Effect of Cu deficiency on the optical bowing of chalcopyrite CuIn1−xGaxSe2

Optical bowing coefficients are used to describe the band gap variation of a composite semiconductor alloy. It is known to be related to the electronic structure and the lattice deformation in the semiconductor alloys. Spectroscopic ellipsometry study shows that the optical bowing coefficient of slightly Cu-poor polycrystalline Cu0.9In1−xGaxSe2 is larger than that of stoichiometric polycrystalline CuIn1−xGaxSe2 and band gaps are larger when Cu becomes poor. This can be explained by an increase in valence band offset due to reduced p-d coupling and an increase of perturbation potential ΔV due to lattice deformation.