Optical Spectroscopic Analysis of ICP Nitridation for SIS Junctions With AlN Barriers

Abstract

High quality, low-leakage superconductorinsulatorsuperconductor (SIS) junctions with Nb electrodes and aluminum oxide barriers are widely reported in the literature, and have become integral in the design and fabrication of various superconducting circuits. However, as current densities are increased, aluminum oxide based tunnel barriers show excess leakage currents due to defects and pinholes in the barrier layer. First reported by our group in 2007, AlN tunnel barriers grown via inductively coupled plasma (ICP) nitridation of Al overlayers are a promising alternative, producing low-leakage Nb/Al-AlN/Nb SIS junctions, with current densities in excess of 30 kA/cm 2 . A correlation between junction quality and plasma dissociation has been reported for Nb/Al-AlN/Nb junctions produced via an electron cyclotron resonance (ECR) plasma. In this work, a quantitative measure of the relative dissociation (RD) of an ICP is determined through the use of a commercially available Ocean Optics USB4000 Optical Spectrometer. The effects of various ICP parameters on the RD, and a correlation between the RD and quality of the resulting Nb/Al-AlN/Nb junctions are reported.Index Terms-Aluminum nitride, inductively coupled plasma, superconductorinsulatorsuperconductor (SIS), spectroscopy. 1051-8223

In situ spectroscopic ellipsometry of SIS barrier formation

“…F While the oxidation of Al films is well researched and the room temperature growth is solely a function of oxygen pressure and exposure time, plasma nitridation is an inherently more complex process, with multiple nitrogen species available for AlN growth. Correlation between plasma conditions and the quality of Nb/Al-AlN/Nb junctions produced with an ECR plasma have been reported, and most recently a similar trend has been observed by our research group for AlN junctions grown via ICP [2][3][4]. As new ICP nitridation conditions are investigated during optimization of growth processes, we note the AlN growth rate is heavily dependent upon process parameters.…”

Surface morphology control of Nb thin films by biased target ion beam deposition

“…The preparation of single-crystal like niobium thin films has been a goal of experimentation for many years due to the extensive use of these materials in superconducting devices such as Josephson junctions [1][2][3][4]…”

“…The preparation of single-crystal like niobium thin films has been a goal of experimentation for many years due to the extensive use of these materials in superconducting devices such as Josephson junctions [1][2][3][4] . A recently proposed Josephson Magnetic Random Access Memory (JMRAM) devices require very smooth surface of superconducting Nb electrodes on top of which magnetic multilayer structures are deposited [3][4][5][6] , and ideally the root-mean-square roughness of Nb thin films should be less than 0.3 nm to ensure the proper magnetic characteristics of magnetic multilayers on top.…”