Different Relaxation Modes of Epitaxial Thin Film Alloys With A Large Size Effect
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J. THIBAULT*, C. DRESSLER, P. BAYLE-GUILLEMAUD CEA-Grenoble / D1partement de Recherche Fondamentale sur la Mati~re Condens~e/SP2M, 17 rue des Martyrs, 38054 GRENOBLE-FRANCE. * Member of the CNRS.
ABSTRACT In epitaxial layers, the mechanism of stress relaxation under consideration is in general dislocations. This paper will present experimental evidences for other modes of relaxation. which may occur in some situations especially where a strong size effect or misfit is present. In fact the stress is the driving force for intermixing, twinning, and phase transformation, which are not expected to occur in the bulk. All these experimental results have been sustained by numerical simulations that will also be presented.
INTRODUCTION In strongly stressed epitaxial systems, the stress relaxation does not necessarily occur via the "classical" dislocation mechanism. On the contrary, the system may find transformations that correspond to metastable states in the bulk but are driven by the stress. The AuNi system grown on (001) Au is an illustrative example of this situation. Au and Ni exhibit a large size effect and the phase diagram shows a large miscibility gap with no miscibility at room temperature. This paper will present the different relaxation modes encountered in Au/Ni multilayers, Ni thin films or Au1 -xNix alloys grown on top on a (001) Au buffer. The samples were grown by MBE at room temperature on a MgO substrate. A detail description of the growth process has been already published [1]. It will be shown that the expected dislocation mechanism occurs only for alloys with a low misfit i.e. with a low Ni content. Otherwise, intermixing, twinning or phase transformations take place. Furthermore, it will be shown that ordering occurs with annealing treatment. This study has been performed using both high-resolution electron microscopy and Xrays [2]. This paper will present the HREM results. The HREM observations were carried out on a Jeol 4000EX electron microscope (Cs = 1 mm) operating at 400 kV.
RELAXATION BY INTERFACIAL MIXING IN Au/Ni MULTILAYERS The results presented in this paragraph were published in [3, 4]. The main results are the following ones. Despite the 13.7 % misfit defined as (aAu-aNi)/aAu, a coherent growth of Ni on
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Mat. Res. Soc. Symp. Proc. Vol. 578 © 2000 Materials Research Society
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Figure 1: HREM image (a) of a Au/Ni multilayer
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