N. J. Wu

Electric-pulse-induced reversible resistance change effect in magnetoresistive films

A large electric-pulse-induced reversible resistance change active at room temperature and under zero magnetic field has been discovered in colossal magnetoresistive (CMR) Pr0.7Ca0.3MnO3 thin films. Electric field-direction-dependent resistance changes of more than 1700% were observed under applied pulses of ∼100 ns duration and as low as ±5 V magnitude. The resistance changes were cumulative with pulse number, were reversible and nonvolatile. This electrically induced effect, observed in CMR materials at room temperature has both the benefit of a discovery in materials properties and the promise of applications for thin film manganites in the electronics arena including high-density nonvolatile memory.

Thin-film heterostructure solid oxide fuel cells

A micro thin-film solid oxide fuel cell (TFSOFC) has been designed based on thin-film deposition and microlithographic processes. The TFSOFC is composed of a thin-film electrolyte grown on a nickel foil substrate and a thin-film cathode deposited on the electrolyte. The Ni foil substrate is then processed into a porous anode by photolithographic patterning and etching to develop pores for gas transport into the fuel cell. A La0.5Sr0.5CoO3 (LSCO) thin-film cathode is then deposited on the electrolyte, and a porous NiO–YSZ cermet layer is added to the anode to improve the electrode performance. The TFSOFC has stably operated in a temperature ranges as low as 480–570 °C, significantly lower than bulk SOFC’s, and has yielded a maximum output power density of ∼110 mW/cm2 in that temperature range.

Electrical conductivity relaxation studies of an epitaxial La0.5Sr0.5CoO3−δ thin film

Abstract The oxygen surface exchange coefficient kchem of a La0.5Sr0.5CoO3−δ (LSCO) thin film has been determined from electrical conductivity relaxation measurements. The LSCO thin films were deposited on LaAlO3 (LAO) single crystal substrates by pulsed laser deposition (PLD). The electrical conductivity relaxation behavior of the film was measured at high temperature on switching the oxygen partial pressure between 0.01, 0.05, 0.10, 0.30, 0.50 and 1.00 atm. The kchem values were obtained by fitting the conductivity relaxation curves using a surface-limited kinetics model. The results show that kchem increases with temperature and with the oxygen partial pressure after the switch, but is not sensitive to the initial partial pressure. After prolonged heating at 900 °C, kchem increased substantially. The increase is associated with a change in the thin film surface morphology on prolonged heating.

Structural steps to oxidation of Ni(100)

In this paper, we emphasize the temperature‐ and exposure‐dependent development of low‐energy electron diffraction patterns,measured quantitatively during oxidation of Ni(100) at 80 to 400 K. We find a strong temperature dependence in the development of LEED patterns associated with NiO. NiO(111) is favored by adsorption temperatures below 300 K, whereas a (7×7)‐like structure is favored by adsorption temperatures of 300 to 400 K. Room temperature is a ‘‘crossover’’ point between these two forms of the oxide. The final oxide depth is independent of adsorption temperature and, therefore, of epitaxial orientation, between 80 and 400 K. When the sample is heated in vacuum after adsorption, massive rearrangements take place above 500 K. Some of the nickel reverts to metallic nickel covered by a c(2×2) oxygen overlayer, and some forms NiO crystallites which are probably deeper than the initial oxide skin. Effectively, the parent oxide disproportionates into a less‐oxygen‐rich phase and a more‐oxygen‐rich phase...

Pulsed laser deposition of conducting porous La-Sr-Co-O films

Abstract La0.5Sr0.5CoO3−δ (LSCO) films for use in solid oxide fuel cells (SOFC) have been deposited on yttria stabilized zirconia (YSZ)(100) substrates by pulsed laser deposition (PLD). Both epitaxial and porous-polycrystalline films were fabricated. The porous films were formed by deposition at room temperature followed by anneal at from 450–750°C to increase porosity. Resistivity measurements have shown that the porous LSCO films annealed at 550°C yielded the lowest resistivity of 10−2 Ω cm. The epitaxial LSCO films were deposited on YSZ(100) in the temperature range of 450 to 750°C, and showed (110) preferred orientation. The electrical resistivity of the epitaxial LSCO films is 10−3 Ω·cm for films deposited at 550°C.

