Yueqing Wang

Structural and magnetic properties of Zn1−xMnxO (0≤x≤0.40) nanoparticles

Zn1−xMnxO nanoparticles synthesized by a combustion method with Mn concentration x up to 0.40 are carefully characterized by means of x-ray powder diffraction and high resolution transmission electron microscopy. Single-phase solid solution Zn1−xMnxO is achieved with a solubility limit of ∼30 at. % Mn. An almost linear dependence of lattice constant on the Mn content is observed, indicating the homogeneous substitution of Mn2+ for the Zn2+ in wurtzite ZnO. Magnetization at T=5 K and H=5 T exhibits a maximum around the Mn doping concentration of xm=0.125. Room temperature ferromagnetism with low coercive field (Hc∼70±5 Oe) is obtained for the sample with x=0.01. The observed magnetic properties of Zn1−xMnxO can be well understood within the framework of the donor impurity band exchange model.

RESET Distribution Improvement of Phase Change Memory: The Impact of Pre-Programming

Some nonvolatile phase change memory (PCM) cells with 80-nm heating electrodes are found very difficult to RESET at 3 mA, which directly affects the RESET distribution of the PCM. The large crystal grains with hexagonal structure in the active phase change area, discovered by transmission electron microscope, are the major reason. One preprogramming testing method is introduced, and the resistance distributions of the PCM cells before and after preprogramming are presented. Results show that the large hexagonal crystal grains have been eliminated, thus the resistance distributions have been greatly improved after preprogramming.

Understanding the early cycling evolution behaviors for phase change memory application

The RESET current of T-shaped phase change memory cells with 35 nm heating electrodes has been studied to understand the behavior of early cycling evolution. Results show that the RESET current has been significantly reduced after the early cycling evolution (1st RESET) operation. Compared the transmission electron microscope images, it is found that the hexagonal Ge2Sb2Te5 (GST) crystal grains are changed into the grains with face centered cubic structure after the early cycling evolution operation, which is taken as the major reason for the reduced RESET current, confirmed by a two-dimensional finite analysis and ab initio calculations.

Methods to speed up read operation in a 64 Mbit phase change memory chip

A 64Mbit phase change memory chip is fabricated in 40 nm CMOS technology. An improved fully-differential sense amplifier with a bias voltage instead of the reference resistor branch is proposed to diminish the chip area. The transient response capability of the proposed sense amplifier is improved by removing the large parasitic capacitance of bit line in the feedback network. Smaller parasitic capacitance is also obtained by the separated programming and reading transmission gates to speed up the read operation. The hierarchical bit line architecture is used to reduce the length of bit line, and thus favorable read performance can be achieved.

Current Controlled Relaxation Oscillations in Ge2Sb2Te5-Based Phase Change Memory Devices*

The relaxation oscillation of the phase change memory (PCM) devices based on the Ge2Sb2Te5 material is investigated by applying square current pulses. The current pulses with different amplitudes could be accurately given by the independently designed current testing system. The relaxation oscillation across the PCM device could be measured using an oscilloscope. The oscillation duration decreases with time, showing an inner link with the shrinking threshold voltage Vth. However, the relaxation oscillation would not terminate until the remaining voltage Von reaches the holding voltage Vh. This demonstrates that the relaxation oscillation might be controlled by Von. The increasing current amplitudes could only quicken the oscillation velocity but not be able to eliminate it, which indicates that the relaxation oscillation might be an inherent behavior for the PCM cell.

Test system of current pulse in phase change memory devices

Up to now, there is no direct test system of current pulse in phase change memory (PCM). The traditional test system uses direct current or voltage pulses to do the set operation and voltage pulses to do reset operation. In this work, a new test system is introduced. This system can give current source pulses to the PCM device to do set operation. The test results are presented and analyzed.

Unusual magnetization process of Gd5Ge4: Effects of impurities of Gd

Unusual magnetization process has been observed in Gd5Ge4 prepared with commercial Gd of 99.9 wt %. First, the zero-field-cooled M(H) curves below 25 K contain a significant low-field ferromagnetic (FM) component followed by an unusually broadened field-induced antiferromagnetic (AFM)–FM transition, which was not observed in previous reports no matter if the sample owns a residual FM component or not. Second, the M(H) curves at 4.2 K collected using pulsed magnetic fields reveal some unusual dynamics on the low-field FM state and the following AFM–FM transition. Thus, the present sample of Gd5Ge4 is a new example showing complexity of the influence from impurities of Gd upon the magnetization process.

Investigation of data retention under current bias for phase change memory

With Phase-change memory (PCM), information can be stored as different resistance states even when not powered. In order to accurately characterize the reliability of PCM devices, data retention has to be tested carefully. In this paper, a new test method is applied to measure the data retention of T-shaped PCM devices. This method makes it possible to accelerate crystallization in the amorphous area by using current bias. The new method works based on the field-induced crystallization theory, and could be able to gather fast and detailed information about high-resistance state’s failure process, and at the same time, it could avoid issues related to high temperature. Experimental data for T-shaped PCM devices based on Ge2Sb2Te5 are presented and analyzed. An exponential trend-line of failure time t versus reciprocal bias current 1/I shows only negligible deviation of the measured data points, enabling the extrapolation of the retention behavior for ten-year lifetime. A maximum disturb current value of 5.08 μA is extracted to guarantee the ten years data retention requirement for PCM applications.

Comprehensive analysis and optimization of interface in device

The interface which should correspond to Ohmic contact between the TiN bottom electrode and the TiN adhesive layer is investigated. However, from the measured V-I curve, a non-linear relationship is observed. The previous research and the replotted V-I curve using double-logarithmic scale demonstrate that an oxide layer at the interface is the major reason for the non-linear relationship and that the conduction mechanism here follows the Space-Charge-Limited- Current mechanism. To eliminate the interface effect, a pulse current with a compliance is introduced. A phenomenon is observed that negative resistance occurs because of the capture of filament in the oxide layer. As the width of pulse current increases, the interface effect is eliminated due to the formation of a permanent conducting filament. And , the VI curve shows a linear relationship, representing that the interface corresponds to Ohmic contact and the interface effect has been eliminated efficiently.

Flexible Block Management with Data Migration Wear-Leveling Algorithm for Phase Change Memory

Phase change memory (PCM) is regarded as a powerful competitor for future non-volatile memory applications. However, a key drawback is its limited write endurance. This paper proposes a flexible block management method with data migration wear-leveling algorithm (FBDM). The proposed method divides blocks into two halves meanwhile blocks can be split and merged flexibly. Data migration wear-leveling algorithm has been presented to extend the life of PCM. We simulated our method using different traces and compare it with previous methods. Simulation results show that the proposed method outperforms comparison methods in terms of wear evenness and overhead reduction.