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Nanofabricated SiO{sub 2}-Si-SiO{sub 2} Resonant Tunneling Diodes

Description: Resonance Tunneling Diodes (RTDs) are devices that can demonstrate very high-speed operation. Typically they have been fabricated using epitaxial techniques and materials not consistent with standard commercial integrated circuits. The authors report here the first demonstration of SiO{sub 2}-Si-SiO{sub 2} RTDs. These new structures were fabricated using novel combinations of silicon integrated circuit processes.
Date: April 6, 2000
Partner: UNT Libraries Government Documents Department

Indentation and oxidation studies on silicon nitride joints

Description: Si nitride ceramics have been joined with a Y oxide-SiO{sub 2} interlayer. A 1:2 molar ratio of Y{sub 2}O{sub 3} to SiO{sub 2} was chosen to obtain the desired Y{sub 2}Si{sub 2}O{sub 7} stoichiometry, which should give the interlayer better oxidation resistance compared to other interlayer materials. Mechanical characterization of the joints performed by indentation shows it to have good room temperature strength.
Date: May 1, 1996
Creator: Gopal, M.; De Jonghe, L.C. & Thomas, G.
Partner: UNT Libraries Government Documents Department

Prediction of release-etch times for surface-micromachined structures

Description: A one-dimensional model is presented which describes the release-etch behavior of sacrificial oxides in aqueous HF. Starting from first principles and an empirical rate law, release etch kinetics are derived for primitive geometries. The behavior of complex three-dimensional structures is described by joining the solutions of constituent primitives and applying appropriate boundary conditions.The two fitting parameters, k{sub 1} and k{sub 2}, are determined from the simplest structure and describe the more complex structures well. Experimental validation of the model is presented with data for all of the geometries and four types of sacrificial oxides.
Date: May 1, 1997
Creator: Eaton, W.P.; Jarecki, R.L. & Smith, J.H.
Partner: UNT Libraries Government Documents Department

Shrinkage and microstructural development during drying of organically modified silica xerogels

Description: We have studied the different driving forces behind syneresis in MTES/TEOS gels by aging them in different H{sub 2}O/EtOH pore fluids. We show using shrinkage, density, contact angle, and N{sub 2} sorption measurements, the influence of gel/solvent interactions on the microstructural evolution during drying. Competing effects of syneresis (that occurs during aging) and drying shrinkage resulted in the overall linear shrinkage of the organically modified gels to be constant at {approximately}50%. Increasing the hydrophobicity of the gels caused the driving force for syneresis to change from primarily condensation reactions to a combination of condensation and solid/liquid interfacial energy. In addition the condensation driven shrinkage was observed to be irreversible, whereas the interfacial free energy driven shrinkage was observed to be partially reversible. Nitrogen sorption experiments show that xerogels with the same overall extent of shrinkage can have vastly different microstructures due to the effects of microphase separation.
Date: July 1, 1996
Creator: Raman, N.K.; Wallace, S. & Brinker, C.J.
Partner: UNT Libraries Government Documents Department

Monolithic integration of waveguide structures with surface-micromachined polysilicon actuators

Description: The integration of optical components with polysilicon surface micromechanical actuation mechanisms show significant promise for signal switching, fiber alignment, and optical sensing applications. Monolithically integrating the manufacturing process for waveguide structures with the processing of polysilicon actuators allows actuated waveguides to take advantage of the economy of silicon manufacturing. The optical and stress properties of the oxides and nitrides considered for the waveguide design along with design, fabrication, and testing details for the polysilicon actuators are presented.
Date: March 1, 1996
Creator: Smith, J.H.; Carson, R.F.; Sullivan, C.T. & McClellan, G.
Partner: UNT Libraries Government Documents Department

Determination of mineral abundances in samples from the exploratory studies facility using x-ray diffraction

