42 Matching Results

Search Results

Advanced search parameters have been applied.

GaN Metal Oxide Semiconductor Field Effect Transistors

Description: A GaN based depletion mode metal oxide semiconductor field effect transistor (MOSFET) was demonstrated using Ga{sub 2}O{sub 3}(Gd{sub 2}O{sub 3}) as the gate dielectric. The MOS gate reverse breakdown voltage was > 35V which was significantly improved from 17V of Pt Schottky gate on the same material. A maximum extrinsic transconductance of 15 mS/mm was obtained at V{sub ds} = 30 V and device performance was limited by the contact resistance. A unity current gain cut-off frequency, f{sub {tau}}, and maximum frequency of oscillation, f{sub max} of 3.1 and 10.3 GHz, respectively, were measured at V{sub ds} = 25 V and V{sub gs} = {minus}20 V.
Date: March 2, 1999
Creator: Ren, F.; Pearton, S.J.; Abernathy, C.R.; Baca, A.; Cheng, P.; Shul, R.J. et al.
Partner: UNT Libraries Government Documents Department

Effect of Temperature on GaGdO/GaN Metal Oxide Semiconductor Field Effect Transistors

Description: GaGdO was deposited on GaN for use as a gate dielectric in order to fabricate a depletion metal oxide semiconductor field effect transistor (MOSFET). This is the fmt demonstration of such a device in the III-Nitride system. Analysis of the effect of temperature on the device shows that gate leakage is significantly reduced at elevated temperature relative to a conventional metal semiconductor field effeet transistor (MESFET) fabricated on the same GaN layer. MOSFET device operation in fact improved upon heating to 400 C. Modeling of the effeet of temperature on contact resistance suggests that the improvement is due to a reduction in the parasitic resistances present in the device.
Date: October 14, 1998
Creator: Abernathy, C.R.; Baca, A.; Chu, S.N.G.; Hong, M.; Lothian, J.R.; Marcus, M.A. et al.
Partner: UNT Libraries Government Documents Department

Nuclear fuels technologies fiscal year 1996 research and research development test results

Description: During fiscal year 1996, the Department of Energy`s Office of Fissile Materials Disposition (OFMD) funded Los Alamos National Laboratory (LANL) to investigate issues associated with the fabrication of plutonium from dismantled weapons into mixed-oxide (MOX) nuclear fuel for disposition in nuclear power reactors. These issues can be divided into two main categories: issues associated with the fact that the plutonium from dismantled weapons contains gallium, and issues associated with the unique characteristics of the PuO{sub 2} produced by the dry conversion process that OFMD is proposing to convert the weapons material. Initial descriptions of the experimental work performed in fiscal year 1996 to address these issues can be found in Nuclear Fuels Technologies Fiscal Year 1996 Research and Development Test Matrices`. However, in some instances the change in programmatic emphasis towards the Parallex program either altered the manner in which some of these experiments were performed (i.e., the work was done as part of the Parallex fabrication development and not as individual separate-effects tests as originally envisioned) or delayed the experiments into Fiscal Year 1997. This report reviews the experiments that were conducted and presents the results. 7 figs., 14 tabs.
Date: November 8, 1996
Creator: Beard, C.A.; Blair, H.T.; Buksa, J.J. & Butt, D.P.
Partner: UNT Libraries Government Documents Department

An assessment of the validity of cerium oxide as a surrogate for plutonium oxide gallium removal studies

Description: Methods for purifying plutonium metal have long been established. These methods use acid solutions to dissolve and concentrate the metal. However, these methods can produce significant mixed waste, that is, waste containing both radioactive and chemical hazards. The volume of waste produced from the aqueous purification of thousands of weapons would be expensive to treat and dispose. Therefore, a dry method of purification is highly desirable. Recently, a dry gallium removal research program commenced. Based on initial calculations, it appeared that a particular form of gallium (gallium suboxide, Ga{sub 2}O) could be evaporated from plutonium oxide in the presence of a reducing agent, such as small amounts of hydrogen dry gas within an inert environment. Initial tests using ceria-based material (as a surrogate for PuO{sub 2}) showed that thermally-induced gallium removal (TIGR) from small samples (on the order of one gram) was indeed viable. Because of the expense and difficulty of optimizing TIGR from plutonium dioxide, TIGR optimization tests using ceria have continued. This document details the relationship between the ceria surrogate tests and those conducted using plutonia.
Date: March 1, 1999
Creator: Kolman, D.G.; Park, Y.; Stan, M.; Hanrahan, R.J. Jr. & Butt, D.P.
Partner: UNT Libraries Government Documents Department

