Reduction Of Oxygen Vacancies Research Articles

Effect of non-magnetic Al3+ doping on structural, optical, electrical, dielectric and magnetic properties of BiFeO3 ceramics

Pure BiFeO3 (BFO) and Al doped BFO samples were synthesized via citrate precursor method and sintered at 500°C for two hours. Effect of Al doping on the structural, optical, electrical, dielectric and magnetic properties were investigated. X-ray diffraction (XRD) confirmed the distorted rhombohedral structure without any merging of peaks which indicates no structural transformation. Average crystallite size was found to be in the range 28–39nm. Field emission scanning electron microscopy (FESEM) images illustrated the dense, agglomerated, spherically shaped with reduced grain size nanoparticles. Increased value of dielectric constant with low dielectric tangent loss was observed for the Al doped BFO samples. The value of dielectric constant was found to be 51 at 100kHz for x = 0.1 sample. Temperature dependent dielectric constant showed a dielectric anomaly, indicating the antiferromagnetic transition. The remanent polarization and the corresponding coercive field for x = 0.1 was found to be 0.0625µC/cm2 and 56.154kV/cm at an operating voltage of 1000V. The improved electrical properties with low leakage current density were ascribed to the stabilization of the pervoskite structure and reduced oxygen vacancies.

Characterization and modeling of SET/RESET cycling induced read-disturb failure time degradation in a resistive switching memory

Read operation induced disturbance in SET-state in a tungsten oxide resistive switching memory is investigated. We observe that the reduction of oxygen vacancy density during read-disturb follows power-law dependence on cumulative read-disturb time. Our study shows that the SET-state read-disturb immunity progressively degrades by orders of magnitude as SET/RESET cycle number increases. To explore the cause of the read-disturb degradation, we perform a constant voltage stress to emulate high-field stress effects in SET/RESET cycling. We find that the read-disturb failure time degradation is attributed to high-field stress-generated oxide traps. Since the stress-generated traps may substitute for some of oxygen vacancies in forming conductive percolation paths in a switching dielectric, a stressed cell has a reduced oxygen vacancy density in SET-state, which in turn results in a shorter read-disturb failure time. We develop an analytical read-disturb degradation model including both cycling induced oxide trap creation and read-disturb induced oxygen vacancy reduction. Our model can well reproduce the measured read-disturb failure time degradation in a cycled cell without using fitting parameters.

Evidence of superparamagnetism and improved electrical properties in Ba and Ta co-doped BiFeO3 ceramics

Ba and Ta co-doped bismuth ferrites with the composition Bi0.8Ba0.2Fe1-xTaxO3 (where x = 0, 0.05, 0.10 and 0.15) were synthesized employing a solid state reaction method. Rietveld refinement of XRD patterns revealed that Ba doping caused structural transformation in perovskite BFO from rhombohedral (R3c) to tetragonal (P4mm) with up to 10% Ta doping. However, with 15% Ta substitution (BBFTO-15) the ceramics exhibited a mixture of tetragonal and rhombohedral structures. Curve fitting of room temperature magnetization vs. magnetic field data showed that the shape of the hysteresis loops for these ceramics were the result of coexistence of single domain and superparamagnetic contributions. For 10% Ta substitution (BBFTO-10), the contribution from superparamagnetic particles was dominant. Polarization with respect to electric field measurements did not yield typical hysteresis loops; instead evidence of high leakage was present in BBFTO-0 and BBFTO-5 while that of BBFTO-10 and BBFTO-15 exhibited a lossy dielectric nature. The I-V characteristics confirmed that Ta doping reduced the leakage current by greater than two orders of magnitude compared to BBFTO-0. It is believed that the reduction in oxygen vacancy in the doped samples was key in reducing the leakage current. As a matter of fact, XPS data revealed a reduction in oxygen vacancy concentration and the percentage of Fe3+was also found to increase with increasing Ta doping. Dielectric constant and dielectric loss were also found to decrease with the increase of Ta doping. Impedance spectroscopy confirmed the contribution from multiple impedance components including evidence of a dipolar relaxation that was found in BBFTO-5 and BBFTO-10. Defect associates were proposed to be responsible for such relaxations.