Description: Tuff samples collected from the Exploratory Studies Facility (ESF) were X-rayed to estimate relative mineral abundances. X-ray analysis was performed on sub-samples of specimens collected from both the Single Heater Test (SHT) and Drift Scale Heater Test (DST) that were used for thermomechanical measurements, as well as samples collected from cores retrieved from boreholes in the Drift Scale Test Area. The abundance of minerals that could affect the behavior of the host rock at repository relevant temperatures is of particular interest. These minerals include crystobalite, which undergoes a phase transition and volume change at elevated temperature (-250 `C), and smectite and clinoptilolite that can dehydrate at elevated temperature with accompanying volume reduction. In addition, the spatial distribution of SiO, polymorphs and secondary minerals may provide evidence for deducing past fluid pathways. The mineral abundances tabulated here include data reported previously in three milestone reports (Roberts and Viani, 1997a,b; Viani and Roberts, 1996) but re-analyzed (see below), as well as previously unreported data. Previous X-ray diffraction analyses of samples from the ESF (Roberts and Viani, 1997a; Viani and Roberts, 1996) utilized the matrix flushing method of Chung (1974) and an internal intensity standard (corundum) to quantify the abundances of the phases present. Although the method is adequate for obtaining relative abundances, its accuracy and precision is limited by the inherent differences between the external standards used to compute the reference intensity ratio and the mineral phases in the sample. In a subsequent report (Roberts and Viani, 1997b) mineral abundances were obtained using the Rietveld method of whole X-ray pattern fitting (Snyder and Bish, 1989; Young, 1993). The Rietveld technique has the potential to be both more accurate and more precise for estimating mineral abundances (Snyder and Bish, 1989).
Date: January 13, 1998
Creator: Roberts, S. & Viani, R.
Partner: UNT Libraries Government Documents Department

Surface functionalization of silica microparticles for capillary electrochromatography (CEC)

Description: We derivatized small (0.5 -3 {mu}m) silica particles by silating their surfaces with long-chain alkyl substituted silanes. These functionalized particles were packed into 100 {mu}m capillaries and used as stationary phases for capillary electrochromatography. The particles supported electroosmotic flow in mixtures of acetonitrile and aqueous buffer (4 mM sodium tetraborate or 2mM TRIS). The columns were used to separate mixtures of organic analytes demonstrating the effectiveness of the functionalized stationary phase.
Date: January 1, 1997
Creator: Shepodd, T.J.; Anex, D.S. & Rognlien, J.
Partner: UNT Libraries Government Documents Department

Index of refraction versus oxygen partial pressure for tantalum oxide and silicon dioxide films produced by ion beam deposition

Description: Tantalum oxide and silicon oxide films were made using an ion beam sputtering system. It was found that even though these films were deposited from oxide targets, additions of oxygen were necessary to achieve stoichiometry and hence index of refraction. It was observed that the tantalum oxide target changed color from white to gray, indicating that the oxygen was being depleted from the target. The addition of oxygen to the chamber during deposition replenished the target and improved film stoichiometry. The deposition rate decreased with increasing oxygen partial pressure. It was experimentally determined that by varying the oxygen partial pressure and keeping all other variables fixed, the index of refraction of the film changed in a predictable manner. That is, as the oxygen partial pressure was increased, the index decreased rapidly initially and then reached a saturation point where it stayed fixed with oxygen content. With this data a coating process can be set up using the minimum amount of oxygen (thus increasing filament lifetime) to produce a fully stoichiometric film that has a fixed index. This paper will present the details of these observations and results.
Date: April 30, 1998
Creator: Goward, W.D.; Petersen, H.E.; Dijaili, S.P. & Walker, J.D.
Partner: UNT Libraries Government Documents Department

Cycling Endurance of SONOS Non-Volatile Memory Stacks Prepared with Nitrided SiO(2)/Si(100) Intefaces

Description: The effects of nitrided SiO{sub 2}/Si(100) interfaces upon cycling endurance in silicon-oxide-nitride-oxide-silicon (SONOS) non-volatile memory transistors are investigated. Analysis of MOSFET sub-threshold characteristics indicate cycling degradation to be a manifestation of interface state (D{sub it}) generation at the tunnel oxide/silicon interface. After 10{sup 6} write/erase cycles, SONOS film stacks prepared with nitrided tunnel oxides exhibit enhanced cycling endurance with {Delta}D{sub it}=3x10{sup 12} V{sup -1}cm{sup -2}, compared to {Delta}D{sub it}=2x10{sup 13} V{sup -l}cm{sup -2} for non-nitrided tunnel oxides. Additionally, if the capping oxide is formed by steam oxidation, rather than by deposition, SONOS stacks prepared with non-nitrided tunnel oxides exhibit endurance characteristics similar to stacks with nitrided tunnel oxides. From this observation it is concluded that latent nitridation of the tunnel oxidehilicon interface occurs during steam oxide cap formation.
Date: January 11, 1999
Creator: Habermehl, S.; Nasby, R.D. & Rightley, M.J.
Partner: UNT Libraries Government Documents Department

