Kui-juan Jin

Switchable diode effect and ferroelectric resistive switching in epitaxial BiFeO3 thin films

Current-voltage hysteresis and switchable rectifying characteristics have been observed in epitaxial multiferroic BiFeO3 (BFO) thin films. The forward direction of the rectifying current can be reversed repeatedly with polarization switching, indicating a switchable diode effect and large ferroelectric resistive switching. With analyzing the potential barriers and their variation with ferroelectric switching at the interfaces between the metallic electrodes and the semiconducting BFO, the switchable diode effect can be explained qualitatively by the polarization-modulated Schottky-like barriers.

High sensitivity of positive magnetoresistance in low magnetic field in perovskite oxide p–n junctions

Large positive magnetoresistance (MR) and high MR sensitivity in low magnetic fields have been discovered in the Sr-doped LaMnO3 and Nb-doped SrTiO3 p-n junctions fabricated by laser molecular-beam epitaxy. The MR ratios, defined as DeltaR/R-0, DeltaR = R-H-R-0, are observed as large as 11 % in 5 Oe, 23 % in 100 Oe, and 26 % in 1000 Oe at 290 K; 53% in 5 Oe, 80 % in 100 Oe, and 94 % in 1000 Oe at 255 K. The MR sensitivities are 85 Omega/Oe at 290 K, 246 Omega/Oe at 255 K, and 136 Omega/Oe at 190 K, respectively, with the applied magnetic field changed from 0 to 5 Oe. The positive MR ratios and high MR sensitivities of the p-n junctions are very different from that of the LaMnO3 compound family

Dember effect induced photovoltage in perovskite p-n heterojunctions

An unusual and rather large transient lateral photovoltage (LPV) has been observed in La0.9Sr0.1MnO3∕SrNb0.01Ti0.99O3 and La0.7Sr0.3MnO3∕Si heterojunctions under the nonuniform irradiation of pulsed laser. The irreversible LPVs on both sides of a p-n junction challenge the well established model for LPV in conventional semiconductor p-n junctions, which can be well explained by the Dember effect. Much larger LPV is observed in La0.7Sr0.3MnO3∕Si than that in La0.9Sr0.1MnO3∕SrNb0.01Ti0.99O3. Similar results measured from both substrates of SrNb0.01Ti0.99O3 and Si also support such a Dember effect. Much larger LPVs in heterojunctions than those in simple samples (SrNb0.01Ti0.99O3 or Si) suggest a potential application of Dember effect in heterostructures.

Evidence for a Crucial Role Played by Oxygen Vacancies in LaMnO3 Resistive Switching Memories

LaMnO(3) (LMO) films are deposited on SrTiO(3):Nb (0.8 wt%) substrates under various oxygen pressures to obtain different concentrations of oxygen vacancies in the films. The results of X-ray diffraction verify that with a decrease of the oxygen pressure, the c-axis lattice constant of the LMO films becomes larger, owing to an increase of the oxygen vacancies. Aberration-corrected annular-bright-field scanning transmission electron microscopy with atomic resolution and sensitivity for light elements is used, which clearly shows that the number of oxygen vacancies increases with the decrease of oxygen pressure during fabrication. Correspondingly, the resistive switching property becomes more pronounced with more oxygen vacancies in the LMO films. Furthermore, a numerical model based on the modification of the interface property induced by the migration of oxygen vacancies in these structures is proposed to elucidate the underlying physical origins. The calculated results are in good agreement with the experimental data, which reveal from a theoretical point of view that the migration of oxygen vacancies and the variation of the Schottky barrier at the interface with applied bias dominate the resistive switching characteristic. It is promising that the resistive switching property in perovskite oxides can be manipulated by controlling the oxygen vacancies during fabrication or later annealing in an oxygen atmosphere.

Picosecond photoelectric characteristic in La0.7Sr0.3MnO3∕Si p-n junctions

Ultrafast photoelectric effects have been observed in La0.7Sr0.3MnO3∕Si p-n junctions fabricated by laser molecular-beam epitaxy. The rise time was ∼210ps and the full width at half-maximum was ∼650ps for the photovoltaic pulse when the junction was irradiated by a 1064nm laser pulse of 25ps duration. The photovoltaic sensitivity was as large as 435mV∕mJ for a 1064nm laser pulse. No such photovoltaic signal was observed with irradiation from a 10.6μm CO2 laser pulse. The results reveal that this phenomenon is an ultrafast photoelectric effect.

