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

The structural and optical properties of nonpolar a-plane (11-20) GaN and In _0.18GaN _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 ω-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.