臺灣博碩士論文加值系統

熱燈絲化學氣相沉積法是一種近年來備受矚目的製程技術，可應用於多種元件級矽或矽合金薄膜的製作上。它是一種運用加熱的燈絲對反應氣體進行解離，以達到沉積效果的技術。相較於普及的電漿輔助化學氣相沉積，熱燈絲化學氣相沉積法具有低製程溫度、高沉積速率、低設備成本、大面積成長、高氣體使用率、無電漿轟擊以及易於控制薄膜結晶率等優點，是一種極具發展潛力的半導體製程技術。本論文主要之目的可分為兩部分，第一部分為開發並優化以熱燈絲化學氣相沉積法製作元件級矽薄膜之製程，第二部分則是將此以熱燈絲化學氣相沉積法製作之元件級矽薄膜應用於製作太陽電池上。製程開發所研究的材料包括本質矽薄膜(含非晶、微晶與多晶)、摻雜矽薄膜(含n型與p型)以及p型碳化矽，探討的製程參數則包含燈絲溫度、基板溫度、腔體壓力與反應氣體比例等。為探討製程參數與成膜品質之關聯，所成長矽膜之微結構與光電特性都被分析與探討，以求優化製程參數。
在本研究中，吾人成功開發以熱燈絲化學氣相沉積法製作元件級本質非晶矽膜，該膜能隙為1.6至1.7 eV、暗電導為2.3×10-11 Ω-1cm-1、光電導為6.1×10-4 Ω-1cm-1。透過氫氣比例調變，吾人亦成功以0.8 nm/s的沉積速率製作出元件級之多晶矽薄膜，以拉曼光譜與穿透式電子顯微鏡鑑定，該膜於厚度1 um時的結晶指數達93 %、晶粒尺寸大於150 nm、優選方向為(220)、能隙為1.1 eV、暗電導為8.2×10-8 Ω-1cm-1、光電導為1.1×10-5 Ω-1cm-1。透過摻雜氣體B2H6與PH3添加，也成功以熱燈絲化學氣相沉積法製作元件級之p型與n型矽薄膜。該n型矽膜的暗電導為0.292 Ω-1cm-1、活化能為0.036 eV且能隙為1.95 eV；該p型矽膜的暗電導為0.15 Ω-1cm-1、活化能0.05 eV且能隙為2.18 eV。於元件級碳化矽薄膜之開發，研究重點聚焦於通入之氫氣比例對膜質之影響。隨著氫氣比例上升，沉積率下降但結晶率上升，其結晶的優選方向為(111)、(220)與(311)，修正製程參數後，將可得到具有摻雜濃度為1.03 × 1020 cm-3、活化能為0.14 eV 且暗電導為3.44 × 10-2 Ω-1cm-1特性之元件級p型碳化矽薄膜。
經過製程優化後，我們將以上熱燈絲化學氣相沉積法製作之元件級矽膜實際應用於太陽電池之製作與驗證上。採用之元件為單面矽異質接面太陽電池，結構為：上電極/摻雜矽膜射極/本質矽緩衝層/單晶矽吸收層/下電極。此種元件透過鍍膜於單晶矽晶片上以形成pn接面，本質緩衝層可用於改善接面特性，由於此元件有高效率與低成本的優勢，近年受到高度矚目。我們研究的元件結構包含：(1) n型微晶矽成長於p型晶片上；(2)雷射退火之 n型多晶矽成長於p型晶片上；(3)雷射擴散圖形化之n型選擇性電極成長於p型晶片上；以及(4) p型碳化矽成長於n型晶片上。
對於n型微晶矽成長於p型晶片結構之矽異質太陽電池，透過調變摻雜氣體(PH3)的比例，吾人得優化n型射極層之特性，藉此實現轉換效率為13.35 %的矽異質接面太陽電池。為進一步提升射極之膜特性，雷射再結晶技術被用於改善n型微晶矽薄膜之晶界缺陷，其改善之成果可透過拉曼光譜與穿透式電子顯微鏡驗證。最佳雷射之功率後，n型微晶矽成長於p型晶片結構之矽異質太陽電池效率可提升至14.2 %。再者，雷射擴散技術也被用於改善射極與上電極銦錫氧化物之接觸特性，透過功率優化與擴散圖形設計，n型微晶矽成長於p型晶片結構之矽異質太陽電池效率可再提升至14.31 %。而針對p型微晶碳化矽成長於n型晶片之矽異質太陽電池，透過調變製程之氫氣比例，可以改善p型微晶碳化矽光穿層的特性，經過一系列測試後，當氫氣流量為75 %時可以得到最佳轉換效率14.5 %。以上結果證實了以熱燈絲化學氣相沉積法製作之元件級矽膜應用於太陽電池製作之可行性，此結果對於日後開發熱燈絲化學氣相沉積法於太陽電池量產技術上是十分重要的指標。

Hot-wire chemical vapor deposition (HWCVD) is a promising technique for depositing device-quality thin amorphous, polycrystalline, and epitaxial silicon films at lower temperatures and higher deposition rates. With this technique, deposition species are generated by decomposition reaction of the source gases on the heated filament. Comparing with conventional plasma-enhanced CVD, main advantages of HWCVD are as follows: (1) low deposition temperature, (2) high deposition rate, (3) low equipment cost, (4) large area deposition, (5) high gas utilization, (6) absence of ion bombardment and easy control of the film crystallinity by varying composition of the gas.
