Using electron tunneling, we report the first direct spectroscopic measurement of the Coulomb correlation gap in the density of states N(e) of a nonmetallic doped semiconductor Si:B. In agreement with analytic models, N(e) 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.

Electron tunneling experiments are used to probe Coulomb correlation effects in the single-particle density of states ͑DOS͒ of boron-doped silicon crystals near the critical density n c of the metal-insulator transition ͑MIT͒. At low energies (р0.5 meV͒, a DOS measurement distinguishes between insulating and metallic samples with densities 10 to 15 % on either side of n c . However, at higher energies (ϳ1 meVрр50 meV͒ the DOS of both insulators and metals show a common behavior, increasing with energy as ⑀ m where m is roughly 0.5. The observed characteristics of the DOS can be understood using a classical treatment of Coulomb interactions combined with a phenomenological scaling ansatz to describe the length-scale dependence of the dielectric constant as the MIT is approached from the insulating side. ͓S0163-1829͑99͒12427-5͔

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Electron tunneling gives an empirical description of how Coulomb correlations evolve across the metal-insulator transition (MIT) in Si:B. For clearly insulating or metallic samples, the tunneling conductance displays a parabolic or a square-root shape, respectively. Just below the MIT, the conductance shows some metallic features before the samples become truly metallic. Very close to the MIT, the conductance spectra are unusually broad and frequency dependent. We interpret this last feature as a result of long-wavelength screening arising from a divergence of the dielectric constant. [S0031-9007(96)01403-2]

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