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Superconducting Properties of the NbN and TiN alloy system.

Abimael Santos-Cotto

Published Jan 1, 2019• 0 citations
Publisher

Abstract

The field of quantum computation has advanced greatly in the recent years, with materials being tuned to address issues like qubit lifetime, and coherence time. Superconducting qubits based on Josephson Junctions are a promising approach for improved qubit design, yet they are made with polycrystalline and amorphous materials. Transition metal nitrides, like TiN and NbN, offer good chemical and mechanical properties as well as the possibility of a lattice matched junction, with AlN as the insulating layer. We have characterized MBE grown NbxTi1-xN films (with x = 0, 0.45, and 0.78) and a Nb2N film by XRD and AFM. This thesis will focus on the superconducting properties: critical temperature (Tc), superconducting transition width (ΔTc), and critical field (Hc). Two methods have been used: four-point probe DC measurements in an adiabatic demagnetization refrigerator (ADR) were used for Tc and ΔTc measurements, and a magnetic properties measurement system (MPMS) which employs a SQUID magnetometer was employed for Tc and Hc measurements. Values of Tc ranged from 4.2 K for TiN to 15.2 K for the x= 0.45 film, and 14.8 for x=0.78, while the Nb2N had Tc of 9.9 K, as found by DC measurements. These values agreed with those found by MPMS. The films’ critical fields were found to be ~69.6 Oe for the TiN film and ~250 Oe for x=0.78. These results start to demonstrate the trends of the superconducting properties of alloyed NbxTi1-xN superconductors.

