Fluctuations and correlations in sputtering and defect generation in collision cascades in Si

This paper concerns the investigation of certain statistical properties of collision cascades in a Si target, induced by Ge ions in the energy range 1-200 keV. The first and second moments, i.e. the mean and the variance of the number of defects and the sputter yield were calculated as functions of the projectile energy. Two-point correlation functions of the defect depth distribution and temporal correlations in sputtering have also been calculated. Such investigations have been made in the past mostly for amorphous materials. In this paper both amorphous and crystalline structures are investigated. All calculations were made by the binary collision code MARLOWE. The effect of the crystalline structure on the calculated statistical quantities of the cascade, in particular the effect of channelling, can be studied quantitatively. It was found that for crystalline structures the energy dependence of the relative variance of the defect number and the sputter yield, as well as the structure of the two-point correlations, follow similar trends as those in an amorphous solid. The magnitude of the relative variance can be both higher and lower for the crystalline material as compared to the amorphous one, depending on the bombarding angle, projectile energy, and the type of quantity considered (defects or sputtering). On the whole, the fluctuations appear to be larger in the crystalline material. (C) 2000 Elsevier Science B.V. All rights reserved.