Oxygen-deficient Films Research Articles

Strain-oxygen vacancies coupling in topotactic (La,Sr)CoO3-δ thin films

Oxygen defect engineering is a widely used approach for tuning physical properties in oxides. Multivalent transition metal oxide La0.7Sr0.3CoO3-δ (LSCO) shows oxygen vacancy-driven metal-to-insulator transition (MIT) due to topotactic phase transition and its high oxygen vacancy tolerance. Here, we introduce strain as a new degree of freedom to study the strain-oxygen vacancy coupling effects and elucidate its impact on the electronic property in oxygen-deficient LSCO epitaxial thin films grown on SrTiO3 (100) single crystal. By combining the experimental results with density functional theory plus U (DFT+U) calculations, we reveal that 2.1 % in-plane tensile strain can stabilize the insulating state of LSCO with a surprisingly low concentration of oxygen vacancies, <0.5 %. This study reveals that the MIT in LSCO is governed by the combination of oxygen vacancies and strain, offering the potential for additional tuning knob of the material's electronic properties.

Controlled assembly and synthesis of oxygen-deficient W18O49 films based on solvent molecular strategy for electrochromic energy storage smart windows

18O49 is a promising electrochromic material with abundant oxygen vacancy and high carrier mobility. However, the electrochromism of W18O49 is not satisfactory, the regulation and synthesis of high-performance W18O49 have become the research hotspot. In this work, it is found that the main chain and side chain structure of solvent molecules can control the assembly and synthesis of W18O49 film, so as to achieve excellent electrochemical and electrochromic properties. The porous reticular W18O49 film synthesized in short-chain molecular solvent shows the excellent transmittance modulation (81.6 % at 633 nm, 77.0 % at 1050 nm), and exhibits the high energy storage of 41.7 mF cm−2 at 0.1 mA cm−2. Solvent molecular strategy provides a new way for the synthesis and regulation of W18O49 electrochromic films and promotes the further development of high-performance electrochromic materials.

Enriched electron donor sites and non-overlapping small polaron tunneling electrical conduction in oxygen-deficient β-Ga2O3 thin film on p-Si (100)

Enriched electron donor sites and non-overlapping small polaron tunneling electrical conduction in oxygen-deficient β-Ga2O3 thin film on p-Si (100)

Investigation on Synaptic Adaptation and Fatigue in ZnO/HfZrO-Based Memristors under Continuous Electrical Pulse Stimulation

This study investigates the behavior of memristive devices characterized by oxygen-deficient ZnO and HfZrO films under continuous pulse stimulation. This dynamic reflects the adaptability observed in neural synapses when repeatedly subjected to stress, ultimately resulting in a mitigated response to pressure. Observations show that the conductivity of memristors increases with the augmentation of continuous electrical pulses. However, the momentum of this growth trend gradually diminishes, highlighting the devices’ capability to adapt to repetitive pressure. This adjustment correlates with the transition of biological synapses from short-term to persistent memory stages, aligning with the principles of the Ebbinghaus memory model. The architecture of memristors, integrating ZnO and HfZrO in a layered manner, holds promising prospects in replicating the inherent synaptic features found in biological organisms.

Influence of thickness scaling on the electronic structure and optical properties of oxygen deficient BaBiO3-δ thin films grown on SrTiO3-buffered Si(001) substrate

BaBiO3 has attracted a lot of research attention since it was discovered as the parent compound for the high-Tc superconducting BaPbxBi1–xO3 and Ba1–xKxBiO3. In its pure state, BaBiO3 is an insulator due to the presence of a breathing distortion of the BiO6 octahedra. The distortion is attributed to the valency of Bi in the compound being charge-ordered in the form of Bi3+ and Bi5+ along the lattice, resulting in alternating expanded or contracted BiO6 octahedra. The interaction between the electronic properties and the thickness of the thin film is crucial to study. We conducted a thorough study to investigate the effect of the thickness reduction of BaBiO3-δ grown on SrTiO3-buffered Si substrates on the optical properties as well as the Bi electronic structure of the thin films. We conclude that modifications in the valency of Bi in the ultra-thin film regime result in an optically conducting layer.

Resistive Avalanches in La1-xSrxCoO3-δ (x = 0, 0.3) Thin Films and Their Reversible Evolution by Tuning Lattice Oxygen Vacancies (δ).

