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Chinese Physics Letters

Studies of all-niobium electrode Josephson tunnel junctions with amorphopus silicon barriers

Meng Xiaofan, Wang Xiewen

Published Aug 1, 1987• 0 citations
Publisher

Abstract

This paper reports the fabrication and properties of small Nb-(a-Si)-Nb-(a-Si)-Nb Josephson tunnel junctions. The smallest junction area is about 0.3μm 2 . A thin a-Si film deposited by electron beam evaporation was oxidized and used for the tunnel barriers. TEM and XPS investigations indicate that the barrier layers consist of SiO x and Si. The junctions have been used for RF SQUID with an energy resolution of 1.9 × 10 -28 J/Hz.

DOI
10.1088/0256-307x/4/8/009

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This paper reports the fabrication and properties of small Nb-(a-Si)-Nb-(a-Si)-Nb Josephson tunnel junctions. The smallest junction area is about 0.3μm 2 . A thin a-Si film deposited by electron beam evaporation was oxidized and used for the tunnel barriers. TEM and XPS investigations indicate that the barrier layers consist of SiO x and Si. The junctions have been used for RF SQUID with an energy resolution of 1.9 × 10 -28 J/Hz.
Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, V m (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.
Tunnel-junction dc SQUIDs are being fabricated from Nb films using electron-beam lithography to define linewidths as small as 0.5 μm. The barriers in the Nb-NbOx-Nb junctions are grown in an oxygen plasma discharge, and the Pd2Si resistive shunts are formed by annealing Pd on the Si substrates. These devices have excellent storage and recycling properties. At a measurement frequency of 40 kHz, a SQUID with an inductance of about 1 nH, a critical current of about 4 μA, and a parallel shunt resistance of about 15 Ω had an intrinsic energy resolution of 2.5×10−32 J Hz−1 at 4.2 K and 1.1×10−32 J Hz−1 at 1.6 K, in good agreement with a model of the dc SQUID.
The Josephson junction is a crucial element in superconducting devices, and niobium is a promising candidate for the superconducting material due to its large energy gap relative to aluminum. AlOx has long been regarded as the highest quality oxide tunnel barrier and is often used in niobium-based junctions. Here, we propose ZrOx as an alternative tunnel barrier material for Nb electrodes. We theoretically estimate that zirconium oxide has excellent oxygen retention properties and experimentally verify that there is no significant oxygen diffusion leading to NbOx formation in the adjacent Nb electrode. We develop a top–down, subtractive fabrication process for Nb/Zr–ZrOx/Nb Josephson junctions, which enables scalability and large-scale production of superconducting electronics. Using cross-sectional scanning transmission electron microscopy, we experimentally find that depending on the Zr thickness, ZrOx tunnel barriers can be fully crystalline with chemically abrupt interfaces with niobium. Further analysis using electron energy loss spectroscopy reveals that ZrOx corresponds to tetragonal ZrO2. Room temperature characterization of fabricated junctions using Simmons’ model shows that ZrO2 exhibits a low tunnel barrier height, which is promising in merged-element transmon applications. Low temperature transport measurements reveal sub-gap structure, while the low-voltage sub-gap resistance remains in the megaohm range.
We report on a technology for the fabrication of sub-micrometer sized cross-type Josephson tunnel junctions in niobium technology. We present the fabrication scheme and properties of cross-type junctions with linear dimensions from 10 down to 0.6 µm. Sidewall passivation of the junctions is achieved by anodization as well as by planarizing the junctions with SiO in a self-aligned deposition step. The measured ratio of the sub-gap resistance to the normal resistance is about 35. Because of their low sub-gap current and low parasitic capacitance such junctions are well suited for applications like high resolution SQUIDs.