Structural and piezoelectric properties of ultra-thin ScxAl1-xN films grown on GaN by molecular beam epitaxy

ScxAl1-xN (x=0.18-0.40) thin films of similar to 28nm thickness grown on metal polar GaN substrates by molecular beam epitaxy are found to exhibit smooth morphology with less than 0.5nm roughness and predominantly single-phase wurtzite crystal structure throughout the composition range. Measurement of the piezoelectric d(33) coefficient shows a 150% increase for lattice-matched Sc0.18Al0.82N relative to pure aluminum nitride, whereas higher Sc contents exhibit lower piezoelectric coefficients. The electromechanical response of the epitaxial films correlates with the crystal quality and the presence of zinc blende inclusions, as observed by high-resolution electron microscopy. It is further found that the polarity of the epitaxial ScxAl1-xN layers is locked to the underlying substrate. The measured electromechanical properties of epitaxial ScxAl1-xN, their relation to the atomic crystal structure and defects, and its crystal polarity provide useful guidance toward the applications of this material.