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This is a copy of the slides presented at the meeting but not formally written up for the volume.
Many important oxidative reactions, such as CO oxidation, take place on metal surfaces at high temperatures and partial pressures. Understanding the atomic processes involved in these catalyzed reactions are of great importance and may be achievable by observations of the adsorbate-induced surface structure under temperatures and pressures relevant to working catalysts. Many of the prior studies, however, have only considered quenched-in structures with no dynamic interaction between the metal surface and the gas phase. This presentation describes in-situ synchrotron x-ray studies of the Cu (001) surface as a function of pO2, the oxygen partial pressure, and temperature. We utilize a controlled-flow reaction chamber specially constructed to mount onto an eight-circle diffractometer at the Advanced Photon Source. The chamber allows the flow of oxygen, hydrogen, and argon mixtures with pO2 ranging from 760 to 1×10-12 Torr and sample temperatures variable from 25 to 1000 °C. After reaching a critical pO2, oxygen adsorbs onto the initially clean Cu (001) surface, resulting in the rapid nucleation and growth of c(2×2)-O domains. Domain formation is concurrent with a small in-plane surface contraction and a large out-of-plane surface expansion associated with a compressive adsorbate-induced surface stress. The often reported (2√2×√2)R45 reconstruction is observed only below ~ 150 °C. Relationships between the different surface structures, subsurface oxygen, surface stress, and surface reactivity will be discussed.
This is a copy of the slides presented at the meeting but not formally written up for the volume.
In situ synchrotron x-ray techniques are ideal for providing insight into the oxidation behavior of metals and alloys. Here, we present results from an in situ study of the early-stage processes during oxidation of (001) single crystal Cu-Ni alloys, of interest, for example, as catalysts of a number of important chemical reactions. Grazing incidence x-ray diffraction observations in controlled, elevated temperature oxidizing conditions reveal that, with increasing oxygen partial pressure (pO2), epitaxial cube-on-cube-oriented NiO islands form first, followed by other epitaxial orientations of NiO. As the pO2 continues to be increased, epitaxially-oriented Cu2O islands will also eventually nucleate and grow. In some cases, evidence is seen for the formation of internal, as well as surface oxide islands. Total reflection x-ray fluorescence observations provide a complementary sensitive measure of the changes in the alloy surface composition in response to changes in the composition of the gas environment in contact with the sample. Evidence is seen for Ni surface segregation under intermediate pO2 conditions where NiO, but not Cu2O nucleates and grows. In addition to describing the oxidation behavior of Cu-Ni alloys as functions of alloy composition, temperature, and pO2, we also will discuss the possible effects of the presence of Cu2O islands or oxygen-induced surface structure(s) on the activity of Cu or Cu-Ni surfaces in catalyzing the methanol oxidation reaction.
The DJEHUTY project is an intensive effort at the Lawrence Livermore National Laboratory (LLNL) to produce a general purpose 3-D stellar structure and evolution code to study dynamic processes in whole stars.
We review critical physics affecting the observational characteristics of those supernovae that occur in massive stars. Particular emphasis is given to 1) how mass loss, either to a binary companion or by a radiatively driven wind, affects the type and light curve of the supernova, and 2) the interaction of the outgoing supernova shock with regions of increasing pr3 in the stellar mantle. One conclusion is that Type II-L supernovae may occur in mass exchanging binaries very similar to the one that produced SN 1993J, but with slightly larger initial separations and residual hydrogen envelopes (∼1 Mʘ and radius ∼ several AU). The shock interaction, on the other hand, has important implications for the formation of black holes in explosions that are, near peak light, observationally indistinguishable from ordinary Type II-p and lb supernovae.
High quality Ga-face and N-face AlGaN/GaN based heterostructures have been grown by plasma induced molecular beam epitaxy. By using Ga-face material we are able to fabricate conventional heterojunction field effect transistors. Because the N-face material confines electrons at a different heterojunction, the resulting transistors are called inverted. The Ga-face structures use a high temperature AlN nucleation layer to establish the polarity. Structures from these materials, relying only on polarization induced interface charge effects to create the two-dimensional electron gases, are used to confirm the polarity of the material as well as test the electrical properties of the layers. The resulting sheet concentrations of the two dimensional electron gases agree very well with the piezoelectric theory for this materials system. Hall mobilities of the two-dimensional gases for the N-face structures are as high as 1150 cm2/Vs and 3440 cm2/Vs for 300 K and 77 K respectively, while the Ga–face structures yield room temperature mobilities of 1190 cm2/Vs. Both structures were then fabricated into transistors and characterized. The inverted transistors, which were fabricated from the N-face material, yielded a maximum transconductance of 130 mS/mm and a current density of 905 mA/mm. Microwave measurements gave an ft of 7 GHz and an fmax of 12 GHz for a gate length of 1 µm. The normal transistors, fabricated from the Ga-face material, produced a maximum transconductance of 247 mS/mm and a current density of 938 mA/mm. Microwave measurements gave an ft of 50 GHz and an fmax of 97 GHz for a gate length of 0.25 µm. This shows that using plasma induced molecular beam epitaxy N-face and Ga(Al)-face AlGaN/GaN heterostructures can be grown with structural and electrical properties very suitable for high power field effect transistors.
