Ab initio studies of adatom vacancies on the Si(111)-(7x7) surface

Ab initio total-energy calculations are used to investigate adatom vacancies on the Si(111)-(7x7) surface. In striking contrast to recent experimental estimates, vacancy formation energies are calculated to be 0.9 eV on average, with similar to 0.1-eV variations depending on the type of adatom. We find that faulted or corner adatoms can be removed more easily than unfaulted or edge adatoms, respectively. Structural relaxations induce large changes in the electronic structure of the surface states. Calculation of scanning tunneling microscopy (STM) images show that the predicted variations should be readily observed in differential STM measurements.