Damage creation in silicon single crystals irradiated with 200 keV/atom [formula omitted] clusters

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Silicon wafers of (100) orientation were irradiated with Aun cluster beams (1⩽n⩽7) produced by the 2.5 MV Van de Graaff accelerator of the Institut de Physique Nucléaire de Lyon equipped with a liquid metal source. The incident energy was of 200 keV per gold atom, which corresponds to a slowing-down mainly governed by elastic processes (nuclear energy loss of Au+ ions: 3 keV nm−1). All the irradiations were performed at room temperature with fluences up to 5×1014 Au (at. cm−2). The typical beam currents varied from 1.5 nA for Au+ down to 20 pA for Au7+. The radiation-induced disorder was measured by means of Rutherford backscattering spectrometry in channeling geometry (RBS-C), using a 4He+ beam accelerated at 2 MV. From the fluence evolution of the lattice disorder at the target surface, we evidence that polyatomic projectiles produce more defects per incident atom than single Au+ ions. As an example we measured damage cross-sections per incident Au atom of 12.5 and 2.7 nm2 for Au7+ and Au+ projectiles, respectively. This cluster effect was ascribed to the high density of nuclear energy deposited within the cascade. Transmission electron microscopy (TEM) was performed on samples irradiated at low fluences (109 at. cm−2) in order to visualize each projectile impact.