Celeste Melamed2 1 Elisabetta Arca2 Rachel Woods-Robinson3 Angela Fioretti2 1 Andriy Zakutayev2 Eric Toberer1 2 Adele Tamboli2

2, National Renewable Energy Laboratory, Golden, Colorado, United States
1, Colorado School of Mines, Golden, Colorado, United States
3, University of California, Berkeley, Berkeley, California, United States

In this work, we present a combinatorial study of the structural and optical properties of sputtered ZnGeN2 thin films, with cross-cutting applications in both fundamental materials science and novel device development. While III-N materials such as GaN have revolutionized modern optoelectronics, device structures rely on stringent lattice-matching to avoid deleterious defects. The II-IV-N2 materials, which are structural analogs to III-N materials, offer the possibility of controlled disorder of the cation sublattice which would allow tunable properties at fixed composition. ZnGeN2 is analog and closely lattice-matched to GaN and exhibits a direct bandgap with predicted strong absorption. However, there is disagreement in the literature regarding optical properties, as experimental works to date measure an absorption onset of ~3 eV, which is smaller than the predicted bandgap for ordered material. Additionally, little work to date has attempted to explore properties as a function of cation composition, which has been shown to greatly impact properties in other II-IV-N2 materials such as ZnSnN2. In this work, we present a study of combinatorial ZnGeN2 grown by RF co-sputtering. Spatially resolved characterization was performed in order to correlate structure with properties. X-ray fluorescence reveals cation compositions (given by x = Zn/(Zn+Ge) ) ranging from x=0.40 to x=0.65 and X-ray diffraction shows that films are phase pure and crystallize in the expected cation-disordered wurtzite structure. Pawley refinement of XRD data demonstrates that lattice constants shift up to 3% with cation composition. UV-visible spectroscopy was performed to determine absorption coefficient as a function of incident energy, and reveals an absorption onset shift from 2.8 eV to 2.1 eV with increasing Zn cation composition. These results suggest that large changes in properties are possible within the Zn-Ge-N materials space. While it remains to be determined how these changes will impact applications, this study re-affirms the complexity and potential of thin film ZnGeN2 as a direct- and wide- bandgap optoelectronic material.