Physical mechanism and temperature acceleration of relaxation effects in phase-change memory cells

Abstract

The main reliability issues of phase change memory (PCM) cells are related to the metastable nature of the amorphous phase in the chalcogenide material. The amorphous phase spontaneously evolves during time by crystallization and structural relaxation, that is the short range ordering in the amorphous phase to minimize the defect concentration and free energy. Crystallization physics has been studied by almost years, and application of the standard nucleation theory has been successfully applied to provide extrapolation methodologies for PCM reliability estimation. On the other hand, structural relaxation and its impact on carrier transport in the amorphous chalcogenide are not completely understood yet. The purpose of this paper is to study structural relaxation effects in PCM cells, focusing on a) the physical interpretation of the relaxation and b) the temperature acceleration of the phenomenon, which is essential to provide extrapolation techniques and analytical compact models for prediction of PCM reliability.

Impact of Ge–Sb–Te compound engineering on the set operation performance in phase-change memories

“…Fig. 4 reports indeed a decreasing activation energy of conduction extracted from R measurements made at different T R on the alloys showing increasing Sb% concentration; this is the signature of a decreasing energy gap, assuming the Fermi level to be pinned around midgap due to a large concentration of defects resulting in a significant density of localized states in the energy gap [20]. Moreover as the electronic switching is controlled by a critical power at the threshold condition, the threshold voltage V T is expected to follow R, both at fixed and variable time [12,21], and hence the E A trend with composition.…”

Role of activation energy in resistance drift of amorphous phase change materials

“…Ielmini et al [25][26][27], Lavizzari et al [28] developed a model in which the transport mechanism is assumed to be thermally assisted hopping between electronic defects. In order to describe the change in the low-field resistance over time, the authors postulated an increase in the hopping distance resulting from an annihilation of electronic defects upon structural relaxation.…”

Estimation of amorphous fraction in multilevel phase-change memory cells

“…The presented method is a valuable characterization tool in order to gain more insight into the structural transformation that takes place during MLC operation, while trap density information is critical for understanding reliability issues that arise due to the resistance drift noise of the amorphous phase [8]. Alternative techniques, such as Transmission Electron Microscopy (TEM), provide accurate structural information at small length scales, but are very tedious and require extensive sample preparation.…”