Wide Range Of Oxygen Pressures Research Articles

Reliable operation of Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes at 600 °C

Beta gallium oxide (β-Ga2O3)-based semiconductor heterojunctions have recently demonstrated improved performance at high voltages and elevated temperatures and are, thus, promising for applications in power electronic devices and harsh environment sensors. However, the long-term reliability of these ultra-wideband gap (UWBG) semiconductor devices remains barely addressed and may be strongly influenced by chemical reactions at the p–n heterojunction interface. Here, we experimentally demonstrate operation and evaluate the reliability of Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes during extended operation at 600 °C, as well as after 30 repeated cycles between 25 and 550 °C. The calculated pO2-temperature phase stability diagram of the Ga-Cr-O material system predicts that Ga2O3 and Cr2O3 should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The fabricated Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes show room-temperature on/off ratios >104 at ±5 V and a breakdown voltage (VBr) of −390 V. The leakage current increases with increasing temperature up to 600 °C, which is attributed to Poole–Frenkel emission with a trap barrier height of 0.19 eV. Over the course of a 140-h thermal soak at 600 °C, both the device turn-on voltage and on-state resistance increase from 1.08 V and 5.34 mΩ cm2 to 1.59 V and 7.1 mΩ cm2, respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr2O3/Ga2O3 interface as observed from the time-of-flight secondary ion mass spectrometry analysis. These findings inform future design strategies of UWBG semiconductor devices for harsh environment operation and underscore the need for further reliability assessments for β-Ga2O3-based devices.

Consistent interpretation of isotope and chemical oxygen exchange relaxation kinetics in SrFe0.85Mo0.15O3-δ ferrite.

This paper is devoted to the study of phase composition and kinetic and thermodynamic characteristics of Mo-doped strontium ferrite SrFe0.85Mo0.15O3-δ (SFM15) under oxygen-conducting membrane working conditions. Single-phase SFM15 with a cubic Pm3̄m structure was synthesized using a ceramic method. It was shown that the molybdenum introduction stabilizes the perovskite cubic structure over a wide range of oxygen pressures and temperatures, preventing the bulk phase transition at high temperatures. Oxygen exchange constants, diffusion coefficients and activation energy of oxygen exchange were obtained using oxygen relaxation and isotopic exchange techniques, and the obtained values are consistent with known literature data. It was shown that the surface reaction rates obtained using chemical and tracer relaxation methods are quantitatively comparable with each other, despite significantly different experimental conditions. This result not only confirms the reliability of the data obtained by independent methods, but also allows one to expand the area of physical conditions for studying the kinetics of oxygen transfer where another method has technical or methodological limitations.

Structural and luminescence properties of laser assisted Eu3+ doped BaZrO3 thin films

Eu3+ doped BaZrO3 thin films were deposited on quartz substrates in a wide range of oxygen pressure (1–300 mTorr), using Pulsed laser deposition technique. X-ray diffraction pattern showed the different growth orientation of the films with oxygen pressure (PO2). The different growth mechanisms were drawn to explanin the thin films growth mechanism at the different oxygen pressure. Time of flight secondary ion mass spectroscopy analysis exhibited homogeneous distribution of all the elements throughout the depth of the films. The depth profile anaylsis revealed sharp interface at the low PO2 and broad intermixing region of the films with the substrate at low PO2. Transmittance spectra revealed the different optical bandgap with different PO2. Excitation spectra showed a broad absorption band centered at 260 nm due to the absorption in the defect states. The emission spectra showed intense emission at 597 nm corresponding to the 5D0→7F1 transition of Eu3+. Apart from this, a broad blue-green emission was observed centered at 440 nm due to the trap levels associated with the oxygen vacancy below the conduction band. The energy transfer mechanism between the host to Eu3+ ion was described via a defect state process. Colour coordintes of the emission spectra showed that colour emission of thin films can be tuned by varying the oxygen pressure. The short-excited state lifetime and high value of internal quantum efficiency make these films potential candidates for light emitting diode application.

