Crossover from Coulomb glass to Fermi glass in Si:P

Electronic transport in highly doped but still insulating Si:P at low temperatures is dominated by localized states. Because these states are due to the randomly distributed phosphorus atoms in the silicon crystal, the whole system is disordered and thus Si:P is a perfect model system for so-called electron glasses. We have studied the frequency-dependent conductivity of this system in the THz frequency range. In agreement with previous experimental results and the theoretical predictions by Efros and Shklovskii with increasing energy, we have found a crossover from (interacting) Coulomb glass to (noninteracting) Fermi glass behavior. Additionally, we have observed a strong influence of the Coulomb gap which was not taken into account by previous workers.