Low-temperature Deposition Of Poly-silicon Thin Films By Hot-wire CVD And The Analysis On Characteristics

Recently, extensive researches have been performed on polycrystalline silicon (poly-Si) films for applications in various electronic devices such as thin film transistors (TFTs) and thin film solar cells. Hot-wire CVD (HWCVD) is widely used to deposit a-Si: H and poly-Si films, due to many advantages compared with other methods, including being much simpler and easier to adopt for large-area deposition at a lower cost, low deposition temperature and high deposition rate.This paper presents the development and the latest achievements in this field as obtained by hot-wire chemical vapor deposition (HWCVD) technology. Moreover, the developing trend and prospects of this technique for poly-Si films are predicted. On this basis, the fabrication of high quality poly-Si thin films is investigated in this thesis. Poly-Si films are deposited on the glass and (100) silicon wafer substrates, in where the distance between the filament and the substrate is in the range of 5mm-10mm. The influence of various parameters on the crystal orientation, size and morphology of poly-Si grains is investigated.Experimental results showed that the distance between the filament and the substrate has a great impact on the crystal orientation. When the distance is 5mm, the preferential orientation of the poly-Si films changed from (111) to (220), and then back to (111), as increasing of the filament temperature in the range of 1400℃-1800℃ (corresponding the substrate temperature was 225℃-320℃). Such experimental law, to the authors' best knowledge, is not available in the open literature. When the distance between the filament and the substrate is 8mm or 10 mm, the preferential orientation (111) of poly-Si films has no significant changes with the rise in the filament temperature. In addition, with the increasing distance between the filament and the substrate, the crystalline volume fraction and the grain size of poly-Si films notablely decrease.In the experiment, we found that crystal morphology of poly-Si grains was strongly affect by deposition pressure, and the influence of the SiH₄ concentration in the range of 1%-5% was not obvious. The Poly-Si films with a moderate lateral grain-size of 1μm and a vertical grain size approximately the same as the film thickness (approx. 5μm) can be deposited at a low filament temperature of 1450℃, a low substrate temperature of 235℃, and the deposition pressure of 60Pa.