The structure of the low-coverage Al phases on the Si{100)2X1surface was determined by scanning tunneling microscopy {STM) with varying bias voltage and bias polarity. Surface structures of 2X2, 2X 3, and 2X 5 phases formed at below 350'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.

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