Donor structure and electric transport mechanism in $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$

The electron paramagnetic resonance (EPR) study of $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ crystals gives evidence that donors can be regarded as ${\mathrm{O}}^{2-}$ vacancies trapping single electrons. The Lorentzian line shape of the EPR spectra observed in the range of 5–300 K, which exhibit anisotropic g values, suggests that motional narrowing occurs in this temperature range. For any magnetic-field orientation a single EPR line is observed, indicating that donor electrons are predominantly created in one of the three different oxygen sites in the $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ crystal. A previous transmission electron microscopy study suggested that a break of symmetry in domains of 2–3 nm correlates with a preceding cluster model of oxygen vacancies. From the temperature dependence of the EPR linewidth and the electrical conductivity it is found that the electron conduction in the clusters and/or between them is governed by a tunneling process at low temperatures, whereas at temperatures above 50 K, the transport of electrons through hopping between the clusters is thermally activated.