III-nitrides have revolutionized lighting technology and power electronics. Expanding the nitride semiconductor family to include heterovalent 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 photovoltage spectroscopy measurements of defect energy levels in ZnGeN2 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 ZnGeN2 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 ZnGeN2 films. The resultant elemental maps vs defect spectral energies and intensities suggest that cation antisite complexes (ZnGe–GeZn) form preferentially vs isolated native point defects and introduce a mid-gap band of defect levels that dominate electron–hole pair recombination. Complexing of ZnGe and GeZn antisites manifests as disorder in the cation sub-lattice and leads to the formation of wurtzitic ZnGeN2 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.

ZnGeN2 films were grown on GaN-on-sapphire templates via metalorganic chemical vapor deposition. Energy dispersive x-ray spectroscopy was used to estimate the Zn/(Zn + Ge) composition ratio in the films. This ratio decreased with an increase in growth temperature but increased with an increase in total reactor pressure or the Zn/Ge precursor flow rate ratio. Systematic mapping of these key growth parameters has allowed us to identify the growth window to achieve ZnGeN2 with stoichiometric cation composition. Compositional and statistical analyses performed on data acquired from atom probe tomography provided insight into the local compositional homogeneity. The cations Zn and Ge did not demonstrate segregation or clustering at the sub-nanometer level. Based on x-ray diffraction 2θ–ω scan profiles and transmission electron microscope nano-diffraction patterns, the films with near-stoichiometric cation ratios were single crystalline with planar surfaces, whereas zinc-rich or zinc-poor films were polycrystalline with nonplanar surfaces. The growth direction of the single crystalline ZnGeN2 films on GaN templates was along the c-axis. Room temperature Raman spectra showed features associated with the phonon density of states, indicating the presence of cation disorder in the lattice. A cathodoluminescence peak associated with transitions involving deep level defects was observed around 640 nm. The intensity of this peak increased by almost 2.5 times as the temperature was reduced to 77 K from room temperature. A similar peak was observed in the photoluminescence spectra collected at 80 K.

No abstract

Heterovalent ternary ZnGeN2 thin films were grown on c-, r-, and a-sapphire substrates using metalorganic chemical vapor deposition. The crystal structure of the films was identified by X-ray diffraction spectroscopy. It is consistent with orthorhombic Pna21 assuming perfect ordering of the cations. For a fully disordered cation sublattice, the X-ray diffraction spectra correspond to a wurtzitic crystal. The growth directions were determined to be along the c-axis for the films grown on c- and a-sapphire substrates, and along the orthorhombic [010] axis (wurtzite[112̅0]) for films grown on r-sapphire substrates. The Zn/Ge atomic ratios, determined by energy dispersive X-ray spectroscopy, were observed to decrease as growth temperatures were increased. Growth rates for all substrates decreased by approximately 10% with changes in growth temperatures from 600 to 710 °C. Broad photoluminescence peaks at ∼2.05 eV were observed at room temperature. These are associated with transitions involving deep level defects. Room temperature photoluminescence excitation spectra showed peaks at ∼3.4 eV associated with enhanced absorption near the band gap of the orthorhombic Pna21 phase. Broad tails in the excitation spectra observed to persist to approximately 2.8 eV may be band tailing due to disorder on the cation sublattice. The unintentionally doped films are n-type with carrier concentrations varying from 2 × 1018 cm–3 to 2 × 1019 cm–3, decreasing with the increase in growth temperature. Room temperature Hall mobilities of up to 17 cm2/V·s were obtained.

An alloy of ZnGeN 2 and GaN in equal proportions can form the octet-rule-preserving quaternary heterovalent nitride semiconductor ZnGeGa 2 N 4 . Singlecrystal films of the alloy targeting this composition were deposited on (11̅ 02) Al 2 O 3 (r-plane sapphire), (0001) Al 2 O 3 (c-plane sapphire), and (0001) GaN/Al 2 O 3 by metal−organic chemical vapor deposition using the precursors diethylzinc, germane, trimethylgallium, and ammonia. The growth directions were along the c-axis for films grown on the cplane sapphire and GaN templates, as well as along the orthorhombic [010] axis for films grown on r-plane sapphire. The effects of varying the growth temperature from 550 to 700 °C, choice of substrate, and trimethylgallium and germane flow rates on film composition and morphology were examined by Xray diffraction, field-emission scanning electron microscopy, and atomic force microscopy. The Zn/Ge atomic ratios were observed to decrease with growth temperature but increase with trimethylgallium flow rate. Growth rates, which varied with growth temperature from approximately 1 to 3.5 μm/h, were observed to increase with growth temperature up to 670 °C, then decrease abruptly with further increase in temperature. The growth rates were similar for growth on r-and c-plane sapphire substrates at the lower growth temperatures. However, above 650 °C the growth rates on c-and r-plane sapphire differed by as much as 70%. A broad photoluminescence double peak was observed only for samples grown on r-plane sapphire at the highest growth temperature. Hall measurements show n-type carrier concentrations in the mid-10 18 /cm −3 range and mobilities of a few cm 2 /V-s for material grown on r-sapphire substrates at 670 °C and above.