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(ɛ) of a nonmetallic doped semiconductor Si:B. In agreement with analytic models, N(ɛ) 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.