It has been known that aluminum nitride (AlN) has a wide direct-bandgap (6.2 - 5.8 eV) [1,2] with hexagonalwurtzite crystal structure [3] and unique properties: good thermal conductivity (~3 W/cmK at 300 K) [4], good insulator (>10¹¹ Ω∙cm) [5], high dielectric constant [6], relatively small linear-expansion coefficients (5.3 and 4.2 × 10⁻⁶ K⁻¹ along aand c-axis) [7], high sound velocity (6 km/s) [8] and large hardness [9]. Owing to these properties, AlN films have potential applications to electronic devices [10], surface acoustic wave (SAW) devices [11], actuator [12], transparent hard coatings and AlN composites to light-emitting devices [13]. Also, AlN films have been used as buffer layer for GaN [14] and ZnO [15] film growth. For these applications, X-ray diffraction (XRD) technique have been extensively employed to evaluate the crystalline quality and growth orientation of AlN films which have been grown by various techniques, chemical-vapor atomic-layer deposition (a special type of CVD) [2], metal organic CVD [16], molecular beam epitaxy [17], ion beam enhanced deposition (electron beam evaporation of Al combined with N ion bombardment) [5], reactive radio-frequency (RF) magnetron sputtering deposition [6,10,18], pulsed laser deposition (PLD) [19] on various substrates, sapphire [2,19], Si [5,14-16,18], SiC [17], Al [6], Mo [12] etc. For AlN films grown on Si(111), the authors have shown that oxygen impurities near the substrate surface affect the growth orientation and suggest that the XRD intensity decreases with increasing the stress and nearly diminishes when the stress exceeds 2%, irrespective of the film thickness (27 - 470 nm) [20]. Here, the stress is defined as the difference of the lattice parameters between film and bulk. Use of the stress can be justified based on the fact that c-axis length increases with the residual-stress [18] and temperature dependence of the lattice parameter is similar to that of the residual-stress in terms of pressure [19]. The result does not agree with the lattice relaxation around 50 nm of AlN on SiC [17] and favors the constant stress throughout the AlN film on Si(111) [16]. It is of interest to study whether the suggested stress is useful for the quality evaluation of AlN film grown on different substrates other than Si(111).

In this paper, we have grown AlN on R-plane cut sapphire (R-Al₂O₃), SiO₂-glass and C-plane cut sapphire (C-Al₂O₃) substrates by a reactive RF-sputter deposition method. We have measured XRD, the composition, thickness and impurities, and examined use of the stress for the film quality evaluation. We also have measured surface morphology (grain size, shape and surface smoothness), which may affect the crystalline quality, since films are polycrystalline, and optical absorption. These properties might be important for applications mentioned above. For AlN on R-Al₂O₃, irradiation with high-energy (90 MeV Ni) ions was performed in order to study whether stress relaxation, surface smoothing and bandgap modification occur or not by ion irradiation.

AlN films were grown on R-Al₂O₃, SiO₂-glass and CAl₂O₃ substrates by using a reactive-RF-sputter-deposition method with Al target (purity of 99.999%) in pure N₂ gas of ~0.3 Pa with a method described in [20,21]. A reason for usage of pure N₂ gas is to avoid Ar inclusion into films, considering that conventionally Ar and N₂ mixture gas has been employed. The substrates were subjected to ultrasonic rinse in ethanol prior to the film deposition. XRD with Cu-kα radiation was performed to examine crystalline quality and orientation. The thickness, composition and impurities of films were analyzed by RBS. The growth rate was obtained to be approximately 3 nm/min for AlN on three substrates used in this study. Light impurities such as carbon and oxygen near the film surface were analyzed by using nuclear reaction analysis (NRA), ¹²C(d, p)¹³C and ¹⁶O(d, α)¹⁴N with 1.2 MeV d at the reaction angle of 160˚ [20]. In RBS and NRA, stopping powers are taken after [22] with the AlN density of 3.26 g∙cm⁻³ (4.8 × 10²² Al cm⁻³). Surface morphology was observed by atomic force microscopy (AFM) and optical absorption was measured by using a conventional spectrometer. Irradiation with 90 MeV Ni ions was performed by using a TANDEM accelerator at Japan Atomic Energy Agency at Tokai.