High-resistance State Resistance Research Articles

Performance degradation and I–V model of TiO2-film-based resistive switching memory under proton irradiation

The performance degradation of a TiO2-film-based RRAM (resistive random access memory) is investigated in a proton irradiation experiment with an energy of 25 MeV. The results reveal that the fabricated Au/TiO2 film/Ti devices exhibit typical I–V of bipolar resistive switching behavior under an irradiation of 1 × 1011 protons/cm2. The low-resistance state (LRS) resistance remains nearly constant, but the high-resistance state (HRS) resistance decreases with an increasing proton fluence. The value of the HRS resistance decreases by approximately one order of magnitude when the value of proton fluence reaches 1 × 1011 protons/cm2. Moreover, the SET voltage decreases with a decreasing proton fluence, while the RESET voltage remains almost constant. Material characterization via x-ray photoelectron spectroscopy demonstrates that the decrease in SET voltage and HRS resistance is mainly caused by radiation-induced oxygen vacancies and non-lattice oxygen. Based on the Voltage Threshold Adaptive Memristor model, a mathematical model of the I–V curve which demonstrates the variation in the RRAM resistance and voltage as a function of proton irradiation is constructed. The simulation results conformed to the experimental results under different proton fluences. Our results form a fundamental guide for the study of radiation performance degradation and radiation hardening of the RRAM with oxygen vacancy conducting filament.

(Invited) Reliability and Scaling Perspectives of HfO2-Based OxRAM

Hafnium oxide OxRAM (Oxide Random Access Memory) is a non-volatile memory technology that has been widely investigated over the last 15 years. Its working mechanism relies on the formation and rupture of a tiny conductive filament through a HfO2 layer sandwiched between an active (Ti) and inert (TiN) electrodes. This causes the electrical resistance to very over three orders of magnitudes between a low resistance state (LRS) and a high resistance state (HRS).OxRAM is a strong candidate to replace embedded Flash technology at scaled nodes. This is justified by several considerations but mainly driven by the low cost of integration and very high compatibility with standard CMOS technology. Yet OxRAM faces tremendous challenges in terms of reliability, variability and scaling.In this paper we first show our latest findings in terms of reliability on a Ti/HfO2(5nm)/TiN cell. Controlling LRS and HRS resistance distributions can be achieved with a proper write/erase algorithm design (see Fig. 1) and it is a key point to enable large array integration. A proper write/erase sequence ensures long bit endurance up to 107 cycles. Data retention is then evaluated at four different temperatures (125, 165, 200 and 260 °C) to extract a 10 years retention temperature of 85°C.Next, we discuss OxRAM scaling perspectives. In this context, it is essential to take into account both the resistive cell and its selector (most typically a MOS transistor). As recently shown [2], so far the OxRAM scaling has been mainly limited by the programming current, which determines the MOS selector size. Even if the OxRAM cell is integrated in the BEOL above its selector, the bitcell size is mostly determined by the underlying transistor. In addition, the OxRAM programming voltage dictates the transistor EOT and challenges the transistor reliability for scaled nodes. We demonstrated our OxRAM cell integration with a thin gate oxide FDSOI transistor (GO1/SG) and we show how this paves the OxRAM integration in a 28nm node.Eventually we discuss the ultimate scaling perspective of an OxRAM array in a Cross-Point architecture. To enable such architecture the development of a BEOL selector is mandatory and we focused on Ovonic Threshold Switching device (OTS) for this scope. The OTS works switching between a low-conductive state and a metastable conductive one, which is voltage-triggered. We demonstrated (see Fig. 3) a 1S1R (one selector one resistor) OxRAM cell where the HfO2 memory is co-integrated with a series Ge-Sb-Se-N (GSSN) OTS. We eventually discuss the limits of this architecture showing that a trade-off exists between the half pitch of the Cross-Point array metal lines and the size of the memory bank.REFERENCE[1] G. Sassine et al., 2018 IEEE International Reliability Physics Symposium (IRPS). [2] J. Sandrini et al., 2019 IEEE Electronic Device Meeting (IEDM). [3] D. Alfaro Robayo et al., 2019 IEEE 11th International Memory Workshop (IMW). Figure 1

Piezoresistive Characteristics of Nylon Thread Resistive Memories for Wearable Strain Sensors

A nylon thread (NT) resistive memory is fabricated by performing a simple dip-and-dry solution process using graphene–poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) conductive ink. The piezoresistive characteristics of the NT resistive memory are further evaluated for wearable strain sensors. While a stretching strain (ε) is applied to the NT resistive memory, the relative resistance change of low-resistance state (LRS) is found to be higher than that of high-resistance state (HRS). This result implies that the contribution of the local overlapping interconnection change in graphene and PEDOT:PSS materials to the LRS resistance change is greater than that to the HRS resistance change. In addition, through many cycles of repeatedly stretching and releasing the LRS of the NT resistive memory at a fixed ε = 7.1%, a gauge factor of approximately 22 is measured and achieved for a highly sensitive and durable strain sensor. Finally, the actual integration of the NT resistive memory into textiles can provide resistive memory and piezoresistive sensor applications simultaneously for wearable electronic textiles.

