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A Dissertation Entitled Spectroscopic Ellipsometry Studies of Thin Film Si A Dissertation entitled Spectroscopic Ellipsometry Studies of Thin Film Si:H Materials in Photovoltaic Applications from Infrared to Ultraviolet by Laxmi Karki Gautam Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics _________________________________________ Dr. Nikolas J. Podraza, Committee Chair _________________________________________ Dr. Robert W. Collins, Committee Member _________________________________________ Dr. Randall Ellingson, Committee Member _________________________________________ Dr. Song Cheng, Committee Member _________________________________________ Dr. Rashmi Jha, Committee Member _________________________________________ Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo May, 2016 Copyright 2016, Laxmi Karki Gautam This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of Spectroscopic Ellipsometry Studies of Thin Film Si:H Materials in Photovoltaic Applications from Infrared to Ultraviolet by Laxmi Karki Gautam Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo May 2016 Optimization of thin film photovoltaics (PV) relies on the capability for characterizing the optoelectronic and structural properties of each layer in the device over large areas and correlating these properties with device performance. This work builds heavily upon that done previously by us, our collaborators, and other researchers. It provides the next step in data analyses, particularly that involving study of films in device configurations maintaining the utmost sensitivity within those same device structures. In this Dissertation, the component layers of thin film hydrogenated silicon (Si:H) solar cells on rigid substrate materials have been studied by real time spectroscopic ellipsometry (RTSE) and ex situ spectroscopic ellipsometry (SE). Growth evolution diagrams has been used to guide deposition of materials with good optoelectronic properties in the actual hydrogenated amorphous silicon (a-Si:H) PV device configuration. The nucleation and evolution of crystallites forming from the amorphous phase were studied using near infrared to ultraviolet spectroscopic ellipsometry in situ, during growth for films prepared as a function of hydrogen to reactive gas flow ratio R = [H2] /{[SiH4] + [Si2H6]. Furthermore, the major challenge in Si:H manufacturing is that iii quantitative analysis, characterization, and control of the relative nanocrystalline and amorphous volume fractions within mixed-phase films were covered during these studies. In conjunction with higher photon energy measurements, the presence and relative absorption strength of silicon-hydrogen infrared modes were measured by infrared extended ellipsometry measurements to gain some insight into chemical bonding. Structural and optical models have been developed for the back reflector (BR) structure consisting of sputtered undoped zinc oxide (ZnO) on top of silver (Ag) coated glass substrates. Characterization of the free-carrier absorption properties in Ag and the interface formed when Ag is over-coated with ZnO were also studied by infrared extended spectroscopic ellipsometry. Measurements ranging from 0.04 to 5 eV were used to extract layer thicknesses, composition, and optical response in the form of complex dielectric function spectra (ε = ε1 + iε2) for undoped a-Si:H layers in a substrate n-i-p a- Si:H based PV device structure and on TCO coated glass for p-i-n configurations. iv To my Family. Acknowledgements First of all, I would like to express my sincere gratitude to my supervisor Dr. Nikolas J. Podraza for his valuable guidance, unlimited support, expert mentoring, advice and encouragement throughout this work. His continuous guidance and support helped me to broaden my view and knowledge in subject matter. I would also like to extend my sincere gratitude to my committee members, Dr. Robert W. Collins, Dr. Randall Ellingson, Dr. Song Cheng and Dr. Rashmi Jha, for being in my committee and giving me valuable advices whenever needed. The laboratory support staff also has been very helpful and I would like to acknowledge support and the help I got from Terrance Kahle, Carl Salupo, and Nirupama Adiga. I would like to express my sincere gratitude to Dr. Patrick Hurley and Dr. Robert Ridgeway, and financial support from Air Products and Chemicals Inc. and Department of Energy (DE-EE0000580). I also thank Matthew L. Herold at the Air Force Institute of Technology Wright-Patterson Air Force Base for collaboration providing ZnO films. My deepest appreciation goes to all of my fellow research group members of the ellipsometry group and colleagues at the University of Toledo for motivating me and ensuring me to work in a very pleasant and comfortable environment. Most of all, my heartiest gratitude goes to my parent, my husband, Madhav, and daughter, Manasi, for supporting me under any circumstances. v Table of Contents Abstract .............................................................................................................................. iii Acknowledgements ..............................................................................................................v Table of Contents ............................................................................................................... vi List of Tables ................................................................................................................... ix List of Figures .................................................................................................................... xi 1 Introduction……. .....................................................................................................1 1.1 Motivation and Background ..............................................................................1 1.2 Dissertation Organization ..................................................................................9 2 Experimental Techniques used in Optical Measurements .....................................12 2.1 Deposition of Si:H Thin Films ........................................................................12 2.1.1 Cluster Tool Deposition System .......................................................14 2.1.2 Sputtering Process .............................................................................16 2.1.3 Plasma Enhanced Chemical Vapor Deposition ................................19 2.2 Material Characterization Techniques ............................................................21 2.2.1 Spectroscopic Ellipsometry ..............................................................21 2.2.1.1 Theoretical Formalism .......................................................23 2.2.1.2 Experimental Methods in Spectroscopic Ellipsometry ......33 2.2.1.3 Data Analysis Strategies in Spectroscopic Ellipsometry ...35 2.2.2 Experimental Methods in Solar Cell Characterization .....................45 vi 3 Growth Evolution of Si:H Prepared with Disilane Additives as Studied by Real Time Spectroscopic Ellipsometry ..........................................................................49 3.1 Introduction and Motivation ............................................................................49 3.2 Overview of Deposition Processes and Microstructural evolution..................50 3.2.1 Deposition Processes of Si:H Films ..................................................50 3.2.2 Microstructural Evolution and Phase Diagram .................................52 3.3 Experimental Details ........................................................................................59 3.4 S = 0, and variable R: Si:H Growth Evolution Baseline ..................................61 3.5 S = 0.12 and 1 and verses R: Effect of Disilane on Si:H Growth ....................63 3.6 Summary… ......................................................................................................67 4 Si:H Layer Optimization for Substrate Configuration of Solar Cell .....................69 4.1 Introduction ......................................................................................................69 4.2 Experimental Details ........................................................................................72 4.3 Ag/ZnO BR Substrate ......................................................................................73 4.4 n-layer Si:H on Ag/ZnO Substrate ...................................................................82 4.5 i-layer Si:H on Ag/ZnO/n-Si:H Substrate ........................................................85 4.6 p-layer Si:H on glass/i-layer Substrate ............................................................89 4.7 Summary… ......................................................................................................98 5 Growth and Analysis of Amorphous to Nanocrystalline Transition in Hydrogenated Silicon films ...................................................................................99 5.1 Introduction and Motivation ............................................................................99 5.2 Overview of Microstructural Evolution (a-Si:H to nc-Si:H) .........................102 5.3 Experimental Details ......................................................................................106 vii 5.4 Microstructural evolution of nc-Si:H
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