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Synthesis of Pu-Doped Ceramic

Description: Plutonium-doped zircon containing about 10 wt% Pu was synthesized in this cooperative project between Russia and the United States conducted at the V. G. Khlopin Radium Institute. The sol-gel method was used for starting precursor preparation to provide complete mixing of initial components and to avoid dust formation inside the glove-box. The sol-gel process also gives interim Pu stabilization in the form of amorphous zirconium hydrosilicate (AZHS), which is a result of gel solidification. AZHS is a solid and relatively durable material that can be easy converted into crystalline zircon by pressureless sintering, thus avoiding significant radioactive contamination of laboratory equipment. A methanol-aqueous solution of tetraethoxysilane Si(OC2H5)4, Pu-nitrate, and zirconil oxynitrate was prepared in final stoichiometry of zircon (Zr,Pu)SiO4 80 wt% + zirconia (Zr,Pu)O2 20 wt%. Gelation occurred after 90 hours at room temperature. AZHS with excess of zirconia 20 wt% was obtained as an interim calcine product and then it was converted into zircon/zirconia ceramic by sintering at 1490 to 1500°C in air for different time periods. The samples obtained were studied by SRD and ESEM methods. It was found that both zircon yield and zircon cell parameters that are correlated with Pu incorporation depend on sintering time.
Date: September 2, 1998
Creator: Anderson, E. B
Partner: UNT Libraries Government Documents Department

Superconductivity in doped insulators

Description: It is shown that many synthetic metals, including high temperature superconductors are ``bad metals``, with such a poor conductivity that the usual meanfield theory of superconductivity breaks down because of anomalously large classical and quantum fluctuations of the phase of the superconducting order parameter. It is argued that the supression of a first order phase transition (phase separation) by the long-range Coulomb interaction leads to high temperature superconductivity accompanied by static or dynamical charge inhomogeneIty. Evidence in support of this picture for high temperature superconductors is described.
Date: December 31, 1995
Creator: Emery, V.J. & Kivelson, S.A.
Partner: UNT Libraries Government Documents Department

Formation and properties of porous GaAs

Description: Porous structures on n-type GaAs (100) can be grown electrochemically in chloride-containing solutions. Crystallographic etching of the sample is a precursor stage of the attack. Polarization curves reveal the existanece of a critical onset potential for por formation (PFP). PFP is strongly dependent on the doping level of the sample and presence of surface defects. Good agreement between PFP and breakdown voltage of the space charge layer is found. Surface analysis by EDX, AES, and XPS show that the porous structure consists mainly of GaAs and that anion uptake in the structure can only observed after attackhas been initiated. Photoluminescence measurements reveal (under certain conditions) visible light emission from the porous structure.
Date: June 1, 1996
Creator: Schmuki, P.; Lockwood, D.J.; Fraser, J.W.; Graham, M.J. & Isaacs, H.S.
Partner: UNT Libraries Government Documents Department

Effect of point defects and disorder on structural phase transitions

Description: Since the beginning in 1986, the object of this project has been Structural Phase Transitions (SPT) in real as opposed to ideal materials. The first stage of the study has been centered around the role of Point Defects in SPT`s. Our intent was to use the previous knowledge we had acquired in the study of point defects in non-transforming insulators and apply it to the study of point defects in insulators undergoing phase transitions. In non-transforming insulators, point defects, in low concentrations, marginally affect the bulk properties of the host. It is nevertheless possible by resonance or relaxation methods to study the point defects themselves via their local motion. In transforming solids, however, close to a phase transition, atomic motions become correlated over very large distances; there, even point defects far removed from one another can undergo correlated motions which may strongly affect the transition behavior of the host. Near a structural transition, the elastic properties win be most strongly affected so as to either raise or decrease the transition temperature, prevent the transition from taking place altogether, or simply modify its nature and the microstructure or domain structure of the resulting phase. One of the well known practical examples is calcium-stabilized zirconia in which the high temperature cubic phase is stabilized at room temperature with greatly improved mechanical properties.
Date: June 1, 1997
Creator: Toulouse, J.
Partner: UNT Libraries Government Documents Department

