The role of ledges in vapor/solid phase transformations observed by low-energy electron microscopy and photoemission ele
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I.
INTRODUCTION
ACCORDING to the terrace-ledge-kink modelIll of crystal growth at low supersaturations, atoms from the vapor phase impinging upon a crystal surface adsorb on the terraces, forming a two-dimensional (2-D) gas. These adatoms then diffuse to ledges of monatomic height and adsorb at kink sites. The net result is the migration of monatomic ledges across the surface. For near-equilibrium growth conditions, the model predicts that kink sites exist every few atomic spacings along ledges of all crystallographic directions. Ill Migration of ledges is then independent of crystallographic direction. Sublimation is simply the opposite of crystal growth, and the ledges migrate in the same manner but in the direction opposite of growth. The model does not hold for most conditions typically met in practice, which are far from equilibrium. Here, the kinetics of growth dominates, and growth is usually highly dependent upon crystallographic direction. The migration of monatomic ledges during growth from the vapor and during sublimation, near equilibrium conditions, was observed in situ and in real time by low-energy electron microscopy (LEEM) and by photoemission electron microscopy (PEEM) for the system Cu/ Mo{110}. The results presented here are part of more extensive studies of the epitaxial growth of Cu on Mo{110} and the fcc(111)-bcc(110) interface (fcc = face-centered cubic; bcc = body-centered cubic). [2[ The system is of interest because it may be grown as an artificial metallic superlattice. [3]
M. MUNDSCHAU, formerly with the Technical University of Clausthal, is Staff Scientist, Fritz-Haber-Institut, Berlin, Federal Republic of Germany. E. BAUER, Professor of Physics, is with the Technical University of Clausthal, Clausthal-Zelleffeld, Federal Republic of Germany. W. SWlI~CH, Doctoral Student, is with the Technical University of Clausthal and the Institute of Experimental Physics, University of Wroc/aw, Wroc/aw, Poland. 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
II.
BACKGROUND m L O W . E N E R G Y ELECTRON MICROSCOPY
Low-energy electron microscopy combines low-energy electron diffraction (LEED) with low-energy electron imaging of surfaces. Monatomic ledges were resolved with this technique by Telieps and Bauer in 1985.t4j Since that time, it has verified many fundamental properties of ledges on surfaces and their role in phase transformations, t5,6] The instrument used in our experiments is shown schematically in Figure 1. The electron gun uses a Wpoint cathode operated under thermionic conditions and is similar to point cathodes used in some transmission electron microscopes. Such guns produce highly coherent electron beams.J7,8] The electrons are initially accelerated to 20 keV. This allows use of convent
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