Silicon Refining Through Chemical Vapour Deposition

SILICON REFINING THROUGH CHEMICAL VAPOUR DEPOSITION

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

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.

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...... 81 9.1 SOURCE OF IMPURITIES ...... 82 9.2 IMPURITIES IN THE Cu-Si ALLOYS...... 84 9.2.1 Experimental Setup...... 85 9.2.2 Results...... 85 10. CONCLUSIONS AND FUTURE WORK ...... 88 11. REFERRENCES...... 90 Appendix A: Chemical Analysis On Cu, MG-Si, Cu-Si Alloys, EG-Si and Deposited Si ...... 92 A.1 Metallurgical Grade Silicon...... 92 A.2 Copper Used for Alloying with MG-Silicon...... 93 A.3 Electronic Grade Silicon ...... 94 A.4 Impurities in the 30wt%Si-Cu alloy (Area 1) ...... 95 A.5 Impurities in the 30wt% Si-Cu alloy (Area 2) ...... 96 A.6 Impurities in the 30wt% Si-Cu alloy (Area 3) ...... 97 A.7 Impurities in the 30wt% Si-Cu alloy (Area 4) ...... 98 A.8 Impurities in the 30wt% Si-Cu alloy (Area 5) ...... 99 A.9 Impurities in the 30wt% Si-Cu alloy (Area 6) ...... 100 Appendix B. The GASEOUS SPECIES OF ELEMENTS Al, B, Si, Fe, P, Ti and Mn. 101 Appendix C. silicon refining efficiency...... 102

v LIST OF FIGURES

Figure 1.1 Energy Flux comparison ...... 1 Figure 1.2 U.S. Electricity Generation by Energy Source, 2004 ...... 2 Figure 1.3 Cost comparison between different grades of silicon, 2003 ...... 2 Figure 1.4 Predicted solar grade silicon demand and supply...... 3 Figure 3.1 Schematic drawing of electronic grade silicon production ...... 5 Figure 3.2 A typical plant for the production of silicon metal...... 6 Figure 3.3 Siemens Reactor ...... 8 Figure 3.4 SEM illustration of 25wt%Si-Cu Alloy. Dark phase is the primary Si phase, lighter phase is the Cu-Si matrix...... 10 Figure 3.5 schematic drawing of experimental reactor...... 11 Figure 3.6 Cu-Si Binary Phase Diagram...... 12 Figure 3.7 Relative contribution of the different mechanisms to the overall purifications ...... 14 Figure 3.8 Experimental set-up in Ebner ...... 21 Figure 3.9 Different alloys stacking up...... 21 Figure 4.1 Partial pressure of possible gaseous species from H, Cl, and Si...... 24 Figure 4.2 Sums of the equilibrium partial pressures of silicon gaseous species (HCl-H2- Si mol ratio is 1:1:50) ...... 25 Figure 4.3 Sums of the equilibrium partial pressures of Al, Fe, B, Ti, Mn and P gaseous species...... 27 Figure 4.4 schematic of experimental set-up ...... 27 Figure 4.5 DC Power Supply...... 28 Figure 4.6 Cast Cu-Si alloys in puck and square shapes ...... 29 Figure 4.7 Alloys in rectangle shape...... 30 Figure 4.8 Electronic grade silicon starting rod...... 30 Figure 4.9 Actual reactor set-up...... 31 Figure 4.10 Power supplies (a) Main DC power supply (b) External heating AC power supply...... 32 Figure 4.11 M770S two color infrared sensor ...... 34 Figure 5.1 Basic Experimental Schematic for Si Deposition Calculation...... 37 Figure 6.1 Silicon diameter vs time ...... 43 Figure 6.2 The weight and diameter of the rod in the first few hours reaction ...... 43 Figure 6.3 Diameter vs time with various pumps...... 45 Figure 7.1 Cu-Si phase diagram...... 46 Figure 7.2 SEM pictures of Cu-Si alloys with various Si compositions ...... 47 Figure 7.3 The Cu-Si alloys with 30, 50 and 75 wt%Si used in the experiment...... 48 Figure 7.4 Eutectic liquid drops on the 50wt%Si alloy ...... 52 Figure 7.5 Silicon deposits after 5.5h reaction ...... 52 Figure 7.6 Cu-Si alloys after 5.5 h reaction. Alloys with higher silicon content are less crumbly and swelling...... 53 Figure 7.7 The 50wt%Si alloy after 5.5h reaction...... 54 Figure 7.8 The Cu-Si alloys with 30, 50 and 75 wt%Si used in the experiment...... 55 Figure 7.9 Deposited silicon (a) front view (b) top view (c) bottom view ...... 58

vi Figure 7.10 Sketch of boundary layer created around silicon rod due to a fluid flow...... 59 Figure 7.11 The Cu-Si alloys after 5.5 h reaction...... 60 Figure 7.12 The Cu-Si alloys with 30 and 50wt%Si used in the experiment...... 61 Figure 7.13 Silicon deposit ...... 64 Figure 7.14 The Cu-Si alloys after the experiment...... 65 Figure 7.15 The disintegrated 30wt%Si alloy...... 65 Figure 7.16 The 30wt%Si-Cu alloy core ...... 66 Figure 7.17 The experimental setup for measuring volume of the alloy core ...... 67 Figure 7.18 Schematic of cutting the alloy core ...... 69 Figure 8.1 Macroshots of the alloys during and after the experiments...... 73 Figure 8.2 The 30 and 75wt%Si Alloy cores after the 24 hr experiment ...... 75 Figure 8.3 Sintered crumble layer from the 30wt%Si alloy ...... 76 Figure 8.4 SEM of the crumble layer from the 30wt%Si alloy ...... 76 Figure 8.5 EDX analysis result of a crumble...... 77 Figure 8.6 Diffractogram of the crumbles from the 30wt%Si-Cu alloy...... 78 Figure 8.7 Illustration of the proposed silicon extraction mechanisms ...... 79 Figure 9.1 Cu-Si matrix phase which may retain most impurities ...... 84 Figure 9.2 The 30wt%Si alloy and its slag ...... 85 Figure 9.3 EDX analysis on the boundary between the Si dendrites and the matrix...... 86 Figure 9.4 EDX analysis on the needle structure which is silicon eutectic ...... 86

vii LIST OF TABLES

Table 3.1 Test conditions and measurements before the experiments for Cu-Si alloys with various compositions ...... 15 Table 3.2 Measurements after the 24 h experiments ...... 16 Table 3.3 Test conditions and measurements before experiment for different deposition surface areas...... 17 Table 3.4 Measurements after the experiments for different deposition surface areas .... 18 Table 3.5 Test conditions and measurements before the experiments for the different alloy surface areas ...... 19 Table 3.6 Measurements after experiment for different Cu-Si surface areas ...... 20 Table 4.1 purity level in MG-Si...... 26 Table 4.2 Reactor Tube Dimension ...... 28 Table 4.3 Purity level of silicon staring rods ...... 31 Table 6.1 Measurements after the 10 hrs experiments using different number of pumps 45 Table 7.1 Test conditions and measurements before the experiment...... 49 Table 7.2 Measurements after 5.5 h of experiment for Cu-Si alloy ...... 50 Table 7.3 Test conditions and measurements before the experiment...... 56 Table 7.4 Measurements after 5.5 h of experiment for Cu-Si alloy ...... 57 Table 7.5 Test conditions and measurements before the experiment...... 62 Table 7.6 Measurements after 5.5 h of experiment for Cu-Si alloy ...... 63 Table 7.7 the percentage of Si weight losses from silicon in the alloy for different experiments...... 64 Table 7.8 Densities of the 30wt%Si alloy and its core before and after the reaction...... 68 Table 7.9 Si content in the alloy core ...... 70 Table 8.1 Tests conditions and measurements before the experiments for the different Cu- Si alloys...... 72 Table 8.2 Test conditions and measurements after the experiments for the different Cu-Si alloys...... 74 Table 8.3 Measurements of the crumbles before and after the 24 hr experiment...... 78 Table 9.1 Impurity analysis results by GDMS on the silicon deposit using the 30wt%Si alloy...... 81 Table 9.2 Impurity analysis results by GDMS on the silicon deposit using the 75wt%Si alloy...... 81 Table 9.3 Average ICP analysis results on impurities in the 30wt%Si alloy ...... 82 Table 9.4 Impurity analysis result by GDMS on MG-Si...... 83 Table 9.5 Impurity analysis result by GDMS on Cu used for alloying with MG-Si ...... 83

viii 1. INTRODUCTION

People are more than ever interested in solar energy. Reasons for this are:

• The sun provides a virtually unlimited supply of solar energy • No green house gas emissions • Silicon, the backbone material used in solar cells, is one of the most abundant elements on Earth. • The solar energy received by the earth far exceeds all other potential energy sources (Fig. 1.1)

Flows of Various Energies per Year

10000

1000

100

Energy(ZJ) 10

0.1 Solar Energy Wind Biomass World Energy Consumption

Figure 1.1 Energy Flux comparison [1-5]

In spite of these benefits, solar cells only produced around 0.018% of the total electric energy in the United States in 2004 (Fig. 1.2).

Figure 1.2 U.S. Electricity Generation by Energy Source, 2004 [6]

The reason that solar energy contributes only a small portion of electric generation is that the cost of producing electricity by utilizing solar cells is very high. The high cost of solar cell modulus is partly due to high purity requirement and high energy demand for refining solar grade silicon. According to silicon impurity levels, silicon can be classified into three types: metallurgical grade silicon (MG-Si), solar grade silicon (SG-Si) and electronic grade silicon (EG-Si). Solar grade silicon is the one used for solar cells.

10000

1000

Metallurgical 100 Grade Silicon ppm) 10 y Level (

it 1 Impur 0.1 Solar Grade Silicon

0.01

Electronic 0.001 Grade Silicon 0 20406080100 Cost (US$/kg)

Figure 1.3 Cost comparison between different grades of silicon, 2003 [7]

2 The cost of solar grade silicon is about one third of the total solar cell module cost. Even though the price of solar cell modules is as expensive as $4/watt, in recent years, with more concerns about global warming and energy crisis, more companies have entered into the photovoltaic industry [8].

Solar grade silicon presently mainly comes from off-specification electronic grade silicon. With the explosive growth of the photovoltaic industry, market demand for SG-Si has far exceeded the off-spec EG-Si supply. As a result, the market price of SG-Si has already skyrocketed from 30US$/kg to 260US$[9].

Figure 1.4 Predicted solar grade silicon demand and supply

The SG-Si shortage problem cannot economically be solved by expanding current EG-Si plants. There are so many purification processes involved in EG-Si plants, and purity level of the produced silicon far exceeds the needs of solar cells. Thus, a newly proposed chemical vapor deposition (CVD) process utilizing a Cu-Si alloy as Si feeds is being developed to produce silicon with sufficient purity level in a more cost-effective manner. The role of Cu in the alloy is to trap impurities and catalyze the gas-silicon reaction. A feasibility study of the process was successfully demonstrated at the lab prototype stage [7][10]. However, the mechanism of the reaction and behaviour of the impurities from MG-Si are still unclear.

3 2. OBJECTIVES

The proposed CVD process is a modification of the standard Siemens process, which is used to produce electronic grade silicon. After trichlorosilane (HSiCl3, or TCS) is produced and refined, it is passed into a Siemens reactor and then decomposes into high purity silicon that will deposit on electrically heated silicon rods. The new proposed silicon refining process combines the production of trichlorosilane (HSiCl3, or TCS) and silicon deposition in one single reactor. The motivation of this combination is to cut the cost of the silicon refining process, while still maintains sufficient purity levels.

This thesis focuses on the investigation of rate limiting steps, extraction mechanisms and impurity behaviour of this simple reactor silicon refining process.

4 3. LITERATURE REVIEWS

3.1 ELECTRONIC GRADE SILICON PRODUCTION

A schematic of the electronic grade silicon production is illustrated in Figure 3.1. [11]

Figure 3.1 Schematic drawing of electronic grade silicon production

5 3.1.1 Metallurgical Grade Silicon Production

The earth’s crust contains about 28% silicon bound as SiO2 silicates and a large fraction of this exists as relatively pure SiO2. To produce high purity metallurgical grade silicon,

carbon is utilized to reduce SiO2 in arc furnaces with idealized carbothermal reaction shown in Equation 3.1:

SiO2 + 2C = Si + 2CO [3.1]

A silicon metal plant that is designed based on above equation is shown in Figure 3.2. [12]

Figure 3.2 A typical plant for the production of silicon metal

6 o The hottest area in the furnace is about 2000 C. At this high temperature SiO2 is reduced to molten silicon by reacting with carbon, which is tapped from the furnace through a taphole at the bottom and refined by slag treatment or gas purging. [12]

3.1.2 Trichlorosilane Production

The metallurgical grade silicon produced from arc furnaces reacts with HCl gases in fluidized bed reactors at 300oC to form a silicon-containing gas [10]:

300°C Si(s) + 3HCl( g) ⎯⎯→⎯ HSiCl3(g ) + H 2( g) [3.2]

Approximately 90% of the gas produced from the fluidized bed reactor is HSiCl3, the rest

being mainly SiCl4, which can be converted to HSiCl3 by reacting with H2 in the presence of MG-Si and copper catalyst:

Cu 3SiCl4( g ) + 2H 2( g ) + Si(s) ⎯⎯→ 4HSiCl3(g ) [3.3]

The produced HSiCl3 can be purified to a very high level by distillation, since chlorides formed by other metal impurities have different boiling points. After the purification process, pure HSiCl3 gas is ready for chemical vapor deposition (CVD) process.

