Correlation of structural and optical properties of AlGaN films grown on SiN-treated sapphire by MOVPE

We investigated the structural and optical properties of AlGaN films grown on SiN-treated sapphire substrates, without and with GaN-template, by atmospheric pressure metal organic vapor phase epitaxy. The samples were characterized using high-resolution X-ray diffraction (HR-XRD), time-resolved photoluminescence (TR-PL), and photoreflectance (PR) spectroscopies. Furthermore, the carrier mobility was determined from Hall-effect measurements. When the AlGaN (GaN-template) layer thickness increases up to 0.6 µm (1.3 µm), an increase in the PL decay times is observed and correlated with the transition from 3D to 2D growth mode resulting in a decrease in the dislocations density as obtained from the HR-XRD measurements. Beyond the aforementioned layer thicknesses, we observed a deterioration in the PL transient corresponds to an increase in the density of VAl-related complexes during the relaxation process, which act as non-radiative recombination centers. Our observations strongly suggest that this type of defects influences the carrier transport and carrier recombination process in the AlGaN layers. Furthermore, our results reveal a phenomenological linear relationship between the internal electric field, obtained from the PR measurements, and the dislocations density. This finding predicts an increase in the GaN internal electric field by about 147 KV/cm when the Al content is increased to 7% in the AlGaN layers. We attribute this increase to a rise in the polarization-induced electric field due to Al incorporation in the AlGaN layer. Based on the obtained correlation between the internal electric field and the dislocation density, we propose an experimental approach that can be utilized to determine the internal electric field, at zero dislocation density, which is very important for designing high-efficient electronic and optoelectronic devices.