Stopping Resistance Drift in Phase Change Memory Cells and Analysis of Charge Transport in Stable Amorphous Ge2Sb2Te5

Abstract

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 ohm.cm), 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.