Growth and characterization of ultrathin cobalt silicide films on Si(211) and (311)
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INTRODUCTION
Previous studies of ultrathin cobalt silicide films grown on Si(lll) have shown that the phases formed, as well as the structural and electrical properties of these films, vary with the deposited Co thickness and reaction conditions.1'2 Perhaps the most striking finding to emerge has been the importance of epitaxial phases in the evolution of cobalt silicide films on the high symmetry Si(lll) surface. All of the phases observed in films formed by depositing less than 35 A Co onto room temperature Si are epitaxial. There are two regimes of growth. If less than 10 A of Co is deposited, epitaxial type B CoSi2 (i.e., the lattice of the CoSi2 is rotated 180° with respect to the lattice of the Si substrate) forms immediately. If greater than 10 A of Co is deposited, epitaxial Co2Si forms first which then proceeds to CoSi2 via the formation of two epitaxial variants of CoSi. The temperature of reaction is another very important parameter. Films formed by depositing greater than 10 A of Co and annealing below 600 °C have large atomic scale disorder, as revealed by the magnitude and temperature dependence of film resistivity. Films less than 10 A thick which are annealed below 500 °C are continuous but have very high resistivity. Annealing these thinnest films at 600 °C de-
a)Current
address: IBM T. J. Watson Research Center, Yorktown Heights, New York 10598. b)Current address: Department of Engineering Physics, University of Illinois, Urbana, Illinois 61801. 1032
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 5, May 1990
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creases the resistivity, but pinholes become increasingly prevalent. These findings raise questions concerning the influence of the symmetry of a Si surface on the formation of cobalt silicide. Epitaxial phases have been shown to play a dominant or even exclusive role in the sequence of phases formed in both the Co-Si and NiSi systems.2'3 In the latter case, the favorable energetics of an epitaxial interface have led to the stabilization of a metastable epitaxial phase. The constraints placed on an overlayer by a relatively low symmetry substrate surface differ from those imposed by a high symmetry surface which may be expected to exert a strong influence on the tendency of an overlayer to grow epitaxially. This may have an effect on the sequence of phases formed and may even affect the final phase (for example, by breaking the near equivalence of two competing epitaxial orientations of the same phase). The Si(211) and (311) surfaces are particularly interesting to study in this context. In situ transmission electron microscopy (TEM) studies of Si(HO) surfaces have shown that Si(311), although a rather high index surface, forms as facets with an apparently stable n x l reconstruction, suggesting that this surface has low energy.4 Si(211) has been suggested as an optimum surface for the suppression of antiphase domains in the epitaxial growth of a polar semiconductor (such as GaAs).5 In this study we find that the formation of epitaxial intermediate phas
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