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Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization

Description: Semi-conductor industry relies heavily on silicon (Si). However, Si is not a direct-band gap semi-conductor. Consequently, Si does not possess great versatility for multi-functional applications in comparison with the direct band-gap III-V semi-conductors such as GaAs. To bridge this gap, what is ideally required is a semi-conductor material system that is based on silicon, but has significantly greater versatility. While sparsely studied, the semi-conducting silicides material systems offer great potential. Thus, I focused on the growth and structural characterization of ruthenium silicide and osmium silicide material systems. I also characterized iron silicon germanide films using extended x-ray absorption fine structure (EXAFS) to reveal phase, semi-conducting behavior, and to calculate nearest neighbor distances. The choice of these silicides material systems was due to their theoretically predicted and/or experimentally reported direct band gaps. However, the challenge was the existence of more than one stable phase/stoichiometric ratio of these materials. In order to possess the greatest control over the growth process, molecular beam epitaxy (MBE) has been employed. Structural and film quality comparisons of as-grown versus annealed films of ruthenium silicide are presented. Structural characterization and film quality of MBE grown ruthenium silicide and osmium silicide films via in situ and ex situ techniques have been done using reflection high energy electron diffraction, scanning tunneling microscopy, atomic force microscopy, cross-sectional scanning electron microscopy, x-ray photoelectron spectroscopy, and micro Raman spectroscopy. This is the first attempt, to the best of our knowledge, to grow osmium silicide thin films on Si(100) via the template method and compare it with the regular MBE growth method. The pros and cons of using the MBE template method for osmium silicide growth are discussed, as well as the structural differences of the as-grown versus annealed films. Future perspectives include further studies on other semi-conducting silicides material systems in terms ...
Date: December 2013
Creator: Elmarhoumi, Nader M.
Partner: UNT Libraries

Oxidation Behavior and Chlorination Treatment to Improve Oxidation Resistance of Nb-Mo-Si-B Alloys

Description: This thesis is written in an alternate format. The thesis is composed of a general introduction, two original manuscripts, and a general conclusion. References cited within each chapter are given at the end of each chapter. The general introduction starts with the driving force behind this research, and gives an overview of previous work on boron doped molybdenum silicides, Nb/Nb{sub 5}Si{sub 3} composites, boron modified niobium silicides and molybdenum niobium silicides. Chapter 2 focuses on the oxidation behavior of Nb-Mo-Si-B alloys. Chapter 3 contains studies on a novel chlorination technique to improve the oxidation resistance of Nb-Mo-Si-B alloys. Chapter 4 summarizes the important results in this study.
Date: December 19, 2004
Creator: Behrani, Vikas
Partner: UNT Libraries Government Documents Department

Incommensurate spin resonance in URu2Si2

Description: We propose to search for the spin resonance in URu{sub 2}Si{sub 2} at {omega}{sub res} = 4-6meV at the incommensurate wavector Q* = (1 {+-} 0.4, 0, 0). We expect that this spin resonance will set in at temperatures below HO transition and the intensity of this peak will scale as {approx} {Delta}{sub HO} {approx} (T{sub HO} - T). The resonance peak is know to occur in the states with superconducting gap and results in the gapping of the electronic spectrum add ref on SrruO and cel 15. In the case of HO the gap {Delta}{sub HO} results in the partially gapped electron spectrum. That appears to be a sufficient condition, as shown by Wiebe et al to produce a gap in spin excitation spectrum. In addition, we predict a peak in the spin excitation spectrum, as spectral weight redistribution produces the resonance feature. To the best of our knowledge, if the predicted resonance peak indeed occurs, it would be the first case where the spin resonance occurs at an incommensurate vector Q*.
Date: January 1, 2008
Creator: Balatsky, A V; Chantis, A; Dahal, Hari; Zhu, J X & Parker, David
Partner: UNT Libraries Government Documents Department

