Richeng Yu

Direct Observations of Nanofilament Evolution in Switching Processes in HfO2‐Based Resistive Random Access Memory by In Situ TEM Studies

Resistive switching processes in HfO2 are studied by electron holography and in situ energy-filtered imaging. The results show that oxygen vacancies are gradually generated in the oxide layer under ramped electrical bias, and finally form several conductive channels connecting the two electrodes. It also shows that the switching process occurs at the top interface of the hafnia layer.

Direct Observation of Multiferroic Vortex Domains in YMnO3

Topological vortices with swirling ferroelectric, magnetic and structural anti-phase relationship in hexagonal RMnO3 (R = Ho to Lu, Y, and Sc) have attracted much attention because of their intriguing behaviors. Herein, we report the structure of multiferroic vortex domains in YMnO3 at atomic scale using state-of-the-art aberration-corrected scanning transmission electron microscopy (STEM). Two types of displacements were identified among six domain walls (DWs); six translation-ferroelectric domains denoted by α+, γ−, β+, α−, γ+ and β−, respectively, were recognized, demonstrating the interlocking nature of the anti-vortex domain. We found that the anti-vortex core is about four unit cells wide. In addition, we reconstructed the vortex model with three swirling pairs of DWs along the [001] direction. These results are very critical for the understanding of topological behaviors and unusual properties of the multiferroic vortex.

In situ electron holography study of charge distribution in high-κ charge-trapping memory.

Charge-trapping memory with high-κ insulator films is a candidate for future memory devices. Many efforts with different indirect methods have been made to confirm the trapping position of the charges, but the reported results in the literatures are contrary, from the bottom to the top of the trapping layers. Here we characterize the local charge distribution in the high-κ dielectric stacks under different bias with in situ electron holography. The retrieved phase change induced by external bias strength is visualized with high spatial resolution and the negative charges aggregated on the interface between Al₂O₃ block layer and HfO₂ trapping layer are confirmed. Moreover, the positive charges are discovered near the interface between HfO₂ and SiO₂ films, which may have an impact on the performance of the charge-trapping memory but were neglected in previous models and theory.

In situ TEM Observation of Resistance Switching in Titanate Based Device

After decades of efforts, the research on resistance switching (RS) behavior in transition metal oxides has shifted to the stage of verifying the proposed models by direct experimental evidences. In this paper, RS behavior and oxygen content variation of La0.85Sr0.15TiO3/SrTiO3:Nb (LSTO/STON) were investigated by in situ transmission electron microscopy observation and in situ electron energy loss spectrum characterization under external electric field. The oxygen content fluctuation adjusted by applied bias has been investigated and the observed results imply the conductive channels should be formed by the oxygen vacancy at the Pt/LSTO interface. Moreover, in situ TEM characterization displays the advantage - to reveal the origin of various RS behaviors.

Ferromagnetic antiphase domain boundary in Mn-doped hexagonal BaTiO3 multiferroics

The magnetic properties of hexagonal Ba(Ti1−xMnx)O3 (x = 1/9, 1/6) samples specifically treated under different thermal conditions are investigated. A magnetic transformation from paramagnetism of as-grown samples to weak ferromagnetism of post-annealed samples is observed at room temperature. Associated with the structural modulation characteristics and dielectric behaviors, our experimental results reveal that the high-temperature ferromagnetism in Mn-doped hexagonal BaTiO3 derives from RIII types of antiphase domain boundaries. Spin glass transitions are observed at Tsg = 48 K and 44 K in the densely doped as-grown and post-annealed samples, respectively, but the transitions are not intrinsic properties of the hexagonal phases.

Dynamic observation of oxygen vacancies in hafnia layer by in situ transmission electron microscopy

The charge-trapping process, with HfO2 film as the charge-capturing layer, has been investigated by using in situ electron energy-loss spectroscopy and in situ energy-filter image under positive external bias. The results show that oxygen vacancies are non-uniformly distributed throughout the HfO2 trapping layer during the programming process. The distribution of the oxygen vacancies is not the same as that of the reported locations of the trapped electrons, implying that the trapping process is more complex. These bias-induced oxygen defects may affect the device performance characteristics such as the device lifetime. This phenomenon should be considered in the models of trapping processes.

