Showing papers by "Qi Liu published in 2011"

An overview of resistive random access memory devices

Abstract: With recent progress in material science, resistive random access memory (RRAM) devices have attracted interest for nonvolatile, low-power, nondestructive readout, and high-density memories. Relevant performance parameters of RRAM devices include operating voltage, operation speed, resistance ratio, endurance, retention time, device yield, and multilevel storage. Numerous resistive-switching mechanisms, such as conductive filament, space-charge-limited conduction, trap charging and discharging, Schottky Emission, and Pool-Frenkel emission, have been proposed to explain the resistive switching of RRAM devices. In addition to a discussion of these mechanisms, the effects of electrode materials, doped oxide materials, and different configuration devices on the resistive-switching characteristics in nonvolatile memory applications, are reviewed. Finally, suggestions for future research, as well as the challenges awaiting RRAM devices, are given.

Improved Resistive Switching Uniformity in $ \hbox{Cu/HfO}_{2}/\hbox{Pt}$ Devices by Using Current Sweeping Mode

Abstract: In this letter, current sweeping programming mode is proposed as an efficient method to improve the uniformity of the switching properties of resistive memory devices. Based on the measurement results of the reset process of filament-based Cu/HfO2/Pt devices, current sweeping mode (CSM) can significantly reduce the distributions of Roff values, as compared with the standard voltage sweeping mode. The improvement is attributed to the elimination of the intermediate resistive states due to the positive feedback of joule heat generation by the use of current sweeping. Furthermore, the uniform distribution of the Vset values of the set process is also obtained by current sweeping, which stems from the localization of conductive filaments formation and rupture. CSM provides an effective way to achieve uniform resistance state of memory cell.

Improvement of resistive switching characteristics in ZrO2 film by embedding a thin TiOx layer.

Abstract: The stabilization of the resistive switching characteristics is important to resistive random access memory (RRAM) device development. In this paper, an alternative approach for improving resistive switching characteristics in ZrO2-based resistive memory devices has been investigated. Compared with the Cu/ZrO2/Pt structure device, by embedding a thin TiOx layer between the ZrO2 and the Cu top electrode, the Cu/TiOx–ZrO2/Pt structure device exhibits much better resistive switching characteristics. The improvement of the resistive switching characteristics in the Cu/TiOx–ZrO2/Pt structure device might be attributed to the modulation of the barrier height at the electrode/oxide interfaces.

Investigation of resistive switching behaviours in WO3-based RRAM devices

Abstract: In this paper, a WO3-based resistive random access memory device composed of a thin film of WO3 sandwiched between a copper top and a platinum bottom electrodes is fabricated by electron beam evaporation at room temperature. The reproducible resistive switching, low power consumption, multilevel storage possibility, and good data retention characteristics demonstrate that the Cu/WO3/Pt memory device is very promising for future nonvolatile memory applications. The formation and rupture of localised conductive filaments is suggested to be responsible for the observed resistive switching behaviours.

Resistive switching mechanism of Ag/ZrO2:Cu/Pt memory cell

Abstract: Resistive switching mechanism of zirconium oxide-based resistive random access memory (RRAM) devices composed of Cu-doped ZrO2 film sandwiched between an oxidizable electrode and an inert electrode was investigated. The Ag/ZrO2:Cu/Pt RRAM devices with crosspoint structure fabricated by e-beam evaporation and e-beam lithography show reproducible bipolar resistive switching. The linear I–V relationship of low resistance state (LRS) and the dependence of LRS resistance (RON) and reset current (Ireset) on the set current compliance (Icomp) indicate that the observed resistive switching characteristics of the Ag/ZrO2:Cu/Pt device should be ascribed to the formation and annihilation of localized conductive filaments (CFs). The physical origin of CF was further analyzed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). CFs were directly observed by cross-sectional TEM. According to EDS and elemental mapping analysis, the main chemical composition of CF is determined by Ag atoms, coming from the Ag top electrode. On the basis of these experiments, we propose that the set and reset process of the device stem from the electrochemical reactions in the zirconium oxide under different external electrical stimuli.

