Pulsed Laser Atom Probe Tomography Research Articles

Selective Area Regrowth Produces Nonuniform Mg Doping Profiles in Nonplanar GaN p–n Junctions

Nonplanar GaN p–n junctions formed by selective area regrowth were analyzed using pulsed laser atom probe tomography. Dilute Al marker layers were used to map the evolution of the p-GaN growth interface, enabling extraction of time-varying growth rates for nonpolar, semipolar, and polar surfaces from the trench edge to the center, respectively. The Mg dopant concentration is facet-dependent and varies inversely with the growth rate for the semipolar facets that grow rapidly away from the trench sidewalls. The negligible growth on the vertical sidewall of the trench coincides with an order of magnitude higher Mg concentration and substantial clustering of likely inactive dopants. A high Mg concentration is also observed near the regrowth interface of polar and semipolar planes, which we attribute to etching damage. We conclude that device fabrication processes employing selective area regrowth on nonplanar interfaces should consider both the spatial and temporal dependencies of growth rate that lead to nonuniform doping and explore growth conditions that could reduce variations in growth rate when nonuniform doping would adversely affect device performance.

Extreme Ultraviolet Radiation Pulsed Atom Probe Tomography of III-Nitride Semiconductor Materials

Laser-pulsed atom probe tomography (LAPT) is a materials characterization technique that has been widely applied in the study and characterization of III-nitride semiconductors. To date, most of th...

Microstructural Characterization of Sulfurization Effects in Cu(In,Ga)Se2 Thin Film Solar Cells.

Surface sulfurization of Cu(In,Ga)Se2 (CIGSe) absorbers is a commonly applied technique to improve the conversion efficiency of the corresponding solar cells, via increasing the bandgap towards the heterojunction. However, the resulting device performance is understood to be highly dependent on the thermodynamic stability of the chalcogenide structure at the upper region of the absorber. The present investigation provides a high-resolution chemical analysis, using energy dispersive X-ray spectrometry and laser-pulsed atom probe tomography, to determine the sulfur incorporation and chemical re-distribution in the absorber material. The post-sulfurization treatment was performed by exposing the CIGSe surface to elemental sulfur vapor for 20 min at 500°C. Two distinct sulfur-rich phases were found at the surface of the absorber exhibiting a layered structure showing In-rich and Ga-rich zones, respectively. Furthermore, sulfur atoms were found to segregate at the absorber grain boundaries showing concentrations up to ~7 at% with traces of diffusion outwards into the grain interior.

Precipitates in microalloyed ultra-high strength weld metal studied by atom probe tomography

Gas metal arc welding with metal-cored filler wires is frequently used to weld high strength steel constructions for lightweight and transportation applications. In the current study, microalloying is considered as strengthening concept for reaching the required mechanical properties by precipitation hardening. For this purpose, the typical microalloying elements Ti, Nb, V, and Al were added to the filler metal in a comparatively high amount (up to 0.5 m.%). All-weld metal samples with a yield strength of 1000 MPa and more were produced by gas metal arc welding. Laser-pulsed atom probe tomography was used to evaluate the potential of these elements to form clusters or precipitates and strengthen the weld metal. While Al and Nb did not form clusters, a strong tendency for clustering was found for V- and Ti-alloyed samples. The cluster size evolution and changes in chemical composition depending on the microalloying contents are discussed. Furthermore, the challenges arising from local alloying element enrichments and local differences in thermal history in the all-weld metal are addressed regarding sample preparation and data evaluation.

Controlling residual hydrogen gas in mass spectra during pulsed laser atom probe tomography

Residual hydrogen (H2) gas in the analysis chamber of an atom probe instrument limits the ability to measure H concentration in metals and alloys. Measuring H concentration would permit quantification of important physical phenomena, such as hydrogen embrittlement, corrosion, hydrogen trapping, and grain boundary segregation. Increased insight into the behavior of residual H2 gas on the specimen tip surface in atom probe instruments could help reduce these limitations. The influence of user-selected experimental parameters on the field adsorption and desorption of residual H2 gas on nominally pure copper (Cu) was studied during ultraviolet pulsed laser atom probe tomography. The results indicate that the total residual hydrogen concentration, HTOT, in the mass spectra exhibits a generally decreasing trend with increasing laser pulse energy and increasing laser pulse frequency. Second-order interaction effects are also important. The pulse energy has the greatest influence on the quantity HTOT, which is consistently less than 0.1 at.% at a value of 80 pJ.

