Band-structure anomalies of the chalcopyrite semiconductors CuGaX2 versus AgGaX2 (X=S and Se) and their alloys

We have performed systematic first-principles calculations for the structural and electronic properties of chalcopyrite semiconductors AgGaS2, AgGaSe2, CuGaS2, CuGaSe2, and their alloys. We show that, in contrast to conventional semiconductors, the band structures of these compounds exhibit several anomalous behaviors: (i) The band gaps of AgGaX2 are larger than the corresponding CuGaX2 (X=S and Se) compounds, despite the lattice constants of AgGaX2 being much larger than for CuGaX2. (ii) The valence band offsets between common-anion pairs CuGaX2/AgGaX2 are large and negative (i.e., CuGaX2 has higher valence band maximum than AgGaX2), opposite to their II-VI analogs. (iii) The valence band offsets between (MGaS2)-Ga-I/(MGaSe2)-Ga-I (M-I=Cu, Ag) are significantly smaller than their II-VI analogs. (iv) The band gap bowing parameters for the common-anion alloys are larger than the common-cation alloys, following the same trend as the valence band offsets. Moreover, we find that the wave function localization of the conduction band minimum states at the group III site plays an important role on the band gap reduction of the chalcopyrites relative to their binary analogs. The origin of the band structure anomalies observed in this system is explained in terms of the atomic sizes and chemical potentials and the increased structural and chemical freedom of these ternary compounds.