Structural and Electronic Properties of ZnGeAs2

Using a first-principles band-structure method based on density functional theory, we have studied the structural and electronic properties of ZnGeAs₂. In agreement with experimental data, ZnGeAs₂ is found to be nearly lattice matched to its binary analog GaAs. The calculated band structures show that ZnGeAs₂ in the chalcopyrite structure has a direct band gap, about 0.31 eV smaller than the band gap of GaAs. The calculated valence-band offset between ZnGeAs₂ and GaAs is 0.18 eV; thus, the band alignment of the ZnGeAs₂/GaAs system is type I, with both holes and electrons localized on ZnGeAs₂. We also find that at low temperature, (ZnGeAs₂)0.5(GaAs) forms a stable stannite structure. However, the band gap and the mixing energy of the alloy at higher temperature depend sensitively on the local short-range order. The calculated formation energies of the (ZnGeAs₂)n(GaAs)2nsuperlattices along the [001] direction show strong nonmonotonic behavior, with the formation energy ΔHn maximized at n = 2. We compared our results for the ZnGeAs₂/GaAs system to the well-studied CuGaSe₂/ZnSe system. The differences between these two systems are explained in terms of their ionicity and their relative strength of the anion p and cation d couplings.