DOSE DEPENDENCE OF CRYSTALLINITY AND RESISTIVITY IN ION-BEAM SYNTHESIZED COSI2 LAYERS

Epitaxial CoSi2 layers have been fabricated in silicon by implanting 200 keV cobalt ions to doses of 2 x 10(17)-7 x 10(17) Co+ cm-2. For the lower doses (2 x 10(17) and 3 x 10(17) Co+ cm-2) the synthesised layers, after implantation, are silicon-rich and consist of A- and B-type CoSi2 precipitates interwoven by silicon. For the medium doses (4 x 10(17) and 5 x 10(17) Co+ cm-2) an epitaxial aligned layer of CoSi2 is formed after implantation, with any excess cobalt being incorporated in small CoSi inclusions at the peak of the distribution. For the highest doses (6 x 10(17)-7 x 10(17) Co+ cm-2) preferential sputtering of the silicon at the surface of the synthesised layer means that this region is cobalt-rich and grains of CoSi form above the epitaxial CoSi2 layer. As the dose is increased up to 5 x 10(17) Co+ cm-2, the crystallinity of the layer improves, after which it deteriorates again as the thickness of the layer of CoSi grains increases. The degradation in crystallinity is accompanied by a rise in resistivity and this can be correlated to the value of x in CoSi(x). When x not-equal 2 then the resistivity of the layer increases and there is a corresponding deterioration in crystal quality. After annealing the resistivity of the CoSi2 layer decreases and the crystallinity improves as the ratio of Co:Si approaches that in the stoichiometric compound (CoSi2).