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.

I. Introduction

Semiconducting gallium nitride (GaN) is used for many practical applications in active optical and electronic devices. Because GaN belongs to the 6 mm point group of the hexagonal crystallographic system, it also exhibits piezoelectric properties and can find applications in passive surface acoustic wave (SAW) devices. However, in this case, it should be semiinsulating. So far, epitaxial GaN layers on sapphire were used for fabrication of these devices [1]–[6]. Now, high-quality semi-insulating bulk GaN wafers are offered by several companies (Kymatech, Ammono, and Seen Semiconductors). Though prices are still high, progress in technology will lower the cost of the crystal growth [7]. Mass density, dielectric, elastic, and piezoelectric constants of semi-insulating bulk GaN crystal were presented in [8]. However, it was impossible to measure the effect of different metal layers on the SAW parameters, because the delay line used for measurements was coupled to GaN plate through an air gap. This can be done in Z-cut GaN by the SAW synchronous two-port resonator method that can be used for determination of such SAW parameters as velocity, electromechanical coupling coefficient, and reflection coefficient of a single reflecting strip for different metal layers [9]. This paper presents SAW parameters obtained from matching the measured and calculated amplitude responses of the resonator for aluminum and gold electrodes.