Site- and Configuration-Selective Anchoring of Iron–Phthalocyanine on the Step Edges of Au(111) Surface

Abstract: Adsorption behavior of iron–phthalocyanine (FePc) at low submonolayer coverage on a reconstructed Au(111) single crystalline surface was investigated by a combination of low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. A site- and orientation-selective adsorption was found at different temperatures and molecular coverages by means of STM. Further DFT calculations demonstrate that the energy difference between different adsorption configurations leads to the selectivity, and thus the formation of one-dimensional molecular chains on the monatomic step edges in the fcc surface reconstruction domains. The exact adsorption site and configuration of the FePc molecule as well as the simulated STM images are obtained on the basis of DFT calculations, which is in good agreement with experimental observations.

Resistive random access memory cell and memory

Abstract: A Resistive Random Access Memory (RRAM) cell and a memory are disclosed In one embodiment, the RRAM cell comprises a two-state resistor and a resistive switching memory cell connected in series The two-state resistor can supply relatively large currents under both positive and negative voltage polarities As a result, it is possible to reduce leakage paths in a crossbar array of memory cells, and thus to suppress reading crosstalk

Reset Instability in $\hbox{Cu}/\hbox{ZrO}_{2}$ :Cu/Pt RRAM Device

Abstract: The metal oxide solid electrolyte-based RRAM device is a promising candidate for post-Flash nonvolatile memories. The critical operation of such a device is the reset process, and the reliability study of the reset process and its physical understanding are important to RRAM development. This letter reports the observation of the unstable reset behavior in the -based solid electrolyte RRAM. During the reset process, the LRS resistance of the device will first reduce and then increase to reach the HRS. It is also seen that (i.e., the voltage at which a decrease of takes place) decreases with an increase of . This instability might be attributed to the subsequent growth of the filament after the energy barrier is reached to overcome electrochemical reactions during the reset process. Based on these experimental results, a physical model is developed to help explain the dependence of on , which provides an important guideline to the optimization of RRAM operations.

Approaches for improving the performance of filament-type resistive switching memory

Abstract: Resistive random access memory (RRAM) has received significant research interest because of its promising potential in terms of down-scaling, high density, high speed and low power. However, its endurance, retention and uniformity are still imperfect. In this article, the physical mechanisms of filament-type RRAM and the approaches for improving the switching performance, including doping, process optimization and interface engineering, are introduced.

Temperature effect on the submicron AlGaN/GaN Gunn diodes for terahertz frequency

Abstract: The wurtzite AlGaN/GaN Gunn diode with tristep-graded Al composition AlGaN as hot electron injector is simulated by using an improved negative differential mobility model of GaN. The results show that the oscillation mode of Gunn diode gradually shifts from dipole domain mode toward accumulation mode with increase in temperature, and the mode shift closely depends on the injector length. At the temperatures of 300–400 K, 0.6 and 0.4 μm Gunn diodes normally generate the oscillation of dipole domain mode, yielding the fundamental oscillation frequencies of 332–352 GHz and 488–508 GHz, respectively, with the dc/rf conversion efficiencies of 2%–3% and the output power densities of 109–1010 W cm−3. At higher temperatures, the diodes generate the accumulation mode oscillation, and the highest frequency approaches 680 GHz and 977 GHz, respectively, with the dc/rf conversion efficiencies of 0.5%–1%.

Progress in rectifying-based RRAM passive crossbar array

Abstract: Resistive random access memory (RRAM) with crossbar structure is receiving widespread attentions due to its simple structure, high density, and feasibility of three-dimensional (3D) stack. It is an extremely promising solution for high density storage. However, a major issue of crosstalk restricts its development and application. In this paper, we will first introduce the integration methods of RRAM device and the existing crosstalk phenomenon in passive crossbar array, and then focus on the 1D1R (one diode and one resistor) structure and self-rectifying 1R (one resistor) structure which can restrain crosstalk and avoid misreading for the passive crossbar array. The test methods of crossbar array are also presented to evaluate the performances of passive crossbar array to achieve its commercial application in comparison with the active array consisting of one transistor and one RRAM cell (1T1R) structure. Finally, the future research direction of rectifying-based RRAM passive crossbar array is discussed.

Doping technology: An effective way to improve the performances of resistive switching memory

Abstract: There have been concerns about how far the floating gate based non-volatile flash memory (NVM) can be extended and what will become the future directions of memory development. Several emerging new memories, including magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase-change random access memory (PRAM) and resistance random access memory (RRAM), were studied as candidates for the future memories to solve the disadvantages of conventional memories [1]. Among them, the RRAM devices based on binary metal oxide (BMO) have received considerable research interests due to their advantages of simple device structure, high density, excellent scalability, fast switching speed, low energy consumption, and compatibility with the CMOS technology [2].

