Characterization and control of ZnGeN2 cation lattice ordering

Abstract

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.Comment: 13 pages, 7 figure

Band Gaps, Band‐Offsets, Disorder, Stability Region, and Point Defects in II‐IV‐N2 Semiconductors

“…Another recent review is by Martinez et al Our group has also created a website dedicated to these materials at https://sites.google.com/a/case.edu/dmref/home. Until recently the most studied of these materials was ZnGeN 2 , the closest analog of GaN . Since 2012, ZnSnN 2 has also received significant attention as a potential solar cell material based on sustainable elements .…”

“…On the other hand, experimentally, it was found that ZnGeN 2 can occur either in a disordered or ordered form (based on XRD and Raman spectra) depending on the growth temperature, with the disordered form occurring at lower temperature. This indicates the disorder is not a thermodynamic disorder but kinetically controlled by the lower surface mobility of atoms during growth at low temperature.…”

“…The ordering specific superlattice peaks (110) and (101) (in the orthorhombic labeling scheme), discussed in Feldberg et al to be absent from their data, which only occur in ordered Pbn 2 1 and not in wurtzite have not yet been unambiguously demonstrated in any study of ZnSnN 2 we are aware of. They were identified in ZnGeN 2 first by neutron diffraction and only recently by X‐ray diffraction …”

“…While these weak intensity and difficult to observe peaks provide the most direct proof of the ordering, in ZnGeN 2 , one can also use the splitting of several strong XRD diffraction peaks of a powder or polycrystalline sample as evidence of the symmetry breaking in the basal plane, as these arise from the deviation of from the ideal wurtzite value of . For example, the wurtzite hexagonal labeling (110) peak splits into the close lying orthorhombic (400) and (230) peaks . These distortions of the lattice vectors arise from the significant difference between the Zn‐N and Ge‐N bond lengths.…”

On the Nitridation of Zn2GeO4

“…Further to replacing scarce elements, the replacement of one atom type by a combination of two others adds a degree of structural complexity to the structure that can greatly influence the electronic properties. It has, for instance, been demonstrated that cation order/disorder phenomena in Zn B N 2 ( B = Sn, Ge) can reach from fully disordered structures in the wurtzite‐type to fully ordered ones in the β ‐NaFeO 2 ‐structure type and that the order–disorder transition causes a drastic band gap change . However, zinc germanium oxide nitrides (ZGON) have been reported to adopt the wurtzite‐type structure as well, and can hence mimic similar effects.…”