3.1.3 Siemens Reactor

The Siemens process is a kind of chemical vapor deposition (CVD) process. HSiCl3 is flushed into a quartz bell jar reactor shown in Figure 3.3 and reduced by H2 to form bulk polysilicon:

1100°C HSiCl3( g) + H 2( g) ⎯⎯→⎯ Si(s) + 3HCl( g) [3.4]

7 The reduced polysilicon is deposited on a U-shaped silicon filament which is resistively heated to 1100oC.

The major by-product of this process is SiCl4, which is produced by:

SiHCl3(g ) + HCl(g ) → SiCl4(g ) + H 2(g ) [3.5]

SiCl4 can either be recycled back to the fluidized bed reactor forming HSiCl3 again by Equation 3.3 or be used as a silicon source for deposition thin-films of silicon.

Figure 3.3 Siemens Reactor [13]

The polysilicon produced from the Siemens reactor requires further processing, which is called Czochralski (Cz) or Floating Zone (Fz) method for electronic device applications.

8 Cz and Fz are the standard methods employed in the semiconductor industry for single- crystalline silicon growth from polysilicon. [13]. During the whole refining process, the off-specification and chopped off silicon will be used in the photovoltaic industry.

3.2 PROCESSES FOR PURIFICATION OF SOLAR GRADE SILICON

Silicon purification processes can be classified into three basic types:

1. Adaptations and modifications of the Siemens process. Formed of volatile silicon compounds, such as silane or chlorosilanes, which can be purified by distillation and from which high-purity silicon is recovered by pyrolysis.

2. Unconventional chemical pathways. Impurities are removed from soluble silicates or organosilanes, which are then reduced to silicon.

3. Metallurgical routes. Combination of directional solidification, slagging, reactive gas blowing, vacuum degassing, or acid Leaching. [14]

The purity specification required for silicon solar cell can be achieved without a doubt by a Siemens type process, however it is energy intensive and costly.

3.3 Cu-Si ALLOYS AS SILICON SUPPLY SOURCES

Research shows that Cu-Si alloys composed of a primary silicon phase, which is embedded in a Cu-Si matrix (see Figure 3.4). Cu-Si matrix phase can function as a “filter” to trap the impurities from metallurgical grade silicon. With the trapped impurities inside the alloy, pure HSiCl3 is able to form inside the reaction chamber. Due to the high purity level of the TCS formed, the distillation process is not required. Therefore, it should be possible to combine TCS formation and Si deposition in one single reactor. [15]

Figure 3.4 SEM illustration of 25wt%Si-Cu Alloy. Dark phase is the primary Si phase, lighter phase is the Cu-Si matrix.

In the experiments conducted by Tejedor et al [15], hypereutectic Cu-Si alloys containing 25wt% Si were used as the silicon source. The reaction condition was similar to the Siemens process. The reactor was first evacuated to a pressure of 30 to 50 Torres, and

then filled with gaseous HCl and H2. A graphite plate filament (65mm × 25mm ×1mm) was resistively heated to 1050-1300oC and the Cu-Si alloy slabs were radiatively heated to 600-700o. The experimental set-up is showed in Figure 3.5

Figure 3.5 schematic drawing of experimental reactor. [15]

In her work, the silicon deposition rate was around 0.61mm/hr. She claimed that the silicon deposition area was the rate limiting factor. From the Cu-Si phase diagram illustrated in Figure 3.6, it is seen that the η phase Cu-Si matrix is Cu3Si. [16][17] The purposes of alloying MG-Si with Cu are to catalyze the formation of TCS from silicon phase and to trap impurities in the Cu rich eutectic phase.

Figure 3.6 Cu-Si Binary Phase Diagram [16]

Many impurities exist in MG-Si, such as Al, B, Ba, Ca, Cr, Fe, Mg, Mn, Mo, Ni, P, Ti, V and Zr. Tejedor and Olson explained different mechanisms to the overall purification. [15] These impurities can be classified into four groups: impurities retained in slag formation during casting Cu-Si alloys; those retained very efficiently by the Cu3Si phase; those

12 retained with medium efficiency; and those incorporated almost fully into the vapor phase.

Impurities effectively retained by slag formation of Cu2O/SiO2 were Al and Ca.

Impurities retained by the Cu rich alloy with high efficiency were B, Ba, Ca, Cr, Fe, Mg, Mo, and Ni. The lower concentration of these elements found in the transported silicon was claimed to be due to their lower diffusion coefficients in the η-Cu3Si phase.

Less retained impurities are P, Ti, V and Zr. All of these impurities can form chlorides. However, the formation of chlorides on the alloys is favored at high alloy temperatures. Hence the depositions of those impurities onto the Si filament where the temperature is even higher than at the alloys were avoided.

Poorly retained impurities, Al and Mn, reacted with HCl to form AlCl3 and MnCl2, which condensed on the cold wall of the reactor due to their high boiling points [15]

The mass balance chart shown in Figure 3.7 supported Tejedor and Olson’s explanation. In conclusion, the work done by Tejedor et al demonstrated the possibility of using Cu-Si alloys as a Si source for refining MG-Si to SG-Si.

Figure 3.7 Relative contributions of the different mechanisms to the overall purifications [15]

3.4 DEPOSITION VARIABLES

Fan [10] conducted similar experiments which adapted Tejedor’s set-up to investigate variables affecting the silicon deposition. Studying those variables will help find out the rate limiting step in the new CVD process. Deposition rates in a traditional Siemens reactor can be increased by: increasing rod temperature; increasing TCS mole ratio; increasing the flow rate of TCS over the Si rod; increasing deposition area; increasing HCl removing rate over Si rod comparing to TCS and increasing pressure of the reactor. [18] However, TCS in the Siemens reactor is produced in a prior step, so the production of TCS is not a concern in the reactor. But in Fan’s reactor, which combined both formation of TCS and deposition of Si, the production of TCS from Cu-Si alloys has to be considered too. Fan suggested that the Cu-Si alloy composition and its surface area also influence the deposition rate. However, it has to be bear in mind that the combined implementation of these variables is complex. Since by changing one variable, the other

14 variables usually will be affected. And some changes may alter the chemistry of chlorides, such as formation of dichlorosilane, which will affect the deposition rate.

3.4.1 Cu-Si Alloy Compositions

Four tests with similar reaction conditions except composition of the Cu-Si alloys were conducted. Details about specifications of tests were listed in Table 3.1

Table 3.1 Test conditions and measurements before the experiments for Cu-Si alloys with various compositions [10] Test C1 Test C2 Test C3 Test C4 Si content (wt%) 15 25 30 50 Length (cm) 15.5 15.1 15.7 15.2 Si Average 5.0 4.86 5.00 4.95 Starting Diameter (mm) Rods Mass (g) 6.95 6.70 7.11 6.62 Si dendrite Si dendrite Si dendrite Solid Microstructure in Cu3Si in Cu3Si in Cu3Si solution Matrix Matrix Matrix Cu-Si Average Alloys 4.38 4.16 4.34 4.36 Thickness (mm) Length (mm) 83 95 105 104 Mass (g) 145.80 112.39 124.08 84.58 Experimental Duration (h) 24 24 24 24

The Cu-Si alloy used in Test C1 was a hypoeutectic alloy whose microstructure is different from that of hypereutectic alloys used in Tests 2-4.

The amount of silicon recovered from the Cu-Si alloy is shown in Table 3.2. The results are normalized to per length of silicon deposited so that they are comparable.

15 Table 3.2 Measurements after the 24 h experiments [10] Test C1 Test C2 Test C3 Test C4 Deposition 10.4 10.6 10.5 10.5 Length (cm) Average 8.47 8.20 7.45 11.23 Diameter (mm) Diameter 3.48 3.34 2.45 6.28 Gained (mm) Diameter Si Growth Rate 0.145 0.139 0.102 0.262 Starting (mm/h) Rods Mass (g) 13.54 14.21 13.39 23.52 Gained (g) 6.59 7.51 5.29 16.89 Deposition Rate 0.27 0.31 0.22 0.70 (g/h) Deposition Rate/Length 0.026 0.030 0.021 0.067 (g/h/cm) Cu-Si Mass (g) 138.50 102.02 115.67 65.76 Alloys Loss (g) 7.3 10.37 8.41 18.82

It is seen that the Si deposition rate utilizing hypereutectic alloys may depend on the composition of alloys. It appears that the higher Si wt%, the higher the deposition rate. Since with higher Si wt%, alloys contain more Si primary phases, which in turn may suggest the TCS was formed from the Si primary phase. However, because of the result of Test C3, which shows a slower rate than that of Test C2, the influence of Cu-Si alloys compositions on the deposition rate still needs to be verified.

16 3.4.2 Deposition Surface Areas

Three tests conducted by Fan were used to demonstrate the effect of deposition surface area on the silicon deposition rate. Silicon rods with different lengths but same diameter were used in these experiments. The details about the silicon rods and Cu-Si alloys for each test are listed in Table 3.3.

Table 3.3 Test conditions and measurements before experiment for different deposition surface areas Test L1 Test L2 Test L3 Intermediate Testing Condition Long rod Short rod rod Length (cm) 13.2 10.5 8.5 Si Starting Average 4.98 4.99 4.97 Rods Diameter (mm) Mass (g) 6.06 4.81 3.72 Average 9.6 9.7 N/A Cu-Si Thickness (mm) Alloys Length (mm) 94 87 106 Mass (g) 402 388 439 Experimental Duration (h) 27.5 51.5 69

The alloys used were 30wt%Si-Cu alloy. The temperature of the reactor was kept between 560 and 660oC.

The measurements of the silicon rods and Cu-Si alloys after the experiments are listed in Table 3.4. Fan suggested that the deposition rate on a weight basis is linearly to the silicon rod length and the deposition rate per unit deposit length did not vary significantly, which may suggest that TCS production is not a rate limiting step. Since the amount of TCS arrived on the silicon rod surface per unit length was the same.

17 Table 3.4 Measurements after the experiments for different deposition surface areas Test L1 Test L2 Test L3 Deposition Length (cm) 9.0 6.5 4.3 Diameter (mm) 12.38 16.17 17.50 Gained (mm) 7.4 11.2 12.5 Diameter growth rate 0.269 0.217 0.181 (mm/h) Si Rod Mass (g) 25.49 30.80 24.60 Gained (g) 19.4 26.0 20.9 Deposition Rate (g/h) 0.69 0.50 0.30 Deposition Rate/ Length 0.077 0.077 0.070 (g/h/cm) Mass (g) 380.2 407.5 364.2 Cu-Si Alloy Loss (g) 22 32 24 Experimental Duration (h) 27.5 51.5 69

However, the statement made by Fan is not completely accurate. The deposition surface area kept increasing with reaction time, since more and more Si deposited on the Si starting rod. If the TCS production is not rate limiting step and the supply of TCS is always enough for deposition, the diameter growth rate (or layer growth rate) should be the same. By utilizing Si rods as deposition filaments, with increasing diameter of the rod, the deposition rate on a weight basis should increase. Fan’s after experiment measurements clearly showed that the diameter growth rates in the three tests were not the same. The diameter growth rate in Test L1 was the highest among the three tests. Assuming the amount of TCS was the same at the beginning of the three tests (this is a reasonable assumption, since the amount of initial HCl/H2 mixture was the same in the three test as well as the amount of Cu-Si alloys), with increased reaction time, the production of the TCS would decrease because the silicon in the charged alloys kept being consumed in this batch process. When the TCS consumption rate from hot silicon rod side was faster than the TCS formation rate from the Cu-Si alloys, due to keep increasing deposition surface area, the deposition rate started to decrease. If the reaction still continued running after at, the deposition rate would further decrease. Even though the length of the Si rod used in Test L1 was longest, which in turn means largest surface area, the TCS consumption in Test L1 was faster than for the other two tests, but its reaction time was the shortest, which might lead to its largest layer deposition rate.

18 Therefore, TCS production might be the rate limiting step after sometime when the TCS consumption rate is faster than the formation rate. However, more tests are needed to verify this. Since all experimental results in Fan’s investigation were collected after the experiment, there was no instantaneous data during the test to tell how the deposition rate dropped.

3.4.3 Surface Areas of Cu-Si Alloys

The tests with two extremes of surface areas of 30wt%Si-Cu alloys were conducted to demonstrate whether various surface areas of alloys could affect Si deposition rate: 1. A regular cylindrical alloy (small surface area) 2. Very fine pellets (large surface area) Reaction conditions were similar in these two tests except the forms of the Si-Cu alloys. The details about the silicon rods and Cu-Si alloys for each test are listed in Table 3.5.

Table 3.5 Test conditions and measurements before the experiments for the different alloy surface areas [10] Test S1 Test S2 Half cylindrical Test Condition Fine pellets alloy Length (cm) 13.2 13.3 Si Starting Rod Average Diameter (mm) 4.98 5.02 Mass (g) 6.06 6.03 Average Thickness 9.6 N/A (mm) Length (mm) 94 N/A Cu-Si Alloy 0.589-1.65: 234g Pellet Diameter (mm) N/A 1.65-4: 69g Surface Area (cm2) 201 2017 Mass (g) 402 297 Experiment Duration (h) 27.5 27.5

19 Larger surface area should result in a higher silicon extraction rate. Even if extraction reaction is not the rate limiting step, the deposition rate should at least be the same. However, the results of the two tests tabulated in Table 3.6 show instead, a decrease in the deposition rate for the sample with the highest surface area. It was found that the Si pellets were sintered together after the experiments, which might decrease its actual surface area. EDX analysis on the sintered layer also revealed high silicon content, which may suggest that silicon back deposition occurred.

