Layer-by-layer morphology is a crucial signature of the quality of epitaxial thin films. In this study, layer-by-layer growth of an ε-phase gallium oxide (ε-Ga2O3) thin film is demonstrated using metal–organic chemical vapor deposition. A two-step growth method, in which a nucleation layer is grown at 600 °C and an epilayer is grown at 640 °C, is employed to fabricate a high-quality ε-Ga2O3 thin film on a c-plane sapphire substrate. The morphology of the ε-Ga2O3 film is evaluated by atomic force microscope. The density of screw-type threading dislocations determined by an X-ray diffraction rocking curve is as low as 1.8 × 108 cm−2.

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 d 33 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-Ga 2 O 3 . High-quality phase-pure 𝝐-Ga 2 O 3 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 d 33 of 𝝐-Ga 2 O 3 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-Ga 2 O 3 thin films and different vibration modes resonating in the GHz range are observed. The results suggest that 𝝐-Ga 2 O 3 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.

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The ε-Ga2O3 p–n heterojunctions (HJ) have been demonstrated using typical p-type oxide semiconductors (NiO or SnO). The ε-Ga2O3 thin film was heteroepitaxial grown by metal organic chemical vapor deposition (MOCVD) with three-step growth method. The polycrystalline SnO and NiO thin films were deposited on the ε-Ga2O3 thin film by electron-beam evaporation and thermal oxidation, respectively. The valence band offsets (VBO) were determined by x-ray photoelectron spectroscopy (XPS) to be 2.17 eV at SnO/ε-Ga2O3 and 1.7 eV at NiO/ε-Ga2O3. Considering the bandgaps determined by ultraviolet-visible spectroscopy, the conduction band offsets (CBO) of 0.11 eV at SnO/ε-Ga2O3 and 0.44 eV at NiO/ε-Ga2O3 were obtained. The type-II band diagrams have been drawn for both p–n HJs. The results are useful to understand the electronic structures at the ε-Ga2O3 p–n HJ interface, and design optoelectronic devices based on ε-Ga2O3 with novel functionality and improved performance.