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Low temperature carrier transport properties in isotopically controlled germanium

Description: Investigations of electronic and optical properties of semiconductors often require specimens with extremely homogeneous dopant distributions and precisely controlled net-carrier concentrations and compensation ratios. The previous difficulties in fabricating such samples are overcome as reported in this thesis by growing high-purity Ge single crystals of controlled {sup 75}Ge and {sup 70}Ge isotopic compositions, and doping these crystals by the neutron transmutation doping (NTD) technique. The resulting net-impurity concentrations and the compensation ratios are precisely determined by the thermal neutron fluence and the [{sup 74}Ge]/[{sup 70}Ge] ratios of the starting Ge materials, respectively. This method also guarantees unprecedented doping uniformity. Using such samples the authors have conducted four types of electron (hole) transport studies probing the nature of (1) free carrier scattering by neutral impurities, (2) free carrier scattering by ionized impurities, (3) low temperature hopping conduction, and (4) free carrier transport in samples close to the metal-insulator transition.
Date: December 1994
Creator: Itoh, K.
Partner: UNT Libraries Government Documents Department

Neutron transmutation doped (Ntd) germanium thermistors for sub-Mm bolometer applications

Description: The authors report on recent advances in the development of Neutron Transmutation Doped (NTD) semiconductor thermistors fabricated from germanium of natural and controlled isotopic composition. The near ideal doping uniformity which can be achieved with the NTD process, the device simplicity of NTD Ge thermistors and the high performance of cooled junction field effect transistor (FET) preamplifiers have led to the widespread acceptance of these thermal sensors in many radiotelescopes operating on the ground, on high altitude aircraft and on spaceborne satellites. These features also have made possible the development of efficient bolometer arrays which are beginning to produce exciting results.
Date: September 1, 1996
Creator: Haller, E.E.; Itoh, K.M. & Beeman, J.W.
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

Implantation processing of Si: A unified approach to understanding ion-induced defects and their impact

Description: A model is presented to account for the effects of ion-induced defects during implantation processing of Si. It will be shown that processing is quite generally affected by the presence of defect excesses rather than the total number of defects. a defect is considered excess if it represents a surplus locally of one defect type over its compliment. Processing spanning a wide range of implantation conditions will be presented to demonstrate that the majority of the total defects played little or no role in the process. This is a direct result of the ease with which the spatially correlated Frenkel pairs recombine either dynamically or during a post-implantation annealing. Based upon this model, a method will be demonstrated for manipulating or engineering the excess defects to modify their effects. In particular high-energy, self-ions are shown to inject vacancies into a boron implanted region resulting in suppression of transient enhanced diffusion of the dopant.
Date: May 1997
Creator: Holland, O. W. & Roth, E. G.
Partner: UNT Libraries Government Documents Department

Atomic scale characterization of semiconductor interfaces by scanning transmission electron microscopy

Description: Recently, the scanning transmission electron microscope has become capable of forming electron probes of atomic dimensions. Through the technique of Z-contrast imaging, it is now possible to form atomic resolution images with high compositional sensitivity from which atomic column positions can be directly determined. An incoherent image of this nature also allows atomic resolution chemical analysis to be performed, by locating the probe over particular columns or planes seen in the image while electron energy loss spectra are collected. These powerful techniques, combined with atomic-scale calculations, constitute a powerful probe of the structural, kinetic and thermodynamic properties of complex materials. The authors show the direct observation of As segregated to specific sites in a Si grain boundary, and present a candidate model for the structure of the Si/SiO{sub 2} interface.
Date: May 1, 1997
Creator: Pennycook, S.J.; Chisholm, M.F.; Duscher, G.; Maiti, A. & Pantelides, S.T.
Partner: UNT Libraries Government Documents Department

