REAL-TIME OBSERVATION OF STEP MOTION ON SI(111) SURFACE BY SCANNING TUNNELING MICROSCOPY

Scanning tunneling microscopy (STM) was applied for the first time to the real-time observation of the heating-current-induced step motion on the Si(111) surface at temperatures between 1100 and 1142 K near the (1 x 1)-(7 x 7) phase transition. The steps were found to shift in the opposite direction to the heating current direction or electric field. When the DC current flew towards the step-up direction, step bunching was observed: the steps constructed by more than three monolayers were hardly shifted and became centers for step bunching; a single monolayer step shifted easily and caught up with the bunched step (step band); the shifting speed of the single monolayer step (0.1-1.5 nm/s) was dependent both on the separation between the shifting step and the step band and on the number of monolayer steps in the step band. On the contrary, the step-down current caused step de-bunching and shifting of the step band. In order to explain this behavior, we propose an electric-field-induced model together with the presence of pinning centers such as impurities, stresses. or adsorbates.