Atomic structure of Si-SiO{sub 2} interface

Description: Investigations of the atomic structure of Si-SiO{sub 2} interfaces have mostly been performed with high resolution transmission electron microscopy. However, the interpretation of the phase contrast in the amorphous phase at the interface is not unique. The authors investigated the Si-SiO{sub 2} interface by studying the ELNES of the O-K edge with the spatial difference technique with a dedicated STEM with 100kV. Also the interface was studied by Z-contrast imaging with a 300 kV dedicated STEM. Silicon wafers (110) were first thermally oxidised to produce a SiO{sub 2} layer. The thermally grown oxide was used as a substrate for liquid phase epitaxy of silicon.
Date: April 1, 1997
Creator: Duscher, G.; Pennycook, S.J. & Banhart, F.
Partner: UNT Libraries Government Documents Department

H atom probes of radiation chemistry: Solids and liquids

Description: H atoms are ubiquitous in radiation chemistry. Radiolysis of most substances yield H atoms and studies of the mechanisms of their production are as old as the field of radiation chemistry. The problem is that study or products does not easily reveal the chemical mechanisms involved even with the clever use of isotopes. Time-resolved pulsed electron paramagnetic resonance (EPR) was used to study formation and decay kinetics of spin-polarized mobile H atoms in radiolysis of wet fused silica containing {approximately} 1,200 ppm of SiOH groups. Two reactions of H atoms can be distinguished: a slow component corresponding to scavenging of H atoms by metastable paramagnetic centers and a fast component which is ascribed to a reaction of a short-lived small polaron (intrinsic hole) with H atoms.
Date: December 1998
Creator: Trifunac, A. D. & Shkrob, I. A.
Partner: UNT Libraries Government Documents Department

Ion beam synthesis and optical properties of semiconductor nanocrystals and quantum dots

Description: Nanocrystals of semiconductor materials have been fabricated in SiO{sub 2} by ion implantation and subsequent thermal annealing. Strong red photoluminescence (PL) peaked around 750 nm has been observed in samples containing Si nanocrystals in SiO{sub 2}. The Si nanocrystals in the samples with optimized PL intensities are a few nanometers in diameter. Difference in the absorption bandgap energies and the PL peak energies are discussed. Significant influence of implantation sequence on the formation of compound semiconductor nanocrystals are demonstrated with the GaAs in the SiO{sub 2} system. Optical absorption measurements show that Ga particles have already formed in the as-implanted stage if Ga is implanted first. A single surface phonon mode has been observed in the infrared reflectance measurement from samples containing GaAs nanocrystals.
Date: November 1, 1996
Creator: Zhu, J.G.; White, C.W. & Withrow, S.P.
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

A Unitifed Computational Approach to Oxide Aging Processes

Description: In this paper we describe a unified, hierarchical computational approach to aging and reliability problems caused by materials changes in the oxide layers of Si-based microelectronic devices. We apply this method to a particular low-dose-rate radiation effects problem
Date: January 27, 1999
Creator: Bowman, D.J.; Fleetwood, D.M.; Hjalmarson, H.P. & Schultz, P.A.
Partner: UNT Libraries Government Documents Department

Growth and characterization of GE nanocrystals

Description: We have synthesized Ge nanocrystals of sizes 4, 8, and 12 nm by ion-implanting Ge+ ions into thermally grown Si0{sub 2} films and subsequent annealing of the films at 8300 C for 30 min in nitrogen. These films were characterized by x-ray, transmission electron microscopy (TEM), and Raman spectroscopy. A distribution of particle size was identified by TEM in a 1 00 nm band below the surface. Particle sizes were estimated by these 3 techniques.
Date: May 1, 1998
Creator: Guha, S., Naval Research Laboratory
Partner: UNT Libraries Government Documents Department

Stability of Trapped Electrons in SiO(2)

