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arXiv:2409.07227 · IEEE International Conference on Quantum Computing and Engineering (QCE)

Development of TiN/AlN-Based Superconducting Qubit Components

Benedikt Schoof, Moritz Singer, Simon Lang, H. M. Gupta, Daniela Zahn, Johannes Weber, Marc Tornow

Published Sep 15, 2024• 1 citation
arXivHTMLPDF

Abstract

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 TiN$I$AlN$I$TiN junctions yielded values ranging from 150$\mu \mathbf{A}$to 2$\mu \mathbf{A}$, 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.

DOI
10.1109/qce60285.2024.00145

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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.
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.
The development of large-scale quantum processors benefits from superconducting qubits that can operate at elevated temperatures and be fabricated with scalable, foundry-compatible processes. Atomic layer deposition (ALD) is increasingly being adopted as an industrial standard for thin-film growth, particularly in applications requiring precise control over layer thickness and composition. Here, we report superconducting qubits based on NbN/AlN/NbN trilayers deposited entirely by ALD. By varying the number of ALD cycles used to form the AlN barrier, we achieve Josephson tunneling through barriers of different thicknesses, with critical current density spanning seven orders of magnitude, demonstrating the uniformity and versatility of the process. Owing to the high critical temperature of NbN, transmon qubits based on these all-nitride trilayers exhibit microsecond-scale relaxation times, even at temperatures above 300 mK. These results establish ALD as a viable low-temperature deposition technique for superconducting quantum circuits and position all-nitride ALD qubits as a promising platform for operation at elevated temperatures.
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.
Aluminum-based Josephson junctions are currently the main sources of nonlinearity for control and manipulation of superconducting qubits. A constriction-based junction provides an alternative source of nonlinearity that promises new types of protected qubits and the possibility of high-temperature and high-frequency operation through the use of superconductors with larger energy gaps. Junctions made from such superconductors have been challenging to incorporate into superconducting qubits because of difficulty controlling junction parameters and have had extremely low lifetimes, which limited their utility. Here we demonstrate that junctions made using titanium nitride (TiN) are a promising and controllable qubit platform. We use TiN junctions to build superconducting qubits based on quantum phase slips through the junction. We operate the qubit at zero flux where the qubit frequency ( ~ 17GHz) is mainly determined by the inductance of the qubit. We perform readout and coherent control of the superconducting qubit, and measure qubit lifetimes >60μs. Finally, we demonstrate operation of the qubit at temperatures exceeding 300 mK. Our results add the TiN-based junction as a tool for superconducting quantum information processing and opens avenues for new classes of superconducting qubits. Superconducting constrictions are an alternative source of nonlinearity to Al-based Josephson junctions, enabling new functionalities for qubits. Here, the authors demonstrate a high-frequency superconducting qubit in a single TiN thin film, based on coherent quantum phase slip tunneling through a constriction.
Titanium nitride (TiN) is a known superconducting material that is attractive for use as passive components in superconducting circuits for both conventional and quantum information devices. In contrast to conventional synthesis techniques, here, plasma-assisted molecular beam epitaxy is reported to produce high-quality TiN on bare silicon wafers. Using a rf-plasma source to crack the nitrogen molecules and a conventional high-temperature effusion cell for titanium, TiN growth is completed under nitrogen-rich conditions. The growth and nucleation is monitored in situ, while the structure and composition are characterized using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and scanning transmission electron microscopy. The stoichiometric TiN (111) films sit on an amorphous nitride layer with low impurity concentrations. The films superconduct with Tc=5.4 K, and coplanar waveguide resonators are fabricated with a small center width of 6 μm that demonstrate single-photon quality factors approaching 1M and high-power quality factors over 5M without observing saturation.