Academic literature on the topic 'Resistive switching memory'

Integrating nanometer-sized pores into low-k ILD films is one of the approaches to lower the RC signal delay and thus help sustain the continued scaling of microelectronic devices. While increasing porosity of porous dielectrics lowers the dielectric constant (k), it also creates many reliability and implementation issues. One of the problems is the little understood metal ion diffusion and drift in porous media. Here, we present a rigorous simulation method of Cu diffusion based on Master equation with elementary jump probabilities within the contiguous dielectric film, along the pore boundary, from the dielectric matrix to the pore boundary, and from the pore boundary to the matrix material. In view of the diffusional jump distance being as large as 2 nm, the nano-pores being on a similar length scale, and the film thickness being only a few tens of nanometers, the conventional diffusion equation in differential equation form is grossly inadequate and elementary jump frequencies are required for a proper description of the Cu diffusion in porous dielectric. The present atomistic approach allows a consistent implementation of Cu ion drift in electric field by lowering and raising of the diffusion barriers along the field direction. This will help understand the behavior of Cu interconnects under thermal or electric stress at an atomistic level. Another approach to lower the increasing RC delays is to bring memory and logic closer by integrating memory in the BEOL. Resistive RAM is one such memory is not transistor based and thus, does not require a silicon substrate. Thus, it offers the possibility of integration directly into the back-end reducing memory to logic distance from 1000s of µm to a 10s of nm. This 3D integration also allows for increased density as well. However, one barrier in the implementation of RRAM in the back end is the use of expensive as well as non-BEOL native material in conventional Cu/TaOx/Pt resistive devices. In this thesis, we present our research about functionality of RRAM with porous low-k dielectrics (which are a candidate for CMOS ILD), and through the similar elementary jump simulations, discuss the impact of porosity in dielectrics on the functionality of RRAM. Lastly, we present a cheaper replacement for Pt as the counter electrode in RRAM and show that it functions as good as Pt. This work addresses following three areas: 1. Modeling of diffusion in porous dielectrics through elementary jump based simulation. The model is based on random walk theory of elementary particle jumps. Initially, qualitative simulations are conducted without actual parameters. It is shown that Cu diffusion in porous dielectrics decreases quasi-linearly with porosity. Furthermore, it is shown that morphology of the pores may have a greater effect on diffusivity compared to porosity. The simulations are then calibrated with parameters, and the result is shown to yield a similar diffusivity times as actual process time. 2. Modeling of Cu ions drift in porous dielectrics under electric stress. First, the model is explained, and then qualitative simulation results are presented for porous dielectrics with varied porosities and morphologies. 3. Research to find a suitable replacement for Pt as the counter electrode in RRAM devices. The research methodology is discussed and a much cheaper Rh is selected as the potential replacement for Pt. Successful functionality of Rh based resistive devices is presented.<br>Master of Science

Los condensadores ferroeléctricos están formados por dos electrodos metálicos separados por una capa ferroeléctrica, y tienen un gran potencial para dispositivos lógicos y memorias. El carácter ferroeléctrico de la barrera permite la aparición de memoria multinivel con una respuesta (resistencia R) que puede venir determinada por su historia (). El voltaje aplicado previamente permite escribir sobre el condensador distintos estados resistivos. Naturalmente, el éxito de este enfoque depende de la habilidad para fabricar condensadores ferroeléctricos con una gran respuesta RS a temperatura ambiente. El objetivo principal de esta tesis es el estudio del comportamiento RS de capas ferroeléctricas delgadas y ultradelgadas. En particular la distinta respuesta RS que tiene lugar dependiendo del espesor de las capas ferroeléctricas, el tiempo de escritura, amplitud y signo, temperatura y configuración de los contactos. Para ello se han utilizado condensadores ferroeléctricos basados en BaTiO3 epitaxial y se ha desarrollado una configuración completa, presentado en este documento. En condensadores ferroeléctricos basados en BaTiO3 ultrafino los electrones pueden atravesar la barrera mediante efecto túnel. En este caso el efecto de RS se origina como consecuencia de la dependencia de altura de la barrera con el estado de polarización ferroeléctrico. A pesar de que el proceso de “switching” “UP to DOWN” es similar al proceso inverso (“DOWN to UP”), en barreras con efecto túnel estos procesos diferentes generan dinámicas muy distintas. Las diferentes dinámicas consisten en una respuesta rápida para la inversión de la