Mixed-conduction mechanism of Cr₂Ge₂Te₆ film enabling positive temperature dependence of electrical conductivity and seebeck coefficient

A Cr₂Ge₂Te₆ (CrGT) film is known to be a phase change material showing low-energy phase change between amorphous and crystalline phases. A crystalline Cr₂Ge₂Te₆ (CrGT) film obtained from an amorphous phase via metastable crystalline phase has been reported to possess different local structure from the CrGT bulk material. The bulk CrGT is known to show a band conduction, while the conduction mechanism of the CrGT film has not been clear yet. In this study, the conduction mechanism of the CrGT film was discussed based on the temperature dependence of the conductivity (σ), Seebeck coefficient (S), and Hall properties. It was found that the CrGT film shows a variable range hopping conduction below 170 ​K, while it shows a mixed-conduction of band-like conduction and nearest-neighbor hopping conduction above 300 ​K. Such the mixed-conduction mechanism was found to cause the positive temperature dependence of the σ and S in the CrGT film. Furthermore, the thermal conductivity of the CrGT film was measured to be 0.16 ​W/mK, which is much lower than that of the bulk material. The mixed-conduction mechanism and the low thermal conductivity introduced by nanostructuring through sputtering method is suggested to promising strategy to enhance the thermoelectric properties of the CrGT material.