Chemical phase segregation during the crystallization of Ge-rich GeSbTe alloys

Ge-rich Ge-Sb-Te alloys are materials with potential for new non-volatile memories named Phase Change Memories offering an extended range of possible applications. However, the origin of their superior properties, notably their much higher transition temperature and increased thermal stability, is unknown. Using a variety of transmission electron microscopy based techniques, we have investigated the changes that affect the structure and composition of such alloys during thermal annealing. We show that, although Ge-rich Ge-Sb-Te materials can be grown as amorphous layers of homogeneous compositions, the primary effect of annealing is to activate phase separation between stable Ge and Ge-Sb-Te phases. This phase separation starts at 380 degrees C while the material is still amorphous and leads to the nucleation of the first Ge nanocrystals. Increasing the annealing temperature to 400 and then to 450 degrees C allows the crystalline Ge phase to grow by driving the Ge excess out of the matrix, which, finally, leads to the formation of large (30-50 nm) crystals with the face-centered-cubic Ge-Sb-Te structure. After annealing at 500 degrees C for 30 minutes, the layer fully crystallizes and consists of a population of large (50-100 nm) face-centered-cubic Ge-Sb-Te crystals with a stoichiometry close to 225 buried in a matrix composed of small Ge nanocrystals. This study evidences that the superior properties of Ge-rich alloys do not result from the intrinsic properties of some Ge-rich crystalline phases but from kinetic factors. The formation of a two phase Ge/Ge-Sb-Te material involves long range diffusion of atomic species, first and foremost, Ge.