Spinodal nanodecomposition in magnetically doped semiconductors

This review presents the recent progress in computational materials design, experimental realization, and control methods of spinodal nanodecomposition under three- and two-dimensional crystal-growth conditions in spintronic materials, such as magnetically doped semiconductors. The computational description of nanodecomposition, performed by combining first-principles calculations with kinetic Monte Carlo simulations, is discussed together with extensive electron microscopy, synchrotron radiation, scanning probe, and ion beam methods that have been employed to visualize binodal and spinodal nanodecomposition (chemical phase separation) as well as nanoprecipitation (crystallographic phase separation) in a range of semiconductor compounds with a concentration of transition metal (TM) impurities beyond the solubility limit. The role of growth conditions, co-doping by shallow impurities, kinetic barriers, and surface reactions in controlling the aggregation of magnetic cations is highlighted. According to theoretical simulations and experimental results the TM-rich regions appear either in the form of nanodots (the {\em dairiseki} phase) or nanocolumns (the {\em konbu} phase) buried in the host semiconductor. Particular attention is paid to Mn-doped group III arsenides and antimonides, TM-doped group III nitrides, Mn- and Fe-doped Ge, and Cr-doped group II chalcogenides, in which ferromagnetic features persisting up to above room temperature correlate with the presence of nanodecomposition and account for the application-relevant magneto-optical and magnetotransport properties of these compounds. Finally, it is pointed out that spinodal nanodecomposition can be viewed as a new class of bottom-up approach to nanofabrication.Comment: 72 pages, 79 figure

Microstructural features controlling very high cycle fatigue of nitrided maraging steel

Maraging steels belong to the group of ultra-high strength materials and are often used in critical aerospace, automotive and tooling components. By applying a surface treatment such as nitriding, the fatigue and wear resistance can be improved. The microstructural features that influence the (very) high cycle fatigue response of nitrided maraging steels are studied in this work. Although the used steel has practically no inclusions, it was found that small surface imperfections, introduced during processing, may form potential fatigue initiation points. The samples are nitrided during aging in order to form nitrided layers with various thicknesses, microstructures and hardness profiles without formation of a continuous (compound) iron nitride layer. Data from microhardness tests, scanning electron microscopy, electron backscatter diffraction, x-ray diffraction and transmission electron microscopy were used to characterize the microstructure of the layers. Bending fatigue tests were employed to evaluate the fatigue response of the steel. It was found that the best fatigue behavior is obtained in samples with a thin diffusion zone with a narrow constant hardness region. In this zone, coherent disc-shaped nitride precipitates are detected with TEM

Hanford Low-Activity Waste Vitrification: A Review

This Paper Summarizes the Vast Body of Literature (Over 200 Documents) Related to Vitrification of the Low-Activity Waste (LAW) Fraction of the Hanford Tank Wastes. Details Are Provided on the Origins of the Hanford Tank Wastes that Resulted from Nuclear Operations Conducted between 1944 and 1989 to Support Nuclear Weapons Production. Waste Treatment Processes Are Described, Including the Baseline Process to Separate the Tank Waste into LAW and High-Level Waste Fractions, and the LAW Vitrification Facility Being Started at Hanford. Significant Focus is Placed on the Glass Composition Development and the Property-Composition Relationships for Hanford LAW Glasses. Glass Disposal Plans and Criteria for Minimizing Long-Term Environmental Impacts Are Discussed Along with Research Perspectives

Mikroegiturako akatsek, ni-oinarridun heusler aleazioen transformazio martensitikoan eta propietate magnetikoetan duten eraginaren azterketaa

187 p.(eusk) 189 p. (eng.)The thesis deals with the effect that microstructural defects have on the magneticproperties and martensitic transformation in Ni-Mn-Z (Z = Ga, In, Sn)and Ni-Fe-Ga alloys. For this purpose, along with the standard characterizationtechniques, two additional specific nuclear techniques have been employed; 119SnMössbauer Spectroscopy and Positron Annihilation Lifetime Spectroscopy, PALS.For the consecution of this thesis one PALS spectrometer has been designed, builtand optimized. For offline emission Mössbuauer experiments, a setup has been alsobuilt for measuring ion implanted samples. Finally, a high temperature annealingfurnace has been designed and built to carry out the annealing of powdered samplesunder controllable atmosphere

Nuclear Fusion Programme: Annual Report of the Association Karlsruhe Institute of Technology (KIT)/EURATOM ; January 2009 - December 2009 (KIT Scientific Reports ; 7548)

