Torr O2 Research Articles

Ethylene Oxidation on Polycrystalline Platinum over Eight Orders of Magnitude in Ethylene Pressure: A Kinetic Study in the Viscous Pressure Regime

C2H4oxidation on polycrystalline Pt films and foils atT=373–473 K was studied with mass spectrometry in the pressure ranges 10−6–102Torr C2H4and 0.3–1500 Torr O2(1 Torr=133.3 Pa). A new, “spatially resolved gas sampling” method enabled true kinetic data to be collected in the viscous pressure regime.In situmeasurements of surface hydrogen with a capacitance–voltage technique andex situcharacterization with Auger electron spectroscopy and atomic force microscopy were also performed. No structure sensitivity with respect to sample grain size could be seen. The reaction orders in C2H4and O2were +1 and −1 in oxygen excess, −2 and +3 in weak ethylene excess, and −0.5 and +1 in large ethylene excess, respectively. The apparent activation energy was between 30 and 75 kJ/mol for different reactant mixtures. The rate data could be qualitatively fitted to a simple Langmuir–Hinshelwood reaction scheme in the excess regimes, assuming competitive adsorption of C2H4and O2, abundant molecular desorption of the excess reactant, and a strong self-inhibition of C2H4adsorption.

High resolution electron energy loss spectroscopy of MnO(100) and oxidized MnO(100)

Single crystal MnO(100) substrates can be selectively oxidized to produce Mn2O3- and Mn3O4-like surfaces under mild oxidation/reduction conditions readily accessed under ultrahigh vacuum (UHV). MnO(100) yields a characteristic Mn 2p x-ray photoelectron spectroscopy (XPS) satellite structure and appropriate O/Mn concentrations from O 1s/Mn 2p XPS intensity ratios. Its high resolution electron energy loss (HREEL) spectrum shows a series of Fuchs–Kliewer multiple phonon excitations with a single loss energy of 70.9 meV, characteristic of the cubic manganese monoxide structure. However, the HREEL spectral (HREELS) background is high and the phonons are not as well resolved as those typically observed on comparable metal monoxides. Annealing the MnO(100) substrate at 625 K and 5×10−7 Torr O2 slowly forms Mn2O3, as indicated by O 1s and Mn 2p XPS, and does so without significantly altering the symmetry of the MnO(100) low energy electron diffraction pattern. The MnO(100)-Mn2O3 surface can be selectively reduced to Mn3O4-like composition by heating under UHV to 775 K and to MnO(100) at 1000 K. HREEL spectra for the UHV annealed surfaces are well-resolved, and for the MnO(100)-Mn3O4 substrate a second fundamental phonon loss is observed at 55.6 meV as a result of the lower symmetry of the Mn3O4 spinel structure. The UHV-annealed MnO(100) surface appears to be more highly ordered since its HREELS phonon loss peaks are better resolved. It is also somewhat reduced, however, resulting in a less intense phonon spectrum with a fundamental loss energy of only 65.1 meV.

Raman Spectroscopic Study of YBa2Cu4O8 Thin Film Grown from Amorphous Precursor

Raman spectroscopic study was performed on YBa2Cu4O8 (Y124) thin films prepared by in situ annealing of sputtered precursor. The amorphous precursors were deposited at 450° C in 50 mTorr. The films are a-axis oriented. The ratio of the vibrational intensity of oxygen in double Cu–O chain to that of apical oxygen (I O d /I O a ) corresponded well to the volume fraction of the Y124 phase in the films. With decrease in the composition Cu/Y, depending on oxygen pressure during deposition, the double Cu–O chain is suffered by the oxygen defects, which was confirmed by observing decreases in I O d /I O a and the appearance of the 537 cm-1 mode. The Y124 growth around the boundary of the stability region (1≤P O2 ≤200 Torr, 625≤T≤820° C) between Y124 and Y123 indicates that higher temperature and higher oxygen pressure during in situ annealing is preferable for the growth of Y124 films. The Y124 films annealed at 820° C under 200 Torr O2 showed I O d /I O a of 1.00, which is the same as the result for Y124 bulk prepared by a high-pressure technique.

