CROSSOVER FROM MOTT TO EFROS-SHKLOVSKII VARIABLE-RANGE-HOPPING CONDUCTIVITY IN INXOY FILMS

Crossover from Mott variable-range-hopping conductivity to the Coulomb-gap Efros-Shklovskii (ES) variable-range-hopping conductivity has been observed in amorphous indium oxide films. The hopping exponent x congruent-to 0.56 in the activated Coulomb-gap regime is greater than the x = 0.50 value predicted by Efros and Shklovskii. The experimental value of x congruent-to 0.56 is in excellent agreement with the computational calculations of Mobius and Richter, who suggest that x = 0.55. The experimental ratios for T(Mott)/T(ES) congruent-to 54 are in close agreement with the prediction of Castner that T(Mott)/T(ES) = 81. Experimental values for the crossover temperatures, which separate the two hopping regimes, are consistent with predicted values. The Coulomb-pp energy DELTA-CG is estimated to range from a few tenths of a meV for films close to the metal-insulator transition to several meV's for films quite deep in the insulating regime.