  The mechanism responsible for the emission of clusters from heavy ion
irradiated solids is proposed to be thermal spikes. Collision cascade-based
theories describe atomic sputtering but cannot explain the consistently
observed experimental evidence for significant cluster emission. Statistical
thermodynamic arguments for thermal spikes are employed here for qualitative
and quantitative estimation of the thermal spike-induced cluster emission from
silicon, germanium and zinc oxide. The evolving cascades and spikes in
elemental and molecular semiconducting solids are shown to have fractal
characteristics. Power law potential is used to calculate the fractal
dimension.The fractal dimension is shown to be dependent upon the exponent of
the power law interatomic potential. Each irradiating ion has the probability
of initiating a space-filling, multi-fractal thermal spike that may sublime a
localized region near the surface by emitting clusters in relative ratios that
depend upon the energies of formation of respective surface vacancies.
