A Significant Reduction of Impurity Contents in Hydrogenated Microcrystalline Silicon Films for Increased Grain Size and Reduced Defect Density

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Abstract
We have prepared high-purity hydrogenated microcrystalline silicon films (µc-Si:H) on quartz substrates from a mixture of silane and hydrogen using a new type of ultrahigh vacuum plasma enhanced chemical vapor deposition system. In substrate temperatures ranging from 25 to 350°C, we have produced high-crystallinity µc-Si:H, as observed by Raman spectroscopy. At the highest temperature (∼350°C), we obtain larger crystalline Si grains, ∼1000 Å, estimated using Scherrer's formula. Transmission electron microscopy micrographs show that these crystalline grains are conical and extend 7000∼8000 Å in the film growth direction with a lateral size of ∼1000 Å. At a mid-range temperature (∼200°C), a spin density as low as ∼5×1015 cm-3 and the midgap position of the Fermi level imply a substantial reduction of the density of defects states in this pure film. Moreover, this pure film is stable against prolonged light exposure. Implications of these results for the role of impurities in the growth process and optoelectric properties of µc-Si:H are discussed.