Evaluation of Analysis Conditions for Laser-Pulsed Atom Probe Tomography: Example of Cemented Tungsten Carbide.

Cemented tungsten carbide has been analyzed using laser-pulsed atom probe tomography (APT). The influence of experimental parameters, including laser pulse energy, pulse repetition rate, and specimen base temperature, on the acquired data were evaluated from different aspects, such as mass spectrum, chemical composition, noise-to-signal ratio, and multiple events. Within all the applied analysis conditions, only 1 MHz pulse repetition rate led to a strong detector saturation effect, resulting in a largely biased chemical composition. A comparative study of the laser energy settings showed that an ~12 times higher energy was required for the less focused green laser of the LEAPTM 3000X HR system to achieve a similar evaporation field as the finer spot ultraviolet laser of the LEAPTM 5000 XS system.

Experimental Evaluation of the Interrelationships Between Laser Energy, Temperature, Applied Bias, and Measured Composition in Laser Pulsed Atom Probe Tomography

Experimental Evaluation of the Interrelationships Between Laser Energy, Temperature, Applied Bias, and Measured Composition in Laser Pulsed Atom Probe Tomography

Understanding of the field evaporation of surface modified oxide materials through transmission electron microscopy and atom probe tomography

Understanding of triggering the field evaporation of surface ions on the non-conductive materials enables improvement in the mass resolution in laser-pulsed atom probe tomography. This study addresses the influence of surface modification through metallic-capped layers, such as Co, Ni, and Ag, with surrounding bulk MgO tips on the physical mechanisms responsible for field evaporation and on the mass resolving power compared to uncapped bulk MgO. In particular, the field evaporation on the surface regions of Ag-capped bulk MgO tips during analysis was extensively observed by transmission electron microscopy to confirm the overall evaporation sequences occurring at the tip surface. We found that the introduction of such capping layers, especially for Ag-capping, controls both symmetric tip geometry at the surface of the specimens and the mass resolving power of ion species consisting of MgO materials. This implies the improvements in the symmetries of local field distributions and the isotropy of thermal heating across the tip surface. It reveals that Ag-capping with high thermal diffusivity promotes the compositional uniformities between the laser illumination side and the opposite side for MgO samples as well as the reduced fraction of multiple events for oxygen ions between both sides. Moreover, a variation in the thickness of the Ag-capping layer is an additional factor governing a thermal-assisted mechanism of MgO evaporation. Based on our findings, homogeneous thermal heat transfer for MgO emission along the tip axis by Ag-capping layers may be significant in potential methods for improvement.

Novel approach for observing the asymmetrical evolution and the compositional nonuniformity of laser pulsed atom probe tomography of a single ZnO nanowire

The characterization of ZnO nanowires is crucial for developing nanostructured devices together with related compounds and alloys with an atomic-scale regime. This study describes the effects of laser energy on the atom probe tomography analysis of a single ZnO nanowire with a high aspect ratio, diameters of 80?100 nm and lengths of 10 µm. We observed both an asymmetrical evolution in the apex morphology and the compositional nonuniformities of Zn and O ions with respect to the laser energies ranging from 5 to 40 nJ. When the higher laser illumination exposed to the ZnO nanowires, non-uniform field strength becomes noticeable especially at the laser incident side of the samples. Moreover, we measured the charge state ratios of Zn+ and Zn2+ ions as a function of the applied laser energies. Our results proved important for accurate quantitative characterization and better interpretation for the laser-pulsed atom probe tomography of ZnO-based devices.

Nanoscale Measurement of Laser-Induced Temperature Rise and Field Evaporation Effects in CdTe and GaN

The measurement of laser-induced temperature change and its influence on the field evaporation behavior in nanoscale CdTe and GaN specimens was assessed through systematic studies using laser pulsed atom probe tomography. For CdTe, the laser was shown to induce a linear thermal response in the material. Using the determined relationships, a phase map of the field evaporation behavior was created. This shows that at high base temperatures, high laser energies, or low fields, significant Cd sublimation occurs, leading to apparently Te-rich measured compositions. In contrast, the highest fields result in simultaneous evaporation of multiple Te species, leading to apparently Cd-rich measured compositions. For GaN, increasing laser energy reduced the applied bias necessary for a given detection rate, whereas base temperature changes produced no significant effect on the evaporation behavior, indicative of a largely athermal evaporation mechanism. Similarly, the laser energy and bias affected the measured compo...