The Study of Phosphors Efficiency and Homogeneity using a Nuclear Microprobe

Description: Ion Beam Induced Luminescence (IBIL) and Ion Beam Induced Charge Collection (IBICC) have been applied in the study of the luminescence emission efficiency and investigation of the homogeneity of the luminescence emission in phosphors. The IBIL imaging was performed by using sharply focused ion beams or broad/partially-focused ion beams. The luminescence emission homogeneity in samples was examined to reveal possible distributed crystal-defects that may lead to the inhomogeneity of the luminescence emission in samples.The purpose of the study is to search for suitable luminescent thin films that have high homogeneity of luminescence emission, large IBIL efficiency under heavy ion excitation, and can be placed as a thin layer on the top of microelectronic devices to be analyzed with Ion Photon Emission Microscopy (IPEM). The emission yield was found to be low for organic materials, due to saturation of the light output dependence on the energy deposition of heavy ions. The emission yield of a typical Bicron plastic scintillator is about 70 photons/ion/micron. Inorganic materials may have higher IBIL yield under high-energy and heavy-ion excitation, but the challenging problem is the inhomogeneity of the IBIL emission. The IBIL image techniques are applied in the investigation of the homogeneity of a GaN epitaxial thin film, a zircon single crystal and a thin layer coated by Thiogallate(EuII) ceramic.
Date: December 8, 2000
Partner: UNT Libraries Government Documents Department

Refractive Index and Hygroscopic Stability of Al(x)Ga(1-x)As Native Oxides

Description: The authors present prism coupling measurements on Al{sub x}Ga{sub 1{minus}x}As native oxides showing the dependence of refractive index on composition (0.3 {le} x {le} 0.97), oxidation temperature (400 {le} T {le} 500), and carrier gas purity. Index values range from n = 1.490 (x = 0.9, 400) to 1.707 (x = 0.3, 500 C). The oxides are shown to adsorb moisture, increasing their index by up to 0.10 (7%). Native oxides of Al{sub x}Ga{sub 1{minus}x}As (x {le} 0.5) have index values up to 0.27 higher and are less hygroscopic when prepared with a small amount of O{sub 2} in the N{sub 2} + H{sub 2}O process gas. The higher index values are attributed to a greater degree of oxidation of the Ga in the film.
Date: July 8, 1999
Creator: Blum, O.; Epstein, R.J.; HOu, H.; Hall, D.C.; Kou, L.; Luo, Y. et al.
Partner: UNT Libraries Government Documents Department

Controlled VLS Growth of Indium, Gallium and Tin Oxide Nanowiresvia Chemical Vapor Transport

Description: We utilized a vapor-liquid-solid growth technique to synthesize indium oxide, gallium oxide, and tin oxide nanowires using chemical vapor transport with gold nanoparticles as the catalyst. Using identical growth parameters we were able to synthesize single crystal nanowires typically 40-100 nm diameter and more than 10-100 microns long. The products were characterized by means of XRD, SEM and HRTEM. All the wires were grown under the same growth conditions with growth rates inversely proportional to the source metal vapor pressure. Initial experiments show that different transparent oxide nanowires can be grown simultaneously on a single substrate with potential application for multi-component gas sensors.
Date: March 13, 2006
Creator: Johnson, M.C.; Aloni, S.; McCready, D.E. & Bourret-Courchesne, E.D.
Partner: UNT Libraries Government Documents Department

Characterization of novel powder and thin film RGB phosphors for field emissions display application

