First Principles Study of Dihydride Chains on H-Terminated Si(100)-2$\times$1 Surface

STM observation of H-terminated Si(100)-2$\times$1 surface often shows the existence of a single dihydride-chain structure parallel to a step. That chain is located near the S$_{\rm B}$ step, but away from it more than one Si-dimer's distance. In order to discuss the mechanism of the formation of such a structure, we have performed first principles calculations. As a result, we have found that the ``rebonded'' step edges of the clean Si surface turns into one dihydride chain and a ``non-rebonded'' step edge at the time of hydrogen termination. We have also found that the chain adjacent to the step is energetically less stable than in a distant position from the step. We will show these results and discuss how the dihydride chain changes its position beyond a monohydride-Si-dimer row. This study was performed through Special Coordination Funds for Promoting Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.