DOI
10.13016/wxo4-q4b0

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The field of quantum computation has advanced greatly in the recent years, with materials being tuned to address issues like qubit lifetime, and coherence time. Superconducting qubits based on Josephson Junctions are a promising approach for improved qubit design, yet they are made with polycrystalline and amorphous materials. Transition metal nitrides, like TiN and NbN, offer good chemical and mechanical properties as well as the possibility of a lattice matched junction, with AlN as the insulating layer. We have characterized MBE grown NbxTi1-xN films (with x = 0, 0.45, and 0.78) and a Nb2N film by XRD and AFM. This thesis will focus on the superconducting properties: critical temperature (Tc), superconducting transition width (ΔTc), and critical field (Hc). Two methods have been used: four-point probe DC measurements in an adiabatic demagnetization refrigerator (ADR) were used for Tc and ΔTc measurements, and a magnetic properties measurement system (MPMS) which employs a SQUID magnetometer was employed for Tc and Hc measurements. Values of Tc ranged from 4.2 K for TiN to 15.2 K for the x= 0.45 film, and 14.8 for x=0.78, while the Nb2N had Tc of 9.9 K, as found by DC measurements. These values agreed with those found by MPMS. The films’ critical fields were found to be ~69.6 Oe for the TiN film and ~250 Oe for x=0.78. These results start to demonstrate the trends of the superconducting properties of alloyed NbxTi1-xN superconductors.
This paper presents the fabrication and characterization of superconducting qubit components from titanium nitride (TiN) and aluminum nitride (AlN) layers to create Josephson junctions and superconducting resonators in an all-nitride architecture. Our methodology comprises a complete process flow for the fabrication of TiN/AlN/TiN junctions, characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), ellipsometry and DC electrical measurements. We evaluated the sputtering rates of AlN under varied conditions, the critical temperatures of TiN thin films for different sputtering environments, and the internal quality factors of TiN resonators in the few-GHz regime, fabricated from these films. Overall, this offered insights into the material properties critical to qubit performance. Measurements of the dependence of the critical current of the TiNIIIAlNIIITiN junctions yielded values ranging from 150μA\mu \mathbf{A}μAto 2μA\mu \mathbf{A}μA, for AlN barrier thicknesses up to ca. 5 nm, respectively. Our findings demonstrate advances in the fabrication of nitride-based superconducting qubit components, which may find applications in quantum computing technologies based on novel materials.
We investigated full-epitaxial TiN/AlN/TiN Josephson junctions on MgO substrates for superconducting qubit applications. The critical temperature (TC) of TiN film is relatively low at around 5 K, but its lattice constant is about 0.424 nm, which is close to the lattice constant of MgO of 0.421 nm. TiN and AIN films were prepared by DC magnetron sputtering in a load-lock sputtering system with an ultra-high vacuum chamber. The deposition temperature was varied from ambient temperature to 1073 K. In XRD analysis, (200) peaks were observed in both the TiN single layer film and the TiN/AIN/TiN trilayer film. No other XRD peaks were observed in the single layer or trilayer films. The lattice constant of TiN was determined to be 0.4242 nm from XRD analysis, close to the value of 0.4212 nm for MgO. The 150 nm-thick single-layer TiN film on the MgO substrate showed a TCof 5.3 K and a resistivity of 3.5 μΩcm at 10 K. Based on these epitaxial TiN films, we fabricated TiN/AIN/TiN Josephson junctions and measured their current-voltage characteristics. At 1.9 K, the electrical parameters of junctions with JC= 50 A/cm2showed that the gap voltage and the ratio of Rsg/RNwere about 2.5 mV and 2.1, respectively.
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.
Abstract Superconducting qubits are a promising quantum computing platform, but significant challenges remain, such as achieving sufficiently long and stable coherence times and precise control of circuit parameters. Further improvements in material processing are essential to overcome these challenges. This paper introduces our research on superconducting quantum circuit technology based on nitride superconductors, where the core technologies are epitaxial growth of titanium nitride (TiN) on silicon (Si) substrates and epitaxial Josephson junctions using niobium nitride (NbN), aluminum nitride (AlN), and magnesium oxide (MgO). We have established a fabrication process of NbN/AlN/NbN epitaxial junctions on Si (100) substrates using a TiN film as a buffer layer and are working on applying this process to superconducting qubits. We describe recent results on the effect of junction annealing and discuss the advantages and potential of epitaxial Josephson junctions for realizing large-scale superconducting quantum circuits.
Abstract Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminum-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on silicon substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation, resulting in fewer two-level fluctuators, feasibility for epitaxial tunnel barriers that reduce energy relaxation and dephasing, and a larger superconducting gap of ~5.2 meV for NbN, compared to ~0.3 meV for aluminum, which suppresses the excitation of quasiparticles. By replacing conventional MgO by a silicon substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time $${T}_{1}=16.3\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 = 16.3 μ s and a spin-echo dephasing time $${T}_{2}=21.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 2 = 21.5 μ s . These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to the previously reported $${T}_{1}\approx {T}_{2}\approx 0.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 ≈ T 2 ≈ 0.5 μ s of NbN-based qubits on MgO substrates. These results are an important step towards constructing a new platform for superconducting quantum hardware.
The superconducting Josephson phase qubit device was fabricated by a NbN/AlN-based technology with the goal to carry out the spectroscopy of parasitic quantum two-level systems (TLSs) in the AlN tunnel barrier by combining qubit spectroscopy with mechanical strain tuning. So far all existing results on TLS spectroscopy have been obtained on Al-based Josephson qubits and thus only TLSs in amorphous aluminum oxide barriers were studied. Understanding the physics of TLS and TLS-induced decoherence requires a study of individual TLSs in tunnel barriers made from different materials. The layout of the NbN/AlN device was optimized to improve the magnetic flux coupling between the flux bias coil and the qubit loop and to minimize the unwanted coupling to the dc-readout SQUID. The area of the SiO insulation under wiring layer was made as small as possible in order to decrease the number of TLSs which can contribute to the suppression of the coherence time. The NbN/AlN Josephson phase qubit was tested in a 3He/4He dilution refrigerator at a temperature of about 60 mK by measuring Rabi oscillations and the energy relaxation time TD. Rabi oscillations had a mean decay time of about of 3-5 ns. We performed systematic studies of TD by varying the power and duration of a resonant microwave pulse. We observe a large enhancement of the qubit's relaxation time for longer microwave pulses. We explain this effect by microwave induced suppression of spontaneous qubit emission.