ABSORPTION-EDGE SPECTRA OF BORON-RICH AMORPHOUS FILMS CONSTRUCTED WITH ICOSAHEDRAL CLUSTER

Optical absorption edge spectra of amorphous B, B4C and B13P2 films, prepared by electron-beam deposition, were measured with a spectrophotometer in the wave length range of about 300 similar to 700 nm. These were fitted by alpha HBAR omega=A (HBAR omega-E(opt))(n), where alpha, HBAR omega, A and the constant E(opt) are the absorption coefficient, the photon energy, a constant and the optical gap energy, respectively. The exponent n was found to be 3 for the amorphous boron-rich films, though n was 2 for the usual amorphous semiconductors. From a comparison of the temperature dependence with that of crystalline beta-rhombohedral boron (beta-B), the usual selection rule of indirect forbidden type transition did not bold in the amorphous solids of these boron-rich systems. Therefore the origin of n=3 was concluded to be the linear distribution of the densities of states. The value of the exponent n and the relatively small value of A were related to the complicated atomic and electronic structures originating from the B-12 clusters. A large difference between the band gap of the amorphous boron-rich solids and that of the crystalline phases was observed, for example, 1 eV in pure boron.