A Physics-Based Compact Model for CBRAM Retention Behaviors Based on Atom Transport Dynamics and Percolation Theory

A physics-based compact model for retention behaviors of conductive-bridge random access memory (CBRAM) is developed by modeling: 1) the material-dependent metal atom transport from the conductive filament into the electrolyte; and 2) the conduction percolation controlled by the atom concentration. Considering the material properties of the metal and electrolyte, this compact model can well reproduce the retention behaviors of the CBRAM in both low resistance state and high resistance state (HRS) for various material stacks under various operating temperatures. Two types of HRS resistance shift can be reproduced by accurately modeling the percolation paths. The compact model can enable the reliability projection of cells in a crossbar array.

In-Situ Electron Microscopy of Cu Movement in MoOx/Al2O3 Bilayer CBRAM during Cyclic Switching Process

Resistive random access memories (ReRAMs) and the conductive bridging RAMs (CBRAMs) are expected as the non-volatile memories of next generation. The switching operation of CBRAM is understood to occur with formation and rupture of the conductive filament of Cu or Ag based on the electrochemical reactions [1]. This was experimentally confirmed by in-situ transmission electron microscopy (in-situ TEM) [2]. The previous in-situ TEM works investigated mainly filament formation in the SET process giving the low resistance state (LRS), and few works have examined the RESET process achieving the high resistance state (HRS) [3-4]. Considering the realistic ReRAM operation, repetitive SET/RESET switching operations are important to be investigated by in-situ TEM, since the results contain information for switching reliability. In practical CBRAM test devices, the bilayer structure of solid-electrolyte and oxide layers has been adopted for good retention, speed and stability [5]. In this study, in-situ TEM was applied to the bilayer CBRAM, and the change of the Cu microstructure were observed during cyclic SET/RESET switching. The origin of device degradation will be briefly discussed. The CBRAM of Pt/Mo/Cu(30 nm)/MoOx(20 nm)/Al2O3(5 nm)/TiN was prepared on a Si wafer. All layers were deposited by conventional sputtering or reactive sputtering using Ar-O2. It was processed to be small devices (less than some hundreds of nanometers) using the focused ion beam (FIB) technique. Samples were set in a special TEM holder with a piezo actuator, and in-situTEM experiments were performed during the SET/RESET switching cycles. An example is shown in Fig. 1, where 47 SET/RESET operations were repeated. In the left panel of Fig. 1(a), I–V switching curves in the 1st and 42th cycles, where the formation and rupture of Cu filament was clearly observed in the TEM images, are compared. Though the resistance in HRS was relatively low, clear hysteretic switching curves are identified in the characteristics. This low HRS resistance may be caused by the redeposits of electrode materials formed during the FIB process. In the right panel of Fig. 1(a), in-situ TEM images extracted from a video of the 42th cycle are shown. Corresponding states are marked in the I–V graph; state-1 of HRS before SET, state-2 of LRS after SET, and state-3 of HRS after RESET. In the region marked with red circle, dark contrast corresponding to the Cu filament appeared/disappeared in the Al2O3 layer after SET/RESET while a little of dark contrast exists in the Al2O3layer both before SET and after RESET. This result means that the resistance change occurred based on the filament model. Comparing two I–V graphs in Fig. 1(a), decreased resistive switching ratio was obtained in the 42nd switching. This gives important information on the endurance failure. The resistance values (@−1 V) in cyclic switching operation are summarized in the left panel of Fig. 1(b), where data in accidental switching-less cycles were omitted. The resistance in the both HRS and LRS gradually decreased and the on/off window became narrow. In the right panel of Fig.1 (b), TEM images of the initial state and after the 37th RESET are compared. The contrast of the central part of the Cu layer gradually turned brighter during 37 switching cycles. On the other hand, the center of the Al2O3 became darker. This indicates that Cu moved into the MoOx/Al2O3layer at SET did not return to the Cu electrode completely even after RESET. This can induce formation of Cu deposit in the switching layer and holes in the Cu electrode. As the result, resistance decreased with the switching cycle, and endurance failure should occur. This suggests power balance of the SET/RESET process should be kept adequately to achieve high endurance in CBRAMs. Acknowledgement This work is supported by JSPS KAKENHI (15H01706, 16H0433906 and 16K18073) and Nanotechnology Platform Program (Hokkaido Univ. and Kyushu Univ.) organized by MEXT Japan.

