Anisotropy of mass transport on Si(0 0 1) surfaces heated with direct current

We have investigated surface mass transport on Si(0 0 1)-(2×1) surfaces heated with direct current (DC) by observing step “coupling” and step bunching behavior on spherically dimpled surfaces using optical and atomic force microscopy. When the DC is applied along the [1̄ 1 0] crystal direction (parallel or perpendicular to the surface dimer rows), step bunching occurs predominantly in areas of the dimple where the applied current is perpendicular to the local step direction. Surprisingly, when the direct current is applied along the [0 1 0] crystal direction (at 45° to the surface dimer rows), the bunching is strongest in areas of the sample where the applied current is parallel to the local step direction. Steps are also observed to couple such that half the dimple mostly has a (2×1) structure (dimer rows parallel to [1̄ 1 0]) and the other half mostly (1×2) (dimer rows perpendicular to [1̄ 1 0]). However, the dividing line separating the two halves is perpendicular (parallel) to the applied current when the current is applied along the [1̄ 1 0] ([0 1 0]) direction. These observations are consistent with a model in which the force exerted by the applied electric current on a diffusing surface atom is anisotropic in such a way that the force can be parallel, anti-parallel, or sideways to the applied electric field direction, depending on the angle between the electric field and the surface dimer row direction. This would also provide an explanation for the well-known fact that step bunching can occur for both step-up and step-down current on the Si(0 0 1) surface.