Absorption Edge Spectra of Boron-Rich Amorphous Films Constructed with Icosahedral Cluster

概要

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Optical absorption edge spectra of amorphous B, B_4C and B_<13>P_2 films , prepared by electron-beam deposition, were measured with a spectrophotometer in the wave length range of about 300-700 nm. These were fitted by αℏω=A(ℏω-E_<opt>)^n, where α, ℏω, 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 semi-conductors. From a comparison of the temperature dependence with that of crystal-line β-rhombohedral boron (β-B), the usual selection rule of indirect forbidden type transition did not hold in the amorphous solids of these boron-rich systems. There-fore 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, l eV in pure boron.
社団法人日本物理学会の論文
1995-09-15