Electronic transport in amorphous carbon

Description: Electronic transport in a-C films has been the subject of considerable debate. In this study, combined stress relaxation and electrical transport studies were used to identify the transport mechanism in a-C films prepared by pulsed-laser deposition. The stress relaxation was modeled by a first-order kinetic reaction involving transformation of 4-fold coordinated carbon atoms to 3-fold coordinated carbon atoms, and the distribution of activation energies for this process was determined. The activation energies were found to range from about 1 eV to over 2 eV, and using these activation energies, the increase in 3-fold carbon concentration with time-temperature annealing was obtained. Conductivity measurements were also performed as a function of time-temperature annealing. It was found that the conductivity of a-C films is exponentially proportional to increases in 3-fold carbon concentration. This result can be explained by thermally activated hopping along carbon 3-fold chains combined with chain-to-chain tunneling. From the data, a typical chain length was estimated to consist of 13 carbon atoms. The heterogeneous nature of the conductivity may explain the spatially localized electron emission which is observed in a-C assuming a tunnel barrier emission model.
Date: December 1, 1997
Creator: Sullivan, J.P. & Friedmann, T.A.
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

Crystalline-as-deposited CaGa{sub 2}S{sub 4}:Ce via low-temperature metal-organic chemical vapor deposition

Description: A metal-organic chemical vapor deposition (MOCVD) process for the growth of crystalline-as-deposited cerium doped calcium thiogallate (CaGa{sub 2}S{sub 4}:Ce) at substrate temperatures less than 600{degree}C has been developed. Blue-green electroluminescence was observed with a luminance of {approx} 2 cd/m{sup 2} at 40 volts above threshold using 60 Hz drive frequency. Threshold voltages were approximately 200 V.
Date: May 1, 1995
Creator: Smith, D.C.; Samuels, J.A.; Espinoza, B.F.; Apen, E.; Peachey, N.M.; Dye, R.C. et al.
Partner: UNT Libraries Government Documents Department

First principles study of substituents in KTaO{sub 3}

Description: The structural properties, energetics, and dynamics of Ca{sup 2+} and Mn{sup 2+} substituents in KTaO{sub 3} are investigated from first principles. It is found that Ca substitutes for both K and Ta ions. Oxygen vacancies bind to isolated Ca ions residing at Ta-sites, causing off-center Ca displacement and forming large dipoles. There is also evidence that oppositely charged defects may cluster together. The calculations predict that the activation energy for dipole reorientation via oxygen vacancy hopping within the first neighbor shell of Ta-substituting Ca or Mn exceeds 2 eV. On the other hand, Mn{sup 2+} substituting at the K-site displaces off center along the (100) direction, also forming a dipole. This dipole can reorient via Mn hopping motion with an activation energy of {approximately} 0.18 eV, in reasonable agreement with experiments. The authors argue that, in general, metal ion hopping at the A-site, not oxygen vacancy hopping, is responsible for the small activation energies found in experiments.
Date: May 17, 2000
Partner: UNT Libraries Government Documents Department

Scanning tunneling microscopy of Si donors in GaAs

Description: Using scanning tunneling microscopy, we have identified and characterized Si donors (Si{sub Ga}) in GaAs located on the (110) surface and in subsurface layers. Si{sub Ga} on the surface shows localized features with characteristic structures in good agreement with a recent theoretical calculation. Si{sub Ga} in subsurface layers appears as delocalized protrusions superimposed on the background lattice, which are interpreted in terms of the modification of the tunneling due to the tip-induced band bending perturbed by the Si{sub Ga} Coulomb potential.
Date: July 1, 1993
Creator: Zheng, J.F.; Weber, E.R.; Liu, X.; Newman, N.; Ogletree, D.F. & Salmeron, M.B.
Partner: UNT Libraries Government Documents Department

Longevity improvement of optically activated, high gain GaAs photoconductive semiconductor switches

