The Electronic and Magnetic Effects of 3d Transition Metal Impurities in Semiconductors

The subject of this dissertation is concerned almost exclusively with soft x-ray spectroscopy of the 3d transition metals. These relatively common metals, owing to their widespread availability, are already used in all facets of technology. Stainless steel is largely chromium, iron, and nickel; copper wires transmit nearly all of our electricity; nickel is used in the making of margarine. Their wide range of electronic properties, strength, and usefulness in chemical reactions underpins their versatility to the point where they are used in essentially everything we manufacture. Key to this thesis is their ability to induce magnetism. If realized, spintronic technologies would harness the semiconducting electronic properties of a material, while also utilizing the induced magnetic properties to transport spin polarized charge. The ability to advance digital logic (0s and 1s) from its current state as on/off switches controlled solely via current, to the spintronic method of flipping spins to an up or down state, would have vast consequences for the computing world. Heat dissipation and cooling issues would largely vanish, and computing speed would show large improvements, while being non-volatile when power is lost. Succinctly put, the broad goal of my studies focused on how transition metal impurities doped into host semiconductors in small proportions can influence the host material's electronic and magnetic properties. This was accomplished primarily through modelling experimental spectra with theoretical calculations, and then extracting information through their agreement. In doing this, it is possible to determine fundamental quantitative properties of for each 3d ion, and how each ion situates itself within the host lattice. This information can then be linked back to known properties of the material in order to determine which 3d ions, host materials, and synthesis conditions show promise for spintronic or other related technologies. For the study concerning Bi2Te3 it was shown that the various transition metals Cr, Mn, Fe, Co, Ni, and Cu each integrate themselves into the host crystal in a particular fashion. Manganese atoms substitute cleanly into Bi sites; chromium atoms are not absorbed into the bulk, but only the surface; iron prefers a mixture of oxidation states; and for cobalt and nickel a mixture of configurations was found. Similarly, with host materials TiO2 and ZnO, DFT calculations predicted that the probability of substitution by a transition metal atom into a Zn or Ti site decreased in probability as the atomic number of the dopant metal atom increases, with a greater chance of metallic clustering in TiO2. Spectroscopic measurements, along with crystal field calculations confirmed these trends though modelling and direct comparison of calculation and experiment. This allowed us to extract real physical properties of the system, such as oxidation state, local symmetry, and effects d-orbital energies, via the calculation parameters. In the ferromagnetic compound NiFe2O4, the Fe atoms are responsible for the magnetism, but are in three different unique sites of various oxidation states and symmetries. By theoretically modelling x-ray magnetic circular dichroism experiments I have shown how these three sites can be readily distinguished, and how the interplay between their individual contributions to the magnetism are necessary to understand how the bulk magnetism arises. Furthermore, only through modelling the experimental XMCD with calculations can it be understood how aluminum alloying affects the overall magnetism. As more non-magnetic aluminum atoms replace magnetic iron atoms, the overall strength of the magnetism does not continuously decrease, but in fact begins to increase again at a certain point; this unexpected and unintuitive result can only be explained using the methodology described above. Structural changes in regular white TiO2 occur under a high pressure atmosphere that cause it to turn black, as a result of mid band gap states forming. I was able to adapt a generally hard x-ray technique (EXAFS) to the soft x-ray regime using the capabilities of the REIXS beamline at the Canadian Light Source to probe the change in interatomic distances between the white and black materials and observe the undergone structural changes. The shift in atomic distances were then compared to distorted structures of the nominal material and a distortion in the vicinity of an oxygen vacancy were able to solve the dilemma of the nature of the distortion. The Chelyabinsk meteorite had a thermomagnetic analysis performed on it to determine the various Curie temperatures of the magnetic materials contained in it, which consists of nickel and iron. Through comparisons with magnetic phase charts, we showed that the meteorite contains an iron-nickel alloy, which is quite common. But the breakthrough finding that had not been observed before was the discovery of an extremely pure form of iron, which hadn't ever been observed to occur naturally before

Paleomagnetic Evidence for a Partially Differentiated Ordinary Chondrite Parent Asteroid

