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.

Photoluminescence spectra of high-quality bulk AlN crystals are reported. In addition to the expected linear luminescence features like free excitons and donor-bound excitons, nonlinear processes like biexcitons and the exciton-exciton scattering band are seen for higher excitation densities. For temperatures above $\approx 150\text{K}$ the electron-hole plasma becomes clearly visible in the spectra. A detailed analysis of the data yields an exciton binding energy of 55 meV, a biexciton binding energy of 28.5 meV, a band gap of 6.089 eV at low temperature, and a band gap of 6.015 eV at room temperature.

Truly bulk ZnGa₂O₄ single crystals were obtained directly from the melt. High melting point of 1900 ± 20 °C and highly incongruent evaporation of the Zn- and Ga-containing species impose restrictions on growth conditions. The obtained crystals are characterized by a stoichiometric or near-stoichiometric composition with a normal spinel structure at room temperature and by a narrow full width at half maximum of the rocking curve of the 400 peak of (100)-oriented samples of 23 arcsec. ZnGa₂O₄ is a single crystalline spinel phase with the Ga/Zn atomic ratio up to about 2.17. Melt-grown ZnGa₂O₄ single crystals are thermally stable up to 1100 and 700 °C when subjected to annealing for 10 h in oxidizing and reducing atmospheres, respectively. The obtained ZnGa₂O₄ single crystals were either electrical insulators or n-type semiconductors/degenerate semiconductors depending on growth conditions and starting material composition. The as-grown semiconducting crystals had the resistivity, free electron concentration, and maximum Hall mobility of 0.002–0.1 Ωcm, 3 × 10¹⁸–9 × 10¹⁹ cm⁻³, and 107 cm² V⁻¹ s⁻¹, respectively. The semiconducting crystals could be switched into the electrically insulating state by annealing in the presence of oxygen at temperatures ≥700 °C for at least several hours. The optical absorption edge is steep and originates at 275 nm, followed by full transparency in the visible and near infrared spectral regions. The optical bandgap gathered from the absorption coefficient is direct with a value of about 4.6 eV, close to that of β-Ga₂O₃. Additionally, with a lattice constant of a = 8.3336 Å, ZnGa₂O₄ may serve as a good lattice-matched substrate for magnetic Fe-based spinel films.