Abstract :
The (111)B surface of GaAs has been investigated using scanning tunnelling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1)LT, (√19×√19) and (1 × 1)HT with increasing annealing temperature, corresponding to decreasing surface As concentration. We find the (1 × 1)LT to be a mixture of the local structures of the (2 × 2) and (√19×√19) phases as well as some elements of a (3 × 3) structure. This is behaviour consistent with a system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1)LT transition, atomic resolution images of the (√19×√19) surface reveal only a threefold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding. This is responsible for the disorder found in the (√19×√19) reconstruction, since the structure may form in one of two domain rotations. At lower surface As concentration, the (1 × 1)HT surface adopts a random structure containing small domains of a (√7×√7)R19.1° reconstruction.