Effect of ambient oxygen on the photoluminescence of sol–gel-derived nanocrystalline ZrO2:Eu,Nb

The development of inorganic nanophosphors is an active research field due to many applications, including optical gas sensing materials. We found a systematic dependence of the photoluminescence (PL) of europium (Eu3+) impurity ions in zirconia (ZrO2) nanocrystals on the ambient oxygen concentration in a O2/N2 mixture at normal pressure. Europium-doped ZrO2 powders were synthesized via a sol–gel route. Heat-treatment at 1200 °C resulted in a well-developed monoclinic phase (XRD crystallite size of ~50 nm) and an intense PL of Eu3+ ions residing in the dominant phase (Eu3+ was excited directly at 395 or 464 nm). Co-doping with niobium resulted in a narrowing of the PL emission lines. Only Nb5+ was detected by XPS and is believed to charge-compensate Eu3+ activators throughout the material leading to a more regular crystal lattice.At room temperature, the exposure to oxygen suppressed the Eu3+ fluorescence, whereas, at elevated temperatures (300 °C), the effect was reversed. At 300 °C and under a focused continuous laser beam, a substantial PL response (>50%) was achieved when switching 100% of N2 for 100% of O2. PL decay kinetics clearly showed that at 300 °C fluorescence quenching centers were induced within the material by oxygen desorption.The relatively fast (<5 min) and sub-linear PL response to the changes of oxygen concentration shows that ZrO2:Eu,Nb is a promising PL-based oxygen sensing material over a wide-range of oxygen pressures.

Brightly luminescent Pt(II) pincer complexes with a sterically demanding carboranyl-phenylpyridine ligand: a new material class for diverse optoelectronic applications.

A series of three Pt(II) complexes with a doubly cyclometalating terdentate ligand L1, L1H2 = 3,6-bis(p-anizolyl)-2-carboranyl-pyridine, and diethyl sulfide (1), triphenylphosphine (2), and t-butylisonitrile (3) as ancillary ligands were synthesized. X-ray diffraction studies of 1 and 2 show a coordination of the L1 ligand in a C-N-C mode in which the bulky and rigid o-carborane fragment is cyclometalated via a C atom. Importantly, no close intermolecular Pt-Pt contacts occur with this ligand type. The new Pt(II) pincer complexes display very high luminescence quantum yields at decay times of several tens of μs even in solution under ambient conditions. On the basis of the low-temperature (T = 1.3 K) emission decay behavior, the emission is assigned to a ligand centered triplet excited state (3)LC with small (1,3)MLCT admixtures. Because the phosphorescence is effectively quenched by molecular oxygen, optical sensors operating in a wide range of oxygen pressure can be developed. Owing to the very high luminescence quantum yields, the new materials might also become attractive as emitter materials for diverse optoelectronic applications.

Calcium- and Cobalt-Doped Yttrium Chromites as an Interconnect Material for Solid Oxide Fuel Cells

The structural, thermal, and electrical characteristics of calcium- and cobalt-doped yttrium chromites were studied for potential use as interconnect material in high temperature solid oxide fuel cells as well as other high temperature electrochemical and thermoelectric devices. The compositions formed single-phase orthorhombic perovskite structures in a wide range of oxygen pressures. Sintering behavior was remarkably enhanced as a result of cobalt substitution for chromium, and densities 95 and 97% of theoretical density were obtained after sintering at in air, when was 0.2 and 0.3, respectively. The electrical conductivity in both oxidizing and reducing atmospheres was significantly improved with cobalt additions, and was 49 and 10 S/cm at and 55 and 14 S/cm at in air and forming gas, respectively, for . The conductivity increase upon cobalt substitution for chromium was attributed to the charge carrier density increase as confirmed by Seebeck measurements. The thermal expansion coefficient was increased with cobalt content and closely matched to that of an 8 mol % yttria-stabilized zirconia (YSZ) electrolyte for . For , and YSZ were found to be chemically compatible for firing temperatures to at least .

Structure and dielectric function of two- and single-domain ZnO epitaxial films

The differences between two- and single-domain ZnO epitaxial films, grown by reactive pulsed magnetron sputtering, have been studied with respect to their texture development and x-ray coherence length behavior at various substrate temperatures and oxygen partial pressures. The film in-plane ordering depends on the surface pretreatment of the sapphire substrate. After pretreatment in an oxygen radio-frequency plasma, single-domain films form even at a substrate temperature of 100°C in a wide range of oxygen pressures, and at a growth rate up to 1.2nm∕s. The single-domain films show a linear dependence of the x-ray coherence length on the substrate temperature, while a steplike dependence is characteristic of the two-domain films. The ZnO complex dielectric function was obtained using a parametrized semiconductor oscillator model for spectroscopic ellipsometry data analysis. For the films grown at 550°C, the band gap of 3.29±0.01eV is independent of the type of in-plane ordering and variation of the texture. The oscillator broadening correlates with the width of (0002) diffraction peak rocking curve. Both parameters increase at high oxygen pressure and low substrate temperature, which is attributed to a higher defect (dislocation) density.

