Titanium oxide nanoparticles spin-coated onto r-plane sapphire substrate:Effects on structural and optical properties of nonpolar a-plane GaN and InGaN/GaN multiple quantum wells

摘要

The structural and optical properties of nonpolar a-plane (11 - 20) GaN and In_(0.18)GaN_(0.82)/GaN multiple quantum wells (MQWs) grown on TiO_2 nanoparticle (NP)-coated r-plane (1 -102) sapphire substrate have been investigated. A smooth surface morphology was observed by atomic force microscopy (AFM), indicating a good lateral overgrowth on TiO_2 NPs. Transmission electron microscopy (TEM) images revealed a reduction in the density of basal stacking faults (BSFs) and threading dislocations (TDs) in the TiO_2 NP-coated sample. From X-ray diffraction measurements, the broadening of co-scan full width at half maximum (FWHM) was influenced by the tilt of the mosaic due to TiO_2 NPs. A reduced PL-integrated intensity ratio (I_(bsf)/I_(nbe)) in low-temperature photoluminescence (PL) spectra of the a-plane GaN film grown over TiO_2 NPs was consistent with TEM results. The improved room temperature-PL intensity of TiO_2-embedded MQWs is associated with the increase in light extraction efficiency, due to the TiO_2 NPs and air voids. Temperature-dependent PL spectra revealed that the dominant PL emission was attributed to localization of carriers in quantum wire-like regions, where the BSFs intersect the QWs. The observed quantum efficiency at 300 K also showed a good quantum confinement, even though alloy fluctuations might be expected in the QWs.