Thermal annealing effects on the luminescence and scintillation properties of CaMoO4 single crystal grown by Bridgman method

CaMoO4 single crystal with high quality and large size was grown by Bridgman method. The as-grown CaMoO4 crystal presents blue-black color, and the annealed CaMoO4 crystal presents optically transparent. The mechanisms of crystal fading by thermal annealing were explained in detail. The annealing effects on the optical transmission, photoluminescence spectra, X-ray luminescence spectra and luminescence decay time were investigated. The results indicate that thermal annealing not only has positive effects on reducing oxygen vacancies and alleviating the self-absorption phenomenon, but also the luminescent intensity can be enhanced greatly. The annealed CaMoO4 crystal has moderate Stokes shift (1.46 eV). The light output of annealed CaMoO4 single crystal by the excitation of 662 KeV X-ray from a 137Cs source was measured to be 57.9 p.e. per MeV. This work offers detailed and significant data of scintillation properties of CaMoO4 single crystal, which are very helpful for potential applications in searching for rare events especially in neutrinoless double beta decay area.

Electrical and photoluminescence properties of Sm3+ doped Na0.5La0.5Bi8-xSmxTi7O27 ceramics

In this study, Sm3+ doped Na0.5La0.5Bi8-xSmxTi7O27 (NBT-BITL-xSm, x = 0, 0.01, 0.015, 0.02, and 0.03) ceramics were synthesized via a conventional solid-state reaction process. The structural, electrical, and photoluminescence properties of NBT-BITL-xSm ceramics were systematically investigated. The crystal structure of NBT-BITL-xSm was refined using XRD Rietveld refinement and found to possess a single orthorhombic structure at room temperature. Raman spectroscopy revealed that Sm3+ ions preferred to substitute for Bi3+ located in the A-sites of pseudo-perovskite layers, inducing a slight decrease in orthorhombic distortion. Strong characteristic emission peaks of Sm3+ ions were observed in orange-red regions under a 407nm laser source, and the sample with x = 0.015 achieved the optimal photoluminescent property. Dielectric measurements showed double anomaly permittivity peaks at the temperature of 589 and 600∘C (Tm and Tc, respectively). The complex impedance spectrum indicated that the electrical conductivities mainly originated from crystal grains at high temperature. The activation energy was calculated to be 1.37–1.44eV from Arrhenius fitting results. After Sm3+ substitution, the activation energy for conductivity was increased as a result of reduced oxygen vacancies.

High-performance thin-film transistors with solution-processed ScInO channel layer based on environmental friendly precursor

Thin-film transistors (TFTs) with solution-processed scandium (Sc) substituted indium oxide (ScxIn1−xO3, ScInO) thin films based on environmental friendly water-induced precursor were fabricated. As the Sc concentration increases from 0% to 10%, the mobility decreases from 23.7 cm2 V−1 s−1 to 6.4 cm2 V−1 s−1, which is attributed to the non-overlapping of the Sc3+ electron orbit. However, the off current decreases and the turn-ON voltage (VON) shifts towards the positive direction as the Sc content increases, which indicates lower carrier density after incorporation of Sc into In2O3. More interestingly, the incorporation of Sc into In2O3 can effectively improve the electrical stability of the TFT devices under gate bias stress, which is attributed to the reduction of the number of oxygen vacancies due to the relatively low standard electrode potential (−2.36) of Sc and strong bonding strength of Sc–O (680 kJ mol−1). The reduction of oxygen vacancies is confirmed by the x-ray photoelectron spectroscopy (XPS) experiments.

Room‐Temperature Fabricated Amorphous Ga2O3 High‐Response‐Speed Solar‐Blind Photodetector on Rigid and Flexible Substrates

A solution to the fabrication of amorphous Ga2O3 solar‐blind photodetectors on rigid and flexible substrates at room temperature is reported. A robust improvement in the response speed is achieved by delicately controlling the oxygen flux in the reactive radio frequency magnetron sputtering process. Temporal response measurements show that the detector on quartz has a fast decay time of 19.1 µs and a responsivity of 0.19 A W−1 as well, which are even better than those single crystal Ga2O3 counterparts prepared at high temperatures. X‐ray photoelectron spectroscopy and current–voltage tests suggest that the reduced oxygen vacancy concentration and the increased Schottky barrier height jointly contribute to the faster response speed. Amorphous Ga2O3 solar‐blind photodetector is further constructed on polyethylene naphthalate substrate. The flexible devices demonstrate similar photoresponse behavior as the rigid ones, and no significant degradation of the device performance is observed in bending states and fatigue tests. The results reveal the importance of finely tuned oxygen processing gas in promoting the device performance and the applicability of room‐temperature synthesized amorphous Ga2O3 in fabrication of flexible solar‐blind photodetectors.

