Resistivity scaling in epitaxial MAX-phase Ti4SiC3(0001) layers

Abstract

In situ transport measurements on 5.8–92.1 nm thick epitaxial Ti4SiC3(0001) layers are used to experimentally verify the previously predicted low resistivity scaling. Magnetron co-sputtering from three elemental sources at 1000 °C onto 12-nm-thick TiC(111) nucleation layers on Al2O3(0001) substrates yields epitaxial growth with Ti4SiC3(0001) || Al2O3(0001) and Ti4SiC3(101¯0) || Al2O3(21¯1¯0), a low and thickness-independent surface roughness of 0.6 ± 0.2 nm, and a measured stoichiometric composition. The room-temperature resistivity ρ increases slightly with decreasing thickness, from ρ = 35.2 ± 0.4 to 37.5 ± 1.1 μΩ cm for d = 92.1–5.8 nm, and similarly from 9.5 ± 0.2 to 11.0 ± 0.4 μΩ cm at 77 K, indicating only a minor effect of electron surface scattering on ρ. Data analysis with the classical Fuchs–Sondheimer model yields a room-temperature bulk resistivity ρo = 35.1 ± 0.4 μΩ cm in the basal plane and suggests effective mean free paths λ = 1.1 ± 0.6 at 293 K and λ = 3.0 ± 2.0 nm at 77 K if assuming completely diffuse electron surface scattering. First-principles calculations predict an anisotropic Ti4SiC3 Fermi surface and a product ρoλ = 19.3 × 10−16 Ω m2 in the basal plane. This value is six times larger than that predicted previously and five times larger than the measured temperature-independent effective ρoλ = (3.8 ± 2.1) × 10−16 Ω m2. This deviation can be explained by a high experimental electron scattering specularity of p = 0.8 for Ti4SiC3(0001) surfaces. Air exposure causes a 4% room-temperature resistivity increase for d = 5.8 nm, indicating a decrease in the surface scattering specularity Δp = −0.19. The overall results show that Ti4SiC3 is not directly applicable as an interconnect material due to its relatively large ρo. However, the particularly small resistivity scaling with an effective λ that is more than an order of magnitude smaller than that of Cu confirms the potential of MAX phase materials for high-conductivity narrow interconnects.