Chalcogenide Phase-Change Materials: Past and Future

Phase-change (PC), nonvolatile memory (NVM) materials, such as Ge₂Sb₂Te₅ (GST), encode stored digital binary data as structural changes in the material. Reversible, ultrafast (nanosecond) transformations between metastable semiconducting/nonreflective amorphous and near-metallic/reflective crystalline states occur as a result of Joule heating, caused by applied voltage/laser write pulses, in electronic/optical NVMs, respectively. The canonical PCM, GST, was originally discovered and developed for optical NVM; although it also works as an electronic NVM (phase-change random-access memory, PCRAM) material, nevertheless it is not optimal in this role. In this contribution, I will discuss the effects of “doping” (or chemical modification) of GST, with elements such as nitrogen or first-row transition metals, to improve PC characteristics, such as crystal grain size, and optical-reflectivity contrast for optical NVM, and to introduce additional PC functionality, for example, magnetism for electronic NVM, respectively. These studies have been carried out by ab initio molecular-dynamics (AIMD) simulations, in which new compositions of doped PCMs have been obtained by a process of in silico materials discovery. I will also describe a strategy to decrease the rate-limiting crystallization time of PCMs, which does not also decrease long-term data retention in the amorphous state (i.e., deleterious spontaneous crystallization), by means of the use of “priming” prepulses. AIMD simulations indicate the structural-ordering origin of this priming behavior. In this way, we have reduced the crystallization time of GST, from ~10 ns down to ~500 ps, well below the critical switching time of ~1 ns needed to replace volatile Si-based CMOS DRAM by an NVM-equivalent, so-called “universal memory”. Finally, I will outline very recent work which demonstrates that GST PCM cells can also be used to perform “in-memory” Boolean logic operations, thereby combining memory and processing operations in the same cell.