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 x 1 surfac e was determined by scanning tunneling microscopy (STM) with varying b ias voltage and bias polarity. Surface structures of 2 x 2, 2 X 3, and 2 X 5 phases formed at below 350-degrees-C consist of Al-dimer lines perpendicular to the underlying Si-dimer rows. The STM images of the A l-dimer lines taken at positive and negative bias between 1 and 3 V ag ree with those of the theoretical simulation by assuming the parallel Al-dimer structure. Moreover we found that filled states of Al-Si back bonds and empty states of Al-Al dimer bonds of the parallel Al-dimer l ines are observed prominently at -3 and +1 eV at positions on the unde rlying Si-dimer rows and between Si-dimer rows, respectively. Atomic c onfigurations at the ends of the Al-dimer lines combined with the unde rlying Si missing dimer defects are discussed on the basis of the obse rved STM im ages.