Magnetoconductance and photoresponse properties of disordered NbTiN films

We report on the experimental study of phonon properties and electron-phonon scattering in thin superconducting NbTiN films, which are intensively exploited in various applications. Studied NbTiN films with sub-10-nm thicknesses are disordered with respect to electron transport, the IoffeRegel parameter of kFle=2.5–3.0 (kF is the Fermi wave vector, and le is the electron mean free path), the inelastic electron-phonon interaction, and the product qTle฀1 (qT is the wave vector of a thermal phonon). By means of magnetoconductance and photoresponse techniques, we derive the inelastic electron-phonon scattering rate 1/฀e-ph and determine sound velocities and phonon heat capacities. In the temperature range from 12 to 20 K, the scattering rate varies with temperature as 1/฀e-ph฀T3.45±0.05; its value extrapolated to 10 K amounts to approximately 1/16 ps. Making a comparative analysis of our films and other films used in superconducting devices, such as polycrystalline granular NbN and amorphous WSi, we find a systematic reduction of the sound velocity in all these films by about 50% compared to the corresponding bulk crystalline materials. A corresponding increase in the phonon heat capacities in all these films is, however, less than the Debye model predicts. We attribute these findings to reduced film dimensionality and film morphology. DOI: https://doi.org/10.1103/physrevb.104.184514 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-210957 Journal Article Published Version Originally published at: Sidorova, M; Semenov, A D; Hübers, H -W; Gyger, S; Steinhauer, S; Zhang, X; Schilling, A (2021). Magnetoconductance and photoresponse properties of disordered NbTiN films. Physical review B, 104(18):184514. DOI: https://doi.org/10.1103/physrevb.104.184514 PHYSICAL REVIEW B 104, 184514 (2021) Magnetoconductance and photoresponse properties of disordered NbTiN films M. Sidorova , A. D. Semenov, H.-W. Hübers, S. Gyger , S. Steinhauer , X. Zhang , and A. Schilling Department of Physics, Humboldt-Universität zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany Institute of Optical Sensor Systems, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany Department of Applied Physics, KTH Royal Institute of Technology, 106 91 Stockholm, Sweden State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China CAS Center for Excellence in Superconducting Electronics, Shanghai 200050, China Physics Institute, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland (Received 9 August 2021; accepted 11 November 2021; published 22 November 2021) We report on the experimental study of phonon properties and electron-phonon scattering in thin superconducting NbTiN films, which are intensively exploited in various applications. Studied NbTiN films with sub-10-nm thicknesses are disordered with respect to electron transport, the Ioffe-Regel parameter of kF le = 2.5–3.0 (kF is the Fermi wave vector, and le is the electron mean free path), the inelastic electron-phonon interaction, and the product qT le ≪ 1 (qT is the wave vector of a thermal phonon). By means of magnetoconductance and photoresponse techniques, we derive the inelastic electron-phonon scattering rate 1/τe-ph and determine sound velocities and phonon heat capacities. In the temperature range from 12 to 20 K, the scattering rate varies with temperature as 1/τe-ph ∝ T 3.45±0.05; its value extrapolated to 10 K amounts to approximately 1/16 ps. Making a comparative analysis of our films and other films used in superconducting devices, such as polycrystalline granular NbN and amorphous WSi, we find a systematic reduction of the sound velocity in all these films by about 50% compared to the corresponding bulk crystalline materials. A corresponding increase in the phonon heat capacities in all these films is, however, less than the Debye model predicts. We attribute these findings to reduced film dimensionality and film morphology. DOI: 10.1103/PhysRevB.104.184514