Under the condition of low chemical potential, a 2×3 and other new superstructures, the axes of which are in directions different from crystal axes, were observed for Se-treated GaAs(001) surface by scanning tunneling microscopy (STM). These structures were explained by the dimer model where buckling structures of the Se dimers play important roles. The controversial 2×1 dimer structure was also observed; however, the corrugation in the 2×1 cluster was very small (-0.01nm), which is considered to have caused difficulties in the observation of this surface by STM.
