The direct deposition in thin films and the production of very shallow junctions by ultralow energy ion implantation involves the interaction of ions with only the outermost surface layers of a solid. Quantitative structural and composition analysis of the grown or implanted layers requires the use of techniques with extremely high depth resolution. The growth of Si epitaxially on Si (100) substrates prepared by a variety of ion beam and thermal treatments has illustrated the complex radiation effects that occur in the bombardment energy range from 20 to 500 eV. These effects have been studied using medium energy ion scattering in the double alignment mode. With a 50 keV H+ beam, a high resolution electrostatic analyser and incidence and emergence directions aligned with the [111̄] and [3̄3̄1] directions, a depth resolution of 3 Å can be obtained. The effects of ion energy on the structure of grown films and on the damage in the substrate during pretreatment with Cl+ and Ar+ ions will be described.