The development of thermoelectric oxide materials like Ca3Co4O9 and Nb:SrTiO3 as an alternative to SiGe compounds for high temperature applications has attracted large interest in recent years. In comparison to other heavy metal and toxic elements containing thermoelectric materials, complex metal oxides comprise extremely high chemical and temperature stability along with low toxicity and high abundance of the constituent chemical elements. The encouraging thermoelectric properties of Ca3Co4O9, the coupling of the magnetic moments of the Co spins and the quasi-two-dimensional electric transport properties can lead to novel functionalities and applications in thermoelectric and -magnetic devices. In order to build useful devices using thin films of this complex metal oxide integration into silicon technology electrical contacts with minimum electrical resistance are decisive criteria. Moreover, due to high chemical reactivity of Si with oxygen and silicide forming metals like cobalt a diffusion barrier is needed which ideally enables epitaxial growth of Ca3Co4O9 and Ir thin films on a (001)-Si substrate.
We show that Ca3Co4O9/Ir electrical contacts with very low electrical resistivity can be fabricated. Ca3Co4O9/Ir electrical contact pairs were formed by growing a conductor track of Ca3Co4O9 on crossing Ir conductor tracks. The potential drop at individual Ca3Co4O9/Ir electrical contacts was determined by combining four-wire resistance measurements eliminating Ca3Co4O9 conductors track resistances. The I-V characteristics show a slight diode like asymmetry and the corresponding contact resistivities were found to be between 1.6 and 3.6 mΩcm2.
Secondary ion mass spectrometry depth profiles show an approximately 5 nm thick layer of IrOx formed during PLD on the Ir conductor tracks by indiffusion of oxygen.
XRD pole figures of the Ca3Co4O9/Ir/YSZ/Si-substrate and Ca3Co4O9/YSZ/Si-substrate reveal a 12-fold in-plane rotational symmetry on the (001)-Ir and (001)-YSZ buffer layer, but rotated by 15°. This leads to high symmetry grain boundaries with low electrical resistivity where the Ca3Co4O9 track crosses the edge of the Ir contact conductor tracks towards the electrically insulating YSZ buffer layer. This epitaxial relationship can be explained by energetically preferred growth directions of the pseudo hexagonal CoO2 sublayers in monoclinic Ca3Co4O9 on the cubic (001)-YSZ and (001)-Ir surface. This symmetric in-plane orientation between the charge carrying CoO2 sublayer domains results in minimal in-plane resistivity of the Ca3Co4O9 thin film. In addition, we show that the influence of the azimuthal orientation on the temperature dependent Seebeck coefficient of the Ca3Co4O9 thin film is imperceptible.