Tseung-Yuen Tseng

Effect of Top Electrode Material on Resistive Switching Properties of

The influence of top electrode material on the resistive switching properties of ZrO2-based memory film using Pt as a bottom electrode was investigated in this letter. In comparison with Pt/ZrO2/Pt and Al/ZrO2/Pt devices, the Ti/ZrO2/Pt device exhibits different resistive switching current-voltage (I- V) curve, which can be traced and reproduced by a dc voltage more than 1000 times only showing a little decrease of resistance ratio between high and low resistance states. Furthermore, the broad dispersions of resistive switching characteristics in the Pt/ZrO2/Pt and Al/ZrO2/Pt devices are generally observed during successive resistive switching, but those dispersions are suppressed by the device using Ti as a top electrode. The reliability results, such as cycling endurance and continuous readout test, are also presented. The write-read-erase-read operations can be over 104 cycles without degradation. No data loss is found upon successive readout after performing various endurance cycles

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Abstract This paper reviews the recent developments of (Ba,Sr)TiO 3 (BST) thin films for future Gbit era dynamic random access memory (DRAM) applications. The trends of DRAM capacitors in the last decade are briefly described first. Then the technological aspects of BST films such as deposition techniques, post-annealing, physical, electrical and dielectric characteristics of the films, effects of electrode materials, dielectric relaxation and defect analysis and the reliability phenomena associated with the films are briefly reviewed with specific examples from recent literature. The basic mechanisms that control the bulk electrical conduction and the origin of leakage currents in BST films are also discussed. Finally, possible developments of gigabit era DRAM technology are summarized.

Film Memory Devices

Nonvolatile memory technologies in Si-based electronics date back to the 1990s. Ferroelectric field-effect transistor (FeFET) was one of the most promising devices replacing the conventional Flash memory facing physical scaling limitations at those times. A variant of charge storage memory referred to as Flash memory is widely used in consumer electronic products such as cell phones and music players while NAND Flash-based solid-state disks (SSDs) are increasingly displacing hard disk drives as the primary storage device in laptops, desktops, and even data centers. The integration limit of Flash memories is approaching, and many new types of memory to replace conventional Flash memories have been proposed. Emerging memory technologies promise new memories to store more data at less cost than the expensive-to-build silicon chips used by popular consumer gadgets including digital cameras, cell phones and portable music players. They are being investigated and lead to the future as potential alternatives to existing memories in future computing systems. Emerging nonvolatile memory technologies such as magnetic random-access memory (MRAM), spin-transfer torque random-access memory (STT-RAM), ferroelectric random-access memory (FeRAM), phase-change memory (PCM), and resistive random-access memory (RRAM) combine the speed of static random-access memory (SRAM), the density of dynamic random-access memory (DRAM), and the nonvolatility of Flash memory and so become very attractive as another possibility for future memory hierarchies. Many other new classes of emerging memory technologies such as transparent and plastic, three-dimensional (3-D), and quantum dot memory technologies have also gained tremendous popularity in recent years. Subsequently, not an exaggeration to say that computer memory could soon earn the ultimate commercial validation for commercial scale-up and production the cheap plastic knockoff. Therefore, this review is devoted to the rapidly developing new class of memory technologies and scaling of scientific procedures based on an investigation of recent progress in advanced Flash memory devices.

Progress in the developments of (Ba,Sr)TiO3 (BST) thin films for Gigabit era DRAMs

Thin films of (Ba,Sr)TiO3 on Pt/SiO2/Si substrates were deposited using rf magnetron sputtering at various substrate temperatures and O2/(Ar+O2) mixing ratios (OMR). The crystallinity of the films improved significantly as the OMR increased. The dielectric constant increased with increasing OMR and reached a maximum value at 50% OMR. The leakage current density decreased with increasing oxygen flow, but had a minimum value at 40% OMR. The results for the dielectric constant and the leakage current were interpreted in terms of polarization effect and loss theory. The film deposited at 450 °C and 50% OMR exhibited good surface morphology and had a dielectric constant of 375, a tangent loss of 0.074 at 100 kHz, a leakage current density of 7.35×10−9 A/cm2 at an electric field of 100 kV/cm with a delay time of 30 s, and a charge storage density of 49 fC/μm2 at an applied field of 150 kV/cm. The 10 yr lifetime of time-dependent dielectric breakdown studies indicate that a 50% OMR sample has a longer lifetime t...

