Nianduan Lu

Hierarchical micro-/nanoscale domain structure in MC phase of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 single crystal

A hierarchical micro-/nanoscale domain structure is revealed in MC phase of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 single crystal near the morphotropic phase boundary by combination of analytical electron microscopy and polarized light microscopy. The hierarchical domain structure is a self-assembling of nanodomains with tetragonal structure, submicrodomains appearing as monoclinic, and microdomains exhibiting the same monoclinic state due to an average effect. The outstanding piezoelectric properties may result from the cooperative response of the hierarchical micro-/nanoscale domain structure.

Eliminating Negative-SET Behavior by Suppressing Nanofilament Overgrowth in Cation-Based Memory

Negative-SET behavior is observed in various cation-based memories, which degrades the device reliability. Transmission electron microscopy results demonstrate the behavior is caused by the overgrowth of the conductive filament (CF) into the Pt electrode. The CF overgrowth phenomenon is suppressed and the negative-SET behavior is eliminated by inserting an impermeable graphene layer. The graphene-based devices show high reliability and satisfying performance.

Thermoelectric Seebeck effect in oxide-based resistive switching memory

Reversible resistive switching induced by an electric field in oxide-based resistive switching memory shows a promising application in future information storage and processing. It is believed that there are some local conductive filaments formed and ruptured in the resistive switching process. However, as a fundamental question, how electron transports in the formed conductive filament is still under debate due to the difficulty to directly characterize its physical and electrical properties. Here we investigate the intrinsic electronic transport mechanism in such conductive filament by measuring thermoelectric Seebeck effects. We show that the small-polaron hopping model can well describe the electronic transport process for all resistance states, although the corresponding temperature-dependent resistance behaviours are contrary. Moreover, at low resistance states, we observe a clear semiconductor–metal transition around 150 K. These results provide insight in understanding resistive switching process and establish a basic framework for modelling resistive switching behaviour.

Transition of dislocation nucleation induced by local stress concentration in nanotwinned copper

Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries. Maximum strength was observed at a critical twin lamella spacing (∼15 nm) by mechanical testing; hence, an explanation of how twin lamella spacing influences dislocation behaviours is desired. Here, we report a transition of dislocation nucleation from steps on the twin boundaries to twin boundary/grain boundary junctions at a critical twin lamella spacing (12–37 nm), observed with in situ transmission electron microscopy. The local stress concentrations vary significantly with twin lamella spacing, thus resulting in a critical twin lamella spacing (∼18 nm) for the transition of dislocation nucleation. This agrees quantitatively with the mechanical test. These results demonstrate that by quantitatively analysing local stress concentrations, a direct relationship can be resolved between the microscopic dislocation activities and macroscopic mechanical properties of nanotwinned metals.

Thermal crosstalk in 3-dimensional RRAM crossbar array.

High density 3-dimensional (3D) crossbar resistive random access memory (RRAM) is one of the major focus of the new age technologies. To compete with the ultra-high density NAND and NOR memories, understanding of reliability mechanisms and scaling potential of 3D RRAM crossbar array is needed. Thermal crosstalk is one of the most critical effects that should be considered in 3D crossbar array application. The Joule heat generated inside the RRAM device will determine the switching behavior itself, and for dense memory arrays, the temperature surrounding may lead to a consequent resistance degradation of neighboring devices. In this work, thermal crosstalk effect and scaling potential under thermal effect in 3D RRAM crossbar array are systematically investigated. It is revealed that the reset process is dominated by transient thermal effect in 3D RRAM array. More importantly, thermal crosstalk phenomena could deteriorate device retention performance and even lead to data storage state failure from LRS (low resistance state) to HRS (high resistance state) of the disturbed RRAM cell. In addition, the resistance state degradation will be more serious with continuously scaling down the feature size. Possible methods for alleviating thermal crosstalk effect while further advancing the scaling potential are also provided and verified by numerical simulation.

