Deep level defects and cation sublattice disorder in ZnGeN_2

摘要

Ⅲ-nitrides have revolutionized lighting technology and power electronics. Expanding the nitride semiconductor family to include heterova-lent ternary nitrides opens up new and exciting opportunities for device design that may help overcome some of the limitations of the binary nitrides. However, the more complex cation sublattice also gives rise to new interactions with both native point defects and defect complexes that can introduce disorder on the cation sublattice. Here, depth-resolved cathodoluminescence spectroscopy and surface photo-voltage spectroscopy measurements of defect energy levels in ZnGeN_2 combined with transmission electron microscopy and x-ray diffraction reveal optical signatures of mid-gap states that can be associated with cation sublattice disorder. The energies of these characteristic optical signatures in ZnGeN_2 thin films grown by metal-organic chemical vapor deposition are in good agreement with multiple, closely spaced band-like defect levels predicted by density functional theory. We correlated spatially resolved optical and atomic composition measurements using spatially resolved x-ray photoelectron spectroscopy with systematically varied growth conditions on the same ZnGeN_2 films. The resultant elemental maps vs defect spectral energies and intensities suggest that cation antisite complexes (Zn_(Ge)-Ge_(Zn)) form preferentially vs isolated native point defects and introduce a mid-gap band of defect levels that dominate electron-hole pair recombination. Complexing of Zn_(Ge) and Ge_(Zn) antisites manifests as disorder in the cation sub-lattice and leads to the formation of wurtzitic ZnGeN_2 as indicated by transmission electron microscopy diffraction patterns and x-ray diffraction reciprocal space maps. These findings emphasize the importance of growth and processing conditions to control cation place exchange.