Crystalline structure of GeTe layer in GeTe/Sb2Te3 superlattice for phase change memory

Low electric-power switching between set and reset has been confirmed to occur in a GeTe/Sb2Te3 superlattice (SL) film. This sort of operation will be essential for future phase-change random access memories. However, the crystalline structure of the SL, especially the key layer of GeTe, has not been studied sufficiently. The GeTe crystalline structure in the SL was thus investigated in XRD experiments, and the order parameter S was estimated. With the bulk crystalline structure of GeTe, S was about 1.76–1.77, which is quite different from the desirable value of around 1. To solve this unreasonable S, GeTe crystalline structures with conceivable real vacancies were modeled, and their Ss’ were estimated. Consequently, a GeTe crystalline structure with 0.7 ≦ S ≦ 1 was discovered which differed from bulk. It was also verified that the previously thought to be imaginary vacancy layer of GeTe in the SL estimated from a first-principle computer simulation is very likely to be a real vacancy layer.