Enlargement of Crystal Grain Size and Significant Inclusion of Hexagonal Diamond Phase in Ultra Pure Microcrystalline Si
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Enlargement of Crystal Grain Size and Significant Inclusion of Hexagonal Diamond Phase in Ultra Pure Microcrystalline Silicon Toshihiro KAMEI Center for Integrated Systems, Department of Electrical Engineering, Stanford University, Stanford, CA 94305-4075, USA ABSTRACT We have shown significant effects of atmospheric impurities on crystal grain sizes of hydrogenated microcrystalline Si (µc-Si:H), the growth surface of which is totally covered by hydrogen. By reducing impurity concentrations, two growth modes emerge, bordered at 250°C. In particular, at higher temperature growth mode, strong (220) preferential growth takes place, resulting in increased grain size. Appearance of this growth mode seems to be related to surface monohydride. Sharp x-ray diffraction peak in lower angle than (111) is observed, which stems from (10-10) crystal plane of hexagonal diamond Si. Its formation is related to (220) oriented crystallites. Implication of these results is the possibility of Si polytype control. INTRODUCTION The technology of hydrogenated microcrystalline Si (µc-Si:H) for thin film (1-3µm) solar cells is growing rapidly, driven by their features of high efficiency (>10%) and virtually no photo-induced degradation [1, 2]. Entirely low temperature process, normally less than 200°C (at most 350°C), allows one to use inexpensive substrates such as glass and plastic film [3]. µc-Si:H is a composite of nanometer-size Si crystallites and amorphous Si (a-Si:H), which is typically made by plasma decomposition of a gas mixture of SiH4 and H2. Although amorphous phase is involved, the photo-induced degradation of µc-Si:H films is negligible under moderate intensity of light like sunshine. Carriers, even if generated in amorphous region, tend to diffuse into crystallites and subsequently recombine inside crystallite or contribute photocurrent through the percolation path of crystallites. As a result, nonradiative recombination of carriers in amorphous region is suppressed. This mechanism makes photo-induced degradation of µc-Si:H films smaller than expected from crystalline volume fraction [4]. Another important feature of µc-Si:H is that its growth surface is totally covered with hydrogen, as is inferred from the observation of a-Si:H growth surface by infrared reflection absorption spectroscopy [5, 6]. This leads to very low defect density, and facilitates Fermi energy to move according to substitutional impurity doping. It is also argued that this surface hydrogen coverage would reduce surface reactivity with contaminants and, thus, maintain the clean growth surface. As a result, impurity would not affect the crystal growth so much [7]. In this work, we have applied ultra clean plasma deposition process to µc-Si:H growth, and have found significant effects of atmospheric impurity, i.e., O, C, and N, on crystal quality such as grain size even in the case of hydrogen covered surface. In particular, grain sizes of (220) oriented crystallites show rapid increase above 250˚C, which is not seen until this purification is achieved. Th
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