Far-infrared optical absorption and reflectivity of a superconducting NbN film

We report measurements of bolometric absorption and absolute reflectivity of a thin film of NbN on a silicon substrate, above and below the superconducting transition temperature of 13.3 K. The opening of the gap at an energy of 3.9 kB Tc is seen clearly in both types of measurement. The data are compared with the predictions of the Mattis-Bardeen theory. The change in the reflectivity and the absorption at energies above 2.6. is larger than that calculated from the theory. By an analysis of the reflectivity data, it is shown that the theoretical values of the complex superconducting conductivity are too high. Thin films of superconducting NbN are of great current interest because of their very low surface resis­ tance, which is comparable to that of Nb, and relatively high values of Tc (13-18 K). This is important for mi­ crowave device applications. Films made from the re­ cently discovered high-Tc ceramic materials have so far exhibited higher surface resistances. A recent study by Oates et al. 1 reported measurements of the temperature and frequency dependencies of the surface impedance of NbN thin films in the microwave region using a stripline resonator and cavity technique. They satisfactorily fitted the temperature dependence of the 35 GHz surface resis­ tance by use of the weak-coupling dirty-limit Mattis­ Bardeen theory,2 but they also noted that their sensitivity was insufficient to differentiate between weak and strong coupling. In the far-infrared region, which contains the energy gap, measurements of transmission through thin NbN films have been reported by Karecki, Pena, and Per­ kowitz. 3 They found it impossible, however, to satisfac­ torily fit the data with a dirty-limit assumption in which the scattering time 7=0 and the normal conductivity has no frequency dependence. Instead they used their fit to obtain a value of 7. One of the important potential appli­ cations of NbN is in tunnel junctions, and measure­ ments4.5 using these devices have also shown the energy­ gap and phonon density of states. We have recently6 used a bolometric method in which the absorption A is measured directly in an attempt to measure the energy gap in the 10 K organic superconduc­ tor, (BEDT-TTF)2Cu(NCS)2' where BEDT-TTF is bis(ethylenedithio) tetrathiafulvalene. These measure­ ments showed no sign of a superconducting gap, so it was important to test our methods with a better understood conductor. Using the NbN samples, we have successfully observed the energy gap as a function of temperature in thin-film NbN with high resolution and good signal to noise. We then measured the power reflectivity and again saw the gap clearly. We found a large discrepancy with