Unexpected interfacial reactions in Co/Sb2Te3 and Co/GeTe couples

Background: Sb2Te3 and GeTe are important thermoelectric materials. Co/Sb2Te3, Co/GeTe, Ni/Sb2Te3 and Ni/ GeTe couples were prepared by electroplating Co and Ni on Sb2Te3 and GeTe substrates, and reacted at 500 degrees C, 400 degrees C and 300 degrees C. Surprisingly, in the couples reacted at 500 degrees C, in addition to reactions at the Co/Sb2Te3 and Co/GeTe interfaces, reaction phases are also found on the outer surface of the electroplated Co which has no direct contact with the thermoelectric substrate. These unexpected reactions are not found in the couples plated with Ni and in the couples plated with Co reacted at 400 degrees C and 300 degrees C. The compositions of the reaction phases on the outer surface of Co are similar to those at the Co/Sb2Te3 and Co/GeTe interfaces.Methods: Systematic investigations were carried out using three different types of reaction couples.Significant findings: The reaction phases on the outer surface of Co result from Te vaporization of the thermo-electric substrates, which are not completely covered with Co. The Te vapor of Sb2Te3 and GeTe provides the Te reactant source of the reaction phases. However, at temperatures of 400 degrees C and 300 degrees C, the Te vapor pressure is not high enough to cause interfacial reactions on the bare Co surface. The formation of the reaction phases on the outer surface of Co can thus be inhibited with a fully-covered Co layer on the Sb2Te3 and GeTe substrates and by reducing the reaction temperatures. In both the Co/Sb2Te3 and Co/GeTe couples, the reaction layers exhibit linear growth rates with aging time.

Automated summary

The reactions at the outer surface of Co in the Co/Sb2Te3 and Co/GeTe couples at 500°C are caused by Te vaporization of the thermoelectric substrates, which are not fully covered with Co. This phenomenon does not occur in the Ni/Sb2Te3 and Ni/GeTe couples, as well as in the Co/Sb2Te3 and Co/GeTe couples reacted at lower temperatures. The formation of reaction phases on the outer surface of Co can be inhibited by fully covering Co with the thermoelectric substrates and reducing the reaction temperatures.