ROLE OF BOND-STRAIN IN THE CHEMISTRY OF HYDROGEN ON THE SI(100) SURFACE

A scanning tunneling microscopy (STM) study of the chemistry of hydrogen on the Si(100) surface is presented. The structure and stability of the hydrogenated 2 x 1, 3 x 1 and 1 x 1 surfaces are studied. The 2 x 1 and 3 x 1 surfaces are shown to be well ordered and stable in the presence of atomic hydrogen under their respective formation conditions. In contrast, the 1 x 1 structure is poorly ordered and susceptible to spontaneous etching by hydrogen atoms. No uniform 1 x 1 dihydride phase was observed under any conditions. The existence of this intermediate 3 x 1 phase and the susceptibility of the 1 x 1 structure to etching are both shown to be due to the strain associated with the dihydride units on the 1 x 1 surface. The repulsive steric interaction between dihydride units on this surface weakens the Si-H bonds and stabilizes the 3 x 1 surface observed at 400 K. The Si-Si backbonds of these dihydride units are also strained, resulting in a lower barrier to reaction which is responsible for the etching observed on the 1 x 1 surface.