  Resistance drift in phase change materials is characterized in amorphous
phase change memory line-cells from 300 K to 125 K range and is observed to
follow the previously reported power-law behavior with drift coefficients in
the 0.07 to 0.11 range in dark. While these drift coefficients measured in dark
are similar to commonly observed drift coefficients (~0.1) at and above room
temperature, measurements under light show a significantly lower drift
coefficient (0.05 under illumination versus 0.09 in dark at 150K). Periodic
on/off switching of light shows sudden decrease/increase of resistance,
attributed to photo-excited carriers, followed by a very slow response (~30
minutes at 150 K) attributed to contribution of charge traps. Continuation of
the resistance drift at low temperatures and the observed photo-response
suggest that resistance drift in amorphous phase change materials is
predominantly an electronic process.
