Daniele Ielmini

Phase Change Materials and Their Application to Nonvolatile Memories

Phase change materials are materials that exist in at least two structurally distinct solid phases, an amorphous and one (or more) crystalline phases. Many materials display phase change properties in this sense and can be deposited at least as a thin film in an amorphous phase (low temperature deposition, very thin film) or crystalline phase (high temperature deposition, epitaxy). Often the amorphous and crystalline phases have very different optical and electrical properties stemming from the large differences in structure between the amorphous and the crystalline phases. These differences can be used to store information in technological applications if it is possible to switch the material repeatedly between the two phases and if both phases are stable at operating temperature. The transformation of the metastable amorphous phase to the energetically favorable, stable crystalline phase occurs by heating the material above its crystallization temperature for a time

Analytical model for subthreshold conduction and threshold switching in chalcogenide-based memory devices

Chalcogenide materials are receiving increasing interest for their many applications as active materials in emerging memories, such as phase-change memories, programmable metallization cells, and cross-point devices. The great advantage of these materials is the capability to appear in two different phases, the amorphous and the crystalline phases, with rather different electrical properties. The aim of this work is to provide a physically based model for conduction in the amorphous chalcogenide material, able to predict the current-voltage (I−V) characteristics as a function of phase state, temperature, and cell geometry. First, the trap-limited transport at relatively low currents (subthreshold regime) is studied, leading to a comprehensive model for subthreshold conduction accounting for (a) the shape of the I−V characteristics, (b) the measured temperature dependence, (c) the dependence of subthreshold slope on the thickness of the amorphous phase, and (d) the voltage dependence of the activation ener...

Reliability study of phase-change nonvolatile memories

A detailed investigation of the reliability aspects in nonvolatile phase-change memories (PCM) is presented, covering the basic aspects related to high density array NVM, i.e., data retention, endurance, program and read disturbs. The data retention capabilities and the endurance characteristics of single PCM cells are analyzed, showing that data can be stored for 10 years at 110/spl deg/C and that a resistance difference of two order of magnitude between the cell states can be maintained for more than 10/sup 11/ programming cycles. The main mechanisms responsible for instabilities just before failure as well as for final device breakdown are also discussed. Finally, the impact of read and program disturbs are clearly assessed, showing with experimental data and simulated results that the crystallization induced during the cell read out and the thermal cross-talk due to adjacent bits programming do not affect the retention capabilities of the PCM cells.

Self-Accelerated Thermal Dissolution Model for Reset Programming in Unipolar Resistive-Switching Memory (RRAM) Devices

This paper addresses the numerical modeling of reset programming in NiO-based resistive-switching memory. In our model, we simulate electrical conduction and heating in the conductive filament (CF), which controls the resistance of the low resistive (or set) state, accounting for CF thermal-activated dissolution. Employing CF electrical and thermal parameters, which were previously characterized on our NiO-based samples, our calculations are shown to match experimental reset and retention characteristics. Simulations show that reset transition is self-accelerated as a consequence of a positive feedback between the thermal dissolution of the CF and local Joule heating in the CF bottleneck, which can account for the abrupt resistance transition in experimental data. Finally, the model is used to investigate the reduction of the reset current, which is needed for device application.

Electronic switching effect and phase-change transition in chalcogenide materials

The threshold switching mechanism in amorphous chalcogenides is investigated, showing experimental data that once and for all demonstrate its electronic nature. The physical mechanisms responsible for the switching to the highly conductive state are discussed and the impact of cumulative read-out pulses is also investigated, showing that phase-change transitions induced by usual reading operations in phase-change memory cells are completely negligible.

Modeling the Universal Set/Reset Characteristics of Bipolar RRAM by Field- and Temperature-Driven Filament Growth

Resistive switching memory (RRAM) devices generally rely on the formation/dissolution of conductive filaments through insulating materials, such as metal oxides and chalcogenide glasses. Understanding the mechanisms for filament formation and disruption in resistive switching materials is a critical step toward the development of reliable and controllable RRAM for future-generation storage. In particular, the capability to control the filament resistance and the reset current through the compliance current during filament formation may provide a key signature to clarify the switching mechanism. This paper provides a physically based explanation for the universal resistance switching in bipolar RRAM devices. A numerical model of filament growth based on thermally activated ion migration accounts for the resistance switching characteristics. The same physical picture is extended to numerically model the reset transition. The impact of migration parameters and experimental setup on the set/reset characteristics is discussed through numerical simulations.

Electrothermal and phase-change dynamics in chalcogenide-based memories

We analyzed the programming dynamics in phase-change memory (PCM) cells. The chalcogenide phase-change mechanism and phase distribution in the programmed cell is studied by both experiments and a numerical model, which self-consistently addresses the electrical-thermal conduction phase transition. We show that the reset-set transition is strongly coupled to the electronic switching in the amorphous phase, thus supporting the need for a self-consistent electrothermal-phase transition model to correctly account for all experimental evidences.

Study of Multilevel Programming in Programmable Metallization Cell (PMC) Memory

Programmable metallization cell (PMC) memory, also known as conductive bridging RAM (CBRAM), is a resistive-switching memory based on non-volatile formation and dissolution of a conductive filament (CF) in a solid electrolyte. Although ease of fabrication, promising performance and multilevel (ML) capability make the PMC a possible candidate for post-flash non-volatile memories, further physical understanding is required to better assess its true potential. In this work, we investigate the kinetics involved in the programming operation (i.e., transition from the high resistance to the low resistance state), which occurs by voltage-driven ion migration and electrochemical deposition, and results in CF formation and growth. The main kinetic parameters controlling the programming operation are extracted from our electrical data. Also, CF growth and corresponding resistance decrease is shown to be controllable with reasonable accuracy in pulse mode by employing a variable load resistance which can dynamically control the programming kinetics. A semi-analytical physical model is shown to account for experimental data and allows for the engineering of fast and reliable ML programming in one transistor-one resistor (1T-1R) devices.

Filament Conduction and Reset Mechanism in NiO-Based Resistive-Switching Memory (RRAM) Devices

The physical understanding of the programming and reliability mechanisms in resistive-switching memory devices requires a detailed characterization of the electrical and thermal conduction properties in the low-resistance state of the memory cell. The aim of this paper is the characterization of the conductive filament (CF), which controls the localized current flow in the low resistive state of the cell. Based on a new technique for evaluating the CF temperature during operation, we perform a statistical characterization of the critical filament temperature for the reset operation, i.e., the transition to the high-resistance state by the thermal dissolution of the CF. The thermal resistance of the CF and the activation energy for the dissolution mechanism are then evaluated, allowing for a physics-based numerical modeling of the reset operation based on CF thermal breakup.

Resistive Switching by Voltage-Driven Ion Migration in Bipolar RRAM—Part II: Modeling

Resistive-switching memory (RRAM) based on transition metal oxides is a potential candidate for replacing Flash and dynamic random access memory in future generation nodes. Although very promising from the standpoints of scalability and technology, RRAM still has severe drawbacks in terms of understanding and modeling of the resistive-switching mechanism. This paper addresses the modeling of resistive switching in bipolar metal-oxide RRAMs. Reset and set processes are described in terms of voltage-driven ion migration within a conductive filament generated by electroforming. Ion migration is modeled by drift–diffusion equations with Arrhenius-activated diffusivity and mobility. The local temperature and field are derived from the self-consistent solution of carrier and heat conduction equations in a 3-D axis-symmetric geometry. The model accounts for set–reset characteristics, correctly describing the abrupt set and gradual reset transitions and allowing scaling projections for metal-oxide RRAM.