Preparation and characterization of high-surface-area aluminum nitride thin films

Free-standing aluminum nitride (AlN) films were prepared by ammonia heat-treatment of pseudoboehmite [AlO(OH)] gels derived from an alumina sol. Supported films on sapphire substrates were also made by spin-coating the alumina sol, followed by ammonia nitridation. The conversion of the specimens to AlN as a function of heat-treatment temperature was studied using various characterization techniques. For the free-standing films, X-ray diffraction (XRD) showed that upon ammonia treatment, the specimens were first transformed from pseudoboehmite to η-alumina and finally to AlN above 1000 °C. ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy confirmed the appearance of Al[N]₄ coordination at 1000 °C, indicative of AlN. Complete conversion to AlN was achieved after 5 hour nitridation at 1200 °C. The 1200 °C heat-treated films consisted of crystallites of AlN in the size range 0.01-0.15 μm, with pores between 0.03-0.25 μm in diameter, as observed by TEM/electron diffraction analyses. These films had BET surface areas of approximately 25 m²/g. Nitridation of the supported films to AlN occurred at lower temperatures (approximately 900 °C), as shown by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS).