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.

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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.

We present a new approach for scaling-up the growth of β-Ga2O3 single crystals grown from the melt by the Czochralski method, which has also a direct application to other melt-growth techniques involving a noble metal crucible. Experimental and theoretical results point to melt thermodynamics as the crucial factor in increasing the volume of a growing crystal. In particular, the formation of metallic gallium in the liquid phase in large melt volumes causes problems with crystal growth and eutectic or intermetallic phase formation with the noble metal crucible. The larger crystals to be grown the higher oxygen concentration is required. The minimum oxygen concentration ranges from about 8 to 100 vol.% for 2 to 4 inch diameter cylindrical crystals, challenging the use of iridium crucibles in a combination with such high oxygen concentrations. A specific way of oxygen delivery to a growth furnace with the iridium crucible allows to minimize the formation of metallic gallium in the melt and thus obtaining large crystal volumes while decreasing the probability of the eutectic formation.

The onset of optical absorption in In2O3 at about 2.7 eV is investigated by transmission spectroscopy of single crystals grown from the melt. This absorption is not defect related but is due to the fundamental band gap of In2O3. The corresponding spectral dependence of the absorption coefficient is determined up to α = 2500 cm−1 at a photon energy hν = 3.05 eV at room temperature without indication of saturation. A detailed analysis of the hν dependence of α including low‐temperature absorption data shows that the absorption process can be well approximated by indirect allowed transitions. It is suggested that the fundamental band gap of In2O3 is of indirect nature. The temperature dependence of the fundamental band gap is measured over a wide range from 9 to 1273 K and can be well fitted by a single‐oscillator model. Compared to other semiconductors the reduction of the gap with increasing temperature is exceptionally strong in In2O3.