dc conductivity of arsenic-doped silicon near the metal-insulator transition

dc conductivity measurements have been made on uncompensated Si:As in the concentration range 6.85×{}10¹⁸32.8×{}10¹⁸R cm^-3 for 0.5T77 K, although three samples were studied in a dilution refrigerator. Insulating samples exhibited variable-range hopping (VRH) behavior of the form {σ}(T)=σ₀(1/T)ˢ exp[-(T₀/T)ᵖ] for 0.86Nc0.99Nc for T8 K. The most satisfactory overall fit to the data is p{~}(1/4 and s{~}0, namely Mott VRH. A new criterion is given to decide the temperature regimes where Mott or Efros-Shklovskii VRH (p=(1/2) should be observed. The characteristic Mott temperature T₀{∝}(1-N/Nc{)}^{3{ν}}yielded 0.77ν0.97. Strong deviations from VRH behavior were observed for N0.84{N}c. Metallic samples showed a σ(n,T)=σ(n,0)+m(n){T}1/2dependence at sufficiently low temperatures. The results yield σ(n,0)={{σ}}₀$(n/${n}c-1)^μ with σ₀=376 S/cm, 8.55nc8.60×{}10¹⁸ cm^-3, and {μ}=0.60±{}0.05. The coefficient m(n) shows behavior similar to that for Si:P, but shows a second sign crossing for n/nc{~}2.4. The m(n) results seem to confirm the dominance of the Hartree interaction for the many-valley case.