The Karlsruhe Institute of Technology (KIT) is working in the framework of the European Fusion Programme on key technologies in the areas of superconducting magnets, microwave heating systems (Electron-Cyclotron-Resonance-Heating, ECRH), the deuterium-tritium fuel cycle, He-cooled breeding blankets, a He-cooled divertor and structural materials, as well as refractory metals for high heat flux applications including a major participation in the preparation of the international IFMIF project

Investigation of indium-rich InGaN alloys and kinetic growth regime of GaN

Nowadays, light emitting diodes (LEDs) and laser diodes (LDs) are part of our daily life. More and more devices incorporate InGaN-based optoelectronic devices. In fact, since the first demonstration of a candela-class InGaN-based LED in the beginning of the nineties, those LEDs have quickly become popular. The efficiency of blue InGaN LEDs is nowadays very high and when coated with a yellow phosphor, they can efficiently emit white light. Such white LEDs are more and more used for different lighting applications, from home lighting to car lighting. Another application of blue InGaN emitters that is found in many households is the Blu-ray disc, based on a 405 nm LD. The great success of the III-nitrides in the blue range created a strong interest to extend the wavelength range into the green, as efficient green emitters are missing. However, their performance is still low compared to their blue counterparts. The goal of this dissertation is to investigate issues that limit the performance of green InGaN LEDs and LDs. To achieve efficient green emission, the growth temperature needs to be reduced to incorporate more indium into the active region. In this context, the present study is divided into two parts. In the first one, the growth regime of GaN as a function of the temperature is investigated and the physical origin of the surface features is discussed. When lowering the temperature, the so called Ehrlich-Schwöbel barrier, a barrier at the step-edges, causes a different attachment probability for the adatoms arriving from the different sides. This asymmetry can lead to the appearance of three different kinetic surface morphologies: hillocks, fingers, or step-bunching. After presenting the theory, the three different regimes are demonstrated for GaN, and the growth parameters that allow to control the morphology are investigated. Indeed, the different morphologies usually reported for the different growth techniques can be attributed to the different growth parameters commonly used. In the end of the first part, the influence of the underlaying morphology on the properties of the active region of an LED is discussed. In the second part, the impact of the thermal budget on the properties of InGaN quantum wells (QWs) is discussed. Active regions with a high In content can degrade easily when the p-type layer is grown on top, due to being exposed to a too high thermal budget. This is especially critical for green LDs which require thick cladding layers, i.e. a long growth time. The origin of the degradation and the parameters influencing it are discussed first: when an In-rich QW is annealed at a too high temperature, its emission intensity is reduced and dark spots appear on PL maps. The annealing temperature causing this degradation depends on the indium content, the QW thickness and the number of QWs. It is shown that these dark spots exhibit a peculiar emission spectra, with a characteristic emission at 2.75 eV. This emission is tentatively attributed to nitrogen vacancies that are created by the formation of metallic indium clusters in the active region. Then the focus is laid on the beneficial effects that can take place when an In-rich active region is exposed to a moderate thermal budget. Apart from reducing the linewidth of the emission, a strong improvement in photoluminescence intensity can be achieved. This improvement increases for high indium content QWs, i.e. when the growth temperature is low. In fact, this improvement is more pronounced for molecular beam epitaxy grown structures whose active regions are grown at a much lower temperatures than their metal organic vapor phase epitaxy counterparts. This is tentatively ascribed to the reduction of point defects by thermal annealing. As a consequence, a moderate annealing can be beneficial for both to reduce the non-radiative recombinations and the linewidth. It is finally proposed to perform a short annealing step of high indium content QWs before growing the p-type (cladding) layer at low temperature

Bibliography of Lewis Research Center technical publications announced in 1993

This compilation of abstracts describes and indexes the technical reporting that resulted from the scientific and engineering work performed and managed by the Lewis Research Center in 1993. All the publications were announced in the 1993 issues of STAR (Scientific and Technical Aerospace Reports) and/or IAA (International Aerospace Abstracts). Included are research reports, journal articles, conference presentations, patents and patent applications, and theses

Ifjan, éretten, éltesen: 80 kérdés - válasz közel nyolc évtizedről

Gyulai József Széchenyi díjas akadémikus közel nyolc évtizedet átölelő munkásságának története Tóth László a műszaki tudományok doktorának kérdései alapján, mely több érdekes visszaemlékezést tartalmaz a KFKI, valamint a hazai mikrolelktronika történetéből