  Recently theorized hybrid II-IV-N{_2} / III-N heterostructures, based on
current commercialized (In,Ga)N devices, are predicted to significantly advance
the design space of highly efficient optoelectronics in the visible spectrum,
yet there are few epitaxial studies of II-IV-N{_2} materials. In this work, we
present heteroepitaxial ZnGeN{_2} grown on GaN buffers and AlN templates. We
demonstrate that a GaN nucleating surface is crucial for increasing the
ZnGeN{_2} crystallization rate to combat Zn desorption, extending the
stoichiometric growth window from 215 {\degree}C on AlN to 500 {\degree}C on
GaN buffers. Structural characterization reveals well crystallized films with
threading dislocations extending from the GaN buffer. These films have a
critical thickness for relaxation of 20 nm - 25 nm as determined by reflection
high energy electron diffraction (RHEED) and cross-sectional scanning electron
microscopy (SEM). The films exhibit a cation-disordered wurtzite structure,
with lattice constants a = 3.216 {\AA} {\pm} 0.004 {\AA} and c = 5.215 {\AA}
{\pm} 0.005 {\AA} determined by RHEED and X-ray diffraction (XRD). This work
demonstrates a significant step towards the development of hybrid ZnGeN{_2}-GaN
integrated devices.