Nanostructured ZnO with bio-capping for nanofluid and natural dye based solar cell applications

In this paper we report the synthesis of ZnO nanoparticles using oxalic acid and the enhancement in its properties through surface modification by bio-capping using the medicinal plant Scorparia dulcis. The surface modified nanomaterial shows improved optical and morphological properties. It also exhibits enhanced near band edge ultraviolet photoluminescence and markedly reduced green emission. The perfect surface passivation with reduction in oxygen vacancies has resulted in the significant change in photoluminescence. Thermal conductivity measurements have been carried out in the base fluids water and ethylene glycol for different weight percentages of nanostructured ZnO. Significant increase in thermal conductivity ∼10–50% is observed for the nanofluids obtained using the surface modified particles. Photo conversion efficiency has been studied for natural dye based solar cells using bio-capped ZnO for the first time. Improved efficiencies of 10–30% are observed for solar cells sensitized using the natural dyes extracted from Jamun, marigold, pomegranate and mangosteen.

Magnetron sputtered ZnO buffer layer for Sb2Se3 thin film solar cells

Sb2Se3 possesses great potential for low-cost photovoltaics due to its excellent optoelectronic properties. CdS was typically used as electron collection layer due to high electron affinity, relatively high electron mobility, and environmental stability. However, the toxicity was a big issue restricting its development. Here we fabricate ZnO films by magnetron sputtering to replace the toxic CdS layer for non-toxic ZnO/Sb2Se3 heterojunction solar cells. It was found that the annealing atmosphere (ambient air or vacuum) played a significant role for the device performance. We found that post-annealing increased the crystallinity of ZnO to some extent. Ambient air annealing reduced the oxygen vacancies in ZnO and vacuum annealing increased oxygen vacancies. The reduced oxygen vacancies in ZnO resulted in an improved p-n junction between ZnO and Sb2Se3 and better device performance. The power conversion efficiency of ZnO/Sb2Se3 solar cell with ZnO film annealed in ambient air reached 4.08%.

Simultaneous enhancement of room temperature multiferroic properties of BiFeO3 by Nd doping at Bi site and Co doping at Fe site

Room temperature ferroelectric and magnetic properties of undoped BiFeO3 and (Nd, Co) codoped BiFeO3 are reported. The sol–gel synthesized Bi1−XNdXFe0.98Co0.02O3 (x = 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06) ceramics were analysed by XRD pattern, Raman spectrum, X-ray photoelectron spectrum (XPS), ferroelectric loop and magnetic hysteresis loop. The evolution of the changes in the ferroelectric and magnetic properties with gradual tiny increase in the dopant concentration was investigated. XRD patterns and Raman spectra confirm the typical rhombohedral structure for all the samples. The oxidation states of Fe and the existence of oxygen vacancies studied through XPS reveal that enhanced ferroelectric properties of the doped samples are due to the presence of low Fe2+ ions and reduction of oxygen vacancies. The remnant polarisation and maximum polarisation values of Nd 1–5% doped samples are remarkably enhanced compared to BFO. The highest Nd doping 6% showed reduced ferroelectric parameters due to the internal strain caused by the high doping, which was illustrated by its Raman spectrum. Well saturated magnetic hysteresis loops are obtained due to the doping of Nd and Co with significant magnetic parameters. A notable simultaneous improvement in both magnetic and ferroelectric properties has been illustrated for 1–5% of Nd doping.

Opto-electronic properties of Zn(1-x)VxO: Green emission enhancement due to V4+ state

Vanadium incorporation in ZnO modifies the lattice structure. The valence state of V plays an important role, controlling the oxygen content and thereby dimensions of the lattice. Both V4+ and V5+ are more electropositive than Zn2+ and reduce oxygen vacancies, resulting in lattice expansion. However, the sizes of both V4+ and V5+ are smaller than Zn2+, thereby resulting in the lattice contraction. The internal competition of increasing oxygen content and reducing effective crystal radius decides the lattice expansion and contraction. This affects the lattice strain and changes electronic levels, which modify absorption and emission processes in between the valence and conduction bands. A strong green emission band not due to oxygen vacancy but due to defects contributed by vanadium is also dependent on the oxidation state of vanadium. Bandgap also increases with the increase in the V4+ content.

Observation of enhanced multiferroic, magnetoelectric and photocatalytic properties in Sm-Co codoped BiFeO3 nanoparticles

