The formation of a two-dimensional ͑2D͒ electron channel at semiconductor surfaces has been studied by high-luminosity and high energy-resolution ultraviolet photoelectron spectroscopy. A large variety of bandbending sources ͑alkali metals, silver and antimony adatoms, cleavage defects͒ on different narrow-gap III-V͑110͒ substrates ͑InAs, InSb͒ has been used. The measured photoemission spectral density in the semiconductor conduction band shows a steplike structure, consistent with the description of a jelliumlike 2D electron gas confined in a potential well, and it is independent of the band-bending sources. A self-consistent solution of the Poisson and Schrödinger equations gives the energy eigenvalues, the eigenstates, and the spectral density, in excellent agreement with the collection of photoemission results. Moreover, the accumulated charge density ranges between 3ϫ10 11 and 2ϫ10 12 electrons/cm 2 , consistent with previous experimental results on plasmon excitations.

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The overlayer growth and electronic properties of the Bi/GaSb(110) interface and of the twodimensional ordered (1 X 1)and (1 X 2)-Bi layers have been investigated by complementary spectroscopic techniques (high-resolution electron-energy-loss, photoemission, and Auger spectroscopy). Bismuth forms an epitaxial monolayer, followed by island formation (Stranski-Krastanov growth mode) covering an average surface area of 40% at a nominal coverage of 4 ML. The (1X2)-symmetry stable structural phase, obtained after annealing at -220'C, corresponds to an average nominal Bi coverage of about 0.7 ML, suggesting an atomic geometry different from the epitaxial-continued layer structure. The disposal of Bi atoms in the (1X2) structure should build up an "open" layer, as the Ga-related surface exciton quenched in the (1X1) epitaxial monolayer is present in the (1X2) stable phase. The two symmetry phases are characterized by strong absorption features at 1 eV [(1X 1)-Bi] and 0.54 eV [{ 1 X 2)-Bi], related to interband electronic transitions between Bi-induced electronic states. The major Bi-related occupied electronic levels, present in the valence band of the (1 X 1)and (1 X2)-Bi layer, have been detected by angle-integrated ultraviolet photoemission spectroscopy. Both the (1 X 1) and (1 X 2) phases show a metallic nature, with a low density of electronic states at the Fermi level. Schottky barrier heights of 0.20 and 0.14 eV are estimated for the epitaxial (1X1)and (1X2)-symmetry stage, respectively, by analyzing the space-charge layer conditions through the study of the dopant-induced free-carrier plasmon in the GaSb substrate.

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