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 = 0.942Nc to 0.998Nc, where Nc 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/Nc)α. A clear crossover of α from α ≈ 1 to 3.5 has been observed at N ≈ 0.99Nc. 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.
|Physica Status Solidi (B) Basic Research
|Published - 2000 3月
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