Silicon-Germanium Alloys for Photovoltaic Applications

By Ammar Nayfeh and Sabina Abdul Hadi

Silicon-Germanium Alloys for Photovoltaic Applications provides a comprehensive look at the use of Silicon-Germanium alloys Si1-xGex in photovoltaics. Different methods of Si1-xGex alloy deposition are reviewed, including their optical and material properties as function of Ge% are summarized, with SiGe use in photovoltaic applications analyzed. Fabrication and characterization of single junction Si1-xGex solar cells on Si using a-Si as emitter is discussed, with a focus on the effect of different Ge%. Further, the book highlights the use Si1-xGex as a template for lattice matched deposition of III-V layers on Si, along with its challenges and benefits, including financial aspects.

Finally, fabrication and characterization of single junction GaAsxP1-x cells on Si via Si1-xGex is discussed, along with the simulation and modeling of graded SiGe layers and experimental model verification.

• Includes a summary of SiGe alloys material properties relevant for solar research, all compiled at one place
• Presents various simulation models and analysis of SiGe material properties on solar cell performance
• Includes a cost-analysis for III-V/Si solar cells via SiGe alloys

• Language: English
• Publisher: Elsevier Science
• Release date: Mar 9, 2023
• ISBN: 9780323856317

Ammar Nayfeh

Professor Ammar Nayfeh was born in Urbana IL in 1979. He received his bachelor's degree from the University of Illinois Urbana Champaign in 2001 in electrical engineering and his master's degree in 2003 from Stanford University. Dr. Nayfeh earned a Ph.D. in electrical engineering from Stanford University in 2006. His research focused on heteroepitaxy of germanium on silicon for electronic and photonic devices. After his PhD, he joined Advanced Micro Devices as a researcher working in collaboration with IBM. After that he joined a silicon valley startup company, Innovative Silicon (ISi) in 2008. In addition, he was a part time professor at San Jose State University. In June 2010, he joined MIT as a visiting scholar and became a faculty member at the Masdar Institute currently Khalifa University. His research interest focuses on nanotechnology for future more efficient low power electronic and photonic devices. Professor Nayfeh is currently an associate professor in the Department Electrical Engineering and Computer Science (EECS) at Khalifa University. Professor Ammar Nayfeh has authored or co-authored over 100 publications, and holds three patents. He is a member of IEEE, MRS and Stanford Alumni Association. He has received the Material Research Society Graduate Student Award, the Robert C. Maclinche Scholarship at UIUC, and Stanford Graduate Fellowship.

Book preview

Preface

From authors

This book covers two key research and scientific topics. One is the growth of new materials to improve the performance of electrical and optical devices. The other is research into devices for clean and renewable energy. This book combines two specific scientific interests of ours: germanium and solar cell device research. This book was a two-year long project starting in April 2020 (early part of COVID-19 pandemic) and was done jointly by the authors, a professor and his former PhD student. It was a tremendous pleasure to work again together on this project, as this book is largely based on our previous joint research work. During our journey with Silicon–Germanium and solar cells based on it, we found that there is a need for a resource where researchers can find all relevant information at one place, whether they need material properties, SiGe growth methods, simulation methodologies, application, and potential uses. By writing this book, we tried to deliver to the research community and academia such single resource.

From Professor Ammar Nayfeh

This book combines two specific scientific passions of mine: germanium and solar cell device research. My passion for germanium started at Stanford during my PhD and evolved to solar device research since joining Khalifa University. Being able to research new materials to improve solar cells that will help the world deploy more clean energy is a very strong motivation of mine.

For the research done in this book, I want to acknowledge my former PhD student Dr. Ghada Dushaq, for her thesis work on low temperature Ge on Si growth, and, in addition, my former students Dr. Ayman Rezk and Amro Alkhatib for nanomaterial research conducted.

This book was a two-year long project and was done jointly with my first PhD student, Dr. Sabina Abdul Hadi. It was a tremendous pleasure to work again with her on this project as she is a dedicated, hardworking, and motivated researcher that shares the same passions about science, solar, and new materials. Dr. Abdul Hadi's PhD and MSc thesis was the main inspiration for this book. I would like to thank Khalifa University and the government of UAE, for their constant support of research, science, and academia and for giving me the opportunity to pursue my scientific passions.

