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

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 °C, 400 °C and 300 °C. Surprisingly, in the couples reacted at 500 °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 °C and 300 °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. Systematic investigations were carried out using three different types of reaction couples. The reaction phases on the outer surface of Co result from Te vaporization of the thermoelectric 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 °C and 300 °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.