Coulomb gap measurement in non-compensated Si:As

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.