THE AS-TERMINATED RECONSTRUCTIONS FORMED BY GAAS(001) - A SCANNING-TUNNELING-MICROSCOPY STUDY OF THE (2X4) AND C(4X4) SURFACES

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the As-terminated reconstructions formed by GaAs(001) surfaces grown in situ by molecular beam epitaxy (MBE). Specific emphasis has been placed on the transition from a (2 x 4) to c(4 x 4) surface with increasing amounts of As. STM images of the initial (2 x 4) surface, corresponding to the beta phase, showed an ordered structure with unit cells containing two As dimers. With increasing amounts of As, the intensity of the 2/4 streak in the RHEED pattern weakened considerably. Although STM images of this (2 X 4) phase again only showed two As dimers per unit cell, the surface was characterized by a considerable degree of disorder and a large number of kinks. The results are consistent with the (2 X 4) beta phase having a structure with unit cells based on two As dimers and Ga absent from the missing dimer trenches. The kinks formed on the more As-rich (2 x 4) structure are then caused by the additional As occupying these vacant Ga sites producing an electron rich site. Quenching to lower temperatures in the presence of As leads to the c(4 X 4) structure. STM images of this surface indicate that the top layer of the structure is based on rectangular units, which when complete, consist of a total of six As atoms. The wide coverage range for which this reconstruction can be maintained is explained by a varying number of missing As atoms from the basic six atom structural unit. A new structural model is proposed for the c(4 X 4) structure based on its formation from the starting (2 X 4) surface and involves a mixed third layer containing both Ga and As.