Observation of impurity band conduction and variable range hopping in heavily doped (010) β-Ga2O3

Temperature-dependent resistivity and Hall measurements were performed on a heavily Sn-doped (010) beta-Ga2O3 substrate grown by edge-defined film-fed method. At room temperature, the Hall electron mobility was 77.7 cm(2)V(-1)s(-1). A maximum mobility of 95.2 cm(2)V(-1)s(-1) was achieved at T = 185 K. Because the doping concentration is near the Mott critical value of 2.48 x 10(18)cm(-3), Hall electron concentration and mobility data were analyzed using the two-band model, which considers transport through both the conduction band and an impurity band. At T > 200 K, charge transport is limited by free carriers in the conduction band. The donor ionization energy was extracted to be E-CD = 4 meV. At T <= 200 K, the net conductivity is due to electrons in the impurity band, with a concentration 7.6 x 10(18)cm(-3) and a low mobility of similar to 0.09-2.27 cm(2)V(-1)s (-1). Over a very wide temperature range, 1.7-200 K, transport in the impurity band was found to occur via variable range hopping, where the average hopping length increases with decreasing temperature.