Structure, defects, and optical properties of commensurate GaN/ZnGeN2/GaN double heterojunctions

Abstract

Materials solutions are required to enhance radiative recombination in the 520 nm to 620 nm wavelength region, termed the ‘green gap’, where III-N emitters are inefficient and phosphide materials cannot emit due to an indirect transition. The II–IV-N₂ family of materials (II = Zn, Mg and IV = Si, Ge, Sn) provides a potential solution; the compounds are structurally analogous to the III-N materials with direct optical band gaps, and published models suggest heteroepitaxial integration of ZnGeN₂ into GaN LEDs may significantly improve recombination efficiency. In this work we present GaN/ZnGeN₂/GaN double heterojunctions grown by molecular beam epitaxy (MBE). The MBE-grown heterogeneous interfaces are coherent as measured by electron microscopy and X-ray diffraction, and they are chemically abrupt as measured by X-ray energy dispersive spectroscopy mapping. Electron microscopy shows threading dislocations nucleated at both interfaces, indicating the need for further improvement of growth methods. In particular, the first 30 nm of GaN grown on ZnGeN₂ is highly defective, likely due to the low growth temperature used to prevent Zn desorption. Photoluminescence spectroscopy shows signals of unintentional Zn and Ge doping in the GaN and a 2.9 eV Zn:GaN defect band (donor–acceptor pair) convolved with a separate defect band of distinct physical origin (free-to-bound) which originates from the ZnGeN₂ layer. We identify Ge_Zn antisite defects or Ga_Ge impurities, as suggested by previously published defect calculations, as the most likely candidates for this luminescence. This work demonstrates coherent interfaces between ZnGeN₂ and GaN, highlighting defects and associated properties of interest with respect to optoelectronic applications.