Metal-insulator transition and interfacial thermal transport in atomic layer deposited Ru nanofilms characterized by ultrafast terahertz spectroscopy

Abstract Ruthenium nanofilms are a promising material for wide applications in nanoelectronics, such as ultrathin electrodes and metallization. However, little is known about their electrical and thermal properties. Here, we utilize ultrafast optical-pump terahertz-probe (OPTP) spectroscopy to characterize thickness- and substrate-dependent properties of ruthenium nanofilms. Atomic layer deposition produces ruthenium nanofilms with precisely controlled thicknesses and provides different morphologies of ruthenium islands on different substrates. First, we directly observe a thickness-dependent metal–insulator transition, revealed by a sign change in the OPTP signals near a critical film thickness of about 10 nm. This phase transition originates from the formation of electrical percolation networks of ruthenium islands, providing key information for film-thickness optimization in nanodevice designs. Second, OPTP reveals interfacial thermal transport from ruthenium to various substrates (sapphire, fused silica, and MgO). The thermal dissipation exhibits strong substrate dependence, attributed to the different morphologies of ruthenium islands and quality of the ruthenium-substrate interface. The observed dynamics are reproduced by simulation of spatiotemporal temperature distributions, providing the interface thermal conductance, a key design parameter in the thermal management of nanodevices. This work provides novel insight into the physical properties of ruthenium nanofilms and their dependence on the thickness and the morphology of the films.