Studies of ordering on surfaces and in superlattices by molecular beam epitaxy
Molecular beam epitaxy, MBE, was used in two different ways to study the ordering of materials on an atomic scale. In one application the ordering of close packed planes into hcp or fcc structures, or polytype stacking sequences was investigated by periodically alternating growth of Ir and Ru to produce single crystal superlattices. In addition, a novel probe of the kinetics by which a surface of Cu3Au re-orders of was developed. By growing a disordered layer onto a well ordered surface, the kinetics of ordering in layers of almost arbitrarily small thickness can be studied. By eliminating the need to quench the sample this procedure removes the effects of the re-ordering of the bulk and allows for study at a wide range of re-ordering temperatures far below the transition temperature. Single crystal Ir-Ru superlattices have been grown by MBE with their close packed planes perpendicular to the growth direction. The phase diagram which plots structure dependance on the number of Ir monolayers and Ru monolayers in a super lattice has been investigated. The diagram is divided into four regions corresponding to the phases of the superlattices found in the assigned portions of parameter space. The types of phases are: bicrystalline, fcc, hcp, and polytype superlattices, as determined for each superlattice from qualitative features of its x-ray diffraction patterns. More detailed computer modeling was also employed for some superlattices. Composition profiles and stacking sequences were determined by means of two distinct x-ray diffraction scans. These simulations make clear that detailed modeling is necessary to fully determine the stacking sequences. The deduction that bicrystal stacking sequences occur has been confirmed by computer modeling for superlattices in the appropriate region of parameter space. In a conventional experiment, the order-disorder transition of the (111) surface of Cu3Au was studied by means of the intensities of RHEED satellite streaks. The order parameter was found to go to zero continuously at a temperature equal to that of the discontinuous bulk transition, within experimental uncertainty. A novel kinetic experiment was performed to examine re-ordering of (111) Cu3Au adlayers. It was found that a surface layer re-orders independently on a sublayer frozen by its slower bulk kinetics in its initial disordered state. This re-ordering occurs in two stages. Near the critical temperature it achieves approximately the same degree of order as surfaces under which the subsurface layers have equilibrium order.
molecular beam epitaxy (MBE), ordering of materials, atomic scale, kinetics probe
University of Illinois - Urbana-Champaign
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