Switching Voltage and Time Statistics of Filamentary Conductive Paths in HfO2-Based ReRAM Devices

Switching voltage and time statistics of HfO2-based one transistor-one resistor structures are investigated with the aim of clarifying the underlying physical mechanism that governs the formation and rupture of filamentary paths in the insulating layer. From the oxide reliability viewpoint, constant and ramped voltage stress experiments provide strong support to the so-called E-model, which is shown to be in line with current theories relating the reversibility of the conduction states in resistive random access memory devices to ionic drift and ultimately to Kramers' escape rate theory. It is shown how the switching statistics can be used to estimate the width and formation energy of the insulating gap along the filament as well as its temperature

The Efficacy of Programming Energy Controlled Switching in Resistive Random Access Memory (RRAM)

Current state-of-the-art memory technologies such as FLASH, Static Random Access Memory (SRAM) and Dynamic RAM (DRAM) are based on charge storage. The semiconductor industry has relied on cell miniaturization to increase the performance and density of memory technology, while simultaneously decreasing the cost per bit. However, this approach is not sustainable because the charge-storage mechanism is reaching a fundamental scaling limit. Although stack engineering and 3D integration solutions can delay this limit, alternate strategies based on non-charge storage mechanisms for memory have been introduced and are being actively pursued. Resistive Random Access Memory (RRAM) has emerged as one of the leading candidates for future high density non-volatile memory. The superior scalability of RRAMs is based on the highly localized active switching region and filamentary conductive path. Coupled with its simple structure and compatibility with complementary metal oxide semiconductor (CMOS) processes; RRAM cells have demonstrated switching performance comparable to volatile memory technologies such as DRAMs and SRAMs. However, there are two serious barriers to RRAM commercialization. The first is the variability of the resistance state which is associated with the inherent randomness of the resistive switching mechanism. The second is the filamentary nature of the conductive path which makes it susceptible to noise. In this experimental thesis, a novel program-verify (P-V) technique was developed with the objective to specifically address the programming errors and to provide solutions to the most challenging issues associated with these intrinsic failures in current RRAM technology. The technique, called Compliance-free Ultra-short Smart Pulse Programming (CUSPP), utilizes sub-nanosecond pulses in a compliance-free setup to minimize the programming energy delivered per pulse. In order to demonstrate CUSPP, a custom-built picosecond pulse generator and feedback control circuit was designed. We achieved high (108 cycles) endurance with state verification for each cycle and established high-speed performance, such as 100 ps write/erase speed and 500 kHz cycling rate of HfO2-based RRAM cells. We also investigate switching failure and the short-term instability of the RRAM using CUSPP

