High-pressure Oxygen Annealing Research Articles

Enhanced conductivity in Sr doped La3Ni2O7-δ with high-pressure oxygen annealing

The structural, electrical and magnetic properties of a series of La3-xSrxNi2O7-δ polycrystalline samples have been systematically investigated with and without high-pressure oxygen annealing. Without annealing, the introduction of Sr at La site leads to a moderate enhancement of conductivity. The high-pressure oxygen annealing leads to an increase of oxygen content by ∼1.08 %. Consequently, the conductivity of the annealing samples is significantly enhanced comparing to the untreated samples. The La3-xSrxNi2O7-δ samples exhibit metal-like behavior in the whole temperature range with annealing, which is due to the hole doping effect. The present results could contribute to the exploration of possible ambient superconductivity in La3Ni2O7-δ system.

Realizing metallicity in Sr2IrO4 thin films by high-pressure oxygen annealing

Perovskite iridates are a promising material platform for hosting unconventional superconductivity. Transport measurements of Sr2IrO4 thin-film field-effect transistors are expected to provide irrefutable evidence for the existence of superconductivity. However, these experiments have revealed a remarkably robust insulating state over wide electron and hole doping ranges; this finding is in contrast to the case of the bulk material, in which metallicity appears upon moderate electron doping by substituting cations in place of Sr. The nature of this robust insulating state and whether any metallic state can be realized in the Sr2IrO4 thin film are two remaining challenges that preclude further progress in the search for superconductivity in this system. Here, we show that this insulating state is enhanced in Sr2IrO4 thin films by thermal annealing under vacuum conditions, while it can be destroyed upon annealing in an oxygen atmosphere within restricted ranges of oxygen pressure, annealing temperature and ion substitution levels. The resulting films exhibit metallic transport behavior near room temperature and a metal–insulator crossover at ~200 K. Our results point to the potentially important roles of the oxygen vacancies at different atomic sites in the formation of the robust insulating state and the new metallic state and to their interplay in the Sr2IrO4 thin film. This finding opens new possibilities in the search for unconventional superconductivity by further tailoring the as-found metallic state in properly oxygen-annealed Sr2IrO4 thin films.

Effects of high pressure oxygen annealing on Hf0.5Zr0.5O2 ferroelectric device

We report a high-pressure oxygen annealing (HPOA) process to improve the performance of TiN/Hf0.5Zr0.5O2 (HZO)/TiN devices by controlling the number of oxygen vacancies and carbon contaminants. The ferroelectric properties of HZO film after HPOA at 250 °C for 30 min under different oxygen pressures from 0 to 80 bar were evaluated by electrical and structural characterizations. We found that a sample treated with an oxygen pressure at 40 bar exhibited large switchable polarization (2Pr) of approximately 38 and 47 μC cm−2 in its pristine and wake-up states, respectively. Compared to a control sample, an approximately 40% reduction in the wake-up effect was achieved after HPOA at 40 bar. Improved ferroelectric properties of HZO film can be explained by the appropriate amount of oxygen vacancies and reduced carbon contaminants after HPOA.

Increase of YBa2Cu3O7 critical currents by Mo substitution and high-pressure oxygen annealing

We have previously shown for powders that Mo substitution into the CuO chains of YBa2Cu3O7 can create effective pinning centres which significantly increase the critical current density (jc) in 7 T field by a factor of 4 and 10 at 50 and 60 K, respectively (Rogacki et al., 2006) [1]. The present work reports on the influence of the Mo substitution and high-pressure oxygen annealing on the pinning properties and critical currents of YBa2Cu3-xMoxO7-d by comparing pure (x = 0, d > 0) and substituted (x = 0.03, d < 0) single crystals. Pinning properties have been investigated by measurements of magnetization loops and calculations of jc in the ab-plane, in the temperature range from 2 to 90 K and in fields up to 14 T. Depending on the Mo substitution and the oxygen treatment, several types of pinning centres increasing jc have been revealed and analysed in the frame of Dew-Hughes’ and Kramer's models.

High pressure synthesis of a quasi-one-dimensional GdFeO3-type perovskite PrCuO3 with nearly divalent Cu ions.

A new perovskite-type cuprate PrCuO3 has been synthesized by high-pressure oxygen annealing. Synchrotron X-ray powder diffraction and absorption spectroscopy revealed that PrCuO3 crystallizes in the GdFeO3-type structure with cooperative Jahn-Teller distortion, forming one-dimensional chains of corner-shared CuO4 plaquettes with nearly divalent Cu ions.