polarización de un signo, y una respuesta lenta para la inversión de polarización de signo contrario. Esto ha sido asociado a la presencia de campos eléctricos (“imprint”) causados por la asimetría intrínseca del dispositivo. La caracterización de los condensadores ferroeléctricos de BaTiO3, atendiendo al espesor de la barrera ferroeléctrica (t=3-110nm), muestra que el RS puede variar de magnitud y signo en función del espesor de la barrera y del protocolo de escritura. Medidas adicionales de temperatura han sido necesarias para evidenciar la existencia de una contribución al RS proveniente de un desplazamiento iónico asistido por campo eléctrico. Se discute cómo el balance relativo entre los procesos de difusión puramente electrónica e iónica modula la altura de la barrera Schottky y consecuentemente cómo son responsables de las variaciones observadas en la magnitud y signo de la electrorresistencia. En capas ultradelgadas se encuentra que estos procesos son despreciables y una modulación de la barrera de efecto túnel (puramente electrónica) tiene lugar. Aprovechando el conocimiento adquirido a lo largo de la elaboración del presente trabajo, se han utilizado condensadores ferroeléctricos de efecto túnel para implementar un dispositivo CRS (“Complementary Resistive Switching”). El CRS ha sido desarrollado para resolver el problema de corriente de “sneak” de arrays de memoria pasivos, lo cual abre oportunidades para conseguir arrays nanocrossbar de mayor densidad. Mediante el uso de una configuración simple de uniones túnel ferroeléctricas hemos implementado la funcionalidad del CRS que permite la lectura y escritura de estados binarios de idéntico estado de alta resistencia en el estado sin “bias”. Además, se demuestra experimentalmente que esta configuración aporta ventajas notables en cuanto al ahorro energético, y se discute sobre los resultados obtenidos los posibles obstáculos que esta funcionalidad podría enfrentar.<br>Ferroelectric capacitors consist of two metallic electrodes separated by a ferroelectric layer have great potential for memory and logic devices. Here, the ferroelectric character of the barrier should allow to build multilevel of memory with a response (resistance R) that can be dictated by its previous history (cycling voltage V). Previous applied voltage allow writing on the capacitor different resistance states. Naturally, the success of this approach base on the ability to build ferroelectric capacitor with large RS response at room temperature. The ultimate goal of the present thesis is the study of the RS behavior of ferroelectric thin and ultrathin films. In particular, the different RS response depending on parameters such as ferroelectric layer thickness, writing time, amplitude and polarity, device temperature, and contact configuration. For that purpose, epitaxial BaTiO3-based ferroelectric capacitors has been used and a complete set-up has been developed and it is documented in the present thesis. In ultrathin BaTiO3-based ferroelectric capacitors, electrons can tunnel across the ferroelectric barrier. In this case RS results from the different barrier height depending on ferroelectric polarization state. Although, it is assumed that the UP to DOWN switching process it is similar to the DOWN to UP, in tunneling barriers these different processes result in very different dynamics. The different dynamics consists on fast response for one sign of polarization reversal and slow for the other, which has been ascribed to the presence of imprint electric fields caused by the intrinsic device asymmetry. Characterization of BaTiO3-based ferroelectric capacitors, focusing on its dependence on ferroelectric barrier thickness (t = 3-110nm) has revealed that RS can change its magnitude and sign depending on the barrier thickness and writing protocol. Additional temperature-dependent measurements have been instrumental to obtain evidence of the presence of field-assisted ionic motion contributing to RS. It is argued that the relative balance between purely electronic and ionic diffusion processes, modulate the height of the interfacial Schottky barriers and consequently, are responsible of the observed variations of the magnitude and sign of electroresistance. In ultrathin films these processes are found to be negligible and modulation of the tunneling barrier (purely electronic) takes place. Taking benefit of the understanding acquired during the elaboration of the presence work, tunneling ferroelectric capacitors have been used to implement a Complementary Resistive Switch (CRS) device. CRS has been developed to overcome the sneak current path problem of passive memory arrays, which reveal’s opportunities for higher density nanocrossbar arrays. By using a simple arrangement of ferroelectric tunnel junctions, we implemented the CRS functionality that allows effectively writing and reading binary states of identically large resistance state in the unbiased state. Moreover, it is experimentally demonstrated that this arrangement has significant advantages in power saving, and it is discussed on the basis of the obtained results that the possible bottlenecks that this functionality might show.