Single phase multiferroics BiFeO3 (BFO), Bi0.99Sm0.01FeO3 (Sm1) and Bi0.99 Sm0.01Fe0.99Co0.01 O3 (Sm1Co1) were successively synthesized by tartaric acid assisted sol-gel method. The X-ray diffraction patterns analyzed through Rietveld refinement method confirmed substitution driven structural distortion, which was further supported by William- Hall plot. The morphological analysis of BFO and Sm1Co1 could be effectively controlled to form nanoparticles, due to the smaller ionic radii of the dopants cations. The co-doping of Sm and Co in BFO lattice significantly enhanced the magnetic properties, with the saturation magnetization being 8.4 emu/g, which is about 20 times larger than that of pure BFO (0.40 emu/g). The decrease of Neel transition temperature and irreversible behavior of FC/ZFC curves also support improved magnetic properties. With the codoping of Sm-Co, the band gap was decreased, while interestingly, the leakage current density was significantly reduced to about 10−7 A cm−2. A reduction of oxygen vacancies was confirmed by XPS analysis in the case of Sm1Co1 sample. The codoping leads to improved ferroelectric properties and significant enhancement of magnetoelectric coupling coefficient (9mV/cmOe). The photocalalytic activity of Sm1Co1 enhanced significantly and about 92% degradation of methylene blue dye was achieved using this catalyst under visible light irradiation.

Origin of in Situ Domain Formation of Heavily Nb-Doped Pb(Zr,Ti)O3 Thin Films Sputtered on Ir/TiW/SiO2/Si Substrates for Mobile Sensor Applications.

High-quality piezoelectric thin films have recently been in demand for mobile sensor applications. An investigation was conducted to understand the improvements in the piezoelectric and imprint characteristics of heavily Nb-doped lead zirconate titanate thin films with an extensive range of Nb content (up to 14 mol %) beyond the typical solid solubility limit of Nb. The positive effects produced by the unusual doping of Nb were realized by utilizing an in situ sputtering process that did not require a subsequent annealing and poling procedure. An enhanced piezoelectric coefficient, -e31, of -12.87 C/m2 and a stronger shift in the coercive field, Ec,shift, of ∼20 kV/cm, which are ideally useful for mobile sensor applications, were obtained for the 12 mol % Nb-doped films deposited on nonconventional buffer electrodes of Ir/TiW. The reduced oxygen vacancy concentration and preferred domain orientation with a stronger piezoresponse induced by the Nb donor doping contributed to the enhancement of the piezoelectric properties. Potential defect dipoles aligned by a residual stress gradient along columnar structures seemed to induce an internal electric field in the Nb-doped films, leading to the preferred domain orientation, as well as the strong imprint behavior due to a clamping of domain walls.

Oxygen vacancy-induced room temperature ferromagnetism in graphene–SnO2 nanocomposites

Reduced graphene oxide (rGO)–SnO2 nanocomposites have been successfully prepared by one-spot facile hydrothermal method. The nanostructures of the samples were analytically inspected and found that SnO2 are evenly anchored on the surface of rGO sheets. The presence of surface defects was identified by spectroscopic techniques. The EPR results confirmed the presence of singly charged oxygen vacancies on the surface of rGO–SnO2 nanocomposites and found to be accountable for its room temperature ferromagnetism. The ferromagnetic behavior (coercivity, saturation magnetization and remanent magnetization) of rGO–SnO2 nanocomposites is suppressed as compared to pure SnO2. The incorporation of rGO sheets notably reduces oxygen vacancies in SnO2 and alters the room temperature ferromagnetism in rGO–SnO2 nanocomposites.

Epitaxial growth and stress analysis of Zn1-xMgxO films on (111) Si substrates

Zn1-xMgxO films with Mg content varied from 0 to 0.17 were fabricated on Lu2O3/(111) Si substrates by plasma-assisted molecular beam epitaxy. The crystal structure, optical and electrical properties of the as-grown Zn1-xMgxO films were investigated by X-Ray diffraction (XRD), photoluminescence and Hall-effect measurements. Highly c-axis preferred oriented hexagonal Zn1-xMgxO films with six-fold symmetry and tunable bandgap from 3.31 to 3.55eV were identified. The lattice constants and residual stress were analyzed by XRD measurement and Raman spectra. A state of compressive uniaxial residual out-of-plane strain and tensile biaxial in-plane stress were confirmed. Besides, the phonon deformation-potential parameter was calculated and the value decreased as Mg content increasing. It indicates that Mg incorporation has effectively reduced oxygen vacancy. In addition to the use of determining residual stress, Raman spectroscopy is also turned out to be a useful supplementary analysis to investigate the intrinsic defects in thin films.

A solution-processed quaternary oxide system obtained at low-temperature using a vertical diffusion technique

We report a method for fabricating solution-processed quaternary In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) at low annealing temperatures using a vertical diffusion technique (VDT). The VDT is a deposition process for spin-coating binary and ternary oxide layers consecutively and annealing at once. With the VDT, uniform and dense quaternary oxide layers were fabricated at lower temperatures (280 °C). Compared to conventional IGZO and ternary In-Zn-O (IZO) thin films, VDT IGZO thin film had higher density of the metal-oxide bonds and lower density of the oxygen vacancies. The field-effect mobility of VDT IGZO TFT increased three times with an improved stability under positive bias stress than IZO TFT due to the reduction in oxygen vacancies. Therefore, the VDT process is a simple method that reduces the processing temperature without any additional treatment for quaternary oxide semiconductors with uniform layers.

