| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 101 === The requirement of photovoltaic (PV) energy has vastly grown in last decade. To achieve high conversion efficiency with low production cost is the mainstream in the solar cell technology. Crystalline Si (c-Si) based solar cell is very popular since it combines the low-cost material system with the stable and high conversion efficiency for long term usage. Aluminum-induced crystallization (AIC) process is an approach to crystallize amorphous Si (a-Si) by means of contacting Al layer with a-Si. This approach is capable of achieving large grain sizes of over 10 μm or realizing the epitaxial growth on a seeding layer at a very low temperature of 673K. For improving several critical issues in the present solar PV technology and solid state devices, AIC process is very promising due to the features of low reaction temperature, low cost in precursors and facilities, and environmental friendly. In this study, the several limitations of standard AIC process were broke to adapt to the solar cell or industrial requirement. Furthermore, the material system in AIC process was modified to extend the application to other advanced solid state devices. Besides the application results, the theoretical models of our approaches had also been established to reveal the mechanisms of the modified processes.
There’re three parts in this thesis to investigate several critical issues of AIC process for advanced solid state device application. In the first part, AIC process can be accelerated by a factor of about 50 by the doping of Si atoms into the initial Al layer for improving the throughput. This process is known as Si-doped AIC (Si-AIC). The grain size and crystallographic orientation of the grown polycrystalline Si (poly-Si) thin film produced are modified due to the fact that the presence of excess Si in the initial Al layer alters the nucleation and growth behavior of the Si grains as compared with the traditional AIC process. In this part the nucleation mechanism and growth rate of Si grains for Si-AIC are analyzed and quantitatively compared with those for AIC using time-series TEM/EDS images. It is found that the activation energy for grain growth was significantly reduced in the Si-AIC process, by 0.7eV compared with the AIC process.
In the second part we investigate the heavily doping of BSF fabrication at very low temperature. P-type poly-Si film on foreign substrate can be fabricated at temperature lower than 773K by traditional AIC process. However, the ultimate carrier concentration of Si film is limited to approximately 3×1018 cm-3 because of the low solid solubility of Al in Si at temperatures below 773K. In this study, a process called B-AIC is developed in which boron is co-doped with Al to increase the carrier concentration in Si film to ~1019cm-3 at temperature as low as 673K. The carrier concentration can be tuned by the initial thickness of a-Si layer in B-AIC process. Beside the fabrication of poly-Si film on glass, the epitaxial growth of this heavily doped p++-Si film can also be realized on a mono-c-Si wafer via solid phase epitaxy (SPE) mechanism. The AIC/SPE thermodynamic model is also developed in this part.
In the final part, the AIC process featured with SPE mechanism are extended to Si-Ge-Al system for heterogeneous epitaxy. A 300 nm Si1-xGex with tunable Ge content thin film can be epitaxially grew onto (100) mono-c-Si wafer at very low temperature of 673K. The chemical composition, atomic structure and the SPE reaction featured with the dissociation-diffusion-crystallization model of AIC process are carefully revealed and established. The development of this SiGe-AIC-SPE process is very promising to fabricate a low defect density virtual substrate by terrace grading structure approach due to its advantage of well controllable doping profile. Furthermore, the features of simple fabrication process, low material cost and low reaction temperature makes this approach predominant in the virtual substrate market.
Additionally, our several works for revealing the fundamental of AIC process are appended in the appendix.
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