Strong correlations are often manifested by exotic electronic phases and phase transitions. LaCoO3-δ (LCO) is a system that exhibits such strong electronic correlations with lattice-spin-charge-orbital degrees of freedom. Here, we show that mesoscopic oxygen-deficient LCO films show resistive avalanches of about 2 orders of magnitude due to the metal-insulator transition (MIT) of the film at about 372 K for the 25 W RF power-deposited LCO film on the Si/SiO2 substrate. In bulk, this transition is otherwise gradual and occurs over a very large temperature range. In thin films of LCO, the oxygen deficiency (0 < δ < 0.5) is more easily reversibly tuned, resulting in avalanches. The avalanches disappear after vacuum annealing, and the films behave like normal insulators (δ ∼0.5) with Co2+ in charge ordering alternatively with Co3+. This oxidation state change induces spin state crossovers that result in a spin blockade in the insulating phase, while the conductivity arises from hole hopping among the allowed cobalt Co4+ ion spin states at high temperature. The chemical pressure (strain) of 30% Sr2+ doping at the La3+ site results in reduction in the avalanche magnitude as well as their retention in subsequent heating cycles. The charge nonstoichiometry arising due to Sr2+ doping is found to contribute toward hole doping (i.e., Co3+ oxidation to Co4+) and thereby the retention of the hole percolation pathway. This is also manifested in energies of crossover from the 3D variable range hopping (VRH) type transport observed in the temperature range of 300-425 K, while small polaron hopping (SPH) is observed in the temperature range of 600-725 K for LCO. On the other hand, Sr-doped LCO does not show any crossover and only the VRH type of transport. The strain due to Sr2+ doping refrains the lattice from complete conversion of δ going to 0.5, retaining the avalanches.

Tuning optical absorption in perovskite (K,Na)NbO3 ferroelectrics.

The ability to tailor the electronic band structure and optical absorption by appropriate cationic substitution in perovskite oxide ferroelectrics is essential for many advanced electronic and optoelectronic applications of these materials. Here, we explored weak (Ba,Ni)-doping for reducing optical bandgaps in (K,Na)NbO3 ferroelectric films and ceramics. The optical absorption in the broad spectral range of (0.7-8.8) eV was investigated in polycrystalline doped, pure, and oxygen deficient films, in doped epitaxial films grown on different substrates, and in doped ceramics. By comparing optical properties of all films and ceramics, it was established that 1-2 at% of cationic substitutions or up to 10 at % of oxygen vacancies have no detectable effect on the direct (∼4.5 eV) and indirect (∼3.9 eV) gaps. Concurrently, substantial sub-gap absorption was revealed and ascribed to structural band tailing in epitaxial films and ceramics. It was suggested that owing to fundamental strain-property couplings in perovskite oxide ferroelectrics, inhomogeneities of lattice strain can lead to increased sub-gap absorption. The uncovered structurally induced sub-gap optical absorption can be relevant for other ferroelectric ceramics and thin films as well as for related perovskite oxides.

Theoretical approach to defect-induced magnetism in oxygen-deficient γ-Ga2O3 films

Gallium oxide (Ga2O3) is an emerging material with several known polymorphs that exhibit many interesting properties for applications. In this work is theoretically studied and discussed the unconventional ferromagnetism observed in oxygen-deficient thin films of Ga2O3-x (0.42 < x < 0.60) grown on sapphire substrates. The formation of the magnetic moments is demonstrated in good agreement with experimental magnetization measurements using first-principles calculations based on the density functional theory performed for a standard crystal structure of γ-Ga2O3 with oxygen vacancies in different configurations. The random distribution of pairs of oxygen vacancies and rearrangements of them within the spinel-like cubic structure containing tetrahedral and octahedral sites for increasing temperatures are consistent with long-range persistent non-collinear magnetic order up to temperatures that may be exceed the temperature at which full conversion of γ-Ga2O3 to β-Ga2O3 takes place. Our present results also reveal that magnetic irreversibility observed from 10 K to room temperature are possibly determined by the intrinsic disorder in the structure of tetrahedral and octahedral sites that allows numerous possible paths for the migration of oxygen vacancies with low values of activation energies and migration barriers, as indicated by the previously published reports.

On the Structure of Oxygen Deficient Amorphous Oxide Films.

