Direct Observation of the Coulomb Correlation Gap in a Nonmetallic Semiconductor, Si: B

Using electron tunneling, we report the first direct spectroscopic measurement of the Coulomb correlation gap in the density of states N(ε) of a nonmetallic doped semiconductor Si: B. In agreement with analytic models, N(ε) is found to have a nearly parabolic energy dependence, resulting in a "soft" zero at the Fermi energy, with a gap width {≈}0.75 meV. Resistivity measurements show that this energy governs the observed crossover temperature between noninteracting and correlated hopping transport.