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.

The development of three-dimensional (3-D), characterisation techniques with high spatial and mass resolution is crucial for understanding and developing advanced materials for many engineering applications as well as for understanding natural materials. In recent decades, atom probe tomography (APT), which combines a point projection microscope and time-offlight mass spectrometer, has evolved to be an excellent characterisation technique capable of providing 3-D nanoscale characterisation of materials with sub-nanometer scale spatial resolution, with equal sensitivity for all elements. This review discusses the current state, as of APT instrumentation, new developments in sample preparation methods, experimental procedures for different material classes, reconstruction of APT results, the current status of correlative microscopy, and application of APT for microstructural characterisation in established scientific areas like structural materials as well as new applications in semiconducting nanowires, semiconductor devices, battery materials, catalyst materials, geological materials, and biological materials. Finally, a brief perspective is given regarding the future of APT.

IntroductionAtom probe tomography has primarily been used for atomic scale characterization of high electrical conductivity materials [1]. A high electrical field applied to needle-shaped specimens evaporates surface atoms, and a time of flight measurement determines each atom's identity. A 2-dimensional detector determines each atom's original position on the specimen. When repeated successively over many surface monolayers, the original specimen can be reconstructed into a 3-dimensional representation. In order to have an accurate 3-D reconstruction of the original, the field required for atomic evaporation must be known a-priori. For many metallic materials, this evaporation field is well characterized, and 3-D reconstructions can be achieved with reasonable accuracy.Compared with conventional atom probes, the use of a local electrode has been shown to increase the sustainable evaporation rate and field of view [2]. The localized electric field produced by the local electrode enables arrays of specimens to be analyzed, as opposed to a single, electropolished wire needle. Specimen arrays increase throughput by minimizing exchange to UHV and cryogenic temperatures, as well as increasing material statistics through analysis of many specimens. In order to take advantage of these specimen arrays, preparation techniques utilizing in-situ FIB liftout techniques were developed [3]. These techniques allow routine preparation of nominally 100nm diameter specimens. The FIB also enables much improved control of the specimen diameter so the atom probe experiments can be tuned accordingly.The maturation of local electrode and laser pulsed atom probe hardware, as well as FIB specimen preparation techniques, have enabled atom probe analysis of non-traditional materials such as semiconductors, ceramics, and some organic materials to become more commonplace [4]. For most of these materials, the evaporation field is not well characterized. For example, oxides and III-V materials tend to evaporate in clusters of atoms, rather than individual atoms [5]. The physics of cluster evaporation in atom probe experiments are not well understood, and the evaporation field required is also not well characterized. In order to increase the accuracy of the 3-D reconstructions in non-traditional materials, the evaporation field and its progression during an atom probe experiment should be calculated using the specimen geometric features, such as tip radius and shank angle.While a combination FIB and SEM can give some information about atom probe specimen structure, higher resolution characterization of specimens using TEM and STEM can further increase reconstruction accuracy. TEM can image not only the specimen radius and shank angle with higher precision, but also can give the internal structure of interfaces and precipitates. Diffraction and high resolution imaging can give information about the orientation of crystallographic axes with respect to the specimen, and thus allow accurate scaling of the reconstruction in the z-direction. Analyti...

Nitrides join the perovskite club Perovskite structured materials have a variety of uses as photovoltaics, capacitors, and micromechanical actuators, along with other applications. Oxides, halides, and chalcogenides all have large numbers of perovskite structured materials. Examples of perovskite nitrides are conspicuously absent, but Talley et al . managed to synthesize one (see the Perspective by Hong). Lanthanum tungsten nitride in the perovskite structure turns out to be piezoelectric, which is ideal for a variety of applications. Perovskite structured nitrides are very attractive because they could easily integrate with the large number of nitride-based semiconducting devices already in use. —BG

Back contacts can significantly limit CdTe solar cell performance, reducing both open circuit voltage (V oc) and fill factor (FF). Copper is an essential component of effective back contacts, but its presence in the CdTe absorber creates detrimental recombination centers. Rapid thermal processing (RTP) is demonstrated as a highly effective approach for reducing back contact barriers in CdTe solar cells contacted with ZnTe:Cu buffer layers, substantially improving both FF (>73%) and V oc (>850 mV). Current density and quantum efficiency remain essentially unchanged, but a five-fold increase in minority carrier lifetime is observed which is attributed to passivation of recombination sites in the back contact region. Quantitative analysis of secondary ion mass spectrometry shows that the majority of Cu segregates to the Au metallization layer and that the ZnTe buffer appears to inhibit the Cu diffusion into CdTe. 3D imaging of the back contact region using atom probe tomography shows that optimized devices are characterized by preferential segregation of copper to both the Au|ZnTe and CdTe|ZnTe interfaces, perhaps in the form of Cu x Te. With its low thermal budget the RTP process has been successfully applied to multiple device architectures. including devices with certified efficiencies in excess of 16%.