Overview of emerging nonvolatile memory technologies

In this study, we investigate the resistive switching behavior of radio frequency (rf)-sputtered Al 2 O 3 thin films. It is observed that both high-conducting state (ON state) and low-conducting state (OFF state) are stable and reproducible during successive resistive switchings by dc voltage sweeping. The ratio of resistance of the ON and OFF state is over 10 3 . Such a reproducible resistive switching can be performed at 150°C, and the resistance of the ON state can be altered by various current compliances. The conduction mechanisms of the ON and OFF states are demonstrated as ohmic conduction and Frenkel-Poole emission, respectively. Both states, performed by dc voltage sweeping and applying short pulse, are stable over 10 4 s at a read voltage of 0.3 V and the electrical-pulse-induced resistance change (EPIR) phenomenon is demonstrated, which are all important properties for further resistive random access memory application.

Effect of oxygen to argon ratio on properties of (Ba,Sr)TiO3 thin films prepared by radio-frequency magnetron sputtering

A humidity sensor using K2O-doped porous Ba0.5Sr0.5TiO3 ceramic is investigated. This ceramic humidity sensor exhibits a porous structure. The porous ceramic easily absorbs water vapour throughout the pores. The log-conductance against relative humidity (r.h.) sensitivity of this sensor is greater than 4 orders of magnitude in the range of 15∼95% r.h. at 400 Hz and 25°C. The adsorption process of the sensor is very fast. Its adsorption response time in r.h. variation from 15 to 95% is within a few seconds. Charging-discharging and complex impedance analysis techniques are used to analysis the direct current (d.c.) and alternating current (a.c.) response of this device under 50∼95% r.h. The sample can be polarized like electrolytes on charging process due to electrode space charge and grain surface water molecular polarization effects. The degree of polarization is enhanced with increasing r.h. The conduction carriers of this sensor in a humid atmosphere are ions and electrons, and the dominant conduction carrier is the ion. Using complex impedance analysis techniques, an equivalent circuit model associated with “non-Debye” capacitance is built. This model separates the sample into three regions: crystal grain, grain surface and electrode surface. The grain surface resistance and electrode surface resistance decrease sharply with increasing r.h., but crystal grain resistance is not affected by water vapour.

Bistable Resistive Switching in Al2O3 Memory Thin Films

The influence of Ti top electrode material on the resistive switching properties of ZrO2-based memory film using Pt as bottom electrode was investigated in the present study. When Ti is used as top electrode, the resistive switching behavior becomes dependent on bias polarity and no current compliance is needed during switching into high conducting state. This phenomenon is attributed to the fact that a series resistance between Ti and ZrO2 film, composed of a TiOx layer, a ZrOy layer, and even the contact resistance, imposed a current compliance on the memory device. Besides, our experimental results imply that switching the device into high conducting state is a field driven process while switching back into low conducting state is a current driven process.

Analysis of the d.c. and a.c. properties of K2O-doped porous Ba0.5Sr0.5TiO3 ceramic humidity sensor

The multilevel resistive switching (RS) behaviors of the Ti/CuxO/Pt device were investigated by controlling the operated parameters of current and voltage bias in this study. We demonstrated that at least five-level memory states for data storage could be determined by controlling the current compliance, the span of voltage sweeping, and the amplitude of voltage pulse imposed on the memory device. During the dc voltage sweeping mode, not only the multilevel ON-states but also the multilevel OFF-states were achieved for the multilevel storage. The RS mechanism of the Ti/CuxO/Pt device is proposed to be related to the formation/rupture of the conducting filaments, arising from the interfacial oxygen ion migration between the Ti top electrode and CuxO films. Moreover, a possible conduction scenario for the multilevel RS behaviors is also suggested. Owing to all the multilevel memory states are distinguishable and possess the nondestructive readout property, it implies that the Ti/CuxO/Pt device has the promi...

Modified resistive switching behavior of ZrO2 memory films based on the interface layer formed by using Ti top electrode

This study investigates the effects of bias-induced oxygen adsorption on the electrical characteristic instability of zinc tin oxide thin film transistors in different ambient oxygen partial pressures. When oxygen pressure is largest, the threshold voltages showed the quickest increase but the slowest recovery during the stress phase and recovery phase, respectively. This finding corresponds to the charge trapping time constant and recovery time constant, which are extracted by fitting the stretched-exponential equation and which exhibit a relationship with oxygen pressure. We suggest that the gate bias reduces the activation energy of oxygen adsorption during gate bias stress.

Multilevel resistive switching in Ti/CuxO/Pt memory devices

In this study, we propose a simple method to produce the various interface thicknesses within Ti/ZrO2 by changing the thickness of the Ti top electrode. As the Ti thickness increases, the induced interface thickness also increases to degrade the dielectric strength of the ZrO2, further lowering the forming voltage. However, when the interface layer is thick enough, it will trap sufficient charges to build up an opposite electric field to increase the forming voltage. The induced interface thickness is found to obviously affect the bias polarity of the resistive switching behavior and the device reliability. A fluctuant ON process is also demonstrated to be attributed to the competition between the formation and rupture of the conducting filaments.