Formation and healing of defects at the Si(111)7x7 surface under low-energy ion bombardment

We studied the formation and healing of adatom defects on a Si(111)7x7 surface bombarded by 0.5-keV Ar ions. Scanning tunneling microscopy showed that adatoms were missing from the Si(111)7x7 surface. Increasing the temperature during the bombardment increased the percentage of missing adatom sites. However, the percentage saturated at 400 K, then decreased with temperature. This temperature dependence was due to competition between the formation and healing of adatom defects; defect formation dominated at low temperatures, but was overcome by healing at high temperatures. We analyzed the temperature dependence using a rate equation for missing adatoms which included the temperature-independent sputtering and other temperature-dependent formation and healing processes. Activation energies of 0.29 and 0.39 eV were obtained for the temperature-dependent formation and the healing of adatom defects. The temperature-dependent formation was attributed to vacancy-adatom recombination, and the temperature-dependent healing was attributed to the interstitial atom-missing adatom site recombination. These vacancy and interstitial atoms were generated in the collision cascade under the surface. Some of them migrated to the surface and contributed to the temperature-dependent formation and healing of missing adatoms.