Electronic structure of electron dopedSrTiO3:SrTiO3−δandSr1−xLaxTiO3 (original) (raw)
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Bulk electronic structure of SrTiO3: Experiment and theory
Journal of Applied Physics, 2001
Valence electron-energy loss spectroscopy ͑VEELS͒ in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO 3. Assignments of the interband transitions in the electronic structure of bulk SrTiO 3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states. The experimentally determined indirect band gap energy is 3.25 eV, while the direct band gap energy is 3.75 eV. The conduction bands in SrTiO 3 correspond to the bands composed of mainly Ti 3d t 2g and e g states, followed at higher energies by the bands of Sr 4d t 2g and e g states, and free electron like states dominating at energies above 15 eV. The upper valence band ͑UVB͒ contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV. The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 3.25 eV up to 15 eV. The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV. The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV. Interband transitions from the O 2s band to the conduction bands appear at 26 eV. A very narrow band at Ϫ33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons.
Physical Review B, 2013
A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment -and hence internal electric fields -within the LaAlO3 layer. Firstly, the Ti 2p core level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Secondly, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased -agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ∼300meV between n = 2 (insulating interface) and n = 6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell by unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n ≥ 4. We discuss effects which could frustrate this elegant and simple charge transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n = 4 and already for n = 2, mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.
Two-dimensional electron gas inδ-dopedSrTiO3
Physical Review B, 2010
It is shown that Shubnikov-de Haas oscillations in SrTiO 3 ␦-doped with La can be understood as arising from a two-dimensional electron gas of one subband immersed in the space charge layer. Despite the inherent complexity of a subband that is derived from four d-band states near the conduction-band minimum of SrTiO 3 , the quantum oscillations can be modeled quantitatively by recognizing that the magnetic field ͑B͒ induced effective spin splitting and Landau-level splitting are comparable. The oscillations are not strictly periodic in 1 / B, which can be understood as caused by a weak dependence of the electron density on the magnetic field in the subband that produces the observed oscillations.
Insight into the Effects of Fe Addition on the Local Structure and Electronic Properties of SrTiO3
This paper reports an experimental and theoretical investigation of the effects of adding Fe to the perovskite strontium titanate SrTiO 3 . The effects include changes in the short-order range structure as well as in the electronic and electrical properties. X-ray diffraction analysis reveals that the SrTi 1−x Fe x O 3 network shrinks with increasing Fe content, while X-ray absorption spectroscopy measurements of the Ti and Fe K-edge revealed the presence of some under-coordinated TiO 5 units created because of the partial replacement of Fe 3+ ions in the Ti 4+ site. UV−visible absorption spectra indicated a reduction in optical gap with increasing Fe content. The electronic structure and spin densities of STFO with x = 0, 0.0625, 0.125, 0.5, and 1 are calculated by the DFT method at the B3LYP computational level, showing a Jahn−Teller distortion of O atoms surrounding the Fe as well as the formation of under-coordinated TiO 5 units. From the electrical viewpoint, the results show that STFO is a mixed (ionic and electronic) conductor and that the electronic contribution becomes dominant with increasing Fe content.
Dielectric spectra and electrical conduction in Fe-doped SrTiO3
Physical Review B, 2000
The transport and dielectric properties of Sr 0.97 (Ti 1Ϫx Fe x )O 3Ϫ␦ solid solutions have been investigated at temperatures 1.5 KрTр300 K and frequencies 20 Hzрр1 MHz. Depending on x, suppression of the quantum-paraelectric background, dielectric anomalies associated with relaxational processes, variable-rangehopping conduction, and a sublinear power-law increase in the ac conductivity were observed. The results of the present work are the following: ͑i͒ For low concentration Fe doping, the suppression of the paraelectric state is smaller than for Bi and La doping; the higher stability of the quantum paraelectric state is due to the fact that, in contrast to Bi and La, Fe substitutes for Ti sites. ͑ii͒ The observed variable-range-hopping ͑VRH͒ conduction indicates that Fe doping leads to the occurrence of local states in the highly disordered SrTiO 3 lattice; this behavior coexists with the stable quantum paraelectric state. ͑iii͒ A dielectric relaxation process from 50 to 200 K is observed that is related to some defects; it is evidenced that the dielectric relaxation has no direct relation with the VRH mechanism, however, it is very possibly related to a polaron mechanism.
Tuning the electronic effective mass in double-doped SrTiO_{3}
Physical Review B, 2011
We present an approach to tune the effective mass in an oxide semiconductor by a double doping mechanism. We demonstrate this in a model oxide system Sr1−xLaxTiO 3−δ , where we can tune the effective mass ranging from 6-20me as a function of filling or carrier concentration and the scattering mechanism, which are dependent on the chosen lanthanum and oxygen vacancy concentrations. The effective mass values were calculated from the Boltzmann transport equation using the measured transport properties of thin films of Sr1−xLaxTiO 3−δ . Our method, which shows that the effective mass decreases with carrier concentration, provides a means for understanding the nature of transport processes in oxides, which typically have large effective mass and low electron mobility, contrary to the tradional high mobility semiconductors.
Enhancement of tetragonality and role of strontium vacancies in heterovalent doped SrTiO3
Applied Physics Letters, 2011
The effect of Sr vacancies on the behavior of strontium titanate with trivalent dopants (La3+, Gd3+, and Y3+) substituting Sr2+ ions is reported. A remarkable shift of the antiferrodistortive transition temperature Ta is revealed by Raman spectroscopy for just a small content of dopant. It is shown that a unique linear dependence of Ta versus tolerance factor is obtained when
Bandgap Controlling of the Oxygen-Vacancy-Induced Two-Dimensional Electron Gas in SrTiO3
Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, charge, spin and orbital degrees of freedom.[1–5] Bandgap, a critical parameter for an oxide insulator, is well determined by those degrees of freedom and in turn directly affects electronic, magnetic and optical properties of the material. Typically, tunability of the bandgap in an oxide insulator can be achieved through chemical doping,[6] which is important for electronic and photonic device applications. Here we report large bandgap enhancement in SrTiO3 (STO) thin films, which can be up to 20% greater than the bulk value, depending on the deposition temperature. There is no significant change in density and cationic ratio of the oxide so the effect is attributed to Sr/Ti antisite defects, an attribution supported by density functional theory calculations. It was found that the bandgap enhancement significantly changes the electronic and magnetic phases in the oxygen-vacancy-induced two-dimensional electron gas at the interface between amorphous LaAlO3 (LAO) and STO. This opens an attractive path to tailor electronic, magnetic and optical properties of STO-based oxide interface systems under intensive focus in the oxide electronics community. Meanwhile, our study provides key insight into the origin of the fundamental issue that STO thin films are difficult to convert into metals by oxygen vacancy doping