A THEORETICAL APPROACH TO THE OPTICAL GAPS OF 1D-SE1-XTEX AND 3D-SE1-XTEX

To study the relation between the interchain and intrachain interactions as well as to compare the chain-like features with the bulk characteristics in chalcogenide alloys, the Te concentration (x) dependence of the optical gap E(g) is theoretically analyzed for Se1-xTex of an isolated chain and a trigonal crystal. In the first place, band structures are calculated for an isolated chain and for a trigonal crystal by the tight-binding approximation. The behaviour of the interaction parameters obtained from our calculations indicates that the chain-like nature of a trigonal crystal becomes less distinguished as the increase of x. Secondly, the electronic density of states of Se1-xTex is calculated by the coherent potential approximation. The results show that the optical gap E(g) for a bulk mixture decreases as x increases while E(g) for a chain does not change very much for x greater-than-or-equal-to 0.2. These results are in good agreement with the experimental data.