P.A.J. Bagot

Structural, electronic and optical properties of m-plane (In,Ga)N/GaN quantum wells: Insights from experiment and atomistic theory

In this paper we present a detailed analysis of the structural, electronic, and optical properties of an

An integrated high temperature environmental cell for atom probe tomography studies of gas-surface reactions: Instrumentation and results

m

The atomic structure of polar and non-polar InGaN quantum wells and the green gap problem

-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.

Atomic-scale Studies of Uranium Oxidation and Corrosion by Water Vapour

An integrated environmental cell has been designed and developed for the latest generation of Atom Probe Tomography LEAP™ instruments, allowing controlled exposure of samples to gases at high temperatures. Following treatment, samples can be transferred through the LEAP vacuum system for subsequent APT analysis, which provides detailed information on changes to chemical microstructures following the reactions with near-atomic resolution. A full description of the cell is presented, along with some sample results on the oxidation of aluminum and two platinum-group alloys, demonstrating the capability of combining exposure/characterization functionality in a single instrument.

Optimisation of sample preparation and analysis conditions for atom probe tomography characterisation of low concentration surface species

We have used high resolution transmission electron microscopy (HRTEM), aberration-corrected quantitative scanning transmission electron microscopy (Q-STEM), atom probe tomography (APT) and X-ray diffraction (XRD) to study the atomic structure of (0001) polar and (11-20) non-polar InGaN quantum wells (QWs). This paper provides an overview of the results. Polar (0001) InGaN in QWs is a random alloy, with In replacing Ga randomly. The InGaN QWs have atomic height interface steps, resulting in QW width fluctuations. The electrons are localised at the top QW interface by the built-in electric field and the well-width fluctuations, with a localisation energy of typically 20meV. The holes are localised near the bottom QW interface, by indium fluctuations in the random alloy, with a localisation energy of typically 60meV. On the other hand, the non-polar (11-20) InGaN QWs contain nanometre-scale indium-rich clusters which we suggest localise the carriers and produce longer wavelength (lower energy) emission than from random alloy non-polar InGaN QWs of the same average composition. The reason for the indium-rich clusters in non-polar (11-20) InGaN QWs is not yet clear, but may be connected to the lower QW growth temperature for the (11-20) InGaN QWs compared to the (0001) polar InGaN QWs.

In-Situ Deuterium Charging for Direct Detection of Hydrogen in Vanadium by Atom Probe Tomography

Understanding the corrosion of uranium is important for its safe, long-term storage. Uranium metal corrodes rapidly in air, but the exact mechanism remains subject to debate. Atom Probe Tomography was used to investigate the surface microstructure of metallic depleted uranium specimens following polishing and exposure to moist air. A complex, corrugated metal-oxide interface was observed, with approximately 60 at.% oxygen content within the oxide. Interestingly, a very thin (~5u2009nm) interfacial layer of uranium hydride was observed at the oxide-metal interface. Exposure to deuterated water vapour produced an equivalent deuteride signal at the metal-oxide interface, confirming the hydride as originating via the water vapour oxidation mechanism. Hydroxide ions were detected uniformly throughout the oxide, yet showed reduced prominence at the metal interface. These results support a proposed mechanism for the oxidation of uranium in water vapour environments where the transport of hydroxyl species and the formation of hydride are key to understanding the observed behaviour.

Research data supporting Gamma Prime Precipitate Evolution During Aging of a Model Nickel Based Superalloy

The practicalities for atom probe tomography (APT) analysis of near-surface chemistry, particularly the distribution of low concentration elements, are presented in detail. Specifically, the challenges of surface analysis using APT are described through the characterisation of near-surface implantation profiles of low concentration phosphorus into single crystal silicon. This material system was chosen to illustrate this surface specific approach as low concentration phosphorus has significant mass spectra overlaps with silicon species and the near surface location requires particular attention to focused ion beam specimen preparation and deposition of various capping layers. Required changes to standard sample preparation procedure are described and the effects of changes in APT analysis parameters are discussed with regards to this specific material system. Implantation profiles of 14 kV phosphorus ions with a predicted peak concentration of 0.2 at .% were successfully analysed using APT using pulsed laser assisted evaporation. It is demonstrated that the most important factor in obtaining the most accurate implantation profile was to ensure all phosphorus mass peaks were as free of background noise as possible, with thermal tails from the Si2+ ions obscuring the P2+ ions being the major overlap in the mass spectrum. The false positive contribution to the phosphorus profiles from hydride species appears minimal at the capping layer/substrate interface. The initial capping layer selection of nickel was successful in allowing the analysis of the majority of the phosphorus profile but nickel and phosphorus mass spectra overlaps prevent optimum quantification of phosphorus at the surface.

Atom Probe Characterization of Oxide Layers Formed on Polycrystalline Nickel Based Superalloys

The study of hydrogen embrittlement is of great interest for several decades, owing to the large reduction in maximum elongation of hydrogen exposed materials [1]. Towards this end, we will be studying the hydrogen interaction with material microstructure. Even though there exist several competing theoretical models, providing a mechanism by which dislocations and cracks are expedited through microstructures [2], there is a lack of experimental evidence to support differing theoretical models, owing to two main limitations. Firstly, hydrogen is weakly interacting with many radiations (e.g. x-ray, electron), and thus is difficult to image. Secondly, the interactions are truly atomistic, and thus require a scale-matched imaging methodology. Thus there exists a need for real-space quantitative analysis for identification of H, at these nano scales. Such information would allow for greater understanding of the dislocation H interaction.

On the effect of boron on grain boundary character in a new polycrystalline superalloy

Excel spreadsheets containing the particle size distributions (PSDs), compositions of each phase, lattice parameters and proximity histograms of composition across the precipitate - matrix interfaces.

On the composition of microtwins in a single crystal nickel-based superalloy

Nickel based superalloys are a class of high-temperature materials used extensively throughout the high pressure sections of jet turbine engines. Two characteristics that make them particularly suited to this application are the ability to maintain strength at high temperatures, combined with the formation of a protective oxide scale which prevents further oxidation damage. These characteristics are prime motivations during the alloy design process.