Quadrupolar effects in PrCu{sub 2}Si{sub 2}

Description: As part of a systematic study of the crystal field (CF) potential in RCu{sub 2}Si{sub 2} compounds (R = Ce, Pr, Nd, Tb, Ho, Er) using inelastic neutron scattering, we have determined the CF level scheme and potential in the antiferromagnet PrCu{sub 2}Si{sub 2} both above and below T{sub N}. There have been recent speculations that the quadrupole moment of PrCu{sub 2}Si{sub 2} has been quenched by the quadrupolar Kondo effect. Using the CF potential derived from our results, we have calculated the quadrupole moment Q{sub 2} of PrCu{sub 2}Si{sub 2} and compared it to the other members of the RCu{sub 2}Si{sub 2} series. The temperature dependence of the quadrupole moment of PrCu{sub 2}Si{sub 2} is significantly weaker-than all the other compounds eg. it is approximately five times smaller than HoCu{sub 2}Si{sub 2}. Therefore, it is the CF potential which is responsible for quenching Q{sub 2} at low temperature rather than a quadrupolar Kondo effect. Furthermore, the CF Schottky contribution to C/T {nu} T{sup 2} is approximately linear above T{sub N} and explains the anomalously high linear term in the specific heat. However, the evolution of the CF potential across the rare earth series provides evidence of an enhanced hybridization contribution to the CF potential of PrCu{sub 2}Si{sub 2}, intermediate between the heavy fermion CeCu{sub 2}Si{sub 2} and the other rare earth compounds.
Date: May 1, 1994
Creator: Osborn, R. & Goremychkin, E. A.
Partner: UNT Libraries Government Documents Department

Cerium heavy-fermion compounds near their T = 0 magnetic-non-magnetic boundary

Description: Measurements of the temperature-dependent specific heat and thermal expansion coefficient near a T = 0 magnetic-non-magnetic boundary, accessed in CeRh{sub 2}Si{sub 2} by application of pressure and in CeRh{sub 2{minus}x}Ru{sub x}Si{sub 2} at ambient pressure by chemical substitution, emphasize the role of disorder in producing non-Fermi-liquid behavior. Interestingly, superconductivity also develops near this boundary in some crystallographically-ordered Ce-based heavy-fermion compounds.
Date: December 1, 1997
Creator: Thompson, J.D.; Hundley, M.F.; Movshovich, R.; Sarrao, J.L.; Graf, T.; Uwatoko, Y. et al.
Partner: UNT Libraries Government Documents Department

Investigation into the joining of MoSi{sub 2} to 316L stainless steel

Description: Partial transient liquid phase joining and low temperature brazing were applied in joining MoSi{sub 2} to 316L ss. Exploratory studies were carried out on various interlayer materials. Mechanical, physical, and chemical compatibilities between various interlayers, brazing material, and substrate materials were investigated. Effect of thermal expansion mismatch between various components of the joint on the overall joint integrity was also studied. Preliminary findings are outlined.
Date: October 1, 1996
Creator: Vaidya, R.U.; Bartlett, A.H.; Conzone, S.D. & Butt, D.P.
Partner: UNT Libraries Government Documents Department

Synthesis and design of silicide intermetallic materials

Description: The overall objective of this program is to develop structural silicide-based materials with optimum combinations of elevated temperature strength/creep resistance, low temperature fracture toughness, and high temperature oxidation and corrosion resistance for applications of importance to the U.S. processing industry. A further objective is to develop silicide-based prototype industrial components. The ultimate aim of the program is to work with industry to transfer the structural silicide materials technology to the private sector in order to promote international competitiveness in the area of advanced high temperature materials and important applications in major energy-intensive U.S. processing industries.
Date: November 1, 1998
Creator: Petrovic, J.J.; Castro, R.G.; Butt, D.P.; Park, Y.; Hollis, K.J. & Kung, H.H.
Partner: UNT Libraries Government Documents Department

Novel SiGe Coherent Island Coarsening: Ostwald Ripening, Elastic Interactions, and Coalescence

Description: Real-time measurements of island coarsening during SiGe/Si (001) deposition reveal unusual kinetics. In particular, the mean island volume increases superlinearly with time, while the areal density of islands decreases at a faster-than-linear rate. Neither observation is consistent with standard considerations of Ostvvald ripening. We attribute our observed kinetics to the effect of elastic interactions in the densely growing island array. Island coalescence likely plays an important role as well.
Date: June 30, 1999
Creator: Chason, E.; Floro, J.A.; Freund, L.B.; Hwang, R.Q.; Lucadamo, G.A.; Sinclair, M. et al.
Partner: UNT Libraries Government Documents Department

