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

AlN thin films have been grown on a-plane sapphire (Al2O3(11 (2) over bar0)) 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 degrees. The epitaxial growth relationships have been determined by the reflection high energy electron diffraction analysis as AlN[1 (1) over bar 00]// Al2O3[0001] and AlN[11 (2) over bar0]// Al2O3[1 (1) over bar 00]. 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)/Al2O3(11 (2) over bar0) 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 degrees, 105 degrees and 180 degrees off the reference Al2O3[ 1 (1) over bar 00] over a 180 degrees 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.