Cryogenic characterization of Josephson junctions

Abstract

Cryogenic characterization is a crucial part of understanding the behavior of low-temperature quantum electronics. Reliable device testing provides the feedback to fabrication process development, facilitating the rapid development of quantum devices. The research presented in this thesis explores the cryogenic testing, analysis, and characterization of a superconducting quantum device, the Josephson junction. This thesis begins with a theoretical description of superconductivity and Josephson junctions, two superconductors separated by a thin insulating battier. Two models of Josephson barriers are presented for use in analysis. The effect of self-induced magnetic field is considered. A numerical simulation is performed to justify neglecting effects of self-induced magnetic field in junctions of diameter less than the Josephson penetration depth Aj. Lincoln Laboratory's Josephson junction fabrication effort is described along with the apparatus used to test junctions at 4.2 K. Custom software used to test these junctions is then presented. The analysis of 4.2 K data is shown with a simple model of a disc as the insulating barrier. 391 valid Josephson junctions are analyzed across 16 wafers in 3 runs.

(cont.) The critical current density J is calculated to be 4.88 ± 2.81 ( ... ) for junctions with expected J of 5 ( ... ). The superconductive energy gap A is calculated to be 1.51 ± 0.31 meV. The process bias 60 is shown to be -0.35 i 0.12 ,tm. Analyzing the junctions with an alternate model taking into account pollution produces an upper bound for barrier pollution depth of approximately 60 nm. Discussion of a 300 mK apparatus is then presented. This apparatus is constructed and presently being incorporated in an existing 300 mK 3He refrigerator. Finally, the results are concluded with a discussion of advantages, and proposed initial experiments for the 300 mK apparatus.