Abnormal crystal structure stability of nanocrystalline Sm2Co17 permanent magnet

Abnormal crystal structure stability is discovered in the single-phase nanocrystalline Sm2Co17 permanent magnet. Three kinds of crystal structures, namely the rhombohedral Th2Zn17-type (2:17 R), the hexagonal TbCu7-type (1:7 H), and the hexagonal Th2Ni17-type (2:17 H), are claimed to exist at room temperature in the Sm2Co17 alloy system. The strong dependence of the magnetic properties on the structure characteristics in the single-phase Sm2Co17 alloy is interpreted in view of the atom space occupancy and the exchange coupling between substructures especially in the nanocrystalline alloy.Abnormal crystal structure stability is discovered in the single-phase nanocrystalline Sm2Co17 permanent magnet. Three kinds of crystal structures, namely the rhombohedral Th2Zn17-type (2:17 R), the hexagonal TbCu7-type (1:7 H), and the hexagonal Th2Ni17-type (2:17 H), are claimed to exist at room temperature in the Sm2Co17 alloy system. The strong dependence of the magnetic properties on the structure characteristics in the single-phase Sm2Co17 alloy is interpreted in view of the atom space occupancy and the exchange coupling between substructures especially in the nanocrystalline alloy.

Crystal structure and magnetic properties of ultrafine nanocrystalline SmCo3 compound

The single-phase ultrafine nanocrystalline SmCo(3) compound with a high coercivity of 33 kOe and a Curie temperature of 925 K was prepared using a simple and efficient method, which took advantages of the concurrent processes of nanocrystallization and densification during spark plasma sintering. The crystal structure of the nanocrystalline SmCo(3) compound was constructed. As compared with the conventional microcrystalline SmCo(3) compound, a large axial ratio c/a = 4.920 and an expansion of the unit cell volume of 2.97% were obtained in the lattice structure of the nanocrystalline SmCo(3). The relationship between the magnetic properties and the nanocrystalline structure was analyzed. A specific magnetic transition from the weak ferromagnetic to the strong ferromagnetic state was discovered in the nanocrystalline SmCo(3) compound, which was considered to be related to the large anisotropic strain in the crystal lattice.

Nanoscale thermodynamic study on phase transformation in the nanocrystalline Sm2Co17 alloy

The characteristics of phase transformation in nanocrystalline alloys were studied both theoretically and experimentally from the viewpoint of thermodynamics. With a developed thermodynamic model, the dependence of phase stability and phase transformation tendency on the temperature and the nanograin size were calculated for the nanocrystalline Sm(2)Co(17) alloy. It is thermodynamically predicted that the critical grain size for the phase transformation between hexagonal and rhombohedral nanocrystalline Sm(2)Co(17) phases increases with increasing temperature. When the grain size is reduced to below 30 nm, the hexagonal Sm(2)Co(17) phase can stay stable at room temperature, which is a stable phase only at temperatures above 1520 K in the conventional polycrystalline alloys. A series of experiments were performed to investigate the correlation between the phase constitution and the grain structure in the nanocrystalline Sm(2)Co(17) alloy with different grain size levels. The experimental results agree well with the thermodynamic predictions of the grain-size dependence of the room-temperature phase stability. It is proposed that at a given temperature the thermodynamic properties, as well as the phase stability and phase transformation behavior of the nanocrystalline alloys, are modulated by the variation of nanograin size, i.e. the grain size effects on the structure and energy state of the nanograin boundaries.

Cu BEOL compatible selector with high selectivity (>107), extremely low off-current (∼pA) and high endurance (>1010)

Selector with high nonlinearity and low leakage current is critical to solve the sneaking current issue in crossbar memory array. In this work, we present a high performance Cu BEOL compatible threshold switching (TS) selector with several outstanding features, such as high nonlinearity (~107), ultra-low off-state leakage current (~pA), robust endurance (> 1010), and sufficient on-state current density (~1 MA/cm2). The observed threshold switching is resulted from the spontaneously rupture of conductive filament in doped HfO2 material. By introducing a tunneling layer in series with the TS layer, the leakage current of the selector is dramatically reduced by more than 5 orders of magnitude. The array level benchmark of this TS selector qualifies its promising potential for 3D storage application.

Demonstration of 3D vertical RRAM with ultra low-leakage, high-selectivity and self-compliance memory cells

Developing high performance self-selective cell (SSC) is one of the most critical issues of the integration of 3D vertical RRAM (V-RRAM). In this work, a four-layer V-RRAM array, with high performance HfO2/mixed ionic and electronic conductor (MIEC) bilayer SSC, was demonstrated for the first time. Several salient features were achieved, including ultra-low half-select leakage (<;0.1 pA), very high nonlinearity (>103), low operation current (nA level), self-compliance, high endurance (>107), and robust read/write disturbance immunity.