ADSORPTION OF HYDROGEN-ATOMS ON THE SILICON(111) (1X1) SURFACE - FIRST-PRINCIPLES CALCULATIONS

With a first-principles method, the energy surface of hydrogen atoms diffusing on the silicon(111) (1 x 1) surface and the energy curves of H atoms approaching the Si(111) (1 x 1) surface from three high-symmetry sites were calculated. The three high-symmetry sites (denoted as the T1, T4 and H3 sites) correspond to positions on top of the first (T1), second (T4) and fourth (H3) layers of the Si atoms. It was found that two of the high-symmetry sites (T4 and H3) correspond to local maxima on the adsorption energy surface while the third one (T1) is the chemisorbed site, i.e. that with the lowest adsorption energy. The completely different characteristics for the three sites of the same symmetry are related to how much the charges transfer from the surface dangling bonds to the H atoms in the adatom structures. It was also found that no local minimum exists on the energy surface and that there is a saddle point. When the coverage varies from one monolayer to 1/2 or 1/4 monolayer, the changes in the adsorption energies for H atoms adsorbed at the three high-symmetry sites are, at most, around 0.2 eV/H. Finally, the energy curves for H atoms approaching the Si surface through the T1 and T4 sites exhibit the familiar strong repulsion near the surface. However the energy curve is soft through the H3 sites even when there is already one monolayer coverage of H atoms at the T1 sites of the surface. The energy barrier for H atoms to penetrate into the bulk through the H3 sites is not larger than 0.3 eV. The large negative formation energies for H atoms residing at several locations beneath the surface demonstrate that penetration of the H atoms into the bulk can easily occur.