Atomic resolution scanning tunnelling microscopy was used to study As-terminated reconstructions of (001) surfaces which had been grown  in situ  by means of molecular beam epitaxy. Particular emphasis was placed on a transition, from (2 x 4) to c(4 x 4), with increasing amounts of As. Scanning tunnelling microscopic images of the initial (2 x 4) surface, which corresponded to the  phase, revealed an ordered structure with unit cells that contained two As dimers. With increasing amounts of As, the intensity of the 2/4 streak in the reflection high-energy electron diffraction patterns weakened considerably. Although scanning tunnelling microscopic images of this (2 x 4) phase still exhibited only two As dimers per unit cell, the surface was characterized by a considerable degree of disorder and a large number of kinks. The results were consistent with a (2 x 4)  phase which had a structure with unit cells that were based upon two As dimers; with Ga absent from the missing dimer trenches. Kinks which formed on the more As-rich (2 x 4) structure were then caused by additional As which occupied these vacant Ga sites, thus producing an electron-rich site. Quenching to lower temperatures, in the presence of As, led to the c(4 x 4) structure. Scanning tunnelling microscopic images of this surface indicated that the top layer of the structure was based upon rectangular units. When these were complete, they consisted of a total of six As atoms. The wide coverage range over which this reconstruction could be maintained was explained by a variation in the numbers of As atoms which were missing from the basic 6-atom structural unit. A new structural model was proposed, for the c(4 x 4) structure, that was based upon its formation from the initial (2 x 4) surface and involved a mixed third layer which contained both Ga and As.

A.R.Avery, D.M.Holmes, J.Sudijono, T.S.Jones, B.A.Joyce: Surface Science, 1995, 323, 91-101