Observations of the formation and kinetics of surface steps during evaporation and condensation
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I. SURFACE DECORATION FOR REVEALING THE STEP KINETICS
IN studying the initial stage of thin film growth in 1958, G.A. Bassett[11 found that a small amount of gold evaporated on the cleavage face of rock salt does not form a homogeneous film but instead small individual crystallites. These crystallites were preferentially formed along surface steps; this is the fact on which the decoration methodt2,3] is based. Generally speaking, this method is a special replica technique: the decoration material is evaporated onto the investigated surface and covered with carbon, and the replica film produced in this way is peeled off and the distribution of the decoration crystallites can be investigated in a transmission electron microscope (TEM). (In special cases, the decorated surface is investigated directly in a scanning electron microscope (SEM).) As found already by Bassett, the decoration method enables surface steps of a height of only one atomic distance to be observed. In many respects, decoration is unequaled by any other surface-imaging method. Apart from the atomic resolution in the vertical dimension, the most striking features of this method are the excellent contrast and the high lateral resolution, down to 4 nm. Decoration primarily requires knowledge, not a sophisticated apparatus. If decoration is applicable to a material, it allows extensive experimental observations of convincing evidence. At present, the decoration of surface steps is possible for about 30 materials. Soon after Bassett's u] observation, the decoration method was successfully applied by Bethge and coworkers, specifically to problems of crystal and thin film growth (surveys in References 4 through 8). The experiments were carded out on alkali and silver halides, silver, and silicon. The observations were interesting for K.W. KELLER is with the Akademie der Wissenschaften der DDR, Institut fuer Festkoerperphysik und Elektronenmikroskopie, PSF 250, Halle DDR 4010, German Democratic Republic. This paper is based on a presentation made in the symposium "The Role of Ledges in Phase Transformations" presented as part of the 1989 Fall Meeting of TMS-MSD, October 1-5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL. METALLURGICAL TRANSACTIONS A
two reasons: first, they revealed step kinetics and molecular processes in crystal growth; and, second, the step patterns produced by crystal growth or decomposition are special examples of dynamic structures, as discussed in Haken's concept of synergetics,tg"l~ As an example, Figure 1 shows step patterns produced by vapor deposition of NaC1 on an NaC1 cleavage surface, tl~ One can see a spiral step and individual and concentric circular steps. Analogous step patterns were observed in previous evaporation experiments, tS] They reveal different ways of step formation: Frank's spiral mechanism and spontaneous and repeated preferential two-dimensional (2-D) nucleation. Here it should be pointed out that the effectiveness
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