ACTIVATED CONDUCTION IN AMORPHOUS CR-SIOX THIN-FILMS

The electrical conductivity of reactively sputtered amorphous Cr-SiOx thin films with Cr-to-Si ratio of 28 to 72 and oxygen concentrations c(o) above the critical concentration c(o)cr of the metal-insulator transition has been measured in the temperature range 50 mK-300 K. The low-temperature conductivity obeys the law sigma(T) = sigma(0) exp [- T0/T)alpha] with alpha changing from 1/4 to 1/2 to 1 with decreasing temperature. The corresponding temperature regions and conductivity parameters depend systematically on c(o). The results are related to a composition-dependent density N(E) of states near E(F), which shows with increasing energy a hard Coulomb gap (alpha = 1), a soft Coulomb gap (alpha = 1/2) and a nearly constant value (alpha = 1/4).