and Photonics Integration

Carl Carl kth royal institute

r of technology euterskiöld Hedlund

Compound semiconductor materials and processing technologies for photonic devices and photonics integration

Doctoral Thesis in Information and Communication Technology Compound semiconductor materials and processing technologies for photonic devices and photonics integration

CARL REUTERSKIÖLD HEDLUND

ISBN 978-91-7873-665-2

TRITA-EECS-AVL-2020:51 2020 KTH www.kth.se Stockholm, Sweden 2020 Compound semiconductor materials and processing technologies for photonic devices and photonics integration

CARL REUTERSKIÖLD HEDLUND

Academic Dissertation which, with due permission of the KTH Royal Institute of Technology, is submitted for public defence for the Degree of Doctor of Philosophy on Friday the 30h October 2020, at 10:00 a.m. in sal C, Electrum, KTH

Doctoral Thesis in Information and Communication Technology KTH Royal Institute of Technology Stockholm, Sweden 2020 © Carl Reuterskiöld Hedlund

ISBN 978-91-7873-665-2 TRITA-EECS-AVL-2020:51

Printed by: Universitetsservice US-AB, Sweden 2020 Contents Abstract …………………………………………………………6

Sammanfattning ……………………………………………….8

Acknowledgements ………………………………………….10

List of Publications ………………………………………….11

1 Introduction ……………………………………………….17

1.1 Part A: GaAs and InP- device growth and processing…...17

1.1.1 Transistor vertical surface emitting lasers …………………..17

1.1.2 Trench confined transistors for spatial light modulator driver electronics…………………………………....………….20

1.2 Part B: Quantum dot devices …………………………………22

1.2.1 Quantum dot photodetectors for long-wavelength infrared detectors ………………………………………………22

1.2.2 Single photon emitters for quantum cryptography…………..23

1.3 Part C: Photonic crystal surface-emitting lasers (PCSELs) …………………………………………………25

1.3.1 Photonic crystal surface emitting lasers ……………………..26

1.3.2 Discrete photonic crystal surface emitting lasers …………..28

2 Experimental methods ………………………………….30

2.1 MOVPE ……………………………………………………………30

2.2 Fast feedback characterization ………………………………32

3 Results and discussion …………………………………34

3.1 Part A: GaAs and InP- device growth and processing …..34

3.1.1 Transistor vertical surface emitting lasers …………………..34

3.1.2 Trench confined transistors for spatial light modulator driver electronics………………………………..………………38

3.2 Part B: Quantum dot devices …………………………………42

3.2.1 Quantum dot photodetectors for long-wavelength infrared detectors ………………………………………………42

3.2.2 Single photon emitters for quantum cryptography ………….45

3.3 Part C: Photonic crystal surface-emitting lasers (PCSELs) …………………………………………………………49

3.3.1 Photonic crystal surface emitting lasers……………………...49

3.3.2 Discrete photonic crystal surface emitting lasers …….…….52

4 Summary, conclusions and outlook ………………….59

5 References ………………………………………………...62

6 Appendix …………………………………………………..77

7 Guide to papers ………………………………………..…95

8 Appended papers ……………………………………..…99

Abstract

The advancement of semiconductor optoelectronics relies extensively on materials and processing technologies of ever-increasing sophistication, such as nanometer-range lithography, epitaxial growth methods with monatomic layer control, and anisotropic etching procedures that allows for the precise sculpturing of device features even in the limit of extreme aspect ratios. However, upcoming application needs puts requirements on optimized designs or device performances, e.g. in terms of integration density, power efficiency, modulation bandwidth or spectral response, which call for innovative and refined methodologies. In the present thesis, we investigate a few different device designs or processing schemes that aims for extended performances or manufacturability as compared to presently available technologies. In specific, we study the design and fabrication of transistor-vertical-cavity surface-emitting lasers (T-VCSELs), the regrowth of InP-based driver electronics in the trenches of arrayed spatial light modulators (SLMs), the epitaxial growth and analysis of quantum dot (QD)-based interband photodetectors, the realization of InGaAs/GaAs QD-based single-photon emitters for the 1.55-μm waveband, as well as the fabrication of discrete and silicon-integrated photonic-crystal surface-emitting lasers (PCSELs). The transistor laser, invented at the University of Illinois around 2006, has received considerable interest due to potential major advantages in modulation bandwidth, noise properties and novel functionality as compared to conventional diode lasers. Here we study the design and fabrication of pnp-type 980-nm AlGaAs/InGaAs/GaAs T- VCSELs. Using an epitaxial regrowth process, an intracavity contacting scheme, and an optimized layer design, continuous-wave (CW) result in terms of threshold, output power and temperature performance comparable to conventional VCSELs could be demonstrated. In addition, the collector-current breakdown mechanism was shown to be due to a band-filling effect rather than an intracavity photon absorption process as previously suggested. A subsequent study regards the epitaxial regrowth for the integration of driver electronics with two-dimensional arrays of spatial light modulators (SLMs). The challenge here relies in controlling the regrowth morphology in the restricted areas that limit the SLM array fill factor. It is shown that the orientation of the SLM array with respect to the crystallographic directions is critical for controlling the regrowth

morphology, with mesa alignments along the <001> directions preferable over the <011> directions. Following this scheme, an optimized etch/regrowth process for top-contacted field-effect transistors is demonstrated. Next, we discuss the development of long-wavelength infrared (LWIR; 8-12 μm) detector elements for thermal imaging. Such detectors have traditionally been realized in the mercury-cadmium-telluride system (MCT; high performance but difficult materials properties resulting in high cost) or using AlGaAs/GaAs quantum-well infrared photodetectors (QWIPs; excellent manufacturing properties but compromised performance figures). In this work we consider interband QD photodetectors based on spatially indirect transitions in the In(Ga)Sb QD/InAs type-II system to combine the respective advantages of MCT detectors and QWIPs. An epitaxial growth process is optimized for photoresponse in the LWIR regime, and the QD properties were also studied using excitation power-dependent PL and spatially resolved current- voltage spectroscopy using a scanning-tunneling microscope. Quantum dot-based structures were also studied for the development of single-photon telecommunication-wavelength emitters. In this case, InAs QDs were formed in an In-rich InGaAs metamorphic buffer layer grown on GaAs substrate. This resulted in narrow and bright micro-photoluminescence emission lines from isolated QDs around 1.55 μm at low temperature, thereby making the application of such QDs an interesting alternative approach to InAs/InP QDs for the realization of single-photon emitters for telecommunication-wavelength fiber-based quantum networks. Finally, we describe the development of silicon-integrated and discrete photonic-crystal surface-emitting lasers (PCSELs). In the former case, a transfer-print process is used to combine an SOI-based PC structure with an InP-based active region. This results in an ultra-shallow device structure and a buried tunnel-junction configuration is used for current injection. In the latter case, the metal-organic vapor-phase epitaxy (MOVPE) growth conditions are tuned to form perfectly encapsulated cavities in the InP matrix. Low-threshold lasing is thereby obtained from optical pumping.

Sammanfattning

Framstegen inom halvledarbaserad optoelektronik baseras i stor utsträckning på alltmer förfinad material och processteknologi, såsom litografi i nanometerskala, epitaxiala tillväxtmetoder med atomär skiktkontroll och starkt anisotropisk etsning av avancerade komponentstrukturer. Uppkommande applikationsbehov ställer emellertid allt större krav på optimerad design och komponentprestanda, t.ex. avseende integrationstäthet, energieffektivitet, modulationsbandbredd eller spektral respons, vilket kräver innovativa och förfinade metoder. I denna avhandling undersöks några olika komponentdesigner och tillverkningsmetoder som syftar till utökad prestanda och/eller tillverkningsförmåga jämfört med nu tillgänglig teknologi. Speciellt studeras design och tillverkning av transistor- vertikalkavitetslasrar (T-VCSELs), epitaxiell återodling av InP-baserad drivelektronik i pixelerade ljusmodulatorer (SLM), epitaxiell tillväxt och analys av kvantpricks (QD)-baserade interband-fotodetektorer, realisering av InGaAs/GaAs QD-baserade single-fotonsändare för telekommunikationsområdet, samt tillverkning av diskreta och kiselintegrerade fotonisk-kristall-baserade ytemitterande lasrar ( PCSEL). Transistorlasern, som uppfanns vid University of Illinois omkring 2006, har rönt stort intresse på grund av möjliga prestandafördelar relaterat till moduleringsbandbredd, brusegenskaper och ny funktionalitet jämfört med konventionella diodlasrar. Här studeras design och tillverkning av pnp-typ 980-nm AlGaAs/InGaAs/GaAs T- VCSEL. Med hjälp av en epitaxiell återodlingsprocess, intrakavitetskontakter och optimerad design, demonstreras T-VCSELs med continuous wave (CW)-resultat i termer av tröskel, uteffekt och temperaturprestanda jämförbara med konventionella VCSELs. Dessutom visas genombrottsmekanismen för kollektorström bero på en bandfyllnadseffekt snarare än en foton-assisterad absorptionsprocess som tidigare föreslagits. I ett annat sammanhang studeras epitaxial återodling för integration av drivelektronik i tvådimensionella pixelerade ljusmodulatorer (SLM). Utmaningen här består i att kontrollera återväxtmorfologin i de begränsade områdena som avskiljer SLM- pixlarna. Det visas att komponenternas orientering med avseende på de kristallografiska riktningarna är avgörande för att kontrollera

återodlingens morfologi. Det visas att en orientering av pixel-raderna längs <001>- snarare än <011> -riktningarna har en fördel i det sammanhanget. Baserat på dessa resultat utvecklas en etsnings/ återodlingsprocess för toppkontakterade fälteffekttransistorer. Därefter diskuteras utvecklingen av långvågiga infraröda (LWIR; 8-12 μm) detektorelement för värmekameror. Sådana detektorer har traditionellt realiserats i kvicksilver-kadmium-telluridsystemet (MCT; högpresterande men svåra materialegenskaper som resulterar i höga kostnader) eller med användning av AlGaAs/GaAs kvantbrunnsbaserad infraröda fotodetektorer (QWIPs; utmärkta tillverkningsegenskaper men mer modest prestanda). I detta arbete undersöks interband-QD- fotodetektorer baserade på rumsligt indirekta övergångar i In(Ga)Sb QD/InAs typ II-systemet för att kombinera prestanda och tillverkningsfördelarna i MCT- och QWIP-teknologierna. En epitaxiell tillväxtprocess optimerades för fotorespons i LWIR-området, och QD- egenskaperna studerades dessutom explicit med excitationsberoende PL och rumsligt upplöst tunnelspektroskopi med hjälp av ett sveptunnelmikroskop. Kvantprickbaserade strukturer studerades också för utveckling av singelfotonemittrar i telekommunikationvåglängdsområdet. I detta fall bildades InAs kvantprickar i ett In-rikt InGaAs metamorfiskt buffertlager odlat på ett GaAs-substrat. Detta resulterade i smala och ljusstarka fotoluminiscensinjer från isolerade kvantprickar vid en vågländ runt 1,55 μm vid låg temperatur. Detta gör tillämpningen av sådana kvantprickar till ett intressant alternativt till InAs/InP kvantprickar för realisering av singelfotonemittrar för telekomvåglängdsbaserade kvantnätverk. Slutligen beskrivs utvecklingen av kiselintegrerade samt diskreta fotonisk-kristall-baserade ytemitterande lasrar (PCSELs). I det förra fallet används en transfer-print-process för att kombinera en SOI-baserad fotonisk-kristall-struktur med InP-baserade aktivt material. Detta resulterar i en extremt kompakt komponentstruktur där en begravd tunneldiod används för ströminjektion. I det senare fallet justeras tillväxtparametrarna i MOVPE-processen för skapa en fotonisk-kristall- struktur med luft-fyllda håligheter i InP-materialet. Lasring uppvisades genom optisk pumpning.

Acknowledgements

First and foremost I would like to thank my main supervisor, Mattias Hammar, who has supported and guided me through my years of research. His passion for research combined with a depth of knowledge in a broad range of fields has been an inspiration for me. We have had many fun lunch discussions together and he has been a good friend both at work and privately. I would also like to especially thank my fellow colleges in the photonics devices group for the friendly atmosphere they have contributed to and their help: Oscar Gustafson, Thomas Zabel, Jesper Berggren, Xingang Yu and Yu Xiang. A big thanks also goes out to the people at HMA for their support and friendliness: Sebastian Lourdudoss, Anand Srinivasan, Yanting Sun, Carl Junesand, Giriprasanth Omanakuttan, Axel Strömberg and Dennis Visser. A special thanks to Cecilia Aronsson for her guidance in processing, without which many things wouldn’t have been possible, and her general cleanroom problem solving knowhow. She has also been a great friend and fun fika companion. A big thanks goes to the ELAB crew who have been a great help, with machine problem solving and fixing things, and who have been very friendly: Sven Valerio, Arman Sikiric, Nils Nordell, Aleksandar Radojcic, Per Wehlin, Anders Ländin and Mikael Sjödin. I would like to thank everyone at EKT for providing a friendly and productive work enviorment: Mikael Östling, Anders Hallén, Carl-Mikael Zetterling, Per-Erik Hellström, Gunnar Malm, Christian Ridder, Yong-Bin Wang, Jiantong Li, Sergeiy Khatsev, Ali Asadollahi, Ahmad Abedin, Sethu Saveda Suvanam, Katarina Smedfors, Konstantinos Garidis, Mattias Ekström, Corrado Capriata, Eugenio Dentoni, Anders Eklund, Ganesh Jayaku-mar, Shouben Hou, Szy-mon Sollami Delekta, Laura Zurauskaite and Anderson Smith. A big thanks to the people at RISE for support and great collaborations: Qin Wang, Olof Öberg, Ingemar Petermann and Michael Salter. A big thank you goes out to the people who work in the cleanroom who have been friendly and helpful: Dan Lantz, Linus Vik, Henry Bleichner, Smilja Becanovic, Carl Asplund, Susann Sehlin, Roger Wiklund, Magnus Lindberg and Tobias Övergaard. And a last but not least a big thank you to my family and friends.

