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

Abstract

Optical absorption edge spectra of amorphous B, B 4 C 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 \(\alpha \hbar \omega =\)A\((\hbar \omega -E_{\rm opt})^{\rm n}\), where α , \(\hbar \omega\) , 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 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. 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.