A Facet‐Dependent Schottky‐Junction Electron Shuttle in a BiVO4{010}–Au–Cu2O Z‐Scheme Photocatalyst for Efficient Charge Separation

Interfacial charge separation and transfer are the main challenges of efficient semiconductor‐based Z‐scheme photocatalytic systems. Here, it is discovered that a Schottky junction at the interface between the BiVO₄ {010} facet and Au is an efficient electron‐transfer route useful for constructing a high‐performance BiVO₄{010}–Au–Cu₂O Z‐scheme photocatalyst. Spectroscopic and computational studies reveal that hot electrons in BiVO₄ {010} more easily cross the Schottky barrier to expedite the migration from BiVO₄ {010} to Au and are subsequently captured by the excited holes in Cu₂O. This crystal‐facet‐dependent electron shuttle allows the long‐lived holes and electrons to stay in the valence band of BiVO₄ and conduction band of Cu₂O, respectively, contributing to improved light‐driven CO₂ reduction. This unique semiconductor crystal‐facet sandwich structure will provide an innovative strategy for rational design of advanced Z‐scheme photocatalysts.     

New Iron‐Cobalt Oxide Catalysts Promoting BiVO4 Films for Photoelectrochemical Water Splitting

Owing to the sluggish kinetics for water oxidation, severe surface charge recombination is a major energy loss that hinders efficient photoelectrochemical (PEC) water splitting. Herein, a simple process is developed for preparing a new type of low‐cost iron‐cobalt oxide (FeCoO_x) as an efficient co‐catalyst to suppress the surface charge recombination on bismuth vanadate (BiVO₄) photoanodes. The new FeCoO_x/BiVO₄ photoanode exhibits a high photocurrent density of 4.82 mA cm^?2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which corresponds to >100% increase compared to that of the pristine BiVO₄ photoanode. The photoanode also demonstrates a high charge separation efficiency of ≈90% with excellent stability of over 10 h, indicating the excellent catalytic performance of FeCoO_x in the PEC process. Density functional theory calculations and experimental studies reveal that the incorporation of Fe into CoO_x generates abundant oxygen vacancies and forms a p‐n heterojunction with BiVO₄, which effectively promotes the hole transport/trapping from the BiVO₄ photocatalyst and reduces the overpotential for oxygen evolution reaction (OER), resulting in remarkably increased photocurrent densities and durability. This work demonstrates a feasible process for depositing cheap FeCoO_x as an excellent OER cocatalyst on photoanodes for PEC water splitting.     

Avalanche‐Discharge‐Induced Electrical Forming in Tantalum Oxide‐Based Metal–Insulator–Metal Structures

Oxide‐based metal–insulator–metal structures are of special interest for future resistive random‐access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite‐like structure, which is induced by an avalanche discharge between the top electrode and the Ta₂O_5‐x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite‐like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta⁴⁺ and Ta⁵⁺, which takes place over the entire former Pt/Ta₂O_5‐x interface of the dendrite‐like structure.     

Effects of the metal gate on the stress-induced traps in Ta2O5/SiO2 stacks

The degradation of Ta₂O₅-based (10 nm) stacked capacitors with different top electrodes, (Al, W, Au) under constant current stress has been investigated. The variation of electrical characteristics after the stress is addressed to gate-induced defects rather than to poor-oxidation related defects. The main wearout parameter in Ta₂O₅ stacks is bulk-related and a generation only of bulk traps giving rise to oxide charge is observed. The post-stress current–voltage curves reveal that stress-induced leakage current (SILC) mode occurs in all capacitors and the characteristics of pre-existing traps define the stress response. The results are discussed in terms of simultaneous action of two competing processes: negative charge trapping in pre-existing electron traps and stress-induced positive charge generation, and the domination of one of them in dependence on both the stress level and the gate used. The charge build-up and the trapping/detrapping processes modify the dominant conduction mechanism and the gate-induced defects are precursors for device degradation. It is concluded that the impact of the metal gate on the ultimate reliability of high-k stacked capacitors should be strongly considered.     