Description: The spectral response, brightness and outgassing characteristics of new, low-voltage phosphors for application in field-emission flat-panel displays, are presented. The tested phosphor materials include combustion synthesized powders and thin films prepared by RF-diode or magnetron sputtering, laser ablation and molecular beam epitaxy. These cathodoluminescent materials are tested with e-beam excitation at currents up to 50 {mu}A within the 200-2000V (e.g. {open_quotes}low-voltage{close_quotes}) and 3-8 kV (e.g. {open_quotes}medium voltage{close_quotes}) ranges. The spectral coordinates are compared to commercial low-voltage P22 phosphors. Phosphor outgassing, as a function of time is measured with a residual gas analyzer at fixed 50 {mu}A beam current in the low-voltage range. We find that levels of outgassing stabilize to low values after the first few hours of excitation. The desorption rates measured for powder phosphor layers with different thickness are compared to desorption from thin films.
Date: December 31, 1996
Creator: Chakhovskoi, A.G.; Hunt, C.E.; Malinowski, M.E.; Felter, T.E. & Talin, A.A.
Partner: UNT Libraries Government Documents Department

Enhanced Luminescence in Epitaxial Oxide Thin-Film Phosphors

Description: Undoped and Mn-doped ZnGa{sub 2}O{sub 4} thin-film phosphors were grown using pulsed laser ablation on (100) MgO single crystal and glass substrates. X-ray results showed the films on (100) MgO are well aligned both out-of plane and in-plane. Epitaxial films show superior photoluminescent intensity as compared to randomly oriented polycrystalline films, indicating that intragranular crystallinity strongIy influences luminescent properties. Li-doped ZnGa{sub 2}O{sub 4} exhibited significantly enhanced photoluminescence intensity.
Date: November 8, 1999
Creator: Lee, Y.E.; Norton, D.P.; Budai, J.D.; Park, C.; Kim, M.; Pennycook, S.J. et al.
Partner: UNT Libraries Government Documents Department

Dielectrics for GaN based MIS-diodes

Description: GaN MIS diodes were demonstrated utilizing AlN and Ga{sub 2}O{sub 3}(Gd{sub 2}O{sub 3}) as insulators. A 345 {angstrom} of AlN was grown on the MOCVD grown n-GaN in a MOMBE system using trimethylamine alane as Al precursor and nitrogen generated from a wavemat ECR N2 plasma. For the Ga{sub 2}O{sub 3}(Gd{sub 2}O{sub 3}) growth, a multi MBE chamber was used and a 195 {angstrom} oxide is E-beam evaporated from a single crystal source of Ga{sub 5}Gd{sub 3}O{sub 12}. The forward breakdown voltage of AlN and Ga{sub 2}O{sub 3}(Gd{sub 2}O{sub 3}) diodes are 5V and 6V, respectively, which are significantly improved from {approximately} 1.2 V of schottky contact. From the C-V measurements, both kinds of diodes showed good charge modulation from accumulation to depletion at different frequencies. The insulator GaN interface roughness and the thickness of the insulator were measured with x-ray reflectivity.
Date: February 1998
Creator: Ren, F.; Abernathy, C. R. & MacKenzie, J. D.
Partner: UNT Libraries Government Documents Department

Garnet phosphors prepared via hydrothermal synthesis

Description: This project studied hydrothermal synthesis as a route to producing green-emitting cathodoluminescent phosphorus isostructural with yttrium aluminum garnet (Y{sub 3}Al{sub 5}O{sub 12}, or YAG). Aqueous precipitation of Y, Gd, Al, Ga, and Tb salts produced amorphous gels, which were heated with water at 600 C and 3,200 bar to produce crystalline YAG:Tb, Y{sub 3}Ga{sub 5}O{sub 12}:Tb, Y{sub 3}Al{sub 3}Ga{sub 2}O{sub 12}:Tb, and Gd{sub 3}Ga{sub 5}O{sub 12}:Tb powders. Process parameters were identified that yielded submicron YAG:Tb and Y{sub 3}Ga{sub 5}O{sub 12}:Tb powders without grinding. Cathodoluminescent efficiencies were measured as functions of power density at 600 V, using both the hydrothermal garnets and identical phosphor compositions synthesized at high temperatures. Saturation behavior was independent of synthetic technique, however, the hydrothermal phosphorus were less susceptible to damage (irreversible efficiency loss) at very high power densities (up to 0.1 W/cm{sup 2}). The fine grain sizes available with hydrothermal synthesis make it an attractive method for preparing garnet phosphorus for field emission, projection, and head-up displays.
Date: May 1, 1996
Creator: Phillips, M.L.F.; Walko, R.J. & Shea, L.E.
Partner: UNT Libraries Government Documents Department

Peculiarities of spin reorientation in a thin YIG film.

