Electrical properties of nominally undoped β-Ga₂O₃ crystals grown by the Czochralski method from an iridium crucible under a carbon dioxide containing atmosphere were studied by temperature dependent conductivity and Hall effect measurements as well as deep level transient spectroscopy. All crystals were n-type with net donor concentrations between 6 × 10¹⁶ and 8 × 10¹⁷cm⁻³. The Hall mobility of electrons was on average 130 cm²/Vs at room temperature and attained a maximum of 500 cm²/Vs at 100 K. The donor ionization energy was dependent on the donor concentration. Extrapolation of this dependence to zero concentration yielded a value of about 36 meV for isolated donors agreeing well with the ionization energy derived from effective-mass theory. Three deep electron traps were found at 0.55, 0.74, and 1.04 eV below the conduction bandedge. The trap at E_C – 0.74 eV was detected in all samples with concentrations of 2 – 4 × 10¹⁶cm⁻³. This concentration is comparable to that of compensating acceptors we have to take into account for an explanation of the temperature dependent electron density. Therefore, under the assumption that the electron trap at E_C – 0.74 eV possesses acceptor character, this trap could be the dominating compensating acceptor in our crystals. Besides, a value of Φ_B = (1.1 ± 0.1) V was determined for the Schottky barrier height of Ni on the (100) surface of n-type β-Ga₂O₃.

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga₂O₃ epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called ε phase; i.e. they exhibited hexagonal P6₃mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga₂O₃ layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5–10 nm size. Each domain has orthorhombic structure with Pna2₁ space group symmetry, referred to as κ-Ga₂O₃. Further XRD analysis carried out on thicker samples (9–10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together – if the domain size falls below the actual resolution of the probing techniques – can be misinterpreted as the disordered structure with its P6₃mc space group symmetry usually referred to as ε-Ga₂O₃ in the current literature. The crystal structure of these Ga₂O₃ layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna2₁ space group and previous observations.

The Schottky barrier height of Au deposited on (100) surfaces of n-type β-Ga₂O₃ single crystals was determined by current-voltage characteristics and high-resolution photoemission spectroscopy resulting in a common effective value of 1.04 ± 0.08 eV. Furthermore, the electron affinity of β-Ga₂O₃ and the work function of Au were determined to be 4.00 ± 0.05 eV and 5.23 ± 0.05 eV, respectively, yielding a barrier height of 1.23 eV according to the Schottky-Mott rule. The reduction of the Schottky-Mott barrier to the effective value was ascribed to the image-force effect and the action of metal-induced gap states, whereas extrinsic influences could be avoided.

Nanoscale semiconductor materials have been extensively investigated as the channel materials of transistors for energy-efficient low-power logic switches to enable scaling to smaller dimensions. On the opposite end of transistor applications is power electronics for which transistors capable of switching very high voltages are necessary. Miniaturization of energy-efficient power switches can enable the integration with various electronic systems and lead to substantial boosts in energy efficiency. Nanotechnology is yet to have an impact in this arena. In this work, it is demonstrated that nanomembranes of the wide-bandgap semiconductor gallium oxide can be used as channels of transistors capable of switching high voltages, and at the same time can be integrated on any platform. The findings mark a step towards using lessons learnt in nanomaterials and nanotechnology to address a challenge that yet remains untouched by the field.