THE PRECIPITATION OF KINKS ON STEPPED SI(111) SURFACES

High resolution low-energy electron diffraction, scanning tunneling microscopy (STM), and transmission electron microscopy (TEM) have been used to study the temperature dependence of a vicinal (stepped) Si(111) surface with a polar angle of 6-degrees from (111) along an azimuth rotated about 10-degrees away from the high symmetry [1BAR1BAR2] direction. At the (1 x 1) to (7 x 7) reconstructive transition, the kinks at the step edges precipitate into a phase with high step density and increased rotation from the [1BAR1BAR2] direction, leaving behind a [1BAR1BAR2] oriented phase with triple-layer height steps and (7 x 7) reconstruction. The inclination and azimuth of the orientation of the kinked phase change continuously with temperature. When the surface is cooled to room temperature, STM images confirm a coexistence between the [1BAR1BAR2] oriented step-tripling phase and the kinked phase. In the step-tripling phase, the steps are very straight (kinks are rare) and oriented in the [1BAR1BAR2] direction. There is a small fraction of single-layer height steps between the predominant triple-layer height steps. All the terraces in this phase are (7 x 7) reconstructed. In the kinked phase, steps with a high density of kinks are bunched together and rotated to a direction about 45-degrees away from [1BAR1BAR2]. TEM images of the surface topography also confirm the phase coexistence on a macroscopic scale. This phase separation (azimuthal faceting) is reversible, and can be understood thermodynamically by analogy with phase separation in a two-component fluid.