Atom probe tomography of interfaces in ceramic films and oxide scales
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ntroduction Engineering ceramics, such as oxides, nitrides, and carbides, represent an important material group used in electronic devices, aerospace components, cutting tools, and in various tribological applications. Crucial for the operation of energyefficient power-generation technologies is to limit the thermal growth of oxides such as zirconia, alumina, and chromia. Atomic-scale characterization of interfaces in these materials is a prerequisite to fully understand their electronic, ionic, mechanical, magnetic, and optical properties.1–3 The only method that can be used to routinely analyze and map individual atoms in a material in three dimensions with nearly atomic resolution and with equal sensitivity for all elements is atom probe tomography (APT). For many years, this technique was mostly used for investigations of metallic materials because of the use of high-voltage pulses to evaporate ions from the specimen surface, which required the material to have high electrical conductivity (>102 S/cm).4,5 Consequently, APT studies of insulating ceramics were not feasible except for investigations concerned with small oxide precipitates embedded in a metallic matrix,6–9 or nanometerthick oxide layers sandwiched between conducting layers.4,10–12 An alternative method to achieve field evaporation of ions using nanosecond laser pulses, proposed in the early 1980s,13 could not be used for quantitative analysis in practice due to
experimental difficulties with laser alignment and the operating parameters (pulsing energy, focus, pulsing rate, and laser wavelength). With the advent of faster laser systems (pico- and femtosecond), the focused ion beam milling technique for specimen preparation, and commercial APT instruments, the doors have been opened for three-dimensional analysis with nearly atomic resolution of insulating oxide and nitride ceramics, semiconductors, and even biological materials.14–16 During the last decade, several successful APT investigations of dielectric ceramics have been reported.17–33 It is recognized, however, that quantitative studies of oxides and nitrides are challenging due to the complexity of the mass spectra (often containing peak overlaps), lower mass resolution and higher proportion of multiple events, and higher background level than for metallic materials, leading to a possible loss of information.34,35 Although it is now clear that field evaporation normally occurs due to heating generated by the laser pulse,36 the mechanism by which materials transparent to light are able to absorb laser energy has been unclear. It appears that the application of a high DC voltage to a tip-shaped dielectric specimen induces a field within a screening distance of a few nm inside the dielectric that increases the light absorption up to values typical for metals. In addition, the bandgap in this zone is expected to decrease down to total band collapse.37,38
K. Stiller, Department of Applied Physics, Division of Materials Microstructure, Chalmers University of Technology, Sweden; [email protected] M. T
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