Oxygen-deficient SrTiO3 Research Articles

Ab initiostudy of the two-dimensional metallic state at the surface of SrTiO3: Importance of oxygen vacancies

Motivated by recent angle-resolved photoemission spectroscopy (ARPES) observations of a highly metallic two-dimensional electron gas (2DEG) at the (001) vacuum-cleaved surface of SrTiO3 and the subsequent discussion on the possible role of oxygen vacancies for the appearance of such a state (Ref 1), we analyze by means of density functional theory (DFT) the electronic structure of various oxygen-deficient SrTiO3 surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies --independently of the oxygen concentration-- redistribute in the surface region and deplete rapidly within a few layers from the surface suggesting the formation of a 2DEG. We discuss the underlying model emerging from such observations.

Impact of structural transitions on electron transport at LaAlO3/SrTiO3 heterointerfaces

We have studied conductive LaAlO3/SrTiO3 heterostructures deposited at different oxygen pressures. Photoluminescence spectra confirm the presence of a significant amount of oxygen vacancies in samples deposited at low oxygen pressures. Power law fitting of resistance versus temperature measurements reveals fundamental characteristics of the conduction mechanism at the interface. A distinct non-Fermi-liquid behavior is observed for samples grown in higher oxygen pressure, which give two-dimensionally confined conducting interfaces, whereas characteristic electron-electron scattering is observed for samples grown in lower oxygen pressures, as seen in bulk doped SrTiO3 (i.e., oxygen deficient SrTiO3). Transitions between different conduction modes occur throughout the studied temperature range (10–270 K) as a result of structural transformations in the near-surface region of the SrTiO3.

Metal-Insulator Transition inSrTiO3−xThin Films Induced by Frozen-Out Carriers

We report optical, electrical and magnetotransport properties of oxygen deficient SrTiO(3) (SrTiO(3-x)) thin films fabricated by pulsed laser deposition technique. The oxygen vacancies (O(vac)) in the thin film are expected to be uniform. By comparing its electrical properties to those of bulk SrTiO(3-x), it was found that O(vac) in bulk SrTiO(3-x) is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the SrTiO(3-x) film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be reexcited by Joule heating, electric and intriguingly magnetic field.

Intriguing photoconductivity behaviors of SrTiO 3−δ thin films with Pt contacts

The photoconductivity spectra of oxygen-deficient SrTiO 3−δ thin films with Pt contacts show two intriguing peaks at and above the band gap (∼3.2 eV). The first (second) peak, located at (above) the band gap, shows hysteretic (linear) photocurrent–voltage characteristics with a long (short) relaxation time. In addition, the first (second) peak shows a weak (strong) dependence on the surface treatment of the substrate as well as the surrounding atmosphere. These results have been discussed in terms of the Schottky contact, built-in voltage, and trapping/recombination process.

Photoluminescence of high energy Ar+-irradiated SrTiO3 single crystal

Abstract We report the structural and photoluminescence characteristics of a high energy Ar + -irradiated SrTiO 3 single crystal. High resolution X-ray reciprocal space mapping, cross-section scanning electron microscopy and photoluminescence measurements suggest that thick amorphous and oxygen-deficient SrTiO 3 layers are formed after irradiation. Due to the highly accelerated Ar + (∼100 keV), electrical conductivity of the oxygen-deficient SrTiO 3 layer seems to be highly reduced, which results in the absence of 440 nm (blue-light) emission at 300 K, and broad and weak 380 nm emission at 20 K.

Electron beam induced current investigations of Pt/SrTiO3−x interface exposed to chemical and electrical stresses

Pt Schottky contacts were fabricated on oxygen deficient SrTiO3−x [001] single crystals. Electron beam induced current (EBIC) and atomic force microscopy images taken on etched SrTiO3 (001) surfaces revealed that the dark {001} oriented lines observed in EBIC correlate with arrays of dislocation etch pits. Annealing contacts in air (at 120 °C for 10 min) changed the dislocation-related EBIC contrast from dark to bright. Electrically stressing the air-annealed Schottky contacts at −10 V for 1 h caused the dislocation-related EBIC contrast to return to dark. The contrast changes are interpreted as arising from oxygen vacancy motion in response to chemical or electrical stresses.

Nonadiabatic small polaron tunneling conduction in reduced Cr-doped SrTiO3−δ thin films

We investigated the electrical conduction and dielectric relaxation of reduced Cr-doped SrTiO3−δ thin films. The electrical conduction behavior indicates the formation of small polarons in oxygen deficient Cr-doped SrTiO3−δ thin films. The associated dielectric relaxation further indicates nonadiabatic small polaron tunneling conduction. A strong electron-phonon interaction is identified based on the large polaron coupling constant, α≈28(α⪢4).

Effects of hydrogen anneals on oxygen deficient SrTiO3−x single crystals

The influence of hydrogen gas anneals on the electrical properties of nominally undoped, oxygen-deficient SrTiO3−x single crystals was investigated. Titanium getter layers and vacuum anneals were used to obtain oxygen-deficient SrTiO3−x with a low electrical resistivity. These crystals showed an optical absorption peak at 2.92 eV and strong midinfrared absorption. Subsequent anneals at 800 °C in forming gas, which contained 10% hydrogen, returned the crystals into the insulating, transparent state. The mechanisms by which hydrogen anneals can compensate for the effects of oxygen vacancies in SrTiO3−x are discussed. The results show that forming gas anneals of stoichiometric SrTiO3 can lead to complex electrical conduction behavior.

