Resistivity scaling and electron surface scattering in epitaxial Co(0001) layers

Abstract

In situ and ex situ transport measurements on epitaxial Co(0001)/Al2O3(0001) layers with thickness d = 7–300 nm are used to quantify the resistivity ρ scaling due to electron surface scattering. Sputter deposition at 300 °C followed by in situ annealing at 500 °C leads to single-crystal layers with smooth surfaces (<1 nm roughness) and an epitaxial relationship: Co[0001]‖Al2O3[0001] and Co[101¯0]‖Al2O3[112¯0]. The measured ρ vs d data are well described by the classical expression by Fuchs and Sondheimer at both 295 and 77 K, yielding a temperature-independent product of the bulk resistivity times the mean free path ρo × λ and an effective room temperature λ = 19.5 ± 1.0 nm. The resistivity increases by 9%–24% upon air exposure for layers with d ≤ 21 nm, indicating a transition from partially specular (p = 0.55 ± 0.05) to completely diffuse (p = 0) surface scattering during native oxide formation. The overall results suggest that Co exhibits a resistivity scaling that is comparable to W and approximately 2× smaller than that of Cu and that the resistance of narrow Co lines can be reduced considerably by engineering the Co-liner interface to facilitate specular electron scattering.