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Enhanced Crystallization of Amorphous Silicon Thin Films by Nano-Crystallite Seeding

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Enhanced Crystallization of Amorphous Silicon Thin Films by Nano-Crystallite Seeding Enhanced Crystallization of Amorphous Silicon Thin Films by Nano-crystallite Seeding A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Jason Trask IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy Prof. Uwe Kortshagen December, 2013 c Jason Trask 2013 ALL RIGHTS RESERVED Acknowledgements I owe the department of mechanical engineering at the University of Minnesota a tremen- dous debt of gratitude, for the Bachelor's degree they granted me in 2005, the friends I acquired along the way through agonizingly long nights in the computer lab, and the job opportunity that it afforded after graduation. I also owe the department for agreeing to take me back for a graduate tour of duty after I contracted a severe case of apathy for that job in 2007. I decided to go back to graduate school for a Master's degree for what I had thought would just be two years. Six years later I left after completing this third and final revision of the document you now hold in your hands. A document which you are about to either a) selectively skim (in which case I must thank you -again- for agreeing to be on my committee) b) put back on the shelves of Walter library after glancing at a few figures, which I can't blame you for (FYI the ones in chapter 6 are the prettiest; they're fancy microscope images of crystals), or c) put back on the shelves in Eckert library and get back to grading final exams after an instructor-purchased lunch. If by some chance you are the lucky successor to the project this document was concerned with, and if I am still alive at the time of your reading this, feel free to contact me by any means necessary and I'll be happy to trouble-shoot apparatus issues with you. Specifically, I'll let you know exactly the size of blunt object needed, and where to hit said apparatus. If that doesn't manage to solve the problem, relying on the skill of the brilliant minds around you is the best route. I surely couldn't have survived without the assistance of colleagues such as David Rowe, Lee Weinkes, Rick Liptak, Curtis Anderson, Rebecca Anthony, Andrew Wagner, Ariel Chatman, Lin Cui, Jeslin Wu, Ting Chen, and Brian Merritt. In regards to non-equipment based support, including data analysis, assessing the i sanity of a hypothesis, or just navigating the psychological rollercoaster that is the graduate experience, your fellow graduate students and staff are the most valuable resource there is. For this, in addition to those previously mentioned, I must also thank Benjamin Adams, Derek Obereit, Ranganathan Gopalakrishnan, Barry Mutlu, Adam Boise, Lance Wheeler, Vince Wheeler, Daniel Rave, Dominic Triana, Geek Cinema, The Velvet Rope Club, Luke Franklin and the GENS network, and last but certainly not least, the grad-father himself John Gardner. If he's still working in the department at the time of your reading this, make sure to pay your respects. From a much needed happy hour after a rough group meeting, to 1 a.m. meals at the village wok while waiting for an experiment to finish, to 3 a.m. meals at a conference in a strange city, if there's anything graduate research will teach you, it's that the people you meet along the way, in the moments between data sets, are by far and away the most significant variables in the entire parameter space. So make sure to leave the lab from time to time, and keep your coffee mug full, because even a sleepless grad-student can still dream. ii Dedication To Randall Trask, a good man that I was fortunate enough to know as a father. And to his grandfather Everett Cincoski: a true engineer if there ever was one. iii Abstract Polycrystalline silicon (poly-Si) has become popular in recent years as a candidate for low cost, high efficiency thin film solar cells. The possibility to combine the stability against light degradation and electronic properties approaching melt-grown, wafer-based crystallline silicon, with the cost advantage of Silicon thin films is highly attractive. To fully realize this goal, efforts have been focused on maximizing grain size while reducing the thermal input involved in a critical \annealing" step. Of the variety of processes involved in this effort, studies have shown that poly-Si films obtained from solid-phase-crystallization (SPC) of hydrogenated amorphous silicon (a-Si:H), grown from non-thermal plasma-enhanced chemical vapor deposition (PECVD), exhibit the potential to achieve the highest quality grain structures. However, reproducible control of grain size has proven difficult, with larger grains typically requiring