X. H. Dai

Bipolar resistive switching behavior of La0.5Sr0.5CoO3−σ films for nonvolatile memory applications

Polycrystalline La0.5Sr0.5CoO3 (LSCO) film is prepared on Pt/Ti/SiO2/Si substrates by pulsed laser deposition in order to explore the resistive switching behavior of the Ag/LSCO/Pt structure. It is found that the oxygen stoichiometric LSCO (LSCO-2) structure does not possess distinct rectifying behavior, while the oxygen deficient La0.5Sr0.5CoO3−σ (LSCO-1) structure exhibits very stable bipolar resistive switching behavior, which is attributed to the oxygen vacancies in the LSCO film. The resistance ratio between high resistance state and low resistance state is about 100, which can be maintained up to 200 cycles and 25 h with no observable degradation, indicating that the Ag/LSCO-1/Pt device possesses very good endurance and retention characteristics. Moreover, the current-voltage characteristic of Ag/LSCO-1/Pt heterostructure can be well explained by the space-charge-limited conduction mechanism. The present results provide essential evidence for the analysis of the switching mechanism and evaluation of...

ADSORPTION AND DISSOCIATION OF O2 ON Ti3Al (0001) STUDIED BY FIRST-PRINCIPLES

The adsorption and dissociation of oxygen molecule on Ti3Al (0001) surface have been investigated by density functional theory (DFT) with the generalized gradient approximation (GGA). All possible adsorption sites including nine vertical and fifteen parallel sites of O2 are considered on Ti3Al (0001) surface. It is found that all oxygen molecules dissociate except for three vertical adsorption sites after structure optimization. This indicates that oxygen molecules prefer to dissociate on the junction site between Ti and Al atoms. Oxygen atoms coming from dissociation of oxygen molecule tend to occupy the most stable adsorption sites of the Ti3Al (0001) surface. The distance of O–O is related to the surface dissociation distance of Ti3Al (0001) surface. The valence electron localization function (ELF) and projected density of states (DOS) show that the bonds of O–O are breakaway at parallel adsorption end structures.

Switching properties of epitaxial La0.5Sr0.5CoO3/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 ferroelectric capacitor

La0.5Sr0.5CoO3/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 (LSCO/NBT/LSCO) ferroelectric capacitors have been successfully fabricated on (001) SrTiO3 substrate, in which the LSCO film is prepared by magnetron sputtering and the NBT film by pulsed laser deposition. Both X-ray diffraction and transmission electron microscopy techniques confirm that the (001) oriented LSCO/NBT/LSCO heterostructure is epitaxially grown on SrTiO3 substrate. The remnant polarization, coercive field and relative dielectric constant of the LSCO/NBT/LSCO capacitor, measured at 250 kV cm−1, are 15.6 μC cm−2, 47 kV cm−1 and 559, respectively. Moreover, the capacitor possesses very good fatigue-resistance, and less pulse width dependence as well as piezoelectric properties (d33 = 145 pm V−1). It is found that the leakage current density of the LSCO/NBT/LSCO capacitor meets well with ohmic conduction behavior at applied fields lower than 55 kV cm−1 and bulk-limited space charge-limited conduction at the fields higher than 55 kV cm−1.

Achievement of two logical states through a polymer/silicon interface for organic-inorganic hybrid memory

A hysteresis loop of minority carrier lifetime vs voltage is found in polystyrenesulfonate (PSS)/Si organic-inorganic hybrid heterojunctions, implying an interfacial memory effect. Capacitance-voltage and conductance-voltage hysteresis loops are observed and reveal a memory window. A switchable interface state, which can be controlled by charge transfer based on an electrochemical oxidation/deoxidation process, is suggested to be responsible for this hysteresis effect. We perform first-principle total-energy calculations on the influence of external electric fields and electrons or holes, which are injected into interface states on the adsorption energy of PSS on Si. It is demonstrated that the dependence of the interface adsorption energy difference on the electric field is the origin of this two-state switching. These results offer a concept of organic-inorganic hybrid interface memory being optically or electrically readable, low-cost, and compatible with the flexible organic electronics.

IMPACTS OF ANNEAL TEMPERATURE ON THE STRUCTURAL, MAGNETIC AND OPTICAL PROPERTIES OF THE Fe:LaSrFeO4 COMPOSITE FILM

We prepared the pulsed laser deposited Fe:LaSrFeO4 (LSFO) composite films on quartz substrates by decomposing the La0.5Sr0.5FeO3 target at room temperature in a high vacuum. Impacts of anneal temperature on the structural and physical properties have been investigated, and the systematic changes were found in structural, magnetic and optical absorption properties upon annealing. The LSFO (110) spacing decreases with the increase of annealing temperature, which can be attributed to the release of intrinsic strain; and there is an increase in spacing for the 750∘C annealed sample, which is ascribed to the oxygen loss in LSFO films.

Control of epitaxial growth at a-Si:H/c-Si heterointerface by the working pressure in PECVD*

The epitaxial-Si (epi-Si) growth on the crystalline Si (c-Si) wafer could be tailored by the working pressure in plasma-enhanced chemical vapor deposition (PECVD). It has been systematically confirmed that the epitaxial growth at the hydrogenated amorphous silicon (a-Si:H)/c-Si interface is suppressed at high pressure (hp) and occurs at low pressure (lp). The hp a-Si:H, as a purely amorphous layer, is incorporated in the lp-epi-Si/c-Si interface. We find that: (i) the epitaxial growth can also occur at a-Si:H coated c-Si wafer as long as this amorphous layer is thin enough; (ii) with the increase of the inserted hp layer thickness, lp epi-Si at the interface is suppressed, and the fraction of a-Si:H in the thin films increases and that of c-Si decreases, corresponding to the increasing minority carrier lifetime of the sample. Not only the epitaxial results, but also the quality of the thin films at hp also surpasses that at lp, leading to the longer minority carrier lifetime of the hp sample than the lp one although they have the same amorphous phase.