Variable-range hopping conduction and metal–insulator transition in amorphous RexSi1−x thin films

Resistivity, ρ(T), of the amorphous RexSi1−x thin films with x = 0.285–0.351 is investigated in the interval of T ~ 300–0.03 K. At x = 0.285–0.324 the activated behavior of ρ(T) is governed by the Mott and the Shklovskii–Efros variable-range hopping (VRH) conduction mechanisms in different temperature intervals and the three-dimensional regime of the hopping. Between x = 0.328 and 0.351 the activationless dependence of ρ(T) takes place. The critical behavior of the characteristic VRH temperatures and of the Coulomb gap, Δ, pertinent to proximity to the metal–insulator transition at the critical value of xc ≈ 0.327, is observed. The analysis of the critical behavior of Δ yields directly the critical exponent of the dielectric permittivity, η = 2.1 ± 0.2, in agreement with the theoretical prediction, η = 2. On the other hand, the values of the critical exponent of the correlation length ν ~ 0.8–1.1 close to the expected value of unity can be obtained from the analysis of the critical behavior of the VRH characteristic temperatures under an additional assumption of a strong underbarrier scattering of hopping charge carriers in conditions, when the concentration of scattering centers considerably exceeds the concentration of sites involved in the hopping.