Site occupation of Si atoms deposited on vicinal GaAs(001)-(2 × 4) surfaces

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si in the presence of an arsenic flux at 560°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 asymmetric (3 X 1) at a Si coverage of ∼ 0.1 ML, before a symmetric (3 X 1) structure was formed at a coverage of ∼ 0.4 ML. Deposition of low coverages of Si (< 0.1 ML) resulted in a disordered surface 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 sites in the missing dimer trenches of the clean (2 X 4) surface. With increasing Si coverage, neighbouring Si atoms in the trenches are eventually covered with As, bridging the missing dimer trenches and forming elongated rectangular units. The number of these units becomes greater as the coverage of Si is increased and there is a change in periodicity from 8 to 12 A along [110] consistent with the appearance of the symmetric (3 X 1) RHEED pattern. Detailed studies carried out on vicinal surfaces, offcut by 1° towards both (111)A and (111)B planes, showed that for coverages up to 0.5 ML of Si, the distribution of the Si is uniform across the surface and there is no preferential interaction with the step edges.