Quantum size effects in hafnium-oxide resistive switching

Discrete changes of conductance of the order of G0 = 2e2/h reported during the unipolar reset transitions of Pt/HfO2/Pt structures are interpreted as the signature of atomic-size variations of the conducting filament (CF) nanostructure. Our results suggest that the reset occurs in two phases: a progressive narrowing of the CF to the limit of a quantum wire (QW) followed by the opening of a spatial gap that exponentially reduces the CF transmission. First principles calculations show that oxygen vacancy paths in HfO2 with single- to few-atom diameters behave as QWs and are capable of carrying current with G0 conductance

Quantum size effects in hafnium-oxide resistive switching

Discrete changes of conductance of the order of G0 = 2e2/h reported during the unipolar reset transitions of Pt/HfO2/Pt structures are interpreted as the signature of atomic-size variations of the conducting filament (CF) nanostructure. Our results suggest that the reset occurs in two phases: a progressive narrowing of the CF to the limit of a quantum wire (QW) followed by the opening of a spatial gap that exponentially reduces the CF transmission. First principles calculations show that oxygen vacancy paths in HfO2 with single- to few-atom diameters behave as QWs and are capable of carrying current with G0 conductance

Robust memristors based on layered two-dimensional materials

Van der Waals heterostructures are formed by stacking layers of different two-dimensional materials and offer the possibility to design new materials with atomic-level precision. By combining the valuable properties of different 2D systems, such heterostructures could potentially be used to address existing challenges in the development of electronic devices, particularly those that require vertical multi-layered structures. Here we show that robust memristors with good thermal stability, which is lacking in traditional memristors, can be created from a van der Waals heterostructure composed of graphene/MoS2–xO x/graphene. The devices exhibit excellent switching performance with an endurance of up to 107 and a high operating temperature of up to 340 °C. With the help of in situ electron microscopy, we show that the thermal stability is due to the MoS2–xO x switching layer, as well as the graphene electrodes and the atomically sharp interface between the electrodes and the switching layer. We also show that the devices have a well-defined conduction channel and a switching mechanism that is based on the migration of oxygen ions. Finally, we demonstrate that the memristor devices can be fabricated on a polyimide substrate and exhibit good endurance against over 1,000 bending cycles, illustrating their potential for flexible electronic applications

Voltage and power-controlled regimes in the progressive unipolar RESET transition of HfO₂-based RRAM

Resistive switching (RS) based on the formation and rupture of conductive filament (CF) is promising in novel memory and logic device applications. Understanding the physics of RS and the nature of CF is of utmost importance to control the performance, variability and reliability of resistive switching memory (RRAM). Here, the RESET switching of HfO₂-based RRAM was statistically investigated in terms of the CF conductance evolution. The RESET usually combines an abrupt conductance drop with a progressive phase ending with the complete CF rupture. RESET1 and RESET2 events, corresponding to the initial and final phase of RESET, are found to be controlled by the voltage and power in the CF, respectively. A Monte Carlo simulator based on the thermal dissolution model of unipolar RESET reproduces all of the experimental observations. The results contribute to an improved physics-based understanding on the switching mechanisms and provide additional support to the thermal dissolution model

Electrical Properties and Biological Synaptic Simulation of Ag/MXene/SiO2/Pt RRAM Devices

Utilizing electronic devices to emulate biological synapses for the construction of artificial neural networks has provided a feasible research approach for the future development of artificial intelligence systems. Until now, different kinds of electronic devices have been proposed in the realization of biological synapse functions. However, the device stability and the power consumption are major challenges for future industrialization applications. Herein, an electronic synapse of MXene/SiO2 structure-based resistive random-access memory (RRAM) devices has been designed and fabricated by taking advantage of the desirable properties of SiO2 and 2D MXene material. The proposed RRAM devices, Ag/MXene/SiO2/Pt, exhibit the resistance switching characteristics where both the volatile and nonvolatile behaviors coexist in a single device. These intriguing features of the Ag/MXene/SiO2/Pt devices make them more applicable for emulating biological synaptic plasticity. Additionally, the conductive mechanisms of the Ag/MXene/SiO2/Pt RRAM devices have been discussed on the basis of our experimental results

Resistive Switching Statistics in MIM structures for Non-volatile memory applications

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.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

Resistive Switching Statistics in MIM structures for Non-volatile memory applications

