Size Effects on the Cross-Plane Thermal Conductivity of Transparent Conducting Indium Tin Oxide and Fluorine Tin Oxide Thin Films

Visibly transparent and electrically conductive oxides are attractive for a wide array of applications. Indium tin oxide (ITO) and fluorine tin oxide (FTO) are the subset of the larger transparent conducting oxide family and possess transmittance in the visible spectrum as well as high electrical conductivity. Even though their unique optical and electrical properties have been thoroughly examined, the thermal transport properties, namely, thermal conductivity in the cross-plane direction, have received much less attention. In this paper, using a series of ITO and FTO thin films comprising a range of thicknesses and grain sizes, we characterize the cross-plane thermal conductivity using time-domain thermoreflectance. We determine the heat capacity of the FTO films from simultaneous measurements of volumetric heat capacity and thermal conductivity on an ~396-nm-thick FTO film. We show that the size effects have a considerable influence on the thermal conductivity from both the perspective of grain boundary and thin film scattering.