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Surface Electron–Ion Mixed Conduction of Ti 0.99 Sc 0.01 O 2−δ Thin Film with Lattice Distortion and Oxygen Vacancies
... By substituting Ti 4+ site by Sc 3+ , large amount of oxygen vacancies was introduced in the Ti0.99Sc0.01O2- layer, and it exhibited the electron-proton mixed conduction with a small activation energy of ~100 meV and a high Hall mobility above 1.6410 2 cm 2 /Vs at room temperature (RT) [12]. These values are much larger than those of TiO2- and amorphous WO3-x thin films which exhibit LTM function [13,14]. ...
... multilayer are estimated to be R (200) of Ti0.99Sc0.01O2- and (111) of Pt [12]. The estimated lattice constant of a-axis in Ti0.99Sc0.01O2- is 4.537 Å, which has smaller lattice constant than the TiO2 bulk crystal due to the lattice mismatch between Pt and Ti0.99Sc0.01O2-. ...
... multilayer. Sharp peak and broad peak of the spectrum are estimated to be the O 2and OHstructures, respectively [12]. The existence of the OHpeak indicates that the oxygen components of the thin films involve the O-H bond. ...
Pulsed-induced resistivity modulation of a Pt/Ti0.99Sc0.01O2-δ/Pt multilayer with a cross-point structure was investigated. This multilayer exhibits nonlinear current–voltage characteristic based on the Schottky barrier at the Pt/Ti0.99Sc0.01O2-δ interface. When the electrical pulses of 1.5 V were applied with short interval time of 10 s, the resistivity modulation corresponding to the long-term-memorization (LTM) were observed. X-ray photoemission spectroscopy showed O-H bond that contributes to electron-proton mixed conduction at the Pt/Ti0.99Sc0.01O2-δ interface. This LTM resistivity modulation is considered to be due to the local proton migration at the Pt/Ti0.99Sc0.01O2-δ interface, and the operation voltage is lower than that of the reported oxide multilayer.
We have investigated the neuromorphic learning and forgetting functions of Pt/Ti0.99Sc0.01O2–δ/Pt multilayer with a cross-point array prepared by RF magnetron sputtering and probed their mechanism. The Ti0.99Sc0.01O2–δ layer with oxygen vacancy ratio of ~2.5% exhibited high electron-proton mixed conduction. The multilayer draws a nonlinear current-voltage curve owing to the Schottky barrier between the upper or lower Pt and Ti0.99Sc0.01O2–δ layers. Two singular current modulations corresponding to the learning long-term memory (LTM) and the short-term memory (STM) functions were observed by applying positive voltage pulses of 0.8 V with interval time of 14 s and 80 s, respectively. Furthermore, the forgetting LTM function of the human brain also exhibited by applying negative voltage pulses of 1.0 V with interval time of 14 s. These neuromorphic current responses are considered to attribute to the collaborative behaviors of electron, proton, and oxygen vacancy at the Schottky barrier.
We investigated the nanoionics-based neuromorphic function of Pt/Ti0.96Co0.04O2-δ/Pt multilayer with cross-point structure prepared by RF magnetron sputtering. This multilayer exhibits the electro-ion mixed conduction and the nonlinear current-voltage characteristic based on the Schottky barrier between Pt and Ti0.96Co0.04O2-δ layers. When the low electrical pulse of 0.8 V was applied with short interval time of 14 s, the current modulation corresponding to the long-term memorization (LTM) was observed, though the current response at the long interval time of 80 s was the short-term memorization (STM). The O 1s photoemission spectrum showed the OH- peak that contributes to the electron-ion mixed conduction. The current responses with both LTM and STM are considered to be due to the local proton migration at near the Schottky barrier.
We discuss the formation of TiO2
thin films via DC reactive magnetron sputtering. The oxygen concentration during sputtering proved to be a crucial parameter with respect to the final film
structure and properties. The initial deposition provided amorphous films that crystallise upon annealing to anatase or rutile, depending on the initial sputtering conditions. Substoichiometric films (TiOx<2), obtained by sputtering at relatively low oxygen concentration, formed rutile upon annealing in air, whereas stoichiometric films formed anatase. This route therefore presents a formation route for rutile films via lower (<500 °C) temperature pathways. The dynamics of the annealing process were followed by in situ ellipsometry, showing the optical properties transformation. The final crystal structures were identified by XRD. The anatase film obtained by this deposition method displayed high carriers mobility as measured by time-resolved microwave conductance. This also confirms the high photocatalytic activity of the anatase films.