Recent developments in Cr{sup 2+}-doped II-VI compound lasers

Description: Transition-metal-doped zinc chalcogenide crystals have recently been investigated as potential mid-IR lasers. Tetrahedrally-coordinated Cr{sup 2+} ions are especially attractive as lasants on account of high luminescence quantum yields for emission in the 2000-3000 nm range. {sup 5}E radiative lifetimes and emission cross sections are respectively {approximately}10 {mu}sec and {approximately}10{sup -18} cm{sup 2}. The associated absorption band peaked at {approximately}1800 nm enables laser-diode pumping of the Cr{sup 2+} systems. Laser demonstrations with ZnS:Cr and ZnSe:Cr (using a MgF{sub 2}:Co{sup 2+} laser pump source) gave slope efficiencies up to 30%. Excited-state-absorption losses appear small, and passive losses dominate. Tuning experiments with a birefringent filter evidence a tuning range covering at least 2280 - 2530 nm. Cr-doped laser samples can be produced by Bridgman growth, seeded physical vapor transport, or diffusion doping.
Date: September 1, 1996
Creator: Page, R.H.; DeLoach, L.D.; Schaffers, K.I., Patel, F.D.; Beach, R.J.; Payne, S.A.; Krupke, W.F. et al.
Partner: UNT Libraries Government Documents Department

Low resistivity ohmic contacts to moderately doped n-GaAs with low temperature processing

Description: A low-temperature process for forming ohmic contacts to moderately doped GaAs has been optimized using a PdGe metallization scheme. Minimum specific contact resistivity of 1.5 {times} 10{sup {minus}6} {minus}cm{sup 2} has been obtained with a low anneal temperature of 250 C. Results for optimizing both time and temperature are reported and compared to GeAu n-GaAs contacts. Material compositions was analyzed by x-ray photoelectron spectroscopy and circuit metal interconnect contact resisitivity to the low-temperature processed PdGe contacts is reported. For the lowest temperature anneals considered, excess Ge on the ohmic contact layer is suspected of degrading interconnect metal contacts, while higher temperature anneals permitted interconnect metal formation with negligible contact resistivity. Atomic force microscopy measurements showed that the PdGe surface morphology is much more uniform than standard GeAu contacts.
Date: December 31, 1994
Creator: Lovejoy, M.L.; Howard, A.J.; Zavadil, K.R.; Rieger, D.J.; Shul, R.J. & Barnes, P.A.
Partner: UNT Libraries Government Documents Department

Next-generation laser for Inertial Confinement Fusion

Description: We report on the progress in developing and building the Mercury laser system as the first in a series of a new generation of diode- pumped solid-state Inertial Confinement Fusion (ICF) lasers at Lawrence Livermore National Laboratory (LLNL). Mercury will be the first integrated demonstration of a scalable laser architecture compatible with advanced high energy density (HED) physics applications. Primary performance goals include 10% efficiencies at 10 Hz and a 1-10 ns pulse with 1 omega energies of 100 J and with 2 omega/3 omega frequency conversion.
Date: September 29, 1997
Creator: Marshall, C.D.; Deach, R.J. & Bibeau, C.
Partner: UNT Libraries Government Documents Department

Investigation of effects of deposition parameters on composition, microstructure,a nd emission of RF sputtered SrS:Eu thin film phosphors

Description: There has been little systematic study of the cause of dead (inactive) layers in II-VI phosphors used in thin film electroluminescent devices. This paper discusses preparation and characterization of rf sputter deposited Eu-doped Sr sulfide (SrS:Eu) thin films for use in a study to determine the cause of the dead layer. (The dead layer`s behavior is likely influenced by thin film composition, crystallinity, and microstructure.) We have deposited SrS:Eu thin films in a repeatable, consistent manner and have characterized properties such as composition, crystallinity, and microstructure as well as photoluminescent (PL) and electroluminescent behavior. The composition was determined using Rutherford backscattering spectrometry and electron microprobe analysis. XRD was used to assess crystalline orientation and grain size, SEM to image thin film microstructure. Measuring the PL decay after subnanosecond laser excitation in the lowest absorption band of the dopant allowed direct measurement of the dopant luminescence efficiency.
Date: December 31, 1996
Creator: Droes, S.R.; Mueller-Mach, R.; Mueller, G.O. & Ruffner, J.A.
Partner: UNT Libraries Government Documents Department

