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Growth Morphology And Surface‐acoustic‐Wave Measurements of AlN Films on Sapphire

Aluminum nitride epitaxial films have been grown on R‐plane sapphire substrates and studied for its piezoelectric properties as applied to surface‐acoustic‐wave device applications. The films reported here are grown by a chemical vapor deposition process involving the reaction of trimethyl aluminum gas with ammonia in the presence of hydrogen at temperatures of approximately 1200 °C. Using a modified epireactor as the growth furnace with a gas injection system that directs gas flow onto the substrate, growth rates of 500 Å/min are achieved producing films over 10 μm in thickness which exhibit excellent mechanical and chemical stability. The R‐plane, of sapphire consists of alternate layers of Al and O with a repetitive sequence of O–Al–O–Al–O. In the two different layers of atoms, the Al atoms occur in a close‐packed configuration while the O atoms are aligned into columns along the direction. The hexagonal wurtzite structure of AlN has the plane consisting of coplanar atoms of Al and N in a close‐packed configuration but segregated into columns aligning with the [0001] direction. A good fit of these two planes is obtained when the columns of atoms are aligned with the sapphire and AlN [0001] directions colinear (i.e., with the AlN c axis pointing in the same direction as the projection of sapphire c axis upon the R plane), giving lattice mismatches of 2.7% along and 11.9% across the direction. A reversal of the AlN [0001] axis however, places the AlN atoms in the interstitial sites of sapphire indicating poor lattice matching.

The as‐grown surface morphology of the AlN films, as examined under a scanning electron microscope, can be divided into two categories. Category I features very distinctive scale‐like facets that are all aligned in one direction while category II inclu- - des the scale‐like facets aligned but pointing in opposing directions along with some spurious nucleations. X‐ray analysis of the sapphire substrate shows the facets to be that of the AlN [0001] axis aligning with the direction of the R plane. Surface‐acoustic‐wave measurements of the AlN films are made by reflective impedance measurements from Al interdigital transducers fabricated upon polished surfaces. There is a definite correlation between the quality of the results and the category of the surface morphology. Results obtained from films of category I were consistently better than those obtained from films of category II leading to the confirmation of the theory that there is a preferred direction of growth of AlN axis upon the R plane for minimum stress with maximum lattice matching. Surface‐acoustic‐wave coupling constant K2 and propagation velocity Vs are measured out to an AlN thickness to transducer wavelength ratio t/λ of 0.75 with typical K2 values of 0.8% and Vs of 6.1 km/s. Preliminary results obtained from a study of the dependence of the K2 versus t/λ curve upon transducer wavelength indicate that the AlN–sapphire interfacial strain extends about 1 μm into the AlN film.


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