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Studies of nucleation, coherent tilt and surface phase transitions by molecular beam epitaxy

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Title: Studies of nucleation, coherent tilt and surface phase transitions by molecular beam epitaxy
Author(s): Huang, Jung-Chun
Doctoral Committee Chair(s): Flynn, C. P.
Department / Program: Physics
Discipline: Physics
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Physics, Molecular Physics, Condensed Matter
Abstract: High-quality single crystals and alloys have been synthesized by molecular beam epitaxy (MBE). These crystals were employed to study the nucleation and strain relief processes during epitaxial growth, as well as phenomena of surface order-disorder transformations.Six distinct orientations of high quality hcp titanium single crystals were prepared on freshly grown bcc transition metals on sapphire substrates. Four different nucleation mechanisms were found to select the eventual crystal orientations. In one special case, the growth of tilted Ti (1012) is identified with nucleation at the step-edges rather than on the terraces of the stepped Ta (211) surface.Nucleation and growth of Au on alkali halides also was investigated. It is found that the growth morphology and the critical thickness at which gold clusters coalesce depend strongly on the kinetic factors such as the growth temperature and deposition rate R. It is observed that the critical thickness h$\sb{\rm c}$ is proportional to R$\sp{1/6}.$ The result is explained in terms of phenomenological theory. This led to estimate the activation energy for adatom diffusion of about 0.78 eV, in good agreement with the global behavior described earlier by Yang and Flynn.By preparing epitaxial layers of various hcp metals it has been possible to explore the anelastic behavior required to grow epitaxial crystals with atomic planes which are tilted away from principal low-index planes, explicit requirements for coherent tilt are formulated using the (211) planes of bcc transition metals as templates, as described earlier by Du and Flynn. fcc transition metals have been successfully prepared in the (110) orientation on the same bcc substrates with a predicable and tunable tilt up to 7$\sp\circ.$ The established principles are further adapted to explain the coherent tilt of bcc metals grown on (1012) sapphire substrates.Order-disorder transitions at the (110) surfaces of MBE grown Cu$\sb3$Au film have been studied using reflection high energy electron diffraction (RHEED). The bulk crystal undergoes a first order transformation at 390$\sp\circ$C. It is observed that the (110) Cu$\sb3$Au exhibits an apparently continuous order-disorder transition, close to that predicted by mean field theory. This is the first confirmation of earlier mean field treatments of the perturbed order near a crystal surface. The kinetics of the surface ordering process also were monitored by RHEED, after initial deposition of several monolayers of disordered Cu$\sb3$Au onto the ordered Cu$\sb3$Au template. The kinetic measurements reveal that the surface ordering occurs in two stages. One, fast process, might correspond to the initial nucleation and growth of ordered domains. The other, slow process, might correspond to the coarsening of the domains. The measured surface activation energies are relatively small compared to the bulk value.
Issue Date: 1992
Type: Text
Language: English
URI: http://hdl.handle.net/2142/20377
Rights Information: Copyright 1992 Huang, Jung-Chun
Date Available in IDEALS: 2011-05-07
Identifier in Online Catalog: AAI9305560
OCLC Identifier: (UMI)AAI9305560
 

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