(Invited) Physical Models of Program and Read Fluctuations in Metal Oxide Resistive RAM

The explosive growth of digital data poses increasing demand for novel hardware capable of storing, searching, and analyzing large amounts of data with high efficiency and low power consumption. In this context, resistive switching memories (RRAM) are under scrutiny for storage-class memories and in-memory computing to alleviate or even suppress the von Neumann bottleneck between memory and the central processing unit (CPU) [1]. RRAM devices provide an ideal solution to solve this dilemma, thanks to their high speed, low power, 3D integration and scalability. However, fundamental issues of switching variability and resistance stability must be understood and solved for a mature RRAM technology. This work addresses program and read variability in RRAM, presenting an overview of physical models to describe and predict statistical variations and their impact on device reliability.Variability issues can originate from cycle-to-cycle variations, cell-to-cell variations, and time-to-time variations. While cell-to-cell variations can be minimized by accurate control of the integration process, cycle-to-cycle and time-to-time variations are typically related to fundamental physical limitations, that need for a physical model and understanding for their solution. Cycle-to-cycle variations consist of switching variations, where the resistance of the low-resistance state (LRS) or high-resistance state (HRS) change from cycle to cycle due to random variations in the conductive path responsible for the state resistance. This is shown in Fig. 1, showing current-voltage characteristics of HfO2-RRAM devices with 1T1R architecture [2]. Devices show cycle-to-cycle variations for all switching parameters, including the set voltage Vset, the reset voltage Vreset, and the resistance of LRS and HRS. Variations decrease at increasing size of the conductive filament which is controlled by the compliance current IC, e.g., from IC = 8 μA (a) to 80 μA (b). These variations can be attributed to 2 fundamental sources of variations, namely shape variations, where the conductive-path shape changes from cycle to cycle at constant size, and number variations, where the size of the conductive path changes due to variations in the number of injected/migrated defects. Statistical physics-based models of the conductive path indicate that the normalized standard deviation of resistance sR/R increases with R for shape variations, and with R0.5 for number variations [3]. Fig. 2 shows the measured σR/R as a function of R, showing a transition from a slope 1 for LRS to a slope 0.5 for HRS in the log-log scale. This suggests that LRS variability is mainly due to shape variations, while HRS variability is due to number variation. The new variability model allows to understand and predict variations at variable set/reset conditions, such as variable voltage, current and time.While cycle-to-cycle program variations can be controlled by accurate program/verify techniques, read fluctuations can take place any time after program, thus cannot be easily suppressed. In particular, noise can lead to distribution broadening and device failure by unwanted mixing between HRS and LRS distributions [4]. A detailed analysis of current noise after the reset operation indicates 2 main contributions to fluctuation, namely random telegraph noise (RTN) and random walk (Fig. 3). The latter consists of sudden jumps of resistance, where the jump frequency decreases with time after programming. Random walk remains one of the main reliability issues of RRAM, since it contributes to the broadening of the resistance distributions after set/reset. Ways to mitigate or compensate the distribution broadening includes algorithms and material engineering. [1] H.-S. Philip Wong and S. Salahuddin, “Memory leads the way to better computing,” Nature Nanotechnology 10, 191 (2015).[2] S. Ambrogio, S. Balatti, A. Cubeta, A. Calderoni, N. Ramaswamy, and D. Ielmini, “Statistical fluctuations in HfOx resistive-switching memory (RRAM): Part I – Set/Reset variability,” IEEE Trans. Electron Devices 61, 2912 (2014).[3] D. Ielmini, “Resistive Switching Memories based on Metal Oxides: Mechanisms, Reliability and Scaling,” Semiconductor Science and Technology (2016).[4] S. Ambrogio, S. Balatti, V. McCaffrey, D. Wang, and D. Ielmini, “Noise-induced resistance broadening in resistive switching memory (RRAM) - Part II: Array statistics,” IEEE Trans. Electron Devices 62, 3812 (2015). Figure 1

Study of radiation hardness of HfO2-based resistive switching memory at nanoscale by conductive atomic force microscopy