Understanding defects in amorphous oxide films and heterostructures is vital to improving performance of microelectronic devices, thin-film transistors, and electrocatalysis. However, to what extent the structure and properties of point defects in amorphous solids are similar to those in the crystalline phase are still debated. The validity of this analogy and the experimental and theoretical evidence of the effects of oxygen deficiency in amorphous oxide films are critically discussed. The authors start with the meaning and significance of defect models, such as "oxygen vacancy" in crystalline oxides, and then introduce experimental and computational methods used to study intrinsic defects in amorphous oxides and discuss their limitations and challenges. To test the validity of existing defect models, ab initio molecular dynamics is used with a non-local density functional to model the structure and electronic properties of oxygen-deficient amorphous alumina. Unlike some previous studies, the formation of deep defect states in the bandgap caused by the oxygen deficiency is found. Apart from atomistic structures analogous to crystal vacancies, the formation of more stable defect states characterized by the bond formation between under-coordinated Al ions is shown. The limitations of such defect models and how they may be overcome in simulations are discussed.

Optimization of deposition conditions to tune optoelectronic properties of MoO3-x films prepared by RF-sputtering technique

Tunable optoelectronic properties of Molybdenum oxide (MoO3-x) make it a suitable applicant for its use as a catalyst and a hole transport layer. Oxygen vacancies play a vital role in deciding the properties of MoO3-x. In this article, the influence of deposition conditions like substrate temperature and gaseous environment on optoelectronic properties of MoO3-x films are investigated. Increase in substrate temperature have formed oxygen deficient MoO3-x films having lesser transmission (∼30%) and higher conductivity (15-200 Ω−1cm−1). Films prepared at the substrate temperature of 400 °C and rf-power 80 W are mostly crystalline and have led to the formation of MoO2 along with MoO3-x which results in highly conducting less transparent film suitable for catalysis application. Further, introduction of oxygen gas during deposition led to the formation of highly transparent (∼80%) and resistive (10−6 Ω−1cm−1) thin films suitable for hole transport layer application. An increase in oxygen concentration and decrease in oxygen vacancy is confirmed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy measurement respectively for films deposited in presence of oxygen gas. These studies imply that stoichiometry of molybdenum oxide can be controlled by varying the film preparation conditions and thus optoelectronic properties can be tuned accordingly.

Short-range order in amorphous oxygen-deficient TaOx thin films and its relation to electrical conductivity

Thin films of tantalum oxide hold promising functional properties for electronic applications such as resistive random-access memory. For this aim, correlating the structure and charge transport properties of oxygen-deficient derivatives is crucial. Here, using electron scattering measurements from nanoscale volumes in a transmission electron microscope (TEM), we report how oxygen content affects short-range order in amorphous TaOx thin films, where 1.34 ≤ x ≤ 2.50. By extracting the bond lengths, we observe that the dominant type of Ta–Ta distances change with decreasing oxygen content from next-nearest-neighbor, ∼3.8 Å, to nearest-neighbor, ∼3 Å. We relate this decrease to the Ta–O polyhedral network within the film, namely decreasing oxygen content increases the presence of TaO5 at the expense of TaO6 polyhedra. The reduction in oxygen content is accompanied by a significant reduction of electrical resistivity of the films from over 4.3 × 103 to (4 ± 0.05)×10−3 Ω × cm. In particular, we observe a sharp percolative decrease in resistivity of three orders of magnitude, at x ∼ 1.9. Ta oxidation states, measured by x-ray photoelectron spectroscopy, suggest that the main polyhedral building block within the TaO2.5 film is TaO6, while in oxygen-deficient films, the relative fractions of TaO5 polyhedra and metallic Ta increase. At even lower oxygen content, x ∼ 1.34, TEM and x-ray diffraction detect crystallites of Ta with cubic and metastable tetragonal structures. We propose that TaO5 polyhedra and Ta crystallites increase conductivity due to direct bonding of Ta atoms, as manifested by nearest-neighbor Ta–Ta bond length, thus enabling conductive paths for charge transport.

Reset-First and Multibit-Level Resistive-Switching Behavior of Lanthanum Nickel Oxide (LaNiO3-x) Thin Films.

The resistive random-access memory (RRAM) with multi-level storage capability has been considered one of the most promising emerging devices to mimic synaptic behavior and accelerate analog computations. In this study, we investigated the reset-first bipolar resistive switching (RS) and multi-level characteristics of a LaNiO3-x thin film deposited using a reactive magnetron co-sputtering method. Polycrystalline phases of LaNiO3 (LNO), without La2O3 and NiO phases, were observed at similar fractions of Ni and La at a constant partial pressure of oxygen. The relative chemical proportions of Ni3+ and Ni2+ ions in LaNiO3-x indicated that it was an oxygen-deficient LaNiO3-x thin film, exhibiting RS behavior, compared to LNO without Ni2+ ions. The TiN/LaNiO3-x/Pt devices exhibited gradual resistance changes under various DC/AC voltage sweeps and consecutive pulse modes. The nonlinearity values of the conductance, measured via constant-pulse programming, were 0.15 for potentiation and 0.35 for depression, indicating the potential of the as-fabricated devices as analog computing devices. The LaNiO3-x-based device could reach multi-level states without an electroforming step and is a promising candidate for state-of-the-art RS memory and synaptic devices for neuromorphic computing.