Carl Reuterskiöld Hedlund October 2020

List of publications

Part A: GaAs and InP- device growth and processing

I. AlGaAs/GaAs/InGaAs pnp-type vertical-cavity surface- emitting transistor-lasers Y. Xiang, C. Reuterskiöld-Hedlund, X. Yu, C. Yang, T. Zabel, M. Hammar and N. Akram, Optics Express, 23 (12), 15680 (2015)

II. Epitaxial growth, fabrication and analysis of vertical-cavity surface-emitting transistor lasers, C. Reuterskiöld-Hedlund, Y. Xiang, X. Yu, C. Yang, T. Zabel, N. Akram and M. Hammar, European Workshop on Metal-Organic Vapor-Phase Epitaxy (MOVPE’2015), June 7-10, Lund, Sweden

III. Trench-confined InP-based epitaxial regrowth using metal- organic vapor-phase epitaxy, C. Reuterskiöld Hedlund, O. Öberg, J.-K. Lim, Q. Wang, M. Salter and M. Hammar, Physica Status Solidi (A), 1700454 (2018)

Part B: Quantum-dot based devices

IV. Photoluminescence and photoresponse from InSb/InAs quantum-dot structures, O. Gustafsson, A. Karim, J. Berggren, Q. Wang, C. Reuterskiöld-Hedlund, C. Ernerheim- Jokumsen, M. Soldermo, J. Weissenrieder, S. Persson, S. Almqvist, U. Ekenberg, B. Noharet, C. Asplund, M. Göthelid, J.Y. Andersson and M. Hammar, Optics Express, 20, 21264-21271 (2012)

V. Auger recombination in In(Ga)Sb/InAs quantum dots, T. Zabel, C. Reuterskiöld Hedlund, O. Gustafsson, A. Karim, J. Berggren, Q. Wang, C. Ernerheim-Jokumsen, J. Weissenrieder, M. Göthelid, and M. Hammar, Appl. Phys. Lett. 106 (1), 013103 (2015)

VI. A stable wavelength-tunable triggered source of single photons and cascaded photon pairs at the telecom C- band, K.D. Zeuner, M. Paul, T. Lettner, C. Reuterskiöld Hedlund, L. Schweickert, S. Steinhauer, L. Yang, J. Zichi, M. Hammar, K.D. Jöns and Val Zwiller, Appl. Phys. Lett. 112, 173102 (2018)

Part C: Photonic-Crystal Surface-Emitting Lasers

VII. Buried tunnel junction current injection for InP-based nanomembrane photonic crystal surface emitting lasers on Silicon, C. Reuterskiöld Hedlund, S.-C. Liu, D. Zhao, W. Zhou, and M. Hammar, Physica Status Solidi A, 1900527 (2019)

VIII. Buried InP/Air hole Photonic Crystal Surface Emitting Lasers, C. Reuterskiöld Hedlund, J. Martins De Pina, M. Hammar, A. Kalapala, Z. Liu and W. Zhou, accepted for publication in Physica Status Solidi, 2020

The work has also led to the following publications and presentations that not have been included in the thesis:

1. Printed Large-Area Single-Mode Photonic Crystal Bandedge Surface-emitting Lasers on Silicon, D. Zhao, S. Liu, H. Yang, Z Ma., C. Reuterskiöld-Hedlund, M. Hammar, and W. Zhou, Scientific Reports, 6, 18860 (2016) 2. Performance optimization of GaAs-based pnp-type vertical- cavity surface-emitting transistor-lasers, Y. Xiang, C. Reuterskiöld-Hedlund, X. Yu, C. Yang, T. Zabel, M. N. Akram and M. Hammar, IEEE Photonics Technology Letters, vol. 27, no. 7, pp. 721– 724, (2015)

3. Heterogeneously Integrated InGaAs and Si Membrane Four Color Photodetector Arrays, L. Menon, H. Yang, S.- J. Cho, S. Mikael, Z. Ma, C. Reuterskiöld-Hedlund, M. Hammar, and W. Zhou, IEEE Photonics Journal 8 (2), 1-7 (2016)

4. Photonic crystal bandedge membrane lasers on Silicon, S.-C. Liu, D. Zhao, Y. Liu, H. Yang, Z. Ma, C. Reuterskiöld- Hedlund, M. Hammar, and W. Zhou, Applied Optics, 56 (31), H67 (2017) 5. Lateral Size Scaling of Photonic Crystal Surface Emitting Lasers on Silicon Substrate, S.-C. Liu, D. Zhao1, X. Ge1, C. Reuterskiöld-Hedlund, M. Hammar, S. Fan, Z. Ma, and W. Zhou, Photonics Journal, 10(3), 4500506 (2018) 6. On-Chip Photonic Crystal Surface-Emitting Membrane Lasers (Invited), W. Zhou, S.-C. Liu, X. Ge, D. Zhao, H. Yang, C. Reuterskiöld-Hedlund, and M. Hammar, IEEE Journal of Selected Topics in Quantum Electronics 25 (3), 4900211 (2019) 7. Reconfigurable Frequency coding of Deterministic Single Photons in the Telecom C–Band, S. Gyger, K.D. Zeuner, K.D. Jöns, A.W. Elshaari, M. Paul, S, Popov, C. Reuterskiöld Hedlund, M. Hammar, O. Ozolins, and V. Zwiller, Optics Express, 27 (10), 14400 (2019) 8. An on–demand entangled photon pair source for deployed fiber–based quantum networks, K.D. Zeuner, K.D. Jöns, L. Schweickert, C. Reuterskiöld Hedlund, C. Nuñez Lobato, T. Lettner, K. Wang, E. Schöll, S. Steinhauer, M. Hammar, and V Zwiller, arXiv preprint arXiv:1912.04782, submitted for publication (2020) 9. Surface emitting 1.5 μm multi-quantum well LED on epitaxial lateral overgrowth InP/Si, G. Omanakuttan, Y.-T. Sun, C. R. Hedlund, C. Junesand, R. Schatz, S. Lourdudoss, V. Paillard, F. Lelarge, J. Browne, J. Justice, and B. Corbett, Optical Materials Express, 10 (7), 1714 (2020) 10. Long-wavelength infrared photoluminescence of InSb and InGaSb quantum dots grown on InAs, O. Gustafsson, A. Karim, C. Reuterskiöld Hedlund, Q. Wang, S. Savage, S. Almqvist, T. Zabel, J. Berggren and M. Hammar, Progress In Electromagnetics Research Symposium (PIERS), Stockholm, Sweden, August 12-15, 2013 11. Printed Photonic Crystal Bandedge Surface-emitting Lasers on Silicon, S. Liu, D. Zhao, H. Yang, Z. Ma, C. Reuterskiöld-Hedlund, M. Hammar, and W. Zhou, Conference on Lasers and Electro-Optics (CLEO’2015), May 12-15, 2015, San Jose, CA, USA 12. Design and Characterization of Photonic Crystal Bandedge Surface-emitting Lasers on Silicon, D. Zhao, S. Liu., H. Yang, Z.

Ma., C. Reuterskiöld-Hedlund., M. Hammar, and W. Zhou, IEEE IPC 2015 13. Solar grade III-V substrates for cost effective high efficiency photovoltaics, Y.-T. Sun, G. Omanakuttan, C. Reuterskiöld Hedlund, M. Hammar and S. Lourdudoss, 32nd European PV Solar Energy Conference and Exhibition (EU PVSEC 2016), 20-24 June, 2016, Munich, Germany 14. Design and Characterization of Photonic Crystal Band Edge Surface Emitting Lasers with III-V membrane on Bulk Silicon, D. Zhao, S.-C. Liu, H. Yang., Z. Ma., C. Reuterskiöld- Hedlund., M. Hammar, and W. Zhou, Conference on Lasers and Electro-Optics (CLEO’2016), June 5-10, 2016, San Jose, CA, USA 15. Room Temperature Photonic Crystal Bandedge Membrane Lasers on SOI Substrates, S.-C. Liu, D. Zhao, H. Yang., Z. Ma., C. Reuterskiöld-Hedlund., M. Hammar, and W. Zhou, Conference on Lasers and Electro-Optics (CLEO’2016), June 5-10, 2016, San Jose, CA, USA 16. Lateral Size Scaling of Photonic Crystal Surface-Emitting Lasers on Si, S.-C. Liu, D. Zhao, H. Yang, C. Reuterskiöld-Hedlund, M. Hammar, S. Fan, Z. Ma and W. Zhou, Conference on Lasers and Electro-Optics (CLEO’2017), May 14-19, 2017, San Jose, CA, USA 17. Photonic Crystal Surface-Emitting Lasers on Bulk Silicon Substrate, S.-C. Liu, D. Zhao, H. Yang, C. Reuterskiöld-Hedlund, M. Hammar, Z Ma and W Zhou, Conference on Lasers and Electro-Optics (CLEO’2017), May 14-19, 2017, San Jose, CA, USA 18. Trench-confined epitaxial regrowth for integrated driver electronics in InP-based spatial light modulator arrays, C. Reuterskiöld Hedlund, O. Öberg, J.-K. Lim, Q. Wang, M. Salter and M. Hammar, Compound Semiconductor Week, CSW’2017, May 14-18, Berlin, Germany 19. Photonic Crystal Surface-Emitting Lasers on Silicon Substrates, S.-C. Liu, D. Zhao, Y. Liu1, H. Yang, C. Reuterskiöld- Hedlund, M. Hammar, Z. Ma, and W. Zhou, 2017 IEEE Photonics Society Summer Topicals Meeting, 10 - 12 July 2017, San Juan, Puerto Rico 20. Electrically Pumped Hybrid III-V/Si Photonic Crystal Surface Emitting Lasers with Buried Tunnel Junction, S.-C. Liu, D. Zhao, C. Reuterskiöld Hedlund, Z. Liu, M. Hammar and W. Zhou, Conference on Lasers and Electro-Optics (CLEO’2018), May 13-18 2018, San Jose, CA, USA

21. Electrically pumped InP nanomembrane-based photonic bandedge lasers on silicon, M. Hammar, C. Reuterskiöld Hedlund, S.-C. Liu, D. Zhao and W. Zhou, Energy Materials and Nanotechnology (EMN) Collaborative Conference on Photonics, Laser sources, April 8-12, 2018, Victoria, Canada (Invited presentation) 22. A stable wavelength-tunable source of triggered single photons at the telecom C-band, K.D. Zeuner, M. Paul, T. Lettner, C. Reuterskiöld Hedlund, L. Schweickert, S. Steinhauer, L. Yang, M. Hammar, K.D. Jöns, J. Zichi and Val Zwiller, 34th International Conference on the Physics of Semiconductors (ICPS), July 29-August 3, Montpellier, France 23. Buried tunnel junction current injection for InP-based nanomembrane photonic crystal surface emitting lasers on Silicon, C. Reuterskiöld Hedlund, S.-C. Liu, D. Zhao, W. Zhou, and M. Hammar, Compound Semiconductor Week, CSW’2019, May 19-23, Nara, Japan 24. Photonic Qubits emitted by semiconductor quantum dots for quantum network applications, K. D. Zeuner, S. Gyger, K. D. Jöns, C. Nunez Lobato, C. Reuterskiöld Hedlund, S. Steinhauer, G. Vall Llosera, K. Wang, M. Hammar, and V. Zwiller, 14th IEEE Nanotechnology Materials and Devices Conference, 27-30 October, Stockholm Sweden 25. On-demand generation of entangled photons in the telecom C-band, K. Jöns et al., SPIE Quantum Nanophotonic Materials, Devices, and Systems 2020, San Diego, CA, USA 26. Lateral Size Scaling of Photonic Crystal Bandedge Lasers on SOI Substrates, D. Zhao, S.-C. Liu, H. Yang, S. Fan, C. Reuterskiöld- Hedlund, M. Hammar, Z. Ma, and W. Zhou, The 12th International Symposium on Photonic and Electromagnetic Crystal structures (PECS-XII), University of York, UK, July 17-21, 2016 27. Photonic Qubits emitted by semiconductor quantum dots for quantum network applications, K. D. Zeuner, S. Gyger, K. D. Jöns, C. Nunez Lobato, C. Reuterskiöld Hedlund, S. Steinhauer, G. Vall Llosera, K. Wang, M. Hammar and V. Zwiller, Graduate summer school on Quantum devices for non-classical light generation and manipulation, September 30 – October 5, 2019, Erice, Italy 28. Discrete and silicon-integrated InP-based photonic-crystal surface-emitting lasers, C. Reuterskiöld Hedlund, J. Martins De Pina., S.-C. Liu., D. Zhao., W. Zhou., and M. Hammar, Optics and Photonics in Sweden (OPS), 16-17 October, 2019, Kista, Sweden

