Karsten Fleck

Analysis of Transient Currents During Ultrafast Switching of

In this letter, bipolar fast-pulse switching in TiO2 -based nanocrossbar devices was investigated. A dedicated measurement setup was used to measure the transient currents during 5-ns resistive switching. Transient peak currents for the set and reset processes were as high as 200 and 230 μA, respectively. The currents observed during fast-pulse switching are explained and simulated by Joule heating, which is needed for fast oxygen-vacancy movement. The measured transient currents enable a further optimization of resistive switches based on TiO2.

\hbox{TiO}_{2}

In this letter, we discuss the kinetics of the resistive switch effect in tantalum oxide thin films. The time to switch from the high resistance state to the low resistance state (SET) was measured by using pulse measurement technique. It was found that the SET switching time has a clear relationship with the power of the leakage current flowing under the applied voltage before the SET. Although the dependence of the SET time on the applied voltage differ from cell to cell, this relationship is universal among the different cells. This implies that the Joule heating effect is the dominant driving factor of the SET, rather than the applied voltage. In addition, this relationship can account for the faster switching speed at an elevated temperature.

Nanocrossbar Devices

The distribution of SET switching time of bipolar switching tantalum oxide thin films is studied using pulse measurement techniques. SET switching times are measured by repeating SET and RESET operations in a single cell. It is found that the distribution measured with a high RESET voltage can be well described by a steep Weibull distribution with a shape parameter >1, but lowering the RESET voltage results in a broad distribution at high cumulative frequencies. Statistical analysis shows that this broadening of the distribution can be attributed to the variation of initial conditions for SET, whereas a steep Weibull distribution points to an aging process leading to SET under a voltage stress. It is also shown that although the power of the leakage current before SET determines the fastest limit of the SET switching speed, those SET in the broadened distributions are delayed due to gradual current increase prior to the SET. The waiting time for the start of the gradual current increase has a correlation with the power of the leakage current, showing that Joule heating effect is still a significant factor in the SET mechanism even if the SET time distribution is broadened due to the variation of initial states programmed by low RESET voltages.

Origin of the SET Kinetics of the Resistive Switching in Tantalum Oxide Thin Films

In this letter, we present a study of the SET kinetics of bipolar switching SrTiO3-based resistive memory devices. Pulse measurements on a timescale from 1 μs to 1 s and voltage sweeps with sweep-rates up to 6 MV/s were performed showing a highly nonlinear correlation between voltage and time. An analytical model is presented that explains the interrelation of both experiments by a comparative analysis of the current- voltage characteristics.

Effect of RESET Voltage on Distribution of SET Switching Time of Bipolar Resistive Switching in a Tantalum Oxide Thin Film

To understand the switching mechanism in resistive switching memories it is important to study the switching kinetics over several orders in time. One open question is the upper limit of the switching speed. In this study, we present a switching kinetics study on Ta2O5-based resistive memories that spans over 15 order of magnitude in time in a single device. Using coplanar waveguide (CPW) devices switching times less than 35 ps are realized, which are still limited by the measurement setup. In addition, the switching event could be unraveled in the sub-ns regime by analyses of the current transients. The switching kinetics of the CPW devices show the same characteristic as 80 × 80 nm2 crossbar devices. Furthermore, we demonstrate multilevel switching with ultrafast sub-ns pulses by variation of the voltage amplitude.

Interrelation of Sweep and Pulse Analysis of the SET Process in SrTiO 3 Resistive Switching Memories

Switchable metal-insulator-metal (MIM) structures are the key elements for future non-volatile resistive RAM (RRAM) devices. Recently this type of memory device has attracted considerable interest due to the prospect of non-volatile data storage combined with low power consumption, excellent scalability and very fast write/read operation. However the physical processes responsible for the fast resistive switching are still under investigation. In this work we will present the replacement of the time consuming quasi-static current driven electroforming process by short voltage pulse induced electroforming. Furthermore resistive switching was measured with voltage pulses down to 5 ns pulse width with an in-situ recording of the current response. The high-frequency measurements provide a deeper insight into the physical background of fast data operation.

Ultrafast switching in Ta 2 O 5 -based resistive memories

Compared to conventional NAND flash resistive switching metal-oxide cells show a number of advantages, like an increased endurance, lower energy consumption, and superior switching speed. Understanding the role of defects for the resistive switching phenomenon in metal oxides is crucial for their improvement and thereby also for their acceptance as a next generation data storage device. Strontium titanate (STO) is considered a model material due to its thoroughly investigated defect chemistry. This paper presents a comparative study of the switching kinetics for three different compositions [Sr]/([Sr]+[Ti]) of 0.57 (Sr-rich), 0.50 (stoichiometric STO), and 0.46 (Ti-rich STO). The STO films, deposited by atomic layer deposition, were integrated in Pt/STO/TiN nanocrossbars with a feature size of 100 nm. By analysis of the transient currents, the switching kinetics are investigated between 10 ns and 104 s for the SET and 10 ns and 100 s for the RESET. A clear influence of the composition on the degree of nonlinearity of the switching kinetics was observed. Applying an analytical model for the oxygen vacancy migration, we were able to explain the differences in the SET kinetics by composition-dependent changes in the thermal conductivity and by a lower activation energy for the Ti-rich sample. This might be utilized in design rules of future ReRAM devices.

Fast pulse analysis of TiO 2 based RRAM nano-crossbar devices

The physical principle of redox based resistive switching memory cells is based on ionic and electronic transport propertips. Both normally reveal a strong temperature dependence. A novel heating setup enables to study the influence of high temperatures on the switching mechanism of these memories. It consists of a 100 nm-wide Pt heating line and includes a Pt/Ta2O5/Ta-based ReRAM cell. It is heated up by inducing a current through the Pt line and employing the Joule heating effect. From this setup unique features are obtained by I-V measurements of the ReRAM cell. For instance, a negative differential resistance during the SET process is observed, which results from the circuitry of the heating structure. In case of the high resistive state, an increase in the current is observed at higher temperatures. In contrast, the current in the low resistive state is temperature independent. The increasing temperatures also lead to a reduction of the switching voltages of the SET and RESET events.

The influence of non-stoichiometry on the switching kinetics of strontium-titanate ReRAM devices

Resistive random access memories based on redox phenomena (ReRAM) combine several advantages. Beside their good scalability, high endurance and fast switching speed they are also very energy efficient. This work presents a study of the SET kinetics of SrTiO3-based resistive switches covering the timescale from <;10 ns up to 104 s. The power-dependence of the SET kinetics and the switching energy are analyzed. It is found that there is a minimum energy that is necessary for switching at a certain time furthermore it is found that devices that otherwise behave very differently have the same minimum switching energies. The experimental findings are discussed theoretically using a 2D axisymmeric finite element simulation model. Based on the simulation results design guidelines to minimize the minimum switching energy are derived.