A high c-axis-oriented and atomically flat-surface aluminum nitride (AlN) film has been successfully deposited on a (0001)sapphire substrate by metalorganic chemical vapor deposition. We evaluated the dependences of surface roughness, tilted mosaicity and twisted mosaicity on the conditions of AlN deposition. It was found that the atomically flat-surface AlN film was deposited under the diffusion-limited area with suppression of vapor phase reaction at a substrate temperature of 1200 C and a V/III ratio of 800. It was also recognized that surface roughness was controlled by gas flow velocity, which is determined by both gas flow rate and pressure in reactor. Mean surface roughness (R a ) of the deposited AlN films was approximately 1A. The full width at half maximum of X-ray rocking curve for (0002) and ð10 1 12ÞAlN were approximately 100 and 2300 arcsec, respectively.

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5-GHz-band surface acoustic wave (SAW) filters for mobile communication were fabricated on atomicallyflat-surface (0001)aluminum nitride/(0001)sapphire (AlN/Al2O3) combination. The SAW devices were fabricated using electron beam lithography and lift-off method. Atomically-flat-surface AlN films were used to reduce SAW propagation loss. Center frequency of the fabricated SAW filter was 5.18 GHz. SAW velocity was 5688 m/s at normalized thickness by wave number (kH) of 9.9. Effective coupling coefficient was 0.1 % and temperature-coefficient of delay was 9 ppm/ • C at kH of 5.9. SAW propagation loss was 0.0053 dB at 5.18 GHz. The atomically-flatsurface (0001)AlN/(0001)Al2O3 combination is promised to be used for 5-GHz-band SAW filter for the application to mobile communication.

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We have investigated the surface acoustic wave (SAW) properties of atomically flat-surface (0001)aluminum nitride on a (0001)sapphire (AlN/-Al 2 O 3 ) combination. SAW propagated along [1 "2 210]AlN/[1 " 1 100]-Al 2 O 3 and [10 "1 10]AlN/[11 " 2 20]-Al 2 O 3 . SAW velocity was measured to be approximately 1.0% higher than that of a conventionally calculated curve. The dispersion of SAW velocity as a function of normalized thickness (kH) was as low as 1.3%. The measured temperature coefficient of delay was 9 ppm/ C at kH values of 5.9 and 9.9. The propagation loss in the case of using an atomically flatsurface AlN film was lower by one order of magnitude than that using a conventional AlN film. The propagation loss at 5.172 GHz was measured to be 0.0053 dB/.