PHYSICAL REVIEW B VOLUME 56,NUMBER 5 1 AUGUST 1997-I Point defect movement and annealing in collision cascades K.Nordlund"and R.S.Averback Materials Research Laboratory.University of Illinois at Urbana-Champaign.Urbana,Illinois 61801 (Received 20 December 1996) The effect of collision cascades on preexisting point defects in crystalline materials was studied by simu- lating 5 keV collision cascades in gold,copper,aluminum,platinum,and silicon.The results indicate that collision cascades do not significantly affect interstitials or vacancies outside the liquid core of the cascade, although in the fcc metals the heating of the crystal due to the cascade causes some thermal migration of the interstitials.Within the liquid cascade core,both interstitials and vacancies move towards the center of the molten region when it resolidifies and recombine or cluster there.At elevated temperatures,random jumps of interstitials during the thermal-spike phase can cause significant additional trapping of interstitials in the liquid. In contrast to the annealing effects of preexisting damage in the fcc metals,in silicon the amount of new damage created by a cascade is roughly independent of the number of initial point defects.The difference is attributed to the nature of the bonding in the materials.[S0163-1829(97)05729-9] L.INTRODUCTION We create the point defects in well-defined distributions and follow their motion during the simulations.Although our Ion irradiation methods are of considerable interest in damage distributions are artificial,they allow us to system- many research and practical applications of materials atically probe the effect of cascades on defects at different processing.2 Since most damage produced in materials dur- locations.We focus on two crystalline materials of contrast- ing ion irradiation derives from a complex process occurring ing properties:gold,a dense material with metallic bonding in collision cascades,much research has been devoted to and close-packed fcc crystal structure,and silicon,a co- studying these events.Experimental work has shed some valently bonded material with the relatively loose-packed light on this problem:however,owing to the difficulty of diamond structure.We also report on simulations of cascades resolving defect structures inside materials,these studies in copper and aluminum to determine whether the differ- have had only limited success.Molecular-dynamics (MD) ences are due to the atom mass or crystal structure,and in computer simulation offers an alternative approach,34 which platinum to determine the effect of the melting point.We has proved successful in providing both a qualitative and believe this choice of materials sets the bounds for possible quantitative description of damage production in solids(see, behavior. e.g.,Refs.5-9). The paper is organized as follows.In Sec.II,we present In most of the simulation studies performed so far,the our simulation procedures in detail.In Sec.III,we first initial state of the lattice in which a collision cascade is ini- present and discuss the results of our simulations for the five tiated has been defect free.However,in practice most ion elements separately,and then compare their common fea- irradiation-induced cascades are produced in regions which tures and differences. have been previously damaged by implanted ions.Therefore, to understand the effect of prolonged implantation on ini- II.SIMULATION AND ANALYSIS METHODS tially crystalline samples,it is important to know how a cas- cade affects preexisting defects. A.General principles Relatively few studies to date have focused on predam- In this study we were primarily interested in elucidating aged sample structures.Sayed e have studied a few defect reactions in collision cascades.Therefore,we set up overlapping cascades in Si at energies of the order of a few our simulation conditions in a manner which can be expected hundred eV.Gao et al.12 very recently worked on multiple to provide an as clear view of the processes involved as overlapping 400 ev to 5 keV cascades in a-iron (a bcc possible.The electronic stopping power is neglected,since it metal),and Foreman et al.3 simulated a few I keV cascades is expected to contribute only little to the slowing down of 5 in Cu with preexisting features like vacancy and interstitial keV self-ions in the materials treated here.18 loops.Kapinos and Bacon have studied the effect of high The initial temperature of the simulation cell was 0 K in vacancy concentrations on vacancy loop formation in Cu,Ni, most runs.Since the interstitial in gold migrates very easily and Fe Both Gao and Foreman report that the preexist- even at low temperatures, 1920 using a 0K ambient cell tem- ing features may be partly annealed when overlapped by a perature is advantageous for clearly distinguishing what part new cascade,a fact well known from experiment.' of the defect motion is due to the collision cascade.Experi- There has been much speculation about the motion of ence in the field shows that the initial state of damage pro- point defects in the heat spike and pressure wave associated duction in collision cascades does generally not differ much with cascades.7 However,no systematic investigation of at temperatures roughly below 100 K,except possibly for these effects has been carried out by molecular dynamics, lengths of replacement collision sequences,21 which are few which is the focus of this work. in number and not of interest here. 0163-1829/97/56(5)/2421(11)/S10.00 56 2421 1997 The American Physical SocietyPoint defect