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

Abstract

Abstract 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 × 10 16 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 ) = σ 0 exp [— ( T 1 / T ) 1/2 ] and not the Mottlaw. The experimental values T 1 are for all samples much smaller than T 1 theor for isolated shallow impurities and scale down according to T 1 = = T 1 *(1 ‐ n / n c ) 1.6±0.4 with T 1 * = 5.9 K by approaching the MIT. This behaviour is due to the divergency of the localization radius a and the static DK ϵ 0 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 ϱ 0 = σ 0 −1 shows also scaling behaviour according to ϱ 0 = ϱ 0 *(1 ‐ n / n c ) 1.2±0.4 and a value of ϱ 0 * = 2.2 Ω cm. At n → n c σ 0 = ϱ 0 −1 reaches the value of Mott's minimal metallic conductivity of σMM = ( Ce 2 / h ) n c 1/3 , with 0.025 ≦ C ≦ 0.05.