Muon spin relaxation studies of the interplay between magnetism and superconductivity in heavy fermion systems

Description: The interplay between magnetism and superconductivity in heavy fermion systems is discussed and the role of muon spin relaxation in elucidating these properties is emphasized. Relevant properties of all six heavy fermion superconductors are briefly surveyed and instances where superconductivity and magnetism compete, coexist and couple with one another are pointed out. Current theoretical concepts underlying these phenomena are highlighted.
Date: October 1, 1993
Creator: Heffner, R. H.
Partner: UNT Libraries Government Documents Department

In situ x-ray diffraction study of CoSi{sub 2} formation during annealing of a Co/Ti bilayer on Si*(100)

Description: X-ray diffraction (XRD) was performed in situ during annealing of a Co/Ti/Si(001) multilayer, which produced an epitaxial CoSi{sub 2} layer. The results indicate that the Ti layer did. not stay intact during the reaction, and thus could not act like a membrane, moderating Co/Si interdiffusion. A strongly textured metastable phase (M) formed prior to CoSi{sub 2} nucleation. This intermediary reaction product was unobservable upon completion of the anneal. We report that nucleation and growth of CoSi{sub 2} on Si(100) took place in the presence of M, a new Co-Ti-Si-(O) phase, located at the metal/Si interface. M might play an important role in the perfection of the silicide. Ti and Co metals intermix already below 300{degrees}C, and there was evidence that metallic Ti precipitated at the surface, commencing at 550{degrees}C.
Date: June 1, 1995
Creator: Selinder, T.I.; Roberts, T.A.; Miller, D.J.; Beno, M.A.; Knapp, G.S.; Gray, K.E. et al.
Partner: UNT Libraries Government Documents Department

Processing and properties of molybdenum silicide intermetallics containing boron

Description: The processing and mechanical properties of Mo-Si-B intermetallic alloys with compositions Mo-26.7Si-7.3B and Mo-12Si-8.5B (at.%) were investigated. The first alloy consisted of the phases Mo{sub 3}Si, Mo{sub 5}Si{sub 3} (T1) and Mo{sub 5}SiB{sub 2} (T2). Attempts to extrude castings of this alloy at 1700 or 1800 C were not successful. Hot isostatic pressing of elemental powders was more promising and room temperature flexure strengths on the order of 200 MPa were reached. The second alloy with the composition Mo-12Si-8.5B could be readily cast and consisted of {alpha}-Mo inclusion in a brittle matrix of Mo{sub 3}Si and T2. A heat treatment of 1 day at 1600C in vacuum improved the room temperature strength and fracture toughness. Values on the order of 500 MPa and 10 MPa m{sup 1/2}, respectively, were obtained. Consistent with ductile phase toughening, limited plastic deformation as well as debonding of the {alpha}-Mo inclusions were seen on fracture surfaces.
Date: November 1, 1998
Creator: Schneibel, J.H.; Liu, C.T.; Heatherly, L. Jr. & Carmichael, C.A.
Partner: UNT Libraries Government Documents Department

Effects of Impurities on Alumina-Niobium InterfacialMicrostructures

Description: Optical microscopy, scanning electron microscopy, and transmission electron microscopy were employed to examine the interfacial microstructural effects of impurities in alumina substrates used to fabricate alumina-niobium interfaces via liquid-film-assisted joining. Three types of alumina were used: undoped high-purity single-crystal sapphire; a high-purity, high-strength polycrystalline alumina; and a lower-purity, lower-strength polycrystalline alumina. Interfaces formed between niobium and both the sapphire and high-purity polycrystalline alumina were free of detectable levels of impurities. In the lower-purity alumina, niobium silicides were observed at the alumina-niobium interface and on alumina grain boundaries near the interface. These silicides formed in small-grained regions of the alumina and were found to grow from the interface into the alumina along grain boundaries. Smaller silicide precipitates found on grain boundaries are believed to form upon cooling from the bonding temperature.
Date: June 20, 2005
Creator: McKeown, Joseph T.; Sugar, Joshua D.; Gronsky, Ronald & Glaeser,Andreas M.
Partner: UNT Libraries Government Documents Department

