  ZnGeN2 and other heterovalent ternary semiconductors have important potential
applications in optoelectronics, but ordering of the cation sublattice, which
can affect the band gap, lattice parameters, and phonons, is not yet well
understood. Here the effects of growth and processing conditions on the
ordering of the ZnGeN2 cation sublattice were investigated using x-ray
diffraction and Raman spectroscopy. Polycrystalline ZnGeN2 was grown by
exposing solid Ge to Zn and NH3 vapors at temperatures between 758 degree C and
914 degree C. Crystallites tended to be rod-shaped, with growth rates higher
along the c-axis. The degree of ordering, from disordered, wurtzite-like x-ray
diffraction spectra to orthorhombic, with space group Pna21, increased with
increasing growth temperature, as evidenced by the appearance of superstructure
peaks and peak splittings in the diffraction patterns. Annealing disordered,
low-temperature-grown ZnGeN2 at 850 degree C resulted in increased cation
ordering. Growth of ZnGeN2 on a liquid Sn-Ge-Zn alloy at 758 degree C showed an
increase in the tendency for cation ordering at a lower growth temperature, and
resulted in hexagonal platelet-shaped crystals. The trends shown here may help
to guide understanding of the synthesis and characterization of other
heterovalent ternary nitride semiconductors as well as ZnGeN2.