29. On-demand generation of entangled photons in the telecom C-band, K.D. Zeuner, K.D. Jöns, L. Schweickert, C. Reuterskiöld Hedlund, C. Nuñez Lobato, T. Lettner, K. Wang, , S. Gyger, E. Schöll, S. Steinhauer, M. Hammar, and V Zwiller, 11th International Conference on Quantum Dots, May 18-22, 2020, Munich, Germany 30. On-demand generation of entangled photons in the telecom C-band, K.D. Zeuner, K.D. Jöns, L. Schweickert, C. Reuterskiöld Hedlund, C. Nuñez Lobato, T. Lettner, K. Wang, , S. Gyger, E. Schöll, S. Steinhauer, M. Hammar, and V Zwiller, Quantum Technology International Conference 2020, April 6-8, 2020, Barcelona, Spain 31. On-demand generation of entangled photons in the telecom C-band, K.D. Zeuner, K.D. Jöns, L. Schweickert, C. Reuterskiöld Hedlund, C. Nuñez Lobato, T. Lettner, K. Wang, S. Gyger, E. Schöll, S. Steinhauer, M. Hammar, and V Zwiller, Quantum Nanophotonic Materials, Devices, and Systems 2020, August 20, 2020, Online

1. Introduction

Compound semiconductor optoelectronic devices rely extensively on the growth and performance of the involved epitaxial materials. Over the years, epitaxial growth techniques such as molecular beam epitaxy (MBE) and metal-organic vapor-phase epitaxy (MOVPE) have therefore developed into the highest degree of sophistication, e.g. allowing for precise thickness and compositional control down to the monolayer regime. This is often referred to as bandgap engineering, indicating the ability to customize the optoelectronic properties for a given application. However, the ever-increasing requests for a multitude of advanced services in the modern society, imposes increasingly tighter application- driven performance requirements and corresponding challenges to the devices optimizations. This has resulted in numerous innovative designs and fabrication concepts. The present thesis investigates several new devices and concepts, including transistor vertical surface emitting lasers (T-VCSELs), type-II quantum-dot (QD)-based interband long-wavelength infrared (LWIR) detectors, trench-confined selective area-growth for dense electronics integration, telecommunication-wavelength QD-based single-photon emitters, and discrete as well as silicon-integrated photonic crystal surface emitting lasers (PCSELs).

1.1 Part A: GaAs and InP- device growth and processing

1.1.1 Transistor vertical-cavity surface-emitting laser

The vertical-cavity surface-emitting laser (VCSEL) was invented and developed in the 1970s by Professor Kenichi Iga et al at the Tokyo Institute of technology [1]. Since then, VCSELs have been extensively used for a variety of applications such as reprographics, sensing and spectroscopy, computer mice, and data communication due to several advantageous properties in terms of modulation bandwidth, energy consumption, small footprint and low-cost fabrication/testing [2]. In communication, VCSELs are widely used for short-range data transfer over optical fiber. The data transfer rate relies on the VCSEL modulation bandwidth which has proven difficult to increase beyond the 30 - 35 GHz range [3] [4]. However, present technology trends call for single-channel transfer rates beyond 100 Gbps [4], making the development of

corresponding VCSEL products a highly requested but also extremely challenging task that requires radically new device concepts. Different development efforts in this direction have included self-injection-locked lasers [5], external modulation [6], transverse-mode coupled-cavity VCSELs [7], and minimized modal volume using high-index-contrast grating mirrors [8], but these schemes are difficult to implement in real- world systems, and compact directly modulated standard VCSEL-like components remain attractive to preserve a simplified electrical, mechanical and optical system interface. In this respect the so-called transistor laser emerged as a potentially promising concept to break the bandwidth bottleneck of diode lasers. The idea to combine the fast switching properties of a transistor with the light emitting capability of a laser was introduced by M. Feng and N. Holonyack in 2004 and a functional transistor laser (T-laser) was demonstrated in 2005 when they introduced a quantum wells (QW) into the base region of a heterojunction bipolar transistor(HBT) [9] [10] [11]. The T-laser rendered significant interest and the first paper that reported room temperature operation of a T-laser was recognized by the American Institute of Physics as one of the five most important papers throughout the history that had been published in their front journal Applied Physics Letters [12].

Figure 1.1: Simplified band diagram of a biased pnp-type T-VCSEL. The inset above the band diagram shows the hole concentration distribution in the base region. The T-VCSEL takes two electrical inputs and outputs both an electrical and an optical signal. Holes are injected into the base from the emitter and will recombine in the MQW/base region or be swept into the collector due to base-collector reverse bias.

The T-laser is a three-terminal electrical device that also has an optical output; it performs electrically like a transistor while simultaneously emitting stimulated light (Fig. 1.1). Different from a conventional semiconductor laser diode, it relies on minority carrier recombination in the QW region and therefore the carrier dynamics is different. In particular, the reverse-biased base-collector junction imposes a carrier gradient over the base/QW region resulting in a reduced carrier lifetime which is of interest since a reduction of carrier lifetime is one mechanism for enhancing the direct modulation bandwidth [13]. Short radiative lifetimes also leads to a resonant-free frequency response [14] and reduced relative intensity noise [15]. Other T-laser attributes of potential interest for improved modulation properties, linearity and simplified driver circuitry relates to options for collector current power monitoring [16] , feedback control [17], and voltage-controlled operation [18]. While edge-emitting T-lasers not are suitable for low-cost mass deployment, transistor-VCSELs (T-VCSELs) would have the same manufacturing and low-power operation advantages as conventional VCSELs. Hence, there is a compelling reason to combine the performance/functionality of the T-laser with the low cost/low power of the VCSEL to form the T-VCSEL. This has spurred development efforts both on the theoretical and experimental side, and initial modeling results confirmed T-VCSEL operational advantages such as increased modulation bandwidth [19], and the Illinois group published a series of papers on 980-nm InGaP/GaAs(InGaAs MQW)/GaAs npn-type T- VCSELs [20] [21] [22] [23]. However, these T-VCSELs had an insufficient current confinement and inappropriate gain-cavity tuning and could only be operated continuous wave (CW) at very low temperature (-75°C) at modest power levels. It was therefore of interest to develop T-VCSELs with adequate current confinement and gain-cavity detuning to investigate the potential of the technology. In paper I and II, we report on the growth, fabrication and analysis of AlGaAs/GaAs(InGaAs MQW)/GaAs pnp-type T-VCSELs with different configurations and demonstrate performance-levels in terms of output power, threshold current and temperature-performance in pair with standard diode-type VCSELs. No firm conclusions could be drawn regarding the overall usefulness of transistor-type lasers, but typical operational features of the devices were observed. In specific, it could be shown that the quenching of the light and run-off in collector current at a particular base current is due to a band-filling process rather than

photon-assisted tunneling as suggested in previous reports as a possible route to high-bandwidth modulation. [19] [24] [Paper II].

1.1.2 Trench confined transistors for spatial light modulator driver electronics

Satellites have classically grown larger and larger with increasing payload capability, starting from Sputnik 1 with a weight of 83 kg to modern communication satellites with sizes of school busses and weight in excess of six tons. On the other hand, due to the miniaturization of technology, useful instrumentation can be put into satellites with a weight of just a few kilograms [25]. Efforts have been made to standardize the shape, weight and most importantly the deployment mechanism of miniaturized satellites [26]. Sorting them based on their sizes, where one cm3 and with 10כ10כso-called standard size CubeSat unit (U) has a size 10 a weight of around 1 kg, a satellite with the size of one 1 U is called a picosatellite, a satellite in the range of 1-10 U is called a nano-satellite, and so forth. Such CubeSat-standardized satellites are launched in large batches to dramatically cut down on the satellite launch-price which opens up the market for smaller actors. An Indian company set the record for number of satellites deployed in 2017 when the Indian Space Research Organization deployed 103 nanosatellites as well as one medium sized satellite into space from one rocket launch [27]. The small dimensions of nanosatellites put strict requirements on the size and power-usage of instrumentation and communication devices. Data from satellites can be sent by radio waves but also by free-space optical communication using lasers. The benefit of using a laser instead of radio waves is that it doesn’t require an antenna or dish, the laser beam cannot easily be intercepted due to its focused beam shape, and it doesn’t consume frequency-band space. However, a notable disadvantage is that the laser needs to be pointed in the right direction. By equipping the satellite with a modulating retroreflector (MRR), a combination of a retroreflector and a spatial light modulator (SLM), an incoming laser beam can be read, modulated and returned. SLMs are basically computer controlled modulators with spatial resolution. The most common example of a SLM is the liquid crystal display (LCD) that has individual pixels that work as polarizers that can be turned on and off, a second static polarizer makes sure that only partially or unpolarized light gets transmitted. Here we use an SLM based on absorption in reverse biased

MQWs that can radically shift their absorption spectrum due to quantum confined stark effect [28] [29]. Very little power is used since the satellite only needs to modulate an incoming beam, and as illustrated in Fig. 1.2 the special shape of the corner retroreflector eliminate the need for sophisticated tracking equipment [30].

Figure 1.2: Schematic of SLM based free space communication from two different positions to a satellite. The corner retroreflector guaranties that any incoming light beam is returned at the same angle. The SLM can dedicate some pixels for detection so it can read the incoming signal and apply further modulation to send data back.

To achieve high modulation-rate, the SLM can be arrayed using insulating trenches for reduced capacitance. In Paper III, we investigate the possibility to fabricate transistors inside such trenches using area- selective epitaxial regrowth. A series of metal-organic vapor-phase epitaxy (MOVPE) experiments were conducted on arrayed and trench- defined structures to understand and control selectivity, growth rate and doping incorporation. It is shown that the orientation of the SLM array with respect to the crystallographic directions is critical for controlling the regrowth morphology, with mesa alignments along the <001> directions preferable over the <011> directions. Following this scheme, an optimized etch/regrowth process for top-contacted field-effect transistors is demonstrated, suggesting a possible route towards trench-confined electronics.

1.2 Part B: Quantum dot devices

1.2.1 Quantum dot photodetectors for long wavelength infrared imaging

Thermal imaging is usually done in the medium-wavelength infrared (MWIR; 3-5 μm) or long-wavelength infrared (LWIR; 8-12 μm) due to the corresponding atmospheric transmission windows. LWIR has better performance in foggy weather, atmospheric turbulence, winter haze and smoky conditions making it suitable for military and surveillance applications in cooler climates or weather conditions [31] [32]. LWIR thermal imaging cameras has so far been dominated by bulk- type mercury-cadmium-telluride (MCT) [33] or GaAs-based quantumwell infrared (QWIP) photodetectors [34]. Here, the MCT technology is the high-performance/high-cost alternative whereas QWIP is a low-cost option but at more modest performance. More recently a new type of infrared-detector material has emerged, consisting of strained type-II heterostructure InAs/GaSb superlattices (often abbreviated T2SL – type- 2 strained superlattice) [35] [36]. These detectors can be tuned for sensitivity ranging from the MWIR to the very-long wavelength infrared (vLWIR; >20 μm) regimes with excellent responsivity. However, the epitaxy of these superlattice structures is very demanding, typically requiring MBE growth. In paper IV we investigate QD-based InSb/InAs detector elements, so-called dot-to-bulk (D2B) [37] detectors with more relaxed growth and material requirements that make them suitable for fabrication using MOVPE, the standard epitaxial growth technology for optoelectronic production. The D2B detector is made up of an InSb/InAs QD superlattice as illustrated in Fig. 1.3. The large lattice mismatch between InAs and InSb lead to QD formation during growth, so called Stranski Krastanov growth mode. There is a broken band alignment (type-III) between InSb and InAs bulk-material, but due to QD size-dependent confinement-energy the photo-excitation interband-transition (QD to bulk) is a staggered (type-II) transition. The QD/bulk type-II band alignment makes the conduction band (CB) states being delocalized which increases the recombination time. The QD size, and the corresponding confinement- and absorption energy, can be tuned with

QD composition and growth conditions. Alloying with Ga for InxGa(1-x)Sb QDs, decreases the lattice mismatch and increases the QDs size, which results in a smaller confinement energy.

Figure 1.3: (left) Illustration of the D2B spatially indirect detection scheme with a superlattice of self-assembled quantum dots. (right) Band diagram

showing the InxGa(1-x)Sb QD/InAs type- II alignment and interband photo excitation [38].

Different D2B device structures were fabricated and characterized with respect to photoluminescence and photocurrent response in the LWIR regime. In addition, direct spectroscopic observation of buried QDs were realized using cross-sectional Scanning Tunneling Microscopy (STM). While photocurrent response could be demonstrated in the long- wavelength regime, it was later on noted that a deep hole confinement imposes a too-high operating temperature for a reasonable signal-to- noise ratio [38]. Instead, it was suggested that the application of a relaxed InSbAs/InAs virtual substrate would allow for more-shallow hole- confinement and thereby a possible route towards high-performance detector elements [38]. In paper V, power-dependent photoluminescence measurements were used to estimate the Auger recombination coefficient for the InSb/InAs and InGaSb/InAs QD systems, and found it to be an order of magnitude lower than for the InGaSb/InAs superlattice system, further indicating the potential benefits of a QD-based solution for LWIR detectors.

