Structural and piezoelectric properties of ultra-thin SC_xAl_(1-x)N films grown on CaN by molecular beam epitaxy

摘要

SC_xAl_(1-x)N(x = 0.18-0.40) thin films of ～28nm thickness grown on metal polar GaN substrates by molecular beam epitaxy are found to exhibit smooth morphology with less than 0.5 nm 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 Sc_(0.18)Al_(0.82)N 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 SC_xAl_(1-x)N layers is locked to the underlying substrate. The measured electromechanical properties of epitaxial SC_xAl_(1-x)N, their relation to the atomic crystal structure and defects, and its crystal polarity provide useful guidance toward the applications of this material.