Analysis of variable-range hopping conductivity in Si : P

The low-temperature electrical conductivity sigma(T) of uncompensated insul ating Si:P with P concentration N just below the metal-insulator transition (MIT), i.e. 3.0 x 10(18) cm(-3) less than or similar to N less than or sim ilar to 3.5 x 10(18) cm(-3), was measured between 0.05 and 5 K. With decrea sing N, sigma(T) shows a crossover from Mott variable-range hopping (VRH) t o Efros-Shklovskii VRH. The data on the insulating side can be described by the universal phenomenological scaling function proposed by Aharony et al. From the N dependence of the Mott temperature T-M a correlation-length exp onent nu = 1.1 is obtained, compatible with the conductivity exponent mu ap proximate to 1.3 for metallic samples. Indeed, the data on both sides of th e MIT can be combined to yield dynamic scaling of sigma(N, T). Upon lowerin g N on the insulating side further, a change from Efros-Shklovskii VRH to s imple activated conduction is observed near N approximate to 2.7 x 10(18) c m(-3). This is attributed to the activation from the lower to the upper Hub bard band, as inferred from a sign change in the thermoelectric power and t he absence of such a feature in compensated Si:(P, B).