  We measure power dissipation in phase change memory (PCM) devices by scanning
Joule ex-pansion microscopy (SJEM) with ~50 nm spatial and 0.2 K temperature
resolution. The temperature rise in the Ge2Sb2Te5 (GST) is dominated by Joule
heating, but at the GST-TiW contacts it is a combination of Peltier and current
crowding effects. Comparison of SJEM and electrical characterization with
simulations of the PCM devices uncovers a thermopower ~350 uV/K for 25 nm thick
films of face centered-cubic crystallized GST, and contact resistance ~2.0 x
10^-8 Ohm-m2. Knowledge of such nanoscale Joule, Peltier, and current crowding
effects is essential for energy-efficient design of future PCM technology.
