E. Miranda

Model for the Resistive Switching Effect in

A physics-based analytical model for the current-voltage (I-V) characteristics corresponding to the low and high resistive states in electroformed metal-insulator-metal structures with HfO2 layers is proposed. The model relies on the Landauer theory for the electron transport in mesoscopic systems. The switching phenomenon is ascribed to the modulation of the constrictions bottleneck cross-sectional area associated with atomic rearrangements within the confinement path. The extracted parameter values allow one to conclude that the length and radius of the region that controls the conduction characteristics are in the nanometer range.

\hbox{HfO}_{2}

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.

MIM Structures Based on the Transmission Properties of Narrow Constrictions

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.

Quantum-size effects in hafnium-oxide resistive switching

Prior to any attempt to model a charge transport mechanism, a precise knowledge of the parameters on which the current depends is essential. In this work, the soft breakdown (SBD) failure mode of ultrathin (3-5 nm) SiO/sub 2/ layers in polysilicon-oxide-semiconductor structures is investigated. This conduction regime is characterized by a large leakage current and by multilevel current fluctuations, both at low applied voltages. In order to obtain a general picture of SBD, room-temperature current-voltage (I-V) measurements have been performed on samples with different gate areas, oxide thicknesses and substrate types. An astounding matching between some of these I-V characteristics has been found. The obtained results and the comparison with the final breakdown regime suggest that the current flow through a SBD spot is largely influenced by its atomic-scale dimensions as occurs in a point contact configuration. Experimental data are also presented which demonstrate that specific current fluctuations can be ascribed to a blocking behavior of unstable SBD conduction channels.

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

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.

Soft breakdown conduction in ultrathin (3-5 nm) gate dielectrics

Abstract When the gate insulator of a metal–oxide–semiconductor structure is subjected to electrical stress, traps or defects are progressively generated inside the oxide that eventually lead to the formation of a low-resistance conducting path between the electrodes. The occurrence of such event can be detected either as a gradual or as a sudden change in the system’s conductance and is associated with the appearance of a localized conduction mechanism in parallel with the area-distributed tunneling current. According to the magnitude and shape of the resulting current–voltage characteristic, the failure mode is usually referred to as soft or hard breakdown. However, because of the random nature of the phenomenon, interpretation and modelling of the electron transport mechanism involved has turned out to be very challenging from the physical point of view. Several models have been proposed to this aim, which can be classified basically according to the underlying mechanism: junction-like, hopping, percolation and tunneling conduction. Within this latter mechanism we can mention direct, Fowler–Nordheim, trap-assisted, resonant, inelastic tunneling and point contact conduction. In this paper, after an overview of a variety of physical and engineering aspects of post-breakdown, we critically examined the foundations and limitations of the proposed conduction models in the light of others and ours experimental results, putting special emphasis on those approaches providing final closed expressions.

Voltage and Power-Controlled Regimes in the Progressive Unipolar RESET Transition of HfO2-Based RRAM

Overcoming challenges associated with implementation of resistive random access memory technology for non-volatile information storage requires identifying the material characteristics responsible for resistive switching. In order to connect the switching phenomenon to the nano-scale morphological features of the dielectrics employed in memory cells, we applied the enhanced conductive atomic force microscopy technique for in situ analysis of the simultaneously collected electrical and topographical data on HfO2 stacks of various degrees of crystallinity. We demonstrate that the resistive switching is a local phenomenon associated with the formation of a conductive filament with a sufficiently small cross-section, which is determined by the maximum passing current. Switchable filament is found to be formed at the dielectric sites where the forming voltages were sufficiently small, which, in the case of the stoichiometric HfO2, is observed exclusively at the grain boundary regions representing low resistant...

Electron transport through broken down ultra-thin SiO2 layers in MOS devices

Back-end-of-line integrated 1 × μm2 TiN/HfO2/Ti/TiN MIM memory devices in a 0.25- μm complementary metal-oxide-semiconductor technology were built to investigate the conduction mechanism and the resistive switching behavior as a function of temperature. The temperature-dependent I- V characteristics in fresh devices are attributed to the Poole-Frenkel mechanism with an extracted trap energy level at φ ≈ 0.2 eV below the HfO2 conduction band. The trap level is associated with positively charged oxygen vacancies. The electroformed memory cells show a stable bipolar switching behavior in the temperature range from 213-413 K. The off -state current increases with temperature, whereas the on-state current can be described by a weak metallic behavior. Furthermore, the results suggest that the I-V cycling not only induces significant changes in the electrical properties of the MIM memory devices, i.e., the increase in the off-state current, but also stronger temperature dependence. The temperature effect on the on-state and off-state characteristics is modeled within the framework of the quantum point-contact model for dielectric breakdown using an effective temperature-dependent confinement potential.

Resistive switching in hafnium dioxide layers: Local phenomenon at grain boundaries

An empirical one parameter-based power law model for the leakage current through one or more soft breakdown spots in ultrathin (<5 nm) gate oxides is presented. Good fit to data can be obtained in nearly five decades of current from 0.5 to 5 V. In addition, it is shown that there exists a slight correlation between the parameters which describe the soft breakdown conduction characteristic and the stressing condition which triggers it.

Impact of Temperature on the Resistive Switching Behavior of Embedded

In 2008, Salahuddin and Datta proposed that a ferroelectric material operating in the negative capacitance (NC) region could act as a step-up converter of the surface potential in a metal-oxide-semiconductor structure, opening a new route for the realization of transistors with steeper subthreshold characteristics (S <; 60mV/dec). In this paper, a comprehensive physics-based surface potential and a drain current model for the NC field-effect transistor are reported. The model is aimed to evaluate the potentiality of such transistors for low-power switching applications. This paper also sheds light on how operation in the NC region can be experimentally detected.