We investigate lattice ordering phenomena for the heterovalent ternaries that are based on the wurtzite lattice, under the constraint that the octet rule be preserved. We show that, with the single exception of a highly symmetric twinned structure, all allowed lattice orderings can be described by a pseudospin model corresponding to the two different stackings of ABAB rows of atoms in the basal plane that occur in the P na21 and P mc21 crystal structures. First-principles calculations show that the difference in the energies of formation between these two structures is 13±3 meV/fu (formula unit) for ZnSnN2 and is an order of magnitude larger for ZnGeN2, and that for both materials the P m31 structure, which contains only octet-rule-violating tetrahedra, has a significantly higher energy of formation and a signficantly lower band gap. We predict almost random stacking and wurtzite-like x-ray diffraction spectra in the case of ZnSnN2, consistent with reported measurements. The octet-rule-preserving model of disorder proposed here predicts a band gap that for ZnSnN2 is relatively insensitive to ordering, in contrast to the prevailing model, which invokes the random placement of atoms on the cation sublattice. The violations of the octet rule in the latter model lead to significant narrowing of the band gap. The Raman and photoluminescence spectra of ZnSnN2 are interpreted in light of the ordering model developed here. The observation that ZnGeN2 orders in the P na21 structure under appropriate growth conditions is consistent with the larger difference in the energies of formation of the P na21 and P mc21 structures for this material. The ordering model presented here has important implications for the optical, electronic and lattice properties of all wurtzite-based heterovalent ternaries.

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

Recent work on heterovalent ternary nitrides, II‐IV‐N2, is reviewed. The authors first provide an overview of the relevant literature, then briefly discuss band gaps, band offsets and the effects and nature of disorder. The authors discuss the energies of formation and evaluate the stability or metastability with respect to competing binary compounds. The Cd‐IV‐N2 compounds are found to be only metastable. For ZnGeN2 we present a revised chemical potential stability region and discuss its effects on the point defect energies of formation. The authors briefly discuss the current status of understanding of the point defects and doping in ZnGeN2.

Mechanical transfer of high performing thin film devices onto arbitrary substrates represents an exciting opportunity to improve device performance, explore non-traditio na l 2 manufacturing approaches, and paves the way for soft, conformal, and flexible electronics. Using a two-dimensional (2D) boron nitride (BN) release layer, we demonstrate the transfer of AlGaN/GaN high-electron mobility transistors (HEMTs) to arbitrary substrates through both direct van der Waals (vdW) bonding and with a polymer adhesive interlayer. No device degradation was observed due to the transfer process, and a significant reduction in device temperature (327 °C to 132 °C at 600 mW) was observed when directly bonded to a silicon carbide (SiC) wafer relative to the starting wafer. With the use of a benzocyclobutene (BCB) adhesion interlayer, devices were easily transferred and characterized on Kapton and ceramic films, representing an exciting opportunity for integration onto arbitrary substrates. Upon reduction of this polymer adhesive layer thickness, the AlGaN/GaN HEMTs transferred onto a BCB/SiC substrate resulted in comparable peak temperatures during operation at powers as high as 600 mW to the as-grown wafer, revealing that by optimizing interlayer characteristics such as thickness and thermal conductivity, transferrable devices on polymer layers can still improve performance outputs.

We report on the effects of applied external strain on AlGaN/GaN two dimensional electron gas structures exhibiting a negative strain dependence of the sheet carrier density (ns). Flexible AlGaN/GaN heterojunctions, grown on two-dimensional boron nitride (BN)-on-sapphire templates, were released from the substrate via strain-induced separation at the weak BN van der Waals interface and then transferred to flexible substrates. By releasing the AlGaN/GaN layer from the substrate, residual strain was removed which allowed for isolation and study of the effects of the externally applied strain. By bending samples, uniaxial strain up to 0.15% was applied as measured by the shifts in the GaN E2H Raman mode. Hall effect measurements revealed a 2.5% decrease in ns with 0.11% applied tensile strain, which is contrary to the increase expected from the piezoelectric effect. The observed decrease in ns is attributed to a relatively large increase in the AlGaN surface barrier height. This effect, which is rarely reported, is observable due to a low surface state density (2.2 × 1012 cm−2 eV–1) in the samples. Illumination was found to dramatically alter the ns-strain dependence, an effect potentially related to detrapping of electrons in the GaN buffer.