STM STUDIES OF ISLAND FORMATION AND SURFACE ORDERING OF SI ON GAAS(001), GAAS(2X4) AND C(4X4) - IMPLICATIONS FOR DELTA-DOPING

Scanning tunnelling microscopy (STM) and reflection high energy electr on diffraction (RHEED) have been used to study the deposition of Si be low 400 degrees C onto GaAs (0 0 1) surfaces grown in situ by molecula r beam epitaxy (MBE). The emphasis is on the island formation and grow th, 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 x 4) and c(4 x 4). For deposition on the c(4 x 4) surface, an asymmetric (3 x 1) RHEED pattern is formed, a consequence of an is otropic ''needle-like'' islands, which grow adjacent to each other a l ong the [1 1 0] direction and produce a three-fold periodic superstruc ture. Individual islands grow by a site exchange process in which the additional As layer of the c(4 x 4) structure acts as a surfactant and enables the Si atoms to occupy Ga sites in the GaAs lattice. In contr ast, Si deposition on the (2 x 4) surface does not lead to any new sur face periodicities as monitored by RHEED. The Si atoms form poorly ord ered clusters distributed randomly across the surface. The site exchan ge process cannot occur in this case as the (2 x 4) surface is termina ted with only one layer of arsenic. Instead, the Si atoms occupy sites on top of the outer arsenic layer.