Modulating Carrier Kinetics in BiVO4 Photoanodes through Molecular Co4O4 Cubane Layers

Understanding the role and immobilization of molecular catalysts on photoelectrodes is essential to use their full potential for efficient solar fuel generation. Here, a Co^II₄O₄ cubane with proven catalytic performance and an active H₂O─Co₂(OR)₂─OH₂ edge-site moiety is immobilized on BiVO₄ photoanodes through a versatile layer-by-layer assembly strategy. This delivers a photocurrent of 3.3 mA cm⁻² at 1.23 V_RHE and prolonged stability. Tuning the thickness of the Co₄O₄ layer has remarkable effects on photocurrents, dynamic open circuit potentials, and charge carrier behavior. Comprehensive-time and frequency-dependent perturbation techniques are employed to investigate carrier kinetics in transient and pseudo-steady-state operando conditions. It is revealed that the Co₄O₄ layer can prolong carrier lifetime, unblock kinetic limitations at the interface by suppressing recombination, and enhance charge transfer. Additionally, its flexible roles are identified as passivation/hole trapping/catalytic layer at respective lower/moderate/higher potentials. These competing functions are under dynamic equilibrium, which fundamentally defines the observed photocurrent trends.     

Degradation behavior of Ta2O5 stacks and its dependence on the gate electrode

Response of 8 nm Ta₂O₅ stacks with Al and Au gate electrodes to voltage stress at room temperature and at 100 °C is investigated. Stress-induced leakage current (SILC) reveals significant gate dependence and distinct difference to SILC in SiO₂. The mechanisms for SILC generation and stress degradation are discussed. Unlike SiO₂, pre-existing traps and positive charge build-up are recognized as a key factor for generation of SILC in Ta₂O₅ stacks.     

Investigation of Ta2O5/SiO2/4H-SiC MIS capacitors

Tantalum pentoxide (Ta₂O₅) deposited by pulsed DC magnetron sputtering technique as the gate dielectric for 4H-SiC based metal-insulator-semiconductor (MIS) structure has been investigated. A rectifying current-voltage characteristic was observed, with the injection of current occurred when a positive DC bias was applied to the gate electrode with respect to the n type 4H-SiC substrate. This undesirable behavior is attributed to the relatively small band gap of Ta₂O₅ of around 4.3 eV, resulting in a small band offset between the 4H-SiC and Ta₂O₅. To overcome this problem, a thin thermal silicon oxide layer was introduced between Ta₂O₅ and 4H-SiC. This has substantially reduced the leakage current through the MIS structure. Further improvement was obtained by annealing the Ta₂O₅ at 900 °C in oxygen. The annealing has also reduced the effective charge in the dielectric film, as deduced from high frequency C-V measurements of the Ta₂O₅/SiO₂/4H-SiC capacitors.     

GeOx/Reduced Graphene Oxide Composite as an Anode for Li‐Ion Batteries: Enhanced Capacity via Reversible Utilization of Li2O along with Improved Rate Performance

A self‐assembled GeO_x/reduced graphene oxide (GeO_x/RGO) composite, where GeO_x nanoparticles are grown directly on reduced graphene oxide sheets, is synthesized via a facile one‐step reduction approach and studied by X‐ray diffraction, transmission electron microscopy, energy dispersive X‐ray spectroscopy, electron energy loss spectroscopy elemental mapping, and other techniques. Electrochemical evaluation indicates that incorporation of reduced graphene oxide enhances both the rate capability and reversible capacity of GeO_x, with the latter being due to the RGO enabling reversible utilization of Li₂O. The composite delivers a high reversible capacity of 1600 mAh g^?1 at a current density of 100 mA g^?1, and still maintains a capacity of 410 mAh g^?1 at a high current density of 20 A g^?1. Owing to the flexible reduced graphene oxide sheets enwrapping the GeO_x particles, the cycling stability of the composite is also improved significantly. To further demonstrate its feasibility in practical applications, the synthesized GeO_x/RGO composite anode is successfully paired with a high voltage LiNi_0.5Mn_1.5O₄ cathode to form a full cell, which shows good cycling and rate performance.     