Las propiedades de conmutación de óxidos de metales de transición y vidrios calcogenuros en estructuras metal-aislante -metal se estudiaron en los años sesenta y setenta. Hoy en día, estas propiedades y materiales se están estudiando con renovado interés debido a que son muy prometedores tanto para dispositivos lógicos como para aplicaciones de memoria. Las memorias de cambio de fase (PCRAM) basadas en transiciones cristalino /amorfo inducidas por calentamiento Joule son ya una realidad comercial. Sin embargo, estos dispositivos sufren de corriente demasiado alta durante la programación y consumo de potencia elevado. En este sentido, la conmutación resistiva (RS) en óxidos de metales de transición se está investigado intensamente debido a su potencial como Memorias Resistivas de Acceso Aleatorio (RRAM). Estas estructuras son ideales para las matrices de memoria de tipo “crossbar” que actualmente están consideradas como los más prometedoras para la implementación del concepto “storage class memroy”. En particular, se considera que estas memorias podrían reemplazar en un futuro a las memorias flash NAND y también eventualmente a las memorias RAM estáticas (SRAM) y dinámicas (DRAM), reduciendo así la jerarquía de memoria en los sistemas de computación. Por otro lado, óxidos electroformados han permitido la primera implementación del dispositivo de estado sólido conocido como memristor , un dispositivo teóricamente propuesto por Chua en 1971 . Este dispositivo es muy prometedor para aplicaciones de lógica reconfigurable y para la aplicación de arquitecturas de computación neuromórficas . Hay tres factores importantes que en la actualidad impiden la transferencia de los resultados de RS a la aplicación industrial , (i) la falta de una adecuada comprensión de la física de los mecanismos físicos de RS, y (ii) la variación estadística de los parámetros de set y reset entre ciclos de operación y de dispositivo a dispositivo, así como (iii) los problemas de fiabilidad , tales como la baja retención a alta temperatura . Esta tesis se centra en dos cuestiones fundamentales: (1) estudiar la física de los mecanismos de conmutación y de conducción del filamento conductor (CF) y (2) la exploración y modelado de las estadísticas de conmutación. La tesis se ha dividido en tres partes principales. La primera de ellas está dedicada a poner de manifiesto la naturaleza del CF , sus propiedades de conducción y de los mecanismos que controlan las transiciones de set y reset. La segunda parte está dedicada al estudio de la variación estadística de los parámetros en los dispositivos RRAM. Partiendo de una implementación basada en celdas del modelo percolativo de la ruptura dieléctrica, se ha propuesto un modelo analítico para las estadísticas de SET y RESET en dispositivos RRAM. La tercera parte está dedicada a poner de manifiesto de tres estados RS efectos para los dispositivos basados en RRAM HfO2 usando tres métodos diferentes de estrés eléctrico, el estrés con rampa de tensión (RVS), estreses sucesivos de rampa de tensión (SVS) , y el estrés a tensión constante (CVS) . En la primera parte , el modelo de Quantum Punto de Contacto (QPC) se ha aplicado al estudio de las propiedades de conducción del CF en los dispositivos RRAM basados en HfO2, tanto en el estado de alta resistencia (HRS) como en el de baja resistencia (LRS). Sobre la base del método de transmisión de Landauer para la conducción a lo largo de constricciones microscópicas estrechas , se ha desarrollada la fórmula del modelo QPC para el caso de múltiples filamentos conductores. Esta ecuación es aplicable tanto al HRS como al LRS, tal como hemos demostrado en dispositivos RRAM basados en HfO2. Posteriormente , el modelo QPC ha sido reformulado en un enfoque multi- escala basado en el acoplamiento a los resultados de simulaciones ab-initio de caminos de vacantes de oxígeno. De esta manera, el modelo se ha simplificado para tener sólo tres parámetros y se ha explicitado una conexión directa con la geometría del CF. Mediante el ajuste de las características I-V experimentales tanto en HRS y los LRS hemos obtenido información indirecta sobre la estructura microscópica del CF en estructuras Pt/Ti/HfO2/Pt y Pt/HfO2/Pt. Para la estructura Pt/HfO2/Pt en modo RS no polar, el CF es simétrico y muy probablemente presenta su mayor constricción en el centro de la capa de óxido. Durante la transición de reset, el CF se estrecha progresivamente hast llegar a un límite de sólo uno o muy pocos caminos de vacantes de oxígeno que conectan los electrodos . Esta etapa es seguida por la apertura de un gap en el