Instabilities of Vicinal SILICON(111) Surfaces

The morphological instabilities of vicinal Si(111) due to a DC electric current and due to the 7 x 7 reconstruction are studied using a variety of diffraction and imaging techniques. The current induced instabilities which occur at sublimation temperatures or under conditions of step -flow produce morphological features on length scales appropriate for optical studies, 5-50mu. Optical microscopy, a laser diffraction technique atomic force microscopy (AFM) and high resolution x-ray diffraction have been applied to these surfaces. In addition to the step bunching instabilities reported by others, these techniques reveal a transverse instability in the step profiles for current flowing in the step-down direction at all temperatures between rm700^circ C and rm1300^circ C. For the same current direction, the step bunching instability is observed below rm1025^circ +/-15^circ C and above rm1230^circ+/-15^circ C. upto at least rm1300^circ C. On the other hand, for current flowing in the step-up direction, the step profiles are stabilized and the step bunching instability is observed between rm1025^circ+/-15^circ C and 1230^circ+/-15^ circ. Stoyanov's theory of the current driven step-bunching instabilities is reviewed and extended to explain the step wandering instabilities by modifying the boundary conditions of a diffusion equation at the step edges. Measurements of the rate of growth of the transverse instability indicate an activation energy of 2.7 +/- 0.1 eV. Near the 7 x 7 reconstruction temperature, T _ c= 870^circ C, a faceting transition occurs. We have investigated the time and temperature dependence of the spinodal decomposition of step bands and the approach to the equilibrium crystal shape. The equilibrium shape profile exhibits critical behavior z ~ x^alpha where lambda= alpha/(alpha-1)=3.0+/-0.1 which is consistent with a Pokrovsky-Talapov transition. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).