longer annealing times. In this work, a novel technique is demonstrated for more effectively controlling the final grain structure of SPC-processed films while simultaneously reducing annealing times. The process utilized involves SPC of a-Si:H thin films containing embedded nanocrystallites, intended to serve as predetermined grain-growth sites, or grain-growth \seeds", during the annealing process. Films were produced by PECVD with a system in which two plasmas were operated to produce crystallites and amorphous films separately. This approach allows crystallite synthesis conditions to be tuned independently from a-Si:H film synthesis conditions, providing a large parameter space available for process optimization, including the effects of particle size, shape, quantity, and location within the film. The work contained here-in outlines the effects of select parameters on the both grain size control and thermal budget. Reproducible control of both grain size and crystallization rate were demonstrated through varying initial seed crystal concentra- tions. Significant reductions in annealing times were demonstrated to occur in seeded films relative to unseeded films, with both seed crystal concentration and seed crys- tal geometry demonstrating significant effects on crystallization rate. Furthermore, the iv development of this technique has resulted in potentially new insights on the mate- rial system involved, with the observation of a potentially unique phase-transformation mechanism. v Contents Acknowledgements i Dedication iii Abstract iv List of Tables ix List of Figures x 1 Introduction 1 2 Background 3 2.1 The Current Energy Landscape . 3 2.2 The Potential Role of Solar Energy . 4 2.3 The Current Industry Leader: Wafer-based Technologies . 5 2.4 Second Generation Solar Cells: Thin Film Technologies . 6 2.4.1 Advantages . 6 2.4.2 Challenges . 6 2.4.3 Current Thin Film Market Leaders: Cadmium Telluride and Copper Indium di-Selenide . 7 2.5 The Primary Alternatives: Silicon Thin Film Technologies . 8 vi 2.5.1 Amorphous Silicon Thin Films . 8 2.5.2 Mixed-phase Silicon Thin Films . 9 2.5.3 Poly-crystalline Silicon: The material system of study in this work . 10 3 Experiment 16 3.1 Dual Plasma Deposition System . 16 3.1.1 Seeded Film Structure . 17 3.1.2 a-Si:H Film Growth . 19 3.1.3 Seed Crystal Synthesis . 23 4 Crystallization Kinetics 25 4.1 Introduction: Motivation for Studies in Crystallization Kinetics . 25 4.2 Raman Crystal Fraction . 27 4.3 Raman Crystal Fraction Study 1: The Effect of Varying Anneal Temper- ature on Similar Films . 28 4.3.1 Crystallization Time . 31 4.3.2 Seeded Grain Growth Rate . 32 4.3.3 Avrami Slope . 34 4.4 Conclusions . 36 5 The Effect of Varying Seed Density 37 5.1 Introduction . 37 5.2 The Effect of Varying Seed Density on Grain Growth Rate . 37 5.3 The Effect of Film Disorder . 41 5.3.1 Short range disorder . 41 5.3.2 Medium Range Disorder . 43 5.3.3 Textured substrates . 46 5.4 The Effect of Varying Seed Density on Final Grain Structure . 49 5.4.1 Relative Grain Size . 49 5.4.2 Electronic Transport . 51 5.5 conclusions . 61 vii 6 The Effect of Seed Crystal Shape on Crystallization Kinetics 62 6.1 Introduction . 62 6.2 Void-induced Crystallization . 62 6.2.1 Void Kinetics . 63 6.2.2 Void formation . 65 6.3 The Effect of Particle Synthesis Power on Film Crystallization Kinetics 67 6.3.1 Seed Crystal Shape . 68 6.3.2 Microstructure . 71 6.3.3 Size Distribution . 72 6.3.4 Crystallization Kinetics . 73 6.4 Conclusions . 75 7 The Effect of Varying Film Properties on Crystallization Kinetics 77 7.1 Introduction: The Motivation for Varying Film Properties . 77 7.1.1 The Relevance of Increasing Film Deposition Rate . 78 7.1.2 The Relevance of Through-thickness Grain Penetration . 78 7.2 The Effect of Varying Pressure and Power . 79 7.2.1 Deposition Rate . 79 7.2.2 Film Properties . 82 7.2.3 Seeded Crystallization Kinetics . 84 7.2.4 Unseeded Incubation Times . 92 7.2.5 Argon dilution . 95 7.2.6 Theoretical Grain Size of High Deposition Rate Seeded Films . 97 7.3 Conclusions and Recommendations . 99 8 Conclusions 100 8.1 Summary of Processing Advantages . 100 8.2 Future Work and Recommendations . 103 References 104 viii List of Tables 3.1 Summary of basic a-Si:H film deposition parameters used for the studies in this work. Values are compared to common literature ranges taken from [25, 26] . 20 3.2 Summary of a-Si:H film properties measured from test case films grown in the plasma box shown in figure 3.1. Values are compared to common literature ranges taken from [25, 26] . 22 5.1 Summary of electronic transport properties of fully crystallized films of varying initial seed density. σ(300K) is dark conductivity averaged over 24 device channels, taken at room temperature.
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