Oxygen enhanced secondary ion emission of Fe and Co by TOF-SIMS and ISS/DR

Abstract

Using secondary ion mass spectrometry (SIMS), ion scattering spectroscopy (ISS), direct recoil (DR) and X-ray photoelectron spectroscopy (XPS), we have studied the mechanism of oxygen enhancement of secondary ion emission. The secondary ion intensities were measured as a function of oxygen exposure for Fe and Co samples under static- and dynamic-SIMS conditions. In the static-SIMS, the intensity of Fe+ increased up to 60 L oxygen exposure and then slowly saturated. However, the intensity of Co+ had an additional peak at low oxygen coverage when compared with the result of Fe+. It sharply increased up to 5 L, dropping off around 20 L and increased again to saturation. Also the intensities of Fe+ and Co+ under dynamic-SIMS conditions showed similar changes to those under static-SIMS conditions. According to XPS spectra, the polycrystalline Fe surface was oxidized immediately on oxygen exposure. However the oxygen on the Co surface was changed from the chemisorbed state at low oxygen doses (0–5 L) to the oxide state at oxygen doses above 5 L. The relative oxygen coverages on Fe and Co samples estimated by DR and ISS intensities showed trends similar to the secondary ion intensities of Fe+ and Co+ as a function of oxygen dose. We applied the bond breaking model and the electron tunneling model for interpretation of the oxygen enhancement effect in the positive secondary ion emission for Fe and Co.