skip to main content

Title: Modeling Reset, Set, and Read Operations in Nanoscale Ge 2 Sb 2 Te 5 Phase‐Change Memory Devices Using Electric Field‐ and Temperature‐Dependent Material Properties
  more » « less
Award ID(s):
Author(s) / Creator(s):
 ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
physica status solidi (RRL) – Rapid Research Letters
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. We stabilize resistance of melt-quenched amorphous Ge2Sb2Te5 (a-GST) phase change memory (PCM) line cells by substantially accelerating resistance drift and bringing it to a stop within a few minutes with application of high electric field stresses. The acceleration of drift is clearly observable at electric fields > 26 MV/m at all temperatures (85 K - 300 K) and is independent of the current forced through the device, which is a strong function of temperature. The low-field (< 21 MV/m) I-V characteristics of the stabilized cells measured in 85 K - 300 K range fit well to a 2D thermally-activated hopping transport model, yielding hopping distances in the direction of the field and activation energies ranging from 2 nm and 0.2 eV at 85 K to 6 nm and 0.4 eV at 300 K. Hopping transport appears to be better aligned with the field direction at higher temperatures. The high-field current response to voltage is significantly stronger and displays a distinctly different characteristic: the differential resistances at different temperatures extrapolate to a single point (8.9x10-8 this http URL), comparable to the resistivity of copper at 60 K, at 65.6 +/- 0.4 MV/m. The physical mechanisms that give rise to the substantial increase in current in the high-field regime also accelerate resistance drift. We constructed field and temperature dependent conduction models based on the experimental results and integrated it with our electro-thermal finite element device simulation framework to analyze reset, set and read operations of PCM devices. 
    more » « less
  2. Phase change memory devices become practical for non-volatile storage at small dimensions due to reduced power and predictable device operation. In larger scale cells, devices can be locally melted due to filament formation and liquid filaments can be retained in parts of the cell for a long time even if most or all of the cells are initially amorphized during long fall-times. The complex amorphization and crystallization dynamics make these large cells more unpredictable and enable their applications as physically unclonable functions (PUF) [1,2]. Computational analysis of the complex amorphization-crystallization dynamics in phase change memory devices with large geometries is important to understand the evolution of phase distributions and temperature profiles during programming of these devices. In this work, we conduct electrothermal finite element simulations of reset operation on a large Ge2Sb2Te5 (GST) cell using the framework we have developed in COMSOL multiphysics [3]-[9] and analyze the complex dynamics of amorphization, nucleation and growth during electrical stress. We input voltage waveforms measured from electrical characterization of on-oxide GST line cells with bottom metal contact pads and Si3N4 capping. A 2D polycrystalline model of the experimentally measured cells (~360 nm wide, ~400 nm long and ~50 nm thick) is constructed in the simulations. Access devices are modeled using the spice models. The simulations capture some of the interplay between changes in the device resistance due to heating and phase changes and current fluctuations. 
    more » « less
  3. We characterized resistance drift in phase change memory devices in the 80 K to 300 K temperature range by performing measurements on 20 nm thick, ∼70–100 nm wide lateral Ge2Sb2Te5(GST) line cells. The cells were amorphized using 1.5–2.5 V pulses with ∼50–100 ns duration leading to ∼0.4–1.1 mA peak reset currents resulting in amorphized lengths between ∼50 and 700 nm. Resistance drift coefficients in the amorphized cells are calculated using constant voltage measurements starting as fast as within a second after amorphization and for 1 h duration. Drift coefficients range between ∼0.02 and 0.1 with significant device-to-device variability and variations during the measurement period. At lower temperatures (higher resistance states) some devices show a complex dynamic behavior, with the resistance repeatedly increasing and decreasing significantly over periods in the order of seconds. These results point to charge trapping and de-trapping events as the cause of resistance drift.

    more » « less
  4. We model the current density in a semiconductor based on the drift-diffusion transport of the charge carriers to accurately determine the thermoelectric effects in the bulk material (Thomson effect) and material junctions (Peltier effect). We utilize the model to perform 2-D finite element simulations of mushroom phase change memory cell with a critical dimension of 20 nm using temperature and electric field dependent material parameters and analyze the contributions of symmetric Joule heating and asymmetric thermoelectric heats during reset and set operations. We investigate the effect of altering the direction of current flow by changing the connection point between the cell and the access device and observe that, corresponding change in thermoelectric effects cause significant difference in operation dynamics, temperature distribution profiles, amorphous volume, energy requirement and resistance contrast between reset and set states. 
    more » « less
  5. Abstract

    Nested idealized baroclinic wave simulations at 4-km and 800-m grid spacing are used to analyze the precipitation structures and their evolution in the comma head of a developing extratropical cyclone. After the cyclone spins up by hour 120, snow multibands develop within a wedge-shaped region east of the near-surface low center within a region of 700–500-hPa potential and conditional instability. The cells deepen and elongate northeastward as they propagate north. There is also an increase in 600–500-hPa southwesterly vertical wind shear prior to band development. The system stops producing bands 12 h later as the differential moisture advection weakens, and the instability is depleted by the convection. Sensitivity experiments are run in which the initial stability and horizontal temperature gradient of the baroclinic wave are adjusted by 5%–10%. A 10% decrease in initial instability results in less than half the control run potential instability by 120 h and the cyclone fails to produce multibands. Meanwhile, a 5% decrease in instability delays the development of multibands by 18 h. Meanwhile, decreasing the initial horizontal temperature gradient by 10% delays the growth of vertical shear and instability, corresponding to multibands developing 12–18 h later. Conversely, increasing the horizontal temperature gradient by 10% corresponds to greater vertical shear, resulting in more prolific multiband activity developing ∼12 h earlier. Overall, the relatively large changes in band characteristics over a ∼12-h period (120–133 h) and band evolutions for the sensitivity experiments highlight the potential predictability challenges.

    Significance Statement

    Multiple-banded precipitation structures are difficult to predict and can greatly impact snowfall forecasts. This study investigates the precipitation bands in the comma head of a low pressure system in a numerical model to systematically isolate the roles of different ambient conditions. The results emphasize that environments with instability (e.g., air free to rise after small upward displacement) and increasing winds with height favor the development of banded structures. The forecast challenge for these bands is illustrated by starting the model with relatively small changes in the temperature field. Decreasing the instability by 10% suppresses band development, while increasing (decreasing) the horizontal temperature change across the system by 10% corresponds to the bands developing 12 h earlier (later).

    more » « less