Karl Dawson

Morphotropic Phase Boundary in the Pb‐Free (1 − x)BiTi3/8Fe2/8Mg3/8O3–xCaTiO3 System: Tetragonal Polarization and Enhanced Electromechanical Properties

Lead-free bismuth-based perovskite oxides with polarization directed along the [001](p) primitive perovskite unit cell edge, analogous to tetragonal PbTiO3, are synthesized at ambient pressure. Enhanced piezoelectric properties, large polarizations, and high depolarization temperatures are observed in the wide morphotropic phase boundary region formed with a rhombohedral phase, with up to 92.5% Bi on the perovskite A site.

Atomic layer deposition of germanium-doped zinc oxide films with tuneable ultraviolet emission

Thin films of germanium-doped zinc oxide have been deposited by atomic layer deposition. The zinc oxide matrix was grown from cyclic pulses of diethylzinc and water vapour over the temperature range of 100–350 °C substrate temperature. Tetramethoxygermanium(IV) was employed as a novel germanium-doping source, which could be incorporated up to 17 at%. At 2.1 at% germanium doping at a deposition temperature of 250 °C, the maximum carrier concentration of 2.14 × 1020 cm−3 coincides with a carrier mobility of approximately 5 cm2 V−1 s−1. No evidence for the formation of nanometre-scale germanium clustering or segregation was observed in the X-ray diffraction patterns or high-resolution transmission electron micrographs of these films. The near band edge photoluminescence shifts to higher energy with increasing germanium incorporation either by the Burstein–Moss mechanism or by alloy formation.

Vacuum ultraviolet photochemical selective area atomic layer deposition of Al2O3 dielectrics

We report the photochemical atomic layer deposition of Al2O3 thin films and the use of this process to achieve area-selective film deposition. A shuttered vacuum ultraviolet (VUV) light source is used to excite molecular oxygen and trimethyl aluminum to deposit films at 60°C. In-situ QCM and post-deposition ellipsometric measurements both show that the deposition rate is saturative as a function of irradiation time. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask and the selectivity of deposition on the illuminated and masked regions of the substrate is a logarithmic function of the UV exposure time. The Al2O3 films exhibit dielectric constants of 8 – 10 at 1 MHz after forming gas annealing, similar to films deposited by conventional thermal ALD.

Friction stir welding of PM2000 ODS alloy

Abstract Friction stir welding has been used to join sheets of a ferritic, oxide dispersion strengthened alloy, PM2000. A stepped spiral probe, polycrystalline cubic boron nitride tool, with a shoulder diameter of 25 mm, was used to weld 4 mm thick plate in a butt joint configuration. The thermomechanically affected zone underwent dynamic recrystallisation during welding; the resultant microstructure consisted of equiaxed ferritic grains containing a dispersion of yttrium aluminium oxides. Microindentation measurements revealed a significant reduction in hardness within the weld zone, when compared to the parent material. The welding process induced an overall coarsening of the yttrium aluminium oxide particles and depletion in their number density. However, the precipitation of secondary phase particles, likely to be oxides, which took place during the welding process, is indicative that an element of mechanical alloying occurs during the welding process. Annealing the welds for 1 h at 1380°C produced a massive recrystallised grain structure in the weld zone and a uniform hardness across the parent and weld was achieved. Transmission electron microscopy showed that, subsequent to annealing, particles were coarser in the weld zone (33 nm mean diameter) than in the parent alloy (24 nm mean diameter). However, electron diffraction and energy dispersive X-ray spectroscopy confirm that the dominant oxide phase, YAlO3 perovskite (YAP), was the same in both regions. Oxide particle size and number densities were not uniform throughout the weld. Focused ion beam prepared surfaces revealed particles within the size range of 50–600 nm diameter in material beneath the tool shoulder/workpiece contact area; the average size of dispersoids in this region was 130 nm.

Preparation of micro-foils for TEM/STEM analysis from metallic powders

A technique has been developed which facilitates the preparation of electro-polished micro-foil transmission electron microscopy (TEM) specimens, which have previously been machined out of ≈100 μm diameter metallic powder particles using a Focussed Ion Beam (FIB) instrument. The technique can be used to create small volume TEM specimens from most metallic powder particles and bulk metal samples. This is especially useful when the matrices are ferritic steels, which are often difficult to image in the electron microscope, since the necessary aberration corrections change as the sample is tilted in the magnetic field of the objective lens. Small samples, such as powder particles, were attached to gold support grids using deposited platinum and were then ion milled to approximately 2 μm thickness in a focussed ion beam (FIB) instrument. Subsequently, the specimen assemblies were electropolished for short durations under standard conditions, to produce large (5 μm×5 μm) electron transparent regions of material. The specimens produced by this technique were free from FIB related artefacts and facilitated atomic resolution scanning-TEM (STEM) imaging of ferritic and nickel matrices containing, for example, yttrium rich oxide nano-dispersoids.

Room Temperature Magnetically Ordered Polar Corundum GaFeO3 Displaying Magnetoelectric Coupling

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO3-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin d5 cations above room temperature in the AFeO3 system. We report the synthesis of a polar corundum GaFeO3 by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO3-type cation ordering by complementary use of neutron, X-ray, and electron diffraction methods. In situ neutron diffraction shows that the polar corundum forms directly from AlFeO3-type GaFeO3 under the synthesis conditions. The A3+/Fe3+ cations are shown to be more ordered in polar corundum GaFeO3 than in isostructural ScFeO3. This is explained by DFT calculations which indicate that the extent of ordering is dependent on the configurational entropy available to each system at the very different synthesis temperatures required to form their corundum structures. Polar corundum GaFeO3 exhibits weak ferromagnetism at room temperature that arises from its Fe2O3-like magnetic ordering, which persists to a temperature of 408 K. We demonstrate that the polarity and magnetization are coupled in this system with a measured linear magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling is a prerequisite for potential applications of polar corundum materials in multiferroic/magnetoelectric devices.