Description: The issue of magnetic orientation transitions in thin films combines interesting physics and importance for applications. We study the magnetic transition and phase diagram of a 0.1{micro}m thick (YLaGd){sub 3}(FeGa){sub 5}O{sub 12} films grown on GGG substrate by liquid phase epitaxy. Observed transitions are compared with those in BiGa:TmIG thin films, studied in previous work by one of the authors. A general picture of orientation transitions in thin films of substituted YIG is discussed.
Date: June 28, 2002
Creator: Bazaliy, Ya. B.; Tsymbal, L. T.; Linnik, A. I.; Dan'shin, N. K.; Izotov, A. I. & Wigen, P. E.
Partner: UNT Libraries Government Documents Department

Collector for recovering gallium from weapons plutonium

Description: Currently, the separation of gallium from weapons plutonium involves the use of aqueous processing using either solvent extraction of ion exchange. However, this process generates significant quantities of liquid radioactive wastes. A Thermally Induced Gallium Removal process, or TIGR, developed by researchers at Los Alamos National Laboratories, is a simpler alternative to aqueous processing. This research examined this process, and the behavior of gallium suboxide, a vapor that is swept away by passing hydrogen/argon over gallium trioxide/plutonium oxide heated at 1100 C during the TIGR process. Through experimental procedures, efforts were made to prevent the deposition of corrosive gallium onto furnace and vent surfaces. Experimental procedures included three options for gallium removal and collection: (1) collection of gallium suboxide through use of a cold finger; (2) collection by in situ air oxidation; and (3) collection of gallium on copper. Results conclude all three collection mechanisms are feasible. In addition, gallium trioxide exists in three crystalline forms, and each form was encountered during each experiment, and that each form will have a different reactivity.
Date: September 1, 1998
Creator: Philip, C. V.; Anthony, R. G. & Chokkaram, S.
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of charge-substituted garnets YCaLnGa{sub 5}O{sub 12} (Ln = Ce,Pr,Tb)

Description: A low temperature method is described for the preparation of the new garnet compounds YCaLnGa{sub 5}O{sub l2} (Ln=Ce, Pr, Th). In this set of compounds (Ca{sup 2+} + Ln{sup 4+}) replaces 2 Y{sup 3+} in the parent gallium based garnet Y{sub 3}Ga{sub 5}O{sub l2} in order to stabilize as effectively as possible the Ln{sup 4+} species in the eight-coordinate ``A`` site of the garnet structure. Characterization of the oxides by x-ray powder diffraction and thermogravimetric analysis is discussed with regard to the structural relationship of the substituted compound to the parent material. The tetravalent ions Pr{sup 4+} and Tb{sup 4+} exhibit increased thermal stability in reducing conditions as compared to the Ln{sup 4+} states in the fluorite (LnO{sub 2}) and perovskite (BaLnO{sub 3}) type structures. This result is discussed with reference to the complex crystal chemistry of these systems.
Date: September 1, 1993
Creator: Gramsch, S. A. & Morss, L. R.
Partner: UNT Libraries Government Documents Department

Synthesis and properties of new family of superconducting copper oxides based on GaO layers

Description: We have discovered the first layered superconducting copper oxide with small, fixed oxidation state cations separating the conducting CuO{sub 2} planes. This material, GaSr{sub 2}Y{sub 1-x}Ca{sub x}Cu{sub 2}O{sub 7}, is similar to YBa{sub 2}Cu{sub 3}O{sub 7} with the square planar copper chains replaced by chains of edge-shared GaO{sub 4} tetrahedra. Thus, oxidation can occur only for the copper ion located in square pyramidal coordination in the CuO{sub 2} plane. The undoped parent compound, x = 0, does not show magnetic order above 4K, probably due to the presence of the thick, ionic region separating the CuO{sub 2} planes. However, this ionic region does not suppress high {Tc} superconductivity ({approximately}70K) for the doped compositions.
Date: December 31, 1991
Creator: Dabrowski, B.; Zhang-McCoy, V.; Radaelli, P.; Mitchell, A. W.; Hinks, D. G.; Vaughey, J. T. et al.
Partner: UNT Libraries Government Documents Department