Finally, I want to dedicate this book to my former PhD advisor at Stanford, Professor Krishna Saraswat, and MIT collaborators, Professor Eugene Fitzgerald and Professor Judy Hoyt. Without their guidance and support throughout the years, this book would not be possible. Also, this book would not be possible without the love and support of my family. This includes Abuhantash family (Azzam, Amal, Ahmad, and Osama); my parents Munir and Hutaf Nayfeh; my amazing children Laith, Leia, and Zaid; and finally, and most importantly of all, my always supportive and loving wife Lama. She was very patient with me during the writing of this book, and I am truly thankful and blessed (behind every great man is an even greater woman).

From Dr. Sabina Abdul Hadi

As a young researcher, having a good reference, be it a book, professor, or a colleague, is a valuable resource that can put you on the right path for success and make your scientific journey full of joy and excitement. With this book, I hope we will give such a reference, where one can find almost everything they need when working with Silicon–Germanium, especially for solar cell applications.

This book would not be possible without my coauthor and former PhD advisor, Professor Ammar Nayfeh. He has been my support and guidance ever since our paths crossed more than a decade ago, for which I can never thank him enough. Professor Nayfeh's enthusiasm and passion for research are infectious and I am deeply grateful for having the opportunity to work with him, a fellow scientist with keen ear for new ideas and eagerness to make them a reality.

I would like to thank Khalifa University and the government of UAE for giving me an opportunity to pursue my research interests during my PhD and MSc studies and to the University of Dubai for their support during writing of this book. Furthermore, I would like to thank Professor Judy L. Hoyt and Professor Eugene Fitzgerald, from MIT, whose knowledge, instrumental insights, and technical support have shaped this research. A lot of research work presented in this book was part of our collaborative efforts with MIT team that included fantastic researchers such as Dr. Pouya Hashemi, Gary Riggott, Dr. Nicole DiLello, Dr. Eva Polyzoeva, and Dr. Timothy Milakovich.

Last, but closest to my heart, I want to thank my family for their endless love, patience, and support during writing of this book and my entire scientific journey. I specially thank my husband Wissam, whose love always had put everything into perspective.

Finally, I dedicate this book to my two favorite ladies, my mother Aida and my daughter Lara. My mother has taught me to be independent and ambitious woman and had always been my loudest supporter. My daughter has taught me patience, perseverance, and pure love, to whom I hope to be a good role model and biggest fan through her life journey.

Chapter 1: About the book

Abstract

New material systems and discoveries are needed to help improve photovoltaic conversion efficiency and cost-effectiveness. These discoveries will be the key enabler in allowing for aggressive use of photovoltaic technologies beyond 2022 and into the next decades. With a global push to curb the negative effects of climate change in full force, the formation of the Paris climate accords in April of 2016, the recent net zero goal from 2021 United Nations Climate Change Conference (COP26), and with many governments changing to a green centric economy, solar energy is most likely the main solution for clean sustainable renewable energy of the future.

Keywords

Climate change; Crystalline silicon-germanium alloy; Net zero goal; Photovoltaic conversion; Solar energy

New material systems and discoveries are needed to help improve photovoltaic conversion efficiency and cost-effectiveness. These discoveries will be the key enabler in allowing for aggressive use of photovoltaic technologies beyond 2022 and into the next decades. With a global push to curb the negative effects of climate change in full force, the formation of the Paris climate accords in April of 2016, the recent net zero goal from 2021 United Nations Climate Change Conference (COP26), and with many governments changing to a green centric economy, solar energy is most likely the main solution for clean sustainable renewable energy of the future [1–3]. This makes the research into new materials for solar energy even more critical. More specifically the vital question is how these new novel materials interact with each other, and how their material properties will affect photovoltaic performance. New material research and development has become the nucleus and driving force of the PV challenge to improve efficiency without increasing cost significantly. This book looks comprehensively at the crystalline silicon-germanium alloys (Si1-xGex) in photovoltaics. c-Si1-xGex photovoltaics has been a key material for high-performance transistors and photodetectors but it turns out c-Si1-xGex has a lot to offer for photovoltaics. This book will cover all the potential uses of c-Si1-xGex in photovoltaics. It will cover topics from deposition methods; optical, electrical, and material properties; single junction Si1-xGex solar cells; TCAD simulation and detailed balance modeling; III-V integration; and a detailed cost analysis.