The primary aim of this dissertation can be divided into two parts: (1) to develop techniques of device-quality silicon films using HWCVD, and (2) to advance applications using silicon films deposited by HWCVD for silicon-base solar cells. A variety of materials, including intrinsic silicon (amorphous, microcrystalline, and polycrystalline), doped silicon (p-type and n-type), and p-type silicon carbide (SiC), was studied to make the films with device-quality. Role of the deposition parameters, mainly including filament temperature, substrate temperature, deposition pressure and gas dilution ratio, were considered to characterize the deposited films. Although the structural, electrical and optical properties can be individually analyzed by means of different characterization techniques, a clear correlation among them was observed. Finally, we produced device-quality intrinsic amorphous silicon films (energy gap (Eg) = 1.6-1.7 eV, dark-conductivity = 2.3×10-11 Ω-1cm-1, and photoconductivity = 6.1×10-4 Ω-1cm-1), poly-Si films (crystalline fraction > 93 %, grain size > 165 nm, Eg = 1.1 eV, dark-conductivity = 8.2×10-8 Ω-1cm-1 and photoconductivity = 1.1×10-5 Ω-1cm-1), n-type microcrystalline silicon (uc-Si) films (dark-conductivity = 0.292 Ω-1cm-1, activation energy (Ea) = 0.036 eV and Eg = 1.95 eV), p-type uc-Si films (dark-conductivity = 0.15 Ω-1cm-1, Ea = 0.05 eV and Eg = 2.18 eV) and p-type SiC films (carrier concentration = 1.03 × 1020 cm-3, Ea = 0.14 eV and dark-conductivity = 3.44 × 10-2 Ω-1cm-1). As the result shown above, those HWCVD deposited silicon films were confirmed to be using for solar cell applications.
In this dissertation, a single-sided silicon heterojunction solar cell was fabricated and characterized with the structure of front contact/doped silicon thin emitter/intrinsic silicon thin buffer/mono-crystalline silicon absorber/rear contact. A doped silicon emitter layer is combined with a thin intrinsic amorphous silicon buffer upon a different type mono-crystalline silicon absorber to form the pn-junction cell. Such heterojunction cells had attracted much attention because of their high efficiency and low-cost fabrication process. The cell structure comprises an n-type uc-Si emitter on p-type wafer, laser-annealed n-type poly-Si emitter on p-type wafer, laser-doping patterned n-type uc-Si selective-emitter on p-type wafer, and finally a p-type uc-SiC emitter on n-type wafer.