CF. La longitud de dicho gap determina la conductancia en el HRS y la puede cambiar en varios órdenes de magnitud. Para la estructura Pt/Ti/HfO2/Pt, el CF es altamente asimétrico, con la parte constrictiva más estrecha cerca de la interfaz entre el HfO2 y el Pt. Se cree que la película de Ti actúa como una capa de extracción de oxígeno y sirve para introducir una alta densidad de vacantes de oxígeno en el HfO2. En el caso de RS bipolar, se ha encontrado un gap en el CF en los dos estados (de mayor dimensión en el HRS) y también se ha puesto de manifiesto una reducción del área efectiva del CF. Para el modo RS unipolar, el número de caminos conductores en el HRS es mucho menor que en el modeo bipolar, aunque el resto de propiedades se mantiene muy parecida. En la segunda parte , hemos partido del modelo percolativo de ruptura basado en celdas como base para proponer un marco general para las estadísticas de conmutación resistiva filamentar. Dicho modelo consta de dos elementos principales: (i) un modelo geométrico basado en celdas para describir la dependencia de la distribución de la RS con la generación de defectos en el CF ; y (ii) un modelo determinista para la dinámica reset y set para describir la relación de la generación de defectos con variables mesurables tales como tensiones y corrientes. El análisis de resultados experimentales obtenidos en muestras Pt/HfO2/Pt han confirmado la validez del modelo estadístico tanto para el set como para el reset. En la tercera parte de la tesis, la transición de reset de las estructuras RRAM basadas en HfO2 se ha investigado en detalle, poniendo especial énfasis en revelar efectos de conmutación resistiva de tres estados. La existencia de un estado intermedio estable se ha puesto de manifiesto experimentalmente. Se ha demostrado que, en dicho estado, el CF se comporta como un cable cuántico (QW). Para ello se han utilizado tres métodos eléctricos diferentes, RVS , SVS y CVS . Este estado de QW se caracteriza por tener la conductancia del orden de la conductancia cuántica G0 ~ 2e2/h. Los tres estados de resistencia que se han puesto de manifiesto son: (1) el LRS, en el que el CF es muy ancho y presenta propiedades de conducción metálicas clásicas; (2) un estado de reset parcial en la que el CF se comporta como un QW y que puede ser tan estrecho como un camino conductor de un solo defecto; y (3) el HRS, en el que un gap se ha abierto en el CF. Un único QW canal de transporte con una conductancia del orden de G0 representa la frontera natural entre el LRS y el HRS. Por último, para mostrar el impacto del estado intermedio sobre las estadísticas de tensión de set y reset, se ha diseñado un test a dos fases, consistentes en una rampa de tensión precedida por un estrés a tensión constante que sitúa un buen número de dispositivos en el estado QW.<br>Switching properties of transition metal oxides and chalcogenide glasses in Metal-Insulator-Metal were studied in the sixties and seventies. Nowadays, these properties and materials are being studied with renewed interest because they are very promising both for logic and memory applications. Phase-change RAM memories based on crystalline/amorphous transitions induced by Joule heating are already a commercial reality. However, these memories suffer from too high programming current and power. In this regard, resistive switching (RS) in formed transition metal oxides is being intensively investigated due to their promising performance as Resistive Random Access Memories (RRAM). These structures are ideal for crossbar memory arrays that are presently considered as the most promising implementation of storage class memory. These memories might replace Flash NAND and also eventually DRAM and SRAM, thus reducing the memory hierarchy. On the other hand, formed oxides have allowed the first solid-state device implementation of the memristor, a device theoretically anticipated by Chua in 1971. This device is very promising for reconfigurable logic applications and for the implementation of neuromorphic computer architectures. There are two important issues which presently hinder the transfer of RS results to industrial application, (i) a lack of adequate understanding of the physics of the mechanisms of RS and (ii) the statistical variation of switching parameters during cycling and from device to device, and reliability issues such as retention at high temperature. This thesis focuses on two key issues: (1) unveiling the physics of the switching and conductance mechanisms of the Conducting Filament (CF) and (2) exploring and modeling the switching statistics. The