Epitaxially grown GaN thin-film SAW filter with high velocity and low insertion loss

This study proposes GaN thin film as a piezoelectric material for SAW (surface acoustic wave) filters. Highly piezoelectric GaN film with a good surface morphology (RMS roughness = 0.7 nm) was obtained on a 2-in (0001)-oriented sapphire substrate by MOCVD growth. The fabricated GaN SAW filter exhibited a very high velocity of 5803 m/s and relatively low insertion loss of -7.7 dB. The attenuation of the center frequency was about 22 dB smaller than those at the tops of the first sidelobes. When the wavelength of the IDT electrode was 60 μm (λ/4 = 15 μm), the center frequency was measured at 96.6 MHz, thereby facilitating a ∼GHz operation when the IDT geometry is designed on a 1 μm scale. The calculated electromechanical coupling factor (K²) was about 4.3 ± 0.3%, which is larger than those obtained from other thin film piezoelectric materials and allows the realization of wider filter fractional bandwidths. TCF (temperature coefficient of frequency) was measured as low as - 18.3 ppm/°C in the range from - 25 to 50 °C. These superior characteristics demonstrate that epitaxially grown GaN thin film can be successfully used for high performance SAW filters.