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Silicon Refining Through Chemical Vapour Deposition

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Silicon Refining Through Chemical Vapour Deposition SILICON REFINING THROUGH CHEMICAL VAPOUR DEPOSITION by Mark (Xiang) Li A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Materials Science and Engineering University of Toronto © Copyright by Mark Xiang Li 2008 SILICON REFINING THROUGH CHEMICAL VAPOUR DEPOSITION Master of Applied Science 2008 Mark Xiang Li Department of Material Science and Engineering University of Toronto ABSTRACT Currently the cost of solar grade silicon accounts for approximately one third of the total solar cell cost, therefore a new silicon refining process is being proposed with the goal of lowering the cost of producing solar grade silicon. In this new process, Si-Cu alloys were used as the silicon source. One to one molar ratio H2-HCl gas mixtures were used as transport agents to extract Si out from the Si-Cu alloy at about 300-700oC, with following reaction taking place: Si+3HCl(g)=HSiCl3(g)+H2(g) While at about 1000-1300oC, pure Si deposits onto a hot silicon rod according to: Si+3HCl(g)=HSiCl3(g)+H2(g) The role of the copper in the alloy was to trap impurities in the Si and catalyze the gas solid reaction. A study on determining the rate limiting step and impurity behavior was done. A possible silicon extraction reaction mechanism was also addressed. ii ACKNOWLEGEMENTS I would like to express my deep and sincere gratitude to all those who gave me the possibility to complete this thesis. I am deeply indebted to my supervisor Prof. Torstein Utigard whose wide knowledge and his logical way of thinking have been of great value for me. His understanding, encouraging and personal guidance have provided a good basis for the present thesis. My warm thanks are due to Anton, Horazio, Gabriel and Alex for valuable advices and friendly help. I would also like to thank ARISE Technologies, as well as University of Toronto Open Fellowship for the financial support. I would like to give my special thanks to my family whose patient love enabled me to complete this work. iii TABLE OF CONTENTS 1. INTRODUCTION .......................................................................................................... 1 2. OBJECTIVES................................................................................................................. 4 3. LITERATURE REVIEWS ............................................................................................. 5 3.1 ELECTRONIC GRADE SILICON PRODUCTION ............................................... 5 3.1.1 Metallurgical Grade Silicon Production ............................................................ 6 3.1.2 Trichlorosilane Production................................................................................. 7 3.1.3 Siemens Reactor................................................................................................. 7 3.2 PROCESSES FOR PURIFICATION OF SOLAR GRADE SILICON ................... 9 3.3 Cu-Si ALLOYS AS SILICON SUPPLY SOURCES............................................... 9 3.4 DEPOSITION VARIABLES.................................................................................. 14 3.4.1 Cu-Si Alloy Compositions............................................................................... 15 3.4.2 Deposition Surface Areas ................................................................................ 17 3.4.3 Surface Areas of Cu-Si Alloys......................................................................... 19 4. EXPERIMENTAL CONDITIONS AND PROCEDURE............................................ 23 4.1 EXPERIMENTAL CONDITION DETERMINATION......................................... 23 4.2 REACTOR DESIGN .............................................................................................. 27 4.2.1 Reactor ............................................................................................................. 27 4.2.2 Cu-Si Alloys..................................................................................................... 29 4.2.3 Silicon Starting Rod......................................................................................... 30 4.2.4 Power Supply................................................................................................... 32 4.2.5 Temperature Measurement Device.................................................................. 33 4.3 EXPERIMENTAL PROCEDURES....................................................................... 34 4.3.1 Prior to Experiments ........................................................................................ 34 4.3.2 During the Experiment..................................................................................... 35 4.3.3 After the Experiment........................................................................................ 36 4.4 EXPERIMENTAL UNCERTAINTIES ................................................................. 36 5. SILICON DEPOSITION CALCULATION................................................................. 37 6. RATE LIMITING STEP............................................................................................... 42 6.1 BY PRODUCTS FORMATION ............................................................................ 44 6.2 MASS FLOW RATE.............................................................................................. 44 7. PRIMARY SILICON REACTION PHASE................................................................. 46 7.1 EXPERIMENT WITH 30, 50 AND 75 Cu-Cu ALLOYS...................................... 48 7.1.1 Experimental Details........................................................................................ 49 7.1.2 Results and Discussions................................................................................... 50 7.1.2.1 Silicon Recovery and Cu-Si Alloy Weight Losses................................... 50 7.1.2.2 Cu-Si Alloys After The Experiment ......................................................... 53 7.1.3 Conclusions...................................................................................................... 54 7.2 REPEATED EXPERIMENT WITH 30, 50 AND 75 WT%Si-Cu ALLOYS........ 55 7.2.1 Experimental Details........................................................................................ 56 7.2.2 Results and Discussions................................................................................... 57 7.2.2.1 Silicon Recovery and Cu-Si Alloy Weight Losses................................... 57 7.2.2.2 Si Deposits ................................................................................................ 58 7.2.2.3 Cu-Si Alloys After The Experiment ......................................................... 59 iv 7.2.3 Conclusions...................................................................................................... 60 7.3 EXPERIMENT WITH 30 AND 50WT%Si-Cu ALLOYS .................................... 61 7.3.1 Experimental Details........................................................................................ 61 7.3.2 Results and Discussions................................................................................... 63 7.3.2.1 Silicon Recovery and Cu-Si Alloy Weight Losses................................... 63 7.3.2.2 Silicon Deposit.......................................................................................... 64 7.3.2.3 Cu-Si Alloy After The Experiment........................................................... 64 7.3.3 Conclusions...................................................................................................... 66 7.4 SILICON DIFFUSION........................................................................................... 67 7.4.1 Density of Alloys Before and After the Experiment ....................................... 67 7.4.1.1 Experimental Setup................................................................................... 67 7.4.1.2 Results....................................................................................................... 68 7.4.2 Compositions of the Alloy Core ...................................................................... 69 7.4.2.1 Experimental Setup................................................................................... 69 7.4.2.2 Results....................................................................................................... 69 7.5 CONCLUSIONS AND FUTURE WORK ............................................................. 70 8. ALLOY CRUMBLES AND SILICON EXTRACTION MECHANISMS.................. 71 8.1 ALLOY CRUMBLES TESTS................................................................................ 71 8.1.1 Experimental Details........................................................................................ 71 8.1.2 Observations and Results................................................................................. 72 8.1.3 Crumbles Microstructures................................................................................ 75 8.2 COMPOSITION OF THE CRUMBLES................................................................ 77 8.2.1 Experimental Setup.......................................................................................... 77 8.2.2 Results.............................................................................................................. 78 8.3 PROPOSED SILICON EXTRACTIOIN MECHANISM...................................... 79 8.4 CONCLUSIONS..................................................................................................... 80 9. IMPURITY STUDY....................................................................................................
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