Abstract
An NH3 molecule is dissociatively adsorbed on Si ( 111 )-7 × 7 at room temperature. The dissociated fragment NH2 can be adsorbed on top of either silicon adatom or rest atom. The probability for NH2 to be adsorbed on top of adatom is confirmed to be 12.1 %. Under the scanning of negative sample biases, the atop adsorbed-H2N can be displaced into the backbond of adatom, forming a metastable state. With further scanning of negative sample biases, the metastable state will be transformed into a final and stable state by liberating an H atom. Based on these observations, we want to know how many inelastic tunneling electrons are needed to induce the transformations in our experiments and, in particular, to study the changes of metastable state under the scanning of both polarities of sample bias. Since our experiments were performed non in-situ, therefore, we first estimated the defect density on Si ( 111 )-7 × 7 surface. About five hours later to the annealing treatment of sample, we found the density of single defect (one dark adatom in a half cell) was about 5%. After the dosage of NH3, we used the following scanning conditions to acquire STM images: 1.5 V and □ 2.0 V of sample bias, 0.05 to 0.3 nA of tunneling current, frame time of about 105 s, and scanning area of 35 × 35 nm2. By varying the tunneling current and fixing the other conditions, we found that the number of tunneling electron to induce both transformations, the initial state (adatom-adsorbed NH2) to metastable state and metastable state to final state, is unit. In other words, the whole transformation from initial state to final state is a two-electron process. On the other hand, we also observed that metastable state has the probability to return back to the initial state or to transform into the final state under the scanning of positive or negative sample bias. This can be interpreted by their potential energies and DOS structures near Fermi level. The initial state only has intense occupied DOS below 1 eV and almost has no unoccupied DOS from 0 to 3 eV. The metastable state has DOS on the both sides of Fermi level, but its potential energy is higher than the other states. These facts explain what we observed in our experiments very well.