Preparation and properties of Ta2O5 film capacitors using TiSi2 bottom electrode

Thin Ta₂O₅ films were grown in a low pressure chemical vapor deposition(LPCVD) reactor. The Al/Ta₂O₅/p-Si and Al/Ta₂O₅/TiSi₂/p-Si capacitors were fabricated and their capacitor characteristics were investigated. During a dry O₂ annealing process of the Ta₂O₅, the oxidation of Si substrate and TiSi₂ layer underneath the Ta₂O₅ layer was observed resulting in formation of SiO₂ and TiO₂ on their surfaces, respectively, due to the oxygen diffusion through the Ta₂O₅ layer. As-deposited 100 nm thick Ta₂O₅ film was found to be crystallized to δ-Ta₂O₅ at a temperature of about 700°C. The crystallized Ta₂O₅ film showed a higher leakage current density for the Al/Ta₂O₅/p-Si capacitor, compared to an amorphous Ta₂O₅ film. For the Al/Ta₂O₅/TiSi₂/p-Si capacitor, on the other hand, the leakage current characteristic was improved as the annealing temperature increased. Capacitance of the capacitor was found to also increase as the annealing temperature increased. The Al/Ta₂O₅/TiSi₂/p-Si capacitor, however, has failed to show the better capacitor characteristics over the conventional Al/Ta₂O₅/p-Si capacitor.     

Effects of annealing conditions on the properties of tantalum oxide films on silicon substrates

The formation of a SiO₂ layer at the Ta₂O₅/Si interface is observed by annealing in dry O₂ or N₂ and the thickness of this layer increases with an increase in annealing temperature. Leakage current of thin (less than 40 nm thick) Ta₂O₅ films decreases as the annealing temperature increases when annealed in dry O₂ or N₂. The dielectric constant vs annealing temperature curve shows a maximum peak at 750 or 800° C resulting from the crystallization of Ta₂O₅. The effect is larger in thicker Ta₂O₅ films. But the dielectric constant decreases when annealed at higher temperature due to the formation and growth of a SiO₂ layer at the interface. The flat band voltage and gate voltage instability as a function of annealing temperature can be explained in terms of the growth of interfacial SiO₂. The electrical properties of Ta₂O₅ as a function of annealing conditions do not depend on the fabrication method of Ta₂O₅ but strongly depend on the thickness of Ta₂O₅ layer.     

Carrier mobility in inversion layers of Si–thin Ta2O5 structures

The behaviour of carrier mobility in the inversion channel of gateless p-MOSFETs with thin (7-50 nm) Ta₂O₅ layers, having a dielectric constant of (23-27) and prepared by rf sputtering of Ta in an Ar-O₂ mixture, has been investigated. It is shown that independently of the high dielectric constant of the layers, the transport properties in the channel are strongly affected by defects in Ta₂O₅/Si system in the form of oxide charge and interface states. These defects act as scattering centers and are responsible for the observed minority carrier mobility degradation. Both, the oxide and the interface state charges are virtually independent on the oxygen content (in the range 10-30%) during the sputtering process. A reduction of the oxide charge and the density of interface states with increasing Ta₂O₅ film thickness was found, which results in the observed increase of the inversion channel mobility with thickness. It is assumed that the bond defects (broken or strained Ta-bonds as well as weak Si-O bonds in the transition region between Ta₂O₅ and Si) are much more probable sources of defect centers rather than Ta and O vacancies or impurities.     

Enhanced visible light-photocatalysis by hydrothermally synthesized thallium-doped bismuth vanadate nanoparticles

Thallium-doped (1.5, 2.3 and 19.3 at%) bismuth vanadate (BiVO₄) and pristine BiVO₄ nanoparticles were hydrothermally synthesized. They were characterized by powder X-ray and selected area electron diffractometries, high resolution scanning electron and transmission electron microscopies, and energy dispersive X-ray, Raman, UV–visible diffuse reflectance and photoluminescence spectroscopies. Tl-doping reduces the band gap energy and recombination of charge carriers. The visible light photocatalytic activity of 19.3% Tl-doped BiVO₄ nanoparticles is larger than those of the other Tl-doped BiVO₄ and pristine BiVO₄ nanoparticles. The synthesized Tl-doped BiVO₄ nanoparticles displaying enhanced photodegradation of dye could find potential applications as visible light photocatalyst for the abatement of various organic pollutants.     

