Temperature Dependent Behavior of Thermal Conductivity of Sub-5 nm Ir film: Defect-electron Scattering Quantified by Residual Thermal Resistivity

Abstract

By studying the temperature-dependent behavior of electron thermal conductivity (k) in a 3.2 nm-thin film, we quantify the extremely confined defect-electron scattering and reveal the intrinsic phonon-electron scattering that is shared by bulk Ir. At low temperatures below 50 K, the thermal conductivity of the thin film has almost two orders of magnitude reduction from that of the bulk Ir. The thermal conductivity of the film increases with increasing temperature while that of the bulk Ir has an opposite trend. We introduce a unified thermal resistivity to interpret this completely different k-T relation. This residual thermal resistivity provides an unprecedented way to quantitatively evaluating defect-electron scatterings in heat conduction. The interfacial thermal conductance across the grain boundaries is found larger than that of the Al/Cu interface. Its value is proportional to temperature largely because of the electron's specific heat. A unified interfacial thermal conductance is defined and firmly proves such relation. The electron reflection coefficient is found to be large (88%) and almost temperature independent. This means most of the electrons which scatter with the grain boundary would be reflected back and the scatterings are not affected by temperature.