X-ray Photoelectron Spectroscopy ofGaP_{1-x}N_x Photocorroded as a Result of Hydrogen Productionthrough Water Electrolysis

Description: Photoelectrochemical (PEC) cells produce hydrogen gas through the sunlight driven electrolysis of water. By extracting hydrogen and oxygen from water and storing solar energy in the H-H bond, they offer a promising renewable energy technology. Addition of dilute amounts of nitrogen to III-V semiconductors has been shown to dramatically increase the stability of these materials for hydrogen production. In an effort to learn more about the origin of semiconductor photocorrosion in PEC cells, three samples of p-type GaP with varying levels of nitrogen content (0%, 0.2%, 2%) were photocorroded and examined by X-ray Photoelectron Spectroscopy (XPS). GaPN samples were observed to be more efficient during the hydrogen production process than the pure GaP samples. Sample surfaces contained gallium oxides in the form of Ga{sub 2}O{sub 3} and Ga(OH){sub 3} and phosphorus oxide (P{sub 2}O{sub 5}), as well as surface oxides from exposure to air. A significant shift in intensity from bulk to surface peaks dramatic nitrogen segregation to the surface during photoelectrochemical hydrogen production. Further investigations, including using a scanning electron microscope to investigate sample topography and inductively coupled plasma mass spectroscopy (ICP-MS) analysis for solution analyses, are under way to determine the mechanism for these changes.
Date: September 27, 2006
Creator: Mayer, Marie A. & /Illinois U., Urbana /SLAC
Partner: UNT Libraries Government Documents Department

Novel High Temperature Materials for In-Situ Sensing Devices

Description: The overriding goal of this project was to develop gas sensor materials and systems compatible with operation at temperatures from 500 to 700 C. Gas sensors operating at these temperatures would be compatible with placement in fossil-energy exhaust streams close to the combustion chamber, and therefore have advantages for process regulation, and feedback for emissions controls. The three thrusts of our work included investigating thin film gas sensor materials based on metal oxide materials and electroceramic materials, and also development of microhotplate devices to support the gas sensing films. The metal oxide materials NiO, In{sub 2}O{sub 3}, and Ga{sub 2}O{sub 3} were investigated for their sensitivity to H{sub 2}, NO{sub x}, and CO{sub 2}, respectively, at high temperatures (T > 500 C), where the sensing properties of these materials have received little attention. New ground was broken in achieving excellent gas sensor responses (>10) for temperatures up to 600 C for NiO and In{sub 2}O{sub 3} materials. The gas sensitivity of these materials was decreasing as temperatures increased above 500 C, which indicates that achieving strong sensitivities with these materials at very high temperatures (T {ge} 650 C) will be a further challenge. The sensitivity, selectivity, stability, and reliability of these materials were investigated across a wide range of deposition conditions, temperatures, film thickness, as using surface active promoter materials. We also proposed to study the electroceramic materials BaZr{sub (1-x)}Y{sub x}O{sub (3-x/2)} and BaCe{sub (2-x)}Ca{sub x}S{sub (4-x/2)} for their ability to detect H{sub 2}O and H{sub 2}S, respectively. This report focuses on the properties and gas sensing characteristics of BaZr{sub (1-x)}Y{sub x}O{sub (3-x/2)} (Y-doped BaZrO{sub 3}), as significant difficulties were encounter in generating BaCe{sub (2-x)}Ca{sub x}S{sub (4-x/2)} sensors. Significant new results were achieved for Y-doped BaZrO{sub 3}, including sensitivities of more than 60 atm{sup -1} for H{sub 2}O vapor at ...
Date: December 31, 2009
Creator: Solzbacher, Florian; Virkar, Anil; Rieth, Loren; Kannan, Srinivasan; Chen, Xiaoxin & Steinebach, Hannwelm
Partner: UNT Libraries Government Documents Department

