ε-Ga₂ O₃ : An Emerging Wide Bandgap Piezoelectric Semiconductor for Application in Radio Frequency Resonators

The explosion of mobile data from the internet of things (IoT) is leading to the emergence of 5G technology with dramatic frequency band expansion and efficient band allocations. Along with this, the demand for high-performance filters for 5G radio frequency (RF) front-ends keeps growing. The most popular 5G filters are constructed by piezoelectric resonators based on AlN semiconductor. However, AlN possesses a piezoelectric constant d₃₃ lower than 5 pm V^-1 and it becomes necessary to develop novel semiconductors with larger piezoelectric constant. In this work, it is shown that strong piezoelectricity exists in ε-Ga₂ O₃ . High-quality phase-pure ε-Ga₂ O₃ thin films with a relatively low residual stress are prepared. A switching spectroscopy piezoelectric force microscope (SS-PFM) measurement is carried out and the piezoelectric constant d₃₃ of ε-Ga₂ O₃ is determined to be ≈10.8-11.2 pm V^-1 , which is twice as large as that of AlN. For the first time, surface acoustic wave (SAW) resonators are demonstrated on the ε-Ga₂ O₃ thin films and different vibration modes resonating in the GHz range are observed. The results suggest that ε-Ga₂ O₃ is a great material candidate for application in piezoelectric devices, thanks to its wide bandgap, strong piezoelectric property, small acoustic impedance, and low residual stress.