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Electronic structure of Ti 1− x Fe x O 2−δ thin films with oxygen vacancies probed by soft X-ray spectroscopy
Abstract
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.
... TiO 2 thin film with Anatase structure can control from an insulator phase to semiconductor or metal phase by lightly carrier doping. [1][2][3][4][5][6][7][8][9][10] The carrier doping is made by substitution of different ion on the Ti site and O site or removing the O constituent accompanied with oxygen vacancy formation. Typical materials are Nb-doped TiO 2 (Ti 1−x Nb x O 2 ), 1,2) Ta-doped TiO 2 (Ti 1−x Ta x O 2 ), 3) and TiO 2−δ with oxygen vacancies. ...
... Typical materials are Nb-doped TiO 2 (Ti 1−x Nb x O 2 ), 1,2) Ta-doped TiO 2 (Ti 1−x Ta x O 2 ), 3) and TiO 2−δ with oxygen vacancies. 10) Furthermore, Fe-doped TiO 2 (Ti 1−x Fe x O 2 ), 6,7) and Co-doped TiO 2 (Ti 1−x Co x O 2 ) 4,5) are also known as diluted magnetic semiconductors. These conducting thin films are expected to be a transparent electrode of flat panel display and spintronics device. ...
... Therefore, the electrical and structural properties of carrier-doped TiO 2 thin film have been extensively studied in the research area of applied physics. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Since the atomic switching device using Ag 2 S thin film has been realized by Terabe et al., 17) similar studies have been reported in WO 3−x , 18) Ta 2 O 5 , 19,20) and TiO 2−δ , [11][12][13][14][15] which have the electron-ion mixed conduction. Such a phenomena can apply the non-volatile memory device such as resistance random access memory based on Redox reaction. ...
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.
... Conduction properties of undoped TiO 2 thin film with anatase or rutile structure can be controlled from an insulator phase to semiconductor or metal phase by substitution of different ion on the Ti 4+ site. [1][2][3][4][5][6][7][8][9][10] Typical materials are Nb 5+ -doped TiO 2 (Ti 1Àx Nb x O 2 ), [2][3][4][5] Ta 5+ -doped TiO 2 (Ti 1Àx -Ta x O 2 ) 6) and Co-doped TiO 2 (Ti 1Àx Co x O 2À ). 7,8) Hitosugi et al. has reported that the anatase-type Ti 0.95 Ta 0.05 O 2 epitaxial thin film prepared by pulsed laser deposition exhibits a low resistivity of 2:5 Â 10 À4 Ω cm, carrier density of 1:4 Â 10 21 cm −3 and high internal transmittance of 95% in the visible light region at room temperature (R.T.). ...
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.
Ti1-xFexO2-δ thin films have been deposited on Nb-doped SrTiO3 substrate by RF magnetron sputtering. The as-deposited thin films prepared at Tsub = 300 ~ 500°C exhibit anatase structure. The lattice constant depends on the crystallization temperature. The substitution of Fe ions is confirmed by photoemission spectra. The Ti 2p X-ray absorption spectra exhibit that Ti 3d-band occupancy is increased as a result of the oxygen vacancies. The thin film prepared at 500°C has the Ti 3d -DOS at the Fermi level. These findings indicate that Ti 3d electrons created by the oxygen vacancies contribute to the electrical conductivity.
The electronic structures of BiFeO (BF) and Mn-doped BiFeO (BF(Mn)) have been studied by X-ray absorption spectroscopy (XAS) and soft-X-ray emission spectroscopy (SXES). The BF and BF(Mn) have the mixed valence state of Fe{sup 2+} and Fe{sup 3+}. The valence band is mainly composed of O 2p state hybridized with the majority-spin t{sub 2g} and e{sub g} orbitals of Fe 3d state. The conduction band is composed of the minority-spin t{sub 2g} and e{sub g} orbitals of Fe 3d. The band gaps of BF and BF(Mn) are estimated to be 1.3 eV and 2.7 eV, respectively. The increase of band gap with Mn substitution contributes to the change of bandwidth of valence band.
