THEORETICAL SUPPORT TO THE DOUBLE-LAYER MODEL FOR POTASSIUM ADSORPTION ON THE SI(001) SURFACE

K adsorption on the Si(001)-2 x 1 surface is studied by first-principles molecular dynamics based on a norm-conserving pseudopotential. The theoretically optimized structure for full coverage (THETA = 1.0) agrees fairly well with the double-layer model analyzed by x-ray photoelectron diffraction. At THETA = 0.5, the configuration with K adsorbed along the trough is more stable than the Levine model. The mean desorption energies for the two peaks in the thermal-desorption-spectrum are well accounted for by the present calculation. The calculated work-function change also agrees quantitatively with the observed one. Based on these analyses, it is concluded that the double-layer model should bc appropriate for the saturation coverage of K on the Si(001) surface.