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

Abstract

The experimentally observed energy band gap difference (ΔE1) between the I–III3–VI5 and I–III–VI2 and the energy band gap difference (ΔE2) 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 VCu and [In(Ga)Cu+2+2 VCu−1] defect pair and to the effect of the p–d hybridization. The nearly linear variation of ΔE1 and ΔE2 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.