투과전자현미경을 이용한 SrRuO3/SrTiO3 양극성 저항변화 소자의 메커니즘 규명

학위논문 (석사)-- 서울대학교 대학원 : 재료공학부, 2015. 8. 김미영.Nowadays, as Information technology (IT) makes rapid advance and the contemporary memory devices have reached its limit in application, high potential memory devices are needed which are not based on the existed memory devices way. Therefore, a number of more powerful and functional nonvolatile memory (NVM) have been extensively explored, and resistive random access memory (ReRAM) device is becoming one of the candidates which displays distinct advantages such as fast switching speed, large resistance ratio, low riving voltage and simple structure. ReRAM is based on the resistive switching (RS) phenomenon, of which the resistance of the metal-insulator-metal (MIM) structured device could be repeatedly set to different resistance by external electric field and these resistance states are corresponding to the binary data storage of 0 and 1. ReRAM can be clarified into two categories by resistive switching behavior, unipolar resistive switching which resistance depends only on magnitude of voltage and bipolar resistive switching which resistance is only depends on the polarity of voltage. In unipolar mode, the mechanism of the resistive switching has been already researched enough and it was revealed that formation and rupture of conductive path, filament, in insulating thin film is the origin of resistive switching phenomenon. However, in case of bipolar mode, there are lots of models which explain the resistive switching. One of them is the formation and rupture of the filament as well as unipolar switching. Many researchers have been proposed for homogeneous bipolar resistive switching by the change at the interface state. In this mechanism, microstructure of interface is the most important factor to determine the resistance change, however, there are not enough experimental evidences for microstructural change directly. Here, we investigated the SrRuO3 and SrTiO3 single crystal junction device as the most ideal system for investigation of homogeneous bipolar resistive switching model for TEM experiment. We confirmed the resistive switching and electrical property by I-V curve and did TEM, STEMEELS analysis to observe the microscopic mechanism. Beside, by using the In-situ STM/TEM holder, we observed the operation of the device and the microscopic change simultaneously. The variation of oxygen vacancy concentration at the interface which has been used for explaining the homogeneous bipolar resistive switching was not detected by electron energy loss spectrum in this experiment. Therefore, these results mean that there is another possibility of determining the resistive switching or the amount of oxygen vacancies is too small to detect in well-defined interface. Finally, the finding may be of help to know about the homogeneous bipolar resistive switching mechanism.Contents Chapter 1 Introduction Chapter 2 Literature Research 2.1ReRAM 2.1.1 ReRAM background 2.1.2 Unipolar switching 2.1.3 Bipolar switching 2.2 Resistive switching mechanism 2.1.1 Unipolar resistive switching mechanism 2.1.2 Bipolar resistive switching mechanism (a) electro-chemical migration of ion (b) fermi level pinning effect by interface states (c) charge defect trap/detrap (d) tunneling path (e ) oxygen vacancy deficient effect 2.3 Structure and electrical characteristics of SrTiO3 Chapter 3 Experimental Method 3.1 Fabrication of the device 3.1.1 Single crystalline bipolar system (SRO/STO) 3.1.2 Electrical property measurement (I-V switching) 3.1.3 TEM sample preparation 3.2 TEM experiment 3.2.1 Normal TEM 3.2.2 in-situ STM/TEM (I-V switching) 3.2.3 STEM-EELS Chapter 4 Result of Experiment 4.1Electrical property 4.1.1 I-V measurement and Resistance change 4.1.2 Retention 4.2 Investigation of the reason of resistance change for TEM 4.2.1 High resolution image of each resistance state 4.2.2 STEM-EELS analysis of each resistance state 4.3 Unraveling the reason of resistance change in SRO/STO system Chapter 5 Conclusion Reference AbstractMaste

Resistive Switching in Transition Metal Oxides for Integrated Non-volatile Memory

Transition metal oxides (TMOs) exhibit characteristic resistance changes when subjected to high electric fields due to the creation, drift and diffusion of defects, and this resistive-switching response is of interest for future non-volatile memory applications. Indeed, resistive random access memories (ReRAM) are considered promising alternatives to conventional charge storage-based devices because of their low production cost, simple fabrication, and excellent scalability. However, the realization of reliable ReRAM devices and their integration in large-scale arrays requires further understanding of the switching mechanisms and the development of new strategies for improving integrated device functionality. The aim of this work is to understand the role of the material structure on device reliability and to investigate the integration of passive selector elements with memory devices for use in memory cross-bar arrays. The thesis begins by investigating the properties of relevant oxide films (ALD HfO2 and plasma deposited NbOx) and then addresses three technologically relevant problems. Specifically these include: 1) understanding how the roughness of metal/dielectric interfaces affects dielectric breakdown and switching behaviour; 2) exploring methods for reducing the operating current of selector and memory/selector devices and 3) investigating the effect of operating conditions on the switching response of devices. The first of these studies is based on Pt/Ti/HfO2/Pt devices and combines experimental methods and finite element modelling to understand the effect of the Pt/HfO2 interface roughness on the electroforming and switching response. Atomic force microscopy (AFM) showed that the roughness of Pt electrodes deposited by electron-beam evaporation increased with film thickness due to facetted grain growth. Results show that roughness leads to a reduction in the electroforming voltage of HfO2, an increase in the failure rate of devices, and a corresponding reduction in resistive switching reliability. Conventional wisdom suggests that these effects result from local electric field enhancement in the vicinity of electrode asperities. However, the effect on electroforming voltage is much less than estimated from simple geometric considerations. Comparison with finite-element modelled showed high-aspect-ratio asperities can produce field enhancements of more than an order of magnitude but that the generation and redistribution of defects moderates this effect prior to dielectric breakdown. As a consequence, the effect of field enhancement is less than anticipated from the initial electric-field distribution alone. It is argued that the increase in the device failure rate with increasing electrode roughness derives partly from an increase in the film defect density and effective device area and that these effects contribute to the reduction in breakdown voltage. The second study showed that the leakage current in NbO2-x selector (1S) elements is shown to be reduced by the properties of an adjacent memory (1M) element when integrated into a hybrid selector-memory device structure. This is shown to result from current confinement in conductive filaments formed in the memory layer. Finite element modelling of the selector-memory structures is used to confirm the observations and to explore material dependencies. The thermal and electrical conductivities of the memory layer are shown to influence the threshold current, but the dominant effect is due to current confinement. The final study explores the effect of device operating conditions on its operation and identifies an alternative approach for reducing the forming and RESET current in integrated memory/selector devices. This study is based on Pt/Nb/HfO2/Pt devices which require a very "soft" electroforming process. Such devices are shown to undergo configurable switching controlled by the SET compliance current. When operated at a low compliance-current (~100 µA), devices show uniform bipolar resistive switching behaviour. As the compliance current is increased (~500 µA), the switching mode changes to integrated threshold-resistive (1S1M) switching, and at still higher currents (~1 mA), it changes to symmetric threshold switching (1S) characteristic of threshold switching in NbO2-. These switching transitions are shown to be consistent with the development of an NbO2- interlayer at the Nb/HfO2 interface that is limited by the set compliance current due to its effect on oxygen transport and local Joule heating. The proposed mechanism is supported by finite element modelling of the 1S1M response assuming the presence of such an interlayer. These findings help to understand role of interface reactions in controlling device performance and provide a means for the self-assembly of integrated 1S1M resistive random access memory structures

