High performance NiO/β-Ga2O3 heterojunction pn diodes were realized by applying a sputtered p-type NiO film onto a lightly doped n-type β-Ga2O3 epitaxial layer. Taking advantage of the high barrier height against carriers within the pn heterojunction, the demonstrated device exhibited a high breakdown voltage (VB) of 1059 V without optimized electric field management techniques, and before breakdown the reverse leakage current density remained below 1 μA/cm2. Simultaneously, a relatively low specific on-resistance (Ron, sp) of 3.5 mΩ∙cm2 was achieved. The built-in potential of the heterojunction that determined by a capacitance-voltage (C-V) measurement was around 2.4 eV. As discussed in terms of the energy band diagram of a type-II heterojunction, the conduction band and valence band offsets at the NiO/β-Ga2O3 hetero-interface were estimated to be 1.2 and 2.3 eV, respectively.

High hole concentration was achieved in Mg-doped Al_xGa_1−xN (x ∼ 0.4) by using indium-surfactant-assisted delta doping method. A maximum carrier concentration of 4.75 × 10¹⁸cm⁻³ was obtained, which is three times higher than that of the conventionally delta-doped sample. Sheet resistivity as low as 2.46 × 10⁴ Ω/sq was realized, benefiting from the high hole concentration (p). Analysis results show that the Mg incorporation is effectively enhanced, while the compensation ratio and acceptor activation energy (E_A) are significantly reduced by using In surfactant. It was also found that the In surfactant may induce stronger valence-band modulation, contributing to the decrease of E_A and the increase of p.

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‐Ga₂O₃. High‐quality phase‐pure ε‐Ga₂O₃ 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‐Ga₂O₃ 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‐Ga₂O₃ thin films and different vibration modes resonating in the GHz range are observed. The results suggest that ε‐Ga₂O₃ 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.

This work presents the fabrication and characterization of a high-performance vertical Nickel oxide (NiO)/Beta gallium oxide ( β-Ga ₂ O ₃ ) heterojunction p-n diode. The heterojunction was fabricated by sputtering a p-NiO thin film onto an epitaxial n ^- - β-Ga ₂ O ₃ drift layer. The sputtered NiO, revealed to be polycrystalline, showed an improved crystallinity after annealed in an oxygen ambient, while a seamless contact with the underneath single crystalline β-Ga ₂ O ₃ was observed. A high hole concentration of 1- 3×10 ¹⁹ cm ^-3 and an improved Ohmic contact were also obtained by the annealing process, as confirmed by the Hall and TLM measurements, respectively. A high breakdown voltage ( V _B ) of 1.4 kV and a low specific ON-resistance ( R _on,sp ) of 2.7 mΩ·cm ² were simultaneously achieved for the fabricated diodes, yielding a superior Baliga's figure-of-merit (BFoM) of 0.73 GW/cm ² . From the temperature-dependent current-voltage ( I- V) measurements, two carrier transport mechanisms, namely interface recombination and multistep tunneling, have been identified in the forward conduction of the NiO/ β-Ga ₂ O ₃ heterojunction. The fast recovery property of the device, which was revealed by a double-pulse test, further confirmed the purposed carrier transport mechanisms.