High-pressure oxygen annealing of Al2O3 passivation layer for performance enhancement of graphene field-effect transistors

High-pressure annealing in oxygen ambient at low temperatures (∼300 °C) was effective in improving the performance of graphene field-effect transistors. The field-effect mobility was improved by 45% and 83% for holes and electrons, respectively. The improvement in the quality of Al2O3 and the reduction in oxygen-related charge generation at the Al2O3-graphene interface, are suggested as the reasons for this improvement. This process can be useful for the commercial implementation of graphene-based electronic devices.

Growth and structural characterization of large superconducting crystals of La2−xCa1+xCu2O6

Large crystals of La$_{2-x}$Ca$_{1+x}$Cu$_2$O$_{6}$ (La-Ca-2126) with $x=0.10$ and 0.15 have been grown and converted to bulk superconductors by high-pressure oxygen annealing. The superconducting transition temperature, $T_c$, is as high as 55~K; this can be raised to 60~K by post-annealing in air. Here we present structural and magnetic characterizations of these crystals using neutron scattering and muon spin rotation techniques. While the as-grown, non-superconducting crystals are single phase, we find that the superconducting crystals contain 3 phases forming coherent domains stacked along the $c$ axis: the dominant La-Ca-2126 phase, very thin (1.5 unit-cell) intergrowths of La$_2$CuO$_4$, and an antiferromagnetic La$_8$Cu$_8$O$_{20}$ phase. We propose that the formation and segregation of the latter phases increases the Ca concentration of the La-Ca-2126, thus providing the hole-doping that supports superconductivity.

Phase transformation in ultrathin Y–Ba–Cu–O films by high-pressure oxygen annealing

To examine the thermodynamic phase stability of Y–Ba–Cu–O superconductors in ultrathin film form, we study the effects of high-pressure oxygen annealing and Cu enrichment on YBa2Cu3O[Formula: see text] films ranging from 8 nm to 15 nm in thickness. The films are grown epitaxially on SrTiO3 substrates by pulsed laser-ablated deposition, and post-annealed in 475 atm of oxygen at 800[Formula: see text]C while subjected to varying concentrations of excess CuO. X-ray diffraction (XRD) on the annealed films shows evidence for suppression of YBa2Cu3O[Formula: see text] and formation of Y2Ba4Cu8O[Formula: see text]. This structural transformation towards a higher-oxidation phase is expectedly correlated with film thinness and Cu enrichment.

Structure and Magnetic Properties of Cr2O3/CrO2Nanoparticles Prepared by Reactive Laser Ablation and Oxidation under High Pressure of Oxygen

<TEX>$Cr_2O_3$</TEX> nanoparticles were prepared via one-step reactive laser ablation of Cr in oxygen. The metastable <TEX>$CrO_2$</TEX> phase was obtained through the subsequent oxidation of <TEX>$Cr_2O_3$</TEX> nanoparticles under <TEX>$O_2$</TEX> with gas pressures of up to 40 MPa. The as-prepared <TEX>$Cr_2O_3$</TEX> nanoparticles are spherical or rectangular in shape with sizes ranging from 20 nm to 50 nm. High oxygen pressure annealing is effective in producing meta-stable <TEX>$CrO_2$</TEX> from as-dried <TEX>$Cr_2O_3$</TEX> nanoparticles, and the <TEX>$Cr_2O_3$</TEX> nanoparticles exhibit a weak ferromagnetic behavior with an exchange bias of up to 11 mT that can be ascribed to the interfacial exchange coupling between uncompensated surface spins and the antiferromagnetic core. The <TEX>$Cr_2O_3/CrO_2$</TEX> nanoparticles exhibit an enhanced saturation magnetization and a reduced exchange bias with an increasing faction of <TEX>$CrO_2$</TEX> due to the elimination of uncompensated surface spins over the <TEX>$Cr_2O_3$</TEX> nanoparticles when exposed to a high pressure of <TEX>$O_2$</TEX> and/or possible phase segregation that results in a smaller grain size for both <TEX>$Cr_2O_3$</TEX> and <TEX>$CrO_2$</TEX>.

Reducing dielectric loss in CaCu3Ti4O12 thin films by high-pressure oxygen annealing

High pressure oxygen annealing could reduce the dielectric loss in CCTO films as well as induce obvious difference in strain status.