1.2.2 Single photon emitters for quantum cryptography

Cryptography generally either use asymmetric public key distribution or symmetric private key distribution. In the latter case, both sender and receiver uses the same key to encrypt and decrypt. If a symmetric key, like a randomized one time pad, only is used once it is impossible to break [39]. However, keys are generally reused so they suffer from key exhaustion, where every time the key is used it leaves itself more vulnerable to attack [40]. The main problem with private key encryptions is the key distribution. Every line of communication needs a

unique key-pair so trusted third parties (TTPs) are often used for key distribution [41]. Asymmetric key distribution work by having two keys, one public and one private. The public key is distributed openly and is used to encrypt messages. The private key is used to decrypt the messages and is not distributed. There are several mathematical problems that the keys can be built on, for example prime number factorization and specifically the polynomial time it takes to compute [42]. While Asymmetric keys are hard to crack, there is no mathematical proof that there isn’t an algorithm to factorize more efficiently. Also, it is susceptible to “man in-the-middle” attacks where the public key can be intercepted and replaced with a false public key. Quantum cryptography relies on several principles of quantum mechanics, such as the Heisenberg uncertainty principle or the principle of quantum entanglement. In a general setup, an encrypted message is sent over a public channel (such as an optical or wireless network) while a secret key for deciphering the message is sent over a quantum channel. Several protocols have been developed for the quantum key distribution (QKD), e.g. BB84 protocol [43] that relies on the uncertainty principle or the E91 protocol [44] that is based on entanglement. The former scheme uses photon polarization to transmit the information. For this to work it is necessary to develop on-demand single photon sources. Semiconductor QDs are excellent candidates for such emitters, and very encouraging results have been realized in the InAs(QD)/GaAs materials system for transmission <1 μm [45] [46]. However, to make use of existing infrastructure and to enable long-distance communication, telecommunication wavelength emitters of 1.55 μm would be preferred. In Paper VI, we demonstrate InGaAs/GaAs QDs with bright and tunable emission in the 1.55-μm wavelength band using a metamorphic buffer- layer scheme where the lattice constant of the GaAs substrate gradually is increased by means of a strain-relaxed graded InGaAs buffer layer [47]. A high repetition rate is ensured using a GaAs/AlAs distributed Bragg reflector and the exact wavelength can be tuned using piezoelectric- induced straining.

1.3 Part C: Photonic crystal surface-emitting lasers

A Photonic crystal (PC) is a structure with a periodic variation of the refractive index that can affect the propagation of light in a controlled way depending on its design. An example of a one-dimensional PC structure consists of the distributed Bragg reflectors (DBRs) used to define the lasing cavity in the T-VCSEL discussed in part A of this thesis. These DBRs have layer thicknesses and refractive indices designed for total reflection within a certain wavelength interval due to constructive interference between reflections from the alternating interfaces. This highly-reflectivity wavelength band that doesn’t allow any propagation of light in the forward direction can be referred to as a photonic bandgap. Photonic crystals can also be made in two and three dimensions with a range of interesting physical properties which has formed an active and rich research fields since they were first proposed in 1987, independently by Eli Yablonovitch [48] and Sajeev John [49] . Two-dimensional PCs are widely considered for applications in thin film photonics where PC structures are defined by means of high-resolution lithography and anisotropic etching, and can e.g. be designed for wave guides, filters and lasers [50] . As the name suggests, 2D PCs are periodic in two directions and homogeneous in the third. They can e.g. consist of a square lattice of dielectric columns, and as such they can exhibit photonic bandgaps in the in-plane directions. In this way, by appropriate removal of a number of lattice rods, light can be confined in a certain region leading to spontaneous emission control and in-plane lasing [51]. Although such lasers would be very attractive for photonic integrated circuits, the performance suffers from out-of-plane losses. The idea behind the PCSEL concept is to instead take account of this vertical leakage for lasing in the surface-normal direction [52]. In this case, a two-dimensional standing- wave cavity mode is formed from which large-area singlemode laser emission is granted by out-of-plane Bragg reflections as dictated by the vertical radiation loss. The interaction between the light and PC lattice is described by the photonic crystal band diagram. We here consider a (defect-free) square PC lattice with circular holes as illustrated in Fig. 1.4 (a) and providing the basis for the PCSELs studied in Paper VII and VIII. Figure 1.4 (b) shows the photonic crystal band diagram along certain symmetry directions in the irreducible Brillouin zone [53]. Here the normalized

frequency (ω) of the light is plotted against the Bloch wave vector (k||). The indicated bandedge modes A-D corresponding to zero gradient points in the band structure (dω/dk=0), i.e. zero group velocity, define the resonant modes of the laser cavity. From a detailed analysis of the in- plane electric field components, the radiation loss, quality factor and threshold gain of each mode can be calculated. The mode with the smallest losses, i.e. lowest threshold gain, will be the first one to reach threshold and will determine the lasing wavelength. The modes that are most likely to lase can also be predicted by studying the symmetry properties of the in-plane electric field distribution. In such an analysis, modes C and D are expected to couple strongly to the radiation modes due to their symmetric electric field distribution while modes A and B only would couple weakly due to their anti-symmetric distribution [F]. Therefore, modes A and B are most likely to contribute to the lasing. By modifying the shapes of the air holes, it is possible to control the output efficiency of the laser [54].

a) b) c)

Figure: 1.4: (a) Illustration showing a photonic crystal consisting of round air holes in a square lattice. (b) Corresponding photonic band structure [54]. The dashed line between the gray and white areas, represents the light line that separates air-region oscillatory and evanescent modes. (c) Zoomed-in view around the Γ point [54].

1.3.1 Si-integrated photonic crystal surface emitting lasers

The direct integration of lasers on silicon wafers, the missing cornerstone for a full-blown silicon-photonics solution [55], has long been considered as a holy grail. Due to its indirect bandgap, silicon is inherently an inefficient light emitter and a variety of innovative approaches have been proposed in the literature. Silicon-based lasers have been demonstrated using stimulated Raman scattering [56], tensile- strained and heavily n-doped Ge [57] and GeSn active region [58], but these approaches are still far from practical implementations and the

most promising results have so far been obtained from heterogeneous integration of III/V-based active materials on Si. While the direct heteroepitaxial growth of III/V material on silicon would be a very attractive solution, persistent materials problems make it challenging to realize high-performance lasers [59] [60], although recent results using InGaAs quantum dots grown directly on silicon have yielded very promising results [61] [62]. The main direction of development during recent years has instead relied on wafer bonding to transfer pre-grown III/V stacks to the silicon-on-insulator substrate for further fabrication into laser diodes or other photonic device structures. While this has led to promising results, including Intel’s volume production of 100 Gb/s fully integrated optical transceiver modules [63], it also suffers from some drawbacks, e.g. related to the size-mismatch between III/V and Si-based wafers and inefficient use of the epitaxial material. Transfer printing is a technology that uses elastomeric stamps to pick materials or device coupons from a source wafer and place them with micrometer precision on a target substrate [64]. In order to use this technology to fabricate a hybrid InP/Si PCSEL, two important pre- conditions that puts specific demands on the contacting scheme must be fulfilled. First, the surface-emitting device also needs to be double top- contacted, i.e. both the anode and cathode contacts need to be assessed from the top surface while not intersecting the emitted light. This means that the contacts need to be displaced laterally and there needs to be some funneling mechanism built into the device structure. Secondly, the transferred active layer needs to be positioned very close to the silicon- based PC structure, which puts constraints on the thickness of the lateral feeding layer for the bottom contact.

Figure 1.5: Illustration of a heterogeneously integrated InP-based PCSEL on Si.

In paper VII we investigate a buried-tunnel junction (BJT) injection scheme (see Fig. 1.5). The device is grown in two steps, where the active area is masked and the surrounding BTJ structure is removed and replaced with an n-type blocking layer. An intricate trade-off is that between a sufficient blocking capability in the surrounding region while maintaining a thin overall device thickness. Different BJT LED test structures were fabricated to investigate the TJ current spreading capability, blocking-layer breakdown behavior, surface-leakage currents and series resistance. A fully fabricated silicon-integrated PCSEL based on this scheme showed narrow-line emission at very low current density.

1.3.2 Discrete photonic crystal surface emitting laser

A PCSELs optical output power scales with area without compromising its single-mode optical output [53], this makes it an interesting technology for material processing, bright light source for laser pumping [65] and sensing applications such as LIDAR [66]. GaAs based near-infrared PCSELs have shown impressive results, demonstrating watt class output power [53], beam steering [67], polarization control [68] and beam tailoring [69]. The integrated photonic crystal is fabricated using electron-beam lithography (EBL) and anisotropic etching followed by an epitaxial regrowth step to either create

a low-index semiconductor-semiconductor grating [70] or to embed air voids in the semiconductor material [71]. The latter method has a higher refractive index contrast, and thereby higher chance of Bragg diffraction but weaker coupling, while the former configuration have improved heat dissipation and reliability properties. Recently an InP based PCSEL was demonstrated by McKenzie et al. that emitted at 1.5 μm. These devises made use of an infilled InP/InGaAsP PC structure and showed pulsed electrically pumped lasing [72]. Paper VIII deals with the fabrication of InP based PCSELs based on an InP/air void PC layer, and special attention is paid to the formation of the embedded voids. It is demonstrated that the as-etched circular holes reshapes towards a more square-like character by exposing {100} or {110]-type facets depending on the detailed growth conditions. This behavior may not be ideal for PCSEL applications, given that the devices typically depend critically on the detailed hole shapes [54], and may suggest the application of an alternative grating material such as InGaAsP used in the all- semiconductor PCSELs by McKenzie et al. [72]. The regrowth behavior (degree of infilling of the etched holes), however, is well explained from predictions based on adatom diffusion dependency on growth temperature, V/III ratio and growth rate, and should be applicable even for other PC materials. The devices were characterized by optical pumping in a micro-photoluminescence setup equipped with a cryostat on a micrometer translation stage for the sample holder. Lasing oscillation was demonstrated for two different device designs (low- and high-temperature operation) and the characteristics was investigated as function of temperature, device size (laser spot size set by defocusing), polarization and lateral position within the PC lattice. We also fabricated a batch of devices for electrically injected operation, using backside lithography to align a top-contacted anode and top-contacted anode for bottom emission. Corresponding LEDs (that is, PCSEL devices without integrated PC layer) showed low-voltage operation and electroluminescence of around 2 mW at 50 mA [73], but no lasing was observed for the PCSEL structures even for drive currents of up to 1 A. The situation may be improved from a device tuning for low- temperature operation and/or pulsed operation.

2 Experimental details

2.1 Metal organic vapor phase epitaxy

Metal organic vapor-phase epitaxy was invented in the 1960s [74], and is a technique used for crystal growth. Years of innovation on reactor design and increased purity of source materials has enabled MOVPE technology to produce complex layer structures with a high degree of uniformity and precision in layer morphology, thickness and composition. A typical MOVPE system as schematically depicted in Fig. 2.1 consists of three parts: (a) Gas mixing system; (b) Reactor; and (c) Pump and scrubber system.

Figure 2.1: Schematic of MOVPE system, showing from the left side the gas mixing system where process gas from bubblers and gas cylinders get mixed in the run vent system. The reactor where the heated susceptor sits. The susceptor holds the wafer and temperature is measured by thermocouple (light grey rod). Finally after the reactor comes the throttle valve, pump and scrubber system.

In the gas mixing system, the flow of source gases, typically hydrides such as AsH3 for the group V elements and metalorganic precursors such as Ga(CH3)2 (trimethylgallium) for the group III elements, are precisely controlled using mass-flow controllers and mixed with a carrier gas such as H2 or N2. Hydrides come in pressurized gas bottles and are mixed with the carrier gas in the supply line to the reactor, whereas the metalorganics comes in liquid or solid form (small crystallites) in

bubblers through which the carrier gas is led and saturated. The source gases then travels (hydrides, metalorganics and dopants separately) to the reactor via the run-vent manifold. This is a stack of pneumatic three- way valves that can redirect the gas either into the reactor or bypassing the reactor directly into the pump (vent line). It is designed for small dead volumes and is positioned close to the reactor, which together with a sufficiently high gas velocity (set by the carrier gas) allows for sharp heterostructure interfaces [75]. The reactor consists of a quartz tube containing a graphite susceptor that holds the substrate on which growth is to occur. In a so- called cool wall-system only the susceptor is heated. This is accomplished by means of IR irradiation (lamp heating), RF induction or resistive heating. The reactor pressure and temperature are typically around 100 mbar and 450-750°C, respectively. After the gases has gone through the reactor, they pass a scrubber system that decomposes any toxic gases that remain in the exhaust. There are flammable, explosive and toxic gases involved in the process, so many detectors and interlocked safety systems are required to operate an MOVPE system. The growth process consists of several steps, including gas-phase decomposition of the reactants, diffusion to the surface, adsorption, surface reactions and desorption. Notably, these are thermally activated, sometimes catalytically accelerated, processes leading to a temperature- dependency of the growth process as discussed below. At appropriate growth conditions (dictated by growth parameters such as substrate temperature, reactor pressure, growth rate and V/III input flow ratio), the adatoms predominantly adsorb at pre-existing surface steps, which thereby leads to a “step-flow” growth mode and a planar growth front that is most ideal for realizing sharp interfaces. In this thesis, all growth was conducted using an Aixtron AIX 200/4 low-pressure MOVPE system with capacity for the simultaneous growth on three two-inch wafer. A deciding advantage of this system for these studies relies on its flexibility of changing all graphite and quartz ware between runs. The studies presented here involved arsenides, posphides and antimonides, that otherwise would be associated with severe cross-contamination effects. In addition, parallel to these growths an industrial user used the same system for gold-assisted of GaAs-based nanowires with state-of-the-art results [76].

