Crystal quality improvement of sputtered AlN film on sapphire substrate by high-temperature annealing

Thermal annealing process of sputtered AlN films with different thicknesses on sapphire substrates was studied in N-2 ambient. Crystal quality of these AlN films was significantly improved after 1400-1700 A degrees C annealing. For the 200-nm AlN film, the full width at half maximum (FWHM) of the (0002)-plane X-ray rocking curve (XRC) was improved from 157.3 to 103.6 arcsec after 1650 A degrees C annealing, while the FWHM of the (102)-plane XRC was significantly improved from 3890 to 353.1 arcsec. Transmission electron microscopy further confirmed that the crystal quality improvement was related to solid-phase reaction at high temperature, allowing columnar structure AlN to recrystallize and thus reducing threading dislocations in the films. Surface morphology with high-density AlN islands transformed to step-and-terrace morphology. X-ray photoelectron spectroscopy measurement also indicated that C and O impurities were decreased by the thermal annealing process. Film strain changed from tensile to compressive stress after annealing. The optical transmissivities of these AlN films did not decrease after annealing, and the optical transmissivity band edge slopes increased, providing further evidence of improved crystal quality.