Block-by-Block Deposition of Complex Oxide Films
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MRS BULLETIN/SEPTEMBER 1994
We then introduce the concept of blockby-block growth and illustrate it with examples such as the La2-I(Ba^r)iCuO4 (214) and the DyBa2Cu3O7_s (123) compounds. In our description, we place particular emphasis on the thermodynamical quantities which—at the high substrate temperatures used (close to the melting point3)— govern the behavior at the growth front, and on the particular advantages of this
Figure 1. The 123 unit cell on top of SrTiO3.
growth technique for obtaining highquality, precipitate-free, thin films. Finally, we provide some key ingredients that may lead to the growth of new materials at a film-substrate interface. Current Growth Methods Every growth mode starts with the deposition of the composing species, which diffuse across the substrate until they encounter a pinning site such as an atomic step or a surface defect. As time and deposition proceeds, clusters form at these sites until a critical size is reached and a crystallite is nucleated. Further growth then proceeds either by lateral additions to the nuclei (Van der Merwe growth), by additions in three dimensions (Volmer-Weber growth), or by a combination of the two. This is a good description of the growth processes for simple materials, but does not seem appropriate for layered materials with large unit cells, multiple components, and a complex phase diagram. In the specific case of codepositing the 123 compound, the three basic materials— Cu, Ba, and Dy are deposited on the substrate under a flow of molecular or activated oxygen. The process described in the previous paragraph can now occur simultaneously for each of those phases in the phase diagrams that are stable under the deposition conditions—there are more than ten different phases4—with the additional complication that these phases can react with one another. For the 123 compound, little information currently exists about the many different sequences of cluster formation and phase reaction that can all lead to the nucleation of a 123 crystallite under growth conditions. This information is crucial for selecting the sequence of events that yields defect- and precipitatefree single-crystal thin films. We will now examine the changes occurring at the surface during a layer-by-layer deposition process. The 123 unit cell, on top of the SrTiO3 substrate, shown in Figure 1, consists of six monolayers. The goal of any layer-by-layer deposition method is to supply monolayers of the desired species using a sequence that is compatible with the naturally occurring one, i.e., ../BaO/CuO2/Dy/CuO2/BaO/CuO/.... Let us consider briefly, from a purely thermodynamic point of view, what can happen during the deposition of each monolayer. After the deposition of the two first monolayers, it is highly probable that the Cu and Ba atoms deposited will build a BaCu-O phase, following the logic of the phase diagram. The subsequent deposition of Dy-O alters this picture completely since at that point the actual surface stoichiometry is DyBaCuOr, which is close
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Block-by-Blo
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