Dependence of the structural and electrical properties of ultrathin cobalt silicide films on formation conditions
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J. L. Batstone Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, United Kingdom
i.C. Hensel, M. Cerullo, and F. C. Unterwald AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 2 May 1988; accepted 25 August 1988) We have studied the dependence of the electrical and structural properties of ultrathin cobalt silicide films on the annealing temperature and deposited Co thickness. If less than 10 A of Co is deposited, epitaxial type B CoSi2 forms immediately. As the deposited thickness approaches 10 A, small amounts of Co2Si are observed. If greater than 10 A of Co is deposited, epitaxial Co2Si forms at room temperature, which proceeds either via the reaction Co2Si —> CoSi —* CoSi2 or via Co2Si —» CoSi2 during annealing. In these thicker films our results suggest that the formation of type A CoSi2 is correlated with the presence of Co2Si; the presence of CoSi as an intermediate phase is correlated with the occurrence of type B CoSi2. Both film thickness and reaction temperature strongly influence the electrical transport in these films such that very high resistivities are encountered when films either become very thin or are reacted at low temperatures. In the former case the size effect is responsible whereas in the latter the transport properties are dominated by extensive atomic-scale disorder. I. INTRODUCTION We recently reported that coherent, electrically continuous films of CoSi2 could be grown on Si(lll) with thicknesses as small as 10 A.' These films were formed by the evaporation of Co onto Si substrates under ultra high vacuum (UHV) conditions followed by anneals at ~600 °C. We observed a pronounced dependence of the structural properties of the layers on the thickness of the silicide. Films less than —30 A thick were coherent with the Si lattice and displayed only the type B orientation, such that the lattice of the silicide was rotated by 180° with respect to the lattice of the Si substrate. They also had a high density of pinholes, covering ~20% of the area of the substrate. Films thicker than 30 A contained misfit dislocations and had both type A (i.e., the lattice of the silicide was aligned with respect to the lattice of the substrate) and B oriented grains. The pinhole density in these thicker films was very low (=slO6 cm" 2 ). The resistivities of the films increased dramatically with decreasing thickness in a manner which was qualitatively consistent with a quantum size effect. The results of this work posed some intriguing questions. While dislocation-free films of CoSi2 could be grown, it was not established that such films could be grown pinhole-free. In addition, the resistivity of the thinnest films was very high, and it was not clear whether this was an intrinsic property of thin CoSi2 films or an artifact of film preparation. Various in situ surface science studies of Co deposition on Si have previously been performed. Angle-resolved photoemission and x-ray photoelectron spectroscopy have indicated the presence of a CoSi2-like layer
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