Threshold switching and phase transition numerical models for phase change memory simulations

Abstract

A comprehensive numerical model for chalcogenide glasses is presented, coupling a physically based electrical model able to reproduce the threshold switching with a local nucleation and growth algorithm to account for the phase transition dynamics. The main ingredients of the chalcogenide physics are reviewed and analyzed through simplified analytical models, providing a deeper insight on the origin of the threshold switching mechanism in chalcogenide glasses. A semiconductorlike three-dimensional full-coupled numerical implementation of the proposed model is finally presented and its capabilities to quantitatively reproduce the key elements of the Ge2Sb2Te5 chalcogenide physics are demonstrated in the framework of phase change memory device simulations.

Crystal growth within a phase change memory cell

“…2a), finiteelement simulations indicate that the maximum temperature is reached very close to the bottom electrode [27][28][29] . This is due to the significant asymmetry between the dimensions of the top and bottom electrodes.…”

Threshold switching via electric field induced crystallization in phase-change memory devices

“…3(a) is the I-V curve obtained using a "conventional" simulation approach, i.e., assuming the amorphous phase conductivity is electric field dependent but that the electric field does not contribute to the free energy (Method I); as expected, characteristic threshold switching is observed with the threshold voltage being around 1 V, in line with published experimental results for this type of device. 5,6,27 Also shown in Fig. 3(a) is the I-V curve obtained when the field dependent conductivity has been removed from the simulation (curve B); here no switching is evident even for relatively high applied voltages (3 V case shown, but no switching was observed even when the maximum voltage applied was increased to 4 V).…”

“…Mainstream understanding of this switching phenomenon is that it is initiated electronically via the influence of high electric fields on the interband trap states. [5][6][7] However, recent work has suggested that field induced (crystal) nucleation could instead be responsible, [7][8][9][10][11][12] and models for such field-induced nucleation were able to explain several experimental observations on PCM devices, such as the occurrence of relaxation oscillations 8,9 and the relationship between switching voltage (and temperature) and switching delay time. 10 Most models of field-induced nucleation presented to date have concentrated on the role of the electric field in lowering the nucleation barrier and the associated critical nucleus size, an approach extended recently by ourselves to include a fuller kinetic treatment that can identify field ranges where electric field effects might play a significant role in the crystallization of "bulk" samples.…”

“…The rate at which each of these three events occur is determined by the system energy, which is usually described in terms of the Gibbs free energy G, where G ¼ (Ar À Vg) and A and V are the surface area and volume, respectively, of a crystal cluster, r is the surface energy, and g is the bulk free energy difference (2012) between phases. Traditionally (i.e., in most theoretical treatments of crystallization in phase-change materials and devices 2,5,14 ), the bulk energy difference term g is considered to be purely temperature dependent (for example, as…”

The Design of Rewritable Ultrahigh Density Scanning-Probe Phase-Change Memories

“…Their potential application as alternatives to silicon solid state memories in the form of the so-called PCRAM (phase-change RAM) devices is also a source of intense recent research [15]- [18]. In both optical storage and PCRAM device applications a particular attraction of phase-change materials is their re-writeability, since the basic storage mechanism of switching between amorphous and crystalline phases is inherently reversible.…”