Structures of low-coverage phases of Al on the Si(100) surface observed by scanning tunneling microscopy.

The structure of the low-coverage Al phases on the Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface was determined by scanning tunneling microscopy (STM) with varying bias voltage and bias polarity. Surface structures of 2\ifmmode\times\else\texttimes\fi{}2, 2\ifmmode\times\else\texttimes\fi{}3, and 2\ifmmode\times\else\texttimes\fi{}5 phases formed at below 350 \ifmmode^\circ\else\textdegree\fi{}C consist of Al-dimer lines perpendicular to the underlying Si-dimer rows. The STM images of the Al-dimer lines taken at positive and negative bias between 1 and 3 V agree with those of the theoretical simulation by assuming the parallel Al-dimer structure. Moreover, we found that filled states of Al-Si backbonds and empty states of Al-Al dimer bonds of the parallel Al-dimer lines are observed prominently at -3 and +1 eV at positions on the underlying Si-dimer rows and between Si-dimer rows, respectively. Atomic configurations at the ends of the Al-dimer lines combined with the underlying Si missing dimer defects are discussed on the basis of the observed STM images.