Characterization of Quantum Heterostructures. A Simulation Method Combining the Evaluation of Lattice and Electrons Conf
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G.MATTEI AND A.M.MAZZONE C.N.R.-Istituto LAMEL, via Gobetti 101, 1-40129-Bologna, Italy. ABSTRACT This work analyzes the characterization of the structural properties of quantum heterostructures by HREM. A simulation of atomistic type is used to construct a Fe/Ag bilayer few nm thick and the HREM image of this structure is calculated by using a standard dynamical diffraction method. The comparison of the two calculations indicates that the interfacial disorder spans few layers and can be resolved against the background of two well-registered lattices.
INTRODUCTION There is currently a considerable interest for the properties of layered structures formed by the intercalation of few atomic planes of semiconductors or metals. From an applicative point of view these structures should be widely used by the electronic and recording industries. From a theoretical point of view the thrust is the identification of two-dimensional domains leading to a new generation of quantum mechanical effects, These effects should appear as enhanced
electron currents or gigantic magnetic moments and consequently be highly beneficial to all practical purposes. At the current state of the art most of the lack of the understanding of these effects appears to arise from an inadequate knowledge of the structural properties of these devices. From a theoretical point of view, in fact, many theories are being developed which invoke surfacespanning, or modulated, Fermi vectors. These concepts are borrowed from thick samples and their validity is uncertain for thin films. Furthermore, from an experimental point of view, observations using electron microscopy techniques are obviously complicated. By STM, in fact, images can be obtained which reproduce the electronic states and their interpretation in terms of the lattice structure represents a complex task [1]. A similar problem arises in the case of HREM. In fact, a part from technical problems, such as specimens manipulation, in quantum heterostructures disorder is by necessity limited to a few atomic planes and these dimensions are comparable with the ones of the entire structure. The validity of the HREM analysis consequently requires that unwanted diffractions do not arise within the ordered regions and/or at the boundaries between ordered and disordered regions. In this paper we attempt to elucidate to some extent either the structural properties of representative metallic quantum heterostructures or the application of HREM to their characterization. Following the approach adopted in [2] a molecular dynamics simulation method is used to simulate a layer-by-layer deposition process of Fe on Ag and the atomic arrangement in the heterostructure. The corresponding spatial distribution of the valence electrons is calculated by using an extended Debye-Hueckel approximation. The HREM image of the heterostructure is constructed from a standard dynamical diffraction method [3]. 257
Mat. Res. Soc. Symp. Proc. Vol. 389 ©1995 Materials Research Society
The comparison of the two calculations shows tha
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