Influence of electronic correlations on the frequency-dependent hopping transport in Si:P

At low energy scales charge transport in the insulating Si:P is dominated by activated hopping between the localized donor electron states. Thus, theoretical models for a disordered system with electron-electron interaction are appropriate to interpret the electric conductivity spectra. With a newly developed technique we have measured the complex broadband microwave conductivity of Si:P from 100 MHz to 5 GHz in a broad range of phosphorus concentration n/nc from 0.56 to 0.95 relative to the critical value nc = 3.5 × 10 cm corresponding to the metal-insulator transition driven by doping. At our base temperature of T = 1.1 K the samples are in the zero-phonon regime where they show a super-linear frequency dependence of the conductivity indicating the influence of the Coulomb gap in the density of the impurity states. At higher doping n → nc, an abrupt drop in the conductivity power law σ1(ω) ∼ ω is observed. The dielectric function ε1 increases upon doping following a power law in (1 − n/nc). Dynamic response at elevated temperatures has also been investigated.