| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 104 === Reducing the solar cell manufacture cost makes the concentrator photovoltaic (CPV) has more competitive in the market. One of the potential method is to use a single crystal silicon substrate (sc-Si) substituted germanium or gallium arsenide (GaAs) substrate. Growth of Si1-xGex epitaxial layer on single crystal Si substrate with different Ge concentration makes the single crystal Si substrate have Ge properties including lattice constant and so on, which maybe significant reduce the CPV manufacture cost. In this study, we will use the aluminum-induced solid phase epitaxy (AI-SPE) process to fabricate Si1-xGex epitaxial layer with different Ge concentration under low-temperature. In previous study, the mechanism of aluminum-induced crystallization (AIC), it used to grow poly-Si or poly-Ge, has well studied and build up. However, researchers have a superficial knowledge of aluminum-induced solid phase epitaxial. We believe that the mechanism of AI-SPE should be build-up if we would like to control the reaction process.
In this study, we will firstly discuss the effect of (1) pre-doping Ge position, (2) pre-doping Ge concentration, and (3) reaction temperature during AI-SPE process. According to the results, the Ge concentration indeed can be well controlled via pre-doping Ge technique after AI-SPE process, and the optimal reaction temperature should higher that 400˚C.
Moreover, we will use in-situ heating transmission electron microscopy to observer the AI-SPE reaction process, and the analysis of the thermodynamics exactly supports the finding from in situ TEM. Based on these results, the mechanism of AI-SPE can be concluded into five steps: (1) The covalent bond of a-Si1-xGex will be weakened and formed "free atoms" by electrons that it is surrounding the surface of aluminum (Screening effect); (2) The free atoms driven by the diffusion driving force, it will diffuse through aluminum grain boundaries to the interface between aluminum and single crystal Si substrate, and the free atoms will thermodynamically stable accumulate at interface until its thickness reaches the critical thickness (Diffusion); (3) As the accumulated thickness reaching the critical thickness, the system will become unstable. The a-Si1-xGex will generate a new crystalline phase to reduce the free energy making the system become stable, that is, Si1-xGex nuclei. the crystal orientation of Si1-xGx nuclei will affect by single crystal Si substrate and hetero-epitaxial grow on it (Nucleation); (4) The free atoms will continuously diffuse to the interface between aluminum and single crystal Si substrate, and supply to the nucleus for vertical and lateral growth. Simultaneously, the stress existing in the aluminum film generated during free atoms diffusion will release, thus, the aluminum will move to the position of a-Si1-xGex (Grain growth); (5) Finally, to form a continuous silicon-germanium epitaxial layer on the single crystal Si substrate. Based on the above study results, we finally successful prepared a germanium virtual substrate via multi-run aluminum-induced solid-phase epitaxy process, which can provide to CPV or integrated circuit as a low-cost substrate or template.
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