Science doi:10.1126/science.1201938 (2011)

The surface of a rewritable digital video disk is covered in a material that can reversibly switch between crystalline and amorphous phases when irradiated by a laser. An electronic memory based on such a phase-change material would have numerous advantages, including non-volatility. However, electronic phase-change memories have required high programming currents to generate the heat necessary for a phase transformation. Now, Eric Pop and colleagues at the University of Illinois at Urbana-Champaign have constructed an electronic phase-change memory that can be programmed with currents 100 times smaller than those required by state-of-the-art devices.

The key to the advance was reducing the volume of the active phase-change material. Gaps as small as 20 nm in width were created in carbon nanotubes with diameters of less than 6 nm, and then filled with a chalcogenide phase-change material. Voltages applied across the nanotubes resulted in large electric fields across these gaps, causing sufficient Joule heating to switch the chalcogenide from a resistive to a conducting state while drawing as little as 1 μA of current. Switching back to a resistive state took as little as 5 μA. This translated to a minimum energy per bit of 100 femtojoules, with the potential to scale to lower values for smaller gap sizes.