Electrical properties of thermoelectric cobalt Ca_3Co_4O_9 epitaxial heterostructures

摘要

Heterostructures fabricated from layered cobalt oxides offer substantial advantages for thermoelectric applications. C-axis-oriented Ca_3Co_4O_9 (CCO) thin films on SrTiO_3 substrates and Ca_3Co_4O_9/SrTi_(0.993)Nb_(0.007)O_3 p-n heterojunctions were fabricated by pulsed laser deposition. The measurements of in-plane resistivity, thermopower, and magnetic properties performed on the Ca_3Co_4O_9 thin films were found to be comparable to ab-plane those of the single crystals due to good orientation of the films. The temperature dependence of the electrical transport properties of Ca_3Co_4O_9/SrTi_(0.993)Nb_(0.007)O_3 p-n heterojunction was also investigated. The junction shows two distinctive transport mechanisms at different temperature regimes under forward bias: tunneling across the Schottky barrier in the temperature range of 100-380 K, and tunneling mechanism at low bias and thermal emission mechanism at high bias between 10 and 100 K. However, for the case of low reverse bias, the trap assisted tunneling process should be considered for the leakage current. Negative magnetoresistance effect is observed at low temperatures, related to the electron spin-dependent scattering and the interface resistance of the heterostructures.