Thermoelectric Properties of Oxygen-Tuned ALD-Grown [Ca₂CoO₃]_0.62[CoO₂] Thin Films

Abstract

Thin films of the p-type thermoelectric misfit-layered oxide, [Ca 2 CoO 3 ] 0.62 [CoO 2 ], were prepared for the first time by means of the atomic layer deposition (ALD) technique using Ca(thd) 2 , Co(thd) 2 , and O 3 as precursors. As-deposited films were amorphous; however, with heat treatment in an O 2 gas flow, well-crystallized highly c-axis-oriented [Ca 2 CoO 3 ] 0.62 [CoO 2 ] films were obtained. The oxygen content of the O 2 -annealed film was further controlled through a reductive N 2 -annealing. Because the degree of reduction was dependent on the annealing temperature, the choice of N 2 -annealing temperature provided us with a tool for precise tuning of the oxygen content. With decreasing oxygen content, the lattice parameter (c) and the Seebeck coefficient (S) were found to increase. The room-temperature S values were 113 and 128 μV/K for the oxygen-richest sample and the mostreduced sample, respectively.

SrCoO 3−δ thin films by atomic layer deposition

“…A natural explanation would be the effect of steric hindrance as the Co(thd) 2 precursor is adsorbed as Co(thd) while Mn(thd) 3 is most likely adsorbed as Mn(thd) 2 . In line with this, in case of the thermoelectric misfit-layered Ca 3 Co 4 O 9 phase, the pulsing ratio of 3:4 for the Ca(thd) 2 and Co(thd) 2 precursors yielded the desired Ca:Co ratio of 3:4 in the films [23]. The non-equal growth has been observed for e.g.…”

“…In ALD processes reported so far for binary and ternary cobalt oxides, Co(thd) 2 [22][23][24][25][26] Co(Cp) 2 [25,27,28] and Co(acac) 3 [29,30] have been primarily used as precursors for cobalt. In our preliminary experiments, we first tested Co(thd) 2 but it turned out not to work well with Sr(thd) 2 : the process was very sensitive to the large temperature gradient in the reactor, resulting in somewhat inhomogeneous films.…”

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

“…In addition, thin films can promote the reduction of thermal conductivity by phonon scattering effect [6], and facilitates Micro-Electro-Mechanical System (MEMS) processing towards on-chip devices [7]. Epitaxial or c-axis oriented Ca 3 Co 4 O 9 thin films have been prepared by various techniques, such as radiofrequency (RF) sputtering [8], pulsed laser deposition (PLD) [9,10], atomic layer deposition (ALD) [11] and topotactic ion-exchange method [12]. The in-plane power factor of a pure Ca 3 Co 4 O 9 thin film has been reported to reach 0.51 mW m −1 K −2 at around 920 K, which is higher than the value of textured bulk ceramics, and comparable to that of single crystals [13].…”

Mechanism of Formation of the Thermoelectric Layered Cobaltate Ca3Co4O9 by Annealing of CaO–CoO Thin Films

“…Less resistive transport of charge carriers (e.g. 2 mΩcm at room temperature) [10] is practically realized in the basal plane of single crystalline [ 12 ] and metal-organic CVD, [ 13 , 14 ] as well as chemical solution deposition methods followed by annealing, [ 15 , 16 , 17 ] and reactive solid-phase epitaxy by topotactic-ion exchange. [18,19] However, PVD has broad advantages compared to chemical methods since PVD operates far from thermodynamic equilibrium and offers atomistic control of the deposition flux.…”