ICT2014 Abstracts

The diffusion barrier with robust thermal stability to reduce interfacial ion diffusion between copper (Cu) and lead telluride (PbTe) is important for the use of high-temperature thermoelectric module. However, low Gibbs energy of formation of PbTe facilitates the generation of telluride-based compound near barrier/PbTe interface that largely degrades the diffusion blocking ability of barrier layer. In addition to common used nickel (Ni) barrier, the titanium metal (Ti) with low work function of 3.9 eV is much lower than 5.1 eV of Ni. The low metal work function and small conduction band offset on thermoelectric PbTe with narrow bandgap of 0.32 eV is favorable to form an ohmic contact. To avoid the Ti oxidation issue, relatively stable bilayer structures such as Ti/Au or Ti/WN have been proposed and used in thermoelectric BiSbTe system. However, a further study on interface diffusion mechanism for high-temperature thermoelectric PbTe system is less mentioned. Thus, we investigate the interfacial thermodynamic properties between Ti barrier and thermoelectric PbTe using first-principles calculations in this work. We calculated the interface formation energies of Ti/PbTe film structures to reveal the energetic favorable interface configurations. The formation energy was computed by relaxing 2x2x1 supercell with constrained PbTe bulk, where a unit cell was defined to include eight Pb, eight Te, and four Ti atoms. The interface was modeled by a superlattice including one interface and an around 1.6 nm vacuum layer on top. Then the total energy calculation was performed by the ab initio code VASP with a plane-wave basis to investigate the interface structure stability. From the simulated electronic properties, the more energetically stable PbTe structure was obtained after capping a one-atomic-layer Ti metal layer. To simultaneously verify these simulation results, the related process experiments with detailed material analysis are also carried out at various thermal budgets.