thesis has been divided into three main parts. The first one is dedicated to reveal the nature of the CF, its conduction properties and the mechanisms which control its formation and disruption. The second part is dedicated to study the statistical variation of switching parameters of RRAM devices. Departing from the cell-based percolation model of gate dielectric breakdown to propose an analytical model for set and reset statistics in RRAM devices. The third part is dedicated to reveal three-state RS effects for HfO2-based RRAM devices using three different electrical stress methods, namely the Ramped Voltage Stress (RVS), the Successive Voltage Stress (SVS), and the Constant Voltage Stress (CVS). In the first part, the Quantum Point Contact (QPC) model has been applied to study the conduction properties of CF in HfO2-based RRAM devices both in the High Resistance State (HRS) and the Low Resistance State (LRS). On the basis of Landauer transmission approach to conduction along narrow microscopic constrictions, the formula of QPC model for multiple breakdown paths has been obtained in MOS devices. This equation can be applicable to both the HRS and the LRS in HfO2-based RRAM devices. Subsequently, the QPC model has been reformulated in a multi-scale approach based on coupling it to the results of ab-initio simulations of oxygen vacancy paths. In this way the model has been simplified to have only three parameters and has been given a direct link to the geometry of the CF. Fitting of the experimental I-V characteristics in both HRS and the LRS provides indirect information about the microscopic structure of the CF for Pt/Ti/HfO2/Pt and Pt/HfO2/Pt structures. For nonpolar Pt/HfO2/Pt structure, the CF is symmetry where the most constrictive part is in the center of the CF. Starting from a very wide CF in the LRS, the width of the CF in its narrowest part reduces to a limit where only one or few oxygen vacancy paths connect the electrodes. This stage is followed by the opening of a gap that the thickness of the most conductive single vacancy path determines the CF conductance in the HRS. For Pt/Ti/HfO2/Pt structure, the CF is highly asymmetric, with the narrowest constrictive part near the bottom of interface. The Ti film is believed to act as an oxygen extraction layer and to introduce a high density of oxygen vacancies in the HfO2. In the LRS, the CF area is rather large and there is one re-oxidized vacancy gap for bipolar RS mode, then the gap increases to two or three vacancies and the CF is narrower than the LRS. For the unipolar RS mode, the number of paths in the HRS is much less than bipolar RS mode, this is to say, the unipolar RS mode is more effective than bipolar RS mode. In the second part, we have departed from the cell-based percolation model of oxide BD to propose a general framework to deal with the statistics of CF-based resistive switching which is composed of two elements: (i) a cell-based geometrical model to describe the dependence of the RS distribution on the defect generation in the CF; and (ii) a deterministic model for the reset and set dynamics to describe the relation of the defect generation with measurable variables such as the voltages and currents. The experimental results based on the Pt/HfO2/Pt sample for reset and set statistics have confirmed the validity of the general statistics method and the physical analytical model. In the third part of the thesis, the reset transition of HfO2-based RRAM structures has been investigated in detail with emphasis on revealing three-state resistive switching effects. A rather stable intermediate state is revealed and shown to have the properties of a Quantum wire (QW) by using three different electrical methods, RVS, SVS and CVS. This QW state is characterized by having conductance of the order of the quantum of conductance G_0~2e^2/h and represents a natural boundary between two different electron transport regimes. Three resistance states are revealed: (1) the LRS, corresponding to a wide CF with classical metallic properties; (2) a partial reset state in which the CF behaves as a QW and which can be as narrow as a single-defect conducting path; and (3) the HRS, in which a physical gap has been opened in the CF. A single transport channel QW with a conductance ~G0 represents the natural boundary between two different reset states. Two-step reset experiments consisting in a low-voltage CVS stage followed by a conventional RVS cycle has been designed to show the impact of the intermediate state on the reset voltage and reset current statistical distributions.