Nanoscale coherent intergrowthlike defects in a crystal ofLa1.9Ca1.1Cu2O6+δmade superconducting by high-pressure oxygen annealing

Superconductivity with ${T}_{c}=53.5$ K has been induced in a large ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_{2}{\mathrm{O}}_{6}$ (La-2126) single crystal by annealing in a high partial pressure of oxygen at 1200 \ifmmode^\circ\else\textdegree\fi{}C. Using transmission electron microscopy techniques, we show that a secondary Ca-doped ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ (La-214) phase, not present in the as-grown crystal, appears as a coherent intergrowthlike defect as a consequence of the annealing. A corresponding secondary superconducting transition near 13 K is evident in the magnetization measurement. Electron energy-loss spectroscopy reveals a pre-edge peak at the O-$K$ edge in the superconducting La-2126 phase, which is absent in the as-grown crystal, confirming the hole doping by interstitial oxygen.

Improvement of Negative Bias Temperature Illumination Stability of Amorphous IGZO Thin-Film Transistors by Water Vapor-Assisted High-Pressure Oxygen Annealing

The effect of high-pressure annealing on the performance and negative bias temperature illumination stress (NBTIS)-induced instability of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) is investigated. IGZO TFTs that were annealed in a H2O vapor-assisted high-pressure O2 atmosphere exhibited significantly improved field-effect mobility and stability against NBTIS compared with those annealed in only high-pressure O2 or air. Annealing under high-pressure O2 in the presence of H2O vapor effectively reduces oxygen-related defects, which act as subgap states within the bandgap. This phenomenon affects band alignments, including the bandgap and conduction band offset [Δ(EC–EF)] of IGZO semiconductors, and is the basis for the improved performance and stability of the TFTs.

Effect of High-Pressure Oxygen Annealing on Electrical Characteristics of Metal–Alumina–Nitride–Oxide–Silicon-Type Flash Memory Devices

We have investigated the effect of high-pressure oxygen annealing (HPOA) on metal–alumina–nitride–oxide–silicon (MANOS)-type flash memory devices. HPOA devices show significant improvement of retention characteristics at elevated temperatures under positive gate bias owing to the reduction of charge loss rate through the blocking oxide. Considering the improvement of electrical characteristics and the change in the Al chemical state, this was attributed to the effective removal of traps and the recovery of the stoichiometry of Al2O3 layer by oxygen passivation. We also confirmed this effect in a Si–Al2O3–Pt capacitor. As a result, we consider that HPOA could be a crucial process for future MANOS-type memory devices.

Effects of Oxygen Annealing on Dielectric Properties of LuFeCuO4

We investigated the effect of oxygen annealing on dielectric properties of LuFeCuO4 mainly by dielectric measurements and transmission electron microscopy experiments. It was found that the leakage current density decreases with high-pressure oxygen annealing and the size of nanodomains with orientational polarization increases, which is characterized by the short-range ordering of Fe3+ and Cu2+ ions on a triangular lattice. In addition, the absolute value of relative permittivity ε' decreases with oxygen annealing and ε' exhibits a broad peak in the temperature window between room temperature and 550 K, which is similar to that observed in relaxor ferroelectric materials. The present experimental results suggest that the number of oxygen vacancies has a crucial effect on dielectric properties of LuFeCuO4.

Improved magnetic trapped field in thin-wall YBCO single-domain samples by high-pressure oxygen annealing

Oxygen high pressure (up to 16 MPa) has been introduced in the oxygen-annealing step necessary to make the YBa 2Cu 3O x phase superconducting (change x from 6 to about 7). It enables a displacement in the equilibrium phase diagram towards higher temperatures, which means the possibility to achieve the same final oxygen content as the one at low temperature, but with the benefit of higher diffusion rates. Initial development made on thin bars (1.5–3 mm thick) has confirmed the interest of using a high pressure of oxygen. TEM observations have shown an increase of twin density associated with higher Jc. This is in agreement with other works claiming the possibility of higher Jc by twin engineering, and more precisely by twin refinement while annealing at high temperature. We report the successful application of this process without any adjustment to so-called thin-wall single-domain samples. These samples are obtained by growing a crystal on a pellet already shaped with an array of holes. The advantage is that, as far as diffusion processes are concerned, the typical length is not anymore the diameter of the sample, but the thickness of the walls between holes. The trapped field of 16 mm diameter Y123 thin-wall single-domain samples was doubled (0.6 T vs. 0.3 T at 77 K) in a rather short annealing time (about 3 days). Microstructures as well as magneto-optical observations of plain and thin-wall samples evidenced a reduction of cracks in the thin-wall samples. Improved performances were confirmed by further characterizations performed from 77 down to 20 K using the pulse-field facilities of the LNCMP at Toulouse.