The textures and accretion ages of chondrites have been used to argue that their parent asteroids never differentiated. Without a core, undifferentiated planetesimals could not have generated magnetic fields through dynamo activity, so chondrites are not expected to have experienced such fields. However, the magnetic remanence carried by the CV chondrites is consistent with dynamo-generated fields, hinting that partially differentiated asteroids consisting of an unmelted crust atop a differentiated interior may exist. Here, we test this hypothesis by applying synchrotron X-ray microscopy to metallic veins in the slowly-cooled H6 chondrite Portales Valley. The magnetic remanence carried by nanostructures in these veins indicates this meteorite recorded a magnetic field over a period of tens to hundreds of years at ~100 Myr after solar system formation. These properties are inconsistent with external field sources such as the nebula, solar wind, or impacts, but are consistent with dynamo-generated fields, indicating that the H chondrite parent body contained an advecting metallic core and was therefore partially differentiated. We calculate the thermal evolution of the chondritic portions of partially differentiated asteroids that form through incremental accretion across 10^5 - 10^6 years, finding this can agree with the measured ages and cooling rates of multiple H chondrites. We also predict the cores of these bodies could have been partially liquid and feasibly generating a dynamo at 100 Myr after solar system formation. These observations contribute to a growing body of evidence supporting a spectrum of internal differentiation within some asteroids with primitive surfaces.NASA US Department of Energ

Paleomagnetic evidence for a partially differentiated ordinary chondrite parent asteroid

The textures and accretion ages of chondrites have been used to argue that their parent asteroids never differentiated. Without a core, undifferentiated planetesimals could not have generated magnetic fields through dynamo activity, so chondrites are not expected to have experienced such fields. However, the magnetic remanence carried by the CV chondrites is consistent with dynamo‐generated fields, hinting that partially differentiated asteroids consisting of an unmelted crust atop a differentiated interior may exist. Here, we test this hypothesis by applying synchrotron X‐ray microscopy to metallic veins in the slowly cooled H6 chondrite Portales Valley. The magnetic remanence carried by nanostructures in these veins indicates that this meteorite recorded a magnetic field over a period of tens to hundreds of years at ∼100 Myr after solar system formation. These properties are inconsistent with external field sources such as the nebula, solar wind, or impacts, but are consistent with dynamo‐generated fields, indicating that the H chondrite parent body contained an advecting metallic core and was therefore partially differentiated. We calculate the thermal evolution of the chondritic portions of partially differentiated asteroids that form through incremental accretion across 105 to 106 years, finding this can agree with the measured ages and cooling rates of multiple H chondrites. We also predict that the cores of these bodies could have been partially liquid and feasibly generating a dynamo at 100 Myr after solar system formation. These observations contribute to a growing body of evidence supporting a spectrum of internal differentiation within some asteroids with primitive surfaces

Paleomagnetic evidence for a partially differentiated ordinary chondrite parent asteroid

The textures and accretion ages of chondrites have been used to argue that their parent asteroids never differentiated. Without a core, undifferentiated planetesimals could not have generated magnetic fields through dynamo activity, so chondrites are not expected to have experienced such fields. However, the magnetic remanence carried by the CV chondrites is consistent with dynamo‐generated fields, hinting that partially differentiated asteroids consisting of an unmelted crust atop a differentiated interior may exist. Here, we test this hypothesis by applying synchrotron X‐ray microscopy to metallic veins in the slowly‐cooled H6 chondrite Portales Valley. The magnetic remanence carried by nanostructures in these veins indicates this meteorite recorded a magnetic field over a period of tens to hundreds of years at ∼100 Myr after solar system formation. These properties are inconsistent with external field sources such as the nebula, solar wind, or impacts, but are consistent with dynamo‐generated fields, indicating that the H chondrite parent body contained an advecting metallic core and was therefore partially differentiated. We calculate the thermal evolution of the chondritic portions of partially differentiated asteroids that form through incremental accretion across 105 ‐ 106 years, finding this can agree with the measured ages and cooling rates of multiple H chondrites. We also predict the cores of these bodies could have been partially liquid and feasibly generating a dynamo at 100 Myr after solar system formation. These observations contribute to a growing body of evidence supporting a spectrum of internal differentiation within some asteroids with primitive surfaces

Materials sciences research

Research projects involving materials research conducted by various international test facilities are reported. Much of the materials research is classified in the following areas: (1) acousto-optic, acousto-electric, and ultrasonic research, (2) research for elucidating transport phenomena in well characterized oxides, (3) research in semiconductor materials and semiconductor devices, (4) the study of interfaces and interfacial phenomena, and (5) materials research relevant to natural resources. Descriptions of the individual research programs are listed alphabetically by the name of the author and show all personnel involved, resulting publications, and associated meeting speeches

The Twenty-Fifth Lunar and Planetary Science Conference. Part 1: A-G

Papers from the conference are presented, and the topics covered include the following: planetary geology, meteorites, planetary composition, meteoritic composition, planetary craters, lunar craters, meteorite craters, petrology, petrography, volcanology, planetary crusts, geochronology, geomorphism, mineralogy, lithology, planetary atmospheres, impact melts, volcanoes, planetary evolution, tectonics, planetary mapping, asteroids, comets, lunar soil, lunar rocks, lunar geology, metamorphism, chemical composition, meteorite craters, and planetary mantles

