Atomistic Aspects of Silicide Reactions Studied with STM
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UHV conditions. [4, 5,6, 7,8, 9, 10] In the case of nickel and cobalt silicides, atomically perfect interfaces are readily prepared, and both their electronic and structural properties can be controlled with appropriate growth conditions. [11, 12] Indeed, new methods of molecular beam epitaxy involving template layers and room temperature growth by codeposition have been empirically developed to improve the structure of the silicide overlayers. [13, 14] However, a fundamental understanding of these growth processes is lacking. In this paper, we describe a "wide angle view" of the silicide reaction, in which we consider extremes of temperature and coverage not normally encountered in studies of the contact reaction. We choose Co/Si(1 11) as a model system representative of transition metal silicides in general, and epitaxial systems in particular. We are able to identify several metastable states that occur in the course of the contact reaction, as well as a true equilibrium state at very high temperature. Specifically these states are: near-surface interstitials, monolayer islands of CoSi 2 with a 7-fold (111) interface and a 2x2 array of silicon adatoms, CoSi 2 with an 8-fold (111) interface, a 4I7x•/7 intermediate layer and a 'lx ' lattice gas of ring-clusters. The role of these various states in the reaction path during annealing is outlined. EXPERIMENTAL The STM instrument used for these studies features a dual lever-approach with "stabilizing foot" and tripod scanner. [15] Samples were prepared from 0.1 ohm-cm As doped
239 Mat. Res. Soc. Symp. Proc. Vol. 320. @1994 Materials Research Society
wafers rinsed in methanol, degassed at 1000K and flashed to 1450K. Metal deposition was accomplished with a miniature electron bombardment evaporator, running at a nominal power of 15 Watts and a rate of 0.10 ± 0.01Ml/minute, calibrated in situ with a crystal thickness monitor and ex situ with RBS. Thicknesses are described here in terms of substrate Ml, where lM1 = 7.83 x 1014 cm 2 for the Si(111) surface. Temperatures were measured with an infrared pyrometer using an emissivity correction of r = 0.3 and calibrated with a heating plate and thermocouple. The estimated accuracy is ± 25C at 320C. Depositions were made "off stage" while holding the sample at temperature by resisitive heating. STM analysis was performed at room temperature approximately 1 hour after quench-cooling the sample. RHEED and Auger measurements were performed in a separate chamber with new samples and close replication of sample preparation conditions. NEAR-SURFACE INTERSTITIALS In Fig. 1 we show the progress of the silicide reaction at room temperature in a sequence of 3 images. Starting with the highest coverage of 0.81M1 (panel D), we see island features that are approximately 20 A wide and 2.0 A high. We believe these are silicide islands, as reported in numerous non-imaging experiments at somewhat higher coverage. [4,8,10,16,17] There is no sign of the original 7x7 surface. For a coverage of 0.18M1 (panel C), we see atomic scale features sugg
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