Effects of interfacial polarization on the dielectric properties of BiFeO3 thin film capacitors

Epitaxial BiFeO3∕La0.7Sr0.3MnO3 (BFO/LSMO) heterostructures were grown on SrTiO3 (001) substrates. Dielectric properties of the BFO thin films were investigated in an In/BFO/LSMO capacitor configuration. The capacitance of the capacitor shows strong dependences on measuring frequency and bias voltage especially in low frequency region (⩽1MHz). By means of complex impedance analysis, it is found that the interfacial polarization caused by space charges in the film/electrode interfaces plays an important role in the dielectric behavior of the capacitor. Our results indicate that the influences of film/electrode interfaces might not be neglected on the dielectric properties of the BFO thin film capacitors.

Transient lateral photovoltaic effect in p-n heterojunctions of La0.7Sr0.3MnO3 and Si

A transient lateral photovoltaic effect (LPVE) has been observed in p-La0.7Sr0.3MnO3∕n-Si heterojunctions. Under the nonuniform irradiation of a pulsed laser, the LPVE shows high sensitivity to the spot position on the La0.7Sr0.3MnO3 surface. A mechanism based on the well established model for the LPVE in conventional semiconductors has been applied to explain the LPVE in the heteroepitaxial junctions of perovskite-type metal oxides. The large LPVE in the heteroepitaxial junctions is expected to make the perovskite-type metal oxide a new and faster candidate for position-sensitive photodetectors.

Positive colossal magnetoresistance in a multilayer p–n heterostructure of Sr-doped LaMnO3 and Nb-doped SrTiO3

A positive colossal magnetoresistance (CMR) has been discovered in an epitaxial multilayer p–n heterostructure fabricated with Sr-doped LaMnO3 and Nb-doped SrTiO3 by laser molecular-beam epitaxy. In contrast to the negative CMR of the LaMnO3 compound family, positive CMR is observed in the temperature range from 100 to 300 K. The largest value of the magnetoresistance (MR) ratio (ΔR/R0,ΔR=RH−R0), 517%, is one order of magnitude larger than that of simple p–n junctions of the same materials previously reported. A very large MR ratio, 297%, remains in a low field of 0.01 T. Even at a temperature as high as 300 K, a MR ratio as large as 17.3% is still observed.

Influence of Ce doping on leakage current in Ba0.5Sr0.5TiO3 films.

Undoped and Ce-doped Ba0.5Sr0.5TiO3 (BST) thin films were prepared by pulsed-laser deposition onto a Nb-doped SrTiO3 (STON) substrate. The Ce concentration, ranging from 0.5 to 1.0 at.%, was found to have a strong influence on the electric properties of films at room temperature. We find that, with a positively biased Pt electrode, the leakage current controlled by BST/STON interface can be described by a space-charge-limited-current model. When the Pt electrode is negatively biased, the leakage current controlled by the BST/Pt interface can be explained by the Schottky emission mechanism. In both cases the Ce-doped BST thin films exhibited a lower leakage current (1.2 × 10 −4 and 5.0 × 10 −5 versus 3.4 × 10 −2 Ac m −2 at 450 kV cm −1 ;4 .0 × 10 −4 and 4.0 × 10 −5 versus 6.2 × 10 −3 Ac m −2 at −450 kV cm −1 ) than undoped BST films. The reduction of the leakage current is ascribed to the effect of acceptor Ce 3+ doping, determined by x-ray photoelectron spectroscopy measurement. (Some figures in this article are in colour only in the electronic version)

Ultraviolet fast-response photoelectric effect in tilted orientation SrTiO3 single crystals

Ultraviolet photoelectricity based on the vicinal cut as-supplied SrTiO3 single crystals has been experimentally studied in the absence of an applied bias at room temperature. An open-circuit photovoltage of 130 ps rise time and 230 ps full width at half maximum was observed under the irradiation of a 355 nm pulsed laser of 25 ps in duration. The dependence of the photoelectric effect on the tilting angles was studied, and the optimum angle is 20.9 degrees. Seebeck effect is proposed to elucidate the tilting angle dependence of laser-induced photovoltage. This work demonstrates the potential of SrTiO3 single crystals in ultraviolet detection. (c) 2006 American Institute of Physics.