material is now in the amorphous "ON" state, where the material is still amorphous, but in a pseudo-crystalline electric state. In crystalline state, the IV characteristics is ohmic. There had been debate on whether threshold switching was an electrical or thermal process. There were suggestions that the exponential increase in current at threshold voltage must have been due to generation of carriers that vary exponentially with voltage such as impact ionization or tunneling. Nano-timescale phase change Recently, much research has focused on the material analysis of the phase-change material in an attempt to explain the high speed phase change of GeSbTe. Using EXAFS, it was found that the most matching model for crystalline GeSbTe is a distorted rocksalt lattice and for amorphous a tetrahedral structure. The small change in configuration from distorted rocksalt to tetrahedral suggests that nano-timescale phase change is possible as the major covalent bonds are intact and only the weaker bonds are broken. Using the most possible crystalline and amorphous local structures for GeSbTe, the fact that density of crystalline GeSbTe is less than 10% larger than amorphous GeSbTe, and the fact that free energies of both amorphous and crystalline GeSbTe have to be around the same magnitude, it was hypothesized from density functional theory simulations that the most stable amorphous state was the spinel structure, where Ge occupies tetrahedral positions and Sb and Te occupy octahedral positions, as the ground state energy was the lowest of all the possible configurations. By means of Car-Parrinello molecular dynamics simulations this conjecture have been theoretically confirmed. Nucleation-domination versus growth-domination Another similar material is AgInSbTe. It offers higher linear density, but has lower overwrite cycles by 1-2 orders of magnitude. It is used in groove-only recording formats, often in rewritable CDs. AgInSbTe is known as a growth-dominated material while GeSbTe is known as a nucleation-dominated material. In GeSbTe, the nucleation process of crystallization is long with many small crystalline nuclei being formed before a short growth process where the numerous small crystals are joined together. In AgInSbTe, there are only a few nuclei formed in the nucleation stage and these nuclei grow bigger in the longer growth stage such that they eventually form one crystal. References Alloys Chalcogenides DVD Germanium compounds Non-oxide glasses Optical materials Antimony compounds Tellurium compounds