Effect of AlN buffer layer on the microstructure and bandgap of AlN films deposited on sapphire substrates by pulsed laser deposition

AlN buffer layer is proposed to improve the growth of AlN films on the sapphire substrate by pulsed laser deposition. The buffer layers were pre-deposited under vacuum for different time, which was aimed to suppress the negative nitridation effect in the initial growth stage, and their effects on the surface morphology, crystal structure and bandgap of AlN films were characterized. It is found that AlN-buffered films exhibit a single (0002) preferred orientation and the crystallinity improves as the pre-deposition time increases from 5 to 20 min. Al-polarity AlN films are obtained at the pre-deposition time of 5 and 10 min, while the polarity inversion from the Al- to N-polarity is observed at 20 min. Based on the analysis of optical transmittance spectra, the bandgap of AlN films decreases with increasing pre-deposition time, which may be resulted from the decrease of axial ratio c/a.