We report on the heterogeneous nucleation of catalyst-free InAs\nnanowires on Si(111) substrates by chemical beam epitaxy. We show that\nnanowire nucleation is enhanced by sputtering the silicon substrate with\nenergetic particles. We argue that particle bombardment introduces\nlattice defects on the silicon surface that serve as preferential\nnucleation sites. The formation of these nucleation sites can be\ncontrolled by the sputtering parameters, allowing the control of\nnanowire density in a wide range. Nanowire nucleation is accompanied by\nunwanted parasitic islands, but careful choice of annealing and growth\ntemperature allows us to strongly reduce the relative density of these\nislands and to realize samples with high nanowire yield.