We describe some characterization methods for the determination of qualitative parameters of superconducting Josephson effect devices. We propose the introduction of a new method to characterize the junctions at the ambient temperature. This method accounts the effects of the single electron tunneling. We also used SEM techniques for technological analysis of the devices.
We describe some characterization methods for the determination of qualitative parameters of superconducting Josephson effect devices. We propose the introduction of a new method to characterize the junctions at the ambient temperature. This method accounts the effects of the single electron tunneling. We also used SEM techniques for technological analysis of the devices.
This work is devoted to the study of tunnel Josephson superconductor-insulator-superconductor (SIS) junctions with a new type of shunting based on the usage of an additional superconductor-insulator-normal metal (NIS) junction located around the SIS junction. Numerical calculations of the parameters of such shunted junctions were carried out and modeling of their IVC (current-voltage characteristics) was performed. The designed samples were manufactured, their parameters were studied. To investigate the behavior of junctions under the influence of high-frequency signals in the sub-THz range, their IVCs were measured. Keywords: Superconducting devices, superconductor-insulator-superconductor tunnel junction, Josephson effect, shunting of Josephson junctions.
This thesis presents results focused on fabrication and electrical characterization of superconducting tunnel junctions and Josephson junctions, e.g. measuring the current–voltage response and 1/f noise from different samples at different temperatures. Junctions were fabricated inside a vacuum chamber where metal evaporation through a resist mask was used to create submicron-sized junctions. The tunneling barrier was common between all different samples, i.e. in-situ oxidized aluminium between the metal depositions. Different material combinations were used, e.g. Al–AlOₓ–Al, Al–AlOₓ–Al:Mn and Al–AlOₓ–Nb. The manganese doping in aluminium (Al:Mn) suppresses the superconductivity making the Al:Mn parts normal, while pure aluminium is superconducting, creating a normal metal–insulator–superconductor (NIS) device. The qualities of junctions were improved, if possible, by vacuum thermal annealing at 400 °C, which was found both to stabilize aluminium and aluminium–manganese based junctions and to reduce the characteristic 1/f noise in them. These results are desired, since for most applications it is important to have a junction with stable parameters over long time and 1/f noise is limiting the sensitivity of many devices. However, the procedure is not for granted, since most other NIS material combinations didn't survive the annealing treatment. We studied Al–AlOₓ–Nb junctions at cryogenic temperatures (≈ 100 mK) either as an SIS' device, or as an NIS device by suppressing the superconductivity of aluminium with magnetic field, and observed excess sub-gap current, which could not be explained by standard tunneling theory with the Dynes model.
Charge states in mesoscopic Josephson devices are considered promising candidates for a solid-state implementation of quantum computing. We studied three superconducting single electron transistors (SSETs) fabricated with small Al/AlO x /Al tunnel junctions, which, with a different choice of the construction parameters, can realize both the quantum bit and the readout system for the qubit charge state. We varied the junction capacitance and barrier transparency, making the Josephson energy E J of the device smaller, comparable and larger than the charging energy E C : the first case is suitable to build the qubit, while the latter can be suitable for the qubit readout.
We present a systematic study of the temperature-dependent transport properties of Nb- and Al-based superconductor--insulator--thin superconductor--ferromagnet--superconductor (SIsFS) Josephson junctions. By varying the superconducting material and the thickness of the intermediate superconducting layer, we identify three distinct transport regimes: a tunnel-dominated SIS-like regime, a temperature-driven crossover, and a complex weak-link regime. A unified Usadel-based description connects these regimes through the self-consistent pair potential of the intermediate s-layer. While for Al-based devices an SIS-like transport regime is observed, a self-consistent calculation of the s/F/S proximity electrode, combined with perturbative tunnel coupling to the outer electrode, accounts for Nb devices with different thickness of the superconducting interlayer. The calculation places the 10-nm-interlayer sample in a proximity-supported, strongly suppressed s state, close to the SInFS limit in which the interlayer is effectively driven normal (\textit{n}). A reduced nonlinear long-junction description captures the same limiting physics. The evolution of I_cR_N(T) thereby provides a direct experimental signature of the three transport regimes. Beyond the fundamental interest, the results establish practical design guidelines: Nb junctions with sufficiently thick s interlayers are well suited for high-speed cryogenic memory elements and tunable superconducting parametric amplifiers, while Al junctions are natural candidates for coherent superconducting quantum circuits
We report on the measurement results of the superconducting-ferromagnetic transistors (SFTs). The devices were made at Northwestern University and Hypres (SeeQC), Inc. (Nevirkovets et al., 2014; 2015). SFT is a multiterminal device with the SIS'FIFS (or SFIFS'IS) structure (where S, I, and F denote a superconductor, an insulator, and a ferromagnetic material, respectively) exploiting intense quasiparticle injection in order to modify the nonlinear I-V curve of a superconducting tunnel junction. SFT is capable of providing voltage, current, and power amplification while having good input/output isolation. We characterized the devices using different measurement techniques. We measured S parameters of the single- and double-acceptor devices at frequencies up to 5 MHz. Importantly, we confirmed that the isolation between the input and output of the device is quite good. However, the techniques typically employed to characterize semiconductor devices do not allow for revealing the full potential of our low-resistive SFT devices, especially those having two acceptors. In the latter case, we also tested the devices using the battery-powered current sources with floating grounds. Analyzing double-acceptor I-V curves recorded at different levels of injection currents, for an optimal load, we deduced a small-signal voltage gain of 33 and a power gain of 2.4. We suggest that further improvement of the SFT device parameters is possible in optimized devices, so that the device potentially may serve as a preamplifier for readout of output signals of cryogenic detectors and be useful as an element of other superconductor-based circuits. In addition, we used scanning transmission electron microscopy to identify some problems in the fabrication of the devices without any planarization.
Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200nm down to 70nm were fabricated from standard Nb∕AlOx∕Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of Δ≈1.35meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles, and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5fF to 4fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100nm×100nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.