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Boron-Induced Step Faceting and Stripe Formation on Si(001)
by J. B. Hannon, N. C. Bartelt, B. S. Swartzentruber, and G. L. Kellogg

MotivationUnderstanding the basic physics that controls the evolution of surface morphology at the 1-100 nm scale is required for the development of predictive models of semiconductor surface stability.  Previous low energy electron microscope (LEEM) and scanning tunneling microscope (STM) investigations show that the (001) surface of heavily boron-doped Si single crystals exhibits an unusual step faceting transition during cooling from high temperatures [D. E. Jones, J. P. Pelz, Y. Hong, E. Bauer, and I. S. T. Tsong, Phys. Rev. Lett. 77, 330 (1996)].  Single-atom high steps first break up into triangles and subsequently form long stripes that span across the single-crystal terraces (Figure 1).  This transition is not observed in lightly doped Si samples.  The motivation for this investigation is to identify the fundamental interactions leading to formation of this striking "striped" surface morphology, and determine the relative importance of step energetics vs. surface stress driving the transition. 

AccomplishmentOur LEEM measurements and modeling studies show that the formation of triangular facets and striped morphology on heavily boron-doped Si(100) surfaces is a direct consequence of a vanishing free energy of formation of the so-called "A-type" steps.  The equilibrium structure of the "normal" Si(100) surface is a periodic array of two-atom-wide rows (dimer rows).  The direction of the dimer rows rotates by 90 degrees when crossing atomic-height steps.  Those steps that are parallel to the upper terrace dimer rows are called "A" steps and those perpendicular are called  "B" steps. We have made three independent LEEM measurements showing that, in the case of heavily boron-doped samples, the B-step energy is relatively constant as a function of temperature, but that the A-step energy vanishes as the temperature is lowered from 1000 to 900 C.  The measurements include the temperature dependence of:  (1) the A- and B- step stiffnesses (the energy per unit length required to bend a step); (2) the apex angle of the triangular facets; and (3) the aspect ratio of elliptical-shaped islands.   The step stiffnesses are determined from a detailed analysis of step fluctuations observed in the LEEM images and are directly related to the step-formation energies.  Step energies are derived from the facet apex angles and equilibrium island shapes through the use of the Wulff construction.   All three measurements, which span overlapping temperature regimes, show a consistent decrease in the A-step energy with decreasing temperature (Figure 2).  From these measurements, we conclude that increasing amounts of boron segregate to the surface as the temperature is lowered and cause the A-step formation energy to decrease and eventually vanish.  The finding that the A and B step stiffnesses vary differently with temperature and the lack of a terrace-width dependence argue against models involving boron-induced changes in the surface stress. 

SignificanceThese  results  provide a clear indication of how small changes in step energetics can lead to dramatic changes in surface morphology.  The conclusion that step energetics drive the transition is counter to the conventional picture that the evolution of surface morphology on semiconductors is due entirely to surface stress  effects.  By manipulating step energies through the use of chemical additives, the findings suggest that one should be able to tailor surface morphology and even induce specific self-assembly

Figure 1.  Low energy electron microscope (LEEM) images showing the step faceting transition and striped phase formation on Si(001) Figure 2.  A-step free energy of formation as a function of temperature as determined by measurements of step fluctuations, equilibrium island shapes, and facet apex angles
Publication: 
J. B. Hannon, N. C. Bartelt, B. S. Swartzentruber, J. C. Hamilton, and G. L. Kellogg, Phys. Rev. Lett. 79, 4226 (1997).
Work supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering
Last modified December 29, 2003
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