Comparing sub-gap and above-gap pulsed radiation sources for measuring oxygen concentration in MgO using atom probe...

This paper explores the experimental conditions required to achieve an apparent stoichiometric composition measurement in MgO using near ultraviolet (NUV; 355 nm) and extreme ultraviolet (EUV; ≈29 nm) pulsed laser sources in atom probe tomography. MgO shows strong composition bias under both NUV and EUV laser pulsing. The expected 1:1 bulk composition can be achieved in both EUV and NUV pulsing at high average nanoscale field (low laser pulse energy), as indicated by either the Mg⁺⁺ charge state fraction or the O⁺ ionic fraction in a constant evaporation rate experiment. We report oxygen-rich composition measurements in MgO for the first time. Regardless of the laser wavelength, pulse frequency, or detection rate, the data falls on a single calibration curve of oxygen concentration as a function of the Mg⁺⁺ charge state fraction except at the highest near ultraviolet laser pulse energy tested. These results indicate that for a material with a potential sub-gap absorption mechanism such as MgO, pulsing with higher photon energy does not confer additional benefit in terms of composition measurement. The average nanoscale field is the primary experimental parameter that can be used to achieve a stoichiometric composition measurement, except for when NUV laser pulsing at high laser pulse energy.