- 學位論文
鍺基光電元件及太陽能電池之研究
Ge-based Optoelectronics and the Study of Photovoltaic Devices
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摘要
在本篇論文之中，著重在鍺基的光電元件及太陽能電池的研究，主要可以分成二大部分，分別探討鍺的光學元件特性以及目前太陽能產業所關注的電池特性分析技術。 長久以來，光電積體電路一直是人類追求的夢想，因為光訊號的傳送比電訊號傳送速度快很多，人們也希望藉由光電積體電路的發展，整合於現有的矽基電子元件來達成更大的市場用途。所以本篇論文前段部分，討論利用相容於積體電路的鍺材料，做出發光波長在1.6微米及1.8微米的鍺基發光源，並研究其直接與間接能隙的躍遷機制及其增強的方法。在室溫下，鍺半導體直接能隙與間接能隙之間的能量差僅為140毫電子伏特，可以藉由N型高摻雜、高溫、高電流密度、高雷射激發功率及外加應力以增強直接能隙的發光性複合。主要增強鍺直接能隙的躍遷機制在於增加直接帶隙(Γ valley)的載子濃度，並降低缺陷及雜質的濃度，以增加直接能隙的發光強度。 本論文的第二大部分討論太陽能電池的特性分析技術。在可再生能源之太陽能電池的研究上，效率的提升最為重要。目前的太陽能模組效率都比單一的太陽能電池效率低很多，主要原因之一在於均勻度及良率不佳，故其特性檢測及缺陷分析扮演著很重要的角色。藉由光激發光、電激發光、能量轉換效率及外部量子效率等多種光學量測方式作為特性檢測及缺陷分析，應用在矽基太陽能電池、薄膜太陽能電池及多接面太陽能電池上。單晶矽太陽能電池之光激發光現象，可在室溫下觀察到1.5微米的紅外光。藉由時間解析的發光特性量測，考慮發光性復合及非發光性復合的種種躍遷機制作為疊代分析，以量測少數載子生命週期。銅銦鎵硒薄膜太陽能電池之光激發光特性顯示出與缺陷能階相關的躍遷，並可進一步分析出施體能階與受體能階。缺陷會造成銅銦鎵硒太陽能電池的短路電流溫度係數為負值，導致高溫下的效率變小。多接面太陽能電池(GaInP/GaInAs/Ge)之光激發光與電激發光特性可作為各接面的特性檢測。GaInP、GaInAs及Ge之能隙躍遷可發出0.7、0.9及1.8 微米的光。電激發光特性搭配適當的濾波片，可用來分析各接面的空間均勻性與缺陷。光激發光與電激發光波長與各層的光吸收範圍相關，可用來量測各接面能隙能量的溫度係數，可幫助在不同的操作溫度下，多接面太陽能電池電流匹配的最佳化設計。
並列摘要
In this dissertation, two important topics are included. One is the optical characteristics of Ge based optoelectronics and the other is the analysis technology of solar cell performance in the photovoltaic industry in recent years. It has been a long time to make the optoelectronic integrated circuits (OEIC) comes true. The transmission of optical signals is faster than electrical signals. With the development of OEIC, the integration of OEIC and Si-based electronics can add extra functionalities for further applications. Part I: Ge can integrate on Si ultra large scale integration (ULSI) circuits and the Ge-based emitter can emit infrared at the wavelength of 1.6μm (direct band gap transition) and 1.8μm (indirect band gap transition). The mechanisms of direct and indirect band gap transitions and the enhancements of direct band gap transition from Ge are studied. The energy difference between direct and indirect band gaps is only 140meV at room temperature. The enhancements of direct band gap transition can be achieved by the high concentration of n type doping, the high pumping level, the elevated temperature, and the strain. The key point of the enhancements is to increase the carrier density in direct valley (Γ valley) and reduce the defect and impurity density. It can enhance the light emission intensity from Ge direct band gap transition. Part II: The analysis technology of solar cells. The improvement of efficiency is most important in renewable solar energy study. Up to now, the efficiency of solar module is much lower than cell due to the uniformity and yield issues. The cell performance determination and defect analysis technique play important roles. The photoluminescence, electroluminescence, efficiency measurement, and external quantum efficiency measurement are used to characterize cell performance and defect information. The analysis models are studied and can be applied to wafer based Si solar cells, thin film solar cells, and multi-junction solar cells in this dissertation. The electroluminescence from Si solar cell at the wavelength of 1.15μm is observed at room temperature. The temporal response of electroluminescence is used to characterize the minority carrier lifetime by fitting the time evolution of radiative recombination using the Shockley-Read-Hall, radiative, and Auger recombination models. Photoluminescence of defect-related Cu(In,Ga)Se2 thin film solar cells show the donor-acceptor transition and band-impurity transition. The donor level and acceptor level can be extracted. The negative temperature coefficient of the short circuit current of Cu(In,Ga)Se2 solar cells may cause more degradation of power conversion efficiency at high temperature due to defects. Electroluminescence and photoluminescence of the monolithic GaInP/GaInAs/Ge triple-junction solar cells are investigated to probe each subcell quality. The band-to-band emissions from the GaInP, GaInAs, and Ge junctions are observed at the wavelengths of 0.7, 0.9, and 1.8 μm, respectively. Electroluminescence can measure the spectral uniformity and defects of individual subcell with selected filter. The photoluminescence and electroluminescence emission are related to absorption edge and the temperature coefficient can be used to design the device structure for current match condition at operation temperature.
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