Effect of Metal Electrodes on Surface Acoustic Wave Properties in Bulk Z-Cut GaN Crystal

The effect of metal electrodes on surface acoustic wave (SAW) properties in bulk semi-insulating Z-cut GaN crystal was measured for the first time. A SAW synchronous two-port resonator was used for these measurements. Such SAW parameters as velocity, electromechanical coupling coefficient, and reflection coefficient of a single reflecting strip were determined by matching the measured and calculated amplitude responses of the resonator for aluminum and gold electrodes at a frequency of about 240 MHz. It was found that for gold electrodes, the reflection coefficient of a single reflecting strip is much larger compared with that of aluminum. Changes of resonance frequency against temperature were measured in the temperature range from -20 °C to 80 °C and linear temperature coefficient of frequency of about -23 ppm/°C was obtained. Isotropic SAW properties in the propagation plane, low temperature coefficient of frequency, and possibility of integration with active devices make the bulk Z-cut GaN crystal attractive for practical applications.