Simulation Studies of Collision Cascades in Liquid in Targets
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SIMULATION STUDIES OF COLLISION CASCADES INLIQUID In TARGETSt 3 3 1 D.Y. LOI, M.H. SHAPIRO 2, T.A. TOMBRELLO , B.J. GARRISON AND N.WINOGRAD 'Division of Physics, Mathematics and Astronomy, Caltech, Pasadena, CA 91125, U.S.A. 2 Physics Dept., California State University, Fullerton, CA 92634, U.S.A. and Division of Physics, Mathematics and Astronomy, Caltech, Pasadena, CA 91125, U.S.A. 3 Chemistry Dept., Pennsylvania State University, University Park, PA 16802, U.S.A.
ABSTRACT Multiple interaction computer simulations have been used to determine the properties of collision cascades in liquid In targets induced by normally incident 5 keV Ar÷ ions. Below the first atomic layer the cascade becomes Thompson-like relatively quickly. However, within the first atomic layer the angular distribution of moving atoms became forward peaked by 150 fs and remained so until ,,-300 fs. Energy and angle resolved (EARN) spectra were calculated for the ejected atoms. The peak of the energy distribution shifted to lower energies at larger ejection angles, and the angular distributions became broader for lower energy particles. Both results agree with recent experimental data, and with a simple model proposed bg Garrison. Our results suggest that the detailed structure of the surface layer is very important in the sputtering process. INTRODUCTION In Thompson's model of sputtering [I] the distribution of ejected atoms is given by 2N(E,e)=
dEdQ
AEcose
.
(E +U)n+l
(1)
where n depends on the atomic cross section and the nature of the collision cascade inside the target, U is the energy cost [2] to remove an atom from the surface, and A is a normalization constant. Deviations from the pure cose dependence have been observed in previous experiments and simulations [3,41.
Mat. Res. Soc. Symp. Proc. Vol. 74. 11987 Materials Research Society
450
Energy distributions are well described by (1) if U and n are considered free parameters, although U is close to the cohesive energy and n is near 2. The energy and polar angle dependencies are decoupled completely in (1). However, recent energy and angle resolved neutral atom (EARN) spectra of ejected atoms show a shift towards lower energy for the peak in the energy distribution as the polar angle becomes more grazing, and a broadening of the polar angle distribution with decreasing energy [5]. Garrison [61 has proposed a modified version of (1) to fit the EARN data, namely: d2N(E,e) dEdO
=
{U + Ecos2e)m/ 2 .
AE cose
(2)
(E + U)m/2+n+l
This formula with its additional free parameter m adequately predicts both the peak position shift in the energy distribution and polar angular distribution broadening. The parameters m and n depend on the nature of the collision cascade inside the target. (1) results from Thomson's assumption of an isotropic velocity distribution inside the target, while Garrison made the ad /hocassumption of a cosmO velocity distribution within the target. Our simulation studies are aimed at determining the nature of the velocity distribution within a liquid In target.
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