Los dispositivos de Memoria Resistiva de Acceso Aleatorio (RRAM) han sido propuestos como posibles candidatos para substituir a las tecnologías actualmente empleadas como dispositivos de memoria no volátil. El origen de esta propuesta se basa en la observación de las extraordinarias propiedades requeridas para el escalamiento de este tipo de dispositivos. En este sentido, una gran variedad de óxidos que exhiben fenómenos de conmutación resistiva se han estudiado últimamente. Sin embargo, la falta de comprensión del mecanismo físico que produce la conmutación resistiva ha limitado principalmente su comercialización. En esta tesis, se exploran las propiedades de conmutación resistiva del óxido complejo La1-xSrxMnO3 y bicapa CeO2-x/La1-x SrxMnO3 para aplicaciones de dispositivos de memoria no volátil. En primer lugar se estudia la técnica de depósito de capas delgadas de La1-xSrxMnO3 y se realizan medidas de caracterización de las propiedades físicas y estructurales con el fin de optimizar al máximo este proceso. Además, se emplean técnicas de microfabricación para obtener dispositivos laterales tipo memristor metal/La1-xSrxMnO3/metal y metal/CeO2-x/La1-x SrxMnO3/CeO2-x/metal en los cuales se evalúa la conmutación resistiva a través de medidas I-V. De acuerdo con los resultados, se propone un mecanismo basado en el intercambio de iones de oxígeno como responsable de la conmutación resistiva de tipo bipolar y complementario inducida en este tipo de dispositivos laterales. Asimismo, basándonos en la conmutación de volumen inducida en dispositivos bicapa metal/CeO2-x/La1-x SrxMnO3/CeO2-x/metal se presenta un dispositivo de tres terminales como parte innovadora de este trabajo. La conducción de corriente a lo largo de la capa de La1-x SrxMnO3 se modula mediante el uso de un electrodo metálico que actúa como terminal de puerta. Como consecuencia de la observación de la conmutación de volumen, se confirma que la capa de CeO2-x actúa como reservorio de oxígeno la cual favorece el intercambio de iones de oxígeno con la capa de La1-xSrxMnO3 y que además modifica las propiedades de conmutación resistiva. También, se demuestra que la conmutación resistiva se produce homogéneamente en el interior de la capa de La1-xSrxMnO3 y que el proceso de electroformado para inducir el cambio de resistencia en los dispositivos bicapa no produce ruptura alguna ni en la capa de CeO2-x ni en la capa de La1-xSrxMnO3. Finalmente, las conclusiones obtenidas de los resultados de este trabajo pueden ser de relevancia para la comprensión de los fenómenos de conmutación resistiva en óxidos complejos.<br>Resistive Random Access Memory (RRAM) devices have been proposed as candidates to replace the actual technologies employed as non-volatile memory devices. The origin of this proposal relies on the observation of the extraordinary properties required for the scaling down of this kind of devices. In this regard, a great variety of oxide materials displaying resistive switching phenomena have been studied lately. However, the lack of understanding of the physical mechanism producing the resistive switching has limited mainly their commercialization. In this thesis, we explore the resistive switching properties of the complex oxide La1-xSrxMnO3 and bilayer CeO2-x/La1-x SrxMnO3 for non-volatile memory applications. First, we study the La1-xSrxMnO3 thin layer deposition technique and perform physical and structural characterization measurements in order to fully optimize this process. In addition, microfabrication techniques are used to obtain the memristor-like metal/La1-xSrxMnO3/metal and metal/CeO2-x/La1-x SrxMnO3/CeO2-x/metal lateral micro-devices where the resistive switching is evaluated through I-V measurements. In line with the results, a mechanism based on the oxygen ion exchange is proposed as responsible of the bipolar and complementary resistive switching induced in this kind of lateral devices. Furthermore, based on volume switching induced in metal/CeO2-x/La1-xSrxMnO3/CeO2-x/metal bilayers, a three-terminal device is presented as innovative part of this work. The current conduction along the La1-xSrxMnO3 layer is modulated by using a metal electrode which acts as a gate terminal. As a consequence of the evaluation of the volume switching, we confirm that the CeO2-x layer acts as an oxygen reservoir favouring the oxygen ion exchange with the La1-xSrxMnO3 layer and modify its resistive switching properties. In addition, we demonstrate that the resistive switching is homogenously produced inside of the La1-xSrxMnO3 layer and that the electroforming process to induce the resistive switching in bilayer devices does not produce any breakdown neither in the CeO2-x nor in the La1-x SrxMnO3 layer. Finally, the conclusions derived from our results can be of relevance for the understanding of the resistive switching phenomena in complex oxides.

