Response of a Material: From Single Ions to Experimental Times and Fluences

Extract

The preceding article by Davies treated the behavior of single ions as they slow down in a solid, colliding with atoms of the solid and often transferring enough energy to initiate atomic displacement cascades. For a 200 keV As ion with a speed ~10⁸ cm/s and a range in Si of ~ 2 × 10⁻⁵ cm, the cascade is completed in less than 10⁻¹² s. The small volume excited in the cascade relaxes over 15 decades or more in time in a series of increasingly deliberate steps. The experimental evidence for most of these decades is indirect. No probes with sufficient spatial and time resolution exist to examine the spatial and temporal evolution of collision cascades created by individual incident ions. The first few decades in time have been followed in metals by molecular dynamics (MD) calculations and also approximated by continuum theory. The results of MD calculations for 5 keV cascades in Ni are shown in Figure 1. The behavior is in-terpreted as indicative of local “melting,” consistent with the concept of a “thermal 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. This constitutes a very 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.