Impact of Site Disorder on Electronic Properties in ZnGeN₂

II-IV-N₂ materials hold promise for use in energy-relevant devices such as light emitting diodes and photovoltaics due to their capacity for tuning electronic structure through cation ordering while maintaining lattice constants closely matched to III-Vs. However, the large number of variables in characterizing films with diffraction and spectroscopy techniques complicates the process of quantifying order in these materials. To aid this characterization, we evaluate site disorder in ZnGeN₂ at short- and long-range through cluster-based Monte Carlo simulations.¹ This method isolates site disorder from other structural effects and captures a picture of the material under various nonequilibrium conditions allowing us to provide parameters for disordered material for device-level simulations. In ZnGeN₂, the non-isovalent character of the disordered species (Zn²⁺ and Ge⁴⁺) subjects the cation ordering to strong short-range order effects which influence band structure by decreasing the band gap relative to ordered ZnGeN₂ from 3.5 eV to 2.0 eV with small but non-dilute fractions of cations present as anti-site pairs.^2,3 The order transition associated with this band gap change creates isolated mid-gap bands in the density of states, which tend to strongly localize. From the simulations, converged configurations are relaxed in volume and lattice parameters through Density Functional Theory as 1,024 atom supercells. We calculate nitrogen coordination as a short-range order parameter and the Bragg-Williams and stretching parameters as long-range order parameters. We use localization of states in the electronic structure to analyze the character of energy gaps as either defect bands or band gaps, which is a point of discussion and presently ill-defined in disordered systems. We then relate the order parameters with the mixing entropy, free energy of the system and electronic properties to draw relations between degree of order and desired electronic properties.<br/>References<br/>J. J. Cordell, G. J. Tucker, A. C. Tamboli, and S. Lany. Band gap analysis and carrier localization in cation-disordered ZnGeN2. submitted.<br/>J. J. Cordell, G. J. Tucker, A. C. Tamboli, and S. Lany. "Role of cation disorder in carrier localization and density of states in ZnGeN₂." arXiv preprint arXiv:2109.05062 (2021).<br/>J. J. Cordell, J. Pan, A. C. Tamboli, G. J. Tucker, and S. Lany. "Probing configurational disorder in ZnGeN₂ using cluster-based Monte Carlo." Physical Review Materials 5, (2021).