3 pages, 3 figures.-- PACS nrs.: 81.16.Rf, 79.20.Rf, 68.66.Hb, 68.35.Ct. ArXiv pre-print available at: http://arxiv.org/abs/cond-mat/0106542 Final publisher version available Open Access at: http://gisc.uc3m.es/~cuerno/publ_list.html We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si (100) substrate with 1.2 keV Ar+ ions at normal incidence. Atomic force and high-resolution transmission electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Under prolonged irradiation (up to 16 h) all dot characteristics remain largely unchanged and a small corrugation develops at long wavelengths. We interpret the formation of the dots as a result of an instability due to the sputtering yield dependence on the local surface curvature. This work has been partially supported by DGES (Spain) Grant Nos. MAT1999-0830-C03-01, MAT2000-0375-C02-02, and BFM2000-0006. Publicado