Near atomic scale quantification of a diffusive phase transformation in (Zn,Mg)O/Al2O3 using dynamic atom probe tomography

Abstract

First Three-Dimensional Atomic Resolution Investigation of Thermally Grown Oxide on a FeCrAl Alloy

Thermally grown Al2O3 scales formed on a FeCrAl alloy, Kanthal APMT, were successfully analysed using pulsed green laser atom probe tomography. The alloy was exposed to 5 vol.% O2 + 95 vol.% N2 at 900 °C for 1 h, and covered by a thin layer of Al2O3. Cr segregation to some of the Al2O3 grain boundaries was observed. The continuous Cr-rich band, which contains a large number of Cr- and/or Fe- enriched nano-sized particles, was studied in detail. Four types of nanoparticles were found and analysed. Oxidation mechanisms at the initial stages are discussed based on the observations.

The influence of experimental parameters and specimen geometry on the mass spectra of copper during pulsed-laser atom-probe tomography.

We have studied the influence of experimental factors and specimen geometry on the quality of the mass spectra in copper (Cu) during pulsed-laser atom-probe tomography. We have evaluated the effects of laser pulse energy, laser pulse frequency, specimen base temperature, specimen tip radius, and specimen tip shank half-angle on the effects of mass resolving power, (m/Δm), at full-width at half-maximum and at full-width at tenth-maximum, the tail size after the major mass-to-charge state (m/n) ratio peaks, and the mass spectra. Our results indicate that mass resolving power improves with decreasing pulse energy between 40 and 80 pJ and decreasing base temperature between 20 and 80 K. The mass resolving power also improves with increasing tip radius and shank half-angle. A pulse frequency of 250 kHz slightly improves the mass resolving power relative to 100 or 500 kHz. The tail size decreases with increasing pulse energy. The mass resolving power improves when the cooling time is reduced, which is influenced by the thermal diffusivity of Cu and the specimen base temperature.

Analysis method of matrix and second phase particles in Zircaloy-2 by atom probe tomography

Properties of second-phase particles (SPPs) influence the corrosion resistance and the anti-hydrogen pickup in Zircaloy-2 claddings. The chemical compositions of Zr(Fe,Cr)2 and Zr2(Fe,Ni) SPPs were investigated by means of pulsed laser atom probe tomography. In order to identify the atomic concentration with accuracy, two approaches were applied to the Zr-matrix and the SPPs: the first by using all the visible peaks of the mass spectrum and the second using the representative peaks with the natural abundance of the corresponding isotopes. It was found that the background and tail effects are minimized by the first approach in case of estimating a single phase. The change in the concentration between the Zr-matrix and the SPPs can be estimated more accurately by the second approach, although Sn concentration in the Zr2(Fe,Ni) SPPs is slightly overestimated.

Quantitative analysis of carbon in cementite using pulsed laser atom probe.

Carbon quantification and the standardisation in a pure cementite were conducted using pulsed-laser atom probe tomography (APT). The results were analysed to investigate a dependence on three distinct experimental parameters; the laser pulse energy, the cryogenic specimen temperature and the laser pulse frequency. All the measurements returned an apparent carbon content of 25.0±1.0at%. Carbon content measurements showed no clear dependence on the cryogenic temperature or the laser pulse frequency. However, the results did demonstrate a strong correlation with the laser pulse energy. For lower laser pulse energies, the analysis returned carbon contents higher than the stoichiometric ratio. It was suggested that this effect is due to pile up of (56)Fe(++) at the detector and as a consequence there is a systematic preferential loss of these ions throughout the course of the experiment. Conversely, in experiments utilising higher laser pulse energies, it was found that the carbon contents were smaller than the stoichiometric ratio. In these experiments an increasing fraction of the larger carbon molecular ions (e.g., C5 ions) were detected as part of a multiple detection events, which could affect the quantification measurements.

