Uniform Superconducting Films Promise Quieter, More Compact Cryoelectronic Devices

Scientists are increasingly investigating highly disordered superconducting thin films for use in advanced cryoelectronic devices. J. Lorenz, S. Linzen, and M. Ziegler, working with colleagues at the Helmholtz-Zentrum Berlin für Materialien und Energie, the University of Konstanz, and the Institute of Solid State Physics, Russian Academy of Sciences, now report exceptional spatial homogeneity in niobium nitride (NbN) films grown by plasma-enhanced atomic layer deposition. Their findings, detailed in this paper, demonstrate remarkably uniform superconducting properties, with order parameter variations of only 2-3% across significant length scales, despite the films exhibiting high disorder and sheet resistance. This level of homogeneity, achieved even as the material approaches the insulator transition, is crucial as it overcomes a key limitation of disordered superconductors and unlocks their potential for creating compact, low-noise components in sensitive electronic applications.

These films, composed of niobium nitride (NbN) and fabricated using plasma-enhanced atomic layer deposition (PE-ALD), maintain remarkably consistent superconducting properties even as they approach the threshold between superconductivity and insulation. This achievement addresses a long-standing challenge in materials science, where increased disorder typically leads to spatial variations in the superconducting order parameter, degrading film performance. The study employed scanning tunneling microscopy to meticulously map the superconducting order parameter across the surface of these NbN films. Analysis revealed minimal variation, a standard deviation of only 2-3%, over distances far exceeding the typical grain size of the material. Simultaneously, the fabricated films exhibit high sheet resistance, reaching up to 1400 Ohm, and correspondingly high sheet kinetic inductance, peaking at approximately 200 pH. This combination of properties positions these films as promising candidates for a range of applications, including superinductors, efficient filters, parametric amplifiers, and potentially, a future quantum current standard. The high kinetic inductance provides effective protection for delicate quantum components and enables amplification of weak microwave signals. By minimising fluctuations in the superconducting order parameter, this research paves the way for more stable and reproducible quantum devices, addressing a critical limitation in the field and potentially unlocking new levels of performance in superconducting quantum technologies. The relationship between resistance and inductance is described by the formula L□= hR□/(2π²∆), where h is Planck’s constant and ∆ is the superconducting order parameter. This advancement builds upon existing techniques like reactive magnetron sputtering, offering improved spatial homogeneity and precise thickness control. Furthermore, the ability to fabricate these films with thicknesses approaching the superconductor-insulator transition opens new avenues for quantum circuit design. The films consistently demonstrated a high degree of spatial homogeneity in the superconducting order parameter, exhibiting standard deviations of only 2-3% across length scales exceeding the film’s grain size. This uniformity is particularly noteworthy given the film thicknesses approach the superconductor-insulator transition, a regime where spatial inhomogeneity is typically expected. Scientists have long sought materials that can efficiently conduct electricity with zero resistance at practical temperatures, a quest central to advances in quantum computing and sensitive detectors. The challenge lies in creating superconducting films that are both highly conductive and remarkably uniform, even when scaled down to the nanoscale. This work, detailing the fabrication and characterisation of niobium nitride films using a refined atomic layer deposition technique, represents a significant step towards overcoming that hurdle. Scanning tunneling microscopy underpinned the investigation of spatial homogeneity in niobium nitride (NbN) thin films fabricated via plasma-enhanced atomic layer deposition (PE-ALD). This technique, which maps surface electronic structure at the nanoscale, was chosen for its ability to resolve variations in the superconducting order parameter, a crucial indicator of film quality. Samples of NbN, deposited on silicon substrates at 380°C, were subjected to detailed analysis, with thicknesses of 25nm, 5nm, and 4nm selected to explore the influence of film thickness on superconducting properties. To preserve the integrity of the films, a carefully controlled transfer process was implemented, sealing samples in situ under a dry nitrogen atmosphere within a glovebox connected to the ALD system, minimising exposure to ambient conditions. Upon introduction to ultra-high vacuum (UHV), the samples underwent degassing at 550 K for 20 minutes to remove any residual adsorbates accumulated during brief air exposure. Platinum-iridium (PtIr) tips, prepared in situ by indenting into a clean silver single crystal, served as the tunneling probes. These tips were selected to ensure a flat density of states, optimising the resolution and accuracy of the tunneling current measurements. STM topography was initially acquired at a bias voltage of 0.8V, maintaining a constant tunneling current of 300 pA, revealing grain diameters ranging from 3nm to 10nm in the 25nm film. Subsequently, scanning tunneling spectroscopy was performed using a lock-in detection scheme at a modulation frequency of 819Hz and a peak-to-peak amplitude of 70 μ.

Films achieved a critical temperature of approximately 15 K and a large superconducting order parameter, minimising unwanted quasiparticle excitations. While disorder generally degrades performance, this research demonstrates how controlled deposition via PE-ALD can yield films with robust superconducting characteristics. The resulting NbN films maintain a critical temperature around 15K and a substantial superconducting order parameter, minimising unwanted quasiparticle excitations. For years, the field has been hampered by the trade-off between kinetic inductance, a desirable trait for certain circuit elements, and material homogeneity. Higher disorder, while boosting inductance, typically introduces defects that disrupt the even flow of superconducting current. This research demonstrates that it is possible to achieve both, through precise control of the deposition process and a surprising level of spatial uniformity. This opens doors not only to improved superconducting circuits but also to more sensitive and reliable single-photon detectors, crucial for quantum communication and imaging. However, the relatively high sheet resistance achieved, while sufficient for some applications, may still limit performance in others. Further research is needed to explore whether this can be reduced without sacrificing uniformity. Moreover, the long-term stability of these films under operational conditions remains an open question. The next phase will likely involve integrating these materials into functional devices and testing their performance in realistic scenarios, alongside exploration of alternative materials and deposition techniques to push the boundaries of superconducting film quality even further.