Description: The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses at 23A, and over 100 pulses at 1kA. This is achieved by improving the ohmic contacts by doping the semi-insulating GaAs underneath the metal, and by achieving a more uniform distribution of contact wear across the entire switch by distributing the trigger light to form multiple filaments. This paper will compare various approaches to doping the contacts, including ion implantation, thermal diffusion, and epitaxial growth. The device characterization also includes examination of the filament behavior using open-shutter, infra-red imaging during high gain switching. These techniques provide information on the filament carrier densities as well as the influence that the different contact structures and trigger light distributions have on the distribution of the current in the devices. This information is guiding the continuing refinement of contact structures and geometries for further improvements in switch longevity.
Date: March 2, 2000
Partner: UNT Libraries Government Documents Department

Defect and dopant diffusion in ion implanted silcon: an atomic scale simulation approach

Description: We present an atomistic approach to the development of predictive process simulation tools. First principles methods are used to construct a database of defect and dopant energetics. This is used as input for kinetic Monte Carlo simulations of ion implantation and dopant diffusion under a wide variety of technologically relevant conditions. Our simulations are in excellent agreement with annealing experiments on 20-80 keV B implants into Si, and with those on 50 keV Si implants into complex B-doped structures. Our calculations produce novel predictions of the time evolution of the electrically active B fraction during annealing.
Date: October 15, 1998
Creator: Carturla, M J; Diaz de la Rubia, T; Lenosky, T J & Theiss, S K
Partner: UNT Libraries Government Documents Department

Properties of H, O and C in GaN

Description: The electrical properties of the light ion impurities H, O and C in GaN have been examined in both as-grown and implanted material. H is found to efficiently passivate acceptors such as Mg, Ca and C. Reactivation occurs at {ge} 450 C and is enhanced by minority carrier injection. The hydrogen does not leave the GaN crystal until > 800 C, and its diffusivity is relatively high ({approximately} 10{sup {minus}11} cm{sup 2}/s) even at low temperatures (< 200 C) during injection by wet etching, boiling in water or plasma exposure. Oxygen shows a low donor activation efficiency when implanted into GaN, with an ionization level of 30--40 meV. It is essentially immobile up to 1,100 C. Carbon can produce low p-type levels (3 {times} 10{sup 17} cm{sup {minus}3}) in GaN during MOMBE, although there is some evidence it may also create n-type conduction in other nitrides.
Date: April 1, 1996
Creator: Pearton, S.J.; Abernathy, C.R. & Lee, J.W.
Partner: UNT Libraries Government Documents Department

Ion-implantation doping of silicon carbide

Description: Because of their commercial availability in bulk single crystal form, the 6H- and 4H- polytypes of SiC are gaining importance for high-power, high-temperature, and high-frequency device applications. Selective area doping is a crucial processing step in integrated circuit manufacturing. In Si technology, selective area doping is accomplished by thermal diffusion or ion-implantation. Because of the low diffusion coefficients of most impurities in SiC, ion implantation is indispensable in SiC device manufacturing. In this paper the authors present their results on donor, acceptor, and compensation implants in 6H-SiC.
Date: October 1, 1997
Creator: Gardner, J.; Edwards, A.; Rao, M.V.; Papanicolaou, N.; Kelner, G. & Holland, O.W.
Partner: UNT Libraries Government Documents Department

Mechanisms of formation of nonlinear optical light guide structures in metal cluster composites produced by ion beam implantation

Description: Ion implantation has been shown to produce a high density of metal colloids in glasses and crystalline materials. The high-precipitate volume fraction and small size of metal nanoclusters formed leads to values for the third-order susceptibility much greater than those for metal doped solids. This has stimulated interest in use of ion implantation to make nonlinear optical materials. On the other side, LiNbO{sub 3} has proved to be a good material for optical waveguides produced by MeV ion implantation. Light confinement in these waveguides is produced by refractive index step difference between the implanted region and the bulk material. Implantation of LiNbO{sub 3} with MeV metal ions can therefore result into nonlinear optical waveguide structures with great potential in a variety of device applications. The authors describe linear and nonlinear optical properties of a waveguide structure in LiNbO{sub 3}-based composite material produced by silver ion implantation in connection with mechanisms of its formation.
Date: November 1, 1997
Creator: Sarkisov, S.S.; Williams, E.K.; Curley, M.; Smith, C.C.; Ila, D.; Venkateswarlu, P. et al.
Partner: UNT Libraries Government Documents Department

