The As-terminated reconstructions formed by GaAs(001): a scanning tunnelling microscopy study of the (2 × 4) and c(4 × 4) surfaces

Abstract

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 × 4) to c(4 × 4) surface with increasing amounts of As. STM images of the initial (2 × 4) surface, corresponding to the β 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 × 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 × 4) β phase having a structure with unit cells based on two As dimers and Gn absent from the missing dimer trenches. The kinks formed on the more As-rich (2 × 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 leadsto the c(4 × 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 × 4) structure based on its formation from the starting (2 × 4) surface and involves a mixed third layer containing both Ga and As.