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We report resistivity measurements in the temperature range 0.15-1.2 K Performed on non-compensated Si:As samples doped by ion implantation on the insulating side of the metal-insulator transition. The data show a crossover of the variable-range-hopping (VRH) resistivity dependence rho(T) from rho is-proportional-to exp(T1/4/T)1/4 (Mott VRH predicted behaviour) to rho is-proportional-to exp(T1/2/T)1/2, which demonstrates the persistence of a Coulomb pp in the density of states even in the vicinity of the critical concentration. The simultaneous observation of the T1/4 and T1/2 parameters allows the determination of the effective size of the pp.

The heat capacities of three specially implanted silicon bolometers have been measured at low and, for the first time, at very low temperatures down to about 10 mK by exposing them to alpha-particles and X-rays. The lattice contribution shows a behaviour with temperature in good agreement with the Debye law, and the percentage of thermalized energy of the alpha-particle is found to be about 70%. The specific heat of the doped part, which dominates at very low temperatures, is not inconsistent with linearity, even if a dependence on a higher power of the temperature seems preferred. The parameter of linearity is found to be considerably larger than expected from extrapolation to very low temperatures of the existing results on silicon P-doped by diffusion.