Guangyue Wang

Correlation of oxygen vacancy variations to band gap changes in epitaxial ZnO thin films

The modulation of oxygen vacancies in ZnO epilayers and its effect on optical band gap have been investigated. The evolution of band gap is correlated with the oxygen vacancy variations induced by readily varying reactive oxygen partial pressure. The oxygen vacancy increased with the decreased oxygen partial pressure, accompanied with a red shift of absorption band edge and of ultraviolet emission peak. The optical band gap was roughly linear dependent on reactive oxygen partial pressure. The band gap narrowing is attributed to the overlap of the nonlocalized oxygen vacancy states with valence band.

Magnetoelectric Coupling Induced by Interfacial Orbital Reconstruction

Reversible orbital reconstruction driven by ferroelectric polarization modulates the magnetic performance of model ferroelectric/ferromagnetic heterostructures without onerous limitations. Mn-d(x2-y2) orbital occupancy and related interfacial exotic magnetic states are enhanced and weakened by negative and positive electric fields, respectively, filling the missing member-orbital in the mechanism of magnetoelectric coupling and advancing the application of orbitals to microelectronics.

Implementation of Complete Boolean Logic Functions in Single Complementary Resistive Switch.

The unique complementary switching behaviour of complementary resistive switches (CRSs) makes them very attractive for logic applications. The implementation of complete Boolean logic functions in a single CRS cell is certainly an extremely important step towards the commercialisation of related logic circuits, but it has not been accomplished to date. Here, we report two methods for the implementation of complete Boolean logic functions in a single CRS cell. The first method is based on the intrinsic switchable diode of a peculiar CRS cell that is composed of two anti-serial bipolar resistive switches with a rectifying high resistance state, while the second method is based directly on the complementary switching behaviour itself of any single CRS cell. The feasibilities of both methods have been theoretically predicted and then experimentally demonstrated on the basis of a Ta/Ta2O5/Pt/Ta2O5/Ta CRS cell. Therefore, these two methods—in particular the complementary switching behaviour itself-based method, which has natural immunity to the sneak-path issue of crossbar logic circuits—are believed to be capable of significantly advancing both our understanding and commercialization of related logic circuits. Moreover, peculiar CRS cells have been demonstrated to be feasible for tri-level storage, which can serve as an alternative method of realising ultra-high-density data storage.

Effect of Electrode Materials on AlN-Based Bipolar and Complementary Resistive Switching

We report the complementary resistive switching (CRS) behaviors in aluminum nitride (AlN)-based memory devices as the promising new material system for large-scale integration of passive crossbar arrays. By utilizing different electrodes (Cu, Pt, and TiN), CRS characteristics are demonstrated in both TiN/AlN/Cu/AlN/TiN electrochemical metallization cells and Pt/AlN/TiN/AlN/Pt ionic resistive switching systems. The instability of Pt/AlN/Cu/AlN/Pt based CRS is explained by the relatively small reset voltage caused by the thermal effects enhanced reset process in the corresponding bipolar resistive switching element. It is concluded that the prerequisite for reliable and stable CRS is that the reset voltage of the bipolar resistive switching element must be much larger than half of the set voltage.

Charge Transfer and Orbital Reconstruction in Strain-Engineered (La,Sr)MnO3/LaNiO3 Heterostructures.

We investigate charge transfer, orbital reconstruction, and the emergence of exchange bias in (La,Sr)MnO3/LaNiO3 heterostructures. We demonstrate that charge transfer from Mn(3+) ions to Ni(3+) ions is accompanied by the formation of hybridized Mn/Ni 3z(2) - r(2) orbits at the interface, instead of strain-stabilized Mn and Ni x(2) - y(2) orbits in the bulk films. In the heterostructures with ultrathin LaNiO3, orbital reconstruction induced by charge transfer results in magnetization frustration of (La,Sr)MnO3 at the interface. But the strain effect exerted by the growth of the LaNiO3 top layer plays a dominant role on orbital reconstruction in the heterostructures with thick LaNiO3, stabilizing 3z(2) - r(2) orbits. In this case, robust spin glass, associated with larger magnetization frustration, accounts for the exchange bias effect. Our work builds a bridge between the microscopic electronic structure and the macroscopic magnetic property, providing the possibility of manipulating the exotic states with the aid of strain engineering in oxide-based electronics.

Interface-modification-enhanced tunnel electroresistance in multiferroic tunnel junctions

We report a large tunnel electroresistance (TER) effect up to ∼104% in La0.67Sr0.33MnO3/BaTiO3/Co (LSMO/BTO/Co) multiferroic tunnel junctions (MFTJs), which couples with well-defined tunnel magnetoresistance. The large TER is related to (LaAlO3)0.3(LaSrTaO6)0.7 substrates which guarantee a high-quality LSMO/BTO interface and robust ferroelectricity in BTO. The insert of 0.5 nm-thick Pt between the Co electrode and BTO barrier further enhances the TER value to 105% and improves the endurance of the MFTJs, ascribed to the shortened screening length and reduced oxidation of BTO/Co interface. Their use would advance the process towards practical MFTJs with four resistance states.

Photon‐Gated Spin Transistor

A new type of spin transistor with an optical gate is proposed with partial exposure of the device, where spin scattering is enhanced under light illumination due to the photon-induced minor spins. Consequently a reproducible transient gate operation of reisitance via optical methods is observed, as ascribed to the nature of spin excitation.