Transient behavior and memory effect of a PbZrxTi1−xO3/YBa2Cu3O7−x three‐terminal device

The transient behavior of a ferroelectric PbZrxTi1−xO3 and superconducting YBa2Cu3O7−x three‐terminal device has been investigated. Four‐state behavior, that is, two polarization states of the PbZrxTi1−xO3 gate and superconducting and normal states of the YBa2Cu3O7−x layer, has been observed. It was shown that the biased superconducting channel can be switched from superconducting state to the normal state by the flowing charge during the polarization switching of the PbZrxTi1−xO3 gate. The nonvolatility of this device based on the different polarization states of the ferroelectric gate has also been demonstrated.

Heterostructures of Pb(ZrxTi1-x)O3 and YBa2Cu3O7-δ on MgO Substrate Prepared by Pulsed Laser Ablation

Ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films were integrated to high-Tc superconducting YBa2Cu3O7-? (YBCO) films in PZT/YBCO/MgO(100) and YBCO/PZT/MgO(100) stacking structures by the pulsed laser ablation technique. The YBCO films in both structures were superconducting and c-axis oriented. The PZT films in both structures were of the perovskite structure having the (100) orientation. The detailed profile of PZT/YBCO interface was determined for the ferroelectric constituents. The ferroelectric properties of the PZT films were characterized in the temperature range of 40 K to 300 K. The remanent polarization was above 15 ?C/cm2 and the coercive field was 46 kV/cm at room temperature. The cycling fatigue and the loss tangent were improved at low temperature.

Pyroelectric, ferroelectric and dielectric properties of Mn and Sb-doped PZT thin films for uncooled IR detectors

Abstract Mn and Sb-doped Pb(Zr,Ti)O 3 (PMSZT)/YBa 2 Cu 3 O 7− y (YBCO) heterostructures grown on Si substrates have been investigated for IR detector arrays. The epitaxial PMSZT films deposited on c -oriented YBCO conducting electrodes show “self polarization” behavior and high IR responsivity even without poling. An asymmetric hysteresis loop and C – V curve show that the original polarization state of the capacitors is pointing towards the top electrode in the PMSZT film, and perpendicular to the YBCO thin film. Doping with Mn and Sb has been shown to enhance the pyroelectric and dielectric properties of PZT for enhanced IR sensitivity. The PMSZT IR detectors were examined for their pyroelectric coefficient and their detectivity D *. The PMSZT thin films have relatively high pyroelectric coefficient of 45 nC cm −2 K at 25°C and 148 nC cm −2 K at 90°C. The PMSZT detector shows detectivity D * values from 2.5×10 8 to 6.0×10 8 cmHz 1/2 W −1 in the 2.5–19.5 μm wavelength range.

Structure and conducting properties of La0.5Sr0.5CoO3-δ films on YSZ

Abstract La0.5Sr0.5CoO3−δ (LSCO) thin films were deposited on yttria stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell electrodes. During the deposition, the substrate temperature was varied from 450 to 750°C, and the oxygen pressure in the chamber was varied from 80 to 310 mTorr. Films deposited at 650°C and an oxygen background pressure of 150 mTorr were mostly (100) oriented. Deposition at higher temperatures or under lower oxygen pressures lead to mostly (110) oriented films. Films with low electrical resistivity of 10−3 Ω·cm were obtained.

Perovskite RRAM devices with metal/insulator/PCMO/metal heterostructures

Perovskite non-volatile resistive random access memory (RRAM) devices with metal/insulator/Pr0.7Ca0.3MnO3 (PCMO)/metal thin film heterostructures have been fabricated and characterized as to performance. The insulator layer was grown as a buffer layer between the active CMR perovskite and the top metal contact. For these buffer layer heterostructure devices, the pulse voltage needed to switch the device is significantly reduced and the resistance-switching ratio is increased as compared to a non-buffered RRAM device. Low temperature magnetic field measurements indicate that the PCMO is the principal material in defining the resistive switching properties of the device. However, the insulating buffer layers used were oxygen ion conductors, and the resultant improvement of switching properties due to the buffer layers may indicate a mechanism where oxygen ions or oxygen vacancies are playing a role