CONTINUOUS-TIME OBSERVATION OF PSEUDO-VACANCY DIFFUSION AT SI(111)-7X7 SURFACES

Scanning tunneling microscopy (STM) is used to study surface diffusion of a special type of point defects at Si(111)-7 x 7 surfaces. These d efects survive even after annealing up to 1250 degrees C. They appear darker than Si adatoms at the tunneling biases ranging from -3 to +3 V , but they are not true vacancies. We found that these vacancy-like de fects (hereafter, we refer to them as pseudo-vacancies) are not caused by adsorption of major contaminants in the vacuum chamber, nor by dop ants. We also observed migration of pseudo-vacancies between nearest n eighboring Si adatom sites at temperatures above 500 degrees C. Most o f the jumps are within a half of the 7 x 7 unit cell. Thousands of STM images were recorded from 520 to 610 degrees C and the activation ene rgies and frequency factors were determined. Varying the tunneling cur rent produces almost no effect on the diffusion, but varying the scann ing speed produces a small effect.