SURFACE RECONSTRUCTIONS AND PHASE-TRANSITIONS ON THE GAAS(111)B SURFACE

The (111)B surface of GaAs has been investigated using scanning tunnel ing microscopy (STM) and a number of different reconstructions have be en found at different surface stoichiometries. In accordance with elec tron diffraction studies, we find the series (2 x 2), (1 x 1)(LT), (ro ot 19 x root 19) and (1 x 1)(HT) with increasing annealing temperature , corresponding to decreasing surface As concentration. The (1 x 1)(LT ) is of particular interest, since it only occurs in a narrow temperat ure window between the two more established reconstructions, the (2 x 2) and the (root 19 x root 19). We find the (1 x 1)(LT) to take the fo rm of a mixture of the local structures of both the (2 x 2) and (root 19 x root 19) phases, rather than having a distinct structure. This is behaviour consistent with a kinetically limited system, dominated by the supply of As adatoms to the surface, and may be an example of a co ntinuous phase transition. Above the (1 x 1)(LT) transition, atomic re solution images of the (root 19 x root 19) surface reveal only a three -fold symmetry of the hexagonal structural units, brought about by ine quivalent surface bonding due to the 23.4 degrees rotation of the surf ace unit cell relative to the substrate, This is responsible for the d isorder found in the (root 19 x root 19) reconstruction, since the str ucture may form in one of two domains. At lower surface As concentrati on, the (1 x 1)(HT) surface adopts a structure combining small domains of a (root 7 x root 7)R 19.1 degrees structure and random disorder, T here is no apparent similarity between the (1 x 1)(LT) and (1 x 1)(HT) structures, which may be due to our measurements being conducted at r oom temperature and without an As flux to control the surface As conce ntration. (C) 1997 Elsevier Science B.V.