Temperature Dependence of Surface Acoustic Wave Characteristics of GaN Layers on Sapphire Substrates

We measured surface acoustic wave (SAW) characteristics along the [01-10] and [2-1-10] directions of 1.5- and 2-µm-thick GaN layers on (0001) sapphire substrates at ambient temperatures between -50 and 200°C. The extracted thermal coefficient of delay (TCD) for the SAW propagating along the [01-10] direction, which is smaller than that for the SAW propagating along the [2-1-10] direction, decreases when GaN layer thickness, h, multiplied by SAW wave number, k, increases. The TCD for h k = 3.9, which might be close to that in bulk GaN, is as small as 28.3 ppm/deg. These results indicate that GaN is promising for SAW device operation over a wide range of ambient temperatures.