Free-standing aluminum nitride (AlN) films were prepared by ammonia heat-treatment of pseudoboehmite [AIO(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 AIN 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 A1N 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 (∼900°C), as shown by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS).