Josephson junctions are used as active devices in superconducting electronics and quantum information technology. Outstanding properties are their distinct non-linear electrical characteristics and a usually sinusoidal relation between the current and the superconducting phase difference across the junction. In general the insertion of ferromagnetic material in the barrier of a Josephson junction is associated with a suppression of superconducting correlations. But also new phenomena can arise which may allow new circuit layouts and enhance the performance of applications. This thesis presents a systematic investigation for two concepts to fabricate Josephson junctions with a rather uncommon negative critical current. Such devices exhibit an intrinsic phase slip of Pi between the electrodes, so they are also known as Pi-junctions. Both studies go well beyond existing experiments and in one system a Pi-junction is shown for the first time. All the thin film junctions are based on superconducting Nb electrodes. In a first approach, barriers made from Si and Fe were investigated with respect to the realisation of Pi-junctions by spin-flip processes. The distribution of Fe in the Si matrix was varied from pure layers to disperse compounds. The systematic fabrication of alloy barriers was facilitated by the development of a novel timing-based combinatorial sputtering technique for planetary deposition systems. An orthogonal gradient approach allowed to create binary layer libraries with independent variations of thickness and composition. Second, Nb | AlOx | Nb | Ni60Cu40 | Nb (SIsFS) double barrier junctions were experimentally studied for the occurrence of proximity effect induced order parameter oscillations. Detailed dependencies of the critical current density on the thickness of s-layer and F-layer were acquired and show a remarkable agreement to existing theoretical predictions. Especially a variation of jc and IcRn by the s-layer thickness up to the value of nonmagnetic SIS junctions is notable. Additionally information on the emergence of superconductivity with the s-layer thickness was acquired.
Josephson junctions are used as active devices in superconducting electronics and quantum information technology. Outstanding properties are their distinct non-linear electrical characteristics and a usually sinusoidal relation between the current and the superconducting phase difference across the junction. In general the insertion of ferromagnetic material in the barrier of a Josephson junction is associated with a suppression of superconducting correlations. But also new phenomena can arise which may allow new circuit layouts and enhance the performance of applications. This thesis presents a systematic investigation for two concepts to fabricate Josephson junctions with a rather uncommon negative critical current. Such devices exhibit an intrinsic phase slip of Pi between the electrodes, so they are also known as Pi-junctions. Both studies go well beyond existing experiments and in one system a Pi-junction is shown for the first time. All the thin film junctions are based on superconducting Nb electrodes. In a first approach, barriers made from Si and Fe were investigated with respect to the realisation of Pi-junctions by spin-flip processes. The distribution of Fe in the Si matrix was varied from pure layers to disperse compounds. The systematic fabrication of alloy barriers was facilitated by the development of a novel timing-based combinatorial sputtering technique for planetary deposition systems. An orthogonal gradient approach allowed to create binary layer libraries with independent variations of thickness and composition. Second, Nb | AlOx | Nb | Ni60Cu40 | Nb (SIsFS) double barrier junctions were experimentally studied for the occurrence of proximity effect induced order parameter oscillations. Detailed dependencies of the critical current density on the thickness of s-layer and F-layer were acquired and show a remarkable agreement to existing theoretical predictions. Especially a variation of jc and IcRn by the s-layer thickness up to the value of nonmagnetic SIS junctions is notable. Additionally information on the emergence of superconductivity with the s-layer thickness was acquired.
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
To increase the integration scale of superconductor electronics, we are developing a new node, titled SFQ7ee, of the fabrication process at MIT Lincoln Laboratory. In comparison to the existing SFQ5ee node, we increased the number of fully planarized superconducting layers to ten and utilized NbN and NbN/Nb kinetic inductors to increase the possible inductor number density above a hundred million inductors per cm2. Increasing the Josephson junction number density to the same level requires implementing self-shunted high-JcJosephson junctions. We investigated the properties of Nb/NbNx/Nb trilayer junctions as a potential replacement of high-JcNb/Al-AlOx/Nb junctions, where NbNxis a disordered, overstoichiometric, nonsuperconducting nitride deposited by reactive sputtering. Dependences of theIcRnproduct and Josephson critical current density, Jc on the NbNx barrier thickness and temperature were studied in the thickness range from 5 nm to 20 nm. The fabricated junctions can be well described by the microscopic theory of SNS junctions, assuming no suppression of the energy gap in Nb electrodes near the NbNxinterfaces and a Cooper pair decay length in the NbNx barrier, ξnTcof about 2.3 nm. Current-voltage characteristics of the junctions are well described by the resistively and capacitively shunted junction (RCSJ) model without excess current. In the studied rangeJc2, the Nb/NbNx/Nb junctions have lower values of the specific resistanceRnA, lower IcRn products, and a stronger dependence of theIcRnon temperature than the self-shunted or critically damped externally shunted Nb/Al-AlOx/Nb junctions with the same critical current density; A is the junction area,Icthe junction critical current, and Rnthe effective shunting resistance.
The authors fabricated high quality Nb∕Al2O3∕Ni0.6Cu0.4∕Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2O3 tunnel barrier allows one to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc≈700 at T=2.11K. The critical current density in the π state was up to 5A∕cm2 at T=2.11K, resulting in a Josephson penetration depth λJ as low as 160μm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces.