2.2 Fast feedback characterization

The MOVPE process is very sensitive to variations in the operating parameters, resulting in some run-to-run variability in growth rate and composition. In particular, the trimethylindium (TMIn) source typically used for growing In-containing compounds (e.g. InP or InGaAs) is problematic in this respect since it is a solid source where the gas saturation depends on the path the it travels through the granulate, but there are also other sources to the growth variations such as temperature irreproducibility or drift in mass flow or pressure controllers. In effect, this necessitates several calibration runs to tune in the growth parameters for a specific structure to be grown, calling for fast-feedback characterization of layer thickness and composition between successive runs. High-resolution x-ray diffraction (HR-XRD) is very useful for measuring the average interatomic distance, the lattice constant. In the present work, a Panalytical X’Pert Pro MRD system was used. This system includes an x-ray mirror and a four-bounce symmetric Germanium (220) monochromator as primary optics. The monochromator is optimized to filter out all radiation except for the Cu-

Kα1 transition, and will ensure low beam divergence while still having reasonably high intensity throughput. This is useful to sort out compositional data for ternary compounds, such as InxGa(1-x)As. However, for quaternary films like InxGa(1-x)AsyP(1-y) or AlxInyGa(1-x-y)As supplementary information is needed as there are two unknowns x and y while XRD only provides a measurement of average interatomic distance. Any measurement that is dependent on composition will suffice, for example a measurement of the band gap energy. Photoluminescence (PL) is a convenient way to measure a semiconductors bandgap and relies on a laser-induced valence to conduction-band excitation-relaxation process. Electrons that are excited with an energy larger than the bandgap will lose excess energy in a fast process due to phonon interaction and Coulomb scattering, and will relax to the conduction band edge. Thereafter, the electrons recombine with a holes under emission of photons with the same energy as the bandgap. In the present work, we have used homebuilt PL-setups as well as a commercial micro-PL system (LabRAM HR 800 confocal Raman/PL microscope) equipped with a cryostat for low-temperature measurements. In addition, PL measurements can provide information

on epi-layer purity, crystalline quality, short-range ordering and interface abruptness.

3 Results and discussions

3.1 Part A GaAs and InP- device growth and processing

3.1.1 Transistor VCSELs

The purpose of the work in Papers I and II was to demonstrate and investigate functional T-VCSELs based on the same building blocks as used for previous developments of telecommunication-wavelength (1.3- μm) VCSELs at the department [77]. This included, epitaxially regrown pnp-blocking layer current confinement, dielectric top DBRs, undoped bottom DBR for reduced losses [78] [79], double intra-cavity top- contacting, and strained InGaAs/GaAs multiple-quantum-well (MQW) active layer, although the emission wavelength was relaxed from the 1.3- μm waveband to around 980 nm. The T-VCSELs described in Papers I and II, also had some additional design variations, including buried tunnel-junction (BTJ) current injection and an asymmetric device structure for reduced extrinsic collector resistance. The design of these pnp-type T-VCSELs is shown in Fig. 3.1.

Dielectric N-type current E α-Si/SiO2 DBR blocking layer P-type GaAs N-type GaAs n+InGaAs/p+GaAs B buried tunnel junction P-type AlGaAs emitter N-type base MQW Active region C N-type base

P-type collector

Undoped AlGaAs/GaAs DBR

Figure 3.1: Illustration of T-VCSEL with PNP (left) and TJ (right) based current confinement.

The basic operating principle of the T-VCSEL is nicely demonstrated in Fig. 3.2. Here, the device characteristics is compared with and without the top DBR [Paper I]. In the latter case, the optical feedback is insufficient for the device to reach threshold for lasing. The modest increase in optical output power with increasing base current is due to spontaneous emission. However, for the complete T-VCSEL structure lasing threshold is reached for a low base current which significantly affects the electrical characteristics. Due to the onset of simulated emission, the base recombination increases stepwise resulting in a drastically reduced current gain as manifested by the reduced increase in collector current. This is a well-known operational feature of T-lasers, referred to as gain compression [80]. Figure 3.3 shows the influence of base region thickness on the device design. Notably, the T- VCSEL is only in its active regime during lasing for a sufficiently large base width. This is of significance since the potential performance- advantages of T-VCSELs relies on the transistor being in its active regime, and hence provides an important design rule for T-VCSELs.

Figure 3.2: Pout, VBE and IC as a function of IB for a 10 μm device with a base region of 200 nm. Top DBR removed (dashed line) and with DBR (solid line).

Figure 3.3: Pout, VBE and IC as a function of IB for base thicknesses of 10, 100 and 200 nm.

Figure 3.4 (a) and (b), shows measured electrical and optical collector diagrams for a fabricated Pnp T-VCSEL. The gain compression beyond threshold is clearly manifested in (a) whereas (b) indicates the potential for voltage-controlled operation. Figure 3.4 (c) shows a partial magnification of the optical collector diagram in (b). At the base current threshold, the output power first stats to increase at a rather high VCE>5V. This is interpreted as due to due to direct injection of electrons by a band- to-band tunneling process from the collector to the base region. Wu et al. [81]suggested a related process, a photon-assisted tunneling of electrons from the collector to the base, to explain the run-off (or breakdown) in collector current with increasing IB and VCE, an effect that they also suggested exploitable for realizing very high modulation rates [82] [83]. In the present work we didn’t find any evidence for such a process, but instead related the collector current-breakdown to a MQW band-filling effect [Paper I].

Figure 3.4 a) Electrical and b) optical collector diagrams for a 200 nm base-region device with a 4 μm aperture. Black color indicates spontaneous emission whereas red color indicates stimulated emission. c) Zoom in of b) close to threshold.

Paper I discusses some of the challenges related to the realization of these devices from an epitaxial standpoint, including the integrity of the MQW active layer situated in a highly doped base region, the realization of high conductivity tunnel junction as an alternative current injection scheme (Fig. 3.1), as well as the controlled morphology of the regrowth around and atop the central mesa [84][Paper II].

3.1.2 Trench confined growth

Paper III investigates the potential for integrating electronic circuit elements in narrow ( 50 μm) trenches using a double area-selective InP- based epitaxial regrowth process. This puts tough requirement on regrowth selectively (dielectric mask versus bare semiconductor), planar morphology, and controllable growth rate and doping incorporation, all of which were studied in a series of experiments. Separate masks were used for the trench formation and for the selective area masking, with the latter only exposing stripes of exposed InP at the bottom of the trenches. One striking observation was that the selectivity of the growth was highly dependent on the orientation of the trenches with respect to the substrate crystallographic orientation. This is clearly observed in the optical micrograph in Fig. 3.5. Here the mask for the selective area growth has deliberately been displaced, making one wall of the trench exposed to growth. Narrow wire-like features have grown on the masked area from the trenches in the ሾͲͳͳሿ direction. From an analysis of the equilibrium regrowth shape, this was related to the lateral growth front as function of trench orientation. For a trench orientation perpendicular to the ሾͲͳͳሿ direction, but not to ሾͲͳͳሿ or ሾͲͲͳሿ directions, the growth is found to protrude towards the mesa wall and it is suggested that this, upon contact with the masked wall, can act as a source for the lateral growth on the surface. So, by orienting the trenches along the [100]-type directions, the overgrowth problem can be avoided.

a) b)

c) d)

Figure 3.8 a) Optical microscope image showing the processed device structure after step 3 in Fig. 3. 1.6. b) SEM image showing a cross-section of the trench and device structure. A short selective InP wet-etching step has been made to visualize the InGaAs layers. c) Zoom-in on the device structure. The partial removal of the InGaAs layer is apparent. d) SEM image at the edge of the trench structure. The mask is visible at the right.

3.2 Part B: Quantum dot devices

3.2.1 Quantum dot interband photon detectors for long-wavelength infrared imaging

The development of D2B detectors (see Sec. 1.2.1) was motivated as an exploratory alternative to the so-called type-2 strained-superlattice (T2SL)-detectors which have emerged as an important contender during the past decade in the thermal imaging market [88]. Such detectors would have potential advantages in performance in terms of extended carrier life times, suppressed Shockley-Read-Hall recombination and reduced Auger generation, in all leading to improved photo response and reduced dark current [89], but also in fabrication. In particular, due to the relaxed requirements on interface abruptness and layer control as compared to T2SL, they should be suitable for MOVPE, the standard technology of choice for optoelectronic production. The development of optoelectronic device structures in general, relies on the interplay between design and simulation, growth and processing experiments, materials analysis and device characterization. An important aspect here regards the microscopic analysis of the active material. Is it, e.g., possible to directly visualize and measure the QDs for a comparison with theory and optical characterization? This may be a deciding input for the device optimization, where any trends in QD size or shape can be related to epitaxial growth parameters with direct consequences for the optical response. However, the direct imaging of Ga(In)Sb/InAs QDs proved difficult. Scanning electron microscopy and atmospheric-ambient AFM do not have sufficient resolution to resolve the QDs, but also transmission electron microscopy (TEM) is challenging due to the cumbersome and sometimes uncertain materials-specific sample preparation. Typically, the cutting, thinning, drilling and sputtering of the sample to make it electron transparent may introduce artefacts in the images that make them difficult to interpret, and it was not possible to resolve these QDs using TEM. Instead, we turned to cross-sectional scanning tunneling microscopy (X-STM), a technique that has been successfully applied to a variety of epitaxial optoelectronic device structures in the past, including T2SL detectors and QD detector structures [90] [91].

The experiments were conducted in an Omicron (variable temperature) VT-STM operated at room temperature under ultra-high vacuum conditions. This setup is equipped with a sample loading and transfer system, e.g. allowing the samples to be moved around for various sample preparation steps. A critical step for the present analysis regards the surface cleanliness and near atomic-layer flatness. In order to facilitate this, an in situ sample cleaving device was constructed and installed in the system. The cleaving device was mounted on a bellow so that it could be inserted or retracted to the sample position, see Fig. 3.9. Some sample pre-preparation was necessary to improve the yield of successful cleaving, including sample thinning be means of grinding and sawing a short slit from the edge in which the cleaving force can be applied.

a) b) c)

Figure 3.9 a-b) Schematic drawing of retractable cleaving device. c) Photograph of cleaving device lined up with slit in sample. Sample is glued onto sample holder that is grabbed by transfer arm.

Figure 3.2.2 shows filled-state (sample bias=-0.6 V) STM images of GaSb/InAs and GaInSb/InAs QD superlattice structures. The overview image in Fig. 3.2.2 a), clearly resolves the superlattice, but although some striation along the lines can be noted it is not possible to see individual QDs. The atomic-resolution images in Fig. 3.10 (c) and (d), indicate QD-like features with an approximate height of 3 monolayers and a lateral extension of 3 and 8 nm for the InSb and In0.4Ga0.6Sb dots, respectively, that would correspond to the lower strain in the latter case.

b) a)

Figure 3.11 a) STS IV sweeps recalculated to ௗூȀௗ௏ as a function of V, where red line is ூȀ௏ bulk InAs and green is from InSb QD region. b) STS imaging different states.

A potentially advantageous property of the D2B detector type relates to the spatial separation of electrons and holes; while the holes are confined to the QD, the electrons are delocalized. This allows for an extended carrier lifetime which improves the extraction of photo- generated carriers. In addition, it is also expected to reduce the Auger recombination rate which would suppress the dark current [93]. A direct measurement of the Auger coefficient, as is the main objective of Paper V, is therefore of greatest relevance. A significant result of this analysis is that the Auger coefficient for the D2B structure is an order of magnitude smaller than for the corresponding T2SL structure [Paper V].

3.2.2 Single photon emitters for quantum cryptography

On-demand single photons generation is emerging as an increasingly important part of future quantum cryptography-based solutions. Quantum dots has previously shown to be an excellent source for single photons [94] [95]. However, few studies have shown QD-based emission in the 1.55-μm minimum-loss telecommunication window suitable for long-haul communication [47]. Here, we use a metamorphic buffer (MMB) layer on a GaAs wafer to act as a virtual substrate in order to control the size and matrix material composition of the InAs QDs for 1.55-μm emission [96]. The MMB layer was realized by linearly increasing the In concentration in InxGa(1-x)As from zero to about 40%. This results in strain relaxation of the MMB layer

by the introduction of misfit dislocations and an increasing lattice constant that allows formation of long-wavelength QDs. From a practical perspective, it is difficult to measure the thickness and composition of the relaxed and graded buffer layer as well as the capping layer using standard XRD analysis. Instead, the end composition is estimated from a reciprocal space map analysis, whereas the composition is assessed using cross-sectional SEM, see Fig. 3.12.

MMB

AlAs

Figure 3.12 SEM image showing cleaved MMB layer with an AlAs layer underneath.