Transport Behaviour of La0.8Sr0.2 AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La0.8Sr0.2AlO3 LSAO) thin films are grown on SrTiO3 (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80–340 K. The optical transmittance spectrum indicates that the LSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.

Time-resolved photoluminescence of n-doped SrTiO3

Following the recent surge of interest in n-doped strontium titanate as a possible blue light emitter, a time-resolved photoluminescence analysis was performed on nominally pure, Nb-doped and oxygen-deficient single-crystal SrTiO3 samples. The doping effects on both the electronic states involved in the transition and the decay mechanism are respectively analyzed by comparing the spectral and dynamic features and the yields of the emission. Our time-resolved analysis, besides shedding some light on the basic recombination mechanisms acting in these materials, sets the intrinsic bandwidth limit of the proposed blue light emitting optoelectronic devices made of Ti-based perovskites heterostructures in the gigahertz range.

Blue luminescence from electron-doped SrTiO3

Blue-light emissions from electron-doped SrTiO3 single crystals at room temperature were observed. Substituting La3+ for Sr2+ and Nb5+ for Ti4+ in SrTiO3 provides electron carriers in Ti 3d conduction bands; these carriers are responsible for the room-temperature blue-light emission. This blue-light emission is essentially the same as that observed in Ar+-irradiated oxygen-deficient SrTiO3. This blue luminescence is independent of the defect type. The chemical substitution of La3+ for Sr2+ changes the temperature of the structural phase transition from cubic to tetragonal symmetry. The relation between the photoluminescence properties and the structural phase transition is also discussed.

Sharp metal-insulator transition in Sr(Ti 1− xV x)O 3− δ thin films on SrTiO 3 substrates

We fabricated Sr(Ti 1− x V x )O 3 composition-spread films on SrTiO 3 (100) substrates by using pulsed laser deposition in ultra high vacuum. Sharp metal-insulator (MI) transitions were observed in the case of Sr(Ti 1− x V x )O 3− δ composition-spread films grown on an oxygen deficient SrTiO 3 (STO) buffer/SrTiO 3 (STO) substrate in the restricted composition x=0.5–1.0 while no MI transition was observed in the case of Sr(Ti 1− x V x )O 3− δ below x=0.5. In the insulating phase below the MI transition, non-linear I– V characteristics were observed. To explain the appearance and disappearance of MI transitions in Sr(Ti 1− x V x )O 3− δ two possibilities were considered: (i) strain effect from the substrate and (ii) charge-transfer at the interface between Sr(Ti 1− x V x )O 3− δ and STO.

Fabrication of submicron-scale SrTiO3−δ devices by an atomic force microscope

By applying a negative voltage to the conducting tip of an atomic force microscope, we modify on submicron-scale semiconducting oxygen deficient SrTiO3−δ thin films grown on LaAlO3 substrates. In comparison with the as-grown film, the modified regions present different electrical and structural properties, which can be exploited to realize submicrometer circuits. After a discussion on the mechanisms of the process, we report a prototype of a SrTiO3−δ-based sidegate field-effect transistor, showing a 4% modulation of channel resistivity with gate voltages up to 40 V.

Doping of SrTiO3 thin films studied by spectroscopic ellipsometry

The knowledge of thickness, optical properties and doping profiles of thin film samples is essential in view of their application in electronic devices. Here we present spectroellipsometric analysis on metallic oxygen deficient SrTiO 3 thin films grown by pulsed laser deposition on SrTiO 3 substrates. Such material is very interesting for applications in oxide electronics, having very large dielectric constant in the insulating state and becoming a metal with very high mobility with doping concentration as low as 10 18 e/cm 3 . First results indicate that SrTiO 3 thin films present a depressed optical absorption in the UV range above the energy gap and an absorption in the infrared region which is enhanced in respect to the bulk material.

Effect of oxygen adsorption on ion beam induced recrystallization of copper films

Oxygen adsorption effects on the crystallization of copper films under the irradiation of 2 MeV Ar + ions were investigated by using two types of substrates, one of which was the oxygen saturated SrTiO 3 (100) and the other was the oxygen deficient SrTiO 3 (100). As-deposited copper films consisted of fine crystals with random orientation. The copper crystals on both types of the substrates were grown without epitaxial relation when ion-irradiated under oxygen atmosphere. The epitaxial growth of copper films was achieved on the oxygen saturated substrate with the sequential procedure of oxygen adsorption and ion irradiation. The epitaxial relationship between the film and the substrate was determined Cu (100)|SrTiO 3 (100) and Cu[001]|SrTiO 3 [001]. The growth mechanism was discussed from the viewpoint of the surface reaction of oxygen with copper.

Possible effect of paramagnetic centers on the superconducting transition in oxygen deficient SrTiO 3

It is suggested that parmagnetic centers, identified by their contribution to the low-temperature hea capacity, are important in determining the apparent dependence of T c on carrier density in reduced SrTiO 3