L. F. Liu

Bipolar switching behavior in TiN/ZnO/Pt resistive nonvolatile memory with fast switching and long retention

Highly stable bipolar resistive switching behaviors of TiN/ZnO/Pt devices were demonstrated for the first time. The excellent memory characteristics including fast switching speed (<20 ns for set and <60 ns for reset), long retention (in the order of 105 s) and non-electroforming process were demonstrated. The bipolar switching behaviors can be explained by formation and rupture of the filamentary conductive path consisting of oxygen vacancies. The excellent bipolar switching behavior can be attributed to the significant amount of oxygen vacancies in ZnO film and the effect of TiN layer serving as an oxygen reservoir.

Highly uniform resistive switching characteristics of TiN/ZrO2/Pt memory devices

We fabricated the TiN/ZrO2/Pt sandwiched resistive switching memory devices. Excellent bipolar resistive switching characteristics, including a large number of switching cycles and highly uniform switching parameters, as well as long retention time were achieved. The improved switching behavior of TiN/ZrO2/Pt could be attributed to the oxygen reservoir effect of TiN electrodes on the formation and rupture of the filamentary conducting paths by modifying the concentration distributions of the oxygen ions and vacancies in ZrO2 thin films. The results demonstrate the feasibility of high performance resistive switching memory devices based on transition metal oxides by using TiN as the top electrode.

Oxide-based RRAM: Unified microscopic principle for both unipolar and bipolar switching

A unified microscopic principle is proposed to clarify resistive switching behaviors of transition metal oxide based resistive random access memories (RRAM) for the first time. In this unified microscopic principle, both unipolar and bipolar switching characteristics of RRAM are correlated with the distribution of localized oxygen vacancies in the oxide switching layer, which is governed by the generation and recombination with dissociative oxygen ions. Based on the proposed microscopic principle, an atomistic simulation method is developed to evaluate critical memory performance, and successfully conduct the device optimization. The experimental data are well in line with the developed simulation method.

Well controlled multiple resistive switching states in the Al local doped HfO2 resistive random access memory device

The resistive switching behaviors in the sandwiched Ti/HfO2/Pt devices with different doping condition were systematically investigated. We show that, comparing with the undoped and the Al layer doped HfO2 devices, significant improvement of switching characteristics is achieved in the Al local doped HfO2 device, including uniformity, reliability, and operation current. As a result, well controlled multiple switching states are obtained in the local doping device by modulating the set current compliance or the maximal reset voltage, respectively. Our results suggest that the switching characteristics of HfO2 device are very closely related to the inducement and controlling of conductive filaments’ growth in the dielectric layer, which can be considered in the optimization of resistive random access memory device design.

Oxide-based RRAM: Physical based retention projection

Based on the retention failure mechanism of high resistance state due to reconstruction of oxygen vacancy filament in the rupture region, a physical model is proposed to quantify the retention failure behavior of oxide-based RRAM devices, supported by experiments. A new data retention evaluation methodology is proposed to predict the failure probability and lifetime of the memory devices.

Anticrosstalk characteristics correlated with the set process for α-Fe2O3/Nb–SrTiO3 stack-based resistive switching device

A resistive switching device based on the stacked α-Fe2O3/Nb–SrTiO3 is proposed and fabricated that demonstrates excellent bipolar resistive switching behaviors including the uniformity, endurance, and retention performance. The Schottky-like current-voltage characteristics correlated with set process were observed in both high resistive states (HRSs) and low resistive states (LRSs) of the device. Importantly, the anticrosstalk characteristic, possessing higher reversed-biased LRS resistance than the forwarded-biased HRS resistance, indicates the potential applications of the stacked α-Fe2O3/Nb–SrTiO3 for multilevel storage in the cross-bar memory arrays. The carrier injection and trapping mechanism is suggested to explain the observed phenomena.

Sub-10 nm low current resistive switching behavior in hafnium oxide stack

In this letter, a tip-induced cell relying on the conductive atomic force microscope is proposed. It is verified as a referable replica of an integrated resistive random access memory (RRAM) device. On the basis of this cell, the functionality of sub-10 nm resistive switching is confirmed in hafnium oxide stack. Moreover, the low current switching behavior in the sub-10 nm dimension is found to be more pronounced than that of a 50 × 50 nm2 device. It shows better ON/OFF ratio and low leakage current. The enhanced memory performance is ascribed to a change in the shape of the conductive filament as the device dimensions are reduced to sub-10 nm. Therefore, device downscaling provides a promising approach for the resistance optimization that benefits the RRAM array design.

Understanding the intermediate initial state in TiO2−δ/La2/3Sr1/3MnO3 stack-based bipolar resistive switching devices

Bipolar resistive switching (RS) behavior was observed in epitaxially deposited TiO2−δ/La2/3Sr1/3MnO3 stacks. An intermediate initial state of this device demonstrated that the original resistive state would switch to either the higher or the lower state, depending only on the electrical polarity of the initial measurement. As the analogue intermediate states were obtained by device fabrication, the direct observation of these switching states was possible, knowing the microscopic physical origin of the RS effect. Based on x-ray photoelectron spectroscopy analysis, an understanding of this interesting phenomenon was proposed, well supporting the oxygen vacancy formation and recombination mechanism.

Unipolar resistive switching and mechanism in Gd-doped-TiO2-based resistive switching memory devices

Gd-doped-TiO2-based resistive switching random-access memory (RRAM) devices with MOM structure of top metal electrode (W, Ti)/Gd-doped-TiO2/Pt/Ti/SiO2/Si were fabricated, and resistive switching characteristics were investigated. The W/Gd-doped-TiO2/Pt RRAM device showed typical unipolar resistive switching behavior irrespective of the bias polarity on the W top electrode. However, no unipolar switching was observed in the Ti/Gd-doped-TiO2/Pt devices when positive bias was applied on the Ti top electrode. A new resistive switching model was proposed to explain the unipolar switching behavior in the W/Gd-doped-TiO2/Pt RRAM device, based on the formation and destruction of conducting filaments which result from pinning and unpinning effects of surface trap states of oxide film on the Fermi level of metal electrode. The model clearly elucidates the mechanism of the low resistance state (LRS) and the high resistance state (HRS) as well as the switching behavior between LRS and HRS in the W/Gd-doped-TiO2/Pt RRAM device. The model also indicates the important role of the metal electrode material in unipolar resistive switching of oxide-based RRAM devices.

Oxygen-Vacancies-Related Room-Temperature Ferromagnetism in Polycrystalline Bulk Co-Doped TiO2

Room-temperature ferromagnetism (RTFM) is observed in the bulk Co x Ti 1 - x O 2 - δ (0.06 ≤ x ≤ 0.1) samples synthesized by the solid-state reaction method for the mixed powder of Ti and Co oxides, followed by a 500°C furnace annealing process in a 10% hydrogen-argon mixture of ambient gases. The X-ray diffraction, X-ray photoelectron spectroscopy, and measurements of magnetic susceptibility, Χ, vs temperature, T, indicate polycrystalline Co-doped TiO 2 anatase without Co clusters was fabricated. A phase transformation from CoTiO 3 to Co x Ti 1 - x O 2 - δ occurs after the hydrogenation process. These results are strong indications for formation of oxygen vacancies near the high spin Co 2 + sites, and the formed oxygen vacancies are essential for the generation of RTFM in Co x Ti 1 - x O 2 - δ . Based on these observations, ferromagnetism in bulk Co x Ti 1 - x O 2 - δ anatase could be attributed to the exchange interaction between Co 2 + mediated by oxygen vacancies near Co 2 + sites, not being caused by Co clusters.