Amorphous InGaMgO Ultraviolet Photo-TFT with Ultrahigh Photosensitivity and Extremely Large Responsivity.

Recently, amorphous InGaZnO ultraviolet photo thin-film transistors have exhibited great potential for application in future display technologies. Nevertheless, the transmittance of amorphous InGaZnO (~80%) is still not high enough, resulting in the relatively large sacrifice of aperture ratio for each sensor pixel. In this work, the ultraviolet photo thin-film transistor based on amorphous InGaMgO, which processes a larger bandgap and higher transmission compared to amorphous InGaZnO, was proposed and investigated. Furthermore, the effects of post-deposition annealing in oxygen on both the material and ultraviolet detection characteristics of amorphous InGaMgO were also comprehensively studied. It was found that oxygen post-deposition annealing can effectively reduce oxygen vacancies, leading to an optimized device performance, including lower dark current, higher sensitivity, and larger responsivity. We attributed it to the combined effect of the reduction in donor states and recombination centers, both of which are related to oxygen vacancies. As a result, the 240-min annealed device exhibited the lowest dark current of 1.7 × 10−10 A, the highest photosensitivity of 3.9 × 106, and the largest responsivity of 1.5 × 104 A/W. Therefore, our findings have revealed that amorphous InGaMgO photo thin-film transistors are a very promising alternative for UV detection, especially for application in touch-free interactive displays.

Zn1−xSixO: Improved optical transmission and electrical conductivity

Si addition in ZnO lattice significantly improves electrical conductivity. The extra charge of Si4+ ion (in comparison to Zn2+) attracts more oxygen in the lattice and reduces oxygen vacancies. Reduction of oxygen vacancies (defects) reduces strain in the lattice. Transparency of visible light (<3.0eV) improves due to reduction of these defects in the wide bandgap (~3.3eV: UV) of ZnO. Extra charge of Si4+ enhances carrier density in the ZnO lattice. Improved carrier density, reduced strain facilitate transport of carriers and therefore conductivity increases. Si incorporation also makes the samples moisture resistant. The material becomes more robust to operate in adverse humid conditions. An ideal transparent conductive oxide (TCO) should be conductive, transmit visible light and able to sustain humid conditions. All these properties are observed in Zn(1−x)SixO material.

Effects of oxygen vacancies on the structural and optical properties of \u03b2-Ga2O3

The structural, electronic, and optical properties of β-Ga2O3 with oxygen vacancies are studied by employing first-principles calculations based on density function theory. Based on the defects formation energies, we conclude the oxygen vacancies are most stable in their fully charge states. The electronic structures and optical properties of β-Ga2O3 are calculated by Generalized Gradient Approximation + U formalisms with the Hubbard U parameters set 7.0 eV and 8.5 eV for Ga and O ions, respectively. The calculated bandgap is 4.92 eV, which is consistent with the experimental value. The static real dielectric constants of the defective structures are increased compared with the intrinsic one, which is attributed to the level caused by the Ga-4s states in the bandgap. Extra peaks are introduced in the absorption spectra, which are related to Ga-4s and O-2p states. Experimentally, β-Ga2O3 films are deposited under different O2 volume percentage with ratio-frequency magnetron sputtering method. The measured results indicate that oxygen vacancies can induce extra emission peaks in the photoluminescence spectrum, the location of these peaks are close to the calculated results. Extra O2 can increase the formation energies of oxygen vacancies and thus reduce oxygen vacancies in β-Ga2O3.

The electrical stability of In-doped ZnO thin films deposited by RF sputtering

The electrical stability of transparent conductive oxides is an important criterion for evaluating their performance, especially when they are employed at elevated temperatures or in long-term operation. In this work, indium-doped ZnO thin films with various doping concentrations were prepared by RF sputtering. The electrical properties, electrical thermal stability, and time stability of films with differing indium contents were investigated. The results showed that the degradation of the films’ conductivity is primarily attributable to the reduction in oxygen vacancies at high temperatures under oxygenated conditions. The aggregation of indium atoms, which cannot replace Zn3+ cations at temperatures above 200 °C, can improve the carrier concentration. Further reaction with oxygen degraded the performance of the films due to the formation of insulating oxides. Long-term analysis showed the IZO films to have quite stable electrical properties. Their conductivity remained almost unchanged after two months at room temperature under normal atmospheric conditions.