Xiaojuan Lian

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.

A Model for the Set Statistics of RRAM Inspired in the Percolation Model of Oxide Breakdown

The set voltage distribution of Pt/HfO2/Pt resistive switching memory is shown to fit well a Weibull model with Weibull slope and scale factor increasing logarithmically with the resistance measured at the set point. Gaining inspiration from the percolation model of oxide breakdown, a physics-based model for the Vset statistics is proposed. The results of the model are completely consistent with the experimental results and demonstrate the need of a strong reset to get large Weibull slope that provides some relief to the strong requirements imposed by the set speed-read disturb dilemma.

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.

An in-depth simulation study of thermal reset transitions in resistive switching memories

An in-depth characterization of the thermal reset transition in RRAM has been performed based on coupling self-consistent simulations to experimental results. A complete self-consistent simulator accounting for the electrical and thermal descriptions of the conductive filaments (CFs) has been developed for the numerical study of the temporal evolution of the reset transition in RRAM. The CFs series resistance, including the contributions of the setup and Maxwell components, has been included in the calculations. Using this simulation tool, we have been able to reproduce many experimental details of the experimental reset data obtained in Cu/HfO2/Pt devices. In doing so, we explained the current steps observed in some reset cycles by considering CFs with several coupled branches that break down at different times. The reset voltage dependence on the initial resistance of the CF has been analyzed and the relevant role played by the CF shape has also been demonstrated. In this respect, devices with a same in...

Boron nitride as two dimensional dielectric: Reliability and dielectric breakdown

Boron Nitride (BN) is a two dimensional insulator with excellent chemical, thermal, mechanical, and optical properties, which make it especially attractive for logic device applications. Nevertheless, its insulating properties and reliability as a dielectric material have never been analyzed in-depth. Here, we present the first thorough characterization of BN as dielectric film using nanoscale and device level experiments complementing with theoretical study. Our results reveal that BN is extremely stable against voltage stress, and it does not show the reliability problems related to conventional dielectrics like HfO2, such as charge trapping and detrapping, stress induced leakage current, and untimely dielectric breakdown. Moreover, we observe a unique layer-by-layer dielectric breakdown, both at the nanoscale and device level. These findings may be of interest for many materials scientists and could open a new pathway towards two dimensional logic device applications.

Set statistics in conductive bridge random access memory device with Cu/HfO2/Pt structure

The switching parameter variation of resistive switching memory is one of the most important challenges in its application. In this letter, we have studied the set statistics of conductive bridge random access memory with a Cu/HfO2/Pt structure. The experimental distributions of the set parameters in several off resistance ranges are shown to nicely fit a Weibull model. The Weibull slopes of the set voltage and current increase and decrease logarithmically with off resistance, respectively. This experimental behavior is perfectly captured by a Monte Carlo simulator based on the cell-based set voltage statistics model and the Quantum Point Contact electron transport model. Our work provides indications for the improvement of the switching uniformity.

Investigation on the RESET switching mechanism of bipolar Cu/HfO2/Pt RRAM devices with a statistical methodology

The RESET switching of bipolar Cu/HfO2/Pt resistance random access memory (RRAM) is investigated. With a statistical methodology, we systematically analyze the RESET voltage (VRESET) and RESET current (IRESET). VRESET shows a U-shape distribution as a function of RON according to the scatter plot of the raw experimental data. After data correction by a series resistance (RS), VRESET is nearly constant, while IRESET decreases linearly with RCF. These behaviours are consistent with the thermal dissolution model of RESET. Moreover, the IRESET and VRESET distributions are strongly affected by the RON distribution. Using a ?resistance screening? method, the IRESET and VRESET distributions are found to be compatible with the Weibull distribution model. The Weibull slopes of the VRESET and IRESET distributions are independent of RCF, indicating that the RESET point corresponds to the initial phase of conductive filament (CF) dissolution, according to our cell-based model for the unipolar RESET of RRAM devices. The scale factor of the VRESET distributions is roughly constant, while that of the IRESET distributions scale with 1/RCF. Accordingly, the RESET switching of the HfO2-based solid electrolyte memory is compatible with the thermal dissolution mechanism, improving our understanding on the physics of resistive switching of RRAM devices.

Multi-scale quantum point contact model for filamentary conduction in resistive random access memories devices

We depart from first-principle simulations of electron transport along paths of oxygen vacancies in HfO2 to reformulate the Quantum Point Contact (QPC) model in terms of a bundle of such vacancy paths. By doing this, the number of model parameters is reduced and a much clearer link between the microscopic structure of the conductive filament (CF) and its electrical properties can be provided. The new multi-scale QPC model is applied to two different HfO2-based devices operated in the unipolar and bipolar resistive switching (RS) modes. Extraction of the QPC model parameters from a statistically significant number of CFs allows revealing significant structural differences in the CF of these two types of devices and RS modes.

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–xOx/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–xOx 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.Memristors that offer good thermal stability, which is lacking in traditional memristors, can be created from a van der Waals heterostructure composed of graphene/MoS2–xOx/graphene.

Compact analytical models for the SET and RESET switching statistics of RRAM inspired in the cell-based percolation model of gate dielectric breakdown

In this work, we depart from the cell-based percolation model of gate dielectric breakdown (BD) to propose analytical models for the SET and RESET statistics in resistive switching memory (RRAM). The SET or RESET statistics model consists of two basic elements: (i) a cell-based geometrical model to describe the dependence of the resistive switching (RS) distribution on the defect generation in the conductive filament (CF), and (ii) a deterministic model for the SET/RESET dynamics to describe the relation of the defect generation with measurable variables such as the SET/RESET voltage and current. The experimental observations in HfO2- and NiO-based RRAM devices can be successfully accounted for by our models for RS statistics. The models set a framework for the consideration of performance-reliability tradeoffs in RRAM.