Low Temperature growth of poly-crystalline film of Silicon-rich Silicon-Germanium by Reactive Thermal Chemical Vapor Dep
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Low Temperature growth of poly-crystalline film of Silicon-rich Silicon-Germanium by Reactive Thermal Chemical Vapor Deposition Kousaku Shimizu, Jianjun Zhang, Jeong-woo Lee, and Jun-ichi Hanna Imaging Science and Engineering Laboratory, Tokyo Institute of Technology ABSTRACT Low temperature growth of poly-SiGe has been investigated by reactive thermal chemical vapor deposition method, which is a newly developed technique for preparing poly-SiGe by using redox reactions in a set of source materials, i.e., Si2H6 and GeF4. In order to prepare silicon-rich poly-SiGe of high mobility, a series of experiment on total pressure, gas flow rates of the source materials and dilution gas of He, and residence time at 450°C has been investigated. At 0.45 Torr, high crystallinity films with high silicon content were prepared, however, homogeneity of film thickness and reproducibility of the film growth was quite low for device application. For overcoming this problem, the growth condition has been studied especially in higher-pressure range of 5-15 Torr. Appropriate choice of the residence time and the gas flow ratios lead to significant improvement in the Si content in the films. Finally, more than 95% of silicon-rich poly-SiGe films, which is p-type, has 7.5 cm2/Vs of Hall mobility and (220) orientation, have been prepared at 10 Torr and 450°C within ±2% fluctuation of reproducibility which is enough to fabricate devices. INTRODUCTION Nowadays, the hydrogenated amorphous silicon (a-Si:H) thin film prepared by rf-grow discharge of silane (SiH4) is established for large-area electronic devices such as thin film transistors (TFTs) for liquid crystal displays, solar cells, and xerographic photoreceptors. The a-Si:H, however, remains a serious problem; i.e. photo-induced degradation of electronic properties, termed as Staebler-Wronski effect [1]. In addition, the a-Si thin films have limitation in the fabricating the devices with a higher switching time over 10 ns because of a small mobility; ~1 cm2/Vs. Polycrystalline Si (poly-Si) films prepared at a temperature where glass substrates can be employed are a promising candidate for such the advanced large-area electronic devices, because of a good compatibility with conventional Si device fabrication processes and of superior electronic properties, e.g., high mobility, effective doping efficiency, and a large absorption coefficient in infra-red region compared with the a-Si:H thin films. Several techniques for preparing poly-Si at the temperature have been intensively investigated, e.g. plasma CVD [2], Hot-wire CVD [3], and photo-CVD [4]. These techniques are still under investigation and have some problems to be solved: poor crystallinity at an early stage of film growth, low uniformity of film thickness or low growth rate. Generally highly homogeneous crystallinity and a low crystal fraction or high crystallinity and high rate growth hardly go together. The former is attributed to be delayed nuclei formation during film growth and their competitive growth of grains each other [5a,b
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