  ZnGeN$_2$ offers a low-cost alternative to InGaN with the potential for
bandgap tuning to span the green gap using cation site ordering. The addition
of oxygen on the anion site creates an additional degree of electronic
tunability. Here, we investigate the structure and optoelectronic properties of
an epitaxial ZnGeN$_{2-x}$O$_{x}$ thin film library grown by combinatorial
co-sputtering on c-Al$_2$O$_3$. Samples exhibit X-ray diffraction patterns and
X-ray pole figures characteristic of a wurtzite (cation-disordered) structure
with the expected 6-fold in-plane symmetry. Transmission electron microscopy
reveals a semi-coherent interface with periodic dislocations that relieve
strain from the large lattice mismatch, and confirms the in-plane and
out-of-plane crystallographic orientation. Room-temperature photoluminescence
exhibits peaks between 2.4 and 2.8 eV which are consistent with a sharp
absorption onset observed by UV-vis spectroscopy. These results demonstrate
low-cost synthesis of optically active yet cation disordered
ZnGeN$_{2-x}$O$_{x}$, indicating a path toward application as a blue-green
emitter.