Resonant inelastic x-ray scattering (RIXS) has recently been a subject of remarkable progress due to the advent of high-brilliance synchrotron radiation sources. The authors present a review of both experimental and theoretical investigations of electrons in solids using this second-order optical process, in which there is coherent absorption and emission of x rays at resonance with electronic excitations. The review starts with some of the fundamental aspects of RIXS, after which are presented typical experimental data and their theoretical interpretation for various materials. The first class of materials considered is semiconductors and insulators (Si, C, and BN), which are typical systems with weak electron correlation, and the data are interpreted based on electronic states described by an energy-band model. Effects of symmetry of electronic states and electron momentum conservation are discussed. At the opposite extreme are rare-earth systems (metals and oxides), in which the 4f electrons are almost localized with strong electron correlation. The observations are interpreted based on the effects of intra-atomic multiplet coupling and weak interatomic electron transfer, which are well described with an Anderson impurity model or a cluster model. In this context a narrowing of spectral width in the excitation spectrum, polarization dependence, and the magnetic circular dichroism in ferromagnetic materials are discussed. The authors then consider transition-metal compounds, materials with electron correlation strengths intermediate between semiconductors and rare-earth systems. In these interesting cases there is an interplay of intra-atomic and interatomic electronic interactions that leads to limitations of both the band model and the Anderson impurity model. Finally, other topics in resonant x-ray emission studies of solids are described briefly.
The resistivity of a very high quality anatase TiO2 doped with 6% of Nb was measured from 300 K down to 40 mK. No sign of superconductivity was detected. Instead, a minute quantity of cation vacancies resulted in a Kondo scattering. Measurements of thermo-electric power and resistivity were extended up to 600 K. The calculated power factor has a peak value of 14 μW/(K2cm) at 350 K, which is comparable to that of Bi2Te3 [Venkatasubramanian et al., Nature 413, 597 (2001)], the archetype thermolectrics. Taking the literature value for the thermal conductivity of Nb doped TiO2 thin films, the calculated figure of merit (ZT) is in the range of 0.1 above 300 K. This value is encouraging for further engineering of the material in order to reach ZT of 1 suitable for high temperature thermoelectrics.
The controlled-valence properties of La1-xSrxFeO3 and La1-xSrxMnO3 are studied by means of soft-x-ray absorption spectroscopy. A comparison between the transition-metal 2p spectra and atomic-multiplet calculations is used to determine the 3d count. The O 1s spectrum is used to characterize changes in unoccupied states that contain oxygen p character. The results indicate that the holes induced by substitution for both series are located in states of mixed metal 3d–oxygen 2p character. The ground state of LaFeO3 is mainly 3d5 and becomes 3d5L (where L denotes a ligand hole) in the La1-xSrxFeO3 series for low Sr concentration. The main component of the ground state of LaMnO3 is 3d4 and becomes a mixture of 3d3 and 3d4L in the La1-xSrxMnO3 series. The trends in controlled- valence properties of similar oxides across the transition-metal series can be rationalized within the framework of the Zaanen-Sawatzky-Allen model.
Anomalous Hall effect (AHE) of a ferromagnetic semiconductor anatase Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> thin film is studied from 10 to 300 K . Magnetic field dependence of anomalous Hall resistance is coincident with that of magnetization, while the anomalous Hall resistance decreases at low temperature in spite of nearly temperature-independent magnetization. Anomalous Hall conductivity σ<sub> AHE </sub> is found to be proportional to the square of Hall mobility, suggesting that charge scattering strongly affects the AHE in this system. The anatase Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> also follows a scaling relationship to conductivity σ<sub>xx</sub> as σ<sub> AHE </sub>∝σ<sub>xx</sub><sup>1.6</sup> , which was observed for another polymorph rutile Ti <sub>1-x</sub> Co <sub>x</sub> O <sub>2-δ</sub> , suggesting an identical mechanism of their AHE.
