First-principles study of steps on the Si(111):H surface

The relaxations and vibrational properties of dihydride steps on the Si(111):H surface are studied from first principles. Besides the expected large relaxation of the ${\mathrm{SiH}}_2$ unit on the step edge, long-range relaxations are found along a zigzag chain deep into the bulk. Three vibrational modes are found with frequencies 10.9, 22.5, and 56.0 ${\mathrm{cm}}^{\mathrm{-}1}$ higher than the Si-H stretching frequency on the terrace. A scissors mode is identified with frequency 262 ${\mathrm{cm}}^{\mathrm{-}1}$ higher than the Si-H wagging frequency. These frequency differences and mode assignments are in good agreement with experiment.