Optical absorption in hydrogenated microcrystalline silicon

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Abstract
Optical absorption measurements between 0.35 and 2.5 eV at 22 degrees C on hydrogenated microcrystalline silicon films prepared in a DC discharge at deposition temperatures (Tdep) between 110 and 450 degrees C, are presented and discussed. Anomalously high optical absorption was observed for films prepared at a floating potential, which is a few orders of magnitude higher than of single-crystal Si for samples prepared at Tdepbetween 260 and 350 degrees C and, in the photon energy range up to 2.0 eV, is also higher than that of amorphous Si. Samples prepared at a floating potential at Tdepor approximately=400 degrees C, and those prepared at a negative substrate bias, show a lower optical absorption, which for the latter films is similar to that of amorphous silicon. Annealing up to 800 degrees C under ultra-high vacuum resulted in a small but reproducible increase in absorption independent of Tdep. Similar annealing experiments followed by absorption measurements in high vacuum indicated no measurable effect of the adsorbed oxygen on the optical absorption. The angular dependence of the forward light scattering from the films was measured at various wavelengths and found to diffuse scattering and absorption in the films. Furthermore, quantitative estimates of the surface roughness of the films from reflection data and scanning electron micrographs were made, and these results are discussed with respect to the enhanced absorption and scattering in the films.