It should be noted, a large percentage of research highlighted in this book is conducted by authors, Professor Ammar Nayfeh and Professor Sabina Abdul Hadi while at Khalifa University. Some of the work was done in collaboration with Professor Judy Hoyt and Professor Eugene Fitzgerald of Massachusetts Institute of Technology (MIT). In addition, the earlier work on the Ge on Si growth was guided by Professor Krishna Saraswat of Stanford University in collaboration with Professor Nayfeh. Fig. 1.1 shows a demonstration of III-V on Si step cell that was developed using c-Si1-xGex technology.

Figure 1.1  Step cell developed by authors Abdul Hadi and Nayfeh.

All the research done was funded by Khalifa University and United Arab Emirates (UAE) University government in line with their 2050 vision:

In 2017, the UAE launched ‘Energy Strategy 2050’, which is considered the first unified energy strategy in the country that is based on supply and demand. The strategy aims to increase the contribution of clean energy in the total energy mix from 25% to 50% by 2050 and reduce carbon footprint of power generation by 70%, thus saving AED 700 billion by 2050. It also seeks to increase consumption efficiency of individuals and corporates by 40% [4].

The book is geared toward graduate students and researchers (both in academia and industry) working on materials in photovoltaics. It includes a summary of c-Si1-xGex material properties relevant for solar research. Also, different simulation models and analysis of c-Si1-xGex material properties on solar cell performance are discussed. Finally, the cost analysis for III-V/Si solar cells via c-Si1-xGex alloys will be useful for companies and future PV planning. The book can be used as MSc or Ph.D. level class textbook. It should be noted that although the book focus is photovoltaics, there is a plethora of useful information for c-Si1-xGex in electronics and photonics.

1.1. Book overview

The book is divided into nine chapters including the current Chapter 1 (about the book).

Chapter 2 covers the motivation for solar energy in the context of climate change. It will give a holistic look on the issues associated with climate change and greenhouse gas effect with the role solar energy can be used to solve these issues. The motivation for new material innovations to increase the efficiency without increasing cost is discussed. Finally, the potential applications of c-Si1-xGex in the future of photovoltaics will be introduced.

Chapter 3 reviews basics of solar cells physics and provides an overview of solar cell concepts, sufficient to prepare the reader for topics of the following chapters. It covers the basic key parameters of solar cell while introducing some basic physics of operations from an electrical, photonic, and material perspective.

Chapter 4 highlights c-Si1-xGex deposition methods over the years, focusing on the optical and material properties. This chapter emphasizes the challenges associated with the growth due to lattice mismatch, while differentiating between various growth modes and deposition tools. A historical look at the key research and growth innovations of c-Si1-xGex deposition over years are discussed.

Chapter 5 provides a history of c-Si1-xGex solar cells fabrication and demonstration. An overview of key research results to date with an emphasis on c-Si1-xGex heterojunction with intrinsic thin layer (HIT) solar cells.

Chapter 6 discusses work on growth of lattice matched III-V on Si via Si1-xGex buffer. Experimental results of actual III-V cells fabricated via Si1-xGex technology are presented here.

Chapter 7 details the modeling and simulations of Si1-xGex buffer-based solar cells, including multi-junction III-V/Si solar cells. Simulation results include theoretical analysis of dual and multi-junction using detailed balance method, with a look into effect of step-cell design on multi-junction performance and optimum bandgap combinations. Furthermore, simulation results using TCAD by Synopsis are presented for different Si1-xGex cells and III-V/Si dual junction cell, including various analyses of those structures, such as effect of antireflective coasting design for III-V/Si cells, effect of Si1-xGex graded buffer on bottom Si cell, or effect of step-cell design, among others.

