Phase Change Characteristics in GeTe-CuTe Pseudobinary Alloy Films

Phase change characteristics in GeTeCuTe pseudobinary alloy films, including GeCu2Te3 (GCT), were investigated. The crystallization temperature of the amorphous film increased with increasing Cu content and then decreased again with further increasing Cu content. X-ray diffraction measurements revealed that the amorphous films with a composition range from 0 to 18.6 atom % Cu crystallized to GeTe single-phase, while the amorphous films with a composition range from 26.2 to 37.9 atom % Cu crystallized to GCT single-phase. The amorphous films with a composition between 21.1 and 24.3 atom % Cu showed a clear two-step crystallization during heating and finally formed the GeTe and GCT mixed-phase. The amount of volume shrinkage upon crystallization of the amorphous film decreased with increasing Cu contents, and the film with around 23 atom % Cu showed almost no volume change after crystallization. Meanwhile, film with over 25 atom % Cu showed volume expansion. It was found that no volume change was achieved by a two-step crystallization, namely, initial crystallization to GCT (volume expansion) with subsequent crystallization to GeTe (volume shrinkage). The film with no volume change also showed no reflectance change upon crystallization. The obtained results suggest that nonstoichiometric Ge(50-x)CuxTe(50) films such as 26.2Cu film showing a small volume change without phase separation are preferable in terms of cyclability of phase change random access memory.