Electron mean free path in epitaxial Ta(001) layers
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Authors
Guan, Daniel
Issue Date
2013-05
Type
Electronic thesis
Thesis
Thesis
Language
ENG
Keywords
Materials engineering
Alternative Title
Abstract
Epitaxial tantalum layers were grown on MgO(001) by ultrahigh vacuum magnetron sputter deposition at 650 °C to thicknesses d ranging from 5 nm to 2 μm. X-ray diffraction including ω-2θ, ω rocking curve, and φ scans confirm that the layers are Ta(001) single crystals, while X-ray reflectivity indicates that their roughness is < 0.1 nm for short lateral length scales. The measured room temperature resistivity increases from 12.6±0.6 μω-cm for large d to 42.1±0.9 μω-cm for d = 5 nm. This increase is well described by the Fuchs-Sondheimer model, yielding a Ta room temperature bulk electron mean free path of λ_294K = 28±1 nm. Corresponding measurements in liquid nitrogen yield λ_77K = 65±5 nm. These values are approximately an order of magnitude larger than 2.35 and 8.36 nm, respectively, expected from the free electron model. This large discrepancy is attributed to the highly non-spherical Ta Fermi surface. More importantly, the ratio of the measured mean free path λ_77K/ λ_294K = 2.32 is 1.5× smaller than the ratio of the measured bulk resistivities ρ_294K/ρ_77K. This suggests that either (i) the mean free path is strongly band dependent, (ii) surface roughness contributes greatly to electron scattering at low temperatures in films of thickness on the order of the mean free path, or (iii) the electron mean free path at low temperature is restricted by the presence of impurities, evidenced by a measured residual resistivity.
Description
May 2013
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY