Co/GeTe interfacial reactions and Co-Ge-Te phase equilibria

Background: GeTe is an important medium-temperature thermoelectric material, and Co is often used as a diffusion barrier layer. This study examines the interfacial reactions between Co and GeTe to verify the suitability of Co as a barrier layer. This study also determines the phase diagram of the Co-Ge-Te ternary system to provide basic material system knowledge and for better understanding the interfacial reactions. Methods: The Co-Ge-Te phase diagram is determined based on the experimental results of this study, those related in the literature, and the phase diagrams of its three binary constituent systems. The Co/GeTe couples are prepared by plating Co layer on the GeTe substrate, and then reacted at 500 °C and 400 °C. Significant findings: It is found the Ge_0.5Te_0.5 is of rhombohedral structure at 500 °C, gradually transforms to a cubic structure with increasing Te content, and the results differ from those in the literature. There are 15 single phases at 500 °C and 400 °C in the Co-Ge-Te ternary system, including two ternary compounds, Co₂Ge₃Te₃ and CoGeTe. In the Co/GeTe couples reacting at 500 °C for 14 days, Co₂Te₃, Co₅Ge₃, CoGe and CoGeTe are formed, and the thickness of the reaction layer is about 80 μm. At 400 °C for 50 days, CoGe₂, Co₅Ge₃ and Co₂Te₃ are formed, and the thickness of the reaction layer is about 12 μm. At both 500 °C and 400 °C, Co is the primary diffusion species in the Co/GeTe reaction couples. The results suggest Co is not a suitable barrier layer for GeTe at 500℃.