Electronic structure and stability of quaternary chalcogenide semiconductors derived from cation cross-substitution of II-VI and I-III-VI2 compounds

Sequential cation cross-substitution in zinc-blende chalcogenide semiconductors, from binary to ternary to quaternary compounds, is systematically studied using first-principles electronic structure calculations. Several universal trends are found for the ternary and two classes of quaternary chalcogenides, for example, the lowest-energy structure always has larger lattice constant a, smaller tetragonal distortion parameter eta=c/2a, negative crystal-field splitting at the top of the valence band, and larger band gap compared to the metastable structures for common-row cation substitution. The band structure changes in the cation substitution are analyzed in terms of the band offsets and band character decomposition, showing that although the band gap decreases from binary II-VI to ternary I-III-VI2 are mostly due to the p-d repulsion in the valence band, the decreases from ternary I-III-VI2 to quaternary I-2-II-IV-VI4 chalcogenides are due to the downshift in the conduction band caused by the wave-function localization on the group IV cation site. We propose that common-row-cation I-2-II-IV-VI4 compounds are more stable in the kesterite structure, whereas the widely assumed stannite structure reported in the literature for experimental samples is most likely due to partial disorder in the I-II (001) layer of the kesterite phase.