Response of a Material: From Single Ions to Experimental Times and Fluences
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lowed in metals by molecular dynamics (MD)1 calculations and also approximated 2 by continuum theory. The results of MD calculations for 5 keV cascades in Ni are 1 shown in Figure I. The behavior is interpreted as indicative of local "melting," consistent with the concept of a "thermal 3 spike" in the material. Extension of MD calculations to times of nanoseconds and longer is still prohibitively expensive in computer time, and application to covalently bonded systems, where the interatomic forces are much more complex than in metals, has not yet been reported. As Davies has noted, the atoms of the cascade rapidly lose their high vibrational excitation to atoms in the surrounding material, and in only a few picoseconds the system cools.3,4 This constitutes a very
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rapid thermal quench—slower only than the quench associated with deposition of individual atoms (or small atomic clusters) on a cold substrate. The quench may produce a small volume of material with a different phase than its surrounding. For example, a small volume of "hot" Si atoms may be highly disordered and may quench to produce a small amorphous Si region in a crystalline Si matrix. Even if the quench does not produce a new phase, it results in "freezing in" a nonequilibrium atomic distribution containing many defects. In the cascade process, atoms are ejected from the center of the cascade, leaving the center rich in vacancies and a surrounding shell (not generally spherical) rich in interstitials. Annealing this defective region requires substantial movement and reorganization of a large number of atoms. This process is strongly temperature dependent, characterized by the dissociation and migrational energies of the defects. During the return to equilibrium, typically relaxation to a crystalline state, sometimes the interstitials and vacancies simply migrate, find each other, and annihilate. But in general, many of the defects interact to form a wide variety of more complex defects such as di-, tri-, tetra- and higher order vacancy clusters and even extended defects such as voids and dislocation loops.5 These defects have been identified in many different ways, ranging from optical6 and magnetic resonance spectroscopy7 to electron micros8 copy. With TEM it is sometimes possible to observe material changes associated 8 with the path of a single ion (and see the following section on Electric Fields and Charged Defects). Of course, these observations are made long after the ion has come to rest. Most other material characterization techniques that are sensitive to the defects created in cascades involve the
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Figure 1. Projection of the atomic configuration in an atomic layer near the center of a collision cascade created by a 5 keV Ni atom in Ni. The panels of the figure are for increasing times, to 3.8 ps. After Diaz de Iα Rubia et al.
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MRS BULLETIN/JUNE 1992
Response of a Material: From Single Ions to Experimental Times and Fluences
overall effect of a substantial fluence of ions (typica
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