THE LOCATION OF SILICON ATOMS AND THE INITIAL-STAGES OF FORMATION OF THE SI GAAS(001) INTERFACE STUDIED BY STM/

Scanning tunnelling microscopy (STM) and reflection high energy electr on diffraction (RHEED) have been used to study the deposition of Si at 560 degrees C in the presence of an arsenic flux on GaAs(001)-(2 X 4) surfaces grown in situ by molecular beam epitaxy (MBE). RHEED studies indicate that for the low Si deposition rates used, the surface struc ture undergoes a series of changes as the Si coverage is increased up to 2.5 monolayers; (2 X 4) --> asymmetric (3 X 1) --> symmetric (3 X 1 ) --> (3 X 2) --> (4 X 2). At low coverages, the Si preferentially occ upies vacant second layer Ga sites in the missing dimer trenches of th e (2 x 4) reconstructed clean surface and a disordered, kinked surface is formed. Adjacent Si atoms in the trenches are eventually covered w ith As, bridging the missing dimer trenches and forming elongated rect angular units with a periodicity along [<(1)over bar 10>] of Angstrom. The number of these units becomes greater as the Si coverage is incre ased and there is a change in periodicity from 8 to 12 Angstrom along [<(1)over bar 10>] consistent with the appearance of the symmetric (3 X 1) structure. Detailed studies on vicinal surfaces misoriented by 1 degrees towards both (111)A and (111)B indicate no preference for the Si to align itself along the step edges. As the Si coverage is increas ed, the Si displaces Ga atoms from the second layer of the structure a nd forms a series of two dimensional islands which appear as ''holes'' in the surface. These islands eventually dominate the surface and res ult in a terminating Si layer and the observation of the (3 x 2) struc ture. Subsequent Si growth occurs by dimer chain formation on top of t his layer and the strings of Si dimers that are formed at high coverag es are characteristic of STM observations of the homoepitaxial growth of Si on Si(001).