Constructing Conductive Interfaces between Nickel Oxide Nanocrystals and Polymer Carbon Nitride for Efficient Electrocatalytic Oxygen Evolution Reaction

Combining transition metal oxide catalysts with conductive carbonaceous material is a feasible way to improve the conductivity. However, the electrocatalytic performance is usually not distinctly improved because the interfacial resistance between metal oxides and carbon is still large and thereby hinders the charge transport in catalysis. Herein, the conductive interface between poorly conductive NiO nanoparticles and semi‐conductive carbon nitride (CN) is constructed. The NiO/CN exhibits much‐enhanced oxygen evolution reaction (OER) performance than corresponding NiO and CN in electrolytes of KOH solution and phosphate buffer saline, which is also remarkably superior over NiO/C, commercial RuO₂, and mostly reported NiO‐based catalysts. X‐ray photoelectron spectroscopy and extended X‐ray absorption fine structure spectrum reveal that a metallic Ni–N bond is formed between NiO and CN. Density functional theory calculations suggest that NiO and CN linked by a Ni–N bond possess a low Gibbs energy for OER intermediate adsorptions, which not only improves the transfer of charge but also promotes the transmission of mass in OER. The metal–nitrogen bonded conductive and highly active interface pervasively exists between CN and other transition metal oxides including Co₃O₄, CuO, and Fe₂O₃, making it promising as an inexpensive catalyst for efficient water splitting.     

Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors

In this paper we report the use of Ta₂O₅ as gate dielectric material for organic thin-film transistors. Ta₂O₅ has already attracted a lot of attention as insulating material for VLSI applications. We have deposited Ta₂O₅ thin-films with different thickness by means of electron-beam evaporation. Being a relatively low-temperature process, this method is particularly suitable for organic thin-film transistor fabrication on plastic substrates. Deposition and patterning are achieved in one step by the use of shadow masks. The dielectric can be evaporated on top of the semiconducting layer. In this way a large variety of structures can be realized. Poly(3-hexylthiophene) was used as semiconducting material in the transistor structure. Such transistors are operating at voltages smaller than −3 V. Having a high dielectric constant (ε_r=21), Ta₂O₅ facilitates the charge carrier accumulation in the transistor channel at much lower electrical fields. The properties of the dielectric material as well as the operation of the organic transistors with a Ta₂O₅ gate dielectric are discussed.     

Improved power conversion efficiency by insertion of RGO–TiO2 composite layer as optical spacer in polymer bulk heterojunction solar cells

We report that the power conversion efficiency (PCE) can be enhanced in polymer bulk heterojunction solar cells by inserting an interfacial electron transporting layer consisting of pristine TiO₂ or reduced graphene oxide–TiO₂ (RGO–TiO₂) between the active layer and cathode Al electrode. The enhancement in the PCE has been analyzed through the optical absorption, current–voltage characteristics under illumination and estimation of photo-induced charge carrier generation rate. It was found that either TiO₂ or RGO–TiO₂ interfacial layers improve the light harvesting, as well as the charge extraction efficiency, acting as a blocking layer for holes, and also reducing charge recombination. The combined enhancement in light harvesting property and charge collection efficiency improves the PCE of the organic solar cell up to 4.18% and 5.33% for TiO₂ and RGO–TiO₂ interfacial layer, respectively, as compared to a value of 3.26% for the polymer solar cell without interfacial layer.     

Ultrathin Lutetium Oxide Film as an Epitaxial Hole‐Blocking Layer for Crystalline Bismuth Vanadate Water Splitting Photoanodes

Here a novel ultrathin lutetium oxide (Lu₂O₃) interlayer is integrated with crystalline bismuth vanadate (BiVO₄) thin film photoanodes to facilitate carrier transport through atomic‐scale interface control. The epitaxial Lu₂O₃ interlayer fabricated by pulsed laser deposition features very few structural defects at the back contact of the heterojunction, and forms a unique band alignment that favors photohole blocking. An optimized interlayer thickness of 1.4 nm significantly enhances charge separation efficiency and photocurrent. Combined with photoelectrochemical characterization, solid‐state electronic, and localized conductive atomic force microscopy measurements, it is revealed that the Lu₂O₃ interlayer modulates the electronic conduction pathways along structural grain boundaries and determines the overall device performance. This study sheds light on the nature of interface‐engineered carrier transport for efficient photoelectrode heterostructure design.     