Silicide Schottky Contacts to Silicon: Screened Pinning at Defect Levels

Description: Silicide Schottky contacts can be as large as 0.955 eV (E{sub v} + 0.165 eV) on n-type silicon and as large as 1.05 eV (E{sub c} {minus} 0.07 eV) on p-type silicon. Current models of Schottky barrier formation do not provide a satisfactory explanation of occurrence of this wide variation. A model for understanding Schottky contacts via screened pinning at defect levels is presented. In the present paper it is shown that most transition metal silicides are pinned approximately 0.48 eV above the valence band by interstitial Si clusters. Rare earth disilicides pin close to the divacancy acceptor level 0.41 eV below the conduction band edge while high work function silicides of Ir and Pt pin close to the divacancy donor level 0.21 eV above the valence band edge. Selection of a particular defect pinning level depends strongly on the relative positions of the silicide work function and the defect energy level on an absolute energy scale.
Date: March 11, 1999
Creator: Drummond, T.J.
Partner: UNT Libraries Government Documents Department

Response of CeRh{sub 2}Si{sub 2} to pressure

Description: Under atmospheric pressure, CeRh{sub 2}Si{sub 2} orders antiferromagnetically at T{sub N} = 35 K, with magnetic entropy S = R{ell}n2 associated with the ordered groundstate. Application of modest pressure ({approx}9 kbar) to CeRh{sub 2}Si{sub 2} suppresses T{sub N} to near zero Kelvin, increases its Sommerfeld coefficient of specific heat by nearly a factor of 3.5 and induces some form of superconductivity below 400 mK that is depressed by a magnetic field at a rate of {minus}80 mK/kG.
Date: September 1, 1995
Creator: Movshovich, R.; Hundley, M.F.; Thompson, J.D.; Smith, J.L.; Graf, T.; Mandrus, D. et al.
Partner: UNT Libraries Government Documents Department

Fracture and fatigue resistance of Mo-Si-B alloys for ultrahigh-temperature structural applications

Description: Fracture and fatigue-crack growth properties are examined for a series of Mo-Mo3Si-Mo5SiB2 containing alloys, which utilize a continuous a-Mo matrix to achieve unprecedented room-temperature fracture resistance (>20 MPaAm). Mechanistically, these properties are explained in terms of toughening by crack trapping and crack bridging by the more ductile a-Mo phase.
Date: August 29, 2003
Creator: Kruzic, J.J.; Schneibel, J.H. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Correlation Between Domain Behavior and Magnetic Properties of Materials

Description: Correlation between length scales in the field of magnetism has long been a topic of intensive study. The long-term desire is simple: to determine one theory that completely describes the magnetic behavior of matter from an individual atomic particle all the way up to large masses of material. One key piece to this puzzle is connecting the behavior of a material's domains on the nanometer scale with the magnetic properties of an entire large sample or device on the centimeter scale. In the first case study involving the FeSiAl thin films, contrast and spacing of domain patterns are clearly related to microstructure and stress. Case study 2 most clearly demonstrates localized, incoherent domain wall motion switching with field applied along an easy axis for a square hysteresis loop. In case study 3, axis-specific images of the complex Gd-Si-Ge material clearly show the influence of uniaxial anisotropy. Case study 4, the only study with the sole intent of creating domain structures for imaging, also demonstrated in fairly simple terms the effects of increasing stress on domain patterns. In case study 5, it was proven that the width of magnetoresistance loops could be quantitatively predicted using only MFM. When all of the case studies are considered together, a dominating factor seems to be that of anisotropy, both magneticrostaylline and stress induced. Any quantitative bulk measurements heavily reliant on K coefficients, such as the saturation fields for the FeSiAl films, H{sub c} in cases 1, 3, and 5, and the uniaxial character of the Gd{sub 5}(Si{sub 2}Ge{sub 2}), transferred to and from the domain scale quite well. In-situ measurements of domain rotation and switching, could also be strongly correlated with bulk magnetic properties, including coercivity, M{sub s}, and hysteresis loop shape. In most cases, the qualitative nature of the domain structures, when properly ...
Date: May 31, 2003
Creator: Leib, Jeffrey Scott
Partner: UNT Libraries Government Documents Department