Based on the top-surface composition and the total thickness, it is possible to calculate the composition profile throughout the MMB layer, as resulting from a linearly increasing TMIn flow with growth time t. We separate the InAs and the GaAs growth rate without considering any parasitic reactions. The TMIn flow is graded from 7 to 180 sccm, implying that the InAs growth rate will change over time while the GaAs growth rate is kept constant. Based on the final composition x we can then describe the final InAs growth rate (݃ூ௡஺௦̴௙௜௡௔௟ሻ as a function of GaAs growth rate (݃ீ௔஺௦ሻas

ݔ ݃כ ൌ ݃ ூ௡஺௦̴௙௜௡௔௟ ሺͳ െ ݔሻ ீ௔஺௦

The InAs growth rate (݃ூ௡஺௦ሻ can now be described as

͹ ͳ͹͵ ݐ ݃כ כ ൅ ݃כ ൌ ݃ ூ௡஺௦ ͳͺͲ ூ௡஺௦೑೔೙ೌ೗ ͳͺͲ ܶ ூ௡஺௦೑೔೙ೌ೗ ͹ ݔ ͳ͹͵ ݐ ݔ ݃כ כ כ ൅ ݃כ כ ൌ ͳͺͲ ሺͳ െ ݔሻ ீ௔஺௦ ͳͺͲ ܶ ሺͳ െ ݔሻ ீ௔஺௦

Here t denotes the elapsed growth time while T is the total time for completing MMB layer. The resulting total thickness ܦெெ஻can then be expressed as

ଶ ݐ ൅ݐሻ כ ܤ ݐ ൅ כ ܣݐൌ݃ீ௔஺௦ሺכ ݐ൅݃ீ௔஺௦כ ெெ஻ ൌ݃ூ௡஺௦ܦ

଻ ௫ ଵ଻ଷ ଵ ௫ In this way we can also . כ כ ൌܤ and כ ൌܣ where ଵ଼଴ ሺଵି௫ሻ ଵ଼଴ ் ሺଵି௫ሻ estimate the GaAs growth rate from the known total thickness and growth time. For comparison, secondary ion mass spectrometry (SIMS) measurements were performed on one sample, as shown in Fig. 3.14, where the calculated In concentration profile has a good fit with the SIMS data.

a) b)

Figure 3.14 a) SIMS measurement data from an InAs(QD)/MMB InGaAs/GaAs sample showing In, Ga and Al concentrations as well as the calculated In fraction. b) Calculated In fraction compared to the SIMS profile.

The detailed shape of the QD has a profound effect on its optical properties, e.g. the exciton fine-structure splitting (FSS) as governed by strain and shape-induced wave function asymmetry [97]. An oval QD shape will lead to different confinement energies in the plane of the QD that leads to an FSS that affects the photon indistinguishability and inhibiting entanglement of emitted photon pairs of significance for their applications quantum information applications. To investigate the ability to control the FSS, QDs were grown at different temperatures. The results can be seen in Fig. 3.15, and an optimum temperature of 545 C is noted [98]. This suggests that QDs at this specific temperature are more

spherical in shape, but the detailed mechanism behind this supposed shape transformation is not known.

Figure 3.15 FSS measurements for QDs grown at a) 515°C, b) 530°C, c) 545°C and d) 560°C.

3.3 Part C: Photonic crystal surface-emitting lasers (PCSELs)

3.3.1 Si-integrated photonic crystal surface emitting lasers

In this section, we discuss design and fabrication aspects of a hybrid Si-InP PCSEL based on transfer printing and buried tunnel junction (BTJ) current injection [Paper VII]. As a basic design rule for PCSELs, the active layer needs to be placed in close proximity to the PC layer to allow for evanescent coupling of the gain [99]. For a top- contacted InP-Si-integrated PCSEL as depicted in Fig. 1.3.2 of Sec. 1.3.1, this puts a tight restriction on the thickness of lateral feeding layer from the bottom contact. In addition, because of the large-area aperture, the requirement of current spreading and uniformity is large on both the top and bottom sides. In contrast to short-wavelength (<1 μm) GaAs-based VCSELs that almost exclusively makes use of oxidation confinement [100], InP-based VCSELs for the longer wavelength (1.3-1.55 μm) regimes typically use a BTJ solution [101]. A major reason for this choice of technology is the lack of a suitable selective oxidation layer in the InP system [102] but the BTJ concept also have other advantages, such as an optical lithography-defined device aperture, improved current injection uniformity and almost exclusively n-type doping in the spreading layers, the latter leading to high lateral conductivity and low optical loss [103]. However, as compared to PCSELs, these small-area telecommunication VCSELs have a small aperture diameter (10 μm as compared to 100:s of μm) and have less restrictions on the layer thicknesses. For a large-area PCSEL, the requirement on an efficient current-injection mechanism becomes significant; a purely aperture-based solution would lead to a significant current crowding in the peripheral part of the device.

Figure 3.16 Figure showing TJ that consists of degenerately doped abrupt PN junction The electron and hole wave function overlap is such that tunneling can occur.

In the central part of the device, defining the aperture or active area, the reverse-biased TJ is highly conductive due to the energy overlap of conduction-band states on the InGaAs:n++-side with valance-band states on the InAlGaAs:p++-side that allows for direct electron tunneling see Fig. 3.16. However, the conductivity is still anisotropic and higher in the lateral than vertical direction, which provides the basis for a uniform injection. The lower bandgaps of the InGaAs and InAlGaAs layers as compared to InP are beneficial to the conductivity of the tunnel junction since they lower the offset between conduction and valance band states, but care must be taken so that they don’t absorb the fundamental laser light around 1.55 μm (equivalent to 0.80 eV). Since InGaAs that is lattice- matched to InP has a bandgap of 0.74 eV at room temperature [104] this would be expected to be highly absorptive and special measures have to be taken. On the p-side, alloying with Al (approximately 14% in the present case) to InAlGaAs increases the bandgap for transparency at 1.55 μm, while the n-type doping itself opens an effective bandgap due band filling and the Moss-Burstein effect [105]. The tunnel junction is removed by wet-chemical etching in the surrounding area after which the whole stack is regrown with n-type InP, hence a ‘buried’ tunnel junction. In this way, the surrounding region has a reverse-biased pn-junction which will block the current and funnel the light to the central region. The InGaAs and InAlGaAs layers were doped

with Si and C, both to 2‧1019 cm-3, and had thicknesses of 20 and 10 nm, respectively, to maintain a high tunneling probability [106]. The InGaAs and InAlGaAs layers were grown at different temperatures of 650 and 520°C and using optimized growth parameters, to allow high doping concentrations and a resulting mirror-like surface morphology. Significant calibration efforts were made to realize 1.55-μm- transparent InAlGaAs:p++ material, where carbon tetrabromide (CBr4) was used as a source for carbon doping. To complicate matters CBr4 removes In and the carbon doping incorporation depends on the input C/In flow ratio [107]. This altogether led to a lengthy layer optimization as illustrated in Table 3.1.

run TMIn_1 TMIn_2 TMGa_1 TMGa_2 CBr4 TMAl TBA_1 AsH3_1 Temp V/III n/p hall mobility 7186 168.3 168.3 37 250 50 510 p 6.00E+19 49 7189 168.3 168.3 10.37 500 50 510 27.71 p 2-7e15 0,7-7,5 7190 168.3 168.3 5.2 250 50 510 55.38 n 5.00E+11 70 7191 168.3 168.3 5.2 250 50 510 55.38 n 1.00E+16 574 7192 168.3 168.3 5.2 250 50 510 32.57 p 4.00E+16 99 7193 168.3 168.3 5.2 250 50 510 32.57 n 6.00E+16 1100 7194 168.3 168.3 45 200 25 470 3.16 p 1.00E+19 55 7236 168.3 168.3 51 200 25 470 2.86 contact problems 7237 168.3 168.3 45 200 25 470 3.16 p 2.00E+19 5 7238 168.3 168.3 45 200 11.3 25 520 3.03 p 1.00E+20 55 7239 168.3 168.3 11 200 11.3 25 520 7.08 contact problems 7256 168.3 168.3 8.9 250 11.3 17 520 3.45 n 4.00E+17 500 7257 168.3 168.3 8.4 250 10.8 9 520 1.86 n 5.00E+17 235 7258 168.3 168.3 1800mbar 10 200 10 7 520 1.9 contact problems 7260 168.3 168.3 8.9 125 11.3 17 520 3.45 n 1.50E+18 25-50 7278 168.3 168.3 7 500 15 7 520 1.42 p 9E+18 7279 168.3 168.3 7 500 22 7 520 1.19 p 8E+18 40 7280 168.3 168.3 1500mbar 10 500 30 7 520 1 p 1E+19 41 7841 105 168.3 1500 mbar 7.17 400 41 7 520 1.03 p 1.89E+19 32 Table 3.1 Summary of calibration runs for realizing the highly doped p- doped InAlGaAs:C layer

Paper VII goes on to characterize the BTJ and blocking layer structure on InP as well as the transfer-printed structure on Si PC. The BTJ structure shows even current spreading while the blocking layer properties are manipulated with the insertion of a undoped layer a scheme which extends its blocking capabilities. The Si-InP PCSEL emission spectra shows linewidth narrowing at low current densities.

3.3.2 Discrete photonic crystal surface emitting laser

For large-area high-power PCSELs it is desirable to have as efficient current-injection as possible, without any constraints from shallow feeding layers resulting from an intracavity double top-contacted approach as in the case of silicon-integrated PCSELs discussed in the previous section. Here, we investigate a discrete fully monolithic InP approach with an encapsulated array of InP/air holes as photonic crystal layer, similar to the GaAs-based PCSELs with multi-watt singlemode output power demonstrated by Noda and coworkers at the University of Kyoto [53]. The discrete PCSEL design investigated here is in principle a rather simple p-i-n MQW LED with light taken out from the backside of the wafer. However, the main difference regards the introduced PC air holes (or voids) in the intrinsic region that provides the feedback necessary for lasing oscillation [50]. Several fabrication aspects need to be investigated to achieve encapsulated voids, such as the stability of etched hole-shapes towards heating, and the detailed evolution of the void formation, e.g. to time the change in growth parameters for planar and doped cladding and contact layers. The PC lattice was designed with lattice constants of 474 and 449 nm, and hole radius of 96 nm. The larger lattice constant is tuned for room temperature emission at approximately 1507 nm while the smaller lattice constant is designed for 77 K-emission around 1410 nm, taking account of the temperature shift in gain of 0.54 nm/K and emission wavelength of 0.10 nm/K [Paper VIII]. The thickness of the PC layer (415 nm) is chosen to be sufficiently thick to allow for a PC grating depth of at least half a wavelength (238 nm) that corresponds to optimized radiation efficiency [108]. The topmost cladding and contact layers are tuned to avoid destructive interference to the standing-wave field from reflectance from the top metal contact [53]. In order to calibrate the etch-depth and epitaxial regrowth, a series of InP test samples were fabricated with SiO2 as hard mask and patterned with e-beam lithography. The pattern included circles with different radii of 80, 96 and 112 nm. The SiO2 hard mask was etched using reactive ion etching (RIE) and the InP was etched using inductively coupled plasma (ICP) with parameters taken from a previous student at the department using the same ICP setup [109], an Oxford Instrument ICP380 Etch

System using CH4/Cl2 chemistry. Table 3.2 provides the ICP etching parameters.

Table 3.2 ICP etching parameters

Cl2 9 sccm

CH4 7,5 sccm

H2 5,5 sccm Chamber 4 mbar pressure Strike pressure 10 mbar RF power 100 W ICP power 1000W Table temp 60°C

The optimized hole shapes can be seen in the SEM micrographs in Fig 3.17. They have mainly straight sidewalls with some tapering towards the bottom. However, the etched hole-shape showed some run-to-run variations with different degrees of under-etching in the upper part of the hole. The reason for this is unknown, but can be due to contamination from other processes in the tool although cleaning and reconditioning measures seemed to have little effect.

a) b)

Figure 3.17 a) ICP etched holes with a radius of 80 nm and a depth of 370 nm b) ICP

etch holes with a radius of 112 nm and a depth of 600 nm. The SiO2 hard-mask can be seen as a dark layer on-top of the InP.

Given the conformity and excellent step-coverage properties of CVD- processes in general, it may appear surprising that it is possible to encapsulate buried air cavities in the growing InP film. However, it is here important to consider the MOVPE growth process as well as the details of the surface topography. Figure 3.18 (a) schematically illustrates

the growth situation. The reactants and carrier gas comes in from the left, forming a laminar flow over the wafer surface. A stagnant layer (depletion layer) is thereby formed over the wafer with gradually decreasing gas velocity towards the surface. The local growth rate depends on the diffusion of reactants through the depletion layer as well as the reaction kinetics at the surface.

Figure 3.18 a) Schematic illustration of laminar gas input flow lines over the PC lattice. The thinning of the lines towards the surface indicates a reduced gas velocity as well as a depletion of the group III species; (b) A zoomed-in view over one PC lattice point suggesting only a minor disturbance of the flow close to the hole opening.

Figure 3.18 (b) is a zoomed-in view of an etched hole. It is clear that the gas-phase diffusion of reactant into the hole will be very limited. Growth may occur within the hole, but that will predominantly be due the diffusion of adatoms on the wafer surface. In this way some growth will occur at the hole edges as illustrated in Fig. 3.19 (a). A plane representing the slowest growth direction (such as {111}B or {110} depending on the hole shape; see Paper III) will thereby develop as indicated in Fig. 3.19 (b- d). Further growth will now proceed in the vertical and lateral direction, narrowing the hole opening with a growth front defined by the intersection of the (100) plane and the stop plane in question. Meanwhile, additional growth may also occur in the hole due to adatom diffusion from the top surface and over the stop plane. The adatom diffusion length is therefore a key parameter in determining the amount of growth within the hole. For this reason, an increased growth temperature, a reduced growth rate or decreased V/III ratio (increasing the surface diffusion of adatoms) will promote infilling and a resulting smaller cavity (Fig. 3.19 c-

d). This is also consistent with the typically observed hole shapes with a sharp angled termination in the growth direction

Figure 3.19 Schematic illustration of the regrowth evolution: (a) Onset of growth with rapidly decreasing gas-phase diffusion down the side walls; (b) The development of “stop planes” corresponding to the lowest growth rate, defining the regrown hole shape. The growth progression down the side walls will depend on the adatom diffusion down the side walls; (c) Closure of the void due to coalescence of the lateral growth over the holes; (d) A situation with increase adatom diffusion length leading to a slightly smaller void than in (c).

