A thickness-dependence study on structure, oxidation, and critical temperature scaling of α -Ta(111) thin films for qubit application

Abstract

Abstract

We present a multi-technique characterization of sputter-grown α-Ta thin films on Al2O3 (001), spanning thicknesses from 2 to 250 nm. Our goal is a multi-faceted understanding of three structurally distinct regions whose properties can impact qubit performance: the substrate-film interface, the bulk Ta layer, and the self-limiting native oxide. Films were deposited at 100 W, 32 mTorr, and 650 °C using a 33 mm target via direct current sputter epitaxy. X-ray diffraction shows mono-oriented α-Ta(111) growth with lattice spacing in agreement with bulk Ta (0.331 nm). X-ray reflectometry and atomic force microscopy reveal smooth surfaces with an average roughness of 0.62 ± 0.22 nm. Films nucleate as small islands whose terrace width increases linearly with thickness up to approximately 150 nm, then saturates. Reflectometry identifies a 0.88 ± 0.14 nm pseudomorphic Ta layer at the substrate interface and a self-limiting amorphous oxide of 2.25 ± 0.20 nm at the film surface, both independent of film thickness beyond 4 nm. Electronic transport measurements using the Van der Pauw method show a smooth evolution of the superconducting critical temperature with thickness, increasing from 2.9 K for 7.5 nm films to 4.2 K for 269.2 nm films, consistent with universal thickness scaling in superconductors. Density functional theory calculations examine oxidation of the α-Ta(111) surface. Increasing oxygen content progressively disrupts crystallinity, and when the Ta:O ratio exceeds 1:1, an amorphous TaO phase forms. The simulated oxide density agrees well with reflectometry, and the calculated structural variation of the amorphous oxide (0.8 nm) matches the measured variation.