The formation energies of oxygen vacancies at different surface and subsurface sites of anatase (101), anatase (001), and rutile (110) surfaces are calculated by the screened-exchange (sX) hybrid functional method. Our results show that the oxygen vacancy is more stable on the surface than subsurface for rutile (110), while it is a more stable subsurface than on the surface for anatase surfaces. These results are similar to those found by simple density functional theory, but now the sX hybrid functional gives the correct defect localizations. The defects introduce a gap state near the conduction band edge. For the most stable oxygen vacancy site at each TiO2 surface, the +2 charge state dominates over a wide range of Fermi energies.
A compact neuromorphic nanodevice with inherent learning and memory properties emulating those of biological synapses is the key to developing artificial neural networks rivaling their biological counterparts. Experimental results showed that memorization with a wide time scale from volatile to permanent can be achieved in a WO3-x-based nanoionics device and can be precisely and cumulatively controlled by adjusting the device's resistance state and input pulse parameters such as the amplitude, interval, and number. This control is analogous to biological synaptic plasticity including short-term plasticity, long-term potentiation, transition from short-term memory to long-term memory, forgetting processes for short- and long-term memory, learning speed, and learning history. A compact WO3-x-based nanoionics device with a simple stacked layer structure should thus be a promising candidate for use as an inorganic synapse in artificial neural networks due to its striking resemblance to the biological synapse.
Nb-doped anatase TiO2 (TNO) polycrystalline films with excellent conductivity and transparency were successfully fabricated by reactive sputtering combined with post annealing in H2 gas. The H2 annealing of as-deposited amorphous films caused an abrupt decrease in resistivity (rho), which was accompanied by crystallization into the anatase structure. A film deposited on an unheated glass substrate with subsequent H2 annealing at 600 °C exhibited a resistivity of 9.5× 10-4 Omega cm and an average optical transmittance of ˜75% in the visible region. This rho value is of the same order as that of epitaxial TNO films, which indicates that sputtering is a promising technique for obtaining large-area TNO films.
The electrical conductivity of dense and nanoporous zirconia-based thin films is compared to results obtained on bulk yttria stabilized zirconia (YSZ) ceramics. Different thin film preparation methods are used in order to vary grain size, grain shape, and porosity of the thin films. In porous films, a rather high conductivity is found at room temperature which decreases with increasing temperature to 120 °C. This conductivity is attributed to proton conduction along physisorbed water (Grotthuss mechanism) at the inner surfaces. It is highly dependent on the humidity of the surrounding atmosphere. At temperatures above 120 °C, the conductivity is thermally activated with activation energies between 0.4 and 1.1 eV. In this temperature regime the conduction is due to oxygen ions as well as protons. Proton conduction is caused by hydroxyl groups at the inner surface of the porous films. The effect vanishes above 400 °C, and pure oxygen ion conductivity with an activation energy of 0.9 to 1.3 eV prevails. The same behavior can also be observed in nanoporous bulk ceramic YSZ. In contrast to the nanoporous YSZ, fully dense nanocrystalline thin films only show oxygen ion conductivity, even down to 70 °C with an expected activation energy of 1.0 ± 0.1 eV. No proton conductivity through grain boundaries could be detected in these nanocrystalline, but dense thin films.
Memory is believed to occur in the human brain as a result of two types of synaptic plasticity: short-term plasticity (STP) and long-term potentiation (LTP; refs 1-4). In neuromorphic engineering, emulation of known neural behaviour has proven to be difficult to implement in software because of the highly complex interconnected nature of thought processes. Here we report the discovery of a Ag(2)S inorganic synapse, which emulates the synaptic functions of both STP and LTP characteristics through the use of input pulse repetition time. The structure known as an atomic switch, operating at critical voltages, stores information as STP with a spontaneous decay of conductance level in response to intermittent input stimuli, whereas frequent stimulation results in a transition to LTP. The Ag(2)S inorganic synapse has interesting characteristics with analogies to an individual biological synapse, and achieves dynamic memorization in a single device without the need of external preprogramming. A psychological model related to the process of memorizing and forgetting is also demonstrated using the inorganic synapses. Our Ag(2)S element indicates a breakthrough in mimicking synaptic behaviour essential for the further creation of artificial neural systems that emulate characteristics of human memory.
