Electrical transport mechanism of the amorphous phase in Cr ₂ Ge ₂ Te ₆ phase change material

A Cr ₂ Ge ₂ Te ₆ (CrGT) phase change material (PCM) was studied. Different from conventional PCMs, it shows an inverse resistance change between a low-resistance amorphous phase and a high-resistance crystalline phase. Moreover, the anomalous low resistivity in the amorphous CrGT is considered to be due to a large carrier density, but the mechanism of electrical transport is still not clear. In this study, the electrical transport mechanism of the amorphous CrGT was discussed based on the temperature dependence of the resistivity, carrier density, mobility, and current-voltage characteristics. Above 300 K, the conduction mechanism of the amorphous CrGT was thermally activated band conduction, which is different from the conventional Ge-Sb-Te PCMs that show Poole-Frenkel conduction in the amorphous phase. Below 300 K, the amorphous CrGT shows hopping conduction, changing from variable range hopping (Mott VRH) to Efros-Shklovskii variable range hopping (ES-VRH) with decreasing temperature. The crossover from Mott VRH to ES-VRH was observed at around 200 K. Furthermore, the Fermi level was not pinned at the center of bandgap; instead, it was located near the valence band.