Single-crystal Josephson junctions for experiments in quantum computation

In the interest of fabricating high-quality Josephson junctions for use in superconducting quantum bits, we have developed techniques for the growth of single-crystal Nb/Al2O3/Nb tri-layers. Each film in the tri-layer is grown by molecular beam epitaxy and examined in situ with standard RHEED analysis. The niobium base electrode grows in the (110) orientation on single-crystal A-plane sapphire and is highly crystalline with an atomically smooth surface. Subsequent growth of the alumina barrier is accomplished in an adjacent chamber by evaporation of aluminum in an oxygen background. In situ XPS studies show complete oxidation of the aluminum, while ex situ AFM reveals an atomically smooth alumina surface. Because the alumina grows in the C-plane direction, the niobium counter-electrode grows (111) and also highly crystalline. Results incorporating these tri-layer structures into Josephson junctions will be reported.