Preparation of Band-Edge Modulated Semiconducting Films and Their Properties

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Band-edge modulated semiconductors are a new type of artificial materials, in which nitrogen composition, chi, in a-Si_<1-x>N_x : H is almost sinusoidally modulated along a direction, z. They were, for the first time, prepared in our laboratory. The edge of conduction band and valence band is modulated with an approximately sinusoidal function. In the case of large amplitude of modulation, optically created electrons and holes may be located around the bottom of the conduction band and the top of the valence band, so that they are affected by modulated potentials around z = 0, these being approximated by a harmonic oscillator potential. Thus, the energy of the conduction band and valence band is quantized along the z direction. The optical gap energy, Eg, measured by optical absorption spectra corresponds to the energy difference between the first quantized levels of both bands. With decreasing the modulation period, L, the observed value of Eg increased in agreement with a theoretical prediction. We have also observed a quantized structure in the photoinduced absorption spectra for samples with L 100 A^^ﾟ and 67 A^^ﾟ, corresponding to optical transitions of a self-trapped hole in weak Si-Si bond into the valence band. This structure corresponds to quantized levels of the valence band. For band-edge modulated films, the hot luminescence has been observed for L 30 290 A^^ﾟ, which has not been observed in bulk films. These band-edge modulated films ar expected to be a promising new material for quantum-electronic devices.