Dose dependence of crystallinity and resistivity in ion beam synthesised CoSi 2 layers

Abstract

Epitaxial CoSi 2 layers have been fabricated in silicon by implanting 200 keV cobalt ions to doses of 2 × 10 17–7 × 10 17 Co + cm −2. For the lower doses (2 × 10 17 and 3 × 10 17 Co + cm −2) the synthesised layers, after implantation, are silicon-rich and consist of A- and B-type CoSi 2 precipitates interwoven by silicon. For the medium doses (4 × 10 17 and 5 × 10 17 Co + cm −2) an epitaxial aligned layer of CoSi 2 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 × 10 17–7 × 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 CoSi 2 layer. As the dose is increased up to 5 × 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 ≠ 2 then the resistivity of the layer increases and there is a corresponding deterioration in crystal quality. After annealing the resistivity of the CoSi 2 layer decreases and the crystallinity improves as the ratio of Co: Si approaches that in the stoichiometric compound (CoSi 2).