Phosphor synthesis routes and their effect on the performance of garnet phosphorus at low-voltages

Description: Garnet phosphors have potential for use in field emission displays (FEDs). Green-emitting Gd{sub 3}Ga{sub 5}O{sub 12}:Tb (GGG:Tb) and Y{sub 3}Al{sub 5}O{sub 12}:Tb (YAG:Tb) are possible alternatives to ZnO:Zn, because of their excellent resistance to burn, low-voltage efficiency, (3.5 lm/W from GGG:Tb at 800 V), and saturation resistance at high power densities. Hydrothermal and combustion synthesis techniques were employed to improve the low-voltage efficiency of YAG:Tb, and Y{sub 3}Ga{sub 5}O{sub 12}:Tb (YGG:Tb). Synthetic technique did not affect low-voltage (100--1,000 V) efficiency, but affected the particle size, morphology, and burn resistance. The small particle size phosphors obtained via hydrothermal (<1 {micro}m) and combustion reactions (<1 {micro}m) would benefit projection TV, high-definition TV (HDTV), and heads-up displays (HUDs), where smaller pixel sizes are required for high resolution.
Date: July 1, 1996
Creator: Shea, L.E.; McKittrick, J. & Phillips, M.L.F.
Partner: UNT Libraries Government Documents Department

Mechanisms of thermal induced gallium removal (TIGR) from plutonium dioxide. Revision 1

Description: This study was initiated in order to determine the advantages of using a mixed-bed rather than a fixed-bed reactor (i.e. furnace) for separation of gallium from PuO{sub 2} by the Thermal Induced Gallium Removal (TIGR) process. The TIGR process is based upon vaporization of gallium suboxide (Ga{sub 2}O). from essentially nonvolatile PuO{sub 2}. The gallium suboxide is formed by passing a reducing gas (i.e. hydrogen) over the PuO{sub 2} particles. Several mechanisms are involved in the reduction and convective vaporization of the gallium suboxide. If the mass transfer of the gallium suboxide across the solid to gas interface significantly affects the processing time, it may be advantageous to use a mixed-bed reactor rather than a fixed-bed reactor. However, due to the difficulty of handling PuO{sub 2} powder, a mixed-bed reactor should be used only if significant advantages can be demonstrated. Based on available data, the results of this study provide strong evidence that a mixed-bed reactor (i.e. furnace) would provide little advantage over a fixed-bed reactor. This is due to the conclusion that the mechanism of internal gallium diffusion within the particle has the predominant affect on the processing time. This is an important conclusion since the use of a mixed-bed would require development of more complex hardware than for a fixed-bed.
Date: August 7, 1998
Creator: DeMuth, S.F.
Partner: UNT Libraries Government Documents Department

Gallium-cladding compatibility testing plan. Phases 1 and 2: Test plan for gallium corrosion tests; Revision 2

Description: This test plan is a Level-2 document as defined in the Fissile Materials Disposition Program Light-Water-Reactor Mixed-Oxide Fuel Irradiation Test Project Plan. The plan summarizes and updates the projected Phases 1 and 2 Gallium-Cladding compatibility corrosion testing and the following post-test examination. This work will characterize the reactions and changes, if any, in mechanical properties that occur between Zircaloy clad and gallium or gallium oxide in the temperature range 30--700 C.
Date: May 1, 1998
Creator: Wilson, D.F. & Morris, R.N.
Partner: UNT Libraries Government Documents Department


Description: A process for the separation of gallium oxide from plutonium dioxide using a ''dry'' process has been developed. The process uses a reducing gas to generate a volatile gallium specie which is collected downstream. The effects of temperature, duration, flow rate, and sample size have been examined. Results indicate that temperature plays a strong role in the efficacy of gallium removal. Other variables have a much smaller effect on gallium removal efficiency. Gallium removal to approximately 1 ppm (atomic) has been observed. Gallium removal to sub-ppm levels appears feasible based on results-to-date.
Date: September 1, 1999
Creator: KOLMAN, D.; GRIEGO, M. & AL, ET
Partner: UNT Libraries Government Documents Department