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.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

Identification and validation of the model consisting of DDX49, EGFR, and T‐stage as a possible risk factor for lymph node metastasis in patients with lung cancer

Abstract Introduction The lymph node metastasis stage of lung cancer is an important decisive factor in the need for postoperative adjuvant treatment and the difference between stage IIIa and stage IIIB that is the necessary information to distinguish whether surgery can be performed or not. The specificity of the clinical diagnosis of lung cancer with lymph node metastasis cannot meet the requirements of preoperative evaluation of surgical indications and prediction of surgical removal range in lung cancer. Methods This was an early experimental laboratory trial. The model identification data included the RNA sequence data of 10 patients from our clinical data and 188 patients with lung cancer from The Cancer Genome Atlas dataset. The model development and validation data consisted of RNA sequence data for 537 cases from the Gene Expression Omnibus dataset. We explore the predictive value of the model on two independent clinical data. Results A higher specificity of diagnostic model for patients with lung cancer with lymph node metastases consisted of DDX49, EGFR, and tumor stage (T‐stage), which were the independent predictive factors. The area under the curve value, specificity, and sensitivity for predicting lymph node metastases were 0.835, 70.4%, and 78.9% at RNA expression level in the training group, and 0.681, 73.2%, and 75.7% at RNA expression level in the validation group as shown as in result part. To verify the predictive performance of the combined model for lymph node metastases, we downloaded the GSE30219 data set (n = 291) and the GSE31210 data set (n = 246) from the Gene Expression Omnibus (GEO) database as the training group and validation group, respectively. In addition, the model had a higher specificity for predicting lymph node metastases in independent tissue samples. Conclusions Determination of DDX49, EGFR, and T‐stage could form a novel prediction model to improve the diagnostic efficacy of lymph node metastasis in clinical application

Facile solution processed MoO3 thin film as hole transportation layer for polymer solar cells

MoO3 hole transportation layer was prepared by a novel spray coating method under low temperature for polymer solar cell. The physical phase of spray coated MoO3 thin film was demonstrated by X-ray diffraction. The surface morphology of solution processed MoO3 (s-MoO3) and thermally evaporated MoO3 (e-MoO3) was characterized by metallurgical microscope and atomic force microscopy. The PSC device based on s-MoO3 HTL shows better photo to electron conversion efficiency (PCE = 2.93%) performance than PEDOT:PSS based devices (PCE = 2.80%), and the high Jsc obtained in s-MoO3 based PSC device indicates that the transportation across the s-MoO3 layer is unhindered. Finally, the stability of PSC devices based on different HTLs has also been investigated

LMNA E82K mutation activates FAS and mitochondrial pathways of apoptosis in heart tissue specific transgenic mice.

The lamin A/C (LMNA), nuclear intermediate filament proteins, is a basic component of the nuclear lamina. Mutations in LMNA are associated with a broad range of laminopathies, congenital diseases affecting tissue regeneration and homeostasis. Heart tissue specific transgenic mice of human LMNA E82K, a mutation causing dilated cardiomyopathy, were generated. Lmna(E82K) transgenic mouse lines exhibited thin-walled, dilated left and right ventricles, a progressive decrease of contractile function assessed by echocardiography. Abnormalities of the conduction system, myocytes disarray, collagen accumulation and increased levels of B-type natriuretic peptide (BNP), procollagen type III α1 (Col3α1) and skeletal muscle actin α1 (Actα1) were detected in the hearts of Lmna(E82K) transgenic mice. The LMNA E82K mutation caused mislocation of LMNA in the nucleus and swollen mitochondria with loss of critae, together with the loss of nuclear envelope integrity. Most interestingly, we found that the level of apoptosis was 8.5-fold higher in the Lmna(E82K) transgenic mice than that of non-transgenic (NTG) mice. In the presence of the LMNA E82K, both of FAS and mitochondrial pathways of apoptosis were activated consistent with the increase of FAS expression, the release of cytochrome c from mitochondria to cytosol and activation of caspase-8, -9 and -3. Our results suggested that the apoptosis, at least for the LMNA E82K or the mutations in the rod region of Lamin A/C, might be an important mechanism causing continuous loss of myocytes and lead to myocardial dysfunction. It could be a potential therapeutic means to suppress and/or prevent inappropriate cardiac cell death in patients carrying LMNA mutation