The effective ionic radii of Shannon & Prewitt [Acta Cryst. (1969), B25, 925-945] are revised to include more unusual oxidation states and coordinations. Revisions are based on new structural data, empirical bond strength-bond length relationships, and plots of (1) radii vs volume, (2) radii vs coordination number, and (3) radii vs oxidation state. Factors which affect radii additivity are polyhedral distortion, partial occupancy of cation sites, covalence, and metallic character. Mean Nb5+-O and Mo6+-O octahedral distances are linearly dependent on distortion. A decrease in cation occupancy increases mean Li+-O, Na+-O, and Ag+-O distances in a predictable manner. Covalence strongly shortens Fe2+-X, Co2+-X, Ni2+-X, Mn2+-X, Cu+-X, Ag+-X, and M-H- bonds as the electronegativity of X or M decreases. Smaller effects are seen for Zn2+-X, Cd2+-X, In2+-X, pb2+-X, and TI+-X. Bonds with delocalized electrons and therefore metallic character, e.g. Sm-S, V-S, and Re-O, are significantly shorter than similar bonds with localized electrons.
The oxygen 1s x-ray-absorption edges of a series of 3d-transition-metal oxides have been measured. The structures at the edge arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of mainly metal character. The spectra can be divided into two regions: The first is a double-peaked sharp structure near threshold, which can be related to the metal 3d states; the second is a broader structure 5–10 eV above the edge and is related to the metal 4s and 4p bands. We attribute the oxygen p character up to 15 eV above threshold to mainly oxygen 2p character. The data are analyzed in terms of ligand-field and exchange splittings. It is shown that the splitting between the two sharp peaks near threshold is related closely to the ligand-field splitting, but the relative intensities of the peaks are not fully explained at the present time.
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.
Ferromagnetic semiconductors are believed to be suitable for future spintronics, because both charge and spin degrees of freedom can be manipulated by external stimuli. One of the most important characteristics of ferromagnetic semiconductors is the anomalous Hall effect. This is because the ferromagnetically spin-polarized carrier can be probed and controlled electrically, leading to direct application for electronics. Control of the Curie temperature and magnetization direction by electronic field, and photo-induced ferromagnetism have been performed successfully using the anomalous Hall effect for group III-V ferromagnetic semiconductors. In these cases, the operation temperature was much below room temperature because of the limited Curie temperature of less than 160 K (ref. 6). Here, we report on the anomalous Hall effect governed by electron doping in a room-temperature transparent ferromagnetic semiconductor, rutile Ti(1-x)Co(x)O(2-delta) (of oxygen deficiency delta). This result manifests the intrinsic nature of ferromagnetism in this compound, and represents the possible realization of transparent semiconductor spintronics devices operable at room temperature.
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.
The bulk electronic state of SrTiO3-δ has been studied
by means of soft-X-ray emission spectroscopy (SXES) and photoemission
spectroscopy (PES) in the Ti 2p energy region. The PES spectrum shows
two peaks in the energy region immediately below the Fermi level
(EF), which sites in an energy band gap. Both peaks
correspond to a coherent part at EF and to an incoherent part
at a binding energy of approximately 1.5 eV. In
t2g-resonanced SXES spectra, Raman scattering of the δ
t2g peak occurring near 2.0 eV arises from a d-d transition
from the occupied incoherent band to the unoccupied coherent band. The
Raman shift reflects the half of the intra-atomic Coulomb energy
(Udd/2).
Here, we report that highly conductive polycrystalline anatase Nb-doped
TiO2 (TNO) thin films can be prepared via crystallization of
amorphous precursors under N2 atmosphere. An optimized TNO
film on a glass substrate exhibited a low resistivity of 8.4 ×
10-4 Ω cm and an absorbance of 6% at a wavelength of
460 nm. These transport and optical properties were comparable to those
of TNO films fabricated by vacuum annealing. This demonstrates the
potential of TNO as an electrode for GaN-based light-emitting diodes.
