Anisotropic Resistivity Size Effect in Epitaxial Mo(001) and Mo(011) Layers

Mo(001) and Mo(011) layers with thickness d = 4-400 nm are sputter-deposited onto MgO(001) and alpha-Al2O3(11 (2) over bar0) substrates and their resistivity is measured in situ and ex situ at room temperature and 77 K in order to quantify the resistivity size effect. Both Mo(001) and Mo(011) layers are epitaxial single crystals and exhibit a resistivity increase with decreasing d due to electron surface scattering that is well described by the classical Fuchs and Sondheimer model. Data fitting yields room temperature effective electron mean free paths lambda(*) = 14.4 +/- 0.3 and 11.7 +/- 0.3 nm, respectively, indicating an anisotropy with a smaller resistivity size effect for the Mo(011) orientation. This is attributed to a smaller average Fermi velocity component perpendicular to (011) surfaces, causing less surface scattering and a suppressed resistivity size effect. First-principles electronic structure calculations in combination with Boltzmann transport simulations predict an orientation dependent transport with a more pronounced resistivity increase for Mo(001) than Mo(011). This is in agreement with the measurements, confirming the effect of the Fermi surface shape on the thin-film resistivity. The predicted anisotropy lambda(001)*/lambda(011)* = 1.57 is in reasonable agreement with 1.66 and 1.23 measured at 77 and 295 K. The overall results indicate that the resistivity size effect in Mo is relatively small, with a measured product of the bulk resistivity times the effective electron mean free path rho(o)lambda* = (7.7 +/- 0.3) and (6.2 +/- 0.2) x 10(-16) Omega m(2) for Mo(001) and Mo(011) layers. The latter value is in excellent agreement with the first-principles-predicted rho(o)lambda = 5.99 x 10(-16) Omega m(2) and is 10% and 40% smaller than the reported measured rho(o)lambda for Cu and W, respectively, indicating the promise of Mo as an alternate conductor for narrow interconnects.

Automated summary

In this study, Mo(001) and Mo(011) layers were sputter-deposited onto MgO(001) and alpha-Al2O3(11 (2) over bar0) substrates with thicknesses ranging from 4 to 400 nm. The resistivity of these layers was measured in situ and ex situ at room temperature and 77 K to investigate the resistivity size effect. Both Mo(001) and Mo(011) layers showed an increase in resistivity as the thickness decreased, which was attributed to electron surface scattering. The measurements and simulations confirmed the anisotropy in resistivity size effect, with the Mo(011) orientation exhibiting a smaller effect compared to Mo(001). The overall results indicated that Mo has a relatively small resistivity size effect, making it a promising conductor for narrow interconnects.