Resistance switching in metal oxides could form the basis for next-generation non-volatile memory. It has been argued that the current in the high-conductivity state of several technologically relevant oxide materials flows through localized filaments, but these filaments have been characterized only indirectly, limiting our understanding of the switching mechanism. Here, we use high-resolution transmission electron microscopy to probe directly the nanofilaments in a Pt/TiO(2)/Pt system during resistive switching. In situ current-voltage and low-temperature (approximately 130 K) conductivity measurements confirm that switching occurs by the formation and disruption of Ti(n)O(2n-1) (or so-called Magnéli phase) filaments. Knowledge of the composition, structure and dimensions of these filaments will provide a foundation for unravelling the full mechanism of resistance switching in oxide thin films, and help guide research into the stability and scalability of such films for applications.
Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From
a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their
magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the
concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating
the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic
moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at
2 kelvin.
Magnetic circular dichroism (MCD) of rutile Ti1-xCoxO2-d is systematically examined with various x and d to reveal a phase diagram for the appearance of ferromagnetism at higher carrier concentration and Co content. The phase diagram exactly matches with that determined from anomalous Hall effect (AHE). The magnetic field dependence of MCD also shows good coincidence with those of the magnetization and AHE. The coincidence of these independent measurements strongly suggests single and intrinsic ferromagnetic origin. Comment: 9 pages, 4 figures
Polycrystalline Zr0.92Y0.08O2 (YSZ) thin films were prepared by RF magnetron sputtering. 80- and 160-nm thin films exhibited (111) orientation and a polycrystalline structure, respectively. The 80-nm thin film had larger distortion than the 160-nm thin film. While the 80-nm thin film and a 120-nm thin film exhibited proton conduction, the 160-nm thin film did not, indicating that surface proton conduction can depend on film thickness. The activation energy of the 80- and 120-nm thin films measured in a wet atmosphere (0.52 eV) was about half of that measured in a dry atmosphere. The enhancement of conductivity for the thin films may be attributed to distortion, which may change the structure around an oxygen vacancy at the YSZ grain surface, accompanied by possible enhancement of H2O adsorption. H2O-annealed thin film had a hydrogen-induced level in the band gap energy region. This is the first observation of hydrogen-induced level at the surface state of the YSZ thin film obtained by X-ray absorption spectroscopy.
Sc-doped TiO2 (Ti1-xScxO2-δ) thin film on Al2O3 substrate were prepared by RF magnetron sputtering using ceramics targets. The Ti1-xScxO2-δ thin films deposited at room temperature were crystallized by postannealing at 800 °C for 1 h in O2 atmosphere. The thin films exhibited the single phase of rutile at x=0, the single phase of anatase at x=0.01 and mixed phases of rutile and anatase at x=0.03, and 0.05. The postannealed thin films were very smooth surface. In the absorption spectra, the fundamental absorption edge of Ti0.99Sc0.01O2-δ thin film with anatase phase shifts to lower wavelength side than those of x=0, 0.03, and 0.05. The small Sc doping for TiO2 thin film contributes to the changes of band gap, lattice constant and crystal structure. The existence of acceptor-doped TiO2 may realize practical application of a wide gap semiconductor diode in blue-light region or fuel cell, which used oxide material.
Anatase TiO2−δ thin film was prepared by RF magnetron sputtering using oxygen radical and Ti-metal target. Degrees of the TiO2−δ crystal orientation in the thin film depends of the oxygen gas pressure () in the radical gun. The (004)- and (112)-oriented TiO2−δ thin films crystallized without postannealing have the mixed valence Ti4+/Ti3+ state. The electrical conductivities, which corresponds to n-type oxide semiconductor, is higher in the case of (004)-oriented TiO2−δ thin film containing with high concentration of oxygen vacancy. The donor band of TiO2−δ thin film is observed at ~1.0 eV from the Fermi level (E
F). The density-of-state at E
F is higher in (004)-oriented TiO2−δ thin film. The above results indicate that the oxygen vacancies can control by changing the of the oxygen radical.