The Nb-doped anatase TiO2 is considered one of the most promising alternative transparent conducting oxides to substitute for indium tin oxide. However, studies have found that the conductivity emerges only in the anatase form, not in the rutile form. We applied the first-principle band structure method for the Nb-doped TiO2 in both polymorphs. The calculation was carried out using the spin-restricted and spin-polarized GGA+U level of the theory. Special care was taken in the calibration of +U parameters to satisfy the generalized Koopman's theorem. A significant difference was found between the spin-polarized and spin-restricted calculations. We noticed that spin polarization was necessary to reproduce the electron trapping in rutile. In addition, electrons are trapped at two lattice Ti atoms adjacent to the Nb-Ti dopant along the [001] direction, as described with the formal charge state of Ti3.5+-Nb5+-Ti3.5+. A careful convergence of the electron trapping character was conducted against the unit cell size based on the Bader population analysis.
The electronic structure of multiferroic BiFeO3 has been studied using soft x-ray emission spectroscopy. The fluorescence spectra exhibit that the valence band is mainly composed of O 2p state hybridized with Fe 3d state. The band gap corresponding to the energy separation between the top of the O 2p valence band and the bottom of the Fe 3d conduction band is 1.3 eV. The soft x-ray Raman scattering reflects the features due to the charge-transfer transition from O 2p valence band to Fe 3d conduction band. These findings are similar to the result of electronic structure calculation by density-functional theory within the local spin-density approximation that included the effect of Coulomb repulsion between localized d states.
A comparison of the electronic structure of rutile (110), anatase (101), and anatase (001) single-crystal surfaces has been made using resonant photoemission and x-ray absorption spectroscopy. Under identical preparative conditions, the anatase (101) surface shows the lowest Ti 3d and 4sp hybridization in the states close to the valence-band maximum of the three surfaces. It also shows the highest concentration of surface-oxygen vacancies. The effect on the electronic structure of modifying the surface preparative route and thus the concentration of surface-oxygen vacancies is examined. The σ-antibonding Ti 3d eg∕O 2p hybridization (probed by XAS) is reduced by the removal of surface-oxygen. Photoemission shows that as the number of surface-defects is increased, the O 2p-Ti 3d t2g π-bonding interaction is disrupted. For the anatase (101) surface it is found that as the number of surface-oxygen vacancies is increased, the Ti 3d and 4sp contributions at the valence-band maximum are reduced. We discuss the correlation between electronic structure and photocatalytic activity of the different polymorphs of TiO2.
Transparent conductive anatase TiO2−xFx(F:TiO2) epitaxial thin films were fabricated by reactive pulsed laser deposition with a solid fluorine source. F-doping as high as 1.6 × 1021 cm−3 (TiO1.95F0.05) was achieved under optimal growth conditions, and the obtained anatase TiO1.95F0.05 film had a low resistivity of 1.6 × 10−3 Ωcm and a high internal transmittance of >95%. Furthermore, the refractive index of a TiO1.95F0.05 film was >0.2 less than that of undoped TiO2. By comparing the refractive indices of F:TiO2 and Nb:TiO2, we concluded that the decreased refractive index in F:TiO2 can be attributed to a reduced electronic polarizability due to increased bond ionicity as well as doped electrons.
Soft X-ray emission studies for several early transition metal compounds, such as Sc, Ti and V compounds, have been studied. Raman scattering was found at the 2p→3d excitation in the transition metal compounds. The elementary excitation of the Raman spectrum is mainly elucidated by the d–d transition within the 3d bands, as well as the interband transition from the O 2p valence band to the conduction band, which consists mainly of transition metal 3d states. The charge transfer transition due to the localized electron picture is dominant in the soft X-ray spectra of Sc compounds. Raman scattering is a powerful tool in the study of the electronic structure of transition metal compounds.