Oxygen adsorption on Mo(112) surface studied by ab initio genetic algorithm and experiment

Density functional theory in combination with genetic algorithm is applied to determine the atomic models of p(1x2) and p(1x3) surface structures observed upon oxygen adsorption on a Mo(112) surface. The authors' simulations reveal an unusual flexibility of Mo(112) resulting in oxygen-induced reconstructions and lead to more stable structures than any suggested so far. Comparison of the stabilities of the predicted models shows that different p(1x2) and p(1x3) structures may coexist over a wide range of oxygen pressures. A pure p(1x2) structure can be obtained only in a narrow region of oxygen pressures. In contrast, a pure p(1x3) structure cannot exist as a stable phase. The results of simulations are fully supported by a multitude of experimental data obtained from low energy electron diffraction, x-ray photoelectron spectroscopy, and scanning tunneling microscopy.

Oxygen Vacancies and Intermediate Spin Trivalent Cobalt Ions in Lithium-Overstoichiometric LiCoO2

Thermal treatments under a very wide range of oxygen pressures were used to probe the composition and defect nature of a lithium-overstoichiometric “Lix0CoO2” (x0 > 1) sample using X-ray powder diffraction, 7Li NMR, and electrochemical tests. It was found that at 900 °C, under atmospheric and elevated oxygen pressures, the lithium-overstoichiometric sample gradually transformed to stoichiometric LiCoO2 by losing excess lithium in the form of Li2O. In addition, it was shown that the defect associated with Co2+ and oxygen deficiency as reported by Gorshkov et al. and Karelina et al. had a different NMR signature than that present in Li-overstoichiometric samples. Therefore, it is believed that oxygen vacancies are present but Co2+ ions are not present in Lix0CoO2 (x0 > 1). This leads to a formula [Li]interslab[CoIII1-3tCo3+(IS)2tLit]slab[O2-t], involving an intermediate spin configuration for 2t Co3+ ions in a square-based pyramidal site. This new model was supported by the NMR and magnetic data of the lithium-overstoichiometric sample and its deintercalated compounds. The effect of the defect on the end-of-discharge voltage profile during cycling was also discussed.

Thin films of Sr2FeMoO6 grown by pulsed laser deposition: preparation and characterization

AbstractWe report the first in situ XPS analysis of thin epitaxial films of the ferromagnetic double perovskite Sr2FeMoO6, which have been prepared by pulsed laser deposition on (100) SrTiO3 substrates under different oxygen pressures. The films consist mainly of the c‐axis‐oriented SFMO phase over a wide range of oxygen pressure (10−8–10−4 mbar) as determined by ex situ x‐ray diffraction measurements. However, different secondary phases are observed, depending on the preparation conditions. When obtained in ultrahigh vacuum conditions (10−8 mbar) the films show the presence of metallic Fe, whereas when prepared under pure oxygen gas flow (pressures above 10−6 mbar ) the Fe3O4 phase is observed. The in situ XPS analysis suggests the formation of the SrFeO3 phase in addition to SFMO, whereas the SrMoO4 phase is formed at oxygen pressures above 10−4 mbar. Copyright © 2002 John Wiley &amp; Sons, Ltd.

