Patrick Stender

Design of a laser-assisted tomographic atom probe at Münster University

To benefit from the latest technical improvements in atom probe analysis, a new tomographic atom probe has been built at the University of Münster, Germany. The instrument utilizes a femtosecond laser system with a high repetition rate combined with the ability of using a micrometer-sized extraction electrode and a wide angle configuration. Since field evaporation is triggered by laser pulses instead of high-voltage pulses, the instrument offers the ability to expand the range of analyzed materials to poorly conducting or insulating materials such as oxides, glasses, ceramics, and polymeric materials. The article describes the design of the instrument and presents characterizing measurements on metals, semiconductors, and oxide ceramic.

Laser-assisted atom probe tomography of oxide materials.

Atom probe tomography provides a chemical analysis of nanostructured materials with outstanding resolution. However, due to the process of field evaporation triggered by nanosecond high voltage pulses, the method is usually limited to conductive materials. As part of recent efforts to overcome this limitation, it is demonstrated that the analysis of thick NiO and WO3 oxide layers is possible by laser pulses of 500 ps duration. A careful analysis of the mass spectra demonstrates that the expected stoichiometries are well reproduced by the measurement. The reconstruction of lattice planes proves that surface diffusion is negligible also in the case of thermal pulses.

Triple junction transport and the impact of grain boundary width in nanocrystalline Cu.

Triple junctions (TJ), singular topological defects of the grain boundary (GB) structure, get a dominant role for grain growth and atomic transport in nanocrystalline matter. Here, we present detailed measurements by atom probe tomography, even of the temperature dependence of TJ transport of Ni in nanocrystalline Cu in the chemical regime of interdiffusion. An unexpected variation of the effective width of merging GBs with temperature is detected. It is demonstrated that proper measurement of TJ transport requires taking into account this remarkable effect. TJ diffusion is found to be a factor of about 200 faster than GB diffusion. Its activation energy amounts to only two-thirds of that of the GB.

Quantitative comparison of energy-filtering transmission electron microscopy and atom probe tomography

Whereas transmission electron microscopy (TEM) is a well established method for the analysis of thin film structures down to the sub-nanometer scale, atom probe tomography (APT) is less known in the microscopy community. In the present work, local chemical analysis of sputtered Fe/Cr multilayer structures was performed with energy-filtering transmission electron microscopy (EFTEM) and APT. The single-layer thickness was varied from 1 to 6nm in order to quantify spatial resolution and chemical sensitivity. While both the methods are able to resolve the layer structure, even at 2nm thickness, it is demonstrated that the spatial resolution of the APT is about a factor of two, higher in comparison with the unprocessed EFTEM data. By calculating the influence of the instrumental parameters on EFTEM images of model structures, remaining interface roughness is indicated to be the most important factor that limits the practical resolution of analytical TEM.

Interface width of immiscible layered elements

Abstract Based on the thermodynamics of inhomogeneous systems, it is expected that the chemical transition at an interface between two immiscible components cannot be atomically sharp. However, measurements by direct local chemical analysis are rare, as utmost spatial resolution on an atomic scale is required. In this article, the temperature dependence of the chemical width of layer interfaces in binary Cu/Ag and ternary Cu/Ni81Fe19 specimens is studied experimentally. Atom probe techniques have been used to achieve sufficient spatial resolution. In both materials the interface width could be measured on a length scale of one to two nanometres. Its temperature dependence is found to be in agreement with the Cahn – Hilliard theory. From the measurements the gradient energy coefficient and the specific interfacial Gibbs energy are quantitatively determined.

Research Update: Inhomogeneous aluminium dopant distribution in magnetron sputtered ZnO:Al thin films and its influence on their electrical properties

The spatial distribution of Al in magnetron sputtered ZnO:Al films has been investigated in depth. Two different kinds of inhomogeneities were observed: an enrichment in the bulk of the film and an enrichment at the interface to the substrate. This has been correlated to the electrical properties of the films: the former inhomogeneities can lead to trap states at the grain boundaries limiting the free carrier mobility. The latter can promote the formation of secondary phases, which leads to an electrical inactivation of the dopant. Furthermore, this effect can contribute to the thickness dependence of the electrical properties of ZnO:Al films.