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.
Pulsed laser deposited films of Co-doped anatase TiO2 are examined for Co substitutionality, ferromagnetism, transport, magnetotransport, and optical properties. Our results show limited solubility (up to ˜2%) of Co in the as-grown films and formation of Co clusters thereafter. For the Ti0.93Co0.07O2-delta sample, which exhibits a Curie temperature (TC) over 1180 K, we find the presence of 20 50 nm Co clusters as well as a small concentration of Co incorporated into the remaining matrix. After being subjected to the high-temperature anneal during the first magnetization measurement, the very same sample shows a TC˜650 K and almost full matrix incorporation of Co. This TC is close to that of as-grown Ti0.99Co0.01O2-delta sample (˜700 K). The transport, magnetotransport, and optical studies also reveal interesting effects of the matrix incorporation of Co. These results are indicative of an intrinsic Ti1-xCoxO2-delta diluted magnetic semiconductor with TC of about 650 700 K.
Growth conditions for pulsed laser deposition of ferromagnetic Ti1-xFexO2 films with the rutile structure have been optimized on the basis of magnetooptical Kerr effect (MOKE) measurements. Thus, it was found that Ti0.94Fe0.06O2 films prepared under very limited growth conditions, at a substrate temperature of 650-675°C and an oxygen pressure of ˜1× 10-6 Torr, show ferromagnetism at room temperature. The optimized films revealed a saturation magnetization of 1.3 muB and a coercive field of 0.14 kOe. From XPS measurements, the valence state of Fe was determined to be 3+, ruling out the possibility that ferromagnetism may arise from Fe3O4 nanoparticles.
We have measured the magnetic and transport properties of FexNbyTi1-x-yO2-delta (x = 0.06, y = 0, 0.1, 0.03) films deposited by pulsed laser deposition at various partial oxygen pressures (PO\scale70%2). The Fe and Nb codoped films have a carrier concentration of >3× 1020 cm-3, which is independent of PO\scale70%2 and far exceeds the critical carrier concentration for ferromagnetic ordering in Fe0.06Ti0.94O2-delta (1× 1019 cm-3). Room-temperature ferromagnetism was observed only when deposition was conducted in reducing atmospheres (i.e., PO\scale70%2
Photoemission and bremsstrahlung isochromat spectra have been measured on TiO2 (rutile) and SrTiO3. The magnitudes of the fundamental band gaps estimated from the combined spectra agree well with the results of the photoabsorption and the energy-band calculations. The profiles of the observed spectra do not reveal the fine structure expected to occur in the density-of-states curves obtained by the energy band calculations, suggesting the existence of a mechanism to smear out the fine structure considerably. The combined spectra are used to interpret the charge-transfer-type satellite manifest in the core-level spectra. For 3s photoemission, satellite features ascribed to the configuration interaction between the 3s3p6 and 3s23p43d states in the atomic notation are also observed.
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.
The valence state of Mn-doped BiFeO3--BaTiO3 ceramics has been probed by soft X-ray absorption spectroscopy (XAS). Mn-doped BiFeO3--BaTiO3 has valence states of Fe3+ and Ti4+, although BiFeO3 and Mn-doped BiFeO3 have mixed valence states of Fe2+ and Fe3+. The Mn 2p-XAS peak of Mn-doped BiFeO3--BaTiO3 locates at a lower energy side than that of Mn-doped BiFeO3 that corresponds to the Mn3+ valence state. These findings may indicate that the Fe3+ valence state of Mn-doped BiFeO3--BaTiO3 is stabilized by charge transfer from the Mn 3d state to the Fe 3d state through the Ti 3d state in BaTiO3.
