Ge-On-Si LASER for Silicon Photonics
Total Page:16
File Type:pdf, Size:1020Kb
Ge-on-Si LASER for Silicon Photonics by Rodolfo E. Camacho-Aguilera B.S./ M.S. Materials Science & Engineering Georgia Institute of Technology, 2008 Submitted to the Department of Materials Science and Engineering in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Materials Science and Engineering at the Massachusetts Institute of Technology June 2013 ©2013 Massachusetts Institute of Technology. All rights reserved. Signature of Author: Department of Materials Science and Engineering April 23rd, 2013 Certified by: Lionel C. Kimerling Thomas Lord Professor of Materials Science and Engineering Thesis Supervisor Accepted by: Gerbrand Ceder Professor of Materials Science and Engineering Chair, Departmental Committee on Graduate Students “Do you want to get lucky or do you want to understand it?” -Lionel C. Kimerling “Germanium paved the way to Silicon microelectronics…now Silicon is paving the way to Germanium optoelectronics” II Ge-on-Si LASER for Silicon Photonics By Rodolfo E. Camacho-Aguilera Abstract Ge-on-Si devices are explored for photonic integration. Importance of Ge in photonics has grown and through techniques developed in our group we demonstrated low density of dislocations (<1x109cm-2) and point defects Ge growth for photonic devices. The focus of this document will be exclusively on Ge light emitters. Ge is an indirect band gap material that has shown the ability to act like a pseudo direct band gap material. Through the use of tensile strain and heavy doping, Ge exhibits properties thought exclusive of direct band gap materials. Dependence on temperature suggests strong interaction between indirect bands, and L, and the direct band gap at . The behavior is justified through increase in photoluminescenceΔ on Ge. The range of efficientΓ emission is to 120 with the first band interaction, and above 400 on the second band interaction. ℃ Low defect concentration in Ge is achieved℃ through chemical vapor deposition at high vacuum (~1x10-8 mbar) in a two-step process. The high temperature growth and low concentration of particles permits epitaxial growth with low defect concentration. Chemical selectivity forbids Ge growth on oxide. Oxide trenches permit the growth on Si for a variety of shapes, without detrimentally affecting the strain of the Ge devices. Dopant concentration above intrinsic growth concentration, ~1x1019cm-3 phosphorus, have been achieved through a series of methods non-CMOS, spin-on dopant; and CMOS, implantation and delta doping. All the techniques explored use enhanced dopant diffusion III observed in Ge under heavy n-type doping. A dopant source, or well, is used to distribute the dopants in the Ge without increasing the defect concentration. The approach lead to the development of electrically injected devices, LEDs and LDs. Ge pnn double heterostructure diodes were made under low, ~1x1018cm-3, and heavy n-type doping, >1x1019cm-3. Both devices showed improved performance compared to pin Ge LED. Furthermore, heavy doped Ge diodes exhibit evidence of bleaching or transparency. The techniques described permitted the development of Ge-on-Si laser with a concentration ~1-2x1019cm-3. It is the first demonstration of a Ge laser optically pumped working under the direct band gap assumption like other semiconductors. It represents the evidence of carrier inversion on an indirect band gap semiconductor. With 50cm-1 gain, the material shows Fabry-Perot cavity behavior. Finally, we demonstrated a fully functioning laser diode monolithically integrated on Si. Ge pnn lasers were made exhibiting a gain >1000cm-1 and exhibiting a spectrum range of over 200nm, making Ge the ideal candidate for Si photonics. IV Acknowledgements This work was supported by the Fully Laser Integrated Photonics (FLIP) program under APIC Corporation, and sponsored by the Naval Air Warfare Center - Aircraft Division (NAWC-AD) under OTA N00421-03-9-002. And the MURI Project for monolithically integrated laser devices supervised by Dr. Gernoke. I was supported by a NSF Graduate Research Fellowship award number 1122374, and the NDSEG fellowship. Both gave me the freedom to choose the research that I found most interesting. Travelling nurtures the mind but the people that are around us nurture our soul. I have been lucky to be in a position where I have been blessed with both sides. I moved from Mexico City to Atlanta, Georgia to study my undergrad career, and 9 years later I am concluding a path in the Massachusetts Institute of Technology. I would like to tell my old self that everything would be fine, and the hard work and any other difficulty is just a door blocking the next big steps, the next big trips, the next big discoveries. More importantly, I would like to tell myself that there are always people that make all the experiences easier and more valuable. I cannot hold in simple words the gratitude I have for all the people that have been on my life that lead me to this point, all these people that made my life more colorful and full of stories. My parents, who have been an inspiration, have supported me through all the difficult point of my academic career and advised me on life. My brothers have also let me know what the meaning is of showing by example, and letting me know that there is always more things to do, and other ways of seeing the same problems. My first advisor, Jud Ready, in Georgia Tech, deserves to be mentioned. He was the first person in academic to believe in me, and who made me an addict to research and discovery. I thank you for all the amazing experiences you let me search, and the doors that working with you has opened. My advisor, Lionel Kimerling, taught me how to rationalize all the problems, to break them down, and to never, NEVER, give up. He is somebody I aspire to be. His ideas aggregated over years of experience permit to have an intuition that allowed my work to happen. I will never V forget the first interview,: “I have three projects, but one of them is extremely difficult and small chance of success”. Thank you for letting me take that small chance. Dr. Jurgen Michel helped with direction of the project and advise in what direction to take in the project, as understanding the behaviors observe in Ge devices electrically and optically. Prof. Jifeng Liu and Dr. Xioachen Sun deserve special thanks, since they developed most of the initial theory on Ge, without you this work would not be possible. Your ingenuity and creativity gave me the foundation and tools to develop this work. Dr. Jonathan Bessette gave me all the knowledge require for characterizing electrically my devices, understand optical behavior, and gave me his friendship to support all the rest. Dr. Marco Romagnoli for his advice to characterize the devices under heavy current, and to successfully achieved a Ge laser. Yan Cai deserves her own special place of honor. She developed hand by hand most of the device characterization, theoretical work and device fabrication for this work. She is a great researcher, a magnificent engineer and good friend. My labmates, who made every day interesting, supported through the characterization and fabrication of the devices. Zhaohong Han helped through the understanding of the Ge bands effects, and even though the time has been short, the conversations progressed to several ideas that will resonate in this work. Neil Patel and Corentin Monmeyran helped to keep my sanity and through interesting conversations to explored new realms of Ge devices. Tim Milakovich helped on the TEM characterization of devices and conversations for the future work on Ge emitters. All the technicians in MTL MIT gave me the foundations to develop my devices, and to keep digging for new techniques and new ideas in old tools. You showed me what is being a thinkerer. Finally I want to thank my friends how are like my family; and Monika Avello. You have made this trip I called grad-school livable and enjoyable. You have been my support and my teachers. Every time I fall, I know I can count on them. I wish only I have been able to offer you as much help as you have offer me over the years. VI Table of Contents Chapter 1. Introduction ........................................................................................................................... 12 1.1 Ge light emitters ............................................................................................................................. 15 1.2 Thesis goal and outline .................................................................................................................. 17 Chapter 2. Background and Theory ...................................................................................................... 19 2.1 Ge Band Structure .......................................................................................................................... 20 2.1.1 Ge Band Engineering .............................................................................................................. 21 2.1.2 Ge n-type dopants ................................................................................................................... 24 2.1.3 Ge heavy doping ..................................................................................................................... 25 2.2 Gain and Light emission on Ge .................................................................................................... 28 2.2.1 Radiative Transitions .............................................................................................................