In Fig. 3.20 we see PL map of a 250x250 μm2 PCSEL device. The device is pumped below threshold and it is observed that there is an enhanced light emission towards the edges of the device. This is due to the finite size of the PC lattice where the symmetry is broken at the edges and more light is propagating towards the edge of the device then towards the center [111]. As a consequence, the out-of-plane Bragg diffraction from two waves propagating in opposite directions don’t cancel as they would in an infinite PC. This leads to more light emission but at the cost of a higher threshold around the edges.

Figure 3.20 PL map of a 250x250 μm2 device. PC symmetry is broken towards the sides of the PC which will results in lower Q factor but stronger light emission as out of plane diffraction is no longer cancelled out by light propagating with a different direction in the PC.

3.3.2.1 Electrically pumped devices

In parallel to the fabrication and characterization of the above discussed optically pumped buried InP/air-hole PCSELs, devices with contacts for electrical injection were also fabricated. These devices were very much inspired by the successful realization of the high-performance GaAs- based PCSELs described in Ref. 53 [53], but with ambitions for the longer-wavelength regime as enabled by the InP materials system. A schematic illustration of the device is shown in Fig. 3.21 (a) and processed device is photographed in Fig. 3.21 (b). From a device design viewpoint, these PCSELs are in all essential discrete light-emitting diodes, complemented with a buried PC layer. The fabrication sequence (Appendix 6.2) and the device functionality were first evaluated in corresponding LEDs, i.e. exactly the same design as the PCSELs but without the buried PC lattice. These tests yielded LED light- current-voltage characteristics as shown in Fig. 3.22.

b) a)

Figure 3.21. (a) Schematic illustration of an InP-based electrically pumped PCSEL. The emission is taken out through the substrate (top side in the figure) and the MQW-based active region is positioned close to the p-type contact, guaranteeing a uniform current injection profile. The epitaxial growth direction is from top to the bottom in the figure; (b) Photograph of a processed PCSEL chip [73].

a) b)

Figure 3.22. (a) LED voltage-current and (b) optical output power-current diagrams on a range of different devices from the same InP base structure [H].

For the PCSEL fabrication, the same epitaxial material as for the optically pumped PCSELs described above were used, but the regrowth for the PC definition was done separately. Unfortunately, only devices tuned for room-temperature operation survived the processing. While the electroluminescence was similar as for the LED devices, no lasing was observed at any temperature between 130 and 293 K, and continuous

electrical pumping up to 1 A in devices ranging between 50×50 and 250×250 μm2. The reason for this is unknown. It is possible that the epitaxial regrowth failed to form a buried PC lattice or that the resulting device design failed to promote low-threshold lasing due to inadequate hole formation. It is possible that pulsed excitation or a low-temperature design would have resulted in working lasers, but there are also other measures that can be taken to improve the odds for working PCSELs, including a more stable InGaAsP grating material, a more robust optical design or improved electrical and thermal management.

4 Summary, Conclusions and outlook

The thesis has investigated new design, materials and fabrication concepts for the development of new photonic device structures and integration schemes. Of specific relevance is the application of the MOVPE growth technology, which has been tuned for different materials systems and growth modes with tough requirements on the electrical, optical and morphological properties.

Specific results include:

The demonstration of three-terminal transistor-VCSELs with static performance on pair with conventional diode lasers in terms of threshold, output power, high-temperature operation and power efficiency.

The regrowth of device quality InGaAs/InP layer structures in narrow trenches, including the two-step regrowth-integration of implantation- free field-effect transistor structures with excellent morphology.

The structural and optical investigation of buried GaInSb/QD self- organized quantum-dot structures with type-II alignment for long- wavelength infrared imaging, including the direct visualization of confined states using scanning tunneling spectroscopy as well as the measurement of the Auger recombination coefficient using excitation power-dependent photoluminescence spectroscopy.

The realization of telecommunication-wavelength single-photon emitters on GaAs substrate using a strain-relaxed InxGa1-xAs metamorphic buffer scheme.

The demonstration of an n++-InGaAs/p++-InAlGaAs buried tunnel junction current injection double intracavity top-contacted active InP membrane for silicon-integrated PCSELs

An epitaxial regrowth scheme optimized for the formation of InP/air hole photonic crystal lattices in which the growth parameters can be set for complete infilling or for perfect encapsulation, and with lasing

oscillation from InP-based PCSELs based on such InP/air hole PCs demonstrated using optical pumping.

The future prospects of these developments vary with the application fields:

The overall usefulness of the transistor-laser concept as such is still to be proven. The development of a robust T-VCSEL device as demonstrated here could be useful test vehicle. An obvious next step would be to investigate the dynamic properties of these devices.

Dense photonics-electronics integration can be expected to become of increasing importance to meet tightened performance requirements imposed by a range of new or refined applications of increasing sophistication. Arrayed photonic device structures with flip-chip contacting of driver or read-out electronics may instead benefit from the direct integration of the electronic circuitry in pixel-separating trenches as described in Paper III.

The D2B LWIR detector concept, discussed in Paper IV and V, will not have any impact on the infrared imaging market in its present shape due to the large hole confinement energy. The application of a “virtual” InAsSb substrate, yielding a lower hole barrier might be helpful in this context [112] [113], but one may expect some challenges related to matrix material homogeneity and crystallinity. On the other hand, the application of these GaInSb/InAs QD may also see other applications, e.g. in mid-infrared emitters [114].

The fabrication of on-demand, high repetition rate single-photon emitters as discussed in Paper VI can be expected to have significance for the development of an enabling technology with large- scale manufacturing capability for future telecommunication network quantum communication. However, many steps in this development are still missing. The next step should consider a better optimized mirror structure or cavity confinement for improved repetition rate, the potential for direct integration with CMOS-based photonic integrated circuitry, e.g. using transfer-print technology as discussed in Paper VII [115], and the option for electrically driven emitters.

The demonstrated BTJ-confined active InP-MQW membrane structure is an important building block for the development of a hybrid InP-Si photonics integration. Further efforts should be devoted to the realization of electrically pumped high-power-efficiency low-threshold PCSELs, or other InP/Si membrane based devices, such as double- PC reflector VCSELs, so-called MR-VCSELs [116], including horizontal emission into lateral waveguides [117].

Compact, high-power, singlemode PCSELs have ample of potential applications as discussed in Sec. 1.3.2. The extension of this technology to the 1.55-μm telecommunication window as discussed in Paper VIII opens up for additional important applications (see e.g. Refs. [118], [119], [120]). This will require an electrically pumped device that is optimized for high-power emission. This work was already initiated, including the demonstration of corresponding LEDs with similar device layout and fabrication process as electrically pumped PCSEL except for the PC layer, as well as preliminary experiments with electrically pumped PCSELs [121]. However, considering the modest temperature stability of etched hole shapes in InP, a different grating material, such as lattice-matched InGaAsP/air holes, should be considered [122]. Since this also would rely on as InP over-layer it is likely that the optimized regrowth parameters reported in Paper VIII would apply also in this case

5 References

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6 Appendix

6.1 TJ LED process list

Step Parameters Comment/tips

1 Epitaxy

2 SiO2 mask deposition

2.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

2.2 Pekka Si OX 90s 315°C 100nm

3 Define Alignment marks

3.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

3.2 Pre bake: APL-HMDS oven Bakes 150C in low pressure

3.3 Negativ Resist spin APL spinner Ma-N 1420, 3000 rpm, 30’’(2um) Bake Hotplate 100C, 120s’

3.4 Emma: Vacuum contact, 550 Mask: Alignment mJ/cm2 (29,5”) marks

3.5 Develop: Ma-D 533/s_65 , 70s Rinse DI-H2O, 1’ more time for more underetch

3.6 Ashing Ariel/ESA, QWIP ASHING CLEAN 5’

3.7 Etch SiO2 Ariel/ESA P129 SIO etch 19nm/m

3.8 Remove resist Aceton/remover MR rem 700 60°C

3.9 Etch 10 nm InP HCL:H3PO4 1:20 15s

3.10 Etch 30 nm InGaAs H3PO4:H2O2:H2O 1:1:8 7 s (0,4 um/min)

3.11 Etch 200 nm InP HCL:H3PO4 1:20 60-80s

3.12 Remove SiO2 BHF 6:1 30+ s

3.13 Clean:Ash Ariel/ESA, QWIP Rinse DI-H2O, 1’ ASHING CLEAN 5’

4 SiO2 mask deposition

4.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

4.2 Pekka Si OX 90s 315°C 100 nm

5 Define TJ Mesa

5.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

5.2 Pre bake: APL-HMDS oven Bakes 150°C in low pressure (Default)

5.3 Resist spin APL spinner AZT S1818 4000 RPM 30s

Hotplate 100°C 90 s

5.4 Emma: Vacuum, 92 mJ/cm2 Mask: Mesa define

5.5 Develop: CD-26, 75 s Rinse DI-H2O, 1’

5.6 Post develop bake 110°C 2 min

5.7 ESA/Ariel Ariel/ESA, QWIP ASHING CLEAN 5’

5.8 Etch SiO2 BHF 6:1 19 s

5.9 Remove resist Aceton/remover MR rem 700 40-60°C

5.10 Etch 10 nm InP HCL:H3PO4 1:20 15 s

5.11 Etch 30 nm InGaAs H3PO4:H2O2:H2O 1:1:8 7 s 0,4 um/min

5.12 Remove SiO2 BHF 6:1 30+ s

5.13 Clean: Ariel/ESA, QWIP Rinse DI-H2O, 1’ ASHING CLEAN 5’

6 Epitaxy Regrowth

6.1 Clean Aceton IPA SEMICO 30s each Sulfuric SEMICO Aceton IPA

6.2 Epi Regrowth

7 Define ring contacts

7.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

7.2 Pre bake: HMDS oven

7.3 Resist spin arnold Ma-N 1420 3000 rpm 30’’

2μm hotplate 100 °C 120’’

7.4 Emma: Vacc contact, 550 mJ/cm2 (27,5”)

7.5 Develop: Ma-D 533s Rinse DI-H2O, 1’

65’’ 0.6 μm undercut

80’’0.8 μm undercut

120’’ 2.1 μm undercut

7.6 Flood exposure 1000 mJ/cm2 (50’’)

7.7 ESA/Ariel (optional) Ariel/ESA, QWIP May worsen undercut ASHING CLEAN 5’ and destroy lift off

8 Deposit metal

8.1 Citric acid, 30” Rinse DI-H2O 60” after Right before evaporation

8.2 Barbara: AuGe/Ni/Ti/Pt/Au Alternativt:Ni/Ge/Au/Ti/ Au 80/100/50/50/110

12/20/30/10/200 nm

(Pd/Ge/Ti/Pd/Ti)

AuGe/Ni/Ti/Pt/Au

RW_TiPtAu

Ti/Pt/Au 30/30/150 nm

8.2 Lift off: Acetone until metal falls off, then NMP/Remover 1165 5’, then DI-H2O 5’

8.3 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

9 Etch down to bottom contact

9.1 Citric acid, 30” Rinse DI-H2O 60” after

9.2 pekka: SiOx valfri 315°C 90’’ per 100nm

9.3 Pre bake: HMDS oven

9.4 Resist spin arnold Positive resist

9.5 Emma: Vacc contact,

9.6 Develop:

9.7 Flood exposure 1000 mJ/cm2 (50’’) optional

9.8 ESA/Ariel Ariel/ESA, QWIP ASHING CLEAN 5’

9.9 SiO2 etch Ariel P129 SiO2 etch 18nm/s

9.10 remove resist Hot aceton/MR REM 700

9.11 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

9.12 Etch down to bottom Wet etch InGaAs InGaAs/InGaAsP H3PO4:H2O2:H2O 1:1:8

InP HCL:H3PO4 1:20

9.13 SiO2 etch Ariel P129 SiO2 etch 18nm/s

10 Define bottom contacts and Deposit bottom contact

10.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

10.2 Pre bake: HMDS oven

10.3 Resist spin arnold Ma-N 1420 3000 rpm 30’’

2μm hotplate 100 °C 120’’

10.4 Emma: Vacc contact, 550 mJ/cm2 (27,5”)

10.5 Develop: Ma-D 533s Rinse DI-H2O, 1’

65’’ 0.6 μm undercut

80’’0.8 μm undercut

120’’ 2.1 μm undercut

10.6 Flood exposure 1000 mJ/cm2 (50’’)

10.7 ESA/Ariel (optional) Ariel/ESA, QWIP May worsen undercut ASHING CLEAN 5’ and destroy lift off

10.8 Citric acid, 30” Rinse DI-H2O 60” after Right before evaporation

10.9 Barbara: AuGe/Ni/Ti/Pt/Au Alternativt:Ni/Ge/Au/Ti/ Au 80/100/50/50/110

12/20/30/10/200 nm

(Pd/Ge/Ti/Pd/Ti)

AuGe/Ni/Ti/Pt/Au

RW_TiPtAu

Ti/Pt/Au 30/30/150 nm

10.10 Lift off: Acetone until metal falls off, then NMP/Remover 1165 5’, then DI-H2O 5’

10.11 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

11 SiO2 deposition and openings in SiO2

11.1 Citric acid, 30” Rinse DI-H2O 60” after

11.2 pekka: SiOx valfri 315°C 90’’ per 100nm

11.3 Pre bake: HMDS oven

11.4 Resist spin arnold Ma-N 1420 3000 rpm 30’’

2μm hotplate 100 °C 120’’

