Resistivity size effect in epitaxial iridium layers

Abstract

The resistivity size effect in Ir is quantified with in situ and ex situ transport measurements at 295 and 77 K using epitaxial layers with thickness d = 5–140 nm deposited on MgO(001) and Al2O3(0001) substrates. Data fitting with the Fuchs–Sondheimer model of the measured resistivity ρ vs d for single-crystal Ir(001)/MgO(001) layers deposited at Ts = 1000 °C yield an effective electron mean free path λeff = 7.4 ± 1.2 nm at 295 K, a room-temperature bulk resistivity ρo = 5.2 μΩ cm, and a temperature-independent product ρoλeff = (3.8 ± 0.6)×10−16 Ω m2, which is in good agreement with first-principles predictions. Layers deposited at Ts = 700 °C and stepwise annealed to 1000 °C exhibit a unique polycrystalline multi-domain microstructure with smooth renucleated 111-oriented grains that are >10 μm wide for d = 10 nm, resulting in a 26% lower ρoλeff. Ir(111)/Al2O3(0001) layers exhibit two 60°-rotated epitaxial domains with an average lateral grain size of 88 nm. The grain boundaries cause a thickness-independent resistivity contribution Δρgb = 0.86 ± 0.19 and 0.84 ± 0.12 μΩ cm at 295 and 77 K, indicating an electron reflection coefficient R = 0.52 ± 0.02 for this boundary characterized by a 60° rotation about the ⟨111⟩ axis. The overall results indicate that microstructural features including strain fields from misfit dislocations and/or atomic-level roughness strongly affect the resistivity size effect in Ir. The measured ρoλeff for Ir is smaller than for any other elemental metal and 69%, 43%, and 25% below reported ρoλ products for Co, Cu, and Ru, respectively, indicating that Ir is a promising alternate metal for narrow high-conductivity interconnects.