Lead Chalcogenide Thin Film Materials and Processing for Infrared Optical Devices By Peter Su B.S.E., Materials Science and Engineering University of Michigan, Ann Arbor, 2015 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 May 2020 ©2020 Massachusetts Institute of Technology. All rights reserved Signature of Author:_____________________________________________________________ Department of Materials Science and Engineering May 08, 2020 Certified by:___________________________________________________________________ Lionel C. Kimerling Thomas Lord Professor of Materials Science and Engineering Thesis Supervisor Certified by:___________________________________________________________________ Anuradha M. Agarwal Principal Research Scientist, Materials Research Laboratory Thesis Supervisor Accepted by:___________________________________________________________________ Frances M. Ross Professor of Materials Science and Engineering Chair, Departmental Committee on Graduate Studies This page intentionally left blank 2 Lead Chalcogenide Thin Film Materials and Processing for Infrared Photonic Devices By Peter Su Submitted to the Department of Materials Science and Engineering on May 8, 2020 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Materials Science and Engineering Abstract Lead chalcogenides are compounds of lead and a combination of the chalcogens sulfur, selenium, and tellurium. They are semiconductors with band gap energies in the infrared region of the electromagnetic spectrum, making them interesting infrared detector materials for both free space and photonic integrated circuit applications. For longer wavelengths of infrared light, they are transparent dielectric materials with extremely large refractive indices, making them potentially interesting materials for free space dielectric metasurfaces. In this work, we study both the processing, properties, and performance of lead chalcogenide materials as thin films and their integration and use in specific infrared photonic devices. For materials development, we examine how we can use and improve lead chalcogenide thin films for infrared detector applications, both through tuning their band gaps via alloying of the binary lead chalcogenide compounds (PbS, PbSe, PbTe), where we show photoconductivity of ternary alloys greater than an order of magnitude above their noise level; and through controlling their 3 majority carrier type and concentration via the addition of PbO to the thin film deposition source material, which can then be utilized to deposit detectors with 2-3 times higher responsivity. For device applications, we integrated PbTe detectors into photonic integrated circuit methane sensors and developed a new patterning method for PbTe thin films for use as transmissive dielectric metasurfaces. Integrating PbTe detectors into the photonic integrated circuit improved the methane sensing performance by 2.5 times, while a ratiometric architecture we designed and tested with an electronic read out circuit tuned to the properties of the integrated PbTe detector promise both the elimination of input laser power fluctuations as a source of spurious signals and 5 times better sensor performance with our testing equipment. Modulating the laser input at even higher frequencies promises even better performance, with an anticipated limit of detection of 0.11 vol% of methane/√퐻푧. For patterning transmissive dielectric metasurfaces, a new fabrication method using nanostencils was designed and tested, creating PbTe metalenses with diffraction limited performance and efficiencies of 40+%. Thesis Supervisors: Lionel C. Kimerling Title: Thomas Lord Professor of Materials Science and Engineering Anuradha M. Agarwal Title: Principal Research Scientist, Materials Research Laboratory 4 Acknowledgements My journey at MIT has been an absolutely amazing one, filled with amazing people doing amazing things, for each other, the broader MIT community, and the world. There are so many people to thank and I will try my best to thank them all without forgetting anybody. First, I must thank my advisors Professor Lionel C. Kimerling and Dr. Anu Agarwal (better known as Kim and Anu) for guiding me through this journey. When I was first looking for a research group to join, I could not help but notice that the Electronic Materials (EMAT) group felt so collaborative and scientifically rigorous. Kim and Anu are the embodiment of that, always generous with their time and always pushing me to go further and extract more data and knowledge from the films and devices. They also embodied another important aspect of EMAT: the support and encouragement of everyone as a whole person and not just a scientist/researcher/grad student. They gave me the freedom and flexibility to explore my other interests through the Graduate Student Council and Science Policy Initiative, and they supported and encouraged me when I made the decision to take a non-traditional path out of my PhD and pursue science policy. Next, I want to thank my other committee members, Professors Juejun Hu and Rafael Jaramillo (better known as JJ and Raf). Both JJ and Raf provided perspectives that I did not hear in my day- to-day research, providing me with new ideas to pursue and questions to answer. Their probing questions were instrumental in pushing my research to the next level. Next, I want to thank all the wonderful members of EMAT and our sister group, Professor JJ Hu’s Photonic Materials (PMAT) group. I could not have asked for a better group of fellow scientists to spend 5 years with. You have all been so generous with your time and effort, always jumping to help me and everyone else in the group solve any issues that we were encountering. I am so glad 5 that I found a group where the group culture is so supportive and collaborative. I need to give a special shoutout to Professor Kazumi Wada. Kazumi has provided so many insights into so many different parts of my work and has asked me so many important and piercing questions that I could probably call him my third thesis advisor. I also have to give a shoutout to all of the older students in the groups for helping teach me so much of what I know. In particular, I want to thank Dr. Zhaohong Han for being my initial tour guide through photonics, nanofabrication, and the lead chalcogenide materials. Without his willingness to answer all of my questions and spend countless hours in the lab watching over me as I practiced fabrication and characterization techniques, I would never have been able to produce the devices that I have been able to make. I also want to thank Cory James and Lisa Sinclair, the two main administrative assistants for the groups during my time at MIT. You have both been a ton of fun to work with and have greatly enabled the smooth functioning of the groups. Next, I want to thank all of my amazing collaborators, who all brought important perspectives, knowledge, and experience to my work. Dr. Piotr Becla has been helping me out the longest out of this group. Piotr’s been working with EMAT for quite a while now, far enough back that I do not know when that collaboration began. He has always been amazingly generous with his time, coming to MIT to help with measurements and designing setups even though he has a full-time research job that is not at MIT. His knowledge and experience greatly informed my work on infrared detectors. When Professor Sabina Merlo from Università di Pavia joined us at MIT, she jumped right into our experimental work on characterizing our lead chalcogenide detectors. Her experience and knowledge were instrumental in getting our monochromator up and running so that we could measure the photoresponse of our lead chalcogenides. Discussions with her were also extremely helpful in interpreting the data we were getting back. Dr. Emanuele Guglielmi from 6 Professor Marco Sampietro’s group at Politecnico di Milano made our work on studying the noise and other detector characteristics of our integrated PbTe detectors possible. He not only designed the electronic readout circuit that we used to get signals from our detectors, but he also measured the noise characteristics which informed our design choices for both the photonic chip and readout circuit. I cannot thank him enough for spending all that time teaching me so much about circuits and electrical characteristics of detectors. Francesco Zanetto recently visited MIT as Emanuele’s successor to work on the next steps on this project, and I cannot wait to see what this collaboration continues to produce. Finally, I want to thank Drs. Harish Bhandari, Pijush Bhattacharya, and Oleg Maksimov of Radiation Monitoring Devices, Inc. (RMD) for their incredible work depositing the ternary lead chalcogenides and for their immense contributions to our discussions about the results of those depositions. Because of the additional element, the ternaries are complicated materials, but Harish, Pijush, and Oleg have always provided important perspectives and insights into what is going on. Next, I want to thank all of the staff of the shared facilities that I used extensively at MIT: the Center for Materials Science and Engineering (CMSE) and Microsystem Technology Laboratories (MTL, which is now transitioning to MIT.nano). They have helped not only train me on the tools needed to do my research, but