STRUCTURE OF THE NA/SI(100)2X1 AND CS/SI(100)2X1 INTERFACES INVESTIGATED BY PHOTOEMISSION EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE

We present a structural investigation of both clean and alkali-metal- (Na,Cs) covered Si(100)2 X 1 surfaces using photoemission extended x-ray-absorption fine structure. We find that the Na-Si bond length is the same for both low (0.4 monolayer) and 1-monolayer coverages at 2.80 +/- 0.1 angstrom, close to the sum of covalent radii, supporting the model of covalent bonding between Na and Si, even below 0.5 monolayer. Furthermore, in contrast to III-V semiconductors, Na and Cs do not significantly modify the structure of the Si(100)2 X 1 surface. We also comment on the dramatic changes in the Si-Si nearest neighbors during the initial stages of oxygen chemisorption on a sodium-covered silicon surface.