Evaluation of A{sub 5}Si{sub 3}Z{sub x} intermetallics for use as high temperature structural materials

Description: Refractory intermetallic silicides are receiving increasing consideration for use as high temperature structural materials. A{sub 5}Si{sub 3} compositions are particularly interesting due to their ability to incorporate a variety of interstitial ternary additions. These ternary additions present a unique opportunity to potentially tailor physical properties. Previous experimental work has shown that these additions can decrease the inherent thermal expansion anisotropy of Ti{sub 5}Si{sub 3}. This paper reviews some of the literature with respect to binary and ternary silicides of the form A{sub 5}Si{sub 3}Z{sub x}. Recent experimental work by the authors on two promising compositions, Ti{sub 5}Si{sub 3} and Mo{sub 5}Si{sub 3}, is discussed. Interstitial additions substantially improve the isothermal oxidation resistance of both Ti{sub 5}Si{sub 3} and Mo{sub 5}Si{sub 3}. The lattice parameter of Ti{sub 5}Si{sub 3} is significantly changed by ternary interstitial additions, indicating the extensive solubility of Ti{sub 5}Si{sub 3} for such additions. The level of similar additions affects the phase assemblage of Mo{sub 5}Si{sub 3}. Hardness and toughness values from indentation measurements for selected compositions are given.
Date: September 1, 1993
Creator: Kim, Y.; Thom, A. J.; Meyer, M. K. & Akinc, M.
Partner: UNT Libraries Government Documents Department

Free Radical Induced Oxidation, Reduction and Metallization of NiSi and Ni(Pt)Si Surfaces

Description: NiSi and Ni(Pt)Si, and of the effects of dissociated ammonia on oxide reduction was carried out under controlled ultrahigh vacuum (UHV) conditions. X-ray photoelectron spectroscopy (XPS) has been used to characterize the evolution of surface composition. Vicinal surfaces on NiSi and Ni(Pt)Si were formed in UHV by a combination of Ar+ sputtering and thermal annealing. Oxidation of these surfaces in the presence of either O+O2 or pure O2 at room temperature results in the initial formation of a SiO2 layer ~ 7 Å thick. Subsequent exposure to O2 yields no further oxidation. Continued exposure to O+O2, however, results in rapid silicon consumption and, at higher exposures, the kinetically-driven oxidation of the transition metal(s), with oxides >35Ǻ thick formed on all samples, without passivation. The addition of Pt retards but does not eliminate oxide growth or Ni oxidation. At higher exposures, in Ni(Pt)Si surface the kinetically-limited oxidation of Pt results in Pt silicate formation. Substrate dopant type has almost no effect on oxidation rate. Reduction of the silicon oxide/metal silicate is carried out by reacting with dissociated NH3 at room temperature. The reduction from dissociated ammonia (NHx+H) on silicon oxide/ metal silicate layer shows selective reduction of the metal oxide/silicate layer, but does not react with SiO2 at ambient temperature.
Date: August 2010
Creator: Manandhar, Sudha
Partner: UNT Libraries

Transient phase formation during the growth of epitaxial CoSi{sub 2} by annealing of CoTi bi-layers of (100) Si

Description: Phase evolution during annealing of Co/Ti bilayers on (100)Si was studied by XRD and analytical electrom microscopy (AEM). XRD performed in situ during annealing revealed a reaction pathway involving the formation of a transient phase when epitaxial CoSi{sub 2} films were grown. AEM was used to identify this phase as a spinel-related phase, isostructural with Co{sub 2}TiO{sub 4}. This phase grows as a result of the presence of the Ti interlayer and a small amount of oxygen from the annealing ambient. Annealing in vacuum or other purified inert gases yielded polycrystalline CoSi{sub 2} films which form via a different reaction pathway that does not involve a spinel phase. This spinel phas may serve both to reduce the native oxide from the underlying Si substrate and to control interdiffusion between Si and Co during the reaction, thereby promoting epitaxial growth.
Date: 1995-11~
Creator: Miller, D. J.; Selinder, T. I. & Gray, K. E.
Partner: UNT Libraries Government Documents Department