Field dependent study on the impact of co-evaporated multihits and ion pile-up for the apparent stoichiometric quantification of GaN and AlN.

For atom probe tomography, multihits and any associated ion pile-up are viewed as an "Achilles" heel when trying to establish accurate stoichiometric quantification. A significant reason for multihits and ion pile-up is credited to co-evaporation events. The impact is the underestimation of one or more elements present due to detector inadequacies when the field evaporated ions are spatially and temporally close. Nitride materials, especially GaN and AlN, have been shown to suffer a strong field dependent compositional bias, with N having the characteristics for being a species prone to ion pile-up. In this paper we have explored through field dependent measurements on GaN and AlN the associated impact of co-evaporated multihits and ion pile-up. To achieve this a normal CAMECA electrode along with a specially modified GRID electrode, which was designed to manipulate co-evaporated ions and hence ion pile-up, were employed. From our results and in-depth analysis, any co-evaporation and associated ion pile-up is found to be either very small, or not species dependent. Thus, ion pile-up cannot be attributed as the cause for the significant N underestimation observed in these materials.