Enhanced ultraviolet emission in photoluminescence of GaN film covered by ZnO nanoflakes

In this study, ultraviolet (UV) photoluminescence (PL) emission of gallium nitride (GaN) was improved using ZnO nanoflakes (NFs) as cap layer. Capped ZnO was synthesized by thermal oxidation at different temperatures (470, 520, and 570 °C). The structures, surface morphologies, and optical properties of the samples were analyzed by X-ray diffraction and Raman spectroscopy. The scanning electron microscopy results showed that oxidation temperature strongly affected the density and formation of ZnO NFs. The PL spectra showed that the UV emission of GaN films was considerably enhanced after being covered by ZnO NFs. However, at a relatively high (∼520 °C) oxidation temperature, the intensity of UV emission decreased. The increase in oxidation temperature of the cap layer widened the PL emission band in the UV region, and a red shift occurred in the peak position of UV emission. This phenomenon may be related to the oxygen inter diffusion between ZnO NFs and GaN films. Unlike the as-grown GaN layers, the oxidized ZnO–capped GaN exhibited considerable enhancement in photocurrent and PL responses. A metal-semiconductor-metal-type UV photodetector was fabricated. The current–voltage characteristics showed that the ratio of the maximum photocurrent to dark current decreased from 75,000 to 2000 as the oxidation temperature of the cap layer increased from 470 °C to 570 °C. This phenomenon indicates the sensitivity of the proposed photodetector to NFs with low oxidation temperatures and high densities.