Energetically and kinetically driven step formation and evolution on silicon surfaces.

摘要

Energetically and kinetically driven step formation and evolution on Si(001) and Si(111) surfaces has been investigated experimentally using scanning tunneling microscopy (STM), atomic force microscopy (AFM), optical microscopy, and low-energy electron microscopy (LEEM). Four systems are investigated:; 1. Detailed STM measurements of boron-doped Si(001) surfaces is presented, along with large-scale AFM and LEEM observations of the well-known boron-induced ‘striped’ phase at elevated temperatures. Boron is shown to induce a variety of related atomic-scale structures, some of which tend to decorate surface step-edges. This, in turn, could provide an explanation for the observed boron-induced reduction in step formation energy. However, the observed boron-accumulation at step-edges does not appear to vary systematically with annealing temperature, leaving the well-known temperature dependence of the striped phase unresolved. Real-time LEEM observations of striped step formation on Si(001) during diborane (B ₂H₆) exposure at elevated temperatures are used to demonstrate the controlled formation of large (>5μm) surface regions with highly uniform striped step structures.; 2. Large-scale step rearrangements have been investigated on Si(001) and Si(111) surfaces heated to sublimation temperatures (>900°C) using a direct current. These surfaces undergo dramatic morphological changes, which are believed to arise from a directional drift of diffusing surface atoms in the presence of an applied electric field. Such ‘electromigration’ phenomena include step ‘bunching’ and step ‘wandering’, as well as a predicted step ‘bending’ instability. Using AFM and optical microscopy, we argue that the direction of surface atom electromigration on Si(001) can be parallel, anti-parallel, or even sideways to the applied electric field, depending on the direction of the applied field with the high-symmetry 110> crystal directions. In addition, the first experimental evidence for the predicted step bending instability is presented.; 3. Sublimation pit formation is studied on Si(001) surfaces heated to ~1000°C. Real-time LEEM and microscopic modeling of step dynamics is used to show that—for a given net sublimation rate—adding a small Si flux during heating increases the stability of atomically flat surface terraces against sublimation pit formation. This makes it practical to produce much larger step-free terraces than have been reported previously.; 4. The anisotropy of surface diffusion on Si(001) has been analyzed from the formation of ‘denuded’ zones during Si(001) two-dimensional homo-epitaxial growth. Comparison with a simple model for surface atom diffusion shows that diffusion is at least 10 times faster along the surface ‘dimer’ rows than across them. Furthermore, contrary to previous reports, Si(001) surface diffusion is shown to be anisotropic at temperatures up to at least 850°C. The apparent contradiction between our results and those reported by previous authors is resolved by observing denuded zone formation for several different growth conditions.