In-Situ Characterization of Growth and Intermixing at a Heteroepitaxial Interface: Fe on Au(001)
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L INTRODUCTION The monitoring of the growth and characterization of thin films has advanced in the past decades through the use of both in situ and ex situ techniques [1]. One of the central topics being studied is the interface property of two crystalline materials. Various techniques, for examples, TEM and X-Ray diffraction have provided valuable information about the smoothness or roughness at the interfaces of many superlattices with resolution of about 50 A [2). Other in situ techniques such as STM [3], RHEED [4], and He atom diffraction [5] have also advanced our understanding of the morphology at the growth front of thin films.
In this paper, we use HRLEED technique [6] to study in situ the growth front and the interface mixing of ultrathin heteroepitaxial films as functions of time and temperature. The quantitative information we obtain at the solid-solid interface is in the regime of less than 5 A, an order of magnitude smaller in scale compared with TEM or X-Ray diffraction studies. We analyze the measured angular profiles with kinematic diffraction theory to give quantitative information on step heights, growth modes, islands, spacings, and vertical layers distributions, atomic place exchange, and inhomogeneity. We confirm the island and spacing distributions extracted from the diffraction using our results from STM images. The method we used is very general and can be applied to the study of other heteroepitaxial interfaces. IL INITIAL STAGE OF GROWTH A. Layer-by-layer Growth Mode vs 3D Island Growth Mode A popular and powerful technique used to monitor the in situ growth is Reflection High Energy Electron Diffraction [4]. In this technique, one measures the diffraction intensity of a 13 Mat. Res. Soc. Symp. Proc. Vol. 318. @1994 Materials Research Society
grazing incident high energy electron beam scattered from a surface growth front. If the growth of the film under study follows the layer-by-layer mode, then the measured peak intensity would show an oscillation as a function of deposition time. The maximum intensity, where constructive interference of high energy electron waves occurs, corresponds to the completion of a monolayer and the minimum intensity, where destructive interference occurs, corresponds to half monolayer. Similarly, diffraction of low energy electrons can also be applied to the study of growth fronts. The experiment has been described elsewhere [7]. Figures la and lb show the peak intensity oscillation of the (00) beam near the out-of-phase diffraction conditions, 25.5 eV and 28 eV, respectively, from two different runs of Fe deposition on a Au(001) surface. Both deposition rates were set about the same by holding the Fe source temperature at about 1000 'C. The oscillation is not so clean cut as compared with many other homoepitaxial systems such as Si on Si [8]. This is because the scattering factor of Au is about three times larger than that of Fe and because of the complication of atomic place exchange and intermixing to be discussed later. Even if the growth is a perfect la
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