Description: Thermally stimulated current and capacitance voltage methods are used to investigate the thermal stability of trapped electrons associated with radiation-induced trapped positive charge in metal-oxide-semiconductor capacitors. The density of deeply trapped electrons in radiation-hardened 45 nm oxides exceeds that of shallow electrons by a factor of {approximately}3 after radiation exposure, and by up to a factor of 10 or more during biased annealing. Shallow electron traps anneal faster than deep traps, and seem to be at least qualitatively consistent with the model of Lelis et al. Deeper traps maybe part of a fundamentally distinct dipole complex, and/or have shifted energy levels that inhibit charge exchange with the Si.
Date: January 29, 1999
Creator: Fleetwood, D.M. & Winokur, P.S.
Partner: UNT Libraries Government Documents Department

A Silicon, Carbon and Deuterium NMR Investigation of Molecular Templating in Amorphous Silicas

Description: The precise pore sizes defined by crystalline zeolite lattices have led to intensive research on zeolite membranes. Unfortunately zeolites have proven to be extremely difficult to prepare in a defect-free thin film form needed for membrane flux and selectivity. We introduce tetrapropylammonium TPA (a structure directing agent for zeolite ZSM-5) into a silica sol and exploit the development of high solvation stresses to create templated amorphous silicas with pore apertures comparable in size to those of ZSM-5. {sup 29}Si and {sup 2}H NMR experiments were performed to evaluate the efficacy of our templating approach. The {sup 29}Si NMR spectrum of the silica matrix was observed by an intermolecular cross-polarization experiment between the {sup 1}H nuclei of TPA and the {sup 29}Si nuclei in the silica matrix. The efficiency of the cross-polarization interaction was used to investigate the degree to which the matrix formed a tight cage surrounding the template molecule. Normally prepared xerogel materials exhibited only weak interactions between the two sets of nuclei. Drying under reduced pressure, where solvation stresses are maximized, resulted in significantly increased interactions. Analogous materials were prepared using fully deuterated TPA. The {sup 2}H NMR wideline spectra consisted of a partially narrowed resonance, corresponding to template molecules which were undergoing restricted rotational motion, and an isotropically narrowed resonance, corresponding to molecules which were undergoing rapid rotational motion. The number of isotropically rotating template molecules decreased for specimens dried under reduced pressure, consistent with improved templating of amorphous silica by TPA.
Date: April 8, 1999
Creator: Alam, T.M,; Assink, R.A.; Brinker, C.J.; Click, C.A. & Naik, S.J.
Partner: UNT Libraries Government Documents Department

Use of Self-Assembling Monolayers to Control Interface Bonding in a Model Study of Interfacial Fracture

Description: The relationships between the extent of interfacial bonding, energy dissipation mechanisms, and fracture toughness in a glassy adhesive/inorganic solid joint are not well understood. We address this subject with a model system involving an epoxy adhesive on a polished silicon wafer containing its native oxide. The extent of interfacial bonding, and the wetting behavior of the epoxy, is varied continuously using self-assembling monolayers (SAMs) of octadecyltrichlorosilane (ODTS). The epoxy interacts strongly with the bare silicon oxide surface, but forms only a very weak interface with the methylated tails of the ODTS monolayer. We examine the fracture behavior of such joints as a function of the coverage of ODTS in the napkin-ring geometry. Various characterization methods are applied to the ODTS-coated surface before application of the epoxy, and to both surfaces after fracture. The fracture data are discussed with respect to the wetting of the liquid epoxy on the ODTS-coated substrates, the locus of failure, and the energy dissipation mechanisms. Our goal is to understand how energy is dissipated during fracture as a function of interface strength.
Date: March 5, 1999
Creator: Kent, M.S.; Matheson, A.; Reedy, E.D. & Yim, H.
Partner: UNT Libraries Government Documents Department

Uniformity model for energetic ion processes using a Kaufman ion source

Description: Processes that use energetic ions for large substrates require that the time-averaged erosion effects from the ion flux be uniform across the surface. A numerical model has been developed to determine this flux and its effects on surface etching of a silica/photoresist combination. The geometry of the source and substrate is very similar to a typical deposition geometry with single or planetary substrate rotation. The model was used to tune an inert ion-etching process that used single or multiple Kaufman sources to less than 3% uniformity over a 30-cm aperture after etching 8 {micro}m of material. The same model can be used to predict uniformity for ion-assisted deposition (IAD).
Date: December 1, 1998
Creator: Smith, D.J.; Warner, J.A. & LeBarron, N.
Partner: UNT Libraries Government Documents Department