Low-temperature preparation of sputter-deposited Pb(Zr 0.52Ti 0.48)O 3 thin films through high oxygen-pressure annealing

This study examined the effect of high oxygen-pressure processing (HOPP) annealing on sputtered Pb(Zr x Ti 1− x )O 3 (PZT) thin films. Three hundred nanometer-thick PZT(52/48) thin films were sputter-deposited on the LNO-coated silicon substrates at a substrate temperature of 200 °C. The as-sputtered PZT(52/48) films were amorphous. On the other hand, after HOPP processing at 400 °C with applying oxygen pressures of 2, 4, 6, and 8 MPa, respectively, all the PZT thin films transformed to a high ( h 0 0)-oriented perovskite phase. The diffraction intensity of the ( h 0 0) peaks increased with increasing oxygen pressure of the HOPP process, indicating better crystallinity. The surface morphology of the PZT thin film after HOPP showed unusual grain shapes compared to the round-shape observed on the conventionally fabricated films. The low-temperature crystallization of HOPP process is associated with a decrease in the nucleation barrier for perovskite phase formation with a high oxygen pressure. The electrical measurements, such as the P– V, C– V, and ε( ω), revealed good ferroelectric properties after HOPP. Better ferroelectric properties of the PZT thin films were obtained by further increasing the oxygen pressure during HOPP. This low-temperature HOPP is compatible with existing silicon-based technology, by incorporating sputtering and low-temperature annealing.

Effects of Defect Control on the Polarization Properties in Bi2WO6-Based Single Crystals

Leakage-current and polarization-switching properties of defect-controlled Bi 2 WO 6 single crystals have been investigated along the crystallographic a axis at 25°C. High-pressure oxygen annealing decreased leakage current, indicating that electron is a detrimental carrier of the leakage-current property. 2-mol% Mn substitution at the W site is shown to be effective for suppressing leakage current density to the order of 108 A/cm2. Mn-doped crystals annealed under high-pressure oxygen atmosphere showed a well saturated polarization hysteresis loop with a remanent polarization of 50 μC/cm2 and a coercive field of 39 kV/cm. It is found that high-pressure oxygen annealing is effective for decreasing coercive field of Mn-doped crystals due to less oxygen vacancies. It is concluded that Mn substitution and the subsequent oxygen annealing enhance polarization switching as well as reduce leakage current in the BW system.

Defect control for polarization switching in Bi2WO6-based single crystals

The authors have investigated the leakage current mechanism and polarization properties of Bi2WO6 single crystals along the a axis at 25°C. High-pressure oxygen annealing led to a decrease in leakage current, showing that electron is a detrimental carrier of the leakage current property. It is demonstrated that 2mol% Mn substitution at the W site is effective for decreasing the leakage current density to the order of 10−8A∕cm2. Mn-doped crystals show a saturated polarization hysteresis loop with a remanent polarization of 50μC∕cm2. Electron spin resonance analysis suggests that Mn3+ acts as an electron acceptor for reducing the leakage current in the Bi2WO6 system.

Growth and characterization of Ca doped Eu-123 whiskers for intrinsic Josephson junctionapplications

Single-crystal whiskers of Eu-123 were grown from Te and Ca doped precursorpellets. The Eu-123 whiskers contained a certain amount of Ca but were notcontaminated with Te. Our composition analysis and superconducting propertymeasurements reveal that the whiskers inherited certain site disorders andoxygen deficiencies in the as-grown state, which is reflected in the very lowTc. High pressure oxygen annealing was required to increase theTc to aboutthe 80 K level. Submicron sized junctions fabricated by focused ion-beametching showed clear multi-branch structure with hysteresis in theI–V curve. The Eu-123 whiskers were found to possess excellent crystalline quality both in the as-grownstate and after high pressure annealing, despite the site disorders. The critical current densityJc of a Eu-123whisker (Tc = 45 K) wasestimated to be 1.43 × 105 A cm−2 at 4.2 K, twice that observed for Y-123 having a similarTc.

Oxygen Stability and Leakage Current Mechanism in Ferroelectric La-Substituted Bi4Ti3O12 Single Crystals

We have investigated the effects of La substitution on the oxygen stability, defects, leakage current and polarization properties in ferroelectric bismuth titanate (Bi4Ti3O12) by measuring the properties of single crystals and by ab initio electronic structure calculations. It is shown that electron holes arising from the incorporation of oxygen at oxygen vacancies act as detrimental carriers for leakage current at room temperature for the crystals of BiT and La-substituted BiT. La substitution is effective for reducing oxygen vacancies and thus electron holes, which is attributed to an excellent insulating property observed for La-substituted crystals. High-pressure oxygen annealing is demonstrated to improve the remanent polarization of La-substituted crystals.