The performance and coherence of superconducting quantum systems are fundamentally tied to the quality of the materials from which they are built. In superconducting transmon qubits, many loss mechanisms and sources of decoherence originate from material imperfections or unwanted oxide formation. Likewise, semiconductor–superconductor hybrid systems depend on high-quality interfaces to ensure strong coupling between electronic states in the two materials. In such hybrid devices, the accessible operating regime is often dictated by the intrinsic material properties of either the semiconductor or the superconductor. Advancing quantum device performance therefore requires exploring new material systems and optimizing their physical characteristics. In this thesis, we present the development of several superconducting material platforms for quantum technologies, including superconducting qubits, single-photon detectors, and semiconductor superconductor hybrid devices. We focus on the growth and characterization of superconducting films and heterostructures, and we establish clear connections between their material properties and the resulting device performance. First, we will present a novel semiconductor-superconductor hybrid material based on in-situ deposited epitaxial layers of Nb and Al on a shallow InAs quantum well. In this structure, the Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs two dimensional electron gas. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms. Then, we discuss a different approach in the next chapter where we develop a hybrid material based on a molecular beam epitaxy grown InAsSb surface quantum well with an in-situ deposited Nb layer directly on top of the quantum well. Relative to conventional Al-InAs based systems, the InAsSb surface quantum well offers a lower effective mass and stronger spin-orbit interaction, while the Nb layer has a higher critical temperature and a larger critical magnetic field. The in-situ deposition of the Nb layer directly on top of the quantum well results in a high-quality interface that enables strong coupling to the InAsSb quantum well. Transport measurements on Josephson junctions reveal an induced superconducting gap of 1.3 meV and a critical current density of 3.75 × 10³A/cm². Furthermore, a planar asymmetric SQUID is realized, exhibiting gate-tunable superimposed oscillations originating from both the individual Josephson junction and the full SQUID loop. The large induced superconducting gap combined with strong spin-orbit interaction position this material as an attractive platform for experiments exploring gate-tunable superconductivity and topological superconducting devices. In the next chapter, we will focus on establishing a link between Nb film crystal properties and their performance in superconducting circuits. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10⁶ at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasi-particle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Then, we explore yet a different material platform by attempting to realize onchip NbTiN single photon detectors on LiNbO₃, an emerging photonic platform with high potential for scalable quantum information processing. We use DC reactive magnetron sputtering to grow high-quality NbTiN thin films and investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature (𝑇𝑐) of 12.3 K grown at a substrate temperature of 825 K. We demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LiNbO3 platform and investigate the obtained interface quality. Finally, we fabricate superconducting nanowire single photon detectors from the NbTiN film on LiNbO₃ and characterize the detector performance. Lastly, we report a gate-tunable Josephson diode effect in hybrid nanowire junctions consisting of a spin-orbit-coupled semiconductor core coated with epitaxial ferromagnetic insulator and superconductor shells. The wires display a hysteretic superconducting window as a function of axial magnetic field. In the superconducting regime, the devices exhibit nonreciprocal current transport, with the diode efficiency showing a strong dependence on back-gate voltage. The effect persists in a remanent magnetization state following a controlled demagnetization procedure, establishing zero-field operation. The combination of results presented in this thesis stress the importance of a material-focused approach in advancing the performance of quantum hardware. The novel hybrid material platforms developed in this thesis expand the operating regime of devices, which could enable future experiments involving gate-tunable superconductivity or topological properties. Similarly, we demonstrate that optimizing superconductor crystal structure benefits quantum devices, such as single-photon detectors and superconducting resonators.
New technology for superconductor integrated circuits has been developed and is presented. It employs diffusion stop-layers (DSLs) to protect Josephson junctions (JJs) from interlayer migration of impurities, improve JJ critical current (Ic) targeting and reproducibility, eliminate aging, and eliminate pattern-dependent effects in Icand tunneling characteristics of Nb/Al/AlOx/Nb junctions in integrated circuits. The latter effects were recently found in Nb-based JJs integrated into multi layered digital circuits. E.g., it was found that Josephson critical current density (Jc) may depend on the JJ's environment, on the type and size of metal layers making contact to niobium base (BE) and counter electrodes (CE) of the junction, and also change with time. Such Jcvariations within a circuit reduce circuit performance and yield, and restrict integration scale. This variability of JJs is explained as caused by hydrogen contamination of Nb layers during wafer processing, which changes the height and structural properties of AlOxtunnel barrier. Redistribution of hydrogen impurities between JJ electrodes and other circuit layers by diffusion along Nb wires and through contacts between layers causes long-term drift of Jc. At least two DSLs are required to completely protect JJs from impurity diffusion effects-right below the junction BE and right above the junction CE. The simplest and the most technologically convenient DSLs we have found are thin (from ~3 nm to ~10 nm) layers of Al. They were deposited in-situ under the BE layer, thus forming an Al/Nb/Al/AlOx/Nb penta-layer, and under the first wiring layer to junctions' CE, thus forming an Al/Nb wiring bi-layer. A significant improvement of Jcuniformity on 150-mm wafer has also been obtained along with large improvements in Jctargeting and run-to-run reproducibility.
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.