Wonjoo Kim

Impact of oxygen exchange reaction at the ohmic interface in Ta2O5-based ReRAM devices

Interface reactions constitute essential aspects of the switching mechanism in redox-based resistive random access memory (ReRAM). For example, the modulation of the electronic barrier height at the Schottky interface is considered to be responsible for the toggling of the resistance states. On the other hand, the role of the ohmic interface in the resistive switching behavior is still ambigious. In this paper, the impact of different ohmic metal-electrode (M) materials, namely W, Ta, Ti, and Hf on the characteristics of Ta2O5 ReRAM is investigated. These materials are chosen with respect to their free energy for metal oxide formation and, associated, their impact on the formation energy of oxygen vacancy defects at the M/Ta2O5 interface. The resistive switching devices with Ti and Hf electrodes that have a negative defect formation energy, show an early RESET failure during the switching cycles. This failure process with Ti and Hf electrode is attributed to the accumulation of oxygen vacancies in the Ta2O5 layer, which leads to permanent breakdown of the metal-oxide to a low resistive state. In contrast, the defect formation energy in the Ta2O5 with respect to Ta and W electrodes is positive and for those highly stable resistive switching behavior is observed. During the quasi-static and transient-pulse characterization, the ReRAM devices with the W electrode consistently show an increased high resistance state (HRS) than with the Ta electrode for all RESET stop voltages. This effect is attributed to the faster oxygen exchange reaction at the W-electrode interface during the RESET process in accordance to lower stability of WO3 than Ta2O5. Based on these findings, an advanced resistive switching model, wherein also the oxygen exchange reaction at the ohmic M-electrode interface plays a vital role in determining of the resistance states, is presented.

Multistate Memristive Tantalum Oxide Devices for Ternary Arithmetic

Redox-based resistive switching random access memory (ReRAM) offers excellent properties to implement future non-volatile memory arrays. Recently, the capability of two-state ReRAMs to implement Boolean logic functionality gained wide interest. Here, we report on seven-states Tantalum Oxide Devices, which enable the realization of an intrinsic modular arithmetic using a ternary number system. Modular arithmetic, a fundamental system for operating on numbers within the limit of a modulus, is known to mathematicians since the days of Euclid and finds applications in diverse areas ranging from e-commerce to musical notations. We demonstrate that multistate devices not only reduce the storage area consumption drastically, but also enable novel in-memory operations, such as computing using high-radix number systems, which could not be implemented using two-state devices. The use of high radix number system reduces the computational complexity by reducing the number of needed digits. Thus the number of calculation operations in an addition and the number of logic devices can be reduced.

3-Bit Multilevel Switching by Deep Reset Phenomenon in Pt/W/TaO X /Pt-ReRAM Devices

The influence of two different ohmic electrodes (W and Ta) on the resistive switching characteristics of TaO_x-based resistive random access memory (ReRAM) devices has been studied. Consistently, higher resistance OFF states have been observed with the W-ohmic electrode under the same operational conditions for all reset stop voltages (V_reset-stop) during both the dc and ac measurements. The deeper reset for samples with W-electrode is attributed to the less negative Gibbs-free energy of W-oxide compared with Ta-oxide, resulting in easier re-oxidation of the filament through oxygen exchange with the W-electrode. The higher R_OFF/R_ON(>10³) with the W-electrode enables 3-bit multilevel cell operation in the Pt/W/TaO_x/Pt ReRAM device. An excellent retention for these eight states is demonstrated at 125°C for 10⁴ s. Furthermore, the TaO_x ReRAM device with both the electrodes shows high endurance up to 10⁶ cycles based on two states.

Multi-valued and fuzzy logic realization using TaOx memristive devices

Among emerging non-volatile storage technologies, redox-based resistive switching Random Access Memory (ReRAM) is a prominent one. The realization of Boolean logic functionalities using ReRAM adds an extra edge to this technology. Recently, 7-state ReRAM devices were used to realize ternary arithmetic circuits, which opens up the computing space beyond traditional binary values. In this manuscript, we report realization of multi-valued and fuzzy logic operators with a representative application using ReRAM devices. Multi-valued logic (MVL), such as Łukasiewicz logic generalizes Boolean logic by allowing more than two truth values. MVL also permits operations on fuzzy sets, where, in contrast to standard crisp logic, an element is permitted to have a degree of membership to a given set. Fuzzy operations generally model human reasoning better than Boolean logic operations, which is predominant in current computing technologies. When the available information for the modelling of a system is imprecise and incomplete, fuzzy logic provides an excellent framework for the system design. Practical applications of fuzzy logic include, industrial control systems, robotics, and in general, design of expert systems through knowledge-based reasoning. Our experimental results show, for the first time, that it is possible to model fuzzy logic natively using multi-state memristive devices.

Forming-free metal-oxide ReRAM by oxygen ion implantation process

We propose a new method for obtaining forming-free ReRAM devices by oxygen ion implantation (O<inf>2</inf> IIP) in the metal oxide film during the device fabrication process. By tuning the implantation dose, as-fabricated devices can be transformed into the ON state. Subsequent standard RESET and SET switching cycles reveal that the forming-free devices switch in a similar way to reference (formed) devices. The devices also show good R<inf>OFF</inf>/R<inf>ON</inf> ratio (>200), retention (10⁴ sec@125°C) and endurance reliability (10⁶ cycles), showing the absence of any device degradation caused by the O<inf>2</inf> IIP process. This method is applied on both (PVD) Ta<inf>2</inf>O<inf>5</inf> and (ALD) HfO<inf>2</inf> nanoscale ReRAM devices, demonstrating the versatile applications of the technique.

Lowering forming voltage and forming-free behavior of Ta 2 O 5 ReRAM devices

In this paper, we investigate the impact of Ta and Ta₂O₅ thickness and of thermal treatment for the Ta₂O₅ layer on the forming and switching characteristics of Pt/Ta₂O₅/Ta/Pt ReRAM devices. The forming voltage (V_FORM) decreases with increasing Ta and decreasing Ta₂O₅ thickness. However, V_FORM saturates (~ 2 V) for thicker Ta layers. Thinner Ta₂O₅ switching layer can further reduce the forming voltage to <; 1 V for 3 nm-thick Ta₂O₅. However, thinner Ta₂O₅ degrades the R_OFF / R_ON ratio and retention of the ReRAM device. On the other hand, thermal treatment of the Ta₂O₅ results in decrease of initial resistance and V_FORM, and rapid thermal anneal at 600°C in O₂ ambient induces forming-free behavior for 70% of the characterized devices. These forming-free devices show highly reliable switching operation up to 10⁶ cycles with R_OFF / R_ON > 10 and retention time of 10⁴ s at 125 °C.

Low-current and high-endurance logic operations in 4F 2 -compatible TaO x -based complementary resistive switches

Non-volatile redox-based resistive memories (ReRAMs) and matched select devices are the key enabler for future ultra-dense passive crossbar arrays. Complementary Resistive Switches (CRSs) inherently comprise a matched selector and memory device, thus offer a highly promising approach to realize memory and logic functionality in a single device. Here, we show the realization of vertically stacked TaOx-based micro- and nano-CRS devices offering high endurance and the lowest currents for CRS cells to date (~200μA). Furthermore, two basic sequential logic operations in a single CRS device are realized with quasi-static sweeps and pulses. The results are confirmed by simulations using a dynamical TaOx switching device model.