Density of states of a two-dimensional electron gas at semiconductor surfaces

The formation of a two-dimensional (2D) 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 defects! on different narrow-gap IIIV~ 110! substrates ~InAs, InSb! 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 Schro¨dinger 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 3x10^11 and 2x10^12 electrons/cm2, consistent with previous experimental results on plasmon excitations.