Bolin Cheng

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

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.

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.

Dielectric properties of Pb(Zr20Ti80)O3/Pb(Zr80Ti20)O3 multilayered thin films prepared by rf magnetron sputtering

A series of Pb(Zr,Ti)O3 (PZT) films with tetragonal/rhombohedral multilayered structures has been grown on Pt/TiO2/SiO2/Si substrates by rf magnetron sputtering at a relative low temperature. All the films comprise 12 periodicities of Pb(Zr20Ti80)O3/Pb(Zr80Ti20)O3 in constant thickness of 40 nm, but the layer thicknesses of tetragonal phase (dT) and rhombohedral phase (dR) in one periodicity are varied. The electric properties of the films are investigated as a function of dT/dR from 10/30 to 35/5. An enhanced dielectric property is observed in the multilayered films. Especially, a optimal value of dT/dR=30/10 is obtained, where the dielectric constant reaches maximum value of 469 at 100 kHz with a loss tangent of 0.037, and the dielectric constant is about five times that of the single tetragonal phase PZT film formed under the identical condition. Moreover, the polarization also increases in the multilayered films, and remarkably, the film of 30/10 exhibits larger remanent polarization, lower coercive v...

Reduction of leakage current by Co doping in Pt(Ba0.5Sr0.5TiO3/Nb-SrTiO3 capacitor

Effect of Co doping on leakage current has been investigated in capacitor consisting of Ba0.5Sr0.5Ti1−xCoxO3 (BSTC, x=0, 0.002, 0.010) thin film, Pt top electrode, and Nb-doped SrTiO3 (STON) bottom electrode. Co doping remarkably decreases the leakage current in BSTC thin film, such as from 9×10−7 A in undoped thin film to 8×10−11 A in 1.0 at. % Co-doped BSTC thin film at bias voltage of 6 V. In the case of the Pt electrode positively biased, the leakage current shows space-charge-limited-current behavior. The trap-filled-limit voltage and the calculated trapped electron density increase with Co concentration in BSTC thin film. The mechanism of the reduction of the leakage current by Co doping is discussed.

Improved dielectric properties and tunability of multilayered thin films of (Ba0.80Sr0.20)(Ti1−xZrx)O3 with compositionally graded layer

Multilayered thin films of (Ba0.80Sr0.20)(Ti1−xZrx)O3 (BSTZ) with compositionally graded layer (CGL) have been fabricated by pulsed laser deposition on Pt/TiO2/SiO2/Si substrate. In each CGL, four individual layers of BSTZ with x=0.36, 0.18, 0.08 and 0 are grown in series with equal thickness. Three kinds of thin-film CGL samples comprising one, two or four CGLs have been elaborated with the final same thickness, and the thickness of each CGL is accordingly varied in different samples to achieve the gradients of composition. The crystalline structures of the CGL thin films are successively characterized using x-ray diffraction and their electrical properties such as ac dielectric properties and polarizations are investigated. Improved dielectric properties and tunability of permittivity have been observed in the multilayered CGL films compared to single-layer BSTZ films prepared in the identical condition. Furthermore, with increasing gradients of compositions in the CGL multilayered films, the dielectric...

Raman spectroscopy studies of Ce-doping effects on Ba0.5Sr0.5TiO3 thin films

Ba0.5Sr0.5TiO3 (BST) thin films are among the best-known ferroelectric and dielectric materials. Ce-doped BST films have been fabricated by pulsed laser deposition in order to enhance their dielectric properties. X-ray diffraction, atomic force microscopy, and Raman spectroscopy have been used to study variations of crystal structure, surface morphologies, and phase stability of Ce-doped BST films, respectively. A strong influence of Ce doping on the properties of the BST films has been observed. First, a small amount of Ce dopant makes easy epitaxial growth of a BST film with a smooth surface on a MgO substrate. Second, residual stress in a BST film on a MgO substrate can be reduced by Ce doping, as demonstrated by the blueshift of phonon peaks in Raman spectroscopy.

Ultraviolet photovoltage characteristics of SrTiO3−δ∕Si heterojunction

A photovoltaic effect is observed in the heterostructure of p-Si∕n-SrTiO3−δ (p: hole carrier type, n: electron carrier type). The current–voltage curve exhibits a good rectifying characteristic similar to that of the traditional diode. The junction shows the open circuit voltage of 126mV∕mJ, the short circuit current of 1.78mA∕mJ, and the response time faster than 10ns for ultraviolet pulsed laser of 25ns in duration at room temperature, suggesting the promising potential of this junction as a new type of ultrafast ultraviolet detectors with high sensitivity for application.

Thermal stability of LaAlO3/Si deposited by laser molecular-beam epitaxy

High-k gate dielectric material LaAlO3 (LAO) films were deposited directly onto silicon substrates by laser molecular-beam epitaxy. The thermodynamic stability of LAO films deposited at different substrate temperatures and of LAO films postannealed at 1000 °C was studied by high-resolution transmission electron microscopy and capacitor–voltage measurements. These studies show that the interfacial reaction between the LAO film and silicon substrate is strongly correlated to the substrate temperature and ambient conditions. In oxygen containing ambient, the interfacial reaction often occurs not only during film deposition but also during the course of postannealing. LAO films annealed at 1000 °C in nitrogen ambient have better thermal stability with silicon than LAO films annealed in oxygen ambient do. Both kinds of films remain amorphous after 1000 °C annealing.

Enhancement of optical nonlinearity in periodic gold nanoparticle arrays

Linear and nonlinear optical properties of periodic triangular Au nanoparticle arrays were investigated. We compared the optical nonlinearity of periodic Au nanoparticle arrays with that of the ultra-thin gold film consisting of randomly distributed spheroidal clusters. A pronounced enhancement of the third-order nonlinear optical susceptibility χ((3)) in Au arrays was observed, and the figure of merit, χ((3))/α, of the periodic nanoparticle arrays is one order of magnitude larger than that of the ultra-thin film. Such an enhancement of the optical nonlinearity could be due to the strong local field near the triangular nanoparticles.