Epitaxial aluminum oxide thin films on niobium (110): A study of their growth and their use in superconducting tunnel-junctions.

摘要

Epitaxial hetero-structures have been turned to in recent years in an attempt to eliminate charge fluctuations in Josephson junction-based quantum bits. Amorphous tunnel-barriers have large numbers of two-level fluctuators that can couple to the quantum states in the device. Single-crystal tunnel-barriers on the other hand show a reduced density of these two-level fluctuators. For my thesis research I studied the hetero-epitaxy of aluminum oxide on niobium, and the use of these aluminum oxide thin films as tunnel-barriers. Single-crystal Nb/Al2O3 and Nb/Al2O3/Nb multi-layers were grown by molecular beam epitaxy and characterized using a variety of materials analysis techniques. Atomically-flat niobium films grown on A-plane sapphire were used for the subsequent hetero-epitaxy of aluminum oxide. Depending on the thermal treatment used during growth, these niobium films had two distinct surface reconstructions. Using in situ RHEED I found that the growth of Al2O3 was highly dependent on the surface reconstruction of the underlying niobium film. Alumina films on annealed Nb had a hexagonal lattice that was under an isotropic tensile strain that relaxed with increasing film thickness. In contrast, alumina films on unannealed Nb showed an asymmetric strain and a pseudo-hexagonal lattice closely resembling that of the underlying niobium. Epitaxial niobium over-layers also tiled these two alumina surfaces differently, with in-plane orientations suggestive of irregular cation sublattices in the oxide film. It was also found using TEM imaging that an atomically sharp interface existed between the aluminum oxide film and the base niobium layer. However, mixing was seen to occur with the deposition of the over-layer, causing a deterioration of the alumina barrier. This mixing had a profound effect on tunnel-junctions fabricated from these tri-layers. Devices showed very poor qualities overall, with current-voltage characteristics indicative of a large density of electrical pinholes. With these findings in mind, some potential solutions are proposed.