SURFACE ATOMIC-STRUCTURE OF THE GAAS(001)(2X4) RECONSTRUCTION

Atomic resolution STM images of the (2 X 4) reconstruction, are presen ted which show the existence of at least four distinct surface structu res. Two of these correspond to the alpha and beta phases previously m odelled from RHEED experiments. The second layer structure within the missing row of the alpha-(2 X 4) phase is clearly resolved in these im ages Evidence is presented of dimer twist as part of the surface energ y minimisation process of one of the alpha phases. We also show images of a previously unreported structure that displays (2 X 4) symmetry.