Atomic-Scale Modeling of Low-Energy Ion-Solid Processes
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ATOMIC-SCALE MODELING OF LOW-ENERGY ION-SOLID PROCESSES
BRIAN W. DODSON Sandia National Laboratories, Albuquerque,
NM
87185-5800
ABSTRACT Various techniques which have been applied to modeling low-energy (10 keV) ion beam interactions begin to break down. In the high-energy regime, the energetic particles are modeled as "soft-spheres", having monotonically decreasing pairwise repulsive interactions. This is quite reasonable, as rapidly moving atoms do not have time to form chemical bonds (to do so, the residence time At of the atoms must be >> h/AE; at typical bonding energies (AE-' eV), At is about 1 femtosecond (fsec); in fact, At for a 100 keV particle is about 0.1 fsec). In addition, the bonds between substrate atoms do not contribute significant inelastic losses, again because of the disparity in energy scales. At the same time, the pairwise interaction approximation is also appropriate because the dominant interactions take place at small interatomic radii. As a result, the "soft-sphere" approximation is quite reasonable in the high-energy regime. When does this "soft-sphere" pairwise interaction approximation break down? Discussion of this multifaceted question will take up a significant part of this review. However, various approaches to this question suggest that the high-energy interaction approximation breaks down noticeably at beam energies of a few hundred eV. In addition, the 'breakeven' energy for substrate sputtering (where the beam flux equals the sputtering rate) is on the order of 1000 eV for many systems. For this review, then, the low-energy regime encompasses beam energies of roughly 10-1000 eV. This criterion also agrees with the requirement, in ion-beam deposition, that sputtering processes are limited enough that a positive net flux to the surface is retained. ATOMIC-SCALE SIMULATION OF LOW-ENERGY ION-SOLID PROCESSES The overall process of interaction of an energetic ion with a solid can be broken down into three major regimes, each occuring on a different timescale. The first is the collisional regime, in which the incoming ion transfers its kinetic energy to the solid. This can take place either through direct nuclear collisions with the atoms of the solid, or by inducing transitions in the electrons of the solid. The nonequilibrium structure of displaced and/or highly energetic atoms formed within the solid in this period is usually called the collision cascade. The underlying crystal structure of
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the solid is often completely disrupted in the cascade, with an accumulation of vacancies in the central region of the cascade and a corresponding number of interstitials generated around the periphery by the action of 'dynamic crowdions'.8 The characteristic timescale for development of this structure is a few tenths of a picosecond in the low-energy regime. The second major process is that of thermalization of the cascade region. Energetic atoms will lose their excess kinetic energy through generation of phonons in the surrounding material. The thermalization regime ends when the c
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