Variable‐Range Hopping in Neutron‐Transmutation‐Doped Gallium Arsenide

First investigations are reported on the temperature dependence of the dc‐conductivity at T = = 0.05 to 300 K on neutron‐transmutation‐doped (NTD) n‐GaAs in the vicinity of the metalinsulator transition (MIT). At a medium compensation of K = 0.60 to 0.77 the MIT takes place at the critical electron concentration of n_c = 2.3 × 10¹⁶ cm⁻³. On the dielectric side of the MIT in the variable‐range hopping regime (VRH) at T ≦ 10 K the conductivity obeys the equation σ(T) = σ₀exp [— (T₁/T)^1/2] and not the Mottlaw. The experimental values T₁ are for all samples much smaller than T₁^theor for isolated shallow impurities and scale down according to T₁ = = T₁*(1 ‐ n/n_c)^1.6±0.4 with T₁* = 5.9 K by approaching the MIT. This behaviour is due to the divergency of the localization radius a and the static DK ϵ₀ at the MIT. The obtained linear In σ — T^−1/2 law over about 2.5 orders of magnitude in temperature variation and more than three orders of magnitude of variation in conductivity is interpreted in the frame of a Coulomb gap with vanishing density of states at the Fermi level, as predicted by Efros and Shklovskii. The pre‐exponential factor ϱ₀ = σ₀⁻¹ shows also scaling behaviour according to ϱ₀ = ϱ₀*(1 ‐ n/n_c)^1.2±0.4 and a value of ϱ₀* = 2.2 Ω cm. At n → n_c σ₀ = ϱ₀⁻¹ reaches the value of Mott's minimal metallic conductivity of σMM = (Ce²/h) n_c^1/3, with 0.025 ≦ C ≦ 0.05.