Luca Vandelli

Metal oxide resistive memory switching mechanism based on conductive filament properties

By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament (CF) features controlling TiN/HfO2/TiN resistive memory (RRAM) operations. The leakage current through the dielectric is found to be supported by the oxygen vacancies, which tend to segregate at hafnia grain boundaries. We simulate the evolution of a current path during the forming operation employing the multiphonon trap-assisted tunneling (TAT) electron transport model. The forming process is analyzed within the concept of dielectric breakdown, which exhibits much shorter characteristic times than the electroforming process conventionally employed to describe the formation of the conductive filament. The resulting conductive filament is calculated to produce a non-uniform temperature profile along its length during the reset operation, promoting preferential oxidation of the filament tip. A thin dielectric barrier resulting from the CF tip oxidation is found to control filament resistance in the high resistive state. Field-driven dielectric breakdown of this barrier during the set operation restores the filament to its initial low resistive state. These findings point to the critical importance of controlling the filament cross section during forming to achieve low power RRAM cell switching.

A Physical Model of the Temperature Dependence of the Current Through

In this paper, we investigate the characteristics of the defects responsible for the leakage current in the SiO2 and SiO2/HfO2 gate dielectric stacks in a wide temperature range (6 K-400 K). We simulated the temperature dependence of the I -V characteristics both at positive and negative gate voltages by applying the multiphonon trap-assisted tunneling model describing the charge transport through the dielectric. In the depletion/weak inversion regime, the current is limited by the supply of carriers available for tunneling. In strong inversion, the temperature dependence is governed by the charge transport mechanisms through the stacks; in particular, in SiO2/HfO2 dielectric stacks, the coupling of the injected carriers with the dielectric phonons at the trap sites is the dominant mechanism. Matching the simulation results to the measurement data allows extracting important trap parameters, e.g., the trap relaxation and ionization energies, which identify the atomic structure of the electrically active defects in the gate dielectric.

\hbox{SiO}_{2}\hbox{/}\hbox{HfO}_{2}

By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament features controlling TiN/HfO2/TiN resistive memory operations. The forming process is found to define the filament geometry, which in turn determines the temperature profile and, consequently, the switching characteristics. The findings point to the critical importance of controlling filament dimensions during the forming process (polarity, max current/voltage, etc.).

Stacks

This paper presents a physics-based compact model for the program window in HfOx resistive random access memory devices, defined as the ratio of the resistances in high resistance state (HRS) and low resistance state (LRS). This model allows extracting the characteristics of the conductive filament (CF) in HRS. For a given forming current compliance limit, the program window is shown to be correlated to the thickness of the reoxidized portion of the CF in HRS, which can be modulated by the reset voltage amplitude. On the other hand, the statistical distribution of the memory window depends exponentially on the barrier thickness variations that points to the critical role of reset conditions for the performance optimization of RRAM devices.

Metal oxide RRAM switching mechanism based on conductive filament microscopic properties

We present a quantitative physical model describing degradation of poly-crystalline HfO2 dielectrics subjected to electrical stress culminating in the dielectric breakdown (BD). The model accounts for the morphology of the hafnium oxide film and considers the interaction of the injected electrons with the atomic defects supporting the charge transport to calculate the 3-D power dissipation and temperature maps across the dielectric. This temperature map, along with that of the electric field, is used to self-consistently calculate the stress-induced defect generation rates in the dielectric during stress. The model quantitatively reproduces the evolution of the currents measured on HfO2 MIM capacitors during constant voltage stress, up to the onset of BD, and the dependencies of the time-dependent dielectric breakdown distributions on stress temperature and voltage. It represents a powerful tool for statistical reliability predictions that can be extended to other high-κ materials, multilayer stacks, and resistive RAM devices based on transition metal oxides.

A Compact Model of Program Window in HfO x RRAM Devices for Conductive Filament Characteristics Analysis

In this work we apply a physical model based on charge transport and molecular mechanics/dynamics simulations to investigate the physical mechanisms governing the RRAM forming and switching operations. The proposed model identifies the major driving forces controlling conductive filament (CF) formation and changes during RRAM switching, thus providing a tool for investigation and optimization of RRAM devices.

Microscopic Modeling of Electrical Stress-Induced Breakdown in Poly-Crystalline Hafnium Oxide Dielectrics

We propose a model describing the operations of hafnium oxide-based resistive random access memory (RRAM) devices at the microscopic level. Charge carrier and ion transport are self-consistently described starting from the leakage current in pristine HfO2. Material structural modifications occurring during the RRAM operations, such as conductive filament (CF) creation and disruption, are accounted for. The model describes the complex processes leading to a formation of the CF and its dependence on both electrical conditions (e.g., current compliance, voltage stress, and temperature) and device characteristics (e.g., electrodes material and dielectric thickness).

Comprehensive physical modeling of forming and switching operations in HfO2 RRAM devices

In this paper we investigate the physical mechanisms governing operations in HfOx RRAM devices. Forming set and reset processes are studied using a model including power dissipation associated with the charge transport, and the corresponding temperature increase, which assists ion diffusion.

Microscopic Modeling of HfO x RRAM Operations: From Forming to Switching

We investigate and quantify the role played by electrons and holes during the erase operation of TANOS memories by means of charge separation experiments and physics-based simulations. Results demonstrate that electron emission via trap-to-band tunneling dominates the first part of the erase operation, whereas hole injection prevails in the remaining part of the transient. In addition, we show that the efficiency of the erase operation is high and constant mainly because of the high energy offset between nitride and alumina valence bands. Our results clearly identify the physical mechanisms responsible for TANOS erase and allow deriving some important guidelines for the optimization of this operation.

Microscopic understanding and modeling of HfO 2 RRAM device physics

In this work we explore the microscopic mechanisms responsible for Random Telegraph Noise (RTN) current fluctuations in HfOx Resistive Random Access Memory (RRAM) devices. The statistical properties of the RTN current fluctuations are analyzed in a variety of reading conditions by exploiting the Factorial Hidden Markov Model (FHMM) to decompose the complex RTN traces in a superimposition of two-level fluctuations. We investigate the physical mechanisms that could be responsible for the RTN current fluctuations by considering two options that are the Coulomb blockade effect and the metastable-to-stable transition of defect assisting the Trap-Assisted-Tunneling (TAT) charge transport. Physics-based simulations show that both options allow reproducing the RTN current fluctuations. The electron TAT via oxygen vacancy defects, responsible for the current in High Resistive State (HRS), is significantly altered by the electric field caused by electron trapping at defects (i.e. neutral interstitial oxygen), not directly involved in charge transport. Similarly, the transition of oxygen vacancies into a stable-slow defect configuration (still unidentified in HfOx) can temporarily switch off the current, thus explaining the RTN.