Spatially and temporally resolved absolute O-atom concentrations in etching plasmas

The detection of atomic O by two-photon-allowed laser-induced fluorescence is implemented to obtain the spatial distribution and the temporal evolution of oxygen atoms in a 10 MHz parallel-plate rf discharge. Absolute concentrations are determined via a titration technique for intensities typically found in 20–100 W, 0.1–3 Torr O2 discharges, and are in the range 4–30×1014 atoms/cm3. The spatial O distribution after initial turn on of the discharge is rather uniform, indicative of a uniform production of O atoms throughout the interelectrode space. Furthermore, the decay of the O concentration reveals information about the reaction of O atoms with surface materials in the plasma environment, while the rise of the population indicates a time-dependent production rate. The dependence of the O-concentration rise and decay rates on discharge pressure, power, and gas composition is explored.

Growth and Stability of MgO/NiO Multilayered Films on Single Crystal NiO(100)

AbstractThe less than 1% lattice mismatch between MgO and NiO makes them ideal candidates for investigating the growth and stability of multilayered oxide films. Ultra-thin multilayers composed of alternating films of MgO and NiO were deposited onto a stoichiometric NiO(100) single crystal substrate at 250°C by evaporating Mg and Ni in 5×10−7 Torr of O2, respectively. The structure of these multilayers was determined using LEED. Reactivity and chemical composition studies of the MgO/NiO interfaces were carried out using XPS and UPS. The MgO/NiO multilayers grow epitaxially on NiO(100), as evidenced by LEED. XPS and UPS analysis indicates attenuation of the NiO or MgO peaks during growth which is consistent with discrete layering. Chemical analysis also reveals negligible intermixing of the MgO and NiO layers during deposition. Results pertaining to the thermal stability of the multilayers show that UHV annealing above 750°C results in significant diffusion of MgO into the NiO(100) substrate.

Reactions of NO3‐naphthalene adducts with O2 and NO2

AbstractThe kinetics of the gas‐phase reaction of the NO3 radical with naphthalene have been investigated at 150 torr O2 + 590 torr N2 and 600 torr O2 + 140 torr N2 at 298 ± 2 K. Relative rate measurements were carried out in reacting NO3N2O5‐naphthalene‐propene‐O2N2 mixtures by longpath Fourier transform infrared absorption spectroscopy. A rate constant ratio for the reactions of O2 and NO2 with the NO3‐naphthalene adduct of k/k < 4 × 10−7 was obtained from the competition between O2 and NO2 for reaction with the NO3‐naphthalene adduct and thermal decomposition of the adduct back to reactants. Atmospheric pressure ionization MS/MS measurements of the nitronaphthalene products of the NO3 radical‐initiated reaction of naphthalene are consistent with the proposed reaction mechanism, and the atmospheric implications of the data are discussed. © 1994 John Wiley & Sons, Inc.

Chemical Bonding, Structure, and Morphology of Mg/∝-Al2O3 and MGO / ∝-Al2O3 Interfaces

AbstractUltra-thin films of Mg and MgO were grown on ∝-Al2O3 (1012) surfaces (r-cut sapphire) and studied using reflection high energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS). When Mg is deposited at 30°C in ultra-high vacuum (UHV), the first monolayer of Mg atoms chemically bonds to the oxygen anions of the sapphire surface. At Mg coverages above a monolayer, a polycrystalline metallic Mg overlayer is formed. Annealing above 250°C in UHV causes the metallic Mg to desorb from the surface. However, annealing above 250°C in 10−6 torr O2 produces a polycrystalline MgO film. This MgO film recrystallizes after annealing in O2 at 900°C for 60 minutes and exhibits a crystallographic orientation of MgO (100) // ∝-Al2O3 (1012). RHEED indicates that the recrystallized MgO layer dewets the sapphire surface and forms islands. When Mg is deposited at 30°C in 10−6 torr O2, a polycrystalline MgO layer is created. This layer also becomes recrystallized and dewets the sapphire surface after extended annealing in O2 at 900°C. No evidence for a MgAl2O4 spinel phase was observed.