Table 3.6 Measurements after experiment for different Cu-Si surface areas Test S1 Test S2 Test condition Cylindrical alloy Pellet alloy Deposition Length 9.04 8.7 (cm) Diameter (mm) 12.38 11.34 Gained (mm) 7.4 6.3 Diameter growth 0.27 0.23 rate (mm/h) Si Rod Mass (g) 25.49 21.12 Gained 19.4 15.1 Deposition Rate 0.71 0.55 (g/h) Deposition 0.078 0.063 Rate/length (g/h/cm) Mass (g) 380.2 278.7 Cu-Si Alloy Loss 21.8 18.3

Another experiment done in Ebner (Linz, Austria) showed that a larger surface area has a positive impact on deposition rates. The reactor set-up is shown in Figure 3.8.

Figure 3.8 Experimental set-up in Ebner

The two tests utilizing 30wt%Si-Cu alloys but different stack ups, illustrated in Figure 3.9, were conducted to demonstrate the influence of larger surface areas on deposition rates.

Figure 3.9 Different alloys stacking up

Both reactions were performed for 20 hrs, and the reaction conditions were nearly the same, except the alloy weight in Test #1 that was 30% more than for Test #2. In Test #1 three alloy pucks were placed on the top of each other, and it was difficult for the gas flow to reach the gaps between the pucks. Several copper rings were added into the system in Test #2 to allow the gases to flow around the two pucks. Since the alloys could be exposed to more gas flow in Test #2, which in turn means there was more affective alloy surface area in Test #2 than that in Test #1, more TCS should be produced. The results showed that the Si deposition rate in Test #2 was 43% higher than in Test #1. Thus, this suggested that the TCS production rate might control the overall deposition rate. [20]

22 4. EXPERIMENTAL CONDITIONS AND PROCEDURE

The process set-up used in this investigation is very similar to Fan’s design in which two major reactions are involved:

Silicon Extraction: HSiCl3(g)+H2(g)=Si+3HCl(g) [4.1] Silicon Deposition: Si+3HCl(g)=HSiCl3(g)+H2(g) [4.2]

Both reactions occur in the same reactor but at different locations where the temperatures are significantly different. The advantage of having this kind of reactor is to cut costs of the process compared to the Siemens process by eliminating the fluidized bed reactor and distillation columns used in HSiCl3 production.

4.1 EXPERIMENTAL CONDITION DETERMINATION

In Fan’s and Tejedor’s investigations, the temperatures of silicon extraction reaction and silicon deposition reaction were controlled between 1100-1300oC and between 400- 600oC respectively, according to chemical thermodynamic analysis. To further confirm these conditions, a thermodynamic analysis was performed by utilizing HSC chemistry [21].

Figure 4.1 shows the equilibrium partial pressures of possible gas species formed from H, Cl and Si at different temperatures with excess solid silicon. The plot is used to determine which species are dominant at a certain temperature. The formation of copper gaseous species was also studies, and it was found that its amount was insignificant compare to o the silicon gaseous species. It can be seen that at temperatures below 1100 C, HSiCl3 o and SiCl4 are the major species while above 1100 C, SiCl2 starts to be dominant. It is o o interesting that the SiCl4 pressure at 1100 C is basically the same as at 400 C. This means that at those temperatures, there will be minimal reactions involving SiCl4 after the initial start up period.

23 Therefore, it appears that the most involved species is SiHCl3. It forms up to a pressure of almost 0.09 bars at 500oC, which at 1100oC it can only remain at 0,043 bars, meaning that roughly half of the SiHCl3 will decompose.

Figure 4.1 Partial pressure of possible gaseous species from H, Cl, and Si.

To determine optimum conditions we have to include all SiHxCly gas species. If we assume that we have thermodynamic equilibrium at both of the cold and hot reaction sites, then we can use this information to optimize the pressure condition. The sum of the equilibrium partial pressures of the various Si gaseous species is plotted in Figure 4.2.

1200oC 810oC

Si Gaseous Species Formation Favored

Figure 4.2 Sums of the equilibrium partial pressures of silicon gaseous species (HCl-H2-Si mol ratio is 1:1:50)

It can be seen that the lower the temperature, the more silicon can be extracted into the gas phase. The minimum total pressure is at about 1150-1200oC, which should be close to the optimum deposition temperature. At temperatures above 1200oC, the sum of partial pressures starts to increase so deposition is not favoured anymore. Therefore, thermodynamically speaking, the MG-Si should be kept at low temperature, like 300oC to allow more HSiCl3 formation. However, the actual temperature of the MG-Si was kept between 500-700oC to speed up the reaction kinetically. The silicon starting rod was kept between 1000-1100oC to favour Si deposition. Higher temperature would lead to higher energy losses as well as material issues.

Another consideration is the behavior of the various impurities. Some impurities can also form gaseous chlorides as discussed in Section 3. Would the formation of those impurity chlorides be favorable at the operation temperatures just decided above?

25 Chemical analysis showed that the major impurities in the metallurgical grade silicon used in the process are Fe, Al, Mn, Ti Cu, B and P shown in Table 4.1. The detailed analysis report is given in Appendix A.

Table 4.1 purity levels in MG-Si Elements ppmw Elements ppmw Elements ppmw Elements ppmw Si Major Ta <5 Ag <0.3 Sb <0.1 Fe 2800 Zn 4.5 Cd <0.3 K 0.072 Al 335 V 1.7 Te <0.3 S 0.069 Mn 55 Ge 1.6 Sr 0.23 As 0.065 Ti 35 Co 1.3 Pr 0.2 Pb <0.05 Cu 24 Ce 1.3 W 0.2 Bi <0.05 B 18 Nd 1.2 Nb 0.17 Th 0.039 P 16 Mo 0.71 Y 0.11 U 0.035 Cr 8.7 Mg 0.7 Na 0.1 Li 0.033 Ni 7.1 La 0.52 F <0.1 Zr 7 Cl 0.31 In <0.1 Ca 5.6 Se <0.3 Sn <0.1

The sums of the equilibrium partial pressure of Si, B, Al, Fe, P, Ti and Mn gas species are plotted in Figure 4.3. It is calculated for pure solid phases (Si, B, Al, Fe, P, Ti and Mn) for illustration purposes. The gaseous species of each element are listed in Appendix B. It clearly shows that with increasing temperature, the formations of Al, Fe, Mn, Ti and P gaseous species are more favored. Hence, those impurities should not deposit on the Si starting rod where the temperature is the hottest in the reactor. On the other hand, Boron may be a problem.

Figure 4.3 Sums of the equilibrium partial pressures of Al, Fe, B, Ti, Mn and P gaseous species

4.2 REACTOR DESIGN

The schematic drawing below illustrates the experimental set-up.

Figure 4.4 schematic of experimental set-up

4.2.1 Reactor

27 The silicon starting rod used is electronic grade silicon which will not introduce impurities to contaminate the deposited Si on the rod. The rod will be resistively heated to between 1000-1100oC by a DC power supply, which is shown in Figure 4.5. The radiation emitted from the rod will heat up and keep the Cu-Si alloy between 500-700oC. Attached water-cooling pipe to brass end caps and cooling air at the bottom of the quartz tube are used to prevent overheating. The temperature difference will produce HSiCl3 at the Cu-Si alloy side and transport it from the alloy side where the temperature is low, to the hot silicon filament by natural convection. Finally, the transported HSiCl3 can react with H2 to form solar grade polysilicon onto the hot starting silicon rod.

Figure 4.5 DC Power Supply

The reactor tube is made of quartz for three reasons: first quartz is transparent, through which observations can be done; second, quartz will not block the wavelength which emitted from resistively heated silicon rod, so the temperature of the rod can be monitored by optical pyrometer; and thirdly it is basically inert to HCl-H2 gases at those temperature. The dimension of the reactor is listed in Table 4.2

Table 4.2 Reactor Tube Dimension Outer Inner Volume of Length (cm) Diameter (cm) Diameter (cm) Reactor (cm3) Quart Tube 7.6 7.4 40.6 1746

The brass end caps are used to seal the reactor to make sure there are no gas leakages. One of three holes through the caps will allow the Cu rod connect to the silicon starting rod which can be resistively heated by passing current. The thermocouple and HCl & H2 gases inlet/outlet pipes go through the other two holes.

Chromel external heating elements are coiled outside of the quartz tube to heat the reactor above 700oC at which the silicon-starting rod starts to conduct current to be resistively heated.

4.2.2 Cu-Si Alloys

The investigation done by Fan [10] shows that hypereutectic Cu-Si alloy seems to have a higher silicon growth rate than a hypoeutectic alloy. Therefore Cu-Si alloys used in my experiments are all hypereutectic alloys. Cu and MG-Si were melted in an induction furnace. During melting, they were protected from air by blowing Ar gas above the charge. Once the alloy was molten, it was poured into high purity graphite moulds to form either puck or square form alloys shown in Figure 4.6. Cast alloys would solidify quickly under normal atmosphere and room temperature. Alloys with different compositions 30, 50 and 75wt%Si were made by the same method.

Figure 4.6 Cast Cu-Si alloys in puck and square shapes

Then the cast alloys were shaped to rectangles to have similar dimensions, in other words, similar reaction surface area, shown in Figure 4.7. Alloys were placed on Al2O3 dishes to allow for weight determinations as well as collection of any dust formed.

Figure 4.7 Alloys in rectangle shape

4.2.3 Silicon Starting Rod

The electronic grade silicon used in the process is roughly 5 mm in diameter shown in Figure 4.8. The lengths of the rod varied in different experiments. Chemical analysis results tabulated in Table 4.3 show impurities level in the ppb range (detailed report in Appendix A)

Figure 4.8 Electronic grade silicon starting rod

30 Table 4.3 Purity levels of silicon staring rods ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw Si Major Pb <0.05 W <0.01 Ta <5 Bi <0.05 Th <0.01 Cl 0.33 Fe 0.048 U <0.01 Ge <0.3 As <0.03 Cr 0.009 Se <0.3 K 0.021 Na 0.007 Ag <0.3 Cu 0.018 Ni 0.005 Cd <0.3 B 0.016 S <0.005 Te <0.3 Li <0.01 Al 0.004 F <0.1 P <0.01 Mg 0.003 In <0.1 Ca <0.01 Mn 0.003 Sn <0.1 Zr <0.01 Ti <0.001 Sb <0.1 Nb <0.01 V <0.001 Zn <0.05 Mo <0.01 Co <0.001

An actual reactor set-up picture is shown in Figure 4.9.

6 7

4 24 3 3 1 5

Figure 4.9 Actual reactor set-up

Where, 1. Starting Cu-Si alloy rectangle 2. Silicon starting rod 3. Graphite connector

31 4. Cu rod which connected with wires to the DC power supply

5. Al2O3 dish 6. Chromel external heating element 7. Quartz tube

4.2.4 Power Supply

Two power supplies were used in the experiments:

Main DC power supply provides the energy to resistively heat the Si starting rod (Figure 4.10 (a)) Supplemental AC power supply provides the energy for external heating elements which will heat the reactor to 700oC (Figure 4.10 (b))

(a) Main DC power supply (b) External heating AC power supply

Figure 4.10 Power supplies (a) Main DC power supply (b) External heating AC power supply

The DC power supply can provide max 200 A and 15 V. It can also supply either constant current or constant voltage. Before the Si starting rod could conduct current, the power supply was set to be constant voltage at its max output 15 V. Once the Si starting rod started to conduct as the temperature increased to around 700oC, the power supply

32 was switched to constant current. To maintain the temperature of the Si starting rod in the range of 1000-1100oC, the current was usually kept in between 50-80A according to the length and diameter of the rod as well as the reaction time. The reason that the DC power supply was switched to constant current was to protect the circuit of the DC power supply. With increasing diameter of the Si starting rod due to Si deposition, its resistance will keep decreasing, which is shown in Equation 4.1:

ρ • L R = [4.3] A

Where L is the length of the object and A is the cross-sectional area, and R is resistance. According to Ohm’s Law (Equation [4.4]), voltage (V) is proportional to current (I) and the proportionality constant is resistance (R).

V = R • I [4.4]

Therefore, when R decreases, with a constant current supply, the voltage will drop. The power supply will never overload to burn the circuit. However, if still using constant voltage without switching to constant current, with decreasing R, the current will keep increasing. Once the current passes the max current allowance of 200 A, the fuse on the circuit of the power supply will burn out.

4.2.5 Temperature Measurement Device

The temperatures of the silicon starting rod and the Cu-Si alloy were measured by an Infrared thermometer (shown in Figure 4.11) and a K-type thermal couple respectively.

Figure 4.11 M770S two color infrared sensor

4.3 EXPERIMENTAL PROCEDURES

4.3.1 Prior to Experiments

The silicon starting rods were rinsed by acetone to clean off any possible organics on the surface. Ideally, HF is the best solution to remove the silicon oxide which can hinder or affect silicon deposition on the surface of the silicon rod. However, the usage of HF in the lab is prohibited due to safety concerns. Thus, the silicon starting rods were only cleaned with acetone. The Cu-Si alloys surfaces were ground and polished to sustain a flat surface area to make calculations of the reaction surface area easier.

Prior to each experiment, the dimensions and the weight of the silicon starting rods and the Cu-Si alloys were measured and recorded in the work log. Pictures of the starting rods and the alloys were taken to compare visually with the ones after the reactions.

34 Gas leakage check was performed before each experiment by passing Ar gas through the reactor. Since the process is a batch process, gas leakage would decrease the amount of the gases (HCl and H2) within the reactor, which would affect the reaction rate. Further, any air in leakage would cause problems. After fixing any leakage problem, the experiment was ready to run.

