High resolution low energy resonance depth profiling of18O in near surface isotopic tracing studies

Abstract

The exceptional depth profiling potential of the ≈ 100 eV narrow resonance in the 18O(p, α)15N nuclear reaction at 151 keV is hindered by the low counting rates due to its low cross section. This drawback is in fact associated with Van de Graaff type accelerators, that provide small proton beam currents at these low energies. In this paper we demonstrate experimentally that this disadvantage may be overcome with ion implantation type accelerators, which may deliver high proton currents. The very high depth resolution of this method in the first hundreds Ångströms of solids was put to benefit efficiently and without hindrance in 18O isotopic tracing experiments applied to the systematic study of oxygen transport processes in various oxides. The method is illustrated by the study of various oxidation processes of silicon and of recoil implantation of oxygen into the silicon substrate from very thin thermal oxide layers under As and Sb ion bombardment. Deep 18O profiles could be measured in perpendicular geometry with energy straggling limited resolution. Shallow 18O profiles in SiO2 films with thicknesses below 100 Å could be deduced from excitation curves recorded in grazing incidence geometry.