Investigations in gallium removal

Description: Gallium present in weapons plutonium must be removed before it can be used for the production of mixed-oxide (MOX) nuclear reactor fuel. The main goal of the preliminary studies conducted at Texas A and M University was to assist in the development of a thermal process to remove gallium from a gallium oxide/plutonium oxide matrix. This effort is being conducted in close consultation with the Los Alamos National Laboratory (LANL) personnel involved in the development of this process for the US Department of Energy (DOE). Simple experiments were performed on gallium oxide, and cerium-oxide/gallium-oxide mixtures, heated to temperatures ranging from 700--900 C in a reducing environment, and a method for collecting the gallium vapors under these conditions was demonstrated.
Date: November 1, 1997
Creator: Philip, C.V.; Pitt, W.W. & Beard, C.A.
Partner: UNT Libraries Government Documents Department

The effect of temperature, growth kinetics, and substrate on the microstructure of RF off-axis sputter deposited YBa{sub 2}Cu{sub 3}O{sub 7} thin films

Description: YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) thin films were grown on single crystal substrates by RF off-axis sputter deposition under different growth condition, systematically varied to change the kinetic and thermodynamic processes that determine both the film microstructure and its crystallographic orientation. The effect of substrate temperature, position, and material, total chamber pressure, and RF power on the final film structure was examined. The growth matrix was bounded by temperatures from 640 to 780C, pressures from 50 to 200 mtorr, and power from 50 to 100 watts. Higher growth rates were achieve by increasing the power setting, lowering the total pressure, and moving the substrate farther into the plasma. Care was taken to stay within these parametric stability limits defining the 123-YBCO growth regime. The resulting films were characterized, primarily, by scanning tunneling and atomic force microscopy, x-ray diffraction, and eddy-current measurements. At the extremes of the growth conditions used in this study, ex-situ examination of the films revealed that they spanned the extremes from all c-axis normal to all a-axis normal material, with mixed a-/c-axis normal material in between extremes in deposition parameter limits, with the mixed films containing a-axis material overlaying a c-axis layer. Smooth single orientation films, suitable for multilayer device structure, could be routinely obtained by controlled growth. Highest surface roughness measurements were usually due to incomplete or non uniform a-axis coverage on c-axis films. Combined data obtained from different studies showed that lower temperatures and higher rates favored a-axis growth under the conditions used in these studies. The YBCO films were more unstable to a cross-over from c-axis to a-a-axis growth when deposited on NdGaO{sub 3} and LaAIO{sub 3} than on SrTiO{sub 3}. Under conditions favoring c-axis perpendicular growth, higher temperatures and rates resulted in rounder islands.
Date: July 1, 1995
Creator: Hawley, M.; Houlton, R.J.; Garzon, F.H. & Raistrick, I.D.
Partner: UNT Libraries Government Documents Department


Description: X-ray fluorescence spectroscopy is a widely used method for determining the electronic configuration and local structure of dilute species with high sensitivity. In the dilute limit, and for thin films, the X-ray fluorescence signal is directly proportional to the atomic sub-shell absorption coefficient. However, for concentrated samples, the well-documented self-absorption effect often leads to the severe suppression of XANES (X-ray Absorption Near-Edge Structure) and EXAFS (Extended X-ray Absorption Fine-Structure) amplitudes. Thus to recover the real value of the sub-shell absorption coefficient, it is important to apply correction procedures to the measured fluorescence spectra. In this paper, we describe a new straightforward method to correct for self-absorption effects (the difference in the measured fluorescence signal compared to that of the true sub-shell photoabsorption coefficient) in XANES and EXAFS fluorescence measurements. Using a variety of sample and detector configurations, this method is used to extract the sub-shell absorption coefficient on elemental nickel and thick single-crystals of Gd{sub 3}Ga{sub 5}O{sub 12} and LaAlO{sub 3}.
Date: August 2, 2004
Creator: ABLETT, J.M.; WOICIK, J.C. & KAO, C.C.
Partner: UNT Libraries Government Documents Department