The electronic structure of p-type SrTiO3 has been studied by photoemission spectroscopy (PES). Comparing with the PES of n-type SrTiO3, the Fermi level of p type is lower by about 0.7 eV and a prominent feature of Ti 3d character within the band gap, which is formed in the n-type SrTiO3, cannot be found in p-type SrTiO3. It is suggested that the band structure of p-type SrTiO3 follows the rigid-band model. The resonant photoemission study shows that the Ti 3d partial density of states in the valence band of p-type SrTiO3 is much larger than that of n-type SrTiO3. Furthermore, it is also found that the satellite intensities of several core lines in p-type SrTiO3 are stronger than those in n-type SrTiO3. These facts suggest that the hybridization effect between the Ti 3d and O 2p states becomes stronger in p-type SrTiO3.
Polarization dependence of soft-x-ray Raman scattering was investigated at the Ti 2p absorption edge of TiO2. Strong Raman scattering feature appears about 14 eV below elastic peaks with strong polarization dependence. These Raman scattering structures are charge transfer excitations to the antibonding state between 3d1L-1 and 3d0 states, because they are enhanced when the incident photon energies are tuned at satellite structures of Ti 2p absorption spectrum. Broad Raman scattering structures are found between 3 eV and 10 eV below elastic peaks. They are assigned to be nonbonding type charge transfer excitations or interband transition from O 2p valence to Ti 3d conduction bands, which includes the crystal field splitting in D2h symmetry with two Ti-O bond lengths.
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.
The x-ray absorption near edge structure spectroscopy and first-principles calculations were combined to study the local and electronic structures of rutile Co:TiO2 thin film with room-temperature ferromagnetism. It was revealed that the CoTi2+ substituting the Ti site forms CoTi2+-VO complex with the O vacancy generated in the annealing process. The O vacancy induces a spin-split donor impurity band with a t2g character within the gap region. We proposed that the strong exchange interaction between the localized carriers captured by O vacancy and the substitutional Co ions leads to the ferromagnetism of the Co:TiO2 thin film.
Iron resonant valence band photoemission spectra (VB PES) of Sr substituted LaFe0.75Ni0.25O3−δ have been recorded across the Fe 2p-3d absorption threshold to obtain Fe specific spectral information on the 3d projected partial density of states. Comparison with La1−xSrxFeO3 resonant VB PES literature data suggests that substitution of Fe by Ni forms electron holes which have mainly O 2p character. Substitution of La by Sr increases the hole concentration to an extent that the eg structure vanishes. The variation in the eg and t2g structures is paralleled by the changes in the electrical conductivity.
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.
We present the V2p and O1s X-ray absorption spectra of V2O3, VO2 and V2O5. The V2p spectra show strong multiplet effects caused by large 2p-3d and 3d-3d Coulomb and exchange interactions. The spectra exhibit noticeable chemical shifts and completely different multiplets. The shapes of the multiplet are related to the symmetry and spin of the ground state. The V2p spectrum of V2O3 can be simulated by an atomic-multiplet calculation projected in the appropriate crystal field. The O1s spectra are related to unoccupied Op character mixed in the conduction band. The spectra reflect V3d bands near the edge and V4sp bands at higher energies. The V3d bands are split by crystal field effects. Changes in the relative intensities are related to differences in the metal-ligand hybridization. The extra dispersion of the V3d bands in V2O3 is attributed to larger metal-metal interactions. The O1s spectrum of VO2 compares favorably to a symmetry-projected band structure calculation. Finally, the O1s spectra of VO2 taken at room temperature (insulator phase) and at T ≈ 120°C (metallic phase) show clearly the splitting of the V3d| band across the metal-insulator transition.
Satellite structure in the 2{ital p} absorption spectra of early 3{ital d} transition-metal compounds is reported. The spectral shape depends strongly on the local symmetry of the excited atom and gives evidence that in Ti oxides the excitation on the metal is screened by an exciton transition in the oxygen band and not by a charge-transfer or Mott-Hubbard transition as in the case of narrow-band metal compounds. The relative satellite intensities can be used to assess the anisotropy of the local electronic structure and the influence of the excited electron on the screening of the core-hole potential.