Self-consistent pseudopotential calculations for sodium adsorption on GaAs(110)

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Abstract
We report self-consistent electronic structure, total-energy, and force calculations based on the density-functional theory for the Na adsorption on GaAs(110). In particular we studied three different coverages: FTHETA=1/4, 1/2, and 1 (FTHETA=1 means one adatom per substrate surface atom). We find that the Na core electrons play an important role for the exchange-correlation interaction and we therefore carry the atomic core-electron density of sodium along with the self-consistent calculations. Tests with our formulation of this core-valence exchange-correlation interaction are presented for the free sodium atom, the NaCl crystal, and Na metal. By the sodium adsorption on GaAs(110) the atomic and electronic structure of the substrate is locally changed, depending on the coverage. For FTHETA=1/4 and 1/2 the calculations give that the highest occupied Kohn-Sham band is rather flat and only partially occupied. We show that the Hubbard correlation energy of this band is larger than the bandwidth. We therefore conclude that the system should be described in a Hubbard picture rather than in a Bloch picture. As a consequence, it should be nonmetallic. The calculated values of the Schottky-barrier height and of the variation of photothreshold as a function of coverage are in good agreement with experimental data. From a detailed analysis of the surface electronic structure we explain the different Schottky-barrier behavior for p-type and n-type substrates.