IDENTIFICATION OF THE SI(001) MISSING DIMER DEFECT STRUCTURE BY LOW-BIAS VOLTAGE STM AND LDA MODELING

A JEOL scanning tunneling microscope (STM) has been used to image the clean silicon (001) surface at low sample bias voltages (around -0.4 V). At this bias, many dimer vacancies are highlighted by a bright feature on the neighbouring dimers. On other defects, this situation is reversed; the area around the defect becomes dark at low voltages. In both cases, at higher bias voltages (around -0.8 V), this contrast disappears. For a number of proposed structures of the single dimer vacancy, ab initio calculations of charge density as a function of energy have been used to simulate STM images. These images show significant bias voltage dependence, and the low bias voltage images differ markedly between the structures modelled. On this basis, we identify the rebonded structure with the bright defect, and the non-rebonded structure with the dark defect.