The structural and electrical properties of a c-axis-oriented BaCe0.85Ru0.05Y0.10O3-delta (BCRY) thin film on an Al2O3(0001) substrate depending on film thickness have been studied. The lattice constant of the c-axis decreases with increasing film thickness. The electrical conductivity is higher in the thin film with a small lattice constant. The activation energy (E-A) of the dry BCRY thin film with a high conductivity is 0.26 eV, which corresponds to half of that of the bulk ceramic. The BCRY thin film exhibits electron-ion mixed conduction with a small E-A of 0.18 eV below 400 degrees C in H2O atmosphere. The Ce3+ state created by oxygen vacancies, which locates at the top of the valence band, plays an important role in the electron-ion mixed conduction or proton conduction of the BCRY thin film.
The energies of atomic processes in resistive random access memories (RRAMs) are calculated for four typical oxides, HfO2, TiO2, Ta
2O5, and Al2O3, to define a materials selection process. O vacancies have the lowest defect formation energy in the O-poor limit and dominate the processes. A band diagram defines the operating Fermi energy and O chemical potential range. It is shown how the scavenger metal can be used to vary the O vacancy formation energy, via controlling the O chemical potential, and the mean Fermi energy. The high endurance of Ta
2O5 RRAM is related to its more stable amorphous phase and the adaptive lattice rearrangements of its O vacancy.
The physical properties and electronic structure of c-axis-oriented Ti1−xFexO2−δ thin films have been studied by soft X-ray spectroscopy. The c-axis lattice constant increases with increasing Fe concentration. Fe ions have mixed valence states of Fe2+ and Fe3+ with a high-spin configuration. The intensity of the unoccupied Ti 3d state decreases and that of the occupied Ti 3d state increases owing to oxygen vacancies with Fe substitution. The electronic structure in the band gap region consists of Fe 3d and Ti 3d states, which correspond to the remnant of the lower Hubbard hand. The density of states (DOS) at the Fermi level, which is closely related to electrical conductivity, is composed of the Ti 3d state. The electrical conductivity of heavily doped Ti1−xFexO2−δ thin films decreases owing to the electron correlation of Ti 3d electrons.
The physical properties and electronic structure of Sm-doped CeO2 (Ce0.90Sm0.10O2−δ) in the thin-film form have been studied. The as-deposited thin film exhibits (111) orientation on an Al2O3(0001) substrate. The lattice constant of the thin film is larger than that of the bulk crystal. The Ce0.90Sm0.10O2−δ thin film has the mixed valence states of Ce4+ and Ce3+. The electrical conductivity of Ce0.90Sm0.10O2−δ thin film in the temperature region from 200 to 700 °C is higher than that of the undoped CeO2 thin film and is independent of the oxygen partial pressure, indicating the occurrence of high oxygen-ion conduction. The valence band consists of the O 2p state hybridized with the Ce 4f state. The 40% Ce3+ (4f1L) state in Ce0.90Sm0.10O2−δ thin film, which was estimated from the resonant photoemission spectra, contributes to the high oxygen-ion conduction and low activation energy.
The proton conduction of BaCe0.9Y0.1O3 -δ (BCY) film on Al2O3 (0001) substrate grown by RF magnetron sputtering has been studied as a function of film thickness (20-600 nm) in the temperature region of 300-700 °C. The lattice constants and grain size of the as-deposited film increase with increasing film thickness. The film thickness above ~ 200 nm, which close to the lattice constants of the bulk, shows high proton conductivity with small activation energy (EA) of ~ 0.3 eV below 500 °C. The BCY thin film has the mixed valent states of Ce4 + and Ce3 + created by the charge transfer from O 2p to Ce 4f states. The EA corresponds to the energy difference between the top of valence band and the Fermi level. These results originate to the changes of O-O bond length and oxygen vacancies in the a-c plane of the BCY thin film.
We report the structural and electrical properties of a Sc3+-doped TiO2 (Ti0.99Sc0.01O2−δ) thin film prepared by RF magnetron sputtering. The thin film prepared at 100 °C exhibits the anatase (112) peak, although the thin films prepared at temperature above 200 °C show the rutile (200) peak. The lattice constant of the a-axis decreases with substrate temperature owing to oxygen vacancies. The Sc3+ substitution and anatase structure were examined of the basis of Ti 2p and Sc 2p X-ray absorption spectra. The electrical conductivity of the Sc-doped TiO2 thin film exhibits thermal activation-type behavior and oxygen partial pressure dependence. The conducting carriers at 500 and 600 °C are considered to be mixed hole- and electron-ion conductions, respectively. The conductivity may be closely related to the broad density of state (DOS) in the band-gap-energy region generated by Sc substitution.
