Abstract
Samples of superconductor–insulator–superconductor (SIS) and normal metal–insulator–superconductor (NIS) junctions with superconducting aluminum of different thickness were fabricated and experimentally studied, starting from conventional shadow evaporation with a suspended resist bridge. We also developed alternative fabrication by magnetron sputtering with two-step direct e-beam patterning. We compared Al film grain size, surface roughness, resistivity deposited by thermal evaporation and magnetron sputtering. The best-quality NIS junctions with large superconducting electrodes approached a resistance R(0)/R(V2Δ) factor ratio of 1000 at 0.3 K and over 10,000 at 0.1 K. At 0.1 K, R(0) was determined completely by the Andreev current. The contribution of the single-electron current dominated at V > VΔ/2. The single-electron resistance extrapolated to V = 0 exceeded the resistance R(V2Δ) by 3 × 109. We measured the influence of the magnetic field on NIS junctions and described the mechanism of additional conductivity due to induced Abrikosov vortices. The modified shape of the SINIS bolometer IV curve was explained by Joule overheating via NIN (normal metal–insulator–normal metal) channels.
Highlights
To date, the common material for superconducting electronics is Niobium and its nitrides, NbN and NbTiN
The superconducting properties of Al films, tunnel normal metal–insulator– superconductor (NIS) and SIS junctions and devices can be affected by the magnetic field, which suppresses resistance, and leads to the overheating and degradation of all the parameters of fabricated SINIS detectors and coolers, NIS thermometers, SIS Josephson junctions and SQUIDs
Such films can be fabricated by cross-type lithography with thermal evaporation, or magnetron sputtering with double patterning
Summary
The common material for superconducting electronics is Niobium and its nitrides, NbN and NbTiN. Contrary to Nb, in Al, film contaminations can provide fine tuning of the transition temperature from 1.2 K in pure bulk to the range of 0.1 to 2.4 K [1]. This is the first-type superconductor, and for relatively thick films, there are no penetrated vortices, associated flux pinning/depinning, or accompanied excess noise, contrary to thin aluminum films below 100 nm, which can demonstrate some features of superconductor type-2 [2]. Thin aluminum film in the magnetic field can show some features of the type-2 superconductor with flux pinning, additional losses, current leak, and overheating in tunnel junctions. The presence of an additional conductivity mechanism via Andreev reflection makes the IV curve more complicated compared to single-electron tunneling in ideal NIS junctions
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