The realization of single-mode rib waveguides in standard epitaxial silicon layer on lightly-doped silicon substrate, using ion-implantation to form the lower cladding, is reported. We exploited a standard microelectronic process step, followed by a calibrated thermal treatment in order to activate and drive-in the implanted impurities, so obtaining a spatially confined lower cladding. The implanted buffer layer enhances the vertical confinement and improves the propagation characteristics. The waveguides were designed with a cross-section comparable in size to the mode-field- diameter of standard single-mode optical fiber, so reducing the fiber-waveguide coupling losses. Propagation losses of about 1.2 dB/cm, for (lambda) equals 1.3 micrometers , in the single mode regime, have been measured. This attenuation is about one order of magnitude lower respect to similar standard all-silicon waveguides. This is the best value of attenuation, to our knowledge, for all-silicon single-mode small-cross-section waveguides reported in literature. A numerical analysis has been performed to evaluate the theoretical attenuation and the transverse optical field profiles, both for (lambda) equals 1.3 micrometers and (lambda) equals 1.55 micrometers . As a result of the presence of the ion implanted buffer layer, a strong reduction of propagation losses and an increase of the fundamental mode confinement have been shown. This results in a great enhancement of the coupling efficiency with standard single-mode optical fibers. Moreover, the proposed technique is low-cost, fully compatible with standard VLSI processes, and allows a great flexibility in the integration of guided-wave devices and electronic circuits. Finally, the very high thermal conductivity characterizing these waveguides makes them attractive host-structures for electrically and thermally- controlled active optical devices.