In situ characterisation of Sr2FeMoO6 films prepared by pulsed laser deposition

Thin epitaxial films of the ferromagnetic double perovskite Sr 2 FeMoO 6 (SFMO) have been prepared by pulsed laser deposition on SrTiO 3 (100) substrates under different oxygen pressures. The surface electronic structure of the as-deposited films has been analysed by XPS without exposure to the atmosphere. The films consist mainly of the c-axis oriented SFMO phase over a wide range of oxygen pressure from 10 -8 to 10 -4 mbar. However, the analysis of the films by XPS and XRD revealed the presence of different secondary phases depending on the preparation conditions. The films prepared under ultra-high vacuum conditions show the presence of metallic Fe. The films prepared under pure oxygen gas flow at pressures above 10 -6 mbar did not show any trace of metallic Fe, but Fe 3 O 4 phase. In these films XPS analysis revealed the formation of the SrFeO 3 phase (SFO), or another non reported Sr-containing phase, which is not observed by XRD. At higher oxygen pressures above 10 -4 mbar there is a clear formation of SrMoO 4 phase (SMO). In conclusion, a ultra-high vacuum atmosphere favours the formation of metallic Fe, whereas the increase in oxygen pressure during deposition induces the decomposition of the SFMO phase to SFO and SMO phases. The conditions for the preparation of pure SFMO phase have not been attained so far, which would lie within a very narrow oxygen pressure window. Our best films (with no any other ferromagnetic phase than SFMO) show a saturation magnetisation of about 1 μ B /f.u. at 10 K, and T c close to 200 K. The resistivity of the films has a metallic-like behaviour and remains almost constant in the temperature range from 10 to 350 K to about 5 to 8 10 -4 Ω cm. It was measured a magnetoresistance of about -10% at 10 T and 10 K, which coincides with the values reported in the literature for SFMO films.

Pulsed Laser Deposition of Y1Ba2Cu3O7-δ?Thin Films in High Oxygen Partial Pressures

ABSTRACTPulsed laser deposition of Yba2Cu3O7-δ?(YBCO) coated conductors was studied for the range of P(O2) from 120 mTorr to 1200 mTorr, higher than typically used oxygen partial pressures during deposition. The purpose of the investigation was to determine the sensitivity of YBCO film quality to varying P(O2) for scaled-up fabrication of long-length coated conductors. Deposition at high P(O2) (≥?400 mTorr) gave very high and more consistent critical temperatures (Tc ≈?92 ±?0.4 °K) than results obtained at lower oxygen partial pressures (≤?200 mTorr) as determined by magnetic susceptibility measurements. Typically, the lower partial pressures are used although the laser fluence used in this research (3.2 J/cm2) is higher than typical. Transport Jc's were consistantly high for a wide range of oxygen pressures, 5–8 ×?106 A/cm2 at 77 K, self-field for P(O2) = 200–1200 mTorr. These results indicate that pulsed laser deposition of YBCO is relatively insensitive to P(O2) at the higher pressures of oxygen considered.

Preparation and Characterization of a Zirconia Oxygen Sensor with an Internal Reference for Low-Temperature Operation

Abstract A miniature zirconia oxygen sensor with a Ag-based electrode as a measuring electrode and with a Cu/Cu2O or Pd/PdO electrode as an internal reference electrode is workable at low temperatures even near 550 K in a wide range of oxygen pressures.

Ab initio calculations on the Al2O3(0001) surface

We calculate using a density functional pseudopotential method the atomic and electronic structure of the (0001) surface of α-alumina (Al2O3). The material is studied in the form of a slab with periodic boundary conditions, containing up to eight layers of the stoichiometric Al2O3 units. Five different terminations of the surface are calculated, representing different surface excesses of oxygen, and their free energies are estimated as a function of oxygen partial pressure. Internal relaxations of the atomic positions are obtained. The aluminium terminated surface, which is stoichiometric, has the lowest surface energy for a wide range of oxygen pressures.

Substrate temperature and oxygen pressure dependence of pulsed laser-deposited Sr ferrite films

The effect of substrate temperature and oxygen pressure on the microstructure, magnetic and magneto-optical properties of Sr ferrite (SrM) films grown on (001) single-crystal sapphire substrates by pulsed laser deposition has been investigated. Polycrystalline SrM films with perpendicular magnetic anisotropy could be prepared under a wide range of oxygen pressures and relatively high temperatures, sufficient to crystallise the material. However, an almost exclusive c-axis orientation normal to the film plane could be attained only at a narrow operational window centered at 0.1 mbar and 840°C. The magneto-optical properties of the films were comparable to those of the bulk barium hexaferrite single-crystal material. In addition, results obtained by atomic force microscopy provide convincing evidence that the growth of Sr ferrite on sapphire takes place by a spiral growth mechanism.

