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AlN thin films were grown by using pulsed sputter deposition (PSD), and the cause of the crystallinity change caused by heat treatment was investigated. The AlN thin films were grown on a sapphire substrate and annealed at temperatures of 700 ∼ 1100 • C by flowing nitrogen gas through a horizontal furnace. The change in the surface morphology, the crystallinity, and the vibration mode were investigated. The PSD-grown samples showed relatively good crystallinity with a very flat surface. Furthermore, the effect of residual strain, which is commonly observed in thin AlN films deposited by sputtering, was confirmed using X-ray diffraction. When the heat treatment temperature was above 900 • C, surface oxidation proceeded rapidly, and the surface morphology and the changes in the crystal phase were confirmed. From the results of Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, the oxidation observed during annealing was found to be related to the interstitial Al defects occurring during film formation.

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