Chapter 8 provides an overview of solar cell cost analysis and goes into the cost benefits of using Si1-xGex graded buffers instead of Ge for III-V based multi-junction solar cells. A process flow using step cell is used for the cost analysis, as one of the proposed cost-effective ways for mass production of III-V layers on Si substrates via Si1-xGex graded buffers. Sensitivity analysis with different scenarios is carried out with comparison to costs of standard III-V growth on Ge substrates.

Chapter 9 highlights notable applications of Si1-xGex other than PV. Different applications of Si1-xGex are summarized, including its use in CMOS, optics, III-V integration, or nanotechnology. This chapter does not go into depth of mentioned subjects, but it provides great starting point for a reader for further explorations of Si1-xGex applications.

References

1. Glanemann N, Willner S, Levermann A. Paris climate agreement passes the cost-benefit test. Nat. Commun. 2020;11:110.

2. United Nations in Western Europe, 2021. [Online]. Available: https://unric.org/en/cop26-a-snapshot-of-the-agreement/.

3. The National Academies of Science Engineering Medicine, 2022. [Online]. Available: https://www.nap.edu/resource/other/dels/net-zero-emissions-by-2050/?gclid=Cj0KCQiAqbyNBhC2ARIsALDwAsC-Mpm4Vx6dmKZXzzKG9S41YZikhG2glBdJ4luvkLiuzO2Oy8SolkEaAjPHEALw_wcB#page-top.

4. Efforts towards sustainability. https://u.ae/en/information-and-services/environment-and-energy/water-and-energy/efforts-towards-sustainability, 2021.

Chapter 2: Motivation

Abstract

Solar energy is one of the most vital sources of clean energy available to help reduce and eliminate the negative effects of climate change due to greenhouse gas emissions. Using solar cells made from semiconductor crystals that are engineered in a way to take advantage of the photovoltaic effect, solar energy can be converted to electrical energy elegantly and in a clean way. Climate change and global warming effects are already causing serious problems in the world and will only get worse unless we distance ourselves from greenhouse gas emitting fossil fuels quickly and shift to clean sources of energy. Some recent scientific studies paint a very bleak picture of the future, concluding that the damage due to the current emissions and global warming might already be irreversible. Nevertheless, we must try to preserve the Earth's habitability for future generations and that begins with a commitment to shift to clean and renewable sources of energy. While solar cells are excellent conversion devices, they are not perfect in the conversion process from solar to electrical energy. With higher efficiency, more energy can be produced at lower cost and smaller environmental impact, which is important for meeting rising global energy needs. Photovoltaic cell conversion efficiency is limited by the material properties of the semiconductor crystal. As a result, finding new materials to convert solar energy more efficiently is of utmost importance. In this chapter the use of the solar energy is motivated based on the basic tenants of climate change, global warming, fossil fuel limitations, and the concept of photovoltaics. Moreover, this leads to the motivation for silicon-germanium (Si1− x Ge x ) alloys in current and future photovoltaic designs.

Keywords

Climate change; Emission; Global warming; Greenhhouse gases; Photovoltaics; Solar energy

2.1. Introduction

The negative effects of climate change and global warming are no longer a futuristic issue to deal with. This is the result of the continued and extensive use of fossil fuels and subsequent greenhouse gas emissions such as CO2. The earth has been warming at an alarmingly fast pace the last 40 years due to greenhouse gas emission and the trapping of heat in the atmosphere. To slow down and eliminate this unnatural warming rate a reduction in greenhouse gas emissions is essential. This is accomplished by an increase in the deployment and use of clean renewable energy. Solar energy continues to play a significant role in the clean energy shift due to the abundance and unlimited supply of energy provided by the sun. The use of solar energy is on an exponential rise, thanks to recent government policies to reduce emissions. For example, Fig. 2.1 plots the solar energy production in the United Arab Emirates (UAE) from 2009 to 2019 showing an exponential rise starting in 2016 reaching almost 3500GWh of production in 2019