Dominant Energy Carrier Transitions and Thermal Anisotropy in Epitaxial Iridium Thin Films

High aspect ratio metal nanostructures are commonly found in a broad range of applications such as electronic compute structures and sensing. The self‐heating and elevated temperatures in these structures, however, pose a significant bottleneck to both the reliability and clock frequencies of modern electronic devices. Any notable progress in energy efficiency and speed requires fundamental and tunable thermal transport mechanisms in nanostructured metals. In this work, time‐domain thermoreflectance is used to expose cross‐plane quasi‐ballistic transport in epitaxially grown metallic Ir(001) interposed between Al and MgO(001). Thermal conductivities ranges from roughly 65 (96 in‐plane) to 119 (122 in‐plane) W m−1 K−1 for 25.5–133.0 nm films, respectively. Further, low defects afforded by epitaxial growth are suspected to allow the observation of electron–phonon coupling effects in sub‐20 nm metals with traditionally electron‐mediated thermal transport. Via combined electro‐thermal measurements and phenomenological modeling, the transition is revealed between three modes of cross‐plane heat conduction across different thicknesses and an interplay among them: electron dominant, phonon dominant, and electron–phonon energy conversion dominant. The results substantiate unexplored modes of heat transport in nanostructured metals, the insights of which can be used to develop electro‐thermal solutions for a host of modern microelectronic devices and sensing structures. Thermal anisotropy and cross‐plane quasi‐ballistic thermal transport is uncovered in thin films of epitaxial iridium, revealing a transition between electron‐, phonon‐, and electron–phonon energy conversion regimes at different thicknesses. Compact modeling links the transport across neighboring interfaces and the coupling of both carriers within the metal to provide insight into the interplay of heat carriers in down‐scaled metal nanostructures.