After optimizing the dopant dilution (PH3) for the n-type emitter deposition, a conversion efficiency of 13.35% was achieved for the silicon heterojunction cells with an n-type uc-Si film on the p-type wafer. To improve the n-type emitter properties, a laser crystallization technique is used to reduce the grain boundary defects of HWCVD deposited micro-crystalline n-type films. It was found that the cell performance can be enhanced under an optimum laser power density, where the 14.2% conversion efficiency has been obtained. Furthermore, a laser doping technique was employed to improve the contact resistance between indium-tin oxide and n-type emitter via the formation of the selective emitter structure. By optimizing the laser power density and doping-pattern design, a cell with a selective-emitter structure can achieve an efficiency of 14.31%. For p-type uc-SiC emitter on n-type wafer, a HWCVD deposited p-type uc-SiC film is used as a window layer in n-type crystalline silicon heterojunction solar cells. The effect of hydrogen dilution during p-type silicon carbide deposition on the material properties and cell performance are investigated. The silicon heterojunction cell with an efficiency of 14.5% can be achieved under a hydrogen flow ratio of 75% in the preparation of p-uc-SiC film for. These are very encouraging results for future fabrication of high efficiency heterojunction solar cells by using HWCVD technique.

Abstract (in Chinese) i
Abstract iii
Contents v
Symbols and Specific Substance Index viii
Chapter 1: Introduction 1
1.1 Solar Energy 1
1.1.1 Power of Sun 1
1.1.2 Brief History of Solar Cells 2
1.1.3 Recent Solar Cells 2
1.2 Silicon Thin Film 3
1.3 Hot-Wire Chemical Vapor Deposition 5
1.3.1 Brief History of the Hot-Wire 5
1.3.2 Deposition Mechanism of the Hot-Wire 6
1.3.3 The Difference Between HWCVD and PECVD 9
1.4 Silicon Thin-Film Solar Cell 10
1.5 Silicon Heterojunction Solar Cell 13
1.6 Aim of This Work 15
1.7 Outline of this thesis 15
Chapter 2: Experimental Techniques and Concepts 17
2.1 Hot-wire Chemical Vapor Deposition 17
2.1.1 Hot-wire at NCHU 17
2.1.2 Design of Deposition Conditions 18
2.1.3 Some Important Definitions in Hot-Wire 19
2.2 Material Characterization Techniques 19
2.2.1 Optical Material Characterization 19
2.2.2 Structural Material Characterization 20
2.2.3 Composition Material Characterization 22
2.2.4 Electrical Material Characterization 23
2.3 Solar Cell Characterization 24
2.3.1 Current Density vs. Voltage Measurement (J-V) 24
2.4 Previous work: device-quality silicon films by HWCVD 25
Chapter 3: Laser Annealed n-poly-Si/p-c-Si SHJPV 28
3.1 Introduction 28
3.2 Experimental 29
3.3 Results and Discussion 30
3.4 Conclusion 34
Chapter 4: Point Heavy-Doped n-uc-Si/p-c-Si SHJPV 35
4.1 Introduction 35
4.2 Experimental 36
4.3 Results and Discussion 38
4.4 Conclusion 44
Chapter 5: Effects of n-Wafer Specification on SHJPV 45
5.1 Introduction 45
5.2 Experimental 46
5.3 Results and Discussion 47
5.3.1 Wafer Thickness 47
5.3.2 Bulk Resistivity 48
5.3.3 Minority-Carrier Lifetime 49
5.4 Conclusions 51
Chapter 6: p-uc-SiC/ n-c-Si SHJPV 52
6.1 Introduction 52
6.2 Experimental 53
6.3 Results and Discussion 55
6.4 Conclusion 63
Chapter 7: Conclusion and Future Work 64
7.1 Conclusion 64
7.2 Future Work 66
Reference 67
List of Publications 79

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