4.3.2 During the Experiment

Silicon is a semi-conductor at room temperature, so a current cannot pass through the silicon rod at room temperature. The heating elements coiled outside of the reactor heated up the reactor to the temperature at which the silicon started to conduct. A thermocouple was contacted with the side of the alloy to monitor its temperature, and an optical pyrometer was pointing at the silicon starting rod to gain its real time temperature readings. Argon gas was continuously purged into the reactor during the heating up stage to prevent the graphite connectors from burning with air while the brass end caps were cooled with running tap water. From observation, it is found that as the temperature of the reactor reached to 700oC, the DC power supply could pass a current on the silicon- starting rod, and then the external heating could be shut off. The duration of the heating up stage with Ar flushing (1.5L/min) was around one hour. When the temperatures of the silicon rod and Cu-Si alloy reached 1000-1100oC and 500-650oC ranges respectively, 1:1 mole ratio HCl/H2 gas mixtures were purged into the reactor for a period of time to fill up the reactor. Usually the gas mixtures were fed for 5 mins with 1.5L/min flow rate. After the reactor was filled with the HCl/H2 mixture, the valves were closed. During the experiments, the temperatures of the alloys and the silicon rod were recorded, as well as the pictures of the alloys and the silicon rod.

35 4.3.3 After the Experiment

Upon the completion of the experiment, the DC power supply was shut off and Ar gas was purged through the reactor continuously until the temperature of the reactor cooled to room temperature. The dimensions and masses of the alloys and the silicon rod were measured and recorded. The pictures of the alloys and the silicon rod were also taken for visual comparisons and studies later. Usually microscopy study and chemical analysis were done.

4.4 EXPERIMENTAL UNCERTAINTIES

Nearly every single experiment is irreproducible due to some uncertainties:

1. Oxides on the silicon starting rod. The silicon starting rods were cleaned with acetone prior to the experiments. However, the oxides on the rod could only be removed by HF which is strictly prohibited from the lab for safety concerns. Hence the oxides remaining on the rod would affect the silicon nucleation, which probably can cause the uneven silicon growth on the rod.

2. Temperature control on the silicon rod and the Cu-Si alloy. Cu-Si alloys were radiatively heated by the hot silicon rod. When the temperature of the rod was set to around 1100oC, the temperatures of the alloys could only be controlled in a temperature range. Even 30oC differences on the alloys would lead to varied silicon extraction rate.

3. Cu-Si alloys from different casts. The Cu-Si alloys charged into the reactor were not from the same casting batch. The impurity levels for different casting may be different, and could affect impurity level in silicon deposit.

36 5. SILICON DEPOSITION CALCULATION

In order to monitor the silicon instantaneous growth rate, the data of current and voltage on the silicon starting rod as well as its temperature were recorded by Data Acquisition and optical pyrometer which were linked to a computer. Figure 5.1 shows a schematic drawing of the connection of the Si starting rod and the energy source. (DC power supply)

Figure 5.1 Basic Experimental Schematic for Si Deposition Calculation

Ohm’s Law states that the current (I) passing through a conductor between two points is directly proportional to the potential difference (voltage, V) across the two points, and inversely proportional to the resistance (R) between them. The mathematical equation that describe this relationship is:

V = R • I [5.1]

37 The relationship between resistivity (ρ) and resistance R is

ρ • L R = [5.2] A

Where L is the length of the object and A is the cross-sectional area. Since the cross- section of the silicon rod is a circle, the area of the cross-section is:

2 ⎛ D ⎞ πD 2 A = πr 2 = π ⎜ ⎟ = [5.3] ⎝ 2 ⎠ 4

Where r is the radius of the rod, and D is the diameter of the rod. Therefore, the instantaneous diameter is:

I × L × ρ D = 2 [5.4] Vrod ×π

Conductivity (σ) is the inverse of resistivity (ρ), shown in Equation 5.5:

1 σ = [5.5] ρ

Thus, the instantaneous diameter becomes:

I × L D = 2 [5.6] Vrod ×π ×σ

The voltage in equation 5.6 should be the potential drop on the silicon rod, but the voltage reading from DATA ACQUISITION is the voltage drop of the whole system. It is found that the voltage drop on connecting wires which connect the DC power supply and the reactor is negligible. Hence, to find out the voltage across the silicon rod, the

38 voltage on the graphite connectors have to be found first. The graphite connectors and the silicon starting rod were connected in series. So they all carry the same current, which is the current reading from DATA ACQUISITION. So the ohm’s law becomes:

V = RTotal • I [5.7]

V = Vrod +Vconnectors [5.8]

V Where, R = R + R = [5.9] Total Rod connectors I

L Substitute [5.5] into [5.2] Rrod = [5.10] A0 •σ

Where Ao is rod initial diameter, thus,

V L Rconnectors = − [5.11] I A0 •σ

Due to the nature of silicon’s conductivity that is temperature dependent, Rrod is changing with different temperatures. The Arrhenius equation [10] that gives the relationship between silicon conductivity and temperature is:

− 56000 σ (ohm −1cm−1) = 2.16 ×10 4 exp( ) [5.12] RT

The conductivity of graphite is temperature independent, so its resistance is constant regardless of changing temperatures. Thus, during the reactor’s heating up, the resistance of the graphite connectors can be calculated by Equation [5.10]. Then the voltage on the silicon starting rod can be calculated by:

39 Vrod = V −Vconnectors = V − Rconnector • I [5.13]

With calculated Vrod the instantaneous Si diameter can be found by Equation [5.6]. Then the Si diameter can be calculated. The instant silicon weight can also determined from the silicon instantaneous diameter.

m = ρ Si •V = ρ Si • A • L [5.14]

Where ρ is the density of silicon, around 2.33g/cm3. L is the length of the rod, and A is the cross-sectional area of the rod, that can be calculated from the diameter of the silicon shown in Equation [5.3]. Then the mass of the rod becomes:

ρ •π • D 2 • L m = Si [5.15] 2

If the silicon extraction reaction is the rate limiting step, the mass of silicon deposit and the diameter is linear to the reaction time and the square root of the reaction time, respectively,

mExtraction Contral = κ • t [5.16]

ρ •π • D 2 • L m = Si = κ • t [5.17] Extraction Contral 2

2 2 •κ DExtraction Contral = • t [5.18] ρ Si •π • L

2 •κ DExtraction Contral = • t = α • t [5.19] ρ Si •π • L

40 Where κ and α are constants and t is reaction time.

If the silicon deposition is the rate limiting step, the diameter of silicon and the mass deposit should be linear to the reaction time and the square of the reaction time,

DDeposition Contral = υ • t [5.20]

ρ •π • (υ • t)2 • L m = Si = β • t 2 [5.21] Deposition Control 2

Where ν and β are constants and t is reaction time.

The calculated diameter and mass of the rod from Equation 5.6 and 5.15 will be compared with the diameter and mass determined by different rate limiting step.

41 6. RATE LIMITING STEP

The calculated diameter from the record current and voltage was plotted in Figure 6.1. It shows that after the first 4 hrs the diameter of the rod in blue seemed fit to the curve (in red) which was determined by the Si extraction rate. And the relationship between the diameter of the rod and the reaction duration was found to be:

D = 1.5456 • t + 3.7419 [6.1]

Where D is diameter of the rod, in mm, and t is the reaction duration in hr. This indicated that with limited extraction reaction rate, the amount of HSiCl3 produced was fixed regardless of the enlarged deposition area. So the rate of diameter growth rate would decrease. With silicon growth on the rod, more energy required to keep its temperature to be 1100oC. Then current was manually increased over the reaction. The peaks in this curve were caused by sudden current increases. Both mass and diameter curves were plotted in Figure 6.2 to determine which reaction limited the reaction in the first 4 hrs. The linear fits of both curves were very similar, so it was impossible to conclude in the early reaction stage, which reaction was the rate limiting step. Besides, other factors may also cause the HSiCl3 shortage, such as by-products formation or slow mass flow rate. In conclusion, the silicon extraction reaction probably limited the whole refining process.

42 Si rod diameter vs time 12 11 10 9 (mm)

8 7 Diameter 6 5 4 0 5 10 15 20 25 Reaction time (hr)

Figure 6.1 Silicon diameter vs. time

5 y = 0.465x + 4.7883 7 2 4.5 R = 0.9984 6 y = 0.5999x + 2.4087 2 5 4 R = 0.9981 4 3.5 Weight 3 3 Diameter Linear (Diameter) 2 2.5 Linear (Weight) 1 2 0 00.511.522.533.544.5

Figure 6.2 The weight and diameter of the rod in the first few hours’ reaction

6.1 BY PRODUCTS FORMATION

If lots of indecomposable by-products formed after HCl reacted with Si, the HCl amount could keep decreasing, and the reaction speed will slow down. However, based on a mass balance the average production efficiency in all my tests is above 90% (individual production efficiency detailed in Appendix C), which means the amount of indecomposable by-products formed is not enough to affect the CVD process.

Deposited Si η = > 90% [6.1] production Si losses from alloy

6.2 MASS FLOW RATE

The mass transfer rate was also suspected to be a rate-limiting factor. To demonstrate this, different number of gas pumps were used to vary the gas circulation rate. Figure 6.2 and Table 6.1 show that by adding one more pump, the final diameter growth rate and mass growth rate after 10 hrs experiments increased by 21% and 17%. It is still seen that the growth rate still started decreasing after some time. Hence, faster flow rate for sure improved the growth rate, however it is not the rate limiting factor.

Figure 6.3 Diameter vs. time with various pumps

Table 6.1 Measurements after the 10 hrs experiments using different number of pumps One Pump Two Pumps Difference Diameter growth 0.70 0.85 21% rate (mm/h) Mass growth rate per unit length 0.0226 0.0266 17% (g/h/mm)

Therefore the extraction reaction which produces HSiCl3 was suspected to be the one limited the process. However, to study extraction rate, two questions needed to be answer first: -Will both Si from dendrites and Si from matrix react with HCl? -How could Si in the bulk of the alloy react with HCl, by diffusion?

45 7. PRIMARY SILICON REACTION PHASE

It is known that Cu-Si alloys react with HCl gases forming HSiCl3, but it is still unknown which phase, the silicon dendrite or the Cu-Si matrix, is the primary phase that reacts with HCl gases. With better understanding of the silicon extraction reaction mechanism, it will be easier to modify the process towards the goal of getting optimal silicon composition and reducing the cost by using less copper alloying with MG-Si. It is clear that with higher silicon content in the Cu-Si alloys, the number and the volume fraction of silicon dendrite increase. This can be seen from Cu-Si phase diagram shown in Figure 7.1 by using the lever rule.

Figure 7.1 Cu-Si phase diagram

46 The Scanning electron microscope (SEM) pictures in Figure 7.2 show visual images of Cu-Si alloys with 30, 50 and 75 wt%Si, where the darker phase is silicon dendrites and the lighter phase is Cu-Si matrix.

1mm 1mm

(a) 30wt%Si-Cu alloy (b) 50wt%Si-Cu alloy

11mmmm

Figure 7.2 SEM pictures of Cu-Si alloys with various Si compositions

47 If hypereutectic Cu-Si alloy rectangles with different silicon compositions are used, the alloy with higher Si content contains more Si dendrites and less Cu-Si matrix than the one containing lower Si content alloys. Alloy weight losses actually represent silicon losses, since silicon is the only major element reacting with HCl gases, besides tiny amounts of impurities. Thus, experiments performed in this section were used to demonstrate which silicon phase is the primary phase reacting with HCl gases. The alloy which was found to lose the most weight must be the one that reacted the most with HCl gases.

7.1 EXPERIMENT WITH 30, 50 AND 75 CU-Cu ALLOYS

The main purpose of the experiment utilizing 30, 50 and 75 wt%Si-Cu alloys was to determine which alloy lost the most weight during an experiment. In order to sustain similar reaction conditions, the three alloys were charged in one reactor, as shown in Figure 7.3. The marked lines on the samples were used for subsequent SEM image analysis.

30 wt% Si‐Cu 50 wt% Si‐Cu 75 wt% Si‐Cu

Figure 7.3 The Cu-Si alloys with 30, 50 and 75 wt%Si used in the experiment.

The specification of the test is listed in Table 7.1

48 Table 7.1 Test conditions and measurements before the experiment. Sample #1 Sample #2 Sample #3 Length (cm) 15.0 Si Starting Average 4.95 Rod Diameter (mm) Mass (g) 6.39 Si content (wt%) 30 50 75 Intermediate Least amount of Most amount of Microstructure amount of silicon dendrites silicon dendrites silicon dendrites Alloy Mass (g) 44.66 39.84 24.24 Silicon weight in 13.40 19.92 18.18 the alloy (g) Cu-Si Alloy Average height 14.53 14.71 14.44 (mm) Average length 28.62 28.51 28.07 (mm) Average width 23.82 25.31 23.04 (mm) Average surface 2205.63 2304.97 2122.79 area (mm2) Experimental Duration (h) 5.5

7.1.1 Experimental Details

This experiment was performed in the reactor with dimensions listed in Section 4.1. The silicon rod was cleaned with acetone before the test. The starting up and shutting down procedure is outlined in the Section 4.The test was performed for 5.5 hrs, with HCl/H2 (1:1 ratio) gas purging only one time. The temperature for the Cu-Si alloys during the experiment was controlled between 660-680oC.

49 7.1.2 Results and Discussions

7.1.2.1 Silicon Recovery and Cu-Si Alloy Weight Losses

The amount of silicon recovered from the Cu-Si alloys and the percentage recovery are shown in Table 6.2 as well as the amount of Cu-Si alloys’ weight losses.