Fabrication of Large-Area, Affordable Dual-Function Electrochromic Smart Windows by Using a Hybrid Electrode Coated with an Oxygen-Deficient Tungsten Oxide Ultrathin Porous Film.

Electrochromic (EC) devices are not commercialized extensively owing to their high cost. The best large-area devices in the market suffer from not reaching a distinct dark-colored state. These devices appear more like a blue tinted glass. While a better performance demands the use of appropriate components, the cost-effectiveness of such components is crucial for commercialization. Specifically, the utilization of cost-effective electrodes, thin WO3 coatings, and inexpensive electrolytes are essential for reducing the cost of EC devices. Here, we report a high-performing porous WO3 thin film (∼130 nm) achieved by optimizing the DC sputtering process parameters. This way, an affordable dual-function EC energy-storage device was fabricated, showing 84% transmittance modulation and a high power density of 3036 mW/m2, thus functioning simultaneously as a transparency switching energy-storage device. With a large-area (900 cm2) device, we have demonstrated that the need for expensive ITO electrodes and Li+ ion-based electrolytes can be eliminated by using a hybrid electrode (ITO/Al-mesh) and multivalent Al3+ ion-based electrolytes while not compromising the device performance. The findings of this study may revolutionize the EC device industry and their commercialization owing to inexpensive ingredients and scalable processing.

The role of defects in the persistent photoconductivity of BaSnO3 thin films

Time-dependent photoconductivity (PC) and PC spectra have been studied in oxygen deficient BaSnO3 thin films grown on different substrates. X-ray spectroscopy measurements show that the films have epitaxially grown on MgO and SrTiO3 substrates. While on MgO the films are nearly unstrained, on SrTiO3 the resulting film is compressive strained in the plane. Electrical conductivity in dark is increased in one order of magnitude for the films on SrTiO3 in comparison to the one on MgO. This leads to an increase of PC in the latter film in at least one order of magnitude. PC spectra show a direct gap with a value of eV for the film grown on MgO while on SrTiO3 eV. For both type of films, time-dependent PC curves show a persistent behavior after illumination is removed. These curves have been fitted employing an analytical procedure based on the frame of PC as a transmission phenomenon showing the relevant role of donor and acceptor defects as carrier traps and as a source of carriers. This model also suggests that in the BaSnO3 film on SrTiO3 more defects are created probably due to strain. This latter effect can also explain the different transition values obtained for both type of films.

From transparent to black amorphous zinc oxide thin films through oxygen deficiency control

Despite the fact that zinc oxide is a well-known transparent oxide, several recent studies on “black” ZnO have renewed its potential for photocatalytic applications. We report on the control of oxygen deficiency in ZnO thin films grown at 300 °C on c-cut sapphire single-crystal substrates by pulsed electron beam deposition (PED) through a slight variation of argon pressure in PED. At a pressure of 2 × 10−2 mbar transparent, stoichiometric (ZnO) and crystalline films are obtained, while at 9 × 10−3 mbar black, oxygen-deficient (ZnO0.85) and amorphous films result. Stoichiometry, structural, and optoelectronic properties of transparent and black ZnO thin films were comparatively analyzed as a function of oxygen deficiency. Black ZnO thin films exhibit enhanced absorption in the visible and near-infrared due to oxygen deficiency, thus extending the range of applications of zinc oxide thin films from transparent electronics to solar absorbers and photocatalysis.

Amorphous InGaZnO Thin-Film Transistors with Double-Stacked Channel Layers for Ultraviolet Light Detection.

Amorphous InGaZnO thin film transistors (a-IGZO TFTs) with double-stacked channel layers (DSCL) were quite fit for ultraviolet (UV) light detection, where the best DSCL was prepared by the depositions of oxygen-rich (OR) IGZO followed by the oxygen-deficient (OD) IGZO films. We investigated the influences of oxygen partial pressure (PO) for DSCL-TFTs on their sensing abilities by experiments as well as Technology Computer Aided Design (TCAD) simulations. With the increase in PO values for the DSCL depositions, the sensing parameters, including photogenerated current (Iphoto), sensitivity (S), responsivity (R), and detectivity (D*) of the corresponding TFTs, apparently degraded. Compared with PO variations for the OR-IGZO films, those for the OD-IGZO depositions more strongly influenced the sensing performances of the DSCL-TFT UV light detectors. The TCAD simulations showed that the variations of the electron concentrations (or oxygen vacancy (VO) density) with PO values under UV light illuminations might account for these experimental results. Finally, some design guidelines for DSCL-TFT UV light detectors were proposed, which might benefit the potential applications of these novel semiconductor devices.

