Phase Change Materials: Challenges on the Path to a Universal Storage Device

Phase change materials, in which a material is reversibly switched between an amorphous and crystalline state with corresponding contrast in optical and electronic transport properties, are excellent nonvolatile storage media. Rewritable digital versatile disks (DVDs) and Blu-ray discs are based on such materials in which the optical contrast between the amorphous and crystalline phases enables data storage. Additionally, the large change in electronic transport properties with resistivity contrast of up to six orders of magnitude on crystallization and fast switching speed is at the heart of a new class of nonvolatile data storage devices with electronic read/write operation and potential for future miniaturization. The amorphous state is characterized by saturated covalent bonds, whereas the crystalline phase forms resonant bonds. This bonding mechanism can account for the high electronic polarizabilities that characterize crystalline phase change materials. Interestingly, the relevant electronic states...