Electronic mechanism of STM-induced diffusion of hydrogen on Si(100)

We have observed a scanning tunneling microscopy (STM) induced lateral transfer of a single hydrogen atom on the Si(100) surface. The transfer rate of the hydrogen atom is proportional to the electron dose, indicating an electron-assisted transfer mechanism. Measurements of the relations between the transfer rate and the sample bias and temperature give further support for an electronic mechanism. The bias dependence of the transfer rate shows a peak, and from a first principles electronic structure calculation we show that the position of the peak is related to the energy of a localized surface resonance. We propose that the hydrogen transfer is related to inelastic hole scattering with this surface resonance. We develop a microscopic model for the hydrogen transfer, and using the experimental data we extract information on the resonance lifetime and the transfer yield per resonant electron. The transfer takes place by tunneling through a small excited state transfer barrier. The transfer rate is increased if the hydrogen atom before the resonant excitation is vibrationally excited, and this gives rise to an increasing transfer rate with increasing sample temperature.