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 × 1017 cm−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 EC – 0.74 eV was detected in all samples with concentrations of 2 – 4 × 1016 cm−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 EC – 0.74 eV possesses acceptor character, this trap could be the dominating compensating acceptor in our crystals. Besides, a value of ΦB = (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 Ga 2 O 3 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 P6 3 mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga 2 O 3 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 Pna2 1 space group symmetry, referred to as κ-Ga 2 O 3. 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 togetherif the domain size falls below the actual resolution of the probing techniquescan be misinterpreted as the disordered structure with its P6 3 mc space group symmetry usually referred to as ε-Ga 2 O 3 in the current literature. The crystal structure of these Ga 2 O 3 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 zigzag 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 Pna2 1 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.

Transparent semiconducting β-Ga 2 O 3 single crystals were grown by the Czochralski method from an iridium crucible under a dynamic protective atmosphere to control partial pressures of volatile species of Ga 2 O 3 . Thermodynamic calculations on different atmospheres containing CO 2 , Ar and O 2 reveal that CO 2 growth atmosphere combined with overpressure significantly decreases evaporation of volatile Ga 2 O 3 species without any harm to iridium crucible. It has been found that CO 2 , besides providing high oxygen concentration at high temperatures, is also acting as a minor reducing agent for Ga 2 O 3 . Different coloration of obtained crystals as well as optical and electrical properties are directly correlated with growth conditions (atmosphere, pressure and temperature gradients), but not with residual impurities. Typical electrical properties of the n-type β-Ga 2 O 3 crystals at room temperature are: ρ = 0.1 -0.3 Ωcm, μ n,Hall = 110 -150 cm 2 V -1 s -1 , n Hall = 2 -6×10 17 cm -3 and E Ionisation = 30 -40 meV. A decrease of transmission in the IR-region is directly correlated with the free carrier concentration and can be effectively modulated by the dynamic growth atmosphere. Electron paramagnetic resonance (EPR) spectra exhibit an isotropic shallow donor level and anisotropic defect level. According to differential thermal analysis (DTA) measurements, there is substantially no mass change of β-Ga 2 O 3 crystals below 1200 °C (i.e. no decomposition) under oxidizing or neutral atmosphere, while the mass gradually decreases with temperature above 1200 °C. High resolution transmission electron microscopy (HRTEM) images at atomic resolution show the presence of vacancies, which can be attributed to Ga or O sites, and interstitials, which can likely be attributed to Ga atoms.

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