Diamond radiation detectors I. Detector properties for IIa diamond

Description: The detector properties and carrier dynamics of type IIa diamonds are reasonably well understood. The trends in the electron and hole mobilities have been characterized as a function of temperature, impurity content, electric field and carrier density. The carrier lifetimes are coupled through the nitrogen impurity. This leaves us with typical samples with collection distances of 20 to 50 micrometers. The detailed dynamics of the carriers can be modeled using a rate equation analysis. Much progress has been made in understanding the detector properties of diamond, but continued progress has been limited by the geologic processes used to make the material, for example sample size and no synthesis control. CVD diamond promises to eliminate these restrictions.
Date: May 16, 1997
Creator: Kania, D. R.
Partner: UNT Libraries Government Documents Department

Thermodynamic and nonstoichiometric behavior of the lead-doped and lead-free Bi-2212 systems

Description: EMF measurements of oxygen fugacities as a function of stoichiometry were made on pb-doped and Pb-free superconducting ceramics at 700-815 C using oxygen titration. Equations of oxygen partial pressure vs composition and temperature were derived from the EMF measurements. Thermodynamic assessments of the partial molar quantities {Delta}{bar H}(O{sub 2}) and {Delta}{bar S}(O{sub 2}) for Pb-doped Bi- 2212 and Pb-free Bi-2212 indicate that the solid-state decomposition of these Bi cuprates at low oxygen partial pressure can be represented by the diphasic CuO-Cu{sub 2}O system. Comparison of these results with Pb-doped Bi-2223 in powder and silver sheath form is presented.
Date: February 1, 1997
Creator: Tetenbaum, M.; Hash, M.; Tani, B.S.; Luo, J.S. & Maroni, V.A.
Partner: UNT Libraries Government Documents Department

Superconducting and normal-state properties of novel materials

Description: Interest in solid state physics naturally gravitates towards novel systems such as the copper oxide superconductors or the alkali-doped fullerenes. This paper tackles high temperature superconductivity by extension of the BCS theory for ordinary superconductors, in particular, incorporation of anharmonicity in phonon dynamics and anisotropy in electron-phonon coupling. These refinements can account for many anomalous properties of the cuprates. Phonon anharmonicity is consistent with a small isotope effect at optimal doping and a larger isotope effect in suboptimal systems. Anisotropy in the interaction, a plausible consequence of certain anharmonic models, can circumvent objections to electron-phonon coupling based on transport measurements. Such anisotropy is consistent with gap anisotropy and strong temperature dependence of Hall coefficient. In contrast to cuprates, the doped fullerenes appear understandable within the standard model of single electron band theory and BCS theory. Microscopic parameters derivable from transport and critical field measurements yield a self-consistent picture of a disordered Type-2 BCS superconductor. Isotope effects imply that superconductivity is mediated by carbon phonons opposed to alkali atom vibrations. The novel properties of the fullerenes are generally traceable to their microscopic heterogeneity, being a collection of tightly bound but weakly overlapping molecules. Separation of electronic regimes into weak intermolecular overlap and strong carbon-carbon on-ball bonds yields a superconductor with both a large density of states and a high phonon frequency, properties consistent with a relatively high {Tc}. Disordered nature of intermolecular overlap produces a large residual resistivity and a universal dependence to the Hall coefficient. This disorder is also consistent with the anomalously large carbon isotope effect for heterogeneous isotopic substitution.
Date: September 1, 1994
Creator: Crespi, V.H.
Partner: UNT Libraries Government Documents Department

Local vibrational modes of Se-H complexes in AlSb

Description: Using infrared spectroscopy the authors have observed local vibrational modes (LVMs) arising from Se-H complexes in AlSb. At liquid-helium temperatures, hydrogenated AlSb:Se samples have three stretch mode peaks at 1,606.3, 1,608.6, and 1,615.7 cm{sup {minus}1}, whereas deuterated samples have only one peak at 1,173.4 cm{sup {minus}1}. The anomalous splitting of the se-H stretch mode may be explained by a resonance between the stretch mode and two multi-phonon modes. As the temperature or pressure is increased, the stretch mode and multi-phonon modes show anti-crossing behavior.
Date: September 1, 1996
Creator: McCluskey, M.D.; Hsu, L.; Haller, E.E.; Walukiewicz, W. & Becla, P.
Partner: UNT Libraries Government Documents Department