Table 7.2 Measurements after 5.5 h of experiment for Cu-Si alloy Sample #1 Sample #2 Sample #3 Deposition Length (cm) 11.0 Diameter (mm) 7.20 Diameter Gain (mm) 2.25 Diameter Growth Rate 0.41 (mm/h) Si Rod Weight (g) 11.56 Weight Gain (g) 5.17 Deposition Rate (g/h) 0.94 Deposition Rate/Length 0.085 (g/h/cm) Weight (g) 42.92 38.49 21.69 Weight Loss (g) 1.74 1.35 2.55

Cu-Si Alloy Alloy Weight Loss (%) 3.89% 3.40% 10.54%

Si weight loss from silicon 12.96% 6.79% 14.13% in the alloy (%) Si Recovery (%) 91.7%

Since the experiment was used to demonstrate which Si phase is the primary phase reacting with HCl gases forming HSiCl3, the study on the deposition rate of this experiment was not significantly important. However, it was found that the silicon layer growth rate of this experiment was larger than any layer deposition rates from Fan’s

50 experiments which used a similar set up. The reason might be that the reaction time of this experiment was much shorter than that in Fan’s experiments. This may reveal that the discussion in Section 3.4.2 about TCS production might be the rate limiting step after sometime is correct.

Comparing the results from Sample #1 to #3, it was found that the highest amount of silicon losses corresponded to the highest silicon content alloy (75wt%Si alloy). Although the relationship between silicon weight losses and silicon content was not conclusive, the weight losses for 75wt%Si alloy was much larger than for 30wt%Si alloy. Therefore, it can be deduced that the primary silicon reacting with HCl gases may be the silicon dendrite phase, but the decrease in weight losses of Sample #2 suggested otherwise. The reason may be that the silicon-starting rod was over-heated in the center.

Figure 7.4 shows the 50wt%Si alloy through the observation window of the reactor during the experiment. It is seen that several liquid drops formed on the surface of the alloy. From the Cu-Si phase diagram (Figure 7.1) it was believed that those drops probably were Cu-Si eutectic which have the lowest melting point 803oC and would melt first. This phenomenon was also observed when remelting Cu-Si alloys. The 50wt%Si alloy was placed right underneath the center of the silicon rod shown in Figure 7.3, where its temperature would be hotter than the other two alloys located near to the ends of the silicon rod. The high temperature of the 50wt%Si alloy would result in unfavored silicon extraction as discussed in Section 4.1, which might lead to the alloy’s less Si weight losses.

Figure 7.4 Eutectic liquid drops on the 50wt%Si alloy

The silicon rod after the experiment (Figure 7.5) also evidenced that the temperature of the center of the silicon rod was hotter than its two ends during the test. It is found that the two ends of the rod were thicker than the middle, which means that more silicon deposited on the ends of the rod. If the temperature of the alloy was above 800oC due to the presence of the eutectic droplets, the temperature of the rod would be much more than 1100oC. As discussed in Section 4.1, when the temperature of silicon rods is over than 1100oC, the deposition reaction will be less favored. In other words, less Si deposits on the center of the silicon rod was due to its higher temperature comparing to the two ends.

2cm

Figure 7.5 Silicon deposits after 5.5h reaction

7.1.2.2 Cu-Si Alloys After The Experiment

The reacted Cu-Si alloys are shown in Figure 7.6. It can be seen that the extent of crumbling and swelling of the alloys decreases with the increasing Si content in the alloys.

30 wt% Si‐Cu 50 wt% Si‐Cu 75 wt% Si‐Cu

2 cm

Figure 7.6 Cu-Si alloys after 5.5 h reaction. Alloys with higher silicon content are less crumbly and swelling.

In terms of industry application purposes, the high silicon content Cu-Si alloys are probably a better choice, since with less swelling and crumbling behavior, its shape would be able hold as a whole, which will have less potential to form dusts to clog the gas piping systems. It is interesting to see some yellowish powders on the top of the 50wt%Si alloy and the remained marked lines on 75wt%Si alloy. The marked lines probably acted as protective covers to the alloys. The reason for formation of those yellowish powders shown in Figure 7.7 is unknown.

Figure 7.7 The 50wt%Si alloy after 5.5h reaction

7.1.3 Conclusions

The alloy weight losses and the appearances of the reacted alloys and the silicon rod after the experiment may suggest that the silicon dendrites is the primary silicon phase reacting with HCl gases forming HSiCl3. However, because of the decrease in weight losses of Sample #2, even though with some explanations, no firm conclusion can be stated based on these tests.

54 7.2 REPEATED EXPERIMENT WITH 30, 50 AND 75 WT%SI-CU ALLOYS

To verify that the silicon dendrites is the primary silicon phase reacting with HCl gases, the experiment described in Section 7.1 with 30, 50 and 75wt%Si alloys was repeated. The three alloys were charged in one reactor, which is shown in Figure 7.8.

30wt%Si Alloy 50wt%Si Alloy 75wt%Si Alloy

2 cm

Figure 7.8 The Cu-Si alloys with 30, 50 and 75 wt%Si used in the experiment.

The 50wt%Si alloy was still placed underneath the center of the silicon rod, and 30 and

75wt%Si alloys were near to the two ends of the rod. All the alloys were placed on Al2O3 dishes to collect the crumbly powder from the alloy, as well as to prevent contaminating the quartz tube.

The specification of the test is listed in Table 7.3

55 Table 7.3 Test conditions and measurements before the experiment. Sample #1 Sample #2 Sample #3 Length (cm) 16.0 Si Starting Average 4.91 Rod Diameter (mm) Mass (g) 6.97 Si content (wt%) 30 50 75 Intermediate Least amount of Most amount of Microstructure amount of silicon dendrites silicon dendrites silicon dendrites Alloy Mass (g) 44.83 32.95 25.24 Silicon weight in 13.45 16.48 18.93 the alloy (g) Cu-Si Alloy Average height 14.13 14.21 14.35 (mm) Average length 27.47 27.82 27.71 (mm) Average width 23.46 23.42 23.40 (mm) Average surface 2075.78 2108.15 2115.08 area (mm2) Experimental Duration (h) 5.5

7.2.1 Experimental Details

This experiment was performed in the reactor with dimensions listed in Section 4.1. The silicon rod was cleaned with acetone before the experiment. The starting up and shutting down procedure is outlined in the Section 4.The test was performed for 5.5 hrs, with

HCl/H2 (1:1 ratio) gas purging only one time. The temperature for the Cu-Si alloys during the experiment was controlled between 660-680oC. The temperature of the silicon rod was well kept in the 1030-1100oC ranges.

7.2.2 Results and Discussions

7.2.2.1 Silicon Recovery and Cu-Si Alloy Weight Losses

The amount of silicon recovered from the Cu-Si alloys and the percentage recovery are shown in Table 7.4 as well as the amount of Cu-Si alloys’ weigh losses.

Table 7.4 Measurements after 5.5 h of experiment for Cu-Si alloy Sample #1 Sample #2 Sample #3 Deposition Length (cm) 11.0 Diameter (mm) 7.24 Diameter Gain (mm) 2.33 Diameter Growth Rate 0.42 (mm/h) Si Rod Weight (g) 12.15 Weight Gain (g) 5.18 Deposition Rate (g/h) 0.93 Deposition Rate/Length 0.085 (g/h/cm) Weight (g) 42.42 31.82 22.80 Weight Loss (g) 2.39 1.13 2.44

Cu-Si Alloy Alloy Weight Loss (%) 5.33% 3.43% 9.67%

Si weight loss from silicon 17.77% 6.86% 12.89% in the alloy (%) Si Recovery (%) 86.9%

Comparing the weight losses, it was found that the highest amount of silicon losses still corresponded to the highest silicon content alloy (75wt%Si alloy). The 50wt%Si alloy locating right under the silicon rod still lost least weight among the three alloys. However,

57 unlike the previous experiment of which the results were tabulated in Table 7.2, the difference between weight losses for 30wt%Si alloy and 75wt%Si alloy was not significant. Therefore, this experiment still could not confirm that the silicon dendrite is the primary reacting silicon phase.

7.2.2.2 Si Deposits

The deposited silicon is shown in Figure 7.9. It can be seen that the deposits were not uniform along the rod. The different temperature distributions along the silicon rod during the reaction might be the reason. It is interesting to see that the bottom of the silicon rod was very smooth but the top of the silicon was very rough. This was probably caused by the gas film boundary layer.

2 cm

(a) Silicon deposit at front

2 cm

(b) Silicon deposit at top

2 cm

Figure 7.9 Deposited silicon (a) front view (b) top view (c) bottom view

Due to the reactor’s set up, the gas flowed from the bottom of the silicon rod upwards towards the top of the reactor, if assuming the symmetrical fluid flow, then a boundary layer was created as illustrated in Figure 7.10.

Si Rod

Figure 7.10 Sketch of boundary layer created around silicon rod due to a fluid flow

The boundary layer usually was thinner at the bottom of the silicon and thicker as moving along the circumference of the rod towards to the top. The thinner the boundary layer, the easier the silicon deposition carried by HSiCl3, since more replenished HSiCl3 and H2 could be transferred to the rod and less HCl gases accumulated. Therefore it would result in the smooth surface at the bottom of the rod.

7.2.2.3 Cu-Si Alloys After The Experiment

The reacted Cu-Si alloys were shown in Figure 7.11. It can be seen that the extent of crumbling and swelling of the alloys decreases with the Si content in the alloys. The 30wt%Si alloy was very crumbly, and it even fell apart when taking it out from reactor. The 50wt%Si alloy did not crumble as the 30wt%Si alloy, possibly due to its less participation in the experiments contrast to the other two alloys.

2 cm 2 cm

(a) 30wt%Si alloy (b) 50wt%Si alloy

2 cm

Figure 7.11 The Cu-Si alloys after 5.5 h reaction.

7.2.3 Conclusions

The repeated experiment showed that the highest amount of silicon losses corresponded to the highest silicon content alloy (75wt%Si alloy) again. However, the decrease in weight losses in Sample #2 occurred once more, which was probably caused by uneven temperature distribution again. To support the statement of that the silicon dendrites is the primary phase reacting with HCl gases, a test with different composition hypereutectic alloys but same temperature is required to carry out.

60 7.3 EXPERIMENT WITH 30 AND 50WT%Si-Cu ALLOYS

The purpose of this experiment was to compare the weight losses of 30 and 50wt%Si alloys under similar experimental conditions. The distance between the alloys to the end of the rod was the same to make sure the alloys would have the same temperature from the radiating silicon rod. If the weight losses for the 50wt%Si alloy is larger than for the 30wt%Si alloy, then it is very likely that the silicon dendrites is the primary silicon phase reacting with HCl gases forming HSiCl3.

30 wt%Si alloy 50 wt%Si alloy

Figure 7.12 The Cu-Si alloys with 30 and 50wt%Si used in the experiment.

The specification of the test is listed in Table 7.5

7.3.1 Experimental Details

This experiment was performed in the reactor described in Section 4.1. The reaction condition was very similar to the two experiments in Sections 7.1 and 7.2. The silicon rod was cleaned with acetone before the experiment. The starting up and shutting down procedure is outlined in the Section 4.The test was performed for 5.5 hrs, with HCl/H2 (1:1 mole ratio) gas purging only one time. The temperature for the Cu-Si alloys during the experiment was controlled between 540-600oC.

Table 7.5 Test conditions and measurements before the experiment. Sample #1 Sample #2 Length (cm) 13.0 Si Starting Average 4.85 Rod Diameter (mm) Mass (g) 5.48 Si content (wt%) 30 50 Microstructure Most amount of silicon Microstructure Least amount of silicon dendrites dendrites Alloy Mass (g) 43.49 34.14 Silicon weight in 13.05 17.07 the alloy (g) Cu-Si Alloy Average height 13.86 14.10 (mm) Average length 27.50 28.16 (mm) Average width 23.42 23.56 (mm) Average surface 2055.74 2121.94 area (mm2) Experimental Duration (h) 5.5

62 7.3.2 Results and Discussions

7.3.2.1 Silicon Recovery and Cu-Si Alloy Weight Losses

The final mass changes are tabulated in Table 7.6.

Table 7.6 Measurements after 5.5 h of experiment for Cu-Si alloy Sample #1 Sample #2 Deposition Length (cm) 8.0 Diameter (mm) 7.50 Diameter Gain (mm) 2.65 Diameter Growth Rate 0.48 (mm/h) Si Rod Weight (g) 10.89 Weight Gain (g) 5.41 Deposition Rate (g/h) 0.98 Deposition Rate/Length 0.12 (g/h/cm) Weight (g) 40.75 30.85 Weight Loss (g) 2.74 3.29

Cu-Si Alloy Alloy Weight Loss (%) 6.30% 9.64%

Si weight loss from silicon 21.00% 19.27% in the alloy (%) Si Recovery (%) 89.7%

It is seen that the weight losses were higher for the 50wt%Si alloy than for the 30wt%Si alloy. Since conditions for both samples were identical, it can be conclude that the silicon dendrite is the primary phase reacting with HCl gases. The results from the three experiments listed in Table 7.7 show that the percentages of Si weight losses from silicon in the alloys which had similar temperatures were not significantly different. Results

63 from the Table 7.7 may also reveal that the efficiency of silicon extraction from different Si content hypereutectic Cu-Si alloys was similar once the reaction conditions on the alloys were similar.