A resistive switching random access memory (RRAM) with an HfO2/Ti structure grown on a molybdenum (MO) substrate was fabricated, and a gold (Au) conductive atomic force microscopy (CAFM) tip was used as the top electrode such that the cell area of the resulting RRAM device is as small as 3×10−12cm2. The pre- and post-irradiated resistive switching behaviors of the RRAM device with various HfO2 layer thicknesses were investigated after being subjected to Co60 γ-ray irradiation with different radiation doses. It is found that the forming voltage (Vforming), set voltage (Vset), resistance of high resistance state (RHRS) and resistance of low resistance state (RLRS) of the RRAM device are all radiation dose-dependent. The Vforming, Vset, RHRS and RLRS all decrease as the radiation dose increases due to increasing radiation-induced oxygen vacancies or defects inside the HfO2 layer. Our experimental results indicate that the HfO2-based RRAM cell with an extremely small cell area is not actually radiation hard since the operating voltage will change with Vforming and Vset after irradiation.

Effect of Pulse and dc Formation on the Performance of One-Transistor and One-Resistor Resistance Random Access Memory Devices

We investigate the effect of the formation process under pulse and dc modes on the performance of one transistor and one resistor (1T1R) resistance random access memory (RRAM) device. All the devices are operated under the same test conditions, except for the initial formation process with different modes. Based on the statistical results, the high resistance state (HRS) under the dc forming mode shows a lower value with better distribution compared with that under the pulse mode. One of the possible reasons for such a phenomenon originates from different properties of conductive filament (CF) formed in the resistive switching layer under two different modes. For the dc forming mode, the formed filament is thought to be continuous, which is hard to be ruptured, resulting in a lower HRS. However, in the case of pulse forming, the filament is discontinuous where the transport mechanism is governed by hopping. The low resistance state (LRS) can be easily changed by removing a few trapping states from the conducting path. Hence, a higher HRS is thus observed. However, the HRS resistance is highly dependent on the length of the gap opened. A slight variation of the gap length will cause wide dispersion of resistance.

Effect of current compliance and voltage sweep rate on the resistive switching of HfO2/ITO/Invar structure as measured by conductive atomic force microscopy

The electrical characterization of HfO2/ITO/Invar resistive switching memory structure was studied using conductive atomic force microscopy (AFM) with a semiconductor parameter analyzer, Agilent 4156C. The metal alloy Invar was used as the metal substrate to ensure good ohmic contact with the substrate holder of the AFM. A conductive Pt/Ir AFM tip was placed in direct contact with the HfO2 surface, such that it acted as the top electrode. Nanoscale current-voltage (I-V) characteristics of the HfO2/ITO/Invar structure were measured by applying a ramp voltage through the conductive AFM tip at various current compliances and ramp voltage sweep rates. It was found that the resistance of the low resistance state (RLRS) decreased with increasing current compliance value, but resistance of high resistance state (RHRS) barely changed. However, both the RHRS and RLRS decreased as the voltage sweep rate increased. The reasons for this dependency on current compliance and voltage sweep rate are discussed.

An Increasing High Resistance State Phenomenon in Al/VOx/Cu Resistive Switching Device

In this paper, we reported increasing high resistance state (HRS) phenomenon of Al/VOx/Cu resistive random access memory (RRAM). The current of HRS became lower in current-voltage (I-V) curves and resistance of HRS gradually increased from a lower pristine sate to a higher HRS during cycling operation. Schottky emission theory was employed to investigate the HRS conduction. We found that the slope of HRS I-V curves on ln(J) and sqrt(V) plot decreased after cycling operation. According to Schottky emission theory, we concluded that generation of AlOx layer between Al electrode and VOx film was the cause of increasing HRS resistance. We finally verify the conclusion by data retention under constant voltage stress.

Improvement of resistive switching uniformity for TiO2-based memristive devices by introducing a thin HfO2 layer

Memristive devices have attracted considerable attention as a promising candidate for the next generation nonvolatile random access memory and unconventional computing. One obstacle for wide applications of these devices is their performance variation from device to device and from cycle to cycle. Here, the authors report that by introducing a thin HfO2 layer into the Pt/TiO2/Pt device geometry, the cycle-to-cycle uniformity of the programing voltages and the resistance states are greatly improved. The programing voltages are distributed in much narrower ranges. The uniformities of low-resistance state resistance and high-resistance state resistance were improved by 25 and 20 times, respectively. It is believed that the extra layer of HfO2 has led to fewer potential conducting filaments in the bilayer devices, contributing to the significant improvement in switching uniformity.