Rapid Thermal Oxidation of Lightly Doped Silicon in N2O

ABSTRACTIn this paper, a detailed study is presented for the growth kinetics of rapid thermal oxidation of lightly-doped silicon in N2O and O2 on (100), (110), and (111) oriented substrates. It was found that (110)-oriented Si has the highest growth rate in both N2O and dry O2, and (100) Si has the lowest rate. There is no “crossover” on the growth rate of rapid thermal N2O oxidation between (110) Si and (111) Si as compared to oxides grown in furnace N2O. Pressure dependence of rapid thermal N2O oxidation is reported for the first time. MOS capacitor results show that the low-pressure (40 Torr) N2O-grown oxides have much less interface state generation and charge trapping under constant current stress as compared to oxides grown in either 760 Torr N2O or O2 ambient.

Modification of GaAs/AlGaAs growth-interrupted interfaces through changes in ambient conditions during growth

The effect of the ambient conditions in the growth chamber of the molecular beam epitaxy machine during the growth of GaAs/Al0.35Ga0.65As structures was investigated. Both growth-interrupted (120 s at each heterointerface) and uninterrupted surfaces and interfaces were evaluated using a growth temperature of 580 °C. Two ambient conditions were studied: (a) ∼1×10−10 Torr O2; and (b) ultrahigh vacuum (UHV, ∼5×10−11 Torr, with no intentional introduction of contaminants). A striking difference was observed in both the 1.7 K photoluminescence (PL) spectra of single quantum well (SQW) structures and UHV scanning tunneling microscopy (STM) of surfaces, which were grown under ambient condition (a) as opposed to (b). When consecutive growth-interrupted SQW samples were grown with different well widths (25 and 28 Å) under condition (a), the emission energy splitting into several peaks was observed, indicating discrete thicknesses of the well. However, the peak energies shifted as the laser spot was scanned across each sample. Additionally, the peak energy shifted from sample to sample for the same nominal well width. On the other hand, when SQW samples were grown under condition (b), no variation in the emission energy was observed as the laser was scanned across the sample, or from sample to sample for a given well width. Furthermore, the PL observations are supported by UHV-STM results. UHV-STM images indicated a very rough surface with large islands containing small terraces on top (a bimodal distribution) for condition (a). Conversely, when samples were grown under condition (b), only large islands were observed. For growth interrupted GaAs surfaces, 400 Å×600 Å islands were observed, and for Al0.35Ga0.65As, they were 150 Å×400 Å, with a one-monolayer step in between islands. These data are consistent with abrupt interfaces with only a single-mode distribution for growth-interrupted surfaces. On the other hand, UHV-STM images of uninterrupted GaAs surfaces grown under condition (b) showed islands that were 40–60 Å across. Photoluminesce spectra of a similarly grown SQW sample showed only a single broad emission line, consistent with an interface configuration of many steps which are smaller than the exciton diameter. The results show that interface roughness is sensitive to background O2.

Epitaxial BaTiO3/MgO Structure Grown on GaAs(100) by Pulsed Laser Deposition*

BaTiO3 thin films on MgO encapsulated GaAs(100) were prepared epitaxially, in condition ranging from 600°C to 800°C and from 2×10-4 Torr O2 to 1×10-2 Torr O2, by pulsed laser deposition. The epitaxial relationship between all materials was cube-on-cube. BaTiO3 thin films obtained were c axis oriented tetragonal phase and showed ferroelectric properties. Epitaxial BaTiO3 had quite smooth surface of less than 100 Å in roughness. BaTiO3 grown on 40-Å-thick epitaxial MgO was also epitaxial although defect density in BaTiO3 looked higher than that in BaTiO3 grown on 400-Å-thick MgO. The interfaces of BaTiO3/40-Å-thick MgO/GaAs structure were abrupt and free of secondary phases in spite of the quite thin MgO layer.