11.5 Emma: Vacc contact, 550 mJ/cm2 (27,5”)

11.6 Develop: Ma-D 533s Rinse DI-H2O, 1’

65’’ 0.6 μm undercut

80’’0.8 μm undercut

120’’ 2.1 μm undercut

11.7 Flood exposure 1000 mJ/cm2 (50’’)

11.8 ESA/Ariel Ariel/ESA, QWIP ASHING CLEAN 5’

11.9 SiO2 etch BHF 6:1 19’’ per 100nm

12 Adding contact pads

12.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

12.2 Pre bake: HMDS oven

12.3 Resist spin arnold Ma-N 1420 3000 rpm 30’’

2μm hotplate 100 °C 120’’

12.4 Emma: Vacc contact, 550 mJ/cm2 (27,5”)

12.5 Develop: Ma-D 533s Rinse DI-H2O, 1’

65’’ 0.6 μm undercut

80’’0.8 μm undercut

120’’ 2.1 μm undercut

12.6 Flood exposure 1000 mJ/cm2 (50’’)

12.7 ESA/Ariel (optional) Ariel/ESA, QWIP May worsen undercut ASHING CLEAN 5’ and destroy lift off

12.8 Citric acid, 30” Rinse DI-H2O 60” after Right before evaporation

12.9 Barbara: Ti/Pt/Au Alternativt: Ti/Au

50/50/400 10/200 nm RW_TiPtAu

Ti/Pt/Au 30/30/150 nm

12.10 Lift off: Acetone until metal falls off, then NMP/Remover 1165 5’, then DI-H2O 5’

12.11 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

12.12 Clean: Citric acid, 30” Rinse DI-H2O 60” after

13 Backside metall

13.1 Resist spin to protect arnold Ma-N 1420 3000 rpm frontside 30’’

2μm hotplate 100 °C 120’’

13.2 ESA/Ariel Ariel/ESA, QWIP Backside up ASHING CLEAN 5’

13.3 Clean: Citric acid, 30” Rinse DI-H2O 60” after

13.4 Barbara: AuGe/Ni/Ti/Pt/Au Alternativt: Ti/Au

80/100/50/50/110 10/200 nm RW_TiPtAu

Ti/Pt/Au 30/30/150 nm

13.3 Clean: Hot aceton or mr rem Rinse DI-H2O 60” after 700

14 open light output

14.1 Pre bake: HMDS oven

14.2 Resist spin arnold Ma-N 1420 3000 rpm 30’’

2μm hotplate 100 °C 120’’

14.3 Emma: Vac contact, 550 mJ/cm2 (27,5”)

14.4 Develop: Ma-D 533s Rinse DI-H2O, 1’

65’’ 0.6 μm undercut

80’’0.8 μm undercut

120’’ 2.1 μm undercut

14.5 Flood exposure 1000 mJ/cm2 (50’’)

14.6 ESA/Ariel Ariel/ESA, QWIP ASHING CLEAN 5’

14.7 SiO2 etch BHF 6:1 19’’ per 100nm

14.8 Clean: Hot aceton or mr rem Rinse DI-H2O 60” after 700

6.2 Discrete PCSEL process list

Step Parameters Comment/tips

1 Epitaxy

2 SiO2 mask deposition

2.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

2.2 Pekka Si OX 90s 315°C 100nm

3 Define Alignment marks

3.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

3.2 Pre bake: APL-HMDS oven Bakes 150C in low pressure

3.3 Negativ Resist spin APL spinner Ma-N 1420, 3000 rpm, 30’’(2um) Bake Hotplate 100C, 120s’

3.4 Emma: Vacuum contact, 550 Mask: Alignment mJ/cm2 (29,5”) marks

3.5 Develop: Ma-D 533/s_65 , 70s Rinse DI-H2O, 1’ more time for more underetch

3.6 Ashing Ariel/ESA, QWIP ASHING CLEAN 5’

3.7 Etch SiO2 Ariel/ESA P129 SIO etch 19nm/m

3.8 Remove resist Aceton/remover MR rem 700 60°C

3.9 Etch 200-400 nm InP HCL:H3PO4 1:20

4 E-beam resist and exposure

5 PC etch

5.1 Etch SiO2 Ariel/ESA P129 SIO 20% over-etch etch 19nm/m

5.2 Ashing remove Ariel/ESA, QWIP ebeam resist ASHING CLEAN 5’

5.3 Etch InP Gallus Inp etch

5.4 SiO2 etch BHF 6:1 19’’ per 100nm

5.5 Clean: Hot aceton or mr rem Rinse DI-H2O 60” after 700

6 Regrowth MOVPE

7 Isolation p-side

7.1 Citric acid, 30” Rinse DI-H2O 60” after

7.2 pekka: SiOx valfri 315°C 90’’ per 100nm

7.3 Pre bake: HMDS oven

7.4 Resist spin arnold Positive resist

7.5 Emma: Vacc contact, Backside alignment

7.6 Develop:

7.7 Flood exposure 1000 mJ/cm2 (50’’) optional

7.8 ESA/Ariel Ariel/ESA, QWIP ASHING CLEAN 5’

7.9 SiO2 etch Ariel P129 SiO2 etch 18nm/s

7.10 remove resist Hot aceton/MR REM 700

7.11 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

8 Contact p-side

8.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

8.2 Pre bake: HMDS oven

8.3 Resist spin arnold Ma-N 1420 3000 rpm 30’’

2μm hotplate 100 °C 120’’

8.4 Emma: Vacc contact, 550 Backside alignment mJ/cm2 (27,5”)

8.5 Develop: Ma-D 533s Rinse DI-H2O, 1’

65’’ 0.6 μm undercut

80’’0.8 μm undercut

120’’ 2.1 μm undercut

8.6 Flood exposure 1000 mJ/cm2 (50’’)

8.7 ESA/Ariel (optional) Ariel/ESA, QWIP May worsen undercut ASHING CLEAN 5’ and destroy lift off

8.8 Citric acid, 30” Rinse DI-H2O 60” after Right before evaporation

8.9 Barbara: Ti/Pt/Au Alternativt: Ti/Au

50/50/400 10/200 nm RW_TiPtAu

Ti/Pt/Au 30/30/150 nm

8.10 Lift off: Acetone until metal falls off, then NMP/Remover 1165 5’, then DI-H2O 5’

8.11 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

8.12 Clean: Citric acid, 30” Rinse DI-H2O 60” after

9 via n side

9.1 Citric acid, 30” Rinse DI-H2O 60” after

9.2 pekka: SiOx valfri 315°C 90’’ per 100nm

9.3 Pre bake: HMDS oven

9.4 Resist spin arnold Positive resist

9.5 Emma: Vacc contact,

9.6 Develop:

9.7 Flood exposure 1000 mJ/cm2 (50’’) optional

9.8 ESA/Ariel Ariel/ESA, QWIP ASHING CLEAN 5’

9.9 SiO2 etch Ariel P129 SiO2 etch 18nm/s

9.10 remove resist Hot aceton/MR REM 700

9.11 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

9 contact n side

9.1 Clean: Citric acid, 30” Rinse DI-H2O 60” after

9.2 Pre bake: HMDS oven

9.3 Resist spin arnold Ma-N 1420 3000 rpm 30’’

2μm hotplate 100 °C 120’’

9.4 Emma: Vacc contact, 550 mJ/cm2 (27,5”)

9.5 Develop: Ma-D 533s Rinse DI-H2O, 1’

65’’ 0.6 μm undercut

80’’0.8 μm undercut

120’’ 2.1 μm undercut

9.6 ESA/Ariel (optional) Ariel/ESA, QWIP May worsen undercut ASHING CLEAN 5’ and destroy lift off

9.7 Citric acid, 30” Rinse DI-H2O 60” after Right before evaporation

9.8 Barbara: Ti/Pt/Au Alternativt: Ti/Au

50/50/400 10/200 nm RW_TiPtAu

Ti/Pt/Au 30/30/150 nm

9.9 Lift off: Acetone until metal falls off, then NMP/Remover 1165 5’, then DI-H2O 5’

9.10 Clean: Ariel/ESA, QWIP ASHING CLEAN 5’

7 Guide to papers

I. AlGaAs/GaAs/InGaAs pnp-type vertical-cavity surface- emitting transistor-lasers

Transistor VCSELs are fabricated with design variations in base layer thickness and current injection configuration, and the corresponding device characteristics reveals a complex interdependency between biasing conditions, current distribution and optical output. This can all be accounted for in a phenomenological manner and provide important inputs to the design optimization. In continuous wave operation, the devices show comparable characteristics as conventional diode-type VCSELs in terms of threshold, output power and high- temperature performance.

Contribution: Epitaxy of base structure and regrowth, discussions of analysis, minor inputs to the manuscript

II. Epitaxial growth, fabrication and analysis of vertical-cavity surface-emitting transistor lasers.

This paper briefly discusses some MOVPE-specific features related to the development of the T-VCSELs presented in Paper I. This includes the QW integrity towards the high base doping, active region and tunnel-junction optimization, design- considerations for minimized optical loss, and growth optimization for controlled regrowth morphology.

Contribution: Epitaxy of base structure and regrowth, discussions of results and analysis, coauthoring of the manuscript

III. Trench-confined InP-based epitaxial regrowth using metal- organic vapor-phase epitaxy.

The epitaxial regrowth of electronic circuit elements in narrow trenches separating pixels in arrayed spatial light modulators (SLMs) is investigated. It is found that the orientation of the SLM array against the underlying InP crystallographic directions has a profound influence on the regrowth morphology, and this could be qualitatively accounted for considering the postulated equilibrium regrowth shape for different rectangular mesa orientations. Using optimized conditions, a two-regrowth-step high-electron mobility transistor structure with good control of the layer thicknesses, doping variations and overall morphology is demonstrated.

Contribution: Design of experiments, epitaxial growth, materials characterization, main part of analysis, main author of the manuscript

IV. Photoluminescence and photoresponse from InSb/InAs quantum-dot structures.

So-called dot-to-bulk (D2B) long-wavelength infrared photodetectors are fabricated using MOVPE in the type-II InGaSb QD/InAs system and characterized by PL spectroscopy, photo-current measurements and STM. Photoresponse is demonstrated beyond 8 μm, indication that the D2B technology may have potential for imaging applications in the LWIR regime. The multi-layer active region is imaged using cross-sectional STM, revealing QD-like structures.

Contribution: Cross-sectional STM experiments and analysis

V. Auger recombination in In(Ga)Sb/InAs quantum dots.

Excitation power-dependent PL and scanning tunneling spectroscopy (STS) is applied to the D2B InGaSb QD/InAs structures examined in paper IV to extract information on the Auger recombination coefficient and detailed QD properties. The Auger recombination coefficient is shown to be significantly (factor 10) longer than for corresponding InGaSb superlattice detectors, which corresponds well to the delocalized nature of the conduction electrons, while the STS results gives some indications regarding the QD sizes and well reflects the type-II band lineup in InGaSb QD/InAs system.

Contribution: Cross-sectional STM experiments and analysis, discussions of overall results, coauthoring of the manuscript

VI. A stable wavelength-tunable triggered source of single photons and cascaded photon pairs at the telecom C-band.

Low-temperature single-photon emission at the 1.55-μm telecommunication wavelength is realized using InAs QD /InGaAs/GaAs structures based on a metamorphic buffer (MMB) layer-assisted growth technique. By gradually increasing the In content in the micrometer-thick MMB layer the strain situation is changed as compared to the standard coherent InAs QD/InGaAs/GaAs system, which is sufficient to extend the wavelength from up to 1.3 to 1.55 μm. A predominant single- photon emission is confirmed using auto-correlation measurements and it is also demonstrated that the emission wavelength can be tuned by controlling the strain state using a piezoelectric substrate.

Contribution: Epitaxial growth optimization and characterization of metamorphic buffer layer and quantum dots

VII. Buried tunnel junction current injection for InP-based nanomembrane photonic crystal surface emitting lasers on Silicon.

The MOVPE growth and optimization of a buried tunnel-junction current-injection configuration for ultrathin InP/Si-hybrid nanomembrane PCSELs are investigated. Using a reverse-biased n++-InGaAs/p++-InAlGaAs tunnel junction diode surrounded by an n-InP/p-InP blocking diode, the current injected from laterally displaced anode and cathode contacts are funneled through the central region of the device. This is demonstrated for large-area LEDs as well as processed InP/SOI PCSEL structures, showing good lateral injection uniformity as well as low series resistance and forward voltage.

Contribution: Epitaxy of base layer and regrowth, design and optimization of tunnel junction, fabrication and characterization of test BTJ-LED test structures, parts of the analysis, lead author of the manuscript

VIII. Buried InP/Air hole Photonic Crystal Surface Emitting Lasers.

A monolithic InP-based PCSEL is designed, fabricated and characterized using optical pumping. Special emphasis is paid to the formation of the buried InP/airhole photonic crystal layer. It is thereby demonstrated that the MOVPE regrowth parameters can be set for perfect encapsulation or complete infilling. It is also shown that the InP with etched holes reshapes significantly under heating, which may make it unsuitable as grating material. Low- threshold lasing is demonstrated using optical pumping.

Contribution: Parts of the device design, epitaxial growth, structural optimization of photonic crystal layer, device fabrication and characterization, lead author of the manuscript