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IEEE Transactions on Applied Superconductivity

Effect of Post Deposition Annealing on the Structural and Electrical Properties of NbTiN Thin Films Deposited by Reactive Bias Target Ion Beam Deposition Technique

Tannaz Farrahi, Michael E. Cyberey, Michael B. Eller, Arthur W. Lichtenberger

Published May 9, 2019• 9 citations
Publisher

Abstract

Nb-based superconducting alloys, such as NbN and NbTiN, can have a superconducting energy gap near twice that of elemental Nb, thus, being ideal candidates for low-loss superconducting circuits operating above the gap frequency of Nb. We are particularly interested in these materials for THz-frequency superconducting-insulating-superconducting (SIS) mixers, kinetic inductance detectors, and traveling wave kinetic inductance parametric amplifier devices. We recently reported the use of an alternative synthesis technique, reactive bias target ion beam deposition (RBTIBD), used to realize high-quality NbTiN films and high-energy gap NbTiN/AlN/NbTiN SIS heterostructures. In this paper, the effect of post-deposition annealing on the electrical and structural properties of room temperature deposited RBTIBD NbTiN was investigated. Our room temperature RBTIBD NbTiN films were annealed using rapid thermal annealing from 200 to 1000 °C. The change in structural and electrical properties was characterized by X-ray diffraction, atomic force microscopy, and four-point probe measurements. The annealed films demonstrated an enhancement in transition temperature (Tc), increased crystallinity, larger grain size, and significantly reduced intrinsic stress.