TiO2−δ thin films have been deposited by RF magnetron sputtering using oxygen radicals. The deposition rate of the thin films largely increases with oxygen radical irradiation during the deposition in the metal mode. A as-deposited TiO2−δ thin film on a Pt substrate prepared at the substrate temperature of 300 °C crystallizes without postannealing. The Ti 2p X-ray absorption spectrum indicates that the Ti 3d electron enters the bottom of the conduction band. The Ti 3d states at the Fermi level (EF) and at ~1.5 eV from EF in the band gap energy region are closely related to the electrical conductivity.
It is demonstrated that a Cu2S gap-type atomic switch, referred to as a Cu2S inorganic synapse, emulates the synaptic plasticity underlying the sensory, short-term, and long-term memory formations in the human brain. The change in conductance of the Cu2S inorganic synapse is considered analogous to the change in strength of a biological synaptic connection known as the synaptic plasticity. The plasticity of the Cu2S inorganic synapse is controlled depending on the interval, amplitude, and width of an input voltage pulse stimulation. Interestingly, the plasticity is influenced by the presence of air or moisture. Time-dependent scanning tunneling microscopy images of the Cu-protrusions grown in air and in vacuum provide clear evidence of the influence of air on their stability. Furthermore, the plasticity depends on temperature, such that a long-term memory is achieved much faster at elevated temperatures with shorter or fewer number of input pulses, indicating a close analogy with a biological synapse where elevated temperature increases the degree of synaptic transmission. The ability to control the plasticity of the Cu2S inorganic synapse justifies its potential as an advanced synthetic synapse with air/temperature sensibility for the development of artificial neural networks.
We report x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements of both paramagnetic and ferromagnetic Ti1−xFexO2−δTi1−xFexO2−δ thin films. Fe L3,2L3,2 edge XAS spectra showed that the Fe ions in our samples were in mixed valence states and that the Fe2+/Fe3+ ratio decreased with increasing oxygen partial pressure. Finite XMCD signals were observed only for the Fe2+ state in the ferromagnetic sample. These findings indicate that the observed ferromagnetism in Ti1−xFexO2−δTi1−xFexO2−δ originates from ferromagnetic interactions between Fe2+ local spins.
The nano-grained specimens of yttria-doped zirconia have been fabricated via a combination of low-temperature nano-powder synthesis and room-temperature high-pressure (4 GPa) compaction. The microstructure is essentially free from macroscopic pores but involves interfacial hydrated layers, which facilitate adsorption of water molecules within the specimens. The three kinds of proton-containing species, i.e., surface terminating OH groups, H-bonded H2O molecules and free H2O molecules, have been distinguished from each other by the Thermal Desorption Spectroscopy analysis in terms of thermal stability, and also by the 1H MAS-NMR measurements. The electric conduction in humidified atmospheres is dominated essentially by proton hopping below ca. 800 K, which is verified by the H/D isotope effect and water vapor pressure dependence. The effect of grain growth on the conductivity suggests that the protonic conduction path is the grain boundary or interface. The present study shows a good feasibility of fabricating proton-conducting materials based on nano-grained oxides, even if the bulk property involves negligible proton solubility and conductivity, by the formation of grain boundary network of interfacial hydration layer.
TiO2 thin films were deposited on MgO substrates by reactive RF magnetron sputtering using oxygen radicals. In order to perform the low temperature crystallization and control of crystal structure, the TiO2 thin film was irradiated by highly reactive oxygen radicals during the deposition. When the distance between the substrate and Ti-metal target (S–T distance) was 90mm and substrate temperature were kept at 125°C, the radical irradiated TiO2 film exhibited a (110) orientation with rutile structure. The radical irradiated TiO2 thin film prepared at 300°C was changed from rutile to anatase structure by the adjustment of S–T distance. When the S–T distance was fixed at 110mm, the TiO2 film exhibited a high a-axis orientation with anatase structure. The TiO2 rutile and anatase thin films consisted of small grains with very smooth surface.
