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 t o 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, c orresponding to the beta phase, showed an ordered structure with unit cells containing two As dimers. With increasing amounts of As, the int ensity of the 2/4 streak in the RHEED pattern weakened considerably. A lthough STM images of this (2 X 4) phase again only showed two As dime rs per unit cell, the surface was characterized by a considerable degr ee 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 ki nks formed on the more As-rich (2 x 4) structure are then caused by th e 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 t he top layer of the structure is based on rectangular units, which whe n complete, consist of a total of six As atoms. The wide coverage rang e for which this reconstruction can be maintained is explained by a va rying number of missing As atoms from the basic six atom structural un it. A new structural model is proposed for the c(4 X 4) structure base d on its formation from the starting (2 X 4) surface and involves a mi xed third layer containing both Ga and As.