Fabrication of Large-Grained Polycrystalline Si Thin Films by Controlling Nucleation Sites on Glass Substrates

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Large-grained polycrystalline silicon (poly-Si) thin films on glass substrates are of interest for the fabrication of high-performance thin-film transistors (TFT) and cost-effective solar cells. To obtain large-grained poly-Si thin films, it is important to control the formation of nucleation sites for Si crystal growth on a glass surface prior to film deposition. Glass substrates having nucleation sites with tailored properties, such as size and spacing, are particularly desirable for the solid-phase crystallization (SPC) of amorphous silicon (a-Si) thin films. In this work, we have proposed a new process to fabricate crystalline Ge islands with controlled size and density in a tailored range to form nuclei for SPC of a-Si thin films. The process consists of the deposition of a-Ge thin films on glass substrates at room temperature, the subsequent vacuum annealing for SPC of the deposited a-Ge thin films to form self-assembled crystalline Ge islands, and the oxygen etching for controlling the size and density of Ge islands. From the micro beam reflection high-energy electron diffraction observation of the partially crystallized a-Si thin film using Ge nuclei, we have confirmed that Ge islands act as nuclei for Si crystallization. Marked reductions of the crystallization time and temperature are observed in the annealing experiments of a-Si : H thin films deposited on glass substrates with Ge nuclei. Moreover, it is found that the crystal growth rate of a-Si : H deposited by atmospheric pressure plasma CVD is much larger than that of a-Si deposited by E-beam evaporation. For the location control of Ge nuclei on glass substrates, a nano-imprint technique using crystalline Ge tip array has been demonstrated.