The Twenty-Fifth Lunar and Planetary Science Conference. Part 2: H-O

Various papers on lunar and planetary science are presented, covering such topics as: planetary geology, lunar geology, meteorites, shock loads, cometary collisions, planetary mapping, planetary atmospheres, chondrites, chondrules, planetary surfaces, impact craters, lava flow, achondrites, geochemistry, stratigraphy, micrometeorites, tectonics, mineralogy, petrology, geomorphology, and volcanology

Forty-first Lunar and Planetary Science Conference

Special sessions were: A New Moon: Lunar Reconnaissance Orbiter Results ; Water in the Solar System: Incorporation into Primitive Bodies and Evolution ; A New Moon: LCROSS, Chandrayaan, and Chang-E-1 ; Water in the Solar System: Moon ; A New Moon: Spectral Constraints on Lunar Crustal Composition ; Characterizing Near-Earth Objects ; A New Moon: Lunar Volcanism and Impact. This CD-ROM contains the contents, program, abstracts, and author indexes for the 41st Lunar and Planetary Science Conference.by Lunar and Planetary Institute, NASA Johnson Space Centerconference co-chairs, Stephen Mackwell, Lunar and Planetary Institute [and] Eileen Stansbery, NASA Johnson Space Center.PARTIAL CONTENTS: Roughness and Radar Polarimetry of Lunar Polar Craters: Testing for Ice Deposits / B.J. Thomson, P.D. Spudis, D.B.J. Bussey, L. Carter, R.L. Kirk, C. Neish, G. Patterson, R.K. Raney, H. Winters, and the Mini-RF Team--Formation of Jupiter's Atmosphere from a Supernova-Contaminated Molecular Cloud / H.B. Throop--Ancient Lunar Dynamo: Absence of Evidence is Not the Evidence of Absence / S.M. Tikoo, B.P. Weiss, J. Buz, I. Garrick-Bethell, T.L. Grove, and J. Gattaccaea--Dark Dunes in Ka'u Desert (Hawaii) as Terrestrial Analogs to Dark Dunes on Mars / D. Tirsch, R.A. Craddock, and R. Jaumann--Mars Ice Condensation and Density Orbiter / T.N. Titus, T. Prettyman, A. Brown, T.I. Michaels, and A. Colaprete--The Atacama Desert Cave Shredder: A Case for Conduction Thermodynamics / T.N. Titus, J.J. Wynne, D. Ruby, and N. Cabrol

Formation of carbide derived carbon coatings on SiC

Control over the structure of materials on nanoscale can open numerous opportunities for the development of materials with controlled properties. Carbon, which is one of the most promising materials for nanotechnology, can be produced by many different methods. One of the most versatile, in terms of a variety of structures demonstrated (graphite, porous amorphous carbon, nanotubes, graphene and diamond), is selective etching of SiC and other carbides. Since the Si atoms are extracted layer by layer, atomic level control of the carbon structures can potentially be achieved without changing the size and shape of the sample. Carbon produced by this method is called Carbide-Derived Carbon (CDC).In this work, CDC formation was studied on single crystalline 3C-SiC whiskers and 6H-SiC wafers by chlorination and vacuum decomposition at high temperatures with the goals to better understand the mechanism of carbide-to-carbon transformation and determine conditions for synthesis of desired carbon structures. The reaction kinetics, morphology and shape conservation were investigated at nanoscale. The transformation mechanism of the SiC surface to carbon was discussed in detail accounting to the effects of processing parameters (temperature, and composition of the environment), and material parameters (surface conditions, surface chemistry, crystal face, etc.). The characterization of the carbon structures was performed by using scanning electron microscopy (SEM), Raman spectroscopy and transmission electron microscopy (TEM).We compared chlorination of SiC whiskers with wet etching and showed that chlorination revealed the dislocations, while wet etching resulted in pagoda-like 3-D nanostructures upon selective etching of stacking faults (SFs). The difference in etching mechanisms was discussed.We determined the processing conditions for controlled synthesis of carbon structures like graphene, graphite and carbon nanotubes (CNTs) on the surface of α-SiC wafers by decomposition in low vacuum. CNT brushes grown on SiC showed good mechanical properties and high oxidation resistance. Moreover, we showed that patterns of graphite and catalyst-free nanotubes can be grown simultaneously and directly on a semiconductor SiC wafer. Devices built in this way can be used in applications from sensing to field emission.Ph.D., Materials Science and Engineering -- Drexel University, 200