  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.
