Variable range hopping conduction in neutron-transmutation-doped ⁷⁰Ge:Ga

Variable range hopping conduction in neutron-transmutation-doped, isotopically enriched ⁷⁰Ge:Ga samples for the temperature range T = 20 to 250 mK has been studied in the critical regime for the metal-insulator transition. Eighteen samples investigated had Ga concentrations in the range N = 0.942N_c to 0.998N_c, where N_c is the critical Ga concentration for the metal-insulator transition. The low temperature resistivities ρ of all samples obey the variable range hopping conduction theory of Efros-Shklovskii type with an appropriate temperature dependence in the pre-factor; ρ = ρ₀T^-1/3 exp(T₀/T)^1/2. The critical exponent α of T₀ as a function of N has been determined using the form T₀ ∝ (1 - N/N_c)α. A clear crossover of α from α ≈ 1 to 3.5 has been observed at N ≈ 0.99N_c. The excitation of holes from the lower-to upper-Hubbard bands is the dominant conduction mechanism in the α ≈ 1 region, while the standard variable range hopping of holes from occupied to empty Ga sites takes place in the very vicinity of the transition where α ≈ 3.5. The important role of the doping compensation is proposed in connection with the critical behavior of the electrical conductivity.