STM studies of island formation and surface ordering of Si on GaAs (001), (2×4) and c(4×4): Implications for δ-doping

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si below 400‡C onto GaAs (001) surfaces grown in situ by molecular beam epitaxy (MBE). The emphasis is on the island formation and growth, as well as surface ordering, for submonolayer quantities of Si (up to 0.2 ML) deposited on two different As-rich reconstructions of GaAs (0 0 1) (2 × 4) and c(4 × 4). For deposition on the c(4 × 4) surface, an asymmetric (3 × 1) RHEED pattern is formed, a consequence of anisotropic “needle-like” islands, which grow adjacent to each other along the [1 1 0] direction and produce a three-fold periodic superstructure. Individual islands grow by a site exchange process in which the additional As layer of the c(4 × 4) structure acts as a surfactant and enables the Si atoms to occupy Ga sites in the GaAs lattice. In contrast, Si deposition on the (2 × 4) surface does not lead to any new surface periodicities as monitored by RHEED. The Si atoms form poorly ordered clusters distributed randomly across the surface. The site exchange process cannot occur in this case as the (2 × 4) surface is terminated with only one layer of arsenic. Instead, the Si atoms occupy sites on top of the outer arsenic layer.