Charge Stripes and Antiferromagnetism in Copper-Oxide Superconductors

Description: Superconducting cuprate compounds are obtained by doping holes into antiferromagnetic insulators. Neutron scattering studies have provided evidence that the doped holes tend to segregate into charge stripes, which act like domain walls between antiferromagnetic regions. The interaction between the spatially segregated holes and the magnetic domains may be responsible for the strong pairing interaction found in the cuprates.
Date: December 31, 1997
Creator: Tranquada, J.M.
Partner: UNT Libraries Government Documents Department

VO{sub 2} precipitates for self-protected optical surfaces

Description: A method for forming crystallographically coherent precipitates of vanadium dioxide in the near-surface region of sapphire and the resulting product is disclosed. Ions of vanadium and oxygen are stoichiometrically implanted into a sapphire substrate (Al{sub 2}O{sub 3}), and subsequently annealed to form vanadium dioxide precipitates in the substrate. The embedded VO{sub 2} precipitates, which are three-dimensionally oriented with respect to the crystal axes of the Al{sub 2}O{sub 3} host lattice, undergo a first-order monoclinic-to-tetragonal (and also semiconducting-to- metallic) phase transition at {minus}77 C. This transformation is accompanied by a significant variation in the optical transmission of the implanted region and results in the formation of an optically active, thermally switchable surface region on Al{sub 2}O{sub 3}.
Date: December 1, 1997
Creator: Gea, Laurence A. & Boatner, Lynn A.
Partner: UNT Libraries Government Documents Department

In-situ x-ray diffraction of layered LiCoO{sub 2}-Type cathode materials

Description: The authors have investigated LiNi{sub 0.8}Co{sub 0.2}O{sub 2} (Sumitomo) and LiNi{sub 5/8}Co{sub 1/4}Mn{sub 1/16}Al{sub 1/16}O{sub 2} (Sandia chemical preparation method) cathode powders via in-situ X-ray Diffraction and Cyclic Voltammetry using a coffee-bag type electrochemical cell. Both cathode materials did not show a monoclinic distortion during de-intercalation but sustained the hexagonal structure up to 4.3 V. The doping of Co into the LiNiO{sub 2} structure appears to stabilize this lattice as the hexagonal structure over the full range of charging (up to 4.3 V). The LiNi{sub 5/8}Co{sub 1/4}Mn{sub 1/16}Al{sub 1/16}O{sub 2} cathode material exhibited a 160 mAh/g capacity (to 4.1 V) on its 1{sup st} cycle, while displaying a much smaller volume change (as compared to LiNi{sub 0.8}Co{sub 0.2}O{sub 2}) during de-intercalation. This reduced overall volume change (2.5 vol%) may have important implications for cycle life of this material.
Date: December 9, 1999
Creator: Rodriguez, M.A.; Ingersoll, D. & Doughty, D.H.
Partner: UNT Libraries Government Documents Department

Ion-implantation doping in SiC and its device applications

Description: The latest ion-implantation results on SiC are presented. The authors have performed nitrogen and phosphorus (N/P) co-implantations to obtain very high n-type carrier concentrations, Si and C bombardments for compensating n-type SiC, and V-implantation for compensating p-type SiC. They have also performed N and Al implantations directly into V-doped semi-insulating 6H-SiC substrates. Vertical p-n junction diodes were made by selective area N, P, and N/P implantations into p-type epitaxial layers grown on 6H-SiC substrates.
Date: October 1, 1997
Creator: Rao, M.V.; Gardner, J.; Edwards, A.; Papanicolaou, N.; Kelner, G.; Holland, O.W. et al.
Partner: UNT Libraries Government Documents Department

