Activated conduction in amorphous Cr-SiO_x thin films

Abstract

The electrical conductivity of reactively sputtered amorphous Cr-SO_x thin films with Cr-to-Si ratio of 28 to 72 and oxygen concentrations c_o above the critical concentration c^cr _O of the metal-insulator transition has been measured in the temperature range 50 mK-300 K. The low-temperature conductivity obeys the law σ(T)=σ₀ exp[-T ₀/T)^α with α 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 (α = 1), a soft Coulomb gap (α=1/2) and a nearly constant value (α=1/4).