Oxygen‐Driven Metal–Insulator Transition in SrNbO3 Thin Films Probed by Infrared Spectroscopy

AbstractThe occurrence of oxygen‐driven metal–insulator‐transition (MIT) in SrNbO3 (SNO) thin films epitaxially grown on (110)‐oriented DyScO3 has been reported. SNO films are fabricated by the pulsed laser deposition technique at different partial O2 pressure to vary the oxygen content and their structural, optical, and transport properties are probed. SNO unit cell has been found to shrink vertically as the oxygen content increases but keeping the epitaxial matching with the substrate. The results of Fourier‐transform infra‐red spectroscopy show that highly oxygenated SNO samples (i.e., grown at high oxygen pressure) show distinct optical conductivity behavior with respect to oxygen deficient films, hence demonstrating the insulating character of the formers with respect to those fabricated with lower pressure conditions. Tailoring the optical absorption and conductivity of strontium niobate epitaxial films across the MIT will favor novel applications of this material.

Ionic Liquid Gating Control of Oxygen Vacancies in the La0.8Ba0.2MnO3 Ultrathin Films

In this work, we show ionic liquid field-effect of La0.8Ba0.2MnO3 (LBMO) untrathin films prepared by the sol-gel method. When applying a positive gate voltage (V g) in vacuum, it is found that the film resistivity continuously increases tenfold within hours or minutes depending on the magnitude of V g, accompanied with disappearance of the low-temperature metallic transport behavior. In contrast, the film resistivity changes little in air or under a negative applied V g. Such a difference reveals that the increased resistivity is related to the oxygen depletion in the films under the positive V g, especially at the grain boundaries. After removing the positive V g in vacuum, the room-temperature resistivity begins to fall off and the low-temperature metallic state is partially restored in several tens hours, suggesting oxygen diffusion from the relatively oxygen-rich grains to the seriously oxygen-deficient grain boundaries. Furthermore, the oxygen content in the oxygen-deficient films can be almost fully restored in ten minutes by external annealing (200 °C) or Joule self-heating (21 mA) the films in air. These experimental findings provide an additional method in tuning oxygen vacancies in transition-metal oxide films.

Electronic manifestation of a disorder mediated metal-insulator transition in epitaxial SrRuO3 thin film

Herein, the electronic, magnetic, and transport properties of an oxygen-deficient epitaxial SrRuO3 (SRO) thin film have been investigated. The oxygen vacancies (VO) in the SRO thin-film transform a fraction of Ru4+ ions into Ru3+, resulting in an enhancement of the unit cell volume. Due to the elongated lattice parameters, the electronic bandwidth decreases and gives rise to the enhanced correlation strength. Further, distinctive effects of VO are seen in the temperature-dependent resistivity behavior [ρ(T)]. At low temperatures, the SRO thin film exhibits a resistivity minima near 48 K along with the ferromagnetic to paramagnetic transition (TC~150 K). The resistivity upturn at low temperatures is analyzed using quantum correction in the conductivity and it is found that the weak localization prevails over renormalized electron-electron interactions in the low-temperature resistivity upturn behavior of oxygen-deficient (VO) SRO thin film.

Observation of Josephson-like Tunneling Junction Characteristics and Positive Magnetoresistance in Oxygen Deficient Nickelate Films of Nd0.8Sr0.2NiO3-δ.

Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of NdSrNiO (obtained by reducing Sr doped NdNiO films) and their similarity to the cuprates high temperature superconductors. Here, we report on the observation of a new type of transport in oxygen poor NdSrNiO films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where a Josephson-like tunneling junction characteristic with serial resistance is revealed. We attribute this phenomenon to coupling between superconductive (S) surfaces of the grains in our Oxygen poor films via the insulating (I) grain boundaries, which yields SIS junctions in series with the normal (N) resistance of the grains themselves. The similarity of the observed conductance spectra to the tunneling junction characteristic with Josephson-like current is striking, and seems to support the existence of superconductivity in our samples.