Structure and morphology of Clean and Magnesium - Dosed Sapphire Surfaces

ABSTRACTThe structure and morphology of r-axis (1012) and c-axis (0001) sapphire (a-Al2O3) surfaces were studied with Low Energy and Reflection High Energy Electron Diffraction (LEED and RHEED) and Atomic Force Microscopy (AFM). Following heat treatments at 1000°C in 5×10-7 torr O2, both surfaces exhibited (1×1) diffraction patterns. Regular step-terrace structure was observed on the (1012) surface with the AFM, but no step structure was visible on the (0001) surface. The initial growth of magnesium overlayers dosed onto the (1012) surface at 25°C in ultra-high vacuum was monitored with RHEED and the chemical interactions at the Mg / Al2O3 interface were characterized with X-ray Photoelectron Spectroscopy (XPS). During the initial deposition, very little interaction between mg and the Al2O3 substrate occurs and a very abrupt interface is formed. At larger mg doses, a textured polycrystalline mg overlayer is produced. Annealing the mg film above 300°C causes some oxidation of mg at the Mg / Al2O3 interface, but most of the mg desorbs from the surface.

13C-selective infrared multiple-photon decomposition study of CBrClF2

The infrared multiple-photon single-frequency decomposition (IRMPD) of CBrClF2 was examined as functions of laser wavenumber, laser fluence, and partial pressure of CBrClF2. The initial step was the scission of a C-Br bond. In the presence of O2 the carbon-containing product was CF2O and its subsequent hydrolysis gave CO2. The initial dissociation was highly 13C selective at wavenumbers below 1014 cm−1. CBrClF2 decomposed at relatively low fluences as compared to CHClF2. However, the decomposition yield rapidly decreased with increasing pressure. In the large-scale irradiation experiment using about 8 J pulse at 1 Hz, we obtained a carbon yield of 0.41 μmol per pulse at a 13C-atom fraction of 17% for a mixture of 10 Torr CBrClF2 and 10 Torr O2, and a carbon yield of 0.17 μmol per pulse at a fraction of 29% for a mixture of 20 Torr CBrClF2 and 20 Torr O2. The IRMPD of CHClF2 gave a carbon yield of 0.18 μmol per pulse at 48% for 10 Torr neat CHClF2 and yield of 0.25 μmol at 52% for 20 Torr CHClF2. The large-scale irradiation experiment was also carried out for mixtures of CBr2F2 and O2. CHClF2 is the most productive of 13C.

Formation of highly excited oxygen atoms O(2p33p,5P) and O(2p33s,5S) in electron irradiation of mixtures of rare gases and O2

The 777 nm emission and absorption observed for electron-irradiated He and Ne containing O2 are attributed to the optical transition between O(2p33p,5P) and O(2p33s,5S). He*2(2s 3Σ+u) produces these excited oxygen atoms in its reaction with O2 and the rate constant is (4.2±0.4)×10−10 cm3 s−1. In irradiated Ne and irradiated Ne containing O2, we found the absorption due to Ne*2 (3p 3Πg, v′=1←3s 3Σ+u, v″=0, v′=2←v″=1, and v′=3←v″=2) and the sharp absorption due to O(2p33s,5S) at ∼777 nm, together with the emission of O(2p33p,5P) at the same wavelength. The vibrational constants of Ne*2 for the 3p 3Πg state are ωe=647 cm−1 and ω′exe=28 cm−1. Ne*2(3s 3Σ+u) may produce O(2p33s,5S) in its reaction with O2. On the other hand, the reactions of excited Ne atoms with O2 appear to be responsible for the emission. The formation of O(2p33s,5S) in irradiated Ar containing O2 is explained in terms of the mechanism involving highly excited Ar** (E≥14.3 eV) and O2. The rate constant for the reaction of Ar** with O2 and the deactivation constant of Ar** by Ar were determined to be (3.2±0.2)×10−10 and (8.9±1.3)×10−15 cm3 s−1, respectively. O(2p33s,5S) disappears by its reaction with O2 in He, Ne, and Ar. The rate constants were found to be (1.8±0.2)×10−10 cm3 s−1 in He, (1.7±0.2)×10−10 cm3 s−1 in Ne, and (1.7±0.2)×10−10 cm3 s−1 in Ar. Ar deactivates O(2p33s,5S) at a rate constant of (2.9±0.2)×10−15 cm3 s−1. Similar deactivation rate constants of O(2p33s,5S) by He and Ne are (1.8±0.1)×10−15 and (3.4±0.3)×10−15 cm3 s−1, respectively. The rough estimation of the radiation chemical yield for O(2p33s,5S) gives G=1×10−2 for the mixture of 7600 Torr He and 0.1 Torr O2.

