Dihydride dimer structures on the Si(100):H surface studied by low-temperature scanning tunneling microscopy

Surface reconstructions on the hydrogenated Si(100):H surface are observed and investigated by using a low-temperature (5 K) scanning tunneling microscope (STM). In addition to the well established $2\ifmmode\times\else\texttimes\fi{}1$ and $3\ifmmode\times\else\texttimes\fi{}1$ phases, linear structures extending over one to six silicon dimers along the same dimer row are observed. After a careful analysis of the corresponding STM topographies for both $n$-type and $p$-type doped silicon substrates, we conclude that these structures are dihydride dimers. This assignment is supported by ab initio density-functional calculations of the local density of states of dihydride structures of one or two dimers long. Furthermore, the calculation of the free-energy formation of our observed structure shows that their creation is closely linked with the hydrogenation process. These results demonstrate that the previous assignments of ``split dimer'' and ``bow-tie'' structures to dihydride dimers and dopant pairs, respectively, need to be reconsidered.