Thermal annealing of AlN-capped AlxG1−xN/GaN heterostructures: impact on physical properties

AlN-capped Al_xGa_1−xN/GaN heterostructures were grown on c-axis sapphire substrates using metal–organic chemical vapor deposition (MOCVD). The AlN thin film served as a protective cap layer. The impact of thermal annealing on the physical properties of these structures was investigated by annealing samples under N₂ (2l/min) between 1050 °C and 1200 °C in 50 °C steps. In situ reflectometry confirmed the chemical stability of the samples post-capping. Scanning electron microscopy (SEM) revealed that the annealed surfaces maintained their morphology with the formation of AlN nanograins. The average nanograin size increased by a factor of 1.081 from approximately 38 nm at 1150 °C to 41 nm at 1200 °C. Secondary ion mass spectrometry (SIMS) showed that the Al solid composition in the AlGaN interlayer remained nearly constant after annealing, with some broadening of the Al profile at the AlGaN/GaN interface, indicating Al diffusion. An Arrhenius plot of the SIMS profile broadening yielded an activation energy of E_a = 2.44 eV for Al diffusion. Photoluminescence (PL) spectroscopy showed a clear reduction in the full width at half maximum (FWHM) of the AlGaN UV emission peak after annealing, with the most significant reduction (12 meV) observed at 1050 °C. An Arrhenius plot of the AlGaN PL FWHM yielded an activation energy of E_a = 2.58 eV.