Novel Epitaxial Growth and Surface Order-Disorder Transitions of CU 3 AU(110) and CU 3 AU(111)
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NOVEL EPITAXIAL GROWTH AND SURFACE ORDER-DISORDER TRANSITIONS OF CU3 AU(1 10) AND CU 3 AU(1 11)
J.C.A. Huang, J.A. Dura and C.P. Flynn, University of Illinois at Urbana-Champaign.
Abstract We report the results of a reflection high energy electron diffraction (RHEED) study on the order-disorder transitions at the (111) and (110) surface of Cu 3 Au. Our Cu 3 Au films were epitaxially grown on sapphire substrates with bcc (Nb or Mo-Cr) buffer layers. The (111) and (110) surfaces of Cu 3Au are atomically flat as indicated by the RHEED patterns. We use the normalized integrated intensity of the superstructure streaks to measure the long range order parameter. We observed that the surface layers of both (110) and (111) Cu 3 Au exhibit an apparently continuous order-disorder transition, close to that predicted by mean field theory [5]. The kinetics of surface ordering also were monitored by RHEED after initial deposition of disordered layers, 2 MLs thick, on the ordered Cu 3 Au templates. In studies of the surface reordering process we find that the RHEED pattern is almost entirely sensitive to the surface layer alone.
1. Introduction Bulk Cu3Au is a well known fcc binary alloy with a first order phase transition at temperature Tc=390 0 C [1]. In fully ordered Cu 3Au, Cu atoms sit at the face centered positions (a sites) and Au atoms take the comers (P3sites). Above the transition temperature, Cu and Au atoms occupy a and
13sites
randomly. Phase transitions at the surface are expected to be
different from those in the bulk [5,9,10]. For one thing, the coordination number in the surface layer is smaller than in the bulk. In addition, the atomic interaction parameters near the surface layers may differ from the bulk. Phase transitions at the surface are also expected to be orientation dependent for Cu 3Au. Along the [111] direction, each single layer contains the same proportion of a and 13 sites as the bulk. The transition from the bulk-like ordered phase to the disordered surface phase can, if necessary, be achieved by diffusion within a single layer. Along the [100] or [110]directions, however, the ordered phase alternates CuAu layers with pure Cu layers. In the transition of the [100] and [110] surfaces, there is a coupling between the ordered stacking and the surface, and in general the transition requires interchanges among atoms from different layers.
Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
518
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2. Experimental Procedures In order to study phase transitions at the surface, it is essential to prepare clean and atomically flat surfaces. Surface roughness on the atomic scale can hardly be avoided when using cut and polished crystals. Our Cu 3Au crystals, several thousand Angstroms thick, were prepared by molecular beam epitaxy (MBE). (111) films of Cu 3Au were grown on freshly prepared (110) Nb, and (110) Cu 3 Au films were similarly grown on (211) Cr after an initial 500
A thick
layer of (211) Mo. The deposition rate of Cu 3Au was about 0.05 ML/sec and the
growth temperatur
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