  We electrically characterized melt-quenched amorphized Ge2Sb2Te5 (GST)
phase-change memory cells of 20 nm thickness, ~66-124 nm width and ~100-600 nm
length with and without photoexcitation in 80-275 K temperature range. The
cells show distinctly different current-voltage characteristics in the
low-field (< ~19 MV/m), with a clear response to optical excitation by red
light (wavelength = 613 nm), and high-field (> ~19 MV/m) regimes, with very
weak response to optical excitation. The reduction in carrier activation energy
with photoexcitation in the low-field regime increases from ~10 meV at 80 K to
~50 meV at 150 K (highest sensitivity) and decreases again to 5 meV at 275 K.
The heterojunctions at the amorphous-crystalline GST interfaces at the two
sides of the amorphous region lead to formation of a potential well for holes
and a potential barrier for electrons with activation energies in the order of
0.7 eV at room temperature. The alignment of the steady state energy bands
suggests formation of tunnel junctions at the interfaces for electrons and an
overall electronic conduction by electrons. When photoexcited, the
photo-generated holes are expected to be stored in the amorphous region,
leading to positive charging of the amorphous region, reducing the barrier for
electrons at the junctions and hence the device resistance in the low-field
regime. Holes accumulated in the amorphous region are drained under high
electric fields, hence the cells' response to photoexcitation is significantly
reduced. These results support electronic origin of resistance drift in
amorphous GST.
