Scanning tunnelling microscopy was used to study (001)-(2 x 4) surfaces which had been grown by molecular-beam epitaxy. It was found that, although reflection high-energy electron diffraction methods always yielded a characteristic (2 x 4) pattern, scanning tunnelling microscopic images indicated significant differences in the composition of the surfaces; depending upon the quenching conditions. A new, locally disordered, (1 x 2) structure was observed under As-deficient conditions. This was found to extend from step edges, and was seen in missing-dimer holes, when the top layer of As had been removed so as to expose the second layer. The atoms in this exposed layer were Ga atoms which formed short rows in the [¯110] direction. The 2-fold periodicity was attributed to a vacancy structure in which Ga atoms were located at alternating sites along the [110] direction. These results were hoped to rationalize recent controversies concerning the composition and structure of the As-terminated (2 x 4) surface, as prepared by using de-capping methods.

A.R.Avery, D.M.Holmes, T.S.Jones, B.A.Joyce, G.A.D.Briggs: Physical Review B, 1994, 50[11], 8098-101