Aluminium oxynitride (AlON) films of variable composition were grown by reactive sputter deposition in a N2/O2 ambient at room temperature and studied for device passivation. The films were deposited on Si and 4H-SiC substrates as well as on SiC PiN diodes. The AlON/SiO2/SiC stack provided superior interface properties compared to the AlON/SiC structure. Samples with 8% oxygen content, in the AlON film, and subjected to a UV exposure prior to deposition, exhibited the smallest net positive interface charge. A large net negative interface charge was observed for samples with 10% oxygen content and for the samples with 8% oxygen content and subjected to a RCA1 surface clean, prior to deposition. Diodes passivated with AlON films demonstrated reduced leakage current compared to as-processed diodes.