Fundamental Efficiency Limitations for Low Electron Energy Cathololuminescence

Description: The design of field emission displays is severely constrained by the universally poor cathodoluminescence (CL) efficiency of most phosphors at low excitation energies. As part of the effort to understand this phenomenon, the authors have measured the time decay of spectrally-resolved, pulsed CL and photoluminescence (PL) in several phosphors activated by rare earth and transition metal impurities, including Y{sub 2}O{sub 3}:Eu, Y{sub 2}SiO{sub 5}:Tb, and Zn{sub 2}SiO{sub 4}:Mn. Activator concentrations ranged from {approximately}0.25 to 10%. The CL decay curves are always non-linear on a log(CL)-linear(time) plot--i.e. they deviate from first order decay kinetics. These deviations are always more pronounced at short times and larger activator concentrations and are largest at low beam energies where the decay rates are noticeably faster. PL decay is always slower than that seen for CL, but these differences disappear after most of the excited species have decayed. They have also measured the dependence of steady state CL efficiency on beam energy. They find that larger activator concentrations accelerate the drop in CL efficiency seen at low beam energies. These effects are largest for the activators which interact more strongly with the host lattice. While activator-activator interactions are known to limit PL and CL efficiency in most phosphors, the present data suggest that a more insidious version of this mechanism is partly responsible for poor CL efficiency at low beam energies. This enhanced concentration quenching is due to the interaction of nearby excited activators. These interactions can lead to non-radiative activator decay, hence lower steady state CL efficiency. Excited state clustering, which may be caused by the large energy loss rate of low energy primary electrons, appears to enhance these interactions. In support of this idea, they find that PL decays obtained at high laser pulse energies resemble the non-linear decays seen in the CL data.
Date: August 1, 2000
Partner: UNT Libraries Government Documents Department

Microfabrication of membrane-based devices by deep-reactive ion etching (DRIE) of silicon

Description: Deep reactive ion etching (DRIE) of silicon was utilized to fabricate dielectric membrane-based devices such as microhotplates, valves and flexural plate wave (FPW) devices. Through-wafer DRIE is characterized by fast etch rates ({approximately} 3 {micro}m/min), crystal orientation independence, vertical sidewall profiles and CMOS compatibility. Low-stress silicon nitride, a popular membrane material, has an appreciable DRIE etch rate. To overcome this limitations DRIE can be accompanied by a brief wet chemical etch. This approach has been demonstrated using KOH or HF/Nitric/Acetic etchants, both of which have significantly lower etch rates on silicon nitride than does DRIE. The DRIE etch properties of composite membranes consisting of silicon dioxide and silicon nitride layers are also under evaluation due to the higher DRIE selectivity to silicon dioxide.
Date: August 1, 1998
Creator: Manginell, R.P.; Frye-Mason, G.C.; Schubert, W.K.; Shul, R.J. & Willison, C.G.
Partner: UNT Libraries Government Documents Department

Characterizing and modeling the apparent anomalous behavior of resistivity in Cr-Si-O thin films

Description: The Cr-Si-O material system is of interest for use as a thin film resistor. The films are sputter deposited onto conducting substrates from metal oxide compacts using a reactive gas mixture. the cermet films composition range from 50 to 100 vol.% SiO{sub 2} as determined from elemental measurements of the Cr, Si and O content. In a wide range of resistivities from 10{sup 1} to 10 {sup 14} {omega}-cm measured through the film thickness, an apparent anomalous behavior is found with the Cr, Si and O composition. The anomaly can be deducted to a discontinuous variation of resistivity with film composition near 80 vol.% SiO{sub 2}. The film microstructure is characterized as a distribution of conducting metal-silicide particles within an insulating matrix. The effective medium theory is used to predict the variation of conductivity and successfully models the anomalous resistivity behavior.
Date: March 1, 1998
Creator: Jankowski, A.F.
Partner: UNT Libraries Government Documents Department