Band-structure determination by subgap spectroscopy in thin films of semiconductors

Abstract

Abstract We use the dispersion of the refractive index of subgap light to determine the main parameters of the band structure of thin films of semiconductors. By making a semiempirical calculation of the effect of the final width of the valence and conduction bands, we obtain a variation in the refractive index n(E) with the light energy E, which goes one order beyond the simple two-level one-oscillator formula. By comparison with the experimental transmission curve, we show that it is possible to obtain, with a simple commercial spectrometer, the distance E M between the centres of gravity of the valence and conduction bands and also an estimate of the widths Δ of the bands. We have applied this method to a series of thin films of hydrogenated amorphous silicon and carbonated alloys.