On the band gap anomaly in I-III-VI2, I-III3-VI5, and I-III5-VI8 families of Cu ternaries

The experimentally observed energy band gap difference (Delta E-1) between the I-III3-VI5 and I-III-VI2 and the energy band gap difference (Delta E-2) between the I-III5-VI8 and I-III-VI2 phases of Cu-In-Se, Cu-Ga-Se, Cu-In-Te, and Cu-Ga-Te systems is explained in terms of the relative shift of the conduction band minimum (CBM) and the valence band maximum (VBM) caused due to the presence of the ordered V-Cu and [In(Ga)(Cu)(+2)+2 V-Cu(-1)] defect pair and to the effect of the p-d hybridization. The nearly linear variation of Delta E-1 and Delta E-2 with p-d hybridization of the corresponding I-III-VI2 phase suggests that in selenides the lowering of the VBM predominates over that of the CBM. In the case of the Cu-In-Te system, they are very near the same magnitude, whereas in Cu-Ga-Te the lowering of the CBM predominates over that of the VBM. (C) 2000 American Institute of Physics. [S0003-6951(00)01927-6].