Superconducting and optical properties of ultrathin films of Bi2(Sr,Ca)3Cu2Ox formed at 673 K

Electronic properties of ultrathin films of Bi2(Sr,Ca)3Cu2Ox, successively formed on MgO(100) at a substrate temperature of 673 K using atomic layering deposition of metals followed by oxidation with a 8×10−7 Torr NO2 are studied. It was revealed that the film formed at 673 K showed a semiconductive property, whereas the film annealed at 1093 K with 760 Torr O2 shows a superconducting transition with the onset Tc of 90 K and the zero-resistance Tc of 40 K. Optical absorption and x-ray diffraction studies indicate that the electronic structure of Bi2(Sr,Ca)3Cu2Ox ultrathin film changes from charge-transfer type semiconductor to a superconductor with an in-gap band, due to the increase in carrier concentration caused by annealing with O2.

Epitaxial growth of MgO on GaAs(001) for growing epitaxial BaTiO3 thin films by pulsed laser deposition

MgO buffer layers were deposited on GaAs by pulsed laser deposition for epitaxial growth of BaTiO3. MgO was grown epitaxially on GaAs for the first time; the orientation is (001) on GaAs(001). The best crystallographic quality was obtained at 350 °C in 5×10−6 Torr O2. BaTiO3 films with (001) orientation grew epitaxially on MgO/GaAs. The in-plane epitaxial relationship was BaTiO3[100]∥ MgO[100]∥ GaAs[100] in spite of a large lattice mismatch (25.5%) between MgO and GaAs.

Peroxyl radical scavenging by beta-carotene in lipid bilayers. Effect of oxygen partial pressure.

The effect of the partial pressure of oxygen (pO2) on the antioxidant reactions of all-trans-beta, beta-carotene (BC) was investigated in a soybean phosphatidylcholine liposome system. Peroxyl radicals generated by thermolysis of azo-bis(2,4-dimethylvaleronitrile) at 37 degrees C initiated lipid peroxidation. BC inhibited lipid peroxidation, which was monitored by conjugated diene formation, by up to 70% versus control at 160 and 15 torr O2. In contrast, at 760 torr O2 the maximum inhibition was approximately 40% versus control and inhibition was less reproducible. Peroxyl radicals oxidized BC to 5,6-epoxy-beta,beta-carotene and several unidentified polar products. The rates of both product formation and BC consumption were significantly higher at 160 torr than at 15 torr O2. However, at 160 and 760 torr O2, the rates of product formation and BC depletion were similar. In liposomes without azo-bis(2,4-dimethylvaleronitrile), BC depletion at 160 torr was only 64% that at 760 torr O2. These results suggest that both radical trapping and autoxidation reactions consume BC and that the latter are accelerated by high pO2. Autoxidation consumes BC without scavenging peroxyl radicals and may attenuate BC antioxidant activity, especially at high pO2. The similarity in its antioxidant effects at 15 and 160 torr O2 suggests that BC could provide antioxidant protection to any tissue within the normal physiologic range of pO2.

Micro-Structure Analysis of Co-Ferrite Magnetic Thin Films Prepared by Heat-Treatment Process in an Oxygen.

