The mutual influence of krypton implantation and pre-existing stress states in polycrystalline alpha titanium S. Nsengiyumva a,b, * , T.P. Ntsoane c , A.T. Raji a , M. Topíc d , G. Kellermann e , J.P. Rivière f , D.T. Britton a , M. Härting a a Department of Physics, University of Cape Town, Rondebosch 7701, South Africa b Department of Physics, Kigali Institute of Education, P.O. Box 5039 Kigali, Rwanda c Nuclear Energy Corporation of South Africa (NECSA), P.O. Box 582, South Africa d iThemba LABS, Somerset West 7129, South Africa e Laboratório Nacional de Luz Síncrotron (LNLS), Campinas, Brazil f Laboratoire de Physique des Matériaux UMR6630-CNRS, 86960, France article info Article history: Available online 27 May 2009 Keywords: Synchrotron radiation Stress-assisted diffusion Stress relaxation Drift Rutherford backscattering abstract The stress profile in polycrystalline titanium implanted with krypton ions at different fluences has been determined using synchrotron radiation diffraction. For each fluence, the krypton profile has been mea- sured using Rutherford backscattering geometry. The results were compared to model calculations obtained from the SRIM 2008 computer code. A strong stress relaxation was found for high fluence implantation, whereas for low fluence implantation an additional source of tensile stress was introduced in the near surface region. The projected range of the implanted krypton was significantly reduced com- pared to the expected range. A possible cause of this discrepancy is the drift of implanted ions under the influence of the pre-existing stress gradient. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Defect dynamics in solids can be described by two superimpos- ing mechanisms: drift and diffusion. Diffusion in solids is the ran- dom motion of matter whereas drift occurs when a driving force causes atoms or vacancies to move in a direction determined by the direction of the force. The drift process is similar to charge transport due to an electric field [1]. The driving force of the drift results from the interaction of the defect’s own strain field with a surrounding stress field [2]. The surrounding stress field can be due to the local strain field of other defects [3], or more generally any structural inhomogeneity in the solid, or due to an applied load. Stress migration of point and open-volume defects is an impor- tant problem in a wide variety of applications, such as degradation of metallic interconnects in semiconductor devices [4], metal fati- gue [5] and corrosion type processes [6]. In a beneficial way, the drift of point defects under the influence of a stress can find appli- cations in semiconductor technology [7] and ion beam modifica- tion irradiation [8]. In general the effect of implantation on the pre-existing stress depends on the fluence, flux, implantation energy, implanted spe- cies, target material, temperature and pre-existing stress. For low dose implantation, the effect is generally to relax the existing stress [9–13]. For instance, in argon implanted polycrystalline titanium low dose implantation has shown to reduce both a pre-existing tensile stress [9], and a pre-existing compressive stress [10]. In krypton implanted Ag 50 Co 50 films, a pre-existing tensile stress reduction was observed [14]. In sputtered chromium thin films, the relaxation of tensile stresses and build up of compressive stres- ses with increasing ion fluence was also observed [15]. Mecha- nisms contributing to the relaxation are attributed to defect annealing mechanisms, or localized thermal effects [11], Newto- nian viscous flow [12], creep [16], or deformation induced by the high energy ion [17]. At higher doses an additional stress is gener- ated in the material which is also associated with radiation hard- ening effects and an increase in stiffness [18,19]. To determine the stress, diffraction experiments with low penetration of the X-rays can be applied [20,21]. This study aims to contribute to a better understanding of the basic processes underlying the effects of stress-assisted diffusion and ion-beam-induced stress relaxation in metals. In particular, emphasis is given to the effect of the implantation of krypton ions on the pre-existing stress in polycrystalline alpha titanium. To ob- tain the krypton depth profiles, i.e. the projected range and the associated straggling of the implanted ions, Rutherford backscat- tering spectroscopy is used. The experimental values are compared to the calculated values obtained using the Monte Carlo computer 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.030 * Corresponding author. Address: Department of Physics, University of Cape Town, Rondebosch 7701, South Africa. Tel.: +27 21 650 3366; fax: +27 21 650 3342. E-mail address: schadrack.nsengiyumva@uct.ac.za (S. Nsengiyumva). Nuclear Instruments and Methods in Physics Research B 267 (2009) 2712–2715 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb