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 defects 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 defects (hereafter, we refer to them as pseudo-vacancies) are not caused by adsorption of major contaminants in the vacuum chamber, nor by dopants. We also observed migration of pseudo-vacancies between nearest neighboring Si adatom sites at temperatures above 500 degrees C. Most of 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 energies and frequency factors were determined. Varying the tunneling current produces almost no effect on the diffusion, but varying the scanning speed produces a small effect.