Elastic recoil coincidence spectroscopy (ERCS)

Abstract

Elastic recoil coincidence spectroscopy (ERCS) is a scattering-recoil coincidence technique with a high sensitivity for profiling light elements ranging from H up to O in thin film samples. In this technique He + ions with energies of at least 2 MeV are incident on a thin target, typically 2 μm thick, and the energies of both forward scattered He ions and elastically recoiled light atoms are detected in coincidence using detectors subtending large solid angles. From each pair of energies the depth where scattering has taken place as well as the mass of recoiled atoms can be derived. ERCS is preferably applied to profile light elements in a heavier matrix and in cases where a low beam current (< 1 nA) or a low total ion dose is required to reduce damage, e.g. to study polymer samples, or for microbeam analysis. A gain in counting rates of 2–3 orders of magnitude compared to conventional elastic recoil detection analysis (ERDA) and a depth resolution of 20 nm can be achieved. In a first experiment, we applied ERCS to profile simultaneously C and O in thin polycarbonate foils and in a second experiment we have analyzed O and B in thin B-diffused Si crystals. In this paper we discuss the basic principles of ERCS and describe in detail the derivation of concentration-versus-depth profiles from coincidence spectra measured for polycarbonate and Si samples.