Structural and Physical Properties of the 6M BaIrO3: A New Metallic Iridate Synthesized under High Pressure

The 6M BaIrO(3) with the distorted hexagonal BaTiO(3) structure was synthesized by high-pressure sintering. Through Rietveld refinement of the powder X-ray diffraction data, the lattice parameters of a = 5.7459(1) A, b = 9.9289(2) A, c = 14.3433(2) A, and beta = 91.340(1) degrees were obtained. In the Ir(2)O(9) dioctahedron, the average Ir-O distance and direct Ir-Ir distance were equal to 2.067(19) and 2.719(1) A, respectively. The temperature dependence of electrical resistivity shows that the 6M BaIrO(3) is a new metallic iridate. It is an abnormal metal, being deviated from the Fermi liquid behavior, following a linear relationship of rho versus T below 20 K. Both magnetic susceptibility and specific heat data indicate that it is an exchange-enhanced Pauli paramagnet, because of the electron-electron correlation effect.

Topology breaking of the vortex in multiferroic Y0.67Lu0.33MnO3

Although topological defects, such as domain walls (DWs) or vortices, are naturally protected by topological invariance, yet, we discover an exception that the six-state topology of the vortex with Z2 × Z3 symmetry is broken by a partial edge dislocation (PED) in hexagonal Y0.67Lu0.33MnO3, where the topologies of the four-state vortex or closed DWs emerges. Using aberration-corrected scanning transmission electron microscopy, we found that the PED plays an important role in changing the phase of translation domain. The PED at the vortex core leads to the formation of the four-state vortex, while the ones at closed DWs connect different types of DWs, both corresponding to continuous phase changes. These results indicate that PEDs can change the topology of translation-related domain vortices and more vortices with even domains can be expected.

Structural and Magnetic Properties of LaCoO3/SrTiO3 Multilayers

Structural and magnetic properties of the LaCoO3/SrTiO3 (LCO/STO) multilayers (MLs) with a fixed STO layer of 4 nm but varied LCO layer thicknesses have been systematically studied. The MLs grown on Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) and SrTiO3 (STO) exhibit the in-plane lattice constant of the substrates, but those on LaAlO3 (LAO) show the in-plane lattice constant between those of the first two kinds of MLs. Compared with the LCO single layer (SL), the magnetic order of the MLs is significantly enhanced, as demonstrated by a very slow decrease, which is fast for the SL, of the Curie temperature and the saturation magnetization as the LCO layer thickness decreases. For example, clear ferromagnetic order is observed in the ML with the LCO layer of ∼1.5 nm, whereas it vanishes below ∼6 nm for the LCO SL. This result is consistent with the observation that the dark stripes, which are believed to be closely related to the magnetic order, remain clear in the MLs while they are vague in the corresponding LCO SL. The present work suggests a novel route to tune the magnetism of perovskite oxide films.

Visualization on charge distribution behavior in thickness-scalable HfO 2 trapping layer by in-situ electron holography and Kelvin Probe Force Microscopy technology

In this paper, charge transportation and storage characteristics of HfO2 charge-trapping layer with various thicknesses were systematically investigated for three dimension (3D) memory device application. For the first time, charge transportation behavior is visualized by introducing in-situ electron holography technology. Under programming transient state mode, charge distribution near blocking oxide is clearly observed followed by lateral charge spreading through HfO2 grain boundary. Meanwhile, steady state behaviors are also given by Kelvin Probe Force Microscopy technique. With HfO2 trapping layer is scaled from 10nm into 3nm, although enough charge storage capability guaranteed mainly by Interface trap sites, severe degradation in data retention characteristics becomes bottleneck. Also, it is proved that the interfaces provide dominate trap sites and vertical decay plays a superior role in charge loss. Finally, nano-scale effects of HfO2 trapping layer are demonstrated from experiment which is well agreed with our characterization results. The study may provide optimization for charge trapping structures, especially for high-density 3D NAND flash applications.