A SCANNING-TUNNELING-MICROSCOPY STUDY OF THE DEPOSITION OF SI ON GAAS(001) IMPLICATIONS FOR SI DELTA-DOPING

Atomic resolution scanning tunnelling microscopy (STM) has been used t o study the adsorption of Si on GaAs(001) surfaces, grown in situ by m olecular beam epitaxy (MBE), with a view to understanding the incorpor ation of Si in delta-doped GaAs structures. Under the low-temperature deposition conditions chosen, the clean GaAs surface is characterized by a well-defined c(4 x 4) reflection high-energy electron diffraction (RHEED) pattern, a structure involving termination with two layers of As. Filled states STM images of this surface indicate that the basic structural unit, when complete, consists of rectangular blocks of six As atoms with the As-As bond in the surface layer aligned along the [1 10] direction. Deposition of < 0.05 ML of Si at 400 degrees C onto thi s surface shows significant disruption of the underlying structure. A series of dimer rows are formed on the surface which, with increasing coverage, form anisotropic ''needle-like'' islands which show no tende ncy to coalesce even at relatively high coverages (similar to 0.5 ML). The formation of these islands accompanies the splitting of the 1/2 o rder rods in the RHEED pattern along [110]. As the Si is known to occu py only Ga sites, the Si atoms displace the top layer As atoms of the c(4 x 4) structure, with the displaced As atoms forming dimers in a ne w top layer. The results are consistent with a recently proposed site exchange model and subsequent island formation for surfactant mediated epitaxial growth.