The effect of Cr atoms: From non-stoichiometric Ge-Te to Cr2Ge2Te6

The traditional phase change material Ge-Te presents a large difference with Cr2Ge2Te6 (CrGT) in the phase change properties. Compared with Ge-Te, the two-dimensional van der Waals material CrGT presents a reversed resistance state, in which the crystalline phase is in a high-resistance state and the amorphous phase is in a low-resistance state. Nevertheless, there are some structural similarities between the two materials. CrGT may be regarded as the Cr doping of Ge-Te. It is necessary to investigate the evolution of the structure and phase change properties of Ge-Te to CrGT through Cr doping. In this study, phase change materials Ge40Te60 (GT) and CrGT were fabricated. Subsequently, a series of intermediate state films were fabricated by changing the Ge/Te/Cr cation-to-anion ratio. Surprisingly, the coexistence of the CrGT and GT phases was observed in all intermediate films. As a result, a gradual reversal in the resistance between the amorphous and crystalline states can be observed. Transmission electron microscopy experiments were also performed to characterize the grain growth and the phase separation. Unlike GT, CrGT island-like crystal grains usually have boundaries with regular shapes. The pure CrGT-based device exhibited an excellent device performance. This work may provide a useful guide for further application of the CrGT phase change material.