Effect of thermal annealing on resistance switching characteristics of Pt/ZrO2/TiN stacks

In this study, the effect of thermal annealing on both the physical properties and the resistive switching properties of ZrO2 films deposited by atomic layer deposition (ALD) method were investigated for its potential application to non-volatile memory devices. The ZrO2 films in the Pt/ZrO2/TiN structure exhibited unipolar and bipolar resistance switching behaviors depending on the nature of the bias applied to Pt top electrodes for the electro-forming process. For unipolar switching, the resistance of the high resistance state (HRS) was reduced with increasing annealing temperature, accompanied with the increase of metallic Zr in the annealed ZrO2 films. In contrast, the HRS resistance in the bipolar switching was increased while the low resistance state (LRS) resistance was decreased with increasing annealing temperature, producing a greater change in resistance. SIMS and EDX showed that the thickness of interfacial TiOxNy layer between the ZrO2 and the TiN bottom electrode was enlarged with annealing. The enlarged TiOxNy layer was expected to produce the reduction of LRS resistance with the increase of HRS resistance in the bipolar resistance switching.

An Empirical Model for RRAM Resistance in Low- and High-Resistance States

We present a simple empirical expression describing hafnium-based RRAM resistance at different reset voltages and current compliances. The model that we propose describes filament resistance measured at low ( ~ 0.1 V) reading voltage in both low-resistance state (LRS) and high-resistance state (HRS). The proposed description confirms that conduction in LRS is ohmic (after forming with a sufficiently high current compliance) and is consistent with the earlier description of HRS resistance as controlled by a trap-assisted electron transfer via traps in the oxidized portion of the filament. The length of the nonohmic part of the filament is found to be directly proportional to reset voltage. Moreover, low-frequency noise measurements at different reset voltages evidence a tradeoff between HRS resistance and noise in reading conditions.

Effects of Vanadium Doping on Resistive Switching Characteristics and Mechanisms of $\hbox{SrZrO}_{3}$-Based Memory Films

The effects of vanadium doping on resistive switching (RS) characteristics and mechanisms of RF-sputtered SrZrO3 (SZO)-based thin films are investigated in this paper. The physical and electrical properties of SZO-based thin films are modulated by vanadium doping due to the Zr4+ ion replaced by V5+, further affecting the RS parameters of SZO-based thin films. The conduction mechanisms of SZO-based thin films are dominated by ohmic conduction (hoping conduction) and Frenkel-Poole emission for the low resistance state (LRS) and the high resistance state (HRS), respectively. The turn-on process might be attributed to the formation of conducting filaments consisting of oxygen vacancies with the effective barrier height (φB,eff) in the range of 0.10-0.13 eV, whereas the turn-off process might result from thermally assisted oxidation of oxygen vacancies by the Joule heating effect. Furthermore, the introduction of the high valence cation (V5+) in a Zr4+ site of SZO crystalline structure can suppress the formation of oxygen vacancies due to the charge neutrality restriction. Such suppression leads to the changes in the forming voltage, turn-on voltage, HRS resistance, dielectric constant, and φB,eff with vanadium doping concentration up to 0.2 mol%, which is within the solid solubility limit based on our measured lattice constants and Vegard's law.

Structural Properties and Resistance-Switching Behavior of Thermally Grown NiO Thin Films

We investigated the structural and electrical properties of polycrystalline NiO thin films on Pt electrodes formed by thermal oxidation. A Ni–Pt alloy phase was found at the interface, which could be explained by the oxidation kinetics and reactions of Ni, NiO, and Pt. An increase in the oxidation temperature decreased the volume of the alloy layer and improved the crystalline quality of the NiO thin films. Pt/NiO/Pt structures were fabricated, and they showed reversible resistance switching from a high-resistance state (HRS) to a low-resistance state (LRS) and vice versa during unipolar current–voltage measurements. The oxidation temperature affected (did not affect) the HRS (LRS) resistance of the Pt/NiO/Pt structures. This indicated that the transport characteristics of HRS and LRS should be different.

Influence of carrier injection on resistive switching of TiO2 thin films with Pt electrodes

The influence of electron injection on the electric-pulse-induced resistive switching of Pt∕TiO2 thin film/Pt structure was studied by current-voltage (I-V) measurements. The electron injection was increased by annealing the sample in a N2 atmosphere or measuring the I-V characteristics at high temperatures (>100°C). The switching from the high-resistance state (HRS) to the low-resistance state by a filamentary mechanism was suppressed when the carrier injection by Schottky emission or space-charge-limited conduction (SCLC) was excessive. Interfacial potential barrier played a crucial role in determining the carrier injection. Switching was observed (not observed) when the HRS resistance was low (high) although SCLC was observed.