DOI
10.1109/tasc.2019.2910024

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Nb-based superconducting alloys, such as NbN and NbTiN, can have a superconducting energy gap near twice that of elemental Nb, thus, being ideal candidates for low-loss superconducting circuits operating above the gap frequency of Nb. We are particularly interested in these materials for THz-frequency superconducting-insulating-superconducting (SIS) mixers, kinetic inductance detectors, and traveling wave kinetic inductance parametric amplifier devices. We recently reported the use of an alternative synthesis technique, reactive bias target ion beam deposition (RBTIBD), used to realize high-quality NbTiN films and high-energy gap NbTiN/AlN/NbTiN SIS heterostructures. In this paper, the effect of post-deposition annealing on the electrical and structural properties of room temperature deposited RBTIBD NbTiN was investigated. Our room temperature RBTIBD NbTiN films were annealed using rapid thermal annealing from 200 to 1000 °C. The change in structural and electrical properties was characterized by X-ray diffraction, atomic force microscopy, and four-point probe measurements. The annealed films demonstrated an enhancement in transition temperature (Tc), increased crystallinity, larger grain size, and significantly reduced intrinsic stress.
High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ~670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconducting computing technologies. In this paper, we report on the growth of high-quality face-centered cubic (FCC) NbTiN films through the use of an alternative materials deposition technology-reactive bias target ion beam deposition. This novel technique offers more degrees of freedom compared to conventional sputtering, notably through decoupling the plasma generation and target bias, through use of a hollow-cathode ion source and independently pulse biased sputtering targets. The reported films are of high quality, with increased transition temperatures, reduced normal state resistivity, and surface roughness, compared to our previously reported reactive magnetron sputtered NbTiN films. In particular, at UVA, we are investigating these NbTiN films for wide-bandgap all-NbTiN superconducting-insulating-superconducting mixers, kinetic inductance detectors, and traveling-wave kinetic inductance parametric amplifier devices.
Abstract In this report, we present the superconducting properties of NbTiN films grown by reactive DC magnetron sputtering on MgO substrates. The films were deposited at room temperature, 300 °C, and 600 o C to study the effect of deposition temperature on crystalline structure and superconducting behavior. Superconducting parameters such as transition temperature (T c ), coherence length ( ξ (0)), and upper critical field (B C2 (0)) were obtained from resistance–temperature measurements. A clear dependence of structural and superconducting properties on deposition temperature was observed. Resistance–temperature data were analyzed using Aslamazov–Larkin (AL) and Lowest Landau Level (LLL) scaling theories to describe the superconducting fluctuation mechanism. Magnetoconductance curves extracted from magnetoresistance (MR) measurements were analyzed within the AL + MT fluctuation framework to estimate the dephasing time ( τ ϕ ) in the picosecond range. All these findings are crucial for understanding the response and recovery time of superconducting single-photon detectors. The successful growth and robust parameters of high-temperature-deposited NbTiN films indicate their potential for superconducting photon detectors, kinetic inductance detectors, and other quantum applications.
Nb-based circuits have broad applications in quantum-limited photon detectors, low-noise parametric amplifiers, superconducting digital logic circuits, and low-loss circuits for quantum computing. The current state-of-the-art approach for superconductor-insulator-superconductor (SIS) junction material is the Gurvitch trilayer process based on magnetron sputtering of Nb electrodes with Al-Oxide or AlN tunnel barriers grown on an Al overlayer [1]. However, a current limitation of elemental Nb-based circuits is the low-loss operation of THz circuits operating above the 670 GHz gap frequency of Nb and operation at higher temperatures for projects with a strict power budget, such as space-based applications. NbTiN is an alternative higher energy gap material and we have previously reported on the first NbTiN/AlN/NbTiN superconducting-insulating-superconducting (SIS) junctions with an epitaxially grown AlN tunnel barrier [2]. One drawback of a directly grown tunnel barrier compared to thermal oxidation is control of the barrier thickness and the uniformity of thickness across a substrate, leading to variations in current density (Jc). Semiconductor barriers with smaller barrier heights enable thicker tunnel barriers for a given Jc[3]. GaN is an alternative semiconductor material with a closed-packed Wurtzite crystal structure similar to AlN and it can be epitaxially grown as a tunnel barrier using the Reactive Bias Target Ion Beam Deposition (RBTIBD) technique. This work presents the preliminary results of the first reported high-quality NbTiN/GaN/NbTiN heterojunctions with underdamped SIS I(V) characteristics
Low-Temperature Stabilization of δδδ-NbN Superconducting Thin Films through Energy-Selective Ion Beam Sputtering
Achieving high-quality superconducting thin films at low temperatures is a central challenge for scalable quantum device integration and Complementary Metal-Oxide-Semiconductor (CMOS)-compatible fabrication. Here, we demonstrate a room-temperature magnetron sputtering approach for synthesizing niobium nitride (NbN) thin films using a novel energy-selective ion source that enables independent control of ion energy and ion flux. This capability provides a powerful route to precisely tailor surface kinetics and crystallization pathways, overcoming longstanding limitations in stabilizing high-quality NbN at reduced temperatures. The resulting films were characterized by superconducting transport measurements, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Compared with conventional reactive sputtering, ion beam-assisted growth yields a substantial narrowing of the transition width at the superconducting temperature, indicating improved phase purity and electronic homogeneity. Furthermore, this work introduces energy-selective ion control as a general, non-equilibrium route to stabilize metastable superconducting phases at low temperatures, offering a new design paradigm for thin-film quantum materials.
Integration of superconducting devices on photonic platforms opens up a wide range of functionalities and applications. We report on NbTiN thin films deposited on SiO2, Si3N4, GaAs, LiNbO3, and AlN as well as on a monolayer of hexagonal boron nitride, using a universal reactive co-sputtering recipe. The morphology and the superconducting properties of the NbTiN thin films with a thickness of 10 nm were characterized by atomic force microscopy and electrical transport measurements. Superconducting strip photon detectors were fabricated using a design suitable for waveguide integration and compared in terms of their internal quantum efficiency and detection pulse kinetics. Our results show well-comparable performances for detectors integrated on different platforms, while also demonstrating that reactive co-sputter deposition of NbTiN at room temperature provides a robust method for realizing superconducting devices on various materials.
Integrating NbTiN-based microstrip tuning circuits with traditional Nb superconductor-insulator-superconductor (SIS) junctions enables the low-noise operation regime of SIS mixers to be extended from below 0.7to1.15THz. In particular, mixers incorporating a NbTiN∕SiO2∕NbTiN microstrip tuning circuit offer low-noise performance below 0.8–0.85THz, although their sensitivities drop significantly at higher frequencies. Furthermore, a microstrip geometry in which NbTiN is used as the ground plane material only (NbTiN∕SiO2∕Al) yields significant improvements in the sensitivities of SIS mixers operating up to 1.15THz, with an upper operating frequency that depends upon the quality of the NbTiN layer, and thus its deposition process. Films deposited at room temperature have Tc=14.4K and ρn,20K∼60μΩcm, and offer low-noise performance up to 1THz, whereas films deposited at 400°C have Tc=16K and ρn,20K∼110μΩcm, and offer low-noise performance up to 1.15THz. Taken together, these results demonstrate that the high-frequency surface resistance of a NbTiN layer depends upon the film’s structural properties. Most significantly, the drop in performance that is seen at F>1THz in mixers incorporating NbTiN ground planes deposited at room temperature is attributed to nonhomogeneities in the structural and electrical properties of these films, as is the poor performance of mixers that incorporate NbTiN wiring layers at F>0.85THz. The development of these NbTiN-based microstrip tuning circuits will enable the production of low-noise SIS mixers for the 0.8–0.96- and 0.96–1.12-THz frequency bands of the Heterodyne Instrument for the Far Infrared on board the European Space Agency’s Herschel Space Observatory.