About 20% of major depressive episodes become chronic and medication-refractory and also appear to be less responsive to standard cognitive–behavioural therapy (CBT).
To test whether CBT developed from behavioural activation principles that explicitly and exclusively targets depressive rumination enhances treatment as usual (TAU) in reducing residual depression.
Forty-two consecutively recruited participants meeting criteria for medication-refractory residual depression were randomly allocated to TAU v. TAU plus up to 12 sessions of individual rumination-focused CBT. The trial has been registered (ISRCTN22782150).
Adding rumination-focused CBT to TAU significantly improved residual symptoms and remission rates. Treatment effects were mediated by change in rumination.
This is the first randomised controlled trial providing evidence of benefits of rumination-focused CBT in persistent depression. Although suggesting the internal validity of rumination-focused CBT for residual depression, the trial lacked an attentional control group so cannot test whether the effects were as a result of the specific content of rumination-focused CBT v. non-specific therapy effects.
The atomic-scale structure of grain boundaries (GBs) in yttria-stabilized cubic zirconia (YSZ) was investigated by high-resolution electron microscopy (HREM). Non-stoichiometric oxides have found a wide range of applications and therefore it is of importance to explore the role of GBs and their atomic-scale relaxation modes.  and  tilt GBs were examined by HREM in highly textured thin films of YSZ grown by metal-organic chemical vapor deposition (MOCVD). In addition, a special technique was developed to also allow the HREM study of twist and general GBs. GBs and triple junctions show quite dense arrangements of cation atomic columns. The GB core structures in YSZ can be contrasted to the more open structures in stoichiometric cubic oxides, such NiO, which are characterized by a relatively large GB excess volume. This appears to be due to several factors, including the necessary rearrangement of the oxygen sublattice near GBs in a CsCl2 type structure, the redeployment of oxygen vacancies near GBs, and the segregation of Y to the GB. Relative to stoichiometric oxides, such mechanisms provide additional degrees of freedom for atomic relaxations at GBs and the development of low-energy GBs. These additional relaxation modes, which result in GB cation arrangements more akin to metallic systems, are also reflected by Burgers vector dissociations observed in low-angle YSZ GBs.
In this study thin AlxGa1−xN nucleation layers on sapphire were patterned and overgrown by plasma-induced molecular beam epitaxy (PIMBE) and metalorganic chemical vapor deposition (MOCVD) to obtain adjacent regions of GaN and AlGaN/GaN heterostructures with different polarities. The role of polarity on the structural and electrical properties of epitaxial layers and AlGaN/GaN heterostructures was investigated for samples grown on patterned AlN or GaN nucleation layers. Epitaxial GaN and AlGaN/GaN heterostructures grown on Al-face AlN or N- face GaN nucleation layers were found to be Ga-face or N-face, respectively, independent of the technique used for the overgrowth.
Two dimensional hole and electron gases in wurtzite GaN/AlxGa1-xN/GaN heterostructures are induced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness and doping. We have investigated the structural quality, the carrier concentration profiles and electrical transport properties by a combination of high resolution x- ray diffraction, Hall effect and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail
AlGaN/GaN is a promising system for high power electron devices. Quality of ohmic contacts is a critical parameter in determining the performance of the device. Although we have achieved a transfer resistance (Rc) of 0.35Δmm and ρc of 9.5×10−7 Δcm−2 the morphology and edge acuity of the contacts are poor. The standard ohmic contact recipes consist of a combination of Titanium and Aluminum with Nickel and/or Gold. This is annealed at 800°C-950°C [1-5]. In this work we study ohmic contacts on unintentionally doped Al0.3Ga0.7N/GaN system. We look at ratios of Ti/Al from 0 to 2 to determine which is the optimum ratio in terms of surface morphology and electrical characteristics. From our studies we conclude that morphology of a Ti/Al contact is good over a ratio of 0.3 and the contact resistance is minimized at a Ti/Al of 0.6. The ohmic contacts are improved electrically if a layer of gold is added on top. The best electrical contacts however were obtained with a four layer recipe of Ti/Al/Ti/Au, which gave contact resistance (Rc) around 0.45Δmm, but the morphology of the contacts was poor.
Understanding the role of grain boundaries in controlling heat flow is critical to the success of many envisioned applications of nanocrystalline materials. This study focuses on the effect of grain boundaries on thermal transport behavior in nanocrystalline yttria-stabilized zirconia (YSZ) coatings prepared by metal-organic chemical vapor deposition.