Role of dopant counter-anion functionality in polyaniline salts/blends and implications to morphology

Description: Polyanilines are of particular current interest primarily due to their relative ease of synthesis, low cost and stable conductivity in air. The insulating, polyaniline emeraldine base (PANI-EB) form becomes electrically conducting by preferential protonation or doping the imine nitrogen sites to yield an electrically conducting polyaniline emeraldine salt (PANI-ES). In this paper, wide and small angle X-ray scattering techniques (i.e., WAXS and SAXS) and light microscopy are used to characterize the influence of the dopant`s structure on the morphology of both polyaniline salt and blend. In an attempt to modify the morphology of the PANI-ES, the authors have evaluated a number of doping acids (i.e., methane sulfonic acid (HMSA), butane sulfonic acid (HBSA), dodecyl benzene sulfonic acid (HDBSA) and camphor sulfonic acid (HCSA)) that vary in size and polarity to better understanding the role of the acid in PANI-ES`s morphology and the resulting electrical conductivity. The other goal was to investigate the effect of the counter-anion structure on the nature of the phase separated PANI-ES network. The shape of the PANI-ES network in the host polycaprolactam has important implications on the nature of conduction behavior and the final electrical conductivity of the blend.
Date: April 1, 1997
Creator: Hopkins, A.R.; Rasmussen, P.G.; Basheer, R.A.; Annis, B.K. & Wignall, G.D.
Partner: UNT Libraries Government Documents Department

Cooperative chemical rebonding in the segregation of impurities in silicon grain boundaries

Description: With ab initio calculations the authors show that the experimentally observed large segregation energies of As at Si grain boundaries can be explained by the formation of isolated dimers or ordered chains of dimers of threefold-coordinated As along the cores of grain boundary dislocations. They also find the intriguing possibility that As segregation may drive structural transformation of certain grain boundaries. Recently, they have obtained the first atomic-resolution STEM images of As in a Si grain boundary, consistent with the formation of As dimers. Segregation energy of As dimers was found to be significantly higher in isolated dislocation cores, where larger site-variation in strain than in grain boundaries lead to further lowering of the electronic levels of As deep into the bandgap.
Date: December 1996
Creator: Maiti, A.; Chisholm, M. F.; Pennycook, S. J. & Pantelides, S. T.
Partner: UNT Libraries Government Documents Department

Wave propagation in ordered, disordered, and nonlinear photonic band gap materials

Description: Photonic band gap materials are artificial dielectric structures that give the promise of molding and controlling the flow of optical light the same way semiconductors mold and control the electric current flow. In this dissertation the author studied two areas of photonic band gap materials. The first area is focused on the properties of one-dimensional PBG materials doped with Kerr-type nonlinear material, while, the second area is focused on the mechanisms responsible for the gap formation as well as other properties of two-dimensional PBG materials. He first studied, in Chapter 2, the general adequacy of an approximate structure model in which the nonlinearity is assumed to be concentrated in equally-spaced very thin layers, or 6-functions, while the rest of the space is linear. This model had been used before, but its range of validity and the physical reasons for its limitations were not quite clear yet. He performed an extensive examination of many aspects of the model's nonlinear response and comparison against more realistic models with finite-width nonlinear layers, and found that the d-function model is quite adequate, capturing the essential features in the transmission characteristics. The author found one exception, coming from the deficiency of processing a rigid bottom band edge, i.e. the upper edge of the gaps is always independent of the refraction index contrast. This causes the model to miss-predict that there are no soliton solutions for a positive Kerr-coefficient, something known to be untrue.
Date: December 10, 1999
Creator: Lidorikis, Elefterios
Partner: UNT Libraries Government Documents Department

