SITE OCCUPATION OF SI ATOMS DEPOSITED ON VICINAL GAAS(001)-(2X4) SURFACES

Scanning tunnelling microscopy (STM) and reflection high energy electr on diffraction (RHEED) have been used to study the deposition of Si in the presence of an arsenic flux at 560 degrees C on vicinal GaAs(001) -(2 x 4) surfaces grown in situ by molecular beam epitaxy (MBE). RHEED studies showed that the surface structure changed from (2 x 4) to asy mmetric (3 x 1) at a Si coverage of similar to 0.1 ML, before a symmet ric (3 x 1) structure was formed at a coverage of similar to 0.4 ML. D eposition of low coverages of Si (< 0.1 ML) resulted in a disordered s urface with STM images showing a high degree of kinking of the (2 x 4) surface. The disorder is caused by Si atoms occupying the vacant Ga s ites in the missing dimer trenches of the clean (2 x 4) surface. With increasing Si coverage, neighbouring Si atoms in the trenches are even tually covered with As, bridging the missing dimer trenches and formin g elongated rectangular units. The number of these units becomes great er as the coverage of Si is increased and there is a change in periodi city from 8 to 12 Angstrom along [<(1)over bar 10>] consistent with th e appearance of the symmetric (3 X I) RHEED pattern. Detailed studies carried out on vicinal surfaces, offcut by 1 degrees towards both (111 )A and (111)B planes, showed that for coverages up to 0.5 ML of Si, th e distribution of the Si is uniform across the surface and there is no preferential interaction with the step edges.