Why DVDs work the way they do: The nanometer-scale mechanism of phase change in Ge–Sb–Te alloys

Present-day multimedia strongly relies on re-writable phase-change optical memory. We have found that Ge2Sb2Te5 (GST), the material of choice in DVD-RAM, does not possess the conventional rock-salt structure as previously believed but that Ge and Sb atoms are displaced from the ideal rocksalt positions. Amorphization of both GeTe and GST results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in short-range order. The order–disorder transition in GeTe and GST is primarily due to a flip of Ge atoms from octahedral positions into tetrahedral positions without rupture of strong covalent bonds. The driving force for the flip are strained Ge–Te bonds in the crystalline phase. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions of cycles.