Table 7.7 the percentage of Si weight losses from silicon in the alloy for different experiments Si weight loss from silicon Alloy composition Duration (h) in the alloy (%) 30wt%Si 5.5 12.96% Experiment 7.1 75wt%Si 5.5 14.13% 30wt%Si 5.5 17.77% Experiment 7.2 75wt%Si 5.5 12.89% 30wt%Si 5.5 21.00% Experiment 7.3 50wt%Si 5.5 19.27%

7.3.2.2 Silicon Deposit

The deposited silicon after the experiment was very silvery as seen in Figure 7.13

2 cm

Figure 7.13 Silicon deposit

7.3.2.3 Cu-Si Alloy After The Experiment

Even though both of the 30 and 50wt%Si alloys swelled and crumbled (Figure 7.14), the

64 30wt%Si alloy disintegrated much more than the 50wt% alloy, which could be seen from the amount of yellowish powders formed around the alloy as shown in Figure 7.15. More details are discussed in Section 8.

2 cm 2 cm

(a) The 30wt%Si alloy after the (b) The 50wt% Si alloy after the experiment experiment

Figure 7.14 The Cu-Si alloys after the experiment

1 cm

Figure 7.15 The disintegrated 30wt%Si alloy

After removing the disintegrated crumbly powders, the 30wt%Si-Cu alloy core is showed in Figure 7.16.

1 cm

Figure 7.16 The 30wt%Si-Cu alloy core

The color of the core is very similar to that of the alloy before the reaction, and it is concluded that the core may not be involved in the silicon extraction at all. Details about this will be discussed in the next section.

7.3.3 Conclusions

The experiments confirmed that the silicon dendrite is the primary phase reacting with HCl. However, since the relationship between silicon weight losses and silicon content is not conclusive, it is still unknown whether silicon in Cu-Si matrix reacts with HCl gases or not.

66 7.4 SILICON DIFFUSION

Experiments and analysis were used to determine if the core of the reacted alloy is involved in the extraction reaction or not. Density comparisons and chemical analysis on the alloys before and after the experiment were performed.

7.4.1 Density of Alloys Before and After the Experiment

The appearance of the alloy core was similar to the alloy before the experiment. To demonstrate that the core was not involved in the Si extraction, its density was compared with the one before the experiment.

7.4.1.1 Experimental Setup

First the mass of an alloy sample was determined. The alloy core hanging by a thin wire which connected to the stand, was immerged inside a beaker filled with water. The beaker was sitting on the balance. Before immerging the alloy in, the balance was tarred to zero.

Figure 7.17 The experimental setup for measuring volume of the alloy core

The mass shown on the balance was the mass of the displaced water by the alloy core. According to physics:

mwater = ρ waterValloy [7.1]

mwater Valloy = [7.2] ρ water

3 3 Where ρwater is 0.9982 g/cm , and it can be assumed to be 1 g/cm . Since the unit of the balance is gram and the volume of the thin wire was less than 0.02cm3, which almost could be neglected comparing to the volume of the alloy, then the number shown on the balance display was the volume of the alloy.

7.4.1.2 Results

The densities of the 30wt%Si alloy and its core before and after the experiment are listed in Table 7.8 respectively. It is seen that the difference between the two densities is almost negligible. Therefore, it was evident that the alloy core was not involved in any reactions. To confirm this, chemical analysis was required.

Table 7.8 Densities of the 30wt%Si alloy and its core before and after the reaction. Mass (g) Volume (cm3) Density (g/cm3) 30wt%Si alloy before the 46.08 8.80 5.24 experiment 30wt%Si alloy core after 31.31 5.99 5.23 the experiment

68 7.4.2 Compositions of the Alloy Core

To verify that no practical silicon diffusion occurred from inside the alloy to its surface, the composition of the alloy cores were analyzed by chemical analysis. If the composition of the alloy core was the same as before the process, definitely there would be no net silicon diffusion inside of alloy.

7.4.2.1 Experimental Setup

The remaining of the 30wt%Si alloy core from Experiment 7.1 was cut to several slices, as shown in the schematic drawing in Figure 7.18.

1 2.0mm 2.0mm 2 14.1mm Alloy Core 2.0mm 2.0mm 3 2.0mm 4.0mm

Figure 7.18 Schematic of cutting the alloy core

The slices, numbered 1 to 3, were sent to International Plasma Lab (ICP) for chemical analysis of their compositions. The method used was titration.

7.4.2.2 Results

The titration results for the slices are shown in Table 7.9.

69 Table 7.9 Si content in the alloy core Slice #1 Slice #2 Slice #3 Si content in the 30.57% 30.42% 30.72% alloy core (wt%)

The Si contents in all the 30wt%Si alloy core slices were almost equal to 30wt%, and it evidenced that there were no silicon gradients in the Cu-Si alloy.

7.5 CONCLUSIONS AND FUTURE WORK

The three experiments demonstrated that the silicon dendrite is the primary phase reacting with HCl to form HSiCl3. It was also found that the extent of crumbling and swelling of the alloys decreases with increasing Si content in the alloys. Even though the efficiency of silicon extraction from different Si content hypereutectic Cu-Si alloys was similar, from the viewpoint of reducing costs by alloying less Cu to alloy and reducing the number of times recharging alloys during the process, a high silicon content alloy should be used as a Si source. However, more tests are required to find out the optimal silicon composition of the alloy which will have the least crumbling behavior and enough copper to trap impurities as well as catalyzing the silicon extraction reaction.

70 8. ALLOY CRUMBLES AND SILICON EXTRACTION MECHANISMS

Alloy crumbling is believed not to be good for practical operations since it will lead to issues, such as pipe clogging and product contamination. However, it is also believed that crumbling might help the silicon extraction by increasing the active surface area. From the findings so far, possible silicon extraction mechanisms were also discussed.

8.1 ALLOY CRUMBLES TESTS

The purpose of these sets of experiments was to determine if the formation of crumbly alloy layers affected the silicon extraction rate. As discussed in Section 7, the extent of alloy crumbling depends on the silicon content of the Cu-Si alloys. Two experiments with different silicon content alloys, were performed to show whether the different intensity of crumbles formation could lead to different silicon extraction efficiency. The specifications of each test are listed in Table 8.1.

8.1.1 Experimental Details

The tests were performed for 24 h, with HCl/H2 (1:1 volume ratio) gas purging every 12 hrs. The temperatures for Cu-Si alloys during the experiments were controlled between 550-600oC. The cleaning and operation procedures are outlined in the Section 4.

71 Table 8.1 Tests conditions and measurements before the experiments for the different Cu-Si alloys Test #1 Test #2 Length (cm) 10.5 11.5 Si Starting Deposition Length (cm) 6.0 6.0 Rod Average Diameter (mm) 4.91 4.91 Mass (g) 4.59 4.86 Si content (wt%) 30 75 Alloy Mass (g) 46.08 25.76 Silicon weight in the alloy (g) 13.82 19.32 Cu-Si Alloy Average height (mm) 14.16 14.96 Average length (mm) 28.25 27.56 Average width (mm) 23.56 23.18 Average surface area (mm2) 2133 2156 Experimental Duration (h) 24

8.1.2 Observations and Results

The macroshots of the alloys during the experiments were taken and shown in Figure 8.1. It is seen that the 30wt%Si alloy started crumbling as early as 4 hours after the experiment began, while the 75wt%Si alloy did not seem to crumble. 20 hrs later when the experiments ended, the 30wt%Si alloy was covered by lots of crumbles and it was very hard to tell its original shape. In contrast, even though the 75wt%Si alloy crumbled, it was still possible to see its rectangle shape.

30wt% Si‐Cu 75wt% Si‐Cu Alloys after 4 hrs reaction 30wt% Si‐Cu 75wt% Si‐Cu

Alloys after 24 hrs reaction

Figure 8.1 Macroshots of the alloys during and after the experiments

The amount of silicon recovered from the Cu-Si alloys and percent recovery are shown in Table 8.2.

Table 8.2 Test conditions and measurements after the experiments for the different Cu-Si alloys 30wt%Si Alloy 75wt%Si Alloy Diameter (mm) 9.26 10.56 Diameter Gain (mm) 4.35 5.65 Diameter Growth Rate 0.18 0.24 (mm/h) Si Rod Weight (g) 12.01 14.53 Weight Gain (g) 7.42 9.67 Deposition Rate (g/h) 0.31 0.40 Deposition Rate/Length 0.052 0.067 (g/h/cm)

Weight After Reaction (g) 38.01 15.26

Alloy Core Weight (g) 12.54 12.75

Crumbles Weight (g) 25.47 2.51

Cu-Si Alloy Weight Loss (g) 8.07 10.49

Alloy Weight Loss (%) 17.5% 40.8%

Si weight loss from silicon 58.4% 54.3% in the alloy (%) Si Recovery (%) 91.9% 92.2%

In spite of the weight losses for the 75wt%Si alloy was greater than for the 30wt%Si alloy, the relative amount of Si extraction from 30wt%Si alloy was a little bit larger than from the 75wt%. This was probably caused by the alloys’ crumbling behavior. The alloys initial volumes were the same and after the reaction, and the 30wt%Si alloy core was much smaller than that of the 75wt%Si alloy as shown in the Figure 8.2. The amount of crumbles from the 30wt%Si was ten times more than for the 30wt%Si alloy in a weight

74 basis (25.47g vs. 2.51g). It seemed that the formation of crumbles did not have a negative impact on the silicon extraction.

1 cm

30wt% Si‐Cu 75wt% Si‐Cu

Figure 8.2 The 30 and 75wt%Si Alloy cores after the 24 hr experiment

8.1.3 Crumbles Microstructures

Some crumbles sintered together and formed the layer shown in Figure 8.3, of which the microstructure (Figure 8.4) was observed. It is seen that there was no silicon dendrites left at all. The lighter phase is the Cu-Si phase and the darker phase is gaps between Cu- Si phases, which were filled up with epoxy. This revealed that crumbles might be the leftover Cu-Si matrix from the alloy after all dendrites have reacted with HCl gases. However the silicon in the matrix seemed to react with HCl as well, the EDX analysis results (Figure 8.5) on the crumble layers indicated that the Cu content in crumbles was 95%, much larger than that in the eutectic phases, which was close to 87wt%.

Figure 8.3 Sintered crumble layer from the 30wt%Si alloy

Figure 8.4 SEM of the crumble layer from the 30wt%Si alloy

Elements Wt% Cu 95.4% Si 4.6%

Figure 8.5 EDX analysis result of a crumble

8.2 COMPOSITION OF THE CRUMBLES

Even though it is believed that the silicon dendrite is the primary phase reacting with HCl gases, it is not clear whether the silicon from Cu-Si matrix will react with HCl. The yellowish crumbles on the top of the 30wt% Si alloys after the experiment 8.1 were collected for a 24 hr reaction. The purpose is to determine if the silicon in the Cu-Si matrix could react.

8.2.1 Experimental Setup

The yellowish crumbles from the 30wt%Si alloy were placed into the reactor and tested for 24 hours to see whether the yellowish crumble would react with HCl. Then the powders were sent for XRD analysis. A silicon starting rod was used to produce reliable results. The measurements of crumbles are listed in Table 8.3

77 Table 8.3 Measurements of the crumbles before and after the 24 hr experiment Mass before Mass after Mass Loss Mass Loss Surface Area

Reaction (g) Reaction (g) (g) (%) (mm2) Crumbles 2.6797 2.6455 0.0342 1.28% 131.85

8.2.2 Results

The weight loss was only 1.28% after the 24 hours experiment, which means that the activity of silicon in crumbles reacting with HCl gases was very low. This also indicated that crumbles could react with HCl but at a slow rate. The XRD diffractogram illustrated in Figure 8.6 showed that the major components in crumbles were Cu7Si and Cu9Si, which were κ phase shown in the Cu-Si phase diagram.

Cu9Si

Cu7Si

Figure 8.6 Diffractogram of the crumbles from the 30wt%Si-Cu alloy

78 The reason of the empty section in between 60 to 70 degrees was that all possible CuxSiy intermetallics have no peak in this range. From previous findings and discussions, a possible silicon extraction reaction mechanism can be proposed.

8.3 PROPOSED SILICON EXTRACTIOIN MECHANISM

HCl will selectively react with primary silicon dendrites as long as dendrites are present. The leftover Cu-Si matrix becomes crumbly dusty particles. Once the silicon dendrites on the surface of the alloys are completely consumed, the HCl gases will flow through the space between the crumbles to react with the silicon dendrites on a new reaction surface under the crumbles. The silicon contained in the Cu-Si matrix can only react with HCl gases at a very slow rate due to its low activity by alloying with a large amount of copper. An illustration of the proposed mechanism is shown in Figure 8.7

HCl HCl TCS TCS TCS H2 HCl H2 H2 Cu3Si Cu9Si Cu3Si

Figure 8.7 Illustration of the proposed silicon extraction mechanisms

79 8.4 CONCLUSIONS

The experimental results and analysis show that the crumbling behavior of Cu-Si alloys during the experiment did not stop the silicon extraction reaction. The formation of the crumbles should not be the rate limiting step, since with different amounts of crumbles formation, the change of the efficiency of silicon extraction is not significant. A reaction mechanism is also proposed. HCl will preferentially first react with silicon dendrites. Once the silicon dendrites are reacted away from the top surface, the top surface starts crumbling and HCl gases can flow through the crumbles to react with “fresh” alloy areas underneath.

80 9. IMPURITY STUDY

The purity level of the deposited silicon is one of the most important criteria to determine how efficient the process is. With the goal of increasing the deposition rate, the proper purity level has to be achieved. Therefore studies of impurity behavior were performed to find out where the impurities came from, and where those impurities are located in the alloys. The impurity levels of the silicon deposit for 24 hrs experiments using the 30 and 75wt%Si alloys are tabulated in Table 9.1 and 9.2 respectively. All other unlisted elements are less than 0.01ppmw, except H, C, O and N.