Molecular Rotors Observed by Scanning Tunneling Microscopy

Abstract: The motion of single molecules plays an important role in nanoscale engineering and holds great potential for bottom-up construction of complex devices at single molecular scale. In this chapter we review the recent progress on molecular rotors. We mainly focus on the motions of molecules at solid surfaces based on the most recent results obtained by scanning tunneling microscopy (STM). Several approaches to illustrate how the surface-supported molecular motor systems work are discussed. These results demonstrate solid advances in the field of molecular-based nanotechnology.

Voltage driving or current driving: Which is preferred for RRAM programming?

Abstract: Voltage driving is a commonly used method to program the resistive switching memory. However, a question of whether voltage driving or current driving is preferred has never been answered. For periphery circuit design, one should first consider which kind of source should be adopted. In this work, we systematically evaluated the performance of Cu/HfO 2 /Pt memory device by using current sweeping or voltage sweeping to SET and RESET. The results show that, tight distribution of R off and V set can be achieved by using current driving to RESET, with effectively eliminating the intermediate resistance states. The possible reason for this improvement may arise from a positive-feedback of joule heat generation during RESET process. Besides, uniform R on distribution can also be realized by current driving, due to more localized conductive filaments formation. The results demonstrate that the current driving method is an effective way to solve the uniformity issue of RRAM.

Metal oxide resistive switching memory and method for manufacturing same

Abstract: The present disclosure relates to the microelectronics field, and particularly, to a metal oxide resistive switching memory and a method for manufacturing the same. The method may comprise: forming a W-plug lower electrode above a MOS device; sequentially forming a cap layer, a first dielectric layer, and an etching block layer on the W-plug lower electrode; etching the etching block layer, the first dielectric layer, and the cap layer to form a groove for a first level of metal wiring; sequentially forming a metal oxide layer, an upper electrode layer, and a composite layer of a diffusion block layer/a seed copper layer/a plated copper layer in the groove for the first level of metal wiring; patterning the upper electrode layer and the composite layer by CMP, to form a memory cell and the first level of metal wiring in the groove in the first dielectric layer; and performing subsequent processes to complete the metal oxide resistive switching memory. According to the present disclosure, the manufacture process can be simplified, without incorporating additional exposure steps in the standard process, resulting in advantages such as reduced cost.

Programming resistive switching memory by a charged capacitor

Abstract: Resistive switching memory is a very promising technology for emerging nonvolatile memory applications. Generally, the switching behavior is triggered by a sweep or pulse voltage. In this paper, a charged capacitor is proposed to be used as the external electrical source to program a resistive switching element. From theoretical analyses based on a set behavior model, the capacitor approach can program the device efficiently. Compared with sweep or pulse mode, capacitor driving method can greatly reduce over-programming after the set event, and thus improve the resistance uniformity. The experimental results performed on Cu/ZrO2:Cu/Pt device support this conclusion quite well. The proposed methodology has great value for achieving a reliable resistive switching, which is important for high density or embedded application such as memory, FPGA, DSP, and even neuromorphic systems.

Metal oxide resistor memory and method for preparing the same

Abstract: A method for preparing a metal oxide resistor memory is provided. The method comprises: forming a W plug lower electrode in the MOS device; forming a cap layer (201a), a first dielectric layer (101) and an etch-stop layer (201b) on the W plug lower electrode sequentially; etching the etch-stop layer (201b), the first dielectric layer (101) and the cap layer (201a) to form a first layer metal wiring groove (700); growing a metal oxide material (400), an upper electrode material (500) and a diffusion barrier layer/seed crystal copper/plating copper composite layer (600) sequentially in the first layer metal wiring groove (700); removing the redundant upper electrode material (500) and diffusion barrier layer/seed crystal copper/plating copper composite layer (600) by Chemical Mechanical Polishing method and forming the memory structure and the first layer metal wiring located in the hole of the first dielectric layer (101) at the same time; processing the subsequent processes to finish the preparation of the metal oxide resistor memory. A metal oxide resistor memory is also provided. The method is simple and convenient and there is no need for additional exposal process on the basis of standard process.