HIGH-TEMPERATURE SCANNING-TUNNELING-MICROSCOPY STUDIES ON THE INTERACTION OF O2 WITH SI(111)-(7X7) SURFACES

The interaction of O2 with Si(111)-(7 x 7) surfaces in the pressure range between 5 x 10(-7) and 5 x 10(-4) Pa was investigated in situ, at reaction temperatures between 850 and 1130 K, by scanning tunneling microscopy. The range of experimental parameters includes the active oxidation regime (high temperature O2 etching), the transition to passive oxidation (thermal oxide growth) and reaches into the latter regime. In the active oxidation regime the reaction proceeds via a step flow mode, where Si is effectively removed at the edges of retracting terraces or via nucleation and lateral growth of two-dimensional holes in the topmost bilayer in the inner areas of extended terraces. Examination of the step propagation velocity for different O2 pressures, sample temperatures and local terrace widths shows that this velocity is proportional to the O2 pressure and the terrace width and independent of temperature. The vertical etch rate r(v), in contrast, depends only linearly on the O2 pressure and is independent of the other two parameters. No O(ad) is present on the surface under these conditions. These findings are consistent only with a mechanism involving oxygen adsorption and SiO formation on the entire terraces and subsequent mass transport via diffusion of vacancies in the Si surface layer, which are finally incorporated at steps or in vacancy islands (vacancy diffusion model). Other models proposed or discussed earlier can be ruled out. At higher pressures, in a distinct transition regime, oxide growth begins by heterogeneous nucleation of small oxide clusters at steps and (7 x 7) domain boundaries, simultaneously with the ongoing etch process. These clusters, which are often lined up along the above defects, stabilize steps against etching and can act as a barrier for moving steps. After removal of the first Si bilayer the step flow process is effectively stopped and further reaction proceeds via slow growth of the oxide covered areas and simultaneous Si removal in between those areas. Under reaction conditions where few oxide clusters are formed, the ongoing etch process leads to an apparent three-dimensional growth of the oxide features. For conditions well in the passive oxidation regime homogeneous nucleation of small oxide clusters (20 angstrom width) on the terraces sets in, in addition to the other two reaction paths. It is found to be the dominant process and leads to surfaces covered by a layer of these small oxide clusters. The high density of these features points to a short diffusion length and hence to a low mobility of the O adatoms, different from the high mobility of the vacancies at these temperatures.