Vortex behaviour in a superconducting NbN–AlN multilayered nanocomposite

• Published in: Superconductor science & technology 2006-06, Vol.19 (6), p.612-617
• Main Authors: Lobotka, P, Gazi, S, Vavra, I, Sedlackova, K, Satka, A, Kovac, J, Hasko, D
• Format: Article
• Language: English
• ISSN: 0953-2048; 1361-6668
• DOI: 10.1088/0953-2048/19/6/033

We report on composite films composed of nanograins made of superconductor embedded in an insulating matrix. The thin films were deposited by the sequential reactive sputtering of three non-continuous granular NbN layers separated by two continuous AlN layers on oxidized Si wafers. This 'multilayer' approach allows for the tailoring of both the Josephson coupling and the tunnelling rate of single electrons between NbN nanograins in the vertical direction. The average in-plane size of NbN nanoparticles was 6.4 nm (as revealed by TEM), and tAlN = 2-4 nm. On cooling, the R(T) curves show a steep logarithmic increase of the nanocomposite resistance with a ratio of R8/R300 > 9, which shows that there is no 'metallic' percolation path in a normal state. V -I curves measured at T = 6.15 K exhibit a V~I3 dependence, which implies the Kosterlitz-Thouless transition in 2D superconductors. At low currents, the V -I curves are linear, which is attributed to the flux flow of unpaired vortices. At higher currents, the V -I curves show superposed oscillations, i.e. small voltage peaks separated equidistantly in current with a period of 0.12 mA. The peaks occur in a wide current range from 0.4 to 4 mA. We assume that this effect is caused by phase slips. The fact that the current period is constant in the whole transition region (6.14-6.81 K) implies the geometrical origin of this effect. Several properties indicate the suitability of NbN-AlN multilayered nanocomposite for use in microbolometers or single-photon detectors. The film should generate electromagnetic waves because the peaks also have a negative slope.