The role of nitrogen doping in ALD Ta2O5 and its influence on multilevel cell switching in RRAM

The role of nitrogen doping on the stability and memory window of resistive state switching in N-doped Ta2O5 deposited by atomic layer deposition is elucidated. Nitrogen incorporation increases the stability of resistive memory states which is attributed to neutralization of electronic defect levels associated with oxygen vacancies. The density functional simulations with the screened exchange hybrid functional approximation show that the incorporation of nitrogen dopant atoms in the oxide network removes the O vacancy midgap defect states, thus nullifying excess defects and eliminating alternative conductive paths. By effectively reducing the density of vacancy-induced defect states through N doping, 3-bit multilevel cell switching is demonstrated, consisting of eight distinctive resistive memory states achieved by either controlling the set current compliance or the maximum voltage during reset. Nitrogen doping has a threefold effect: widening the switching memory window to accommodate the more intermed...

The Stability of Fine, Sub-Grain Microstructures Within Carbon Depleted Regions of Dissimilar Metal, Ferritic, Creep Resistant Welds

The duration of post weld heat treatments (PWHT) applied to thick section multi-pass dissimilar metal welds (DMW), involving ferritic creep resistant steels of differing chromium content, are shown to have a considerable impact on the performance of the welded joint. Welding consumables of alloy types P22 and P24 have been used to form joints with P91 base alloy which were subsequently post weld heat treated for varying durations. High resolution transmission electron microscopy (TEM) has been exploited in the characterisation of precipitation in the weld material and the heat affected zone. It has been shown that uphill diffusion of carbon from the low to the higher alloy material during PWHT and creep test conditions occurs in all specimens. Selected area diffraction (SAD) and convergent beam electron diffraction (CBED) studies of carbon extraction replicas reveal extensive dissolution of M23 C6 and M7 C3 carbides in the decarburised zone of the weld alloy subsequent to post weld heat treatments. However, welds completed using Nb and V containing consumables retain a fine distribution of MX precipitation in the carbon depleted regions after PWHT. The retention of these microstructure stabilising carbonitrides facilitates the preservation of an ultra fine sub-grain microstructure, thus avoiding recrytallisation which is invariably observed in post weld heat treated P22:P91 DMWs. Cell size comparisons of the sub-grain microstructures have been investigated utilising channelling contrast back scattered scanning electron images of as welded and post weld heat treated material.Copyright

Bi4O4Cu1.7Se2.7Cl0.3: Intergrowth of BiOCuSe and Bi2O2Se Stabilized by the Addition of a Third Anion

Layered two-anion compounds are of interest for their diverse electronic properties. The modular nature of their layered structures offers opportunities for the construction of complex stackings used to introduce or tune functionality, but the accessible layer combinations are limited by the crystal chemistries of the available anions. We present a layered three-anion material, Bi4O4Cu1.7Se2.7Cl0.3, which adopts a new structure type composed of alternately stacked BiOCuSe and Bi2O2Se-like units. This structure is accessed by inclusion of three chemically distinct anions, which are accommodated by aliovalently substituted Bi2O2Se0.7Cl0.3 blocks coupled to Cu-deficient Bi2O2Cu1.7Se2 blocks, producing a formal charge modulation along the stacking direction. The hypothetical parent phase Bi4O4Cu2Se3 is unstable with respect to its charge-neutral stoichiometric building blocks. The complex layer stacking confers excellent thermal properties upon Bi4O4Cu1.7Se2.7Cl0.3: a room-temperature thermal conductivity (κ) of 0.4(1) W/mK was measured on a pellet with preferred crystallite orientation along the stacking axis, with perpendicular measurement indicating it is also highly anisotropic. This κ value lies in the ultralow regime and is smaller than those of both BiOCuSe and Bi2O2Se. Bi4O4Cu1.7Se2.7Cl0.3 behaves like a charge-balanced semiconductor with a narrow band gap. The chemical diversity offered by the additional anion allows the integration of two common structural units in a single phase by the simultaneous and coupled creation of charge-balancing defects in each of the units.

Voltage Controlled Hot Carrier Injection Enables Ohmic Contacts Using Au Island Metal Films on Ge

We introduce a new approach to creating low-resistance metal-semiconductor ohmic contacts, illustrated using high conductivity Au island metal films (IMFs) on Ge, with hot carrier injection initiated at low applied voltage. The same metallization process simultaneously allows ohmic contact to n-Ge and p-Ge, because hot carriers circumvent the Schottky barrier formed at metal/n-Ge interfaces. A 2.5× improvement in contact resistivity is reported over previous techniques to achieve ohmic contact to both n- and p- semiconductor. Ohmic contacts at 4.2 K confirm nonequilibrium current transport. Self-assembled Au IMFs are strongly orientated to Ge by annealing near the Au/Ge eutectic temperature. Au IMF nanostructures form, provided the Au layer is below a critical thickness. We anticipate that optimized IMF contacts may have applicability to many material systems. Optimizing this new paradigm for metal-semiconductor contacts offers the prospect of improved nanoelectronic systems and the study of voltage controlled hot holes and electrons.