Band-structure anomalies of the chalcopyrite semiconductors $\mathrm{CuGa}{X}_{2}$ versus $\mathrm{AgGa}{X}_{2}$ ($X=\mathrm{S}$ and Se) and their alloys

Band-structure anomalies of the chalcopyrite semiconductors $CuGa X_{2}$ versus $AgGa X_{2}$ ( $X = S$ and Se) and their alloys

Shiyou Chen, X. G. Gong, and Su-Huai Wei

Phys. Rev. B 75, 205209 – Published 31 May 2007

Abstract

We have performed systematic first-principles calculations for the structural and electronic properties of chalcopyrite semiconductors ${AgGaS}_{2}$ , ${AgGaSe}_{2}$ , ${CuGaS}_{2}$ , ${CuGaSe}_{2}$ , 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 $AgGa X_{2}$ are larger than the corresponding $CuGa X_{2}$ ( $X = S$ and Se) compounds, despite the lattice constants of $AgGa X_{2}$ being much larger than for $CuGa X_{2}$ . (ii) The valence band offsets between common-anion pairs $CuGa X_{2} ∕ AgGa X_{2}$ are large and negative (i.e., $CuGa X_{2}$ has higher valence band maximum than $AgGa X_{2}$ ), opposite to their II-VI analogs. (iii) The valence band offsets between $M^{I} {GaS}_{2} ∕ M^{I} {GaSe}_{2}$ ( $M^{I} = Cu$ , 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.