Role of Photoexcitation and Field Ionization in the Measurement of Accurate Oxide Stoichiometry by Laser-Assisted Atom Probe Tomography

The addition of pulsed lasers to atom probe tomography (APT) extends its high spatial and mass resolution capability to nonconducting materials, such as oxides. For a prototypical metal oxide, MgO, the measured stoichiometry depends strongly on the laser pulse energy and applied voltage. Very low laser energies (0.02 pJ) and high electric fields yield optimal stoichiometric accuracy. Correlated APT and aberration-corrected transmission electron microscopy (TEM) are used to establish the high density of corner and terrace sites on MgO sample surfaces before and after APT. For MgO, long-lifetime photoexcited holes localized at oxygen corner sites can assist in the creation of oxygen neutrals that may spontaneously desorb either as atomic O or as molecular O-2. The observed trends are best explained by the relative field-dependent ionization of photodesorbed O or O-2 neutrals. These results emphasize the importance of considering electronic excitations in APT analysis of oxide materials.