movement and annealing in collision cascades K. Nordlund* and R. S. Averback Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ~Received 20 December 1996! The effect of collision cascades on preexisting point defects in crystalline materials was studied by simu￾lating 5 keV collision cascades in gold, copper, aluminum, platinum, and silicon. The results indicate that collision cascades do not significantly affect interstitials or vacancies outside the liquid core of the cascade, although in the fcc metals the heating of the crystal due to the cascade causes some thermal migration of the interstitials. Within the liquid cascade core, both interstitials and vacancies move towards the center of the molten region when it resolidifies and recombine or cluster there. At elevated temperatures, random jumps of interstitials during the thermal-spike phase can cause significant additional trapping of interstitials in the liquid. In contrast to the annealing effects of preexisting damage in the fcc metals, in silicon the amount of new damage created by a cascade is roughly independent of the number of initial point defects. The difference is attributed to the nature of the bonding in the materials. @S0163-1829~97!05729-9# I. INTRODUCTION Ion irradiation methods are of considerable interest in many research and practical applications of materials processing.1,2 Since most damage produced in materials dur￾ing ion irradiation derives from a complex process occurring in collision cascades, much research has been devoted to studying these events.3,4 Experimental work has shed some light on this problem; however, owing to the difficulty of resolving defect structures inside materials, these studies have had only limited success. Molecular-dynamics ~MD! computer simulation offers an alternative approach,3,4 which has proved successful in providing both a qualitative and quantitative description of damage production in solids ~see, e.g., Refs. 5–9!. In most of the simulation studies performed so far, the initial state of the lattice in which a collision cascade is ini￾tiated has been defect free. However, in practice most ion irradiation-induced cascades are produced in regions which have been previously damaged by implanted ions. Therefore, to understand the effect of prolonged implantation on ini￾tially crystalline samples, it is important to know how a cas￾cade affects preexisting defects. Relatively few studies to date have focused on predam￾aged sample structures. Sayed et al.10,11 have studied a few overlapping cascades in Si at energies of the order of a few hundred eV. Gao et al.12 very recently worked on multiple overlapping 400 eV to 5 keV cascades in a-iron ~a bcc metal!, and Foreman et al.13 simulated a few 1 keV cascades in Cu with preexisting features like vacancy and interstitial loops. Kapinos and Bacon have studied the effect of high vacancy concentrations on vacancy loop formation in Cu, Ni, and Fe.14,15 Both Gao and Foreman report that the preexist￾ing features may be partly annealed when overlapped by a new cascade, a fact well known from experiment.16 There has been much speculation about the motion of point defects in the heat spike and pressure wave associated with cascades.17 However, no systematic investigation of these effects has been carried out by molecular dynamics, which is the focus of this work. We create the point defects in well-defined distributions and follow their motion during the simulations. Although our damage distributions are artificial, they allow us to system￾atically probe the effect of cascades on defects at different locations. We focus on two crystalline materials of contrast￾ing properties: gold, a dense material with metallic bonding and close-packed fcc crystal structure, and silicon, a co￾valently bonded material with the relatively loose-packed diamond structure. We also report on simulations of cascades in copper and aluminum to determine whether the differ￾ences are due to the atom mass or crystal structure, and in platinum to determine the effect of the melting point. We believe this choice of materials sets the bounds for possible behavior. The paper is organized as follows. In Sec. II, we present our simulation procedures in detail. In Sec. III, we first present and discuss the results of our simulations for the five elements separately, and then compare their common fea￾tures and differences. II. SIMULATION AND ANALYSIS METHODS A. General principles In this study we were primarily interested in elucidating defect reactions in collision cascades. Therefore, we set up our simulation conditions in a manner which can be expected to provide an as clear view of the processes involved as possible. The electronic stopping power is neglected, since it is expected to contribute only little to the slowing down of 5 keV self-ions in the materials treated here.18 The initial temperature of the simulation cell was 0 K in most runs. Since the interstitial in gold migrates very easily even at low temperatures,19,20 usinga0K ambient cell tem￾perature is advantageous for clearly distinguishing what part of the defect motion is due to the collision cascade. Experi￾ence in the field shows that the initial state of damage pro￾duction in collision cascades does generally not differ much at temperatures roughly below 100 K, except possibly for lengths of replacement collision sequences,21 which are few in number and not of interest here. PHYSICAL REVIEW B VOLUME 56, NUMBER 5 1 AUGUST 1997-I 0163-1829/97/56~5!/2421~11!/$10.00 2421 © 1997 The American Physical Society 56