Development of a focused ion beam (FIB) technique to minimize X-ray fluorescence during energy dispersive X-ray spectroscopy (EDS) of FIB specimens in the transmission electron microscope (TEM)

Droplet-mediated formation of embedded GaAs nanowires in MBE GaAs(1-x)Bi(x) films

We have examined the morphology and composition of embedded nanowires that can be formed during molecular beam epitaxy of GaAs(1-x)Bi(x) using high angle annular dark field ('Z-contrast') imaging in an aberration-corrected scanning transmission electron microscope. Samples were grown in Ga-rich growth conditions on a stationary GaAs substrate. Ga-rich droplets are observed on the surface with lateral trails extending from the droplet in the [110] direction. Cross-sectional scanning transmission electron microscopy of the film reveals epitaxial nanowire structures of composition ∼GaAs embedded in the GaAs(1-x)Bi(x) epitaxial layers. These nanowires extend from a surface droplet to the substrate at a shallow angle of inclination (∼4°). They typically are 4 μm long and have a lens-shaped cross section with major and minor axes dimensions of 800 and 120 nm. The top surface of the nanowires exhibits a linear trace in longitudinal cross-section, across which the composition change from ∼GaAs to GaAs(1-x)Bi(x) appears abrupt. The bottom surfaces of the nanowires appear wavy and the composition change appears to be graded over ∼25 nm. The droplets have phase separated into Ga- and Bi-rich components. A qualitative model is proposed in which Bi is gettered into Ga droplets, leaving Bi depleted nanowires in the wakes of the droplets as they migrate in one direction across the surface during GaAs(1-x)Bi(x) film growth.

FIB Plan View Preparation and Electron Tomography of Ga-Containing Droplets Induced by Melt-Back Etching in Si

Melt-back etching is an effect that can occur for gallium (Ga) containing III/V semiconductors grown on Si. Since this effect influences interfaces between the two compounds and therefore the physical characteristics of the material composition, it is desirable to understand its driving forces. Therefore, we investigated Ga grown on Si (001) via metal organic chemical vapor deposition using trimethyl Ga as a precursor. As a result of the melt-back etching, Ga-containing droplets formed on the Si surface which reach into the Si wafer. The shape of these structures was analyzed by plan view investigation and cross sectional tomography in a (scanning) transmission electron microscope. For plan view preparation a focused ion beam was used to avoid damage to the Ga-containing structures, which are sensitive to the chemicals normally used during conventional plan view preparation. Combining the results of both investigation methods confirms that the Ga-containing structure within the Si exhibits a pyramid shape with facets along the Si {111} lattice planes.

Electron beam-assisted healing of nanopores in magnesium alloys

Nanopore-based sensing has emerged as a promising candidate for affordable and powerful DNA sequencing technologies. Herein, we demonstrate that nanopores can be successfully fabricated in Mg alloys via focused electron beam (e-beam) technology. Employing in situ high-resolution transmission electron microscopy techniques, we obtained unambiguous evidence that layer-by-layer growth of atomic planes at the nanopore periphery occurs when the e-beam is spread out, leading to the shrinkage and eventual disappearance of nanopores. The proposed healing process was attributed to the e-beam-induced anisotropic diffusion of Mg atoms in the vicinity of nanopore edges. A plausible diffusion mechanism that describes the observed phenomena is discussed. Our results constitute the first experimental investigation of nanopores in Mg alloys. Direct evidence of the healing process has advanced our fundamental understanding of surface science, which is of great practical importance for many technological applications, including thin film deposition and surface nanopatterning.

Preparation of high-quality ultrathin transmission electron microscopy specimens of a nanocrystalline metallic powder

This article explores the achievable transmission electron microscopy specimen thickness and quality by using three different preparation methods in the case of a high-strength nanocrystalline Cu-Nb powder alloy. Low specimen thickness is essential for spatially resolved analyses of the grains in nanocrystalline materials. We have found that single-sided as well as double-sided low-angle Ar ion milling of the Cu-Nb powders embedded into epoxy resin produced wedge-shaped particles of very low thickness (<10 nm) near the edge. By means of a modified focused ion beam lift-out technique generating holes in the lamella interior large micrometer-sized electron-transparent regions were obtained. However, this lamella displayed a higher thickness at the rim of ≥30 nm. Limiting factors for the observed thicknesses are discussed including ion damage depths, backscattering, and surface roughness, which depend on ion type, energy, current density, and specimen motion. Finally, sections cut by ultramicrotomy at low stroke rate and low set thickness offered vast, several tens of square micrometers uniformly thin regions of ∼10-nm minimum thickness. As major drawbacks, we have detected a thin coating on the sections consisting of epoxy deployed as the embedding material and considerable nanoscale thickness variations.

A comparison of EDS microanalysis in FIB-prepared and electropolished TEM thin foils

This paper reports the results of a fine-probe EDS microanalytical study of cellular precipitation in a Cu-Ti binary alloy. Compositional profiles across the solute depleted Cu-rich FCC lamellae and the Cu4Ti lamellae within isothermally formed cellular colonies were measured in a FEG-TEM from thin-foil specimens prepared by conventional electropolishing and by a technique using a Ga+ focused ion-beam (FIB). The Cliff-Lorimer ratio method, with an absorption correction, was employed to quantify the compositions. Two FIB samples were prepared with different orientations of the lamellae with respect to the ion-milling direction. The compositional profiles across the Cu-rich FCC lamellae and the Cu4Ti compound lamellae in both the FIB-prepared samples and the electropolished sample were, within experimental error, numerically equivalent. The composition of the Cu4Ti compound phase lamellae was very close to the ideal stoichiometric composition of 20 at % Ti. It is concluded that for this system, and for the specimen preparation procedures used in this study, the Ga+ ion-milling process has had no detectable effect on the chemistry changes across the interlamellar interface at the scale studied. These results indicate that the possible sources of chemical artifacts which include redeposition, preferential sputtering and ion-induced atomic migration can be minimized if several precautions are taken during milling in the FIB. Consistent with previous investigators, it was also found that the ion-milling process does introduce significant structural artifacts (e.g., dislocations) into the softer FCC Cu-rich phase compared with a specimen produced by conventional electropolishing.