This paper presents the perpendicular magnetic properties and microscopic structures of Co-ferrite films prepared by a heat-oxidation process of evaporated Fe-Co multi-layered thin films. The oxi-dation temperature is less than 550°C. Thin films are oxidized in 760 Torr O2 atmosphere under a magnetic field of 1 kG applied in the perpendicular direction to the film surface. Co-ferrite films are evaluated using as follows: (a) the crystal structure is analyzed using an X-ray diffractometer, (b) a SEM is used in observations of surfaces and cross-sections, (c) composition distributions in the film depth direction are investigated by AES and XPS, (d) a VSM is used to measure hysteresis loops, and (e) a magneto-optical property is measured from Kerr-rotation angle. One cause of the perpendicular magnetic anisotropy of the oxide films is thought to be the effect of the columnar grain growth of oxide particles which facilitated by growing vertically along grain boundaries.

The effects of oxidation and air exposure on RbxBa1−xBiO3 superconducting thin films

Using x-ray photoelectron spectroscopy (XPS), we have investigated the effects of oxidation on RbxBa1−xBiO3, the degradation of its surface on exposure to air, and the feasibility of cleaning techniques to reverse this degradation. The sample was a superconducting thin film grown on MgO by molecular-beam epitaxy. XPS spectra of the initial, air-exposed surface show two O 1s peaks. The main O 1s peak increases in intensity upon oxidation at 300 °C for 10 min in 100 Torr O2, and decreases upon air exposure. Also, this peak shifts, together with the Ba and Bi peaks, to lower binding energy upon oxidation. These shifts to lower binding energy upon gain of oxygen (and to higher binding energy upon oxygen loss) can be explained by a rise in the conductivity of the film with rising oxygen content. Carbon and chlorine are observed on the initial air-exposed surface. Higher binding energy contributions in the Rb XPS peaks indicate that impurity compounds containing Rb are also present. Air-exposed surfaces do not show any XPS features at the Fermi level. However, removal of C and Cl by Ar+ sputtering at 1 kV, followed by oxidation, results in the appearance of intensity at the Fermi level. Hence sputter/oxidation treatments may prove useful in restoring the surfaces of films exposed to air (during patterning, for example) prior to overlayer deposition.

Surface reactivity of titanium–aluminum alloys: Ti3Al, TiAl, and TiAl3

Titanium‐based alloys, and Ti–Al alloys in particular, are of special interest due to the technological importance of these materials as structural materials in aggressive environments. The surface chemistry of 1000‐A films of Ti3Al, TiAl, and TiAl3, prepared by codeposition in an ultrahigh vacuum system, has been examined by x‐ray photoelectron spectroscopy (XPS). The alloys are examined as deposited and after treatments from room temperature to 600 °C in either vacuum of 5×10−7 Torr or 5×10−3 Torr O2. The alloys are compared with one another with regard to their surface segregation and oxide overlayer formation. Particular attention is given to the nature of the surface oxide that develops under the various conditions, and low take‐off angle XPS is used to determine the depth distribution of the various oxides present. The oxidation process is controlled by temperature, oxygen pressure, the bulk alloy composition and the free energies of formation for the various oxides found in the overlayers.

The oxidation and oxide overlayer stability of AgMn alloys

AbstractThe oxidation of several compositions (5, 9 and 14% Mn) of polycrystalline Ag‐Mn alloys from room temperature to 650 °C in high vacuum and in 5 m Torr O2 has been investigated by x‐ray photoelectron spectroscopy (XPS). The behavior of the oxidation and the nature of the oxide films are controlled by tempearature, oxygen pressure, bulk alloy composition and the stability of the oxide overlayers. At low temperatures and pressures, the oxidation process is controlled by the relative free energies of oxide formation of the alloy components. At high temperatures the oxide of stabilities play a large role in determining the oxidation state of the oxide overlayer. The chemical state and the morphology of the oxide of manganese formed during the alloy oxidation has been examined. The chemical state and the morphology of the oxide overlayers have been studied by XPS, x‐ray diffraction (XRD) and scanning electron microscopy (SEM). The complex oxidation process has been delineated.