Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams.

We report a morphotropic phase transformation in vanadium dioxide (VO2) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO2 nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO2 nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems.

A fracture criterion for the notch strength of high strength steels in the presence of hydrogen

High strength steels can suffer from a loss of ductility when exposed to hydrogen, and this may lead to sudden failure. The hydrogen is either accommodated in the lattice or is trapped at defects, such as dislocations, grain boundaries and carbides. The challenge is to identify the effect of hydrogen located at different sites upon the drop in tensile strength of a high strength steel. For this purpose, literature data on the failure stress of notched and un-notched steel bars are re-analysed; the bars were tested over a wide range of strain rates and hydrogen concentrations. The local stress state at failure has been determined by the finite element (FE) method, and the concentration of both lattice and trapped hydrogen is predicted using Oriani's theory along with the stress-driven diffusion equation. The experimental data are rationalised in terms of a postulated failure locus of peak maximum principal stress versus lattice hydrogen concentration. This failure locus is treated as a unique material property for the given steel and heat treatment condition. We conclude that the presence of lattice hydrogen increases the susceptibility to hydrogen embrittlement whereas trapped hydrogen has only a negligible effect. It is also found that the observed failure strength of hydrogen charged un-notched bars is less than the peak local stress within the notched geometries. Weakest link statistics are used to account for this stressed volume effect. © 2013 Elsevier Ltd.

Atom probe tomography of SRAM transistors: Specimen preparation methods and analysis

Different FIB-based sample preparation methods for atom probe analysis of transistors have been proposed and discussed. A special procedure, involving device deprocessing, has been used to analyze by APT a sub-30 nm transistor extracted from a SRAM device. The analysis provides three dimensional compositions of Ni-silicide contact, metal gate and high-k oxide of the transistor gate. © 2013 Elsevier B.V. All rights reserved.

Ni silicide nanowires analysis by atom probe tomography

The Ni silicide formed at low temperature on Si nanowire has been analyzed by atom probe tomography (APT) thanks to a special technique for sample preparation. A method of preparation has been developed using the focused ion beam (FIB) for the APT analysis of nanowires (NWs). This method allow for the measurement of the radial distribution when a NW is cut, buried in a protective metal matrix, and finally mounted on the APT support post. This method was used for phosphorous doped Si NWs with or without a silicide shell, and allows obtaining the concentration and distribution of chemical elements in three-dimensions (3D) in the radial direction of the NWs. The distribution of atoms in the NWs has been measured including dopants and Au contamination. These measurements show that δ-Ni2Si phase is formed on Si NW, Au is found as cluster at the Ni/δ-Ni2Si interface and P is segregated at the δ-Ni2Si/ Si NW interface. The results obtained on NWs after silicidation were compared with the silicide on the Si substrate, showing that the same silicide phase δ-Ni2Si formed in both cases (NWs and substrate). © 2013 Elsevier B.V. All rights reserved.

Atom probe tomography for advanced metallization

In microelectronics, the increase in complexity and the reduction of devices dimensions make essential the development of new characterization tools and methodologies. Indeed advanced characterization methods with very high spatial resolution are needed to analyze the redistribution at the nanoscale in devices and interconnections. The atom probe tomography has become an essential analysis to study materials at the nanometer scale. This instrument is the only analytical microscope capable to produce 3D maps of the distribution of the chemical species with an atomic resolution inside a material. This technique has benefit from several instrumental improvements during last years. In particular, the use of laser for the analysis of semiconductors and insulating materials offers new perspectives for characterization. The capability of APT to map out elements at the atomic scale with high sensitivity in devices meets the characterization requirements of semiconductor devices such as the determination of elemental distributions for each device region. In this paper, several examples will show how APT can be used to characterize and understand materials and process for advanced metallization. The possibilities and performances of APT (chemical analysis of all the elements, atomic resolution, planes determination, crystallographic information...) will be described as well as some of its limitations (sample preparation, complex evaporation, detection limit, ...). The examples illustrate different aspect of metallization: dopant profiling and clustering, metallic impurities segregation on dislocation, silicide formation and alloying, high K/metal gate optimization, SiGe quantum dots, as well as analysis of transistors and nanowires. © 2013 Elsevier B.V. All rights reserved.