Table 9.1 Impurity analysis results by GDMS on the silicon deposit using the 30wt%Si alloy

Elements ppmw Elements ppmw Elements Ppmw Elements ppmw Si Major W <0.5 Be <0.05 Cr 0.04 Ta <10 Zn <0.2 Mg <0.05 S 0.027 P 1 As <0.2 Mn <0.05 Ni 0.026 F <1 Sb <0.2 Co <0.05 Al 0.025 Cl <1 Ca 0.17 Zr <0.05 V 0.02 Na 0.85 B 0.15 Nb <0.05 Ti 0.01 Cu 0.75 K 0.14 Mo <0.05 Th <0.01 Ge <0.5 Ga <0.1 Pb <0.05 U <0.01 Ag <0.5 Hf <0.1 Bi <0.05 Sn <0.5 Li <0.05 Fe 0.045

Table 9.2 Impurity analysis results by GDMS on the silicon deposit using the 75wt%Si alloy Elements ppmw Elements ppmw Elements ppmw Elements ppmw Si Major Sn <0.5 Cu <0.1 Zr <0.05 Fe 37 W <0.5 Ga <0.1 Nb <0.05 Cl 22 Ca 0.25 Hf <0.1 Mo <0.05 Ta <10 B 0.24 Al 0.08 Pb <0.05 P 2.5 S 0.2 Cr 0.075 Bi <0.05 V 1 Zn <0.2 Mn 0.07 Ti <0.01 F <1 As <0.2 Li <0.05 Th <0.01 Na 0.66 Sb <0.2 Be <0.05 U <0.01 Ge <0.5 Ni 0.13 Mg <0.05 Ag <0.5 K 0.1 Co <0.05

81 It is seen that in general, the purity level of silicon deposit from the experiment using the 30wt%Si alloy is much higher than with 70wt%Si-Cu alloy, especially the Fe impurity level, and it will be discussed later.

9.1 SOURCE OF IMPURITIES

A Cu-Si alloy is used as the silicon source, so impurities found in the silicon deposit have to be mainly from the Cu-Si alloy. Table 9.3 shows the average result of ICP-MS

(Inductively Coupled Plasma) analysis on impurities levels of the 30wt%Si-Cu alloys over six different areas (the results of each individual area are listed in Appendix A). Impurity levels of unlisted elements are less than 0.01ppmw.

Table 9.3 Average ICP analysis results on impurities in the 30wt%Si alloy Elements ppmw Elements ppmw Elements ppmw Elements ppmw Si Major Co 1.95 Ga 0.34 Eu <0.1 Cu Major Zn 1.79 Sb 0.33 Gd <0.1 Al 465 Nb 1.66 Th 0.33 Tb <0.1 Fe 380 Mo 1.53 Sn 0.29 Dy <0.1 V 87.2 Ag <1 La 0.23 Ho <0.1 Ti 84.6 Pb 0.87 U 0.18 Er <0.1 P <50 Ce 0.76 Pr 0.15 Tm <0.1 Ni 25.8 Nd 0.57 Li <0.1 Yb <0.1 Mn 22.2 Ge <0.5 Sc <0.1 Lu <0.1 Rh <10 Se <0.5 As <0.1 Re <0.1 Na 8.18 Y <0.5 Rb <0.1 Os <0.1 Zr 5.94 Pd <0.5 Sr <0.1 Ir <0.1 Ca 4.87 Te <0.5 Ru <0.1 Pt <0.1 Cr 2.99 Hf <0.5 Cd <0.1 Au <0.1 B 2.98 Ta <0.5 Cs <0.1 Hg <0.1 Mg 2.94 W <0.5 Ba <0.1 Tl <0.1 K 2.19 Bi <0.5 Sm <0.1 Be <0.05

The impurities in the Cu-Si alloys are all from the Cu and the metallurgical grade silicon. The GDMS (Glow Discharge Mass Spectrometer) analysis results on purity level of the two metals were tabulated in Tables 9.4 and 9.5 respectively. All elements left blank are <0.1ppmw. It can be seen that the purity level of the Cu used for alloying with MG-Si

82 was very high and all major impurities were therefore from the MG-Si. It is interesting that the impurity levels of some elements, such as Al, Ti and V, in the alloy were higher than in either Cu or MG-Si. This is probably due to the limitation of the chemical analysis. ICP-MS, which only focuses on very localized areas. The impurities may also come from the sample preparation where Alumina were used for polishing.

Table 9.4 Impurity analysis result by GDMS on MG-Si Elements ppmw Elements ppmw Elements ppmw Elements ppmw Si Major Ta <5 Ag <0.3 Sb <0.1 Fe 2800 Zn 4.5 Cd <0.3 K 0.072 Al 335 V 1.7 Te <0.3 S 0.069 Mn 55 Ge 1.6 Sr 0.23 As 0.065 Ti 35 Co 1.3 Pr 0.2 Pb <0.05 Cu 24 Ce 1.3 W 0.2 Bi <0.05 B 18 Nd 1.2 Nb 0.17 Th 0.039 P 16 Mo 0.71 Y 0.11 U 0.035 Cr 8.7 Mg 0.7 Na 0.1 Li 0.033 Ni 7.1 La 0.52 F <0.1 Zr 7 Cl 0.31 In <0.1 Ca 5.6 Se <0.3 Sn <0.1

Table 9.5 Impurity analysis result by GDMS on Cu used for alloying with MG-Si Elements ppmw Elements ppmw Elements ppmw Elements ppmw Cu Major Te <0.3 Au <0.05 Mo <0.01 Ag 15 Cr 0.27 Zn 0.034 Al 0.008 S 10 As 0.24 Ca 0.028 Si 0.007 Fe 1.7 Bi 0.23 K 0.026 Mg 0.005 Ni 1.5 Mn 0.2 Na 0.01 Ti 0.002 P 1.2 F <0.1 Li <0.01 Co 0.001 Ta <1 Cd <0.1 Be <0.01 V <0.001 Sb 0.7 In <0.1 B <0.01 Pb 0.38 Sn <0.1 Zr <0.01 Se <0.3 Cl 0.098 Nb <0.01

83 9.2 IMPURITIES IN THE Cu-Si ALLOYS

As discussed in Section 3, Cu is used to trap impurities. Thus the chance of finding the impurities in the Cu-Si matrix phase (Figure 9.1) should be much higher than in Si dendrite phase. However, it is very difficult to locate the impurities. EDX (Energy Dispersive X-Ray) Spectroscopy could not find any impurities probably as the impurity levels are below the EDX’s detection limits (0.5wt%, equivalent to 5000ppmw). ICP-MS could not generate a reliable analysis result neither, because the minimum depth of the laser beam used to evaporate the sample is 100 µm. The width of the silicon dendrites is less than 100 µm, so most times the analysis done on silicon dendrites may also contain a large amount of Cu-Si matrix.

200 um

1mm

Figure 9.1 Cu-Si matrix phase which may retain most impurities

84 9.2.1 Experimental Setup

Extra amounts of elements, such as Fe, B, Al, which were major impurities found in the cast alloy, are added into the charge during melting Cu and MG-Si, in order to increase the impurities level beyond the EDX minimum detection limit. To cast 250 gram of 30wt%Si-Cu alloy, 75g of Si, 167g of Cu, 2.5 g of Fe powder, 8.75 g of Al pellets as well as 8.1g of B2O3 powder were charged in a high purity graphite crucible, which was heated up to 1440oC with Ar gas stirring. After cooling there was a large amount of slag illustrated in Figure 9.2 found with the alloy. Then the alloy was observed under SEM and analyzed by EDX.

2 cm

Figure 9.2 The 30wt%Si alloy and its slag

9.2.2 Results

It is seen from Figure 9.3 that the impurities in the silicon dendrites and the Cu-Si matrix were still below the EDX detection limit, but the Fe impurity was found in the eutectic in the matrix (Figure 9.4).

85 Cu3Si Si Dendrite Matrix Si Dendrite and Matrix

200 um

Figure 9.3 EDX analysis on the boundary between the Si dendrites and the matrix

Cu3Si Si Dendrite Matrix Needle Structure in Matrix

200 um

Figure 9.4 EDX analysis on the needle structure which is silicon eutectic

Higher impurities level in the Cu-Si matrix phase indicates that Cu was a very good filter to trap impurities inside the matrix phase. In general the purity levels in silicon deposit produced by the 30wt%Si alloy was higher than the 75wt%Si alloy. The suspected reason

86 is that the decreasing of Cu content in Cu-Si alloys weakens the power of trapping impurities. This may explain why Fe impurity level was so high in the Si deposit using the 75wt%Si alloy. The total impurity level of the silicon produced in the lab from 30wt%Si alloy is less than 15ppmw, and it is very close to the required target.

87 10. CONCLUSIONS AND FUTURE WORK

It is confirmed that silicon was extracted from Cu-Si alloys using an HCl/H2 gas mixture.

The HSiCl3 formed at the alloy then decomposed into pure silicon on a hot pure silicon starting rod. As the silicon rod grew in diameter the deposition area kept increasing. It was found that with time, the silicon mass growth rate almost stayed constant probably due to the constant silicon extraction rate. Hence, the silicon extraction reaction may be the rate limiting step for the entire CVD process. The rate-controlling factor in the first few hours could not be determined, and more work is required in future.

From the results using Cu-Si alloys with varying composition, it was found that the highest amount of silicon extracted corresponds to the highest silicon content alloy (75wt%Si alloy). It is concluded that silicon from the silicon dendrite is the primary silicon reacting with the HCl gases forming HSiCl3. The depletion of the silicon dendrites close to the surface would result in the formation of crumbles which were mainly Cu-Si matrix phase, which would react with HCl gases at a much slower rate compared to the silicon dendrites. The porosity of the crumbles would allow more HCl flowing through the crumbles and reacting with the unreacted alloy underneath.

The density and chemical analysis of the alloy cores after the run, showed that there was no noticeable silicon diffusion occurring in the alloy during the experiment. The alloy cores were covered by flaking particles, and the amount of those crumbly and flaky particles decreases with the Si content in the alloys. However, the different extents of alloy crumbling and swelling did not affect the efficiency of silicon extraction (extracted silicon over total amount of silicon in the alloy).

88 The impurities found in the silicon deposit were mainly from the metallurgical grade silicon. Slag formation during melting of copper and MG-Si to form Cu-Si alloy reduced some impurities. Most impurities were retained in the Cu-Si matrix phase. The purity of the silicon produced by the 30wt%Si alloy was within solar grade requirement, and it was much better than that with the 75wt%Si alloy. This may indicate that the copper in the 75wt%Si alloy was not enough to retain the impurities. GDMS analysis done on those silicon deposit were focused on very localized areas, from which the analysis results may not be very representative. Experiments using 50wt%Si alloy should be performed to verify whether the impurity level in silicon deposit increases with higher silicon content alloy used as a silicon source.

89 11. REFERRENCES

1. V. Smil, Energy at the Crossroads , Organization for Economic Co-operation and Development. http://www.oecd.org/dataoecd/52/25/36760950.pdf. Retrieved on 2007-09-29, p.12 2. C.Archer, Evaluation of Global Wind Power. Stanford. http://www.stanford.edu/group/efmh/winds/Archer2004jd005462.doc. Retrieved on 2008-05-11. 3. H. Lieth and R. Whittaker, Primary Productivity of the Biosphere. Springer- Verlag1 (1975), p. 305-328 4. V. Smil, Energy at the Crossroads: Global Perspectives and Uncertainties. MIT Press, 2004, p. 15 5. Energy Information Administration, International Energy Outlook 2007. (2007- 05). http://www.eia.doe.gov/oiaf/ieo/highlights.html.Retrieved on 2007-09-29. 6. Energy Information Administration, Energy Information Administration Brochures. Released on August 2005. 7. ARISE Technologies Corporation, Silicon Technology. 2008, Available: http://www.arisetech.com/content/view/37/108/ 8. Lawson Hunter Communications, Alternative/Renewable Energy Newsletter. 2006/2007. http://www.arenewsletter.com/Solar.html 9. Silicon prices fatal blow China's photovoltaic industry. http://www.china- cbn.com/s/n/000004/20071218/020000063152.shtml. 10. H.Y. Fan, Solar Grade Silicon Refining By chemical Vapour Deposition. MASc. Thesis, University of Toronto, 2007 11. P. Woditsch, and W. Koch, Solar grade silicon feedstock supply for PV industry, Solar Energy Materials & Solar Cell, Vol 72, 2002. 12. A. Schei, J. K. Tuset, and H. Tveit, Production of High Silicon Alloys. Tapir Forlag, 1998, p.13-20. 13. B. Ceccaroli, and O. Lohne, Solar Grade Silicon Feedstock, Handbook of Photovoltaic Science and Engineering, John Wiley & Sons, 2003, p. 153-204

90 14. M.G. Mauk, J.A.Rand, R. Jonezyk, R. B. Hall, and A. M. Bamett, Solar Grade silicon: The Next Decade, 3rd World Conference on Photovoltaic Energy Conversion, May 11-18, 2003. p. 939-943 15. P. Tejedor and J. M. Olson, Silicon Purification by the Van Arkel-De Boer

