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.
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.
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.
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 δ-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.
Superconducting Hot-electron Bolometric (HE.B) mixers employing thin films ofNbN have become the devices of choice for heterodyne detection at tera,hertz frequen- cies. HEB mixers are inherently insensitive to the bandgap frequency of the superconducting material used in the device, in contrast to SIS mixers,. We have developed a fabrication pro- cess for thin NbN films at the National Institute of Standards and Technology in Boulder, CO. A two-fold approach can be taken. The first is to maximize the critical temperature of the superconducting device by growing thicker films at the expense of IF bandwidth, whereas the second approach focuses on maximizing the IF bandwidth at the expense of the critical tem- perature. So far, we have developed a film-deposition process utilizing our DC reactive mag- netron sputtering chamber. By current-biasing the RF plasma in a mixture of Ar and N2, while using a Nb target, we can control the film stoichiometry and produce films with thicknesses of 5 nm. The films are deposited on MgO substrates that are heated to about 800 °C during depo- sition. A typical critical temperature (TO is about 10 K and the transition width is very small (0.5 K). The films are evaluated by measuring their superconducting characteristics as well as their thickness and surface roughness by means of AFM analysis. Phonon-cooled HEB devices are fabricated from the films to study their performance as HEB mixers. The device fabrication process at UMass/Amherst involves lift-off lithography of the antenna (gold), and Reactive ion etching (R1E) or wet etching of the NbN. The I-V char- acteristics and noise temperatures of the devices were measured in order to determine their quality for PHEB applications.
Abstract A wide-bandwidth and low-noise amplification chain in the microwave regime is crucial for the efficient read-out of quantum systems based on superconducting detectors, such as Microwave Kinetic Inductance Detectors (MKIDs), Transition Edge Sensors (TESs), Magnetic Microcalorimeters (MMCs), and RF cavities, as well as qubits. Kinetic Inductance Travelling Wave Parametric Amplifiers (KI-TWPAs) operated in a three-wave mixing fashion have demonstrated exceptional dynamic range and low-noise performance, approaching the quantum limit. These amplifiers can be fabricated using a single layer of a high kinetic inductance film as weakly dispersive artificial transmission lines, with the ability to control the phase-matched bandwidth through dispersion engineering. In this study, we present the optimisation of the rf sputter-deposition process of NbTiN films using a Nb80%Ti20% target, with the goal of achieving precise control over film characteristics, resulting in high kinetic inductance while maintaining a high transition temperature. The parameter landscape related to the different sputtering conditions, such as pressure, power, and nitrogen flow, has been explored and the film thickness has been used as a fine-tuning parameter to adjust the properties of the final NbTiN films used for the fabrication of KI-TWPAs. As a final result, we have obtained a NbTiN film with a kinetic inductance of 8.5 pH/sq which we have exploited to fabricate KI-TWPA prototype devices, showing promising amplification performance.
Abstract Hafnium is an elemental superconductor which crystallizes in a hexagonal close packed structure, has a transition temperature T C ≃ 400 mK, and has a high normal state resistivity around 90 µ ω cm. In Microwave Kinetic Inductance Detectors (MKIDs), these properties are advantageous since they allow for creating detectors sensitive to optical and near infra-red radiation. In this work, we study how sputter conditions and especially the power applied to the target during the deposition, affect the hafnium T C , resistivity, stress, texture and preferred crystal orientation. We find that the position of the target with respect to the substrate strongly affects the orientation of the crystallites in the films and the internal quality factor, Q i , of MKIDs fabricated from the films. In particular, we demonstrate that a DC magnetron sputter deposition at a normal angle of incidence, low pressure, and low plasma power promotes the growth of compressive (002)-oriented films and that such films can be used to make high quality factor MKIDs with Q i up to 600,000.