A VERY NARROW RESONANCE IN O-18(P,ALPHA) N-15 NEAR 150 KEV - APPLICATION TO ISOTOPIC TRACING .2. HIGH-RESOLUTION DEPTH PROFILING OF O-18

Various depth profiling experiments illustrating the high depth resolutions obtained are presented for Ta2O5 and SiO2 layers with sharp O-18-O-16 interfaces in depth ranges up to some hundreds of angstrom. A further increase of depth resolution is reached by resorting to glancing geometry with respect to the beam. The depth sensitivity of this resonance is actually so high that it is the energy straggling induced by the ubiquitous hydrocarbon contamination layer on the sample surface that limits the near-surface resolution. The experimental curves are interpreted with the stochastic theory of energy loss. The results are compared to those obtained with the previously used resonances for O-18 depth profiling. Applications of these techniques to the isotopic tracer study of the microscopic growth mechanisms of SiO2 on silicon are shown. Operated in UHV this method may yield potentially nanometric or even subnanometric near surface depth sensitivities, as illustrated by detailed theoretical calculations corresponding to ideal but realistic experimental conditions. These results illustrate the usefulness for very high resolution depth profiling of low energy narrow resonances in general due to the corresponding large dE/dx values.