Nanocrystalline TiO2 compacts having initial approximate mean grain sizes of 14 nm and approximate green densities of 70% of theoretical were sintered by short-time exposure in a 2.45 GHz microwave cavity to maximum temperatures of 800, 1000 or 1200 ºC. Sample densities were measured before and after exposure to microwaves using Archimede's method. Transmission electron microscopy and x-ray diffraction were utilized to monitor grain growth and phase changes. Rutherford backscattering was used to monitor any changes in oxygen stoichiometry. The results of this study indicate that enhanced densification behavior is obtained for microwavesintered samples relative to samples sintered using conventional pressureless-sintering techniques.
Quantitative x-ray diffraction measurements of ultrafine-grained (nanocrystalline) Pd and a coarse-grained polycrystalline reference foil were obtained using synchrotron radiation. The Bragg reflection intensity profiles of nanocrystalline Pd were found to be considerably better represented by Lorentzian functions than by Gaussian functions, indicating that a large fraction of the intensity from the Bragg peaks is located in the tails of the reflections. The intensity that is in the Lorentzian-shaped Bragg peaks differs only slightly for different grain-sized materials; thus, the atomic relaxations in the vicinity of grain boundaries do not produce broadly distributed diffuse scattering other than that found in the Bragg reflections. The results of the present work do not support the previously proposed existence of either a “gas-like” grain boundary phase, or large quantities of vacancies or voids within grains of nanocrystalline Pd.
Heteroepitaxial growth of CaF2 films on (511)Si and GaAs films on CaF2/Si(511) structures is investigated. CaF2 films and GaAs films are grown by vacuum evaporation and molecular beam epitaxy, respectively. Ion channeling measurements and replica transmission electron microscopy show that CaF2 films having good crystalline quality and surface steps can be formed by annealing at 900°C for 30 sec after the growth at 550° C. GaAs films having smooth surfaces and good crystalline quality can be grown on the annealed CaF2/Si(511) structures.
Low energy ion damage effects have been investigated in GaAs-A1GaAs two dimensional electron gas (2DEG) materials. The effect of ion mass (He, Ar, Xe) and adsorbed C12 on the charge carrier density and mobility has been studied for ion bombarded 2DEG systems. The 2DEG mobility was significantly reduced by ion damage with the effect becoming more dramatic with smaller ion mass. For the same treatment, the two dimensional carrier density was relatively unaffected. The results of He ion exposure showed serious degradation of the 2DEG with moderate ion dose. Electrical measurements were performed to determine the conducting widths of narrow patterned wires. For the same structural widths (mask width) He defined wires showed smaller electrical widths than Ar milling in the presence of chlorine. Serious limitations to patterning small structures may be imposed using beam processes that include He or other light mass species.
The control of p-type dopants is very important in producing high performance minority carrier devices such as heterojunction bipolar transistors (HBT) and lasers. In this study, an electrical characterization technique is described which is very sensitive to the p-type dopant profile in a heterojunction. Both the placement of the dopant, i.e. the as-grown profile, and thermal diffusion effects have been investigated. The factors which control the initial placement and subsequent diffusion of the dopant species have been determined and used to produce device-quality GaAs/Al0.30Ga0.70As p+/n heterojunctions.
Raman scattering, double-crystal x-ray diffraction, and electron beam electroreflectance have been used to assess the damage produced in undoped (100)-GaAs by boron ion implants and the influence of post-implant anneals. Both conventional furnace and rapid thermal annealing treatments were found to remove much of the lattice strain created by the implants. However, considerable disorder also remains after these anneals.
We report the first investigation of the effect of dry etching parameters on the sidewall carrier recombination rate of GaAs/AIGaAs and lnPilnGaAsP microstructures. Surface recombination was measured as a function of ion voltage and etching time. The increase in recombination rate due to etching can be reversed by subsequent chemical removal of the immediate sidewall layer. By monitoring the recovery in recombination rate as a function of the amount of sidewall layer removed, the effective damage depth is inferred.
Rapid thermal processing has been utilized to diffuse Zn into GaAs from a thin film zinc silicate source prepared by atmospheric pressure chemical vapor deposition (CVD). The zinc source was capped with ∼500 Å of silicon dioxide (SiO2 ). At 750°C for 20 sec, the diffusion of Zn reached a depth of 0.8 μm. For these diffusions, the diffusion constant is concentration dependent and is proportional to the square of the Zn concentration. Above 7500 C, anomalous secondary diffusion fronts were observed in the Zn diffusion profiles. A new model is proposed that explains the diffusion profiles at all temperatures. Ohmic contacts have been made to the above Zn-diffused surfaces using Cr/AuZn/Au metallizations. A typical value of the specific contact resistance is 8.0 × 10−7 ohm-cm2.
For compound semiconductors with the diamond structure, (111) planar channel walls contain different atomic species when viewed from different sides; thus under channeling conditions, the yields of ions backscattered from the different species oscillate with depth in opposite senses for ion beams directed towards the different channel sides. We have studied this effect for 2 MeV He ions in a (111) channel of InP crystals. The application of this channeling phenomenon to the determination of the lattice sites of solute atoms was demonstrated for implanted In and Sb atoms at different depths in GaP.