Epitaxial Growth of Sc(0.09)Al(0.91)N and Sc(0.18)Al(0.82)N Thin Films on Sapphire Substrates by Magnetron Sputtering for Surface Acoustic Waves Applications

Scandium aluminum nitride (Sc(x)Al(1−x)N) films are currently intensively studied for surface acoustic waves (SAW) filters and sensors applications, because of the excellent trade-off they present between high SAW velocity, large piezoelectric properties and wide bandgap for the intermediate compositions with an Sc content between 10 and 20%. In this paper, the growth of Sc(0.09)Al(0.91)N and Sc(0.18)Al(0.82)N films on sapphire substrates by sputtering method is investigated. The plasma parameters were optimized, according to the film composition, in order to obtain highly-oriented films. X-ray diffraction rocking-curve measurements show a full width at half maximum below 1.5°. Moreover, high-resolution transmission electron microscopy investigations reveal the epitaxial nature of the growth. Electrical characterizations of the Sc(0.09)Al(0.91)N/sapphire-based SAW devices show three identified modes. Numerical investigations demonstrate that the intermediate compositions between 10 and 20% of scandium allow for the achievement of SAW devices with an electromechanical coupling coefficient up to 2%, provided the film is combined with electrodes constituted by a metal with a high density.