The electronic structure of TiO2-doped yttria-stabilized zirconia (YSZ) has been studied by soft-X-ray emission spectroscopy (SXES) and X-ray absorption spectroscopy (XAS). The valence band is mainly composed of the O 2p state. The O 1s XAS spectrum exhibits the existence of the Ti 3d unoccupied state under the Zr 4d conduction band. The intensity of the Ti 3d unoccupied state increases with increasing TiO2 concentration. The energy separation between the top of the valence band and the bottom of the Ti 3d unoccupied state is in accord with the energy gap, as expected from dc-polarization and total conductivity measurements.
The electronic structure of lightly Nb-doped SrTiO3 has been investigated using high resolution x-ray absorption spectroscopy (XAS). Below the O 1s threshold, XAS spectra show two features due to empty states whose energy positions match with those of the 3d photoemission spectra in the band gap energy region of the parent undoped SrTiO3. Similar features are also observed in lightly La-doped SrTiO3. These features exhibit systematic temperature dependence, which is well explained by the Fermi-Dirac distribution function. This indicates that the two features in the band gap are real bulk states such as simple donor states.
Nb-doped anatase TiO2 (Ti0.94Nb0.06O2) films with excellent conductivity and transparency were deposited on non-alkali glass by pulsed laser deposition. X-ray diffraction analysis and transmission electron microscopy confirmed that the obtained films were polycrystalline with anatase structure. The films deposited at a substrate temperature of 250 °C with subsequent H2 annealing at 500 °C showed a resistivity of 1.5 × 10-3 Omega\cdotcm at room temperature and an optical transmittance of 60-80% in the visible region. These results indicate that anatase Ti0.94Nb0.06O2 has great potential as a transparent conducting oxide that could replace Sn-doped In2O3 (ITO).
We present electrical transport and optical properties of Ta-doped TiO2 epitaxial thin films with varying Ta concentration grown by the pulsed laser deposition method. The Ti0.95Ta0.05O2 film exhibited a resistivity of 2.5× 10-4 Omega cm at room temperature, and an internal transmittance of 95% in the visible light region. These values are comparable to those of a widely used transparent conducting oxide (TCO), indium tin oxide. Furthermore, this new material falls into a new category of TCOs that utilizes d electrons.
We have constructed a high-resolution synchrotron-radiation angle-resolved photoemission (ARPES) spectrometer combined with a combinatorial laser molecular-beam epitaxy (laser MBE) thin film growth system in order to investigate the electronic structure of transition metal oxide thin films. An ARPES spectrometer GAMMADATA SCIENTA SES-100 was selected for the high-throughput and high-energy and angular-resolution ARPES measurements. A total energy resolution of 6.3 meV and a momentum (an angular) resolution of 0.02 Å−1 (0.2°) were obtained at a photon energy of 40 eV. The system is installed at the high-resolution vacuum-ultraviolet beamline BL-1C or the soft-x-ray undulator beamline BL-2C at the Photon Factory as an end-station. Another distinctive feature of this system is the direct connection from the spectrometer to a laser MBE chamber. Thin film samples can be transferred quickly into the photoemission chamber without breaking ultrahigh vacuum. Laser MBE is one of the best methods to grow thin films of many different transition metal oxides and to achieve well-ordered surfaces, which are indispensable for the ARPES measurements. The capabilities of the system and the importance of the in situ sample transfer between ARPES and laser MBE are demonstrated by studying the band structure of La0.6Sr0.4MnO3 thin films epitaxially grown on SrTiO3 substrates by laser MBE. © 2003 American Institute of Physics.
We have investigated electronic band structure of a transparent
conducting oxide, Nb-doped anatase TiO2 (TNO), by means of
first-principles band calculations and photoemission measurements. The
band calculations revealed that Nb 4d orbitals are strongly hybridized
with Ti 3d ones to form a d-nature conduction band, without impurity
states in the in-gap region, resulting in high carrier density exceeding
1021 cm-3 and excellent optical transparency in
the visible region. Furthermore, we confirmed that the results of
valence band and core-level photoemission measurements are consistent
with prediction by the present band calculations.
