Surface diffusion of single vacancies on Ge(111)-c(2×8) studied by variable temperature scanning tunneling microscopy

The dynamics of single vacancies on Ge(111)-c(2×8) surfaces have been investigated by means of variable temperature scanning tunneling microscopy (STM). These vacancies were deliberately created with the STM at different sample temperatures by slight tip-sample contacts. The activation energies and the preexponential factors for the surface diffusion of the created vacancies have been measured, finding that it is a thermally activated motion that presents a slight anisotropic behavior. The activation energy barrier obtained along the [11̅ 0] direction is E_dy=0.83±0.03 eV, while along [1̅ 1̅ 2] it is E_dx=0.95±0.04 eV. The origin of such anisotropy is discussed in terms of previous experimental results measured only at room temperature as well as previous first-principle calculations of the pristine Ge(111)-c(2×8) surface. Finally, the vacancy extraction procedure has been investigated in a wide temperature range and it is shown for the first time that it is possible to create single vacancies at temperatures as low as 40 K.