【Domestic Papers】High-overtone Bulk Acoustic Resonators and Comb Filters using Epitaxial ε-Ga₂O₃ Films on 4H-SiC

      Researchers from the Sun Yat-sen University have published a dissertation titled "High-overtone Bulk Acoustic Resonators and Comb Filters using Epitaxial ε-Ga₂O₃ Films on 4H-SiC" in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control.

Project Support

      This work was supported in part by the National Key Research and Development Program of China under Grant 2024YFE0205300 and National Natural Science Foundation of China under Grant 62471504.

Background

      With the rapid evolution of 5G technology, communication systems are demanding increasingly higher operating frequencies and faster data transmission speeds. These challenges are driving the need for deeper integration between piezoelectric components and active electronic devices. Piezoelectric acoustic devices, known for their advantages in sensing, filtering, and energy conversion, play a vital role in 5G communication systems. Meanwhile, active devices are essential for signal processing and transmission. The expansion of new frequency bands in the 5G era has raised performance requirements for radio frequency (RF) systems, prompting the development of compact, high-frequency, and multifunction components. Compared to discrete component configurations, monolithic integration of acoustic and electronic devices enables system-on-chip (SoC) solutions that offer improved performance, reduced parasitic effects, lower power consumption, and smaller footprints, leading to overall cost savings. Consequently, there is a growing demand for innovative materials that support such integration.

Abstract

      This work demonstrates novel high-overtone bulk acoustic resonators (HBARs) with only top electrodes using an epitaxial ɛ-Ga₂O₃ piezoelectric film grown on conductive 4H-SiC substrates. The device exhibits a broad frequency response spanning 1 to 8 GHz, with a free spectral range (FSR) of 18.6 MHz between adjacent modes. Key performance metrics include an f∙Q product exceeding 1.2×10¹⁴ Hz at 70 K and over 1.5×10¹³ Hz at 300 K, along with excellent temperature stability characterized by a low temperature coefficient of frequency (TCF) of −15.46 ppm/°C. The acoustic parameters of ε-Ga₂O₃ are extracted, including density of 5001.7 kg/m³, elastic constant C^D₃₃ of 2.82×1011 N/m², longitudinal acoustic wave velocity of 7596 m/s and intrinsic electromechanical coupling coefficient k²_t of 7.9%. Evaluation of the theoretical f∙Q limit and acoustic impedance mismatch reveals substantial potential for further performance enhancement. Additionally, a comb filter was demonstrated by laterally coupling two ɛ-Ga₂O₃ HBARs, achieving over 275 equidistant passbands across an over 5 GHz bandwidth. These results highlight the promise of ɛ-Ga₂O₃-based HBARs for advanced RF applications. Leveraging its excellent piezoelectric and electronic properties, ɛ-Ga₂O₃ enables monolithic integration of acoustic devices with on-chip electronics, paving the way for compact, high-performance RF systems.

Conclusion

      As a next-generation wide-band-gap semiconductor, ɛ-Ga₂O₃ offers outstanding potential for monolithic integration due to its excellent power electronic and piezoelectric properties, along with its compatibility with modern microelectronic fabrication processes. In this study, we proposed and fabricated novel HBARs using epitaxial ɛ-Ga₂O₃ films on conductive SiC substrates. Key acoustic parameters of ɛ-Ga₂O₃ were extracted, including a density of 5001.7 kg/m³, elastic constant of 2.82×10¹¹ N/m², acoustic velocity of 7596 m/s, and an intrinsic electromechanical coupling coefficient of 7.9%. Theoretical f∙Q limits and acoustic impedance matching analysis indicate room for further optimization. Additionally, a comb filter was realized through lateral coupling of two ɛ-Ga₂O₃ HBARs, yielding over 275 passbands across a 5 GHz span. These results demonstrate the significant promise of heteroepitaxial ɛ-Ga₂O₃ piezoelectric films for high-performance RF acoustic devices. Moreover, the potential for monolithic integration with ɛ-Ga₂O₃-based electronics such as MOSFETs and HEMTs opens new avenues for compact, high-performance RF systems.