Tuning resistive switching in complex oxide memristors

The continuous demand of lightweight portable, cheap and low-power devices has pushed the electronic industry to the limits of the current technology. Flash memory technology which represents the mainstream non-volatile memories has experienced an impressive development over the last decade. This led their fabrication down to a 16 nm node and implementation of high-density 3D memory architectures. Due to the scaling limit of Flash technology, the need of new memories that combine the characteristics of a Flash but overcome the scaling limits is increasing. In this surge, oxide-based resistive memories – also called memristors – have emerged as a new family of storage-class memory. The extremely simple physical structure fast response, low cost and power consumption render resistive memories as a valid alternative of the Flash technology and an optimal choice for the next generation memory technology. The nanoscale resistive memories have demonstrated a variety of memory characteristics which depends on the electrochemical properties of the oxide system and several physical parameters including device structure and electrical biasing conditions. This indicates a complex nature of the underlying microscopic switching mechanisms which require a thorough understanding in order to fully benefit from the virtue of this technology. The work presented in this Doctoral Dissertation focuses on the realization and fine tuning the memory characteristics of SrTiO3 based resistive switching memories. A novel synthesis route is adopted to realize highly complementary metal oxide semiconductor (CMOS) compatible nanoscale memristive devices and engineer the composition of the functional SrTiO3 perovskite oxide. By following the novel synthesis approach, SrTiO3 memristive devices with different stoichiometry such as different concentration of oxygen vacancies, metallic dopant species and physical structures are fabricated to achieve multifunctional characteristics of these devices. Rigorous electrical and material characterizations are carried out to analyze the resistive switching performance and understand the underlying microscopic mechanisms. Stable multi-state resistive switching is demonstrated in donor (Nb) doped oxygen-deficient amorphous SrTiO3 (Nb:a-STOx) memories. The dynamics of multi-state switching behavior and the effect of Nb-doping on tuning the resistive switching are investigated by utilizing a combination of interfacial compositional evaluation and activation energy measurements. Furthermore, multiple switching behaviors in a single acceptor (Cr) doped amorphous SrTiO3 (Cr:a-STOx) memory cell are demonstrated. A physical model is also suggested to explain the novel switching characteristics of these versatile memristive devices. A highly transparent and multifunctional SrTiO3 based memory system is fabricated which offers a reliable data storage and photosensitive platform for further transparent electronics. Also a unique photoluminescence mapping is presented as an identification technique for localized conduction mechanism in oxide resistive memories. Finally, SrTiO3 resistive memories are engineered to mimic biological synapses. A hybrid CMOS-memristor approached is presented to demonstrate first implementation of higher order time and rate dependent synaptic learning rules. Furthermore, these artificial synapses are tuned for energy-efficient performance to highlight their potential for the future neuromorphic networks

A new method for estimating the conductive filament temperature in OxRAM devices based on escape rate theory