Triple line diffusion in nanocrystalline Fe/Cr and its impact on thermal stability.

The thermal reaction of iron-chromium multilayers is analyzed by atom-probe tomography. Samples were prepared using ion-beam sputter deposition and cutting by focused ion beams. Isothermal and isochronal annealing sequences were carried out in a vacuum furnace. Effects of atomic transport are observed at temperatures above 773 K. Segregation along line-shaped zones is noticed to very high concentrations. These zones, with a diameter of 1.5 nm, are identified as triple lines of the grain structure. While these defects could not be resolved by TEM, the outstanding potential of a 3D analysis provided by APT allowed their detailed investigation. Evaluating the dependence of the segregation amplitude on time and temperature, the segregation enthalpy and diffusivity of the triple lines are quantified. The segregation enthalpy is determined to be 0.076 eV, which indicates the considerable excess volume at the triple line.

Laser-assisted atom probe analysis of sol–gel silica layers

Semi-conducting nanocrystals embedded in a non-conducting matrix of silicate glass may be used as non-volatile data storage device. Structures of silicate glasses are conveniently produced by a sol-gel process, which offers the possibility to coat tip-shaped substrates with a silica layer. The study presents first results of their local chemical analysis by laser-assisted atom probe. Till date the exact mechanisms of laser pulsing are still controversial. But it is common sense that there is an at least considerable heating effect on the tip, which leads to a short temperature rise and a prolonged cooling period in materials of low heat conductivity. This effect alters the shape of mass peaks and is examined here using a one-dimensional model of heat transport.

Defect analysis by statistical fitting to 3D atomicmaps

In this article we present a statistical fitting method for evaluation of atomic reconstructions which does not require a coarse-graining step. The fitting compares different models of chemical structure in their capability to explain the measured data set by a least square type merit function. Only preliminary qualitative assumptions about the possible chemical structure are required, while accurate quantitative parameters of the chosen model are delivered by fitting. The technique is particularly useful for singular defect structures with very high composition gradients, for which iso-concentration surfaces determined by coarse-graining become questionable or impossible. We demonstrate that particularly detailed information can be gained from triple junctions and grain boundaries.

Concentration Dependence of the Diffusion in the Ni/Cu System

Deviations from the Fickian-laws of diffusion in the case of concentration dependent diffusion coefficients and high composition gradients gain more and more acceptance nowadays. The cause of this phenomenon is the finite permeability of the atomic layers, or in other words “interface control”. The consequences are wide-spreading e.g. linear diffusion kinetics, deviations in the nucleation behavior of reaction products and kinetically determined interface shape in miscible alloys. Furthermore, if the original chemical interface is broader than the optimum width, even a sharpening of the interface by diffusion can be observed. Previous experiments proving these effects used more or less ideal specimens (e.g. single crystalline or amorphous samples with very flat interfaces) and some doubts can be raised whether these effects can be observed in a realistic specimen with a more complex grain structure. In this talk we will present the results of atom probe measurements on sputter deposited Ni/Cu multilayers (containing surface roughness, lattice defects, etc.). Samples with sharp and smeared Ni/Cu interfaces were produced and later annealed. We found an asymmetry on the interface width in the as-prepared specimens depending on the stacking order. After annealing this asymmetry vanished and remarkably the Cu/Ni interface sharpened by diffusion. After short diffusion time, the interface width became independent on the sample origin (sharp or smeared interface) proving the kinetic control of the interface. Atom probe tomography also allows the direct, local investigation of the grain boundary diffusion in any single grain boundaries. Surprisingly the best description of the shortcut transport can be achieved by assuming a concentration-independent grain boundary diffusion coefficient.