Atom probe tomography evaporation behavior of C-axis GaN nanowires: Crystallographic, stoichiometric, and detection efficiency aspects

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.

Characterization of a dielectric/GaN system using atom probe tomography

Characterization of Al2O3 gate dielectric on GaN using pulsed laser Atom Probe Tomography is reported. Atomic layer deposition was used to grow 5 nm of aluminum oxide on Ga-polar GaN. No oxidation of the surface of the semiconductor was observed and the interface was found to be non-abrupt. A significant amount of carbon impurities (1019/cm3) were detected in the dielectric film that matches well with the estimated bulk trap density from C–V measurements. Our experiments suggest possible correlation between trap-related electrical hysteresis and the observed impurity concentration in these films.

Analysis of atomic distribution in as-fabricated Zircaloy-2 claddings by atom probe tomography under high-energy pulsed laser

The properties of second-phase particles (SPPs) in Zircaloy-2 claddings are key factors influencing the corrosion resistance of the alloy. The chemical compositions of Zr (Fe,Cr)2 and Zr2(Fe,Ni) SPPs were investigated by means of pulsed laser atom probe tomography. In order to prevent specimen fracture and to analyse wide regions of the specimen, the pulsed laser energy was increased to 2.0 nJ. This gave a high yield of average of 3×107ions per specimen. The Zr (Fe,Cr)2 SPPs contained small amounts of Ni and Si atoms, while in Zr2(Fe,Ni) SPPs almost all the Si was concentrated and the ratio of Zr: (Fe+Ni+Si) was 2:1. Atomic concentrations of the Zr-matrix and the SPPs were identified by two approaches: the first by using all the visible peaks of the mass spectrum and the second using the representative peaks with the natural abundance of the corresponding atoms. It was found that the change in the concentration between the Zr-matrix and the SPPs can be estimated more accurately by the second method, although Sn concentration in the Zr2(Fe,Ni) SPPs is slightly overestimated.

Atom probe tomography of thermally grown oxide scale on FeCrAl

Thermally grown Al₂O₃ scales formed on a FeCrAl alloy were successfully analyzed using pulsed green laser atom probe tomography. Two types of atom probe tomography specimens, the "thin oxide" type: a thin Al₂O₃ layer (<100 nm) with underlying metal (1 μm), and the "thick oxide" type: only with Al₂O₃ (1 μm), were prepared and analyzed. It was found that the thin oxide type yields poorer mass resolution due to a combined effect of laser absorption and thermal conduction effects. Application of a relatively low laser energy yields a better mass resolution and increased multiple events, however, more accurate quantification results. Although no other oxide phase than Al₂O₃ is expected to form, some iron-oxygen and chromium-oxygen molecular ions were recorded at the Al₂O₃/metal interface due to the large change in evaporation field over this zone.

Atom Probe Tomography of Apatites and Bone-Type Mineralized Tissues

Nanocrystalline biological apatites constitute the mineral phase of vertebrate bone and teeth. Beyond their central importance to the mechanical function of our skeleton, their extraordinarily large surface acts as the most important ion exchanger for essential and toxic ions in our body. However, the nanoscale structural and chemical complexity of apatite-based mineralized tissues is a formidable challenge to quantitative imaging. For example, even energy-filtered electron microscopy is not suitable for detection of small quantities of low atomic number elements typical for biological materials. Herein we show that laser-pulsed atom probe tomography, a technique that combines subnanometer spatial resolution with unbiased chemical sensitivity, is uniquely suited to the task. Common apatite end members share a number of features, but can clearly be distinguished by their spectrometric fingerprint. This fingerprint and the formation of molecular ions during field evaporation can be explained based on the chemistry of the apatite channel ion. Using end members for reference, we are able to interpret the spectra of bone and dentin samples, and generate the first three-dimensional reconstruction of 1.2 × 10(7) atoms in a dentin sample. The fibrous nature of the collagenous organic matrix in dentin is clearly recognizable in the reconstruction. Surprisingly, some fibers show selectivity in binding for sodium ions over magnesium ions, implying that an additional, chemical level of hierarchy is necessary to describe dentin structure. Furthermore, segregation of inorganic ions or small organic molecules to homophase interfaces (grain boundaries) is not apparent. This has implications for the platelet model for apatite biominerals.