Emerging memory technologies are being intensively investigated for extending Moore\'s law in the next decade. The conductive bridge random access memory (CBRAM) is one of the most promising candidates. CBRAM shows unique nanoionics-based filamentary switching mechanism. Compared to flash memory, the advantages of CBRAM include excellent scalability, low power consumption, high OFF-/ON-state resistance ratio, good endurance, and long retention. Besides the nonvolatile memory applications, resistive switching devices implement the function of memristor which is the fourth basic electrical component. This research presents the characterization and modeling of Cu/TaOx/Pt resistive switching devices. Both Cu and oxygen vacancy nanofilaments can conduct current according to the polarity of bias voltage. The volatile resistive switching phenomenon has been observed on Cu/TaOx/delta-Cu/Pt devices and explained by a flux balancing model. The resistive devices are also connected in series and in anti-parallel manner. These circuit elements are tested for chaotic neural circuit. The quantum conduction has been observed in the I-V characteristics of devices, evidencing the metallic contact between the nanofilament and electrodes. The model of filament radial growth has been developed to explain the transient I-V relation and multilevel switching in the metallic contact regime. The electroforming/SET and RESET processes have been simulated according to the mechanism of conductive filament formation and rupture and validated by experimental results. The Joule and Thomson heating effects have also been investigated for the RESET processes.<br>Ph. D.

The Semiconductor industry is investigating high speed, low power consumption, high-density memory devices that can retain their information without power supply. Resistive Random Access Memory (ReRAM) is one of the most attractive candidates as an alternative to conventional flash memory devices due to of its simple metal-insulator-metal (MIM) structures. A compositional gradient of thin film materials produced by the simultaneous combination of elements provides a powerful tool for the combinatorial synthesis of materials. It was applied here to control the composition, structure and morphology of materials in composite devices of ReRAM. This allows the systematic high throughput screening of the intrinsic properties of the materials, as well as the high throughput optimisations of composite thin films that mimic memory device structures. Therefore, the focus of this project is to develop a novel capacitor for ReRAM application. We present here details of the preparation technique and the screening methodologies of this approach by applying the synthesis to various phases of titania, for which there is an extensive literature, as a prelude to the screening of more complex systems. Inert Pt electrodes and active Cu electrodes were deposited on TiO2 as top electrodes using different mask sizes (50 micron and 250 micron). The bottom electrode is Si/ SiO2/ TiO2/ Pt (SSTOP) was constant throughout this project. TiO2 was prepared using evaporative physical vapour deposition (PVD) with a variation of thickness between 10 nm and 300 nm on SSTOP. The synthetic conditions were chosen to produce TiO2 oxygen stoichiometric and sub-stoichiometric amorphous, anatase and rutile materials. The oxides have been fully characterised by X-Ray Diffraction (XRD), X-ray Photo electron Spectroscopy (XPS), Raman Spectroscopy, Four Point Probe (4pp) and Atomic Force Microscopy (AFM). The electrical screening was carried out on capacitor-like structures produced using 250 micron diameter top electrodes deposited using a 14 x 14 array contact mask. Current-Voltage (I-V) measurements were conducted employing a variety of current compliances (IC). The typical I-V switching of the unipolar mode (both state in one polarity) was achieved on all titania phases, whereas the bipolar mode (each state in different polarity) was achieved only on the amorphous phase. The resistance differences between High Resistance State (HRS) and Low Resistance State (LRS) were clearly identified in each system. It was observed that for all the devices investigated, a lower forming field was required on the thicker layer of the active switching layers. Devices with copper electrodes, and composite devices with sub-stoichiometric titania adjacent to the stoichiometric titania could be formed at lower voltages and electric fields. The results obtained here confirm the feasibility of the high-throughput approach to optimise functional nanomaterials and composite device structures for resistive switching memory application.