Abstract
The bandgap of ZnGeN2 changes with the degree of cation site disorder and is sought in light emitting diodes for emission at green to amber wavelengths. By combining the perspectives of carrier localization and defect states, we analyze the impact of different degrees of disorder on electronic properties in ZnGeN2, addressing a gap in current studies, which largely focus on dilute or fully disordered systems. The present study demonstrates changes in the density of states and localization of carriers in ZnGeN2 calculated using bandgap-corrected density functional theory and hybrid calculations on partially disordered supercells generated using the Monte Carlo method. We use localization and density of states to discuss the ill-defined nature of a bandgap in a disordered material and identify site disorder and its impact on the structure as a mechanism controlling electronic properties and potential device performance. Decreasing the order parameter results in a large reduction of the bandgap. The reduction in bandgap is due, in part, to isolated, localized states that form above the valence band continuum associated with nitrogen coordinated by more zinc than germanium. The prevalence of defect states in all but the perfectly ordered structure creates challenges for incorporating disordered ZnGeN2 into optical devices, but the localization associated with these defects provides insight into the mechanisms of electron/hole recombination in the material.
Highlights
Site disorder, the replacement of chemical species on a fixed crystallographic lattice, has recently attracted interest across semiconductor research areas as a means to control optoelectronic properties
To investigate the impact of disorder on the bandgap of ZnGeN2, we utilize disordered structures in large supercells of 1024 atoms.[20]. These structure models incorporate site disorder consisting of cation antisite pairs that numerous defect studies have highlighted as the dominant native defects in ZnGeN2.9,11,21–26 In contrast to a dilute defect model, site disorder accounts for the interaction of ZnGe and GeZn present in high concentrations representative of materials grown under non-equilibrium conditions
We examined the effect of cation disorder on the density and localization of electronic states in ZnGeN2
Summary
The replacement of chemical species on a fixed crystallographic lattice, has recently attracted interest across semiconductor research areas as a means to control optoelectronic properties. The theoretical discussion of differentiating bandgaps and defect states has been held in this context of dilute point defects[27–29] or in fully random systems[30–32] but misses systems with intermediate degrees of order with a few notable exceptions.[33,34] In materials with both dilute and non-dilute defects, Urbach energy[35] describes how the optical absorption of a semiconductor tails off exponentially[36–38] at energies below the bandgap due to transitions from within bands to defect states in the energy gap and at even lower energies directly between defect states in the gap.[39–41]. There is a fundamental difference between optical transition energies in absorption and emission, i.e., the Stokes shift,[49] which is non-radiatively converted to heat These effects add significant uncertainties to bandgap determination in all but the most thoroughly characterized systems (e.g., GaAs,[50] Cu2O,51 ZnO,[52] and GaN53). By comparing the DOS of ZnGeN2 structures from bandgap corrected calculations in 1024 atom cells, we analyze the effect of disorder on the bandgap of the system
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