Challenges of Ta2O5 as high-k dielectric for nanoscale DRAMs

The present status, successes, challenges and future of Ta₂O₅, and mixed Ta₂O₅-based high-k layers as active component in storage capacitors of nanoscale DRAMs are discussed. The engineering of new Ta₂O₅-based dielectrics (doped Ta₂O₅ and multicomponent Ta₂O₅-based high-k dielectrics) as well as of metal/high-k interface in MIM capacitor configuration are identified as critical factors for further reduction of EOT value below 1 nm.     

Amorphous Ta-nanocrystalline RuOx diffusion barrier for lower electrode of high density memory devices

The effects of the amount of RuO₂ added in the Ta film on the electrical properties of a Ta-RuO₂ diffusion barrier were investigated using n⁺⁺-poly-Si substrate at a temperature range of 650–800°C. For the Ta layer prepared without RuO₂ addition, Ta₂O₅ phase formed after annealing at 650°C by reaction between Ta and external oxygen, leading to a higher total resistance and a non-linear I-V curve. Meanwhile, in the case of the Ta film being deposited with RuO₂ incorporation, not only a lower total resistance and ohmic characteristics exhibited, but also the bottom electrode structure was retained up to 800°C, attributing to the formation of a conductive RuO₂ crystalline phase in the barrier film by reaction with the indiffused oxygen because of a Ta amorphous structure formed by chemially strong Ta-O or Ta-Ru-O bonds and a large amount of conductive RuO₂ added. Since a kinetic barrier for nucleation in formation of the crystalline Ta₂O₅ phase from an amorphous Ta(O) phase is much higher than that of crystalline RuO₂ phase from nanocrystalline RuO_x phase, the formation of the RuO₂ phase by reaction between the indiffused oxygen and the RuO_x nanocrystallites is kinetically more favorable than that of Ta₂O₅ phase.     

Monomolecular‐Layer Ba5Ta4O15 Nanosheets: Synthesis and Investigation of Photocatalytic Properties

Monomolecular‐layer perovskite Ba₅Ta₄O₁₅ nanosheets with hexagonal structure have been synthesized by a hydrothermal method. The thickness of the nanosheets is about 1.1 nm, which corresponds to a monolayer of Ba₅Ta₄O₁₅ molecules, with the lateral size ranging from 50 to 200 nm. The optimal conditions for the formation of the nanosheets are maintaining the reactants above 270 °C for 24 h. A dissolution–recrystallization mechanism is suggested based on observations of the factors that influence nanosheet formation, such as reaction time, temperature, and basicity. Formation of Ba₅Ta₄O₁₅ nanosheets takes precedence over other nanostructures under high concentrations of OH^– because the hindering effect of OH^– ions on the c‐axis growth is strong. Thus, the extended growth rate of polyhedrons on one monolayer is much faster than the superposition rate of the monolayer, and the crystal grows more easily along the a‐ and b‐planes. The Ba₅Ta₄O₁₅ nanosheets show a high photocatalytic activity in the degradation of Rhodamine B and gaseous formaldehyde. The layered perovskite probably affects the photocatalytic activity by promoting the charge separation and delocalization of photogenerated electrons and holes.     

The understanding of the memory nature and mechanism of the Ta2O5-gate-dielectric-based organic phototransistor memory

The memory nature and mechanism of the Ta₂O₅-gate-dielectric-based organic phototransistor memory (OPTM) have been studied. The UV–Vis absorption spectra and the X-ray photoelectron spectroscopy indicate that Ta₂O₅ owns positive interfacial charge because of the existence of Ta–OH. The hydroxide results in oxygen deficiency in Ta₂O₅ which is proposed to trap electrons. The characteristics of Ta₂O₅-based capacitor and the energy level alignment at Ta₂O₅–pentacene interface reveal that the electron-injection process is favorable which stimulates the electron-trapping process in Ta₂O₅. The Kelvin probe force microscopy of the Ta₂O₅-pentacene interface certificates the electron-injection and electron-trapping processes as well. It is the positive charges in Ta₂O₅ and energy level alignment that lead to the memory effect of Ta₂O₅-gate-dielectric-based OPTM. Compared to Ta₂O₅, polymethyl methacrylate (PMMA) does not have so strong a positive interface. Accordingly, PMMA films of different thickness are adopted on Ta₂O₅ to tune the Ta₂O₅-pentacene interface, offering control of the memory properties including the memory window and retention time. The understanding of the mechanism is at the forefront of devising high-performance OPTM devices.