Growth and characterization of light emitting ZnS/GaN heterostructures

Abstract

Heterostructures involving ZnS/GaN show promise for the injection of holes from p-GaN into n-ZnS. Utilizing knowledge obtained from ZnS phosphor technology, this combination could result in a new type of multi-color electroluminescent display. Further, this combination provides a very interesting interface. Both ZnS and GaN are very ionic materials. Hence, it is desirable that the interface will be relatively benign, and that charge injection can occur despite the large lattice mismatch and resulting misfit defects that form near the interface. The highly lattice mismatched structures ZnS/GaN and ZnS/Al_2O_3 were grown by molecular beam epitaxy using elemental sources. Growth rates of up to 0.4 μm ph were observed for the lower growth temperatures, with rapidly diminishing rates for temperatures above 350 °C. The GaN substrate consisted of a 3 μm epilayer grown on sapphire (0001) by metalorganic chemical vapor deposition. Reflection high energy electron diffraction observations indicate that the zincblende ZnS layers commonly contain (111) twins, although films with no visible twin spots could be grown at a high substrate temperature. The sulfide layers were characterized using photoluminescence, x-ray diffraction, and scanning electron microscopy (SEM). X-ray peaks typically had widths of 400 arcsec for ω/2θ scans, and somewhat worse for ω scans, indicating mosaic tilt. Photoluminescence spectra of the ZnS films doped with Ag and Al demonstrated the characteristic blue transition near 440 nm. SEM electron channeling patterns indicated that the ZnS films lie at a 30° rotated orientation to the Al_2O_3 (0001) substrate, as does GaN. p-GaN/n-ZnS devices were fabricated using standard photolithography techniques. The current voltage characteristics are reported and preliminary electroluminescence results are discussed for this heterojunction system.