We have used Raman scattering to deduce the volume fraction of crystallinity for the highly phosphorus-doped glow-discharge Si:F:H alloys. The measured critical volume fraction at the onset of rapidly increased conduction in this two-phase system of microcrystallites embedded in an amorphous matrix is 0.18. This value coincides with the theoretical percolation limit and serves to explain the conduction process in these two-phase materials which are useful as contacts in amorphous solar cells.
Infrared spectroscopy has been used to analyze the structural changes in samples prepared by the sol-gel method. Silica gels were prepared from alcoholic solutions of tetraethylorthosilicate (TEOS) with different H2O/TEOS molar ratios. The IR spectra of these gels, in the Si–O bond stretching region, shows that their structure strongly depends on the H2O/TEOS ratio. The relative change in intensity of the Si–O stretching modes, in samples prepared using different H2O/TEOS ratios, are interpreted in terms of different degrees of structural disorder. According to our infrared absorption data, a decrease in the H2O/TEOS ratio from about 7, the SiO2 structure evolves from a three-dimensional network toward a chainlike structure.
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