Thin-film gallium nitride (GaN) is a promising platform for phononic integrated circuits that hold great potential for scalable information processing processors. Here, an unsuspended traveling phononic resonator based on a high-acoustic-index-contrast mechanism is realized in GaN-on-Sapphire with a frequency up to 5 GHz, which matches the typical superconducting qubit frequency. A sixfold increment in quality factor is found when temperature decreases from room temperature (Q =5000) to 7K (Q =30 000), and thus, a frequency-quality factor product of 1.5×1014 is obtained. Higher quality factors should be available when the fabrication process is further optimized. Our system shows great potential in hybrid quantum devices via the so-called circuit quantum acoustodynamics.

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