Angular dependence of surface acoustic wave characteristics in AlN thin films on a-plane sapphire substrates

摘要

AlN thin films have been grown on a-plane sapphire (Al_2O_3(1120)) substrates. X-ray diffraction measurements indicate the films are fully c-plane (0001) oriented with a full width at half maximum of the AlN(0002) rocking curves of 0.76-0.92°. The epitaxial growth relationships have been determined by the reflection high energy electron diffraction analysis as AlN[1100]//Al_2O_3[0001] and AlN[1120]//Al_2O_3[1100]. Angular dependence of important surface acoustic wave (SAW) characteristics, such as the phase velocity and electromechanical coupling coefficient, has been investigated on the AlN(0001)/Al_2O_3(1120) structure. While the SAW is excited at all propagation angles with an angular dispersion of the phase velocity in the range of 5503-6045 m/s, a higher velocity shear-horizontal (SH) mode is observed only at 0°, 105° and 180° off the reference Al_2O_3[1100] over a 180° angular period. The phase velocity of the SH mode shows dispersion (6089-6132 m/s) as a function of the SAW wavelength. Temperature coefficients of frequency are also demonstrated for both modes.