Surface Damage During keV Ion Irradiation: Results of Computer Simulations
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R.S. AVERBACK, MAI GHALY and HUILONG ZHU Department of Materials Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, IL.
ABSTRACT MD simulations have been employed to investigate damage processes near surfaces during keV bombardment of metal targets. For self-ion implantation of Au, Cu, and Pt in the range of 5-20 keV, we have found that the proximity of the surface leads to significantly more damage and atomic mixing in comparison to recoil events occurring in the crystal interior. In some cases, large craters are formed in a micro-explosive event, while in others a convective flow of atoms to the surface creates adatoms and leaves dislocations behind. Both the amount damage created in the surface and its morphology depend sensitively on the details of the energy deposition along individual ion trajectories. The results of these simulations will be summarized and compared to recent scanning tunneling microscopy studies of individual ion impacts in Pt and Ge.
INTRODUCTION Most past descriptions of radiation effects in solids have been built on the assumption of two-body atomic collisions, and for the most part, they neglect the detailed physical and chemical properties of the solid. Part of the reason for this development stems from the pioneering work on defect production by Vineyard and co-workers at Brookhaven National Laboratory using molecular dynamics (MD) computer simulations [1]. They showed that the threshold energy for defect production in metals was greater than -- 25 eV so that the thermodynamics of the solid could presumably be safely ignored. Models based on the assumptions of binary collisions (BC) and linear cascades, such as MARLOWE [2], TRIM [3], and Boltzmann transport theory [4] are justified on this premise. The early Brookhaven work, however, was restricted to very low energy recoil events where thermal spike effects are negligible. Many experiments and computer simulations have since established that the BC model breaks down in predicting defect production in high energy cascades, and that the collective atomic motion in the thermal spike, and in many cases local melting, strongly influence the primary state of damage, including defect production [5], atomic mixing [5], and sputtering [4]. Fig. 1 illustrates this point; it shows the evolution of a cascade in the ordered intermetallic compound P-NiAl. Clearly seen is a region of high structural and chemical disorder that has been found to have many similarities with a liquid [6]. The boundary between the structurally ordered and disordered zones, in fact, is sharp, suggesting the coexistence of two distinct phases. The observation of local melting is now a common finding in MD simulations of energetic cascade events. The kinetic energy distribution of those atoms in the disordered zone provides a convenient means to characterize different time regimes in the cascade dynamics, as shown in Fig. 2, again for P3-NiAl. At times earlier than -: 0.5 ps, the number of atoms in the melt is small, the effective temperature, T,
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