Columnar thin films of undoped ceria were grown by metal-organic chemical vapor deposition. The films, deposited on Pt-coated MgO(100) substrates, display a columnar microstructure with nanometer scale grain size and ∼30% overall porosity. Through-plane (thickness mode) electrical conductivity was measured by AC impedance spectroscopy. Proton conduction is observed below 350-400 °C, with a magnitude that depends on gas-phase water vapor pressure. The overall behavior suggests proton transport that occurs along exposed grain surfaces and parallel grain boundaries. No impedance due to grain boundaries normal to the direction of transport is observed. The proton conductivity in the temperature range of 200-400 °C is approximately four times greater than that of nanograined bulk ceria, consistent with enhanced transport along aligned grain surfaces in the CVD films.
Electrical and optical spectroscopic studies of TiO 2 anatase thin films deposited by sputtering show that the metastable phase anatase differs in electronic properties from the well‐known, stable phase rutile. Resistivity and Hall‐effect measurements reveal an insulator–metal transition in a donor band in anatase thin films with high donor concentrations. Such a transition is not observed in rutile thin films with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self‐trapped excitons is observed in anatase thin films, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin films have a wider optical absorption gap than rutile thin films.
This Letter focuses on the discovery of a transparent conducting oxide (TCO), anatase Ti <sub>1-x</sub> Nb <sub>x</sub> O <sub>2</sub> films with x=0.002–0.2 . The resistivity of films with x≥0.03 is 2–3×10<sup>-4</sup> Ω cm at room temperature. The carrier density of Ti <sub>1-x</sub> Nb <sub>x</sub> O <sub>2</sub> can be controlled in a range of 1×10<sup>19</sup> to 2×10<sup>21</sup> cm <sup>-3</sup> . The internal transmittance for films with x≤0.03 ( 40 nm thickness) is about 97% in the visible light region. The transport and optical parameters are comparable to those of typical TCOs, such as In <sub>2-x</sub> Sn <sub>x</sub> O <sub>3</sub> and ZnO.
The electronic properties of rutile TiO<sub>2</sub> with an ordered arrangement of oxygen vacancies show a transition from a resistive to conductive oxide as a function of vacancy ordering. Vacancy ordering along two different directions [110] and [001], studied by the density functional theory, predicts that the geometries in which the vacancy-to-vacancy interaction is the strongest, within the nearest neighbor coordination, are thermodynamically favorable and of technological importance. The oxygen vacancies induce several occupied defect states of Ti 3 d character, and according to our model, the vacancies are the mediators of electron conduction, while the conductive filament is formed by Ti ions. We propose that the formation of these types of conductive filament is intrinsically connected to the observed defect-assisted tunneling processes and oxide breakdown issues.
Ionic conductivities of ceria-rare earth oxide system were investigated in relation to their structures, electrical conductivities, and reducibilities. Rare earth oxides were soluble in fluorite lattice of ceria at low additive concentrations. The electrical conductivities of ceria-rare earth oxide systems depended on ionic radii of the added cation. Samarium and gadolinium-added ceria samples showed the highest electrical conductivity because of the close ionic radii of Sm3+ and Gd3+ to the radius of Ce4+. These systems exhibited the wide electrolytic region in conductivity versus Po2 plots as compared with ceria-calcia and -yttria systems. The ceria-samarium oxide samples exhibited a high power for solid oxide fuel cell at relatively low temperatures (873–1073 K).
We have studied Y1-xCaxTiO3 by photoemission and inverse-photoemission spectroscopy. Valence-band photoemission spectra show a d-band peak ~1.4 eV below the Fermi level (EF), which evolves into the lower Hubbard band in the x= 0 (d1) limit. The spectra show quasiparticle emission at EF with an extremely small spectral weight, z~0.01, which vanishes as the system approaches either the Mott insulator limit (x=0) or the band insulator limit (x=1). Correspondingly, inverse-photoemission spectra show the upper Hubbard band and a quasiparticle feature in the unoccupied state. The fact that the observed quasiparticle spectral weight is smaller than that of La1-xSrxTiO3 is attributed to the larger U/W, where U is the on-site d-d Coulomb energy and W is the d-band-width. The presence of the ~1.4-eV peak for a nearly empty d band (x~ 1) and the small spectral weight at EF cannot be explained within the Hubbard model, indicating the importance of interactions which are not included in the model, such as the long-range Coulomb interaction and the electron-phonon interaction.