SIH4 CHEMICAL-VAPOR-DEPOSITION ON SI(111)-(7X7) STUDIED BY SCANNING-TUNNELING-MICROSCOPY

The initial growth of silicon on Si(111)-(7 x 7) from SiH4 as the prec ursor gas was studied in the temperature range from 690 to 925 K under ultra-high vacuum (UHV) conditions (UHV-CVD) by scanning tunneling mi croscopy. At the onset of hydrogen desorption at around 690 K the evol ution of a bulk-like monohydride-terminated (1 x 1) structure is obser ved. It is shown that growth proceeds by both the formation of homogen eously distributed adlayer islands and heterogeneous nucleation at tra nslational domain boundaries of the initial surface. During the (7 x 7 )-->(1 x 1):H conversion a site-selective etch reaction is observed, w here center adatoms are etched preferentially. Annealing of the (1 x 1 ):H surface structure results in the reappearance of dimer-adatom stac king-fault (DAS) structures in addition to metastable non-DAS structur es. It is shown that the diffusion barrier for mobile fragments to cro ss over terrace/island edges strongly depends on the step orientation. With increasing substrate temperature, defective (5 x 5) and (7 x 7) DAS structures are formed and growth is dominated by heterogeneous nuc leation at domain boundaries and step flow. Finally, at the highest te mperatures a well-ordered (7 x 7) structure grows exclusively via step flow. Step propagation in the [(1) over bar (1) over bar 2)] directio ns is preferred in the entire temperature range investigated. The stic king probability of SiH4 on presputtered Si(111) is enhanced, but at t he expense of crystalline quality. (C) 1997 Elsevier Science B.V.