dc conductivity measurements have been made on uncompensated Si:As in the concentration range 6.85×${10}^{18}$$0^{18}$R ${\mathrm{cm}}^{\mathrm{-}3}$ for 0.5<T<77 K, although three samples were studied in a dilution refrigerator. Insulating samples exhibited variable-range hopping (VRH) behavior of the form σ(T)=${\sigma }_0$(1/T${)}^s$ exp[-($T_0$/T${)}^p$] for 0.86$N_c$$N_c$ for T<8 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 $T_0$∝(1-N/$N_c$${)}^{3\nu }$ yielded 0.77<ν<0.97. Strong deviations from VRH behavior were observed for N<0.84$N_c$. Metallic samples showed a σ(n,T)=σ(n,0)+m(n)$T^{1/2}$ dependence at sufficiently low temperatures. The results yield σ(n,0)=${\sigma }_0$(n/$n_c$${\mathrm{-}1)}^{\mu }$ with ${\sigma }_0$=376 S/cm, 8.55<$n_c$<8.60×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, 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/$n_c$∼2.4. The m(n) results seem to confirm the dominance of the Hartree interaction for the many-valley case.

• Received 31 October 1988

DOI:https://doi.org/10.1103/PhysRevB.40.1216