Processing challenges for GaN-based photonic and electronic devices

Description: The wide gap materials SiC, GaN and to a lesser extent diamond are attracting great interest for high power/high temperature electronics. There are a host of device processing challenges presented by these materials because of their physical and chemical stability, including difficulty in achieving stable, low contact resistances, especially for one conductivity type, absence of convenient wet etch recipes, generally slow dry etch rates, the high temperatures needed for implant activation, control of suitable gate dielectrics and the lack of cheap, large diameter conducting and semi-insulating substrates. The relatively deep ionization levels of some of the common dopants (Mg in GaN; B, Al in SiC; P in diamond) means that carrier densities may be low at room temperature, and thus contact resistances will be greatly improved provided the metallization is stable and reliable. Some recent work with CoSi{sub x} on SiC and W-alloys on GaN show promise for improved ohmic contacts. The issue of unintentional hydrogen passivation of dopants will also be covered - this leads to strong increases in resistivity of p-SiC and GaN, but to large decreases in resistivity of diamond. Recent work on development of wet etches has found recipes for AlN (KOH), while photochemical etching of SiC and GaN has been reported. In the latter cases p-type materials is not etched, which can be a major liability in some devices. The dry etch results obtained with various novel reactors, including ICP, ECR and LE4 will be compared - the high ion densities in the former techniques produce the highest etch rates for strongly-bonded materials, but can lead to preferential loss of N from the nitrides and therefore to a highly conducting surface. This is potentially a major problem for fabrication of dry etched, recessed gate FET structures.
Date: September 1, 1997
Creator: Pearton, S.J.; Ren, F. & Shul, R.J.
Partner: UNT Libraries Government Documents Department

Surface modification of nickel battery electrodes by cobalt plasma immersion ion implantation and deposition

Description: Nickel hydroxide is the electrochemically active material in the positive electrode of several important rechargeable alkaline-electrolyte batteries. It is believed that divalent Ni(OH){sub 2} is converted to trivalent NiOOH as the electrode is electrochemically oxidized during the battery charging process, and the reverse reaction (electrochemical reduction) occurs during battery discharge, however the details of this process are not completely understood. Because these electrochemical reactions involve surface charge-transfer processes, it is anticipated that surface modification may result in improved battery performance. We used broad-beam metal ion implantation and Metal Plasma Immersion Ion Implantation and Deposition to add cobalt and other species to the nickel electrode surface. The principle of the latter technique is explained in detail. It is shown that implanted and deposited cobalt ions act as a dopant of Ni(OH){sub 2}, and thereby alter its electronic conductivity. This electronic effect promotes lateral growth of NiOOH nodules and more-complete conversion of Ni(OH){sub 2} to NiOOH, which can be interpreted in terms of the nodule growth model. Other dopants such as Au, W, Pb, Ta and Ti{sub 4}O{sub 7} were also tested for suppressing the parasitic oxygen evolution reaction in rechargeable nickel cells.
Date: February 1, 1995
Creator: Anders, S.; Anders, A.; Brown, I.; Kong, F. & McLarnon, F.
Partner: UNT Libraries Government Documents Department

Ion implantation for high performance III-V JFETS and HFETS

Description: Ion implantation has been an enabling technology for realizing many high performance electronic devices in III-V semiconductor materials. We report on advances in ion implantation processing for GaAs JFETs (joint field effect transistors), AlGaAs/GaAs HFETs (heterostructure field effect transistors), and InGaP or InAlP-barrier HFETs. The GaAs JFET has required the development of shallow p-type implants using Zn or Cd with junction depths down to 35 nm after the activation anneal. Implant activation and ionization issues for AlGaAs are reported along with those for InGaP and InAlP. A comprehensive treatment of Si-implant doping of AlGaAs is given based on donor ionization energies and conduction band density-of-states dependence on Al-composition. Si and Si+P implants in InGaP are shown to achieve higher electron concentrations than for similar implants in AlGaAs due to absence of the deep donor level. An optimized P co- implantation scheme in InGaP is shown to increase the implanted donor saturation level by 65%.
Date: June 1, 1996
Creator: Zolper, J.C.; Baca, A.G.; Sherwin, M.E. & Klem, J.F.
Partner: UNT Libraries Government Documents Department