【Domestic Papers】Isotropic performance of surface acoustic wave resonators based on ε-Ga₂O₃(001) thin films

      Researchers from the Sun Yat-sen University have published a dissertation titled " Isotropic performance of surface acoustic wave resonators based on ε-Ga₂O₃(001) thin films" in Applied Surface Science.

Project Support

      This work was supported by the Natural Science Foundation of China(No. 62074167), National Key Research and Development Program (No.2023YFF1500700), the Research and Development Plan Project of Guangdong Province (No. 2021B0101300005).

Background

      Nowadays, SAW devices have found widespread application in sensing, detection, wireless communication and other fields, owing to the small size, low manufacturing cost, high sensitivity and stability. more and more attention has been paid to resonator devices based on semiconductor thin films such as AlN (E_g = 6.2 eV, d₃₃≈5 pm/V) and ZnO (E_g = 3.3 eV, d₃₃≈12.3 pm/V) . However, the piezoelectric coefficient of AlN is too low to provide high electromechanical coupling coefficient in RF resonators, while the bandgap of ZnO is not wide enough to guarantee low leakage current. Recently, ε-Ga₂O₃ (E_g = 4.9 eV, d₃₃≈11 pm/V) has been demonstrated as a novel piezoelectric semiconductor characterized by small acoustic impedance, wide bandgap and high piezoelectric coefficient, making them strong contenders for the applications in RF resonators.

Abstract

      Epsilon-phase gallium oxide (ε-Ga₂O₃) thin film is an emerging piezoelectric semiconductor for the application in RF resonators. Compared with traditional thin films such as AlN (d₃₃≈5 pm/V), ε-Ga₂O₃ possesses a high piezoelectric coefficient (d₃₃≈11 pm/V). As a material belonging to the orthorhombic crystal system, ε-Ga₂O₃ is theoretically expected to exhibit anisotropic properties on the (001) plane. However, in this study, a comprehensive investigation on the angular dependence of surface acoustic wave (SAW) propagation properties reveals that the ε-Ga₂O₃(001) plane is actually highly isotropic. Measurements of electrostatic, mechanical, and thermomechanical characteristics along different in-plane directions on ε-Ga₂O₃(001) present standard deviation smaller than 3 %. The dielectric constant of ε-Ga₂O₃ is extracted to be 11.7 ± 0.1 by the capacitance measurement. Phase velocity and the temperature coefficient of frequency (TCF) for ε-Ga₂O₃ SAW resonators are determined to be 3169 ± 4 m/s and −58.9 ± 1.7 ppm/°C for Rayleigh mode, while it is 5319 ± 11 m/s and −58.9 ± 1.2 ppm/°C for Sezawa mode. The abnormal isotropy of the orthorhombic ε-Ga₂O₃ is explained by the existence of triple rotational domains. The results provide critical basic parameters for the future development and application in ε-Ga₂O₃-based SAW devices.

Highlights

      ● The ε-Ga₂O₃-based surface acoustic wave (SAW) resonators are fabricated and investigated by their electrostatic, mechanical, thermomechanical properties.

      ● Abnormal isotropy performance of SAW devices on ε-Ga₂O₃(001) is observed, which is explained by the existence of triple rotational domains in the ε-Ga₂O₃ thin film.

      ● Basic parameters of ε-Ga₂O₃ including dielectric constant, phase velocity and temperature coefficient of frequency (TCF) are reported in this work.

Conclusion

      In this work, the properties and performances of ε-Ga₂O₃-based SAW resonators with different propagation angles (θ) are investigated on the (001) plane. The structural properties of the ε-Ga₂O₃ thin film are characterized by XRD. The FWHM of the rocking curve is determined to be 0.19◦, indicating the high quality of ε-Ga₂O₃ thin film. The φ-scan of (122) plane demonstrates the existence of triple rotational domains. The performances of the ε-Ga₂O₃-based SAW resonators are examined by static capacitance (C_exp), phase velocity (v), Bode-Q, FOM and TCF, and all the measurements along different propagation directions on (001) plane present standard deviation small enough to be neglected. The dielectric constant of ε-Ga₂O₃ is extracted to be 11.7 ± 0.1. The phase velocity (v) and TCF of ε-Ga₂O₃(001) are determined to be 3169 ± 4 m/s and -58.9 ± 1.7 ppm/ ℃ for Rayleigh mode, while it is 5319 ± 11 m/s and -58.9 ± 1.2 ppm/℃ for Sezawa mode. The experimental results have comprehensively demonstrated the isotropic nature of ε-Ga₂O₃(001) thin films, which could be explained by the existence of triple rotational domains.