Technique using a Cu3Si: Si composite alloy source, Journal of Crystal Growth, vol. 89, p220-226, 1988 16. M. Hansen and K. Anderko, Constitution of Binary Alloys, 2nd ed. New York: McGraw-Hill Book Company Inc., 1958 17. R.W. Olesinski and G. J. Abbaschian, Cu-Si, Phase Diagram of Binary Copper Alloys, edited by P.R.Subramanian, D.J. chakrabarti, and D.E. Caughlin, ASM International, 1994, p.398-405. 18. W.C. P’Mara, R.B. Herring and L.P. Hunt, Handbook of Semiconductor Silicon Technology, Norwich, NY: Noyers Publications/ William 19. R. C. Powell and J. M. Olson, Rate Limiting Step in CVT Purification of Silicon

using a Si:Cu3Si source, Journal of Crystal Growth, vol. 70, p. 218-222, 1984. 20. M. Li, Result Comparison Report, University of Toronto, Sept, 2007. 21. Outokumpu Research Information Services. Outokumpu HSC chemistry for Windows. 5.0

91 APPENDIX A: CHEMICAL ANALYSIS ON Cu, MG-Si, Cu-SI ALLOYS, EG-SI AND DEPOSITED Si

A.1 METALLURGICAL GRADE SILICON

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw H Zn 4.5 Pr 0.20 Li 0.033 Ga Nd 1.2 Be Ge 1.6 Sm B 18 As 0.065 Eu C Se <0.3 Gd N Br Tb O Rb Dy F <0.1 Sr 0.23 Ho Na 0.10 Y 0.11 Er Mg 0.70 Zr 7.0 Tm Al 335 Nb 0.17 Yb Si Major Mo 0.71 Lu P 16 Ru Hf S 0.069 Rh Ta <5 Cl 0.31 Pd W 0.20 K 0.072 Ag <0.3 Re Ca 5.6 Cd <0.3 Os Sc In <0.1 Ir Ti 35 Sn <0.1 Pt V 1.7 Sb <0.1 Au Cr 8.7 Te <0.3 Hg Mn 55 I Tl Fe 2800 Cs Pb <0.05 Co 1.3 Ba Bi <0.05 Ni 7.1 La 0.52 Th 0.039 Cu 24 Ce 1.3 U 0.035

All other elements <0.1ppmw, each

A.2 COPPER USED FOR ALLOYING WITH MG-SILICON

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw H Zn 0.034 Pr Li <0.01 Ga Nd Be <0.01 Ge Sm B <0.01 As 0.24 Eu C Se <0.3 Gd N Br Tb O Rb Dy F <0.1 Sr Ho Na 0.010 Y Er Mg 0.005 Zr <0.01 Tm Al 0.008 Nb <0.01 Yb Si 0.007 Mo <0.01 Lu P 1.2 Ru Hf S 10 Rh Ta <1 Cl 0.098 Pd W K 0.026 Ag 15 Re Ca 0.028 Cd <0.1 Os Sc In <0.1 Ir Ti 0.002 Sn <0.1 Pt V <0.001 Sb 0.70 Au <0.05 Cr 0.27 Te <0.3 Hg Mn 0.20 I Tl Fe 1.7 Cs Pb 0.38 Co 0.001 Ba Bi 0.23 Ni 1.5 La Th Cu Major Ce U

All other elements <0.1ppmw, each

93 A.3 ELECTRONIC GRADE SILICON

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw H Zn <0.05 Pr Li <0.01 Ga Nd Be Ge <0.3 Sm B 0.016 As <0.03 Eu C Se <0.3 Gd N Br Tb O Rb Dy F <0.1 Sr Ho Na 0.007 Y Er Mg 0.003 Zr <0.01 Tm Al 0.004 Nb <0.01 Yb Si Major Mo <0.01 Lu P <0.01 Ru Hf S <0.005 Rh Ta <5 Cl 0.33 Pd W <0.01 K 0.021 Ag <0.3 Re Ca <0.01 Cd <0.3 Os Sc In <0.1 Ir Ti <0.001 Sn <0.1 Pt V <0.001 Sb <0.1 Au Cr 0.009 Te <0.3 Hg Mn 0.003 I Tl Fe 0.048 Cs Pb <0.05 Co <0.001 Ba Bi <0.05 Ni 0.005 La Th <0.01 Cu 0.018 Ce U <0.01

All other elements <0.1ppmw, each

94 A.4 IMPURITIES IN THE 30WT%Si-Cu ALLOY (AREA 1)

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw Li <0.1 As <0.1 Sm <0.1 Be <0.05 Se <0.5 Eu <0.1 B 3.3 X Gd <0.1 X Rb <0.1 Tb <0.1 Na 6.9 Sr <0.1 Dy <0.1 Mg 2.9 Y <0.5 Ho <0.1 Al 400 Zr 8.2 Er <0.1 Si Major Nb 1.9 Tm <0.1 P <50 Mo 3.1 Yb <0.1 X Ru <0.1 Lu <0.1 X Rh <10 Hf <0.5 K 2.3 Pd <0.5 Ta <0.5 Ca 3.8 Ag <1 W <0.5 Sc <0.1 Cd <0.1 Re <0.1 Ti 80 X Os <0.1 V 75 Sn 1.0 Ir <0.1 Cr 3.3 Sb 0.46 Pt <0.1 Mn 22 Te <0.5 Au <0.1 Fe 400 X Hg <0.1 Co 2.1 Cs <0.1 Tl <0.1 Ni 25 Ba <0.1 Pb 0.88 Cu Major La 0.22 Bi <0.5 Zn 1.6 Ce 1.0 Th 0.81 Ga 0.31 Pr 0.15 U 0.18 Ge <0.5 Nd 0.57

95 A.5 IMPURITIES IN THE 30WT% Si-Cu ALLOY (AREA 2)

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw Li <0.1 As <0.1 Sm <0.1 Be <0.05 Se <0.5 Eu <0.1 B 2.6 X Gd <0.1 X Rb <0.1 Tb <0.1 Na 7.5 Sr <0.1 Dy <0.1 Mg 2.9 Y <0.5 Ho <0.1 Al 420 Zr 10 Er <0.1 Si Major Nb 1.5 Tm <0.1 P <50 Mo 1.8 Yb <0.1 X Ru <0.1 Lu <0.1 X Rh <10 Hf <0.5 K 2.5 Pd <0.5 Ta <0.5 Ca 8.2 Ag <1 W <0.5 Sc <0.1 Cd <0.1 Re <0.1 Ti 82 X Os <0.1 V 85 Sn 0.38 Ir <0.1 Cr 2.7 Sb 0.37 Pt <0.1 Mn 23 Te <0.5 Au <0.1 Fe 390 X Hg <0.1 Co 2.1 Cs <0.1 Tl <0.1 Ni 24 Ba <0.1 Pb 0.91 Cu Major La 0.25 Bi <0.5 Zn 1.6 Ce 0.83 Th 0.34 Ga 0.34 Pr 0.15 U 0.18 Ge <0.5 Nd 0.58

96 A.6 IMPURITIES IN THE 30WT% Si-Cu ALLOY (AREA 3)

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw Li <0.1 As <0.1 Sm <0.1 Be <0.05 Se <0.5 Eu <0.1 B 2.8 X Gd <0.1 X Rb <0.1 Tb <0.1 Na 10 Sr <0.1 Dy <0.1 Mg 3.4 Y <0.5 Ho <0.1 Al 540 Zr 3.5 Er <0.1 Si Major Nb 1.9 Tm <0.1 P <50 Mo 1.8 Yb <0.1 X Ru <0.1 Lu <0.1 X Rh <10 Hf <0.5 K 3.1 Pd <0.5 Ta <0.5 Ca 7.9 Ag <1 W <0.5 Sc <0.1 Cd <0.1 Re <0.1 Ti 89 X Os <0.1 V 91 Sn 0.25 Ir <0.1 Cr 3.1 Sb 0.31 Pt <0.1 Mn 24 Te <0.5 Au <0.1 Fe 420 X Hg <0.1 Co 2.0 Cs <0.1 Tl <0.1 Ni 25 Ba <0.1 Pb 0.84 Cu Major La 0.23 Bi <0.5 Zn 2.8 Ce 0.52 Th 0.41 Ga 0.32 Pr 0.15 U 0.16 Ge <0.5 Nd 0.52

97 A.7 IMPURITIES IN THE 30WT% Si-Cu ALLOY (AREA 4)

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw Li <0.1 As <0.1 Sm <0.1 Be <0.05 Se <0.5 Eu <0.1 B 3.7 X Gd <0.1 X Rb <0.1 Tb <0.1 Na 7.3 Sr <0.1 Dy <0.1 Mg 3.1 Y <0.5 Ho <0.1 Al 540 Zr 6.2 Er <0.1 Si Major Nb 2.8 Tm <0.1 P <50 Mo 1.3 Yb <0.1 X Ru <0.1 Lu <0.1 X Rh <10 Hf <0.5 K 2.1 Pd <0.5 Ta <0.5 Ca 8.9 Ag <1 W <0.5 Sc <0.1 Cd <0.1 Re <0.1 Ti 89 X Os <0.1 V 94 Sn 0.38 Ir <0.1 Cr 3.5 Sb 0.31 Pt <0.1 Mn 22 Te <0.5 Au <0.1 Fe 420 X Hg <0.1 Co 2.3 Cs <0.1 Tl <0.1 Ni 26 Ba <0.1 Pb 0.92 Cu Major La 0.23 Bi <0.5 Zn 1.8 Ce 1.0 Th 0.31 Ga 0.34 Pr 0.15 U 0.17 Ge <0.5 Nd 0.54

98 A.8 IMPURITIES IN THE 30WT% Si-Cu ALLOY (AREA 5)

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw Li <0.1 As <0.1 Sm <0.1 Be <0.05 Se <0.5 Eu <0.1 B 3.1 X Gd <0.1 X Rb <0.1 Tb <0.1 Na 6.5 Sr <0.1 Dy <0.1 Mg 2.9 Y <0.5 Ho <0.1 Al 500 Zr 5.9 Er <0.1 Si Major Nb 1.6 Tm <0.1 P <50 Mo 1.8 Yb <0.1 X Ru <0.1 Lu <0.1 X Rh <10 Hf <0.5 K 1.6 Pd <0.5 Ta <0.5 Ca 4.6 Ag <1 W <0.5 Sc <0.1 Cd <0.1 Re <0.1 Ti 85 X Os <0.1 V 87 Sn 0.23 Ir <0.1 Cr 3.1 Sb 0.36 Pt <0.1 Mn 22 Te <0.5 Au <0.1 Fe 385 X Hg <0.1 Co 1.9 Cs <0.1 Tl <0.1 Ni 26 Ba <0.1 Pb 0.88 Cu Major La 0.23 Bi <0.5 Zn 1.7 Ce 0.72 Th 0.28 Ga 0.34 Pr 0.15 U 0.18 Ge <0.5 Nd 0.57

99 A.9 IMPURITIES IN THE 30WT% Si-Cu ALLOY (AREA 6)

ANALYSIS ppmw ANALYSIS ppmw ANALYSIS ppmw Li <0.1 As <0.1 Sm <0.1 Be <0.05 Se <0.5 Eu <0.1 B 2.8 X Gd <0.1 X Rb <0.1 Tb <0.1 Na 10 Sr <0.1 Dy <0.1 Mg 2.9 Y <0.5 Ho <0.1 Al 425 Zr 5.4 Er <0.1 Si Major Nb 1.5 Tm <0.1 P <50 Mo 1.0 Yb <0.1 X Ru <0.1 Lu <0.1 X Rh <10 Hf <0.5 K 2.6 Pd <0.5 Ta <0.5 Ca 3.8 Ag <1 W <0.5 Sc <0.1 Cd <0.1 Re <0.1 Ti 84 X Os <0.1 V 88 Sn 0.22 Ir <0.1 Cr 2.8 Sb 0.28 Pt <0.1 Mn 22 Te <0.5 Au <0.1 Fe 360 X Hg <0.1 Co 1.9 Cs <0.1 Tl <0.1 Ni 26 Ba <0.1 Pb 0.86 Cu Major La 0.24 Bi <0.5 Zn 1.8 Ce 0.75 Th 0.29 Ga 0.34 Pr 0.15 U 0.18 Ge <0.5 Nd 0.59

100 APPENDIX B. The GASEOUS SPECIES OF ELEMENTS Al, B, Si, Fe, P, Ti and Mn

possible Al gas species possible B gas species possible Si gas species possible Fe gas species possible P gas species AlCl(g) BCl(g) SiCl(g) FeCl(g) PCl(g) AlCl2(g) BCl2(g) SiCl2(g) FeCl2(g) PCl2(g) AlCl3(g) BCl3(g) SiCl3(g) FeCl3(g) PCl3(g) Al2Cl4(g) B2Cl4(g) SiCl4(g) Fe2Cl4(g) PCl5(g) Al2Cl6(g) BCl2H(g) SiHCl(g) Fe2Cl6(g) AlClH2(g) BHCl(g) SiHCl3(g) AlCl2H(g) BHCl2(g) SiH2Cl2(g) BH2Cl(g) SiH3Cl(g)

possible Ti gas species possible Mn gas species TiCl(g) MnCl(g)

Ti2Cl2(g) MnCl2(g)

MnCl3(g)

MnCl4(g)

Mn2Cl4(g)

101

APPENDIX C. SILICON REFINING EFFICIENCY

Deposited Si η = production Si losses from alloys

Reaction Number Silicon deposit (g) Si losses from alloys (g) Efficiency (η) 7.1 5.17 5.64 91.7% 7.2 5.18 5.96 86.9% 7.3 5.41 6.03 89.7% 8.1(Test#1) 7.42 8.07 91.9% 8.1(Test#2) 9.67 10.49 92.2% Repeated 8.1 9.97 10.44 95.5% (Test #2)

η 91.7% + 86.9% + 89.7% + 91.9% + 92.2% + 95.5% η = ∑ = ave. production #of Experiments 6

= 91.3%

102