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 similar to 10ns down to similar to 500ps, well below the critical switching time of similar to 1ns 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.