The field evaporation behavior of c-axis GaN nanowires was explored in two different laser-pulsed atom probe tomography (APT) instruments. Transmission electron microscopy imaging before and after atom probe tomography analysis was used to assist in reconstructing the data and assess the observed evaporation behavior. It was found that the ionic species exhibited preferential locations for evaporation related to the underlying crystal structure of the GaN and that the species which evaporated from these locations was dependent on the pulsed laser energy. Additionally, the overall stoichiometry measured by APT was significantly correlated with the energy of the laser pulses. At the lowest laser energies, the apparent composition was nitrogen-rich, while higher laser energies resulted in measurements of predominantly gallium compositions. The percent of ions detected (detection efficiency) for these specimens was found to be considerably below that shown for other materials, even for laser energies which produced the expected Ga:N ratio. The apparent stoichiometry variation and low detection efficiency appear to be a result of evaporation of Ga ions between laser pulses at the lowest laser energies and evaporation of neutral N2 species at higher laser energies. All of these behaviors are tied to the formation of nitrogen-nitrogen bonds on the tip surface, which occurred under all analysis conditions. Similar field evaporation behaviors are therefore expected for other materials where the anionic species readily form a strong diatomic bond.

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Atom probe tomography (APT) is a powerful materials characterization technique capable of ppm chemical resolution and near atomic scale spatial resolution. However, owing to a number of factors, the technique has not been widely applied to insulating materials and even less to complex oxides. In this study, we outline the methodology necessary to obtain high‐quality results on a technologically relevant complex oxide Pb(Zr,Ti)O3 (or PZT) using laser‐assisted APT on both bulk and thin film specimens. We show how, with optimized and well‐controlled conditions, APT complements conventional techniques such as STEM‐EDS. The correlative information can be used to obtain the nanoscale 3‐D chemical information and investigate the nanoscale distribution of cations. Using nearest‐neighbor cluster analysis routines, 5–10 nm segregation of B‐site cations was detected in bulk sintered PZT 53/47 from chemically prepared powders. No statistically significant segregation of B‐site cations was observed in thin film specimens. This work opens new avenues toward understanding the process‐structure properties in complex materials at length scales heretofore unachievable.

Laser‐assisted atom probe tomography (L‐APT) was performed on GaN nanowires (NWs) and axial GaN/InGaN nanowire heterostructures. All samples were grown by MBE on Si(111) substrates. The laser pulse energy (PE) at 355 nm used in L‐APT analysis of GaN NWs was restricted to the range of 2‐50 fJ in order to recover the correct GaN stoichiometry within experimental uncertainty. Higher PE at 355 nm generally returns an apparent (but unphysical) deficit in N composition for GaN. Axial GaN/InGaN NW heterostructures were also grown by MBE. In these experiments a GaN NW segment was first grown at 820 °C followed by GaN‐capped InGaN marker layers grown variously at 585 and 615 °C. The capped InGaN markers were separated by GaN spacer layers. L‐APT revealed that under certain growth conditions, In diffused throughout the quantum well region. These observations were corroborated by HRTEM. Moreover, L‐APT performed on the GaN/InGaN NWs with PE = 2 fJ revealed that regions with dispersed In showed an (unphysical) N deficit but regions where the InGaN marker layers remained localized showed the approximately correct stoichiometry. Interestingly, even when the In markers remain localized, background In is still found in the GaN spacers between the markers. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)