Thermoelectric properties of Ca₃Co₄O₉ and Ca_2.8Bi_0.2Co₄O₉ thin films in their island formation mode

Abstract

Ca3Co4O9 and Ca2.8Bi 0.2Co4O9 thin films were fabricated on LaAlO3 (LAO) substrate using pulsed laser deposition technique and were studied for their thermoelectric (TE) properties in Stranski-Krastanov mode for the first time. The thin films consisted of 3D clusters/islands on a 14-nm thick 2D layer with cluster density being higher for Ca 2.8Bi0.2Co4O9 thin films. The clusters also represent areas of dislocation and therefore act as carrier scattering centers, which leads to a temperature-activated type conductivity. Seebeck coefficient as high as 136 and 163 u V/K was measured for the Ca 3Co4O9 and Ca2.8Bi 0.2Co4O9 thin films, respectively, which is among the highest reported values for this system. The 3D island formation was also found to be useful in reducing the thermal conductivity of the thin film/substrate system by increased phonon scattering. This work shows that the island formation in thin films can be utilized as a means of enhancing TE properties of a thin film system, however, a detailed work including optimization of the film thickness and cluster/inland density is required. thin films, respectively, which is among the highest reported values for this system. The 3D island formation was also found to be useful in reducing the thermal conductivity of the thin film/substrate system by increased phonon scattering. This work shows that the island formation in thin films can be utilized as a means of enhancing TE properties of a thin film system, however, a detailed work including optimization of the film thickness and cluster/inland density is required.

Thermoelectric properties of the misfit cobaltate Ca$_3$Co$_4$O$_9$

“…32, 8 • 10 −16 s. Expressed in a different way, we find σ/τ to be given by ≃ 0.37 • 10 15 (mΩ • cm • s) −1 , which is by about a factor of four larger than the corresponding value of Lemal et al [32]. Note, however, that the reported resistivities vary considerably, ranging from ≃ 2 for single crystals [5] to ≃ 170 mΩ • cm for thin films fabricated on an LAO substrate using the PLD technique [22]. Another group, also using PLD but on (001)-silicon with a thin epitaxial yttria-stabilized zirconia (YSZ) buffer layer, reports ≃ 2 − 13 mΩ • cm, depending on the microstructure [25].…”

Thermoelectric properties of the misfit cobaltate Ca3Co4O9

“…The results are expected to be applicable for not too low temperature, T > ∼ 200 K. In order to explain the measured room-temperature resistivity [4], we have chosen the relaxation time to be given by 3 [32]. Note, however, that the reported resistivities vary considerably, ranging from ≃ 2 for single crystals [5] to ≃ 170 mΩ · cm for thin films fabricated on an LAO substrate using the PLD technique [22]. Another group, also using PLD but on (001)-silicon with a thin epitaxial yttria-stabilized zirconia (YSZ) buffer layer, reports ≃ 2 − 13 mΩ · cm, depending on the microstructure [25].…”