Because of the atomic nature of the system under study, an estimation of the temperature of the conductive filament (CF) in OxRAM devices as a function of the applied bias can only be obtained by means of indirect methods, usually electrothermal simulations. In this paper, a heuristic approach that combines time-dependent dielectric breakdown (TDDB) statistics for the electroformed device with field and temperature-assisted ionic transport within the framework of escape rate theory is presented. Extended expressions for the time-to-failure acceleration law (E-model) and for the Kramers' rate compatible both with the standard models at moderate/high biases and with the principle of detailed balance at equilibrium are proposed. An approximate expression for the CF temperature is reported. For the investigated stress voltage range (0.30 V-0.65 V), the estimated CF temperature at the SET condition is found to be in the range 350 K-600 K

Study on the Connection Between the Set Transient in RRAMs and the Progressive Breakdown of Thin Oxides

In this paper, the transition rate (TR) from the high-resistance state to the low-resistance state of a HfO2-based resistive random access memory (RRAM) is investigated. The TR is statistically characterized by applying constant voltage stresses in the range from 0.45 to 0.65 V. It is found that TR follows a voltage dependence which closely resembles the one exhibited by metal-insulator-semiconductor / metal-insulator-metal structures when subjected to constant voltage stress, but with remarkably different fitting parameters. This result suggests a common underlying mechanism in both evolutionary behaviors. Furthermore, the investigation provides additional evidence supporting the micro-structural changes in the oxide after the forming step as well as the role played by the atomic species during the SET event.Fil: Aguirre, Fernando Leonel. Universidad Tecnológica Nacional. Facultad Regional Buenos Aires. Unidad de Investigación y Desarrollo de las Ingenierías; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodriguez Fernandez, Alberto. Universitat Autònoma de Barcelona; EspañaFil: Pazos, Sebastián Matías. Universidad Tecnológica Nacional. Facultad Regional Buenos Aires. Unidad de Investigación y Desarrollo de las Ingenierías; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Suñé, Jordi. Universitat Autònoma de Barcelona; EspañaFil: Miranda, Enrique. Universitat Autònoma de Barcelona; EspañaFil: Palumbo, Felix Roberto Mario. Universidad Tecnológica Nacional. Facultad Regional Buenos Aires. Unidad de Investigación y Desarrollo de las Ingenierías; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Resistive switching devices with improved control of oxygen vacancies dynamics

L'abstract è presente nell'allegato / the abstract is in the attachmen

Characterisation of Novel Resistive Switching Memory Devices

Resistive random access memory (RRAM) is widely considered as a disruptive technology that will revolutionize not only non-volatile data storage, but also potentially digital logic and neuromorphic computing. The resistive switching mechanism is generally conceived as the rupture/restoration of defect-formed conductive filament (CF) or defect profile modulation, for filamentary and non-filamentary devices respectively. However, details of the underlying microscopic behaviour of the resistive switching in RRAM are still largely missing. In this thesis, a defect probing technique based on the random telegraph noise (RTN) is developed for both filamentary and non-filamentary devices, which can reveal the resistive switching mechanism at defect level and can also be used to analyse the device performance issues. HfO2 is one of the most matured metal-oxide materials in semiconductor industry and HfO2 RRAM shows promising potential in practical application. An RTN-based defect extraction technique is developed for the HfO2 devices to detect individual defect movement and provide statistical information of CF modification during normal operations. A critical filament region (CFR) is observed and further verified by defect movement tracking. Both defect movements and CFR modification are correlated with operation conditions, endurance failure and recovery. Non-filamentary devices have areal switching characteristics, and are promising in overcoming the drawbacks of filamentary devices that mainly come from the stochastic nature of the CF. a-VMCO is an outstanding non-filamentary device with a set of unique characteristics, but its resistive switching mechanism has not been clearly understood yet. By utilizing the RTN-based defect profiling technique, defect profile modulation in the switching layer is identified and correlated with digital and analogue switching behaviours, for the first time. State instability is analysed and a stable resistance window of 10 for >106 cycles is restored through combining optimizations of device structure and operation conditions, paving the way for its practical application. TaOx-based RRAM has shown fast switching in the sub-nanosecond regime, good CMOS compatibility and record endurance of more than 1012 cycles. Several inconsistent models have been proposed for the Ta2O5/TaOx bilayered structure, and it is difficult to quantify and optimize the performance, largely due to the lack of microscopic description of resistive switching based on experimental results. An indepth analysis of the TiN/Ta2O5/TaOx/TiN structured RRAM is carried out with the RTN-based defect probing technique, for both bipolar and unipolar switching modes. Significant differences in defect profile have been observed and explanations have been provided