Zhang Kailiang

Microstructure and piezoelectric properties of c-BN Nano-films deposited on Si by RF sputtering for piezoelectric devices

In this paper, boron nitride (BN) films were deposited on silicon substrates by RF magnetron sputtering under different bias voltages. Microstructure and piezoelectric properties of BN films were characterized by FTIR, SEM, AFM and PFM. The results show that surface of films are of lower roughness, while the highest cubic phase volume fraction appeared (about 95%) under bias voltage 120V. And the deposited c-BN films are of apparent piezoelectric properties both in plane and vertical direction by PFM figures.

Modeling of conducting bridge evolution in bipolar vanadium oxide-based resistive switching memory

We investigate the resistive switching characteristics of a Cu/VOx/W structure. The VOx film is deposited by radio-frequency magnetron sputtering on the Cu electrode as a dielectric layer. The prepared VOx sample structure shows reproducible bipolar resistive switching characteristics with ultra-low switching voltage and good cycling endurance. A modified physical model is proposed to elucidate the typical switching behavior of the vanadium oxide-based resistive switching memory with a sudden resistance transition, and the self-saturation of reset current as a function of compliance current is observed in the test, which is attributed to the growth pattern of the conducting filaments. Additionally, the related conducting mechanism is discussed in detail.

Analysis of the resistive switching behaviors of vanadium oxide thin film

We demonstrate the polarization of resistive switching for a Cu/VOx/Cu memory cell. The switching behaviors of Cu/VOx/Cu cell are tested by using a semiconductor device analyzer (Agilent B1500A), and the relative micro-analysis of I—V characteristics of VOx/Cu is characterized by using a conductive atomic force microscope (CAFM). The I—V test results indicate that both the forming and the reversible resistive switching between low resistance state (LRS) and high resistance state (HRS) can be observed under either positive or negative sweep. The CAFM images for LRS and HRS directly exhibit evidence for the formation and rupture of filaments based on positive or negative voltage. The Cu/VOx/Cu sandwiched structure exhibits reversible resistive switching behavior and shows potential applications in the next generation of nonvolatile memory.

Schottky-barrier modulated HfO 2 -resistive switching memory with ultra-low power

To reduce the Reset current, the HfO2-based RRAM device with Ni top electrode (TE) and TiN bottom electrode (BE) is fabricated. Compared to the devices with Al and Ti top electrodes, the Ni/HfO2/TiN device cell with the high-work-function Ni TE exhibits the ultra-low Reset current (sub-100nA), bipolar resistive switching, consistent switching with a large window and good data retention. In addition, multilevel storage characteristics are demonstrated by setting different compliance currents during set processes. The ultra-low power characteristic is related to the Schottky barrier at the interface of high-work-function Ni TE and n-type HfO2. The mechanisms of resistive switching and current conduction are also analyzed based on the Schottky-barrier modulation.

Improvement on switching uniformity of HFO x -based RRAM device fabricated by CMP

The hafnium oxide (HfOx)-based Resistive Random Access Memory (RRAM) array devices are fabricated by chemical mechanical planarization (CMP) process, which is based on the optimization of slurry and exploration on mechanism of HfOx-CMP. The performance of switching properties post-polishing is investigated from device to device (WIW) as well as compared with those without planarization. The results indicate that the switching uniformity and the dispersion of operational voltages were improved after a two-step planarization, and the mechanism has also been explored.

A high sensitivity alcohol gas sensor based on TiO 2 thin films

In this paper, the alcohol gas sensors based on TiO2 thin films deposited by DC magnetron sputtering were fabricated. The effect of annealing temperature on the sensitivity of the samples was invested by varying the annealing temperature from 250°C to 550°C. The surface morphology and crystal orientation were characterized by Atomic Force Microscope (AFM) and X-ray diffraction (XRD). It was found that the sensitivity of the sample annealed at 450°C was higher than that of other samples.

Synthesis of monolayer MoS 2 with seed promoters by chemical vapor deposition at low temperature

In this paper, large area monolayer MoS2 was prepared with chemical vapor deposition (CVD) utilizing seed promoters, perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA). The process conditions including seed promoter concentration, temperature and the flow rate of carrier gas were optimized, and the obtained MoS2 is characterized by AFM and optical microscopy. The AFM results show that the thickness of MoS2 is about 0.7nm, which indicates that the MoS2 is monolayer. It is found that large area of monolayer MoS2 may be got at relatively low temperature 650°C with low concentration of PTCDA.

Piezoelectric properties of ZnO / BN multilayer structures at the nanometer scale

In order to improve the piezoelectric properties of c-BN films, ZnO films with high coupling coefficient were stacked with it. The microstructure and piezoelectric characteristics of ZnO/BN films were investigated in this paper. The piezoelectric properties of multilayer films were improved dramatically with the piezoelectric coefficient d*33 was much higher and switching behavior of polarization could be more easily observed when compared with single c-BN film. This improvement makes c-BN suitable for high-frequency SAW devices.

Effect of VO x interlayer in Cu /HfO x /TiN cell and its resistive switching mechanism

In order to improve the performance of HfOx-based resistive random access memory (RRAM), a VOx buffer layer was introduced in the Cu/HfOx interface of Cu/HfOx/TiN RRAM cell in this paper. Their resistive switching characteristics (such as I-V characteristics, endurance and retention) and the switching mechanism were investigated. Results show that the VOx buffer layer acts as a barrier which avoids excessive Cu ion reaching to HfOx layer as result to improve the device performances. The current conduction mechanism of low resistive state (LRS) is Ohmic conduction while the high resistive state (HRS) is Schottky emission. Based on the negative temperature coefficient of LRS resistance and conduction mechanism, we believe that the resistive switching between HRS and LRS is attributed to the Cu-CFs formation and rupture.

Simulation study of dimensional effect on bipolar resistive random access memory

The dependency of the RRAM device electrical parameters such as set voltage, reset current and resistance on the RRAM cell dimensional scalability is investigated with Monte Carlo simulation to optimise the power consumption of bipolar RRAM. It is found in the simulation that the switching process in bipolar RRAM is related to the cell dimension in the region of 10 nm to 100 nm in terms of its horizontal length. The suppressing effect of existing conducting filament is also discussed. With optimal cell size sufficient initial resistance and a low forming voltage will be achieved, accelerating the feasibility of the high-density low-power RRAM.