Analysis of Pulsed Excimer Laser Ablation of PZT, Pb And Ti in an Oxygen Ambient Using Energy Dispersive Mass Spectrometry

AbstractA number of factors have been identified as affecting the gas phase oxidation of cations during pulsed laser ablation of lead zirconate titanate (PZT), by using energy dispersive mass spectrometry (EDMS). These phenomena are known to be critical to the deposition of high quality thin films of PZT. However, to isolate the role of each cation species and the influence of its gas phase properties, we have also looked beyond the ablation of the PZT ceramic. In this paper, we detail our observations during pulsed laser ablation of electroceramic and metallic targets, specifically PZT, Pb and Ti. We have observed the evolution and transport of molecular species during gas phase expansion over a wide range of oxygen pressures (10−7 - 10−1 mbar) and laser fluences (0.3 -10 Jcm−2). The yields of the molecular ion species are strongly dependent upon the number density of the background gas. We propose an interpretation of the relative yields of the species based upon their respective gas phase binding energies and their ease of formation based upon the reaction thermodynamics and reaction cross-section of each species.

Solid State Equilibria in the Ba-Cu-O System

Thermodynamic modeling is performed for the Ba-Cu-O system, which is essential to a good understanding of phase and chemical equilibria in the Y-Ba-Cu-O and some other oxide systems containing high-temperature superconductors. A self-consistent set of thermodynamic functions of the phases BaO 2, BaCu 2O 2, BaCuO 2, Ba 2Cu 3O 5+ y and Ba 2CuO 3+ q is obtained. A variety of phase equilibria in the Ba-Cu-O system are calculated for a wide range of oxygen pressures and temperatures. The present thermodynamic data can be readily used for computing the phase equilibria and conditions for thermodynamic stability of oxide superconductors. It is detected that both BaCuO 2 and Ba 2CuO 3+ q have two stability boundaries, one at low temperatures and high oxygen pressures, and the other at high temperatures and low oxygen pressures. Critical analysis of phase equilibria in the Ba-Cu-O system makes it possible to explain a number of conflicting results encountered in the literature. These contradictions arise from solid state reactions between phases, which may be very slow due to kinetic problems.

Optical emission spectra from microwave oxygen plasma produced by surfatron discharge

This paper is devoted to a systematic study of the production of molecular and atomic ions and excited atoms in a microwave oxygen plasma generated by surfatron in a quartz tube (i.d. 6 mm). The content of O 2 + , O+ and O is measured, using the optical emission spectroscopy, in a wide range of oxygen pressures (from 3.5 Pa to 102 Pa) and microwaves powers (from about 30 W to 300 W) delivered into the plasma. It is shown that the content of individual species O2+, O+ and O strongly depends on the conditions, particularly operating pressurep and the microwave powerP, under which plasma is created. It means that the chemical reactivity of microwave plasma also strongly depends onp andP. It is of great importance for many practical applications, e.g. for a reproducible production of thin films with prescribed properties.

Ultrahigh vacuum study of indium oxide GaAs(110) interfaces

Indium oxide/n-GaAs(110) interfaces fabricated by means of reactive evaporation of indium in the presence of oxygen onto ultrahigh vacuum cleaved GaAs(110) have been studied by means of Auger electron spectroscopy and electron energy loss spectroscopy. The results show that the growth of the indium oxide layers at room temperature under a wide range of oxygen pressures followed the Stransky–Krastanov model. In all cases the presence of In clusters at the interface was observed, and at the high-pressure regime (∼ 1 × 10−4 Torr) some oxidation of the GaAs surface was noted as well. These results can be correlated to our earlier report [J. Appl. Phys. 69, 1494 (1991)] on the performance of diodes produced under high vacuum conditions.

Quantitative SIMS analysis based on the “combined LTE-bond breaking” model

A semi-theoretical model for quantitative SIMS analysis, which is based on the experimental results of secondary ion emission on the metallic surface exposed to oxygen, is discussed in this paper. The improvement upon the “combined LTE-bond breaking” idea comes from the fact that the ion yields of metallic elements will be enhanced by increasing oxygen pressure, but the enhancement for the metallic element with higher electronegativity is more remarkable. The amount of positive and negative secondary ions produced in bond breaking is not only determined by how many oxides are formed on the surface, but by how strong the oxide bonds are. If the bond breaking process is considered as an ionization process in which elemental ionization potentials are supposed to be lower than their normal values, and if both the amount of oxides and the bond strength are assumed to be functions of the electronegativity the theoretical equation of the positive secondary ion yield on the surface exposed to oxygen is obtained. In the equation, the quantity of exposure to oxygen and the elemental electronegativity are included. This quantitative method can be used under a wide range of oxygen pressure and some satisfactory analytical results have been obtained.