ELECTRONIC-STRUCTURE OF THE NA-ADSORBED SI(100)2X1 SURFACE STUDIED BY INVERSE AND DIRECT ANGLE-RESOLVED PHOTOEMISSION

The surface electronic band structure of the Na-saturated Si(100)2 X 1 surface has been studied with angle-resolved inverse and direct photoemission. Single-domain 2 X 1-reconstructed surfaces were obtained by using vicinal samples. At a sodium coverage of roughly half the saturation coverage, an overlayer-derived empty surface state appears at the GAMMABAR point 1.5 eV above the Fermi level. With increasing Na coverage, it moves downward in energy to 0.7 eV at saturation, where it displays a weak upward dispersion along both the GAMMABAR-JBAR and the GAMMABAR-JBAR direction. The filled dangling-bond surface band of the clean surface is split by the Na adsorption into two peaks, which also move downward in energy with increasing coverage. As a result, the surface band structure stays semiconducting at saturation with a band gap of about 2.1 eV. This electronic structure is similar to that of the Si(100)2 X 1-K surface. However, significant differences exist regarding energy positions and dispersions, indicating a weaker interaction within the overlayer and a stronger alkali-substrate interaction in the case of Na, which is also consistent with recent theoretical studies.