Optical spectroscopy of InGaN epilayers in the low indium composition regime

Description: Photoluminescence (PL) spectroscopy was carried out on a series of Si-doped bulk InGaN films in the low indium (In) composition regime. Room temperature PL showed a factor of 25 increase in integrated intensity as the In composition was increased from 0 to 0.07. Temperature dependent PL data was fit to an Arrhenius equation to reveal an increasing activation energy for thermal quenching of the PL intensity as the In composition is increased. Time resolved PL measurements revealed that only the sample with highest In (x = 0.07) showed a strong spectral variation in decay time across the T=4K PL resonance, indicative of recombination from localized states at low temperatures. The decay times at room temperature were non-radiatively dominated for all films, and the room temperature (non-radiative) decay times increased with increasing In, from 50--230 psec for x = 0--0.07. The data demonstrate that non-radiative recombination is less effective with increasing In composition.
Date: January 10, 2000
Creator: Crawford, M.H.; Han, J.; Banas, M.A.; Myers, S.M. Jr.; Petersen, G.A. & Figiel, J.J.
Partner: UNT Libraries Government Documents Department

The equilibrium state of hydrogen in gallium nitride: Theory and experiment

Description: Formation energies and vibrational frequencies for H in wurtzite GaN were calculated from density functional theory and used to predict equilibrium state occupancies and solid solubilities for p-type, intrinsic, and n-type material. The solubility of deuterium (D) was measured at 600--800 C as a function of D{sub 2} pressure and doping and compared with theory. Agreement was obtained by reducing the H formation energies 0.2 eV from ab-initio theoretical values. The predicted stretch-mode frequency for H bound to the Mg acceptor lies 5% above an observed infrared absorption attributed to this complex. It is concluded that currently recognized H states and physical processes account for the equilibrium behavior of H examined in this work.
Date: April 17, 2000
Partner: UNT Libraries Government Documents Department

Local Electronic Structure and High Temperature Superconductivity

Description: It is argued that a new mechanism and many-body theory of superconductivity are required for doped correlated insulators. Here they review the essential features of and the experimental support for such a theory, in which the physics is driven by the kinetic energy.
Date: February 8, 1999
Creator: Emery, V. J. & Kivelson, S. A.
Partner: UNT Libraries Government Documents Department

A Method to Improve Activation of Implanted Dopants in SiC

Description: Implantation of dopant ions in SiC has evolved according to the assumption that the best electrical results (i.e., carrier concentrations and mobility) is achieved by using the highest possible processing temperature. This includes implantation at > 600 C followed by furnace annealing at temperatures as high as 1,750 C. Despite such aggressive and extreme processing, implantation suffers because of poor dopant activation, typically ranging between < 2%--50% with p-type dopants represented in the lower portion of this range and n-types in the upper. Additionally, high-temperature processing can led to several problems including changes in the stoichiometry and topography of the surface, as well as degradation of the electrical properties of devices. A novel approach for increasing activation of implanted dopants in SiC and lowering the activation temperature will be discussed. This approach utilizes the manipulation of the ion-induced damage to enhance activation of implanted dopants. It will be shown that nearly amorphous layers containing a small amount of residual crystallinity can be recrystallized at temperatures below 900 C with little residual damage. It will be shown that recrystallization traps a high fraction of the implanted dopant residing within the amorphous phase (prior to annealing) onto substitutional sites within the SiC lattice.
Date: January 16, 2001
Creator: Holland, O.W.
Partner: UNT Libraries Government Documents Department

Selectively excited blue luminescence in heavily Mg doped p-type GaN

Description: The emission at {approx}2.8 eV from heavily doped p-GaN, known as the blue luminescence (BL), has been studied by selective excitation using a dye laser tunable between 2.7-3.0 eV. The peak position and intensity of the BL are found to exhibit an unusual dependence on the excitation photon energy. We have explained our results with a shallow-donor and deep-acceptors pair recombination model which includes potential fluctuations induced by heavy doping. We found a ''critical energy'' of {approx}2.8 eV for the BL. Electron-hole pairs with energies above this energy are able to achieve quasi-thermal equilibrium while those with energies below 2.8 eV are strongly ''localized''.
Date: December 31, 2000
Creator: Colton, John S. & Yu, Peter Y.
Partner: UNT Libraries Government Documents Department