Temperature dependent NbN thin film resistance for nitrogen partial pressures in the range 26%–31% for films grown at a temperature of with a thickness of 22 nm. The critical temperature and transition width are extracted using the mean value and width of the 10–90% resistance-transition width.
Overview of measured superconductivity metrics for NbN films on GaAs with thicknesses of 4–6 nm (marked as ) and 21–23 nm (marked as ) as a function of and . (a) Critical temperature (lower panels) and transition width (upper panels) measured for 5 nm (red symbols) and 22 nm (black/blue symbols) thick NbN films as a function of N2 partial pressure for (black symbols) and (blue/red symbols). In addition, the transition temperatures for 5 nm films, shifted by the factor extracted from fig 3 , are given in grey. Maxima corresponding to the δ and ϵ phase, respectively, are marked in the lower panel. (b) Measured growth temperature dependent data. The onset of GaAs dissociation is indicated by the dashed line, whereas the gradual transition from the QG to the SD regime is indicated by the red/blue arrow.
Critical temperature (bottom panel) and transition width (top panel) as a function of film thickness for NbN films grown at and C. Blue triangles correspond to as-grown films with sputtering times of and 55 s. The red symbols correspond to selectively thinned films, originating from a single 19 nm film, which was etched down for 0 s, 30 s, 60 s, and 90 s, respectively. Both methods are schematically shown in the inset on the top. The data points within the dashed bubble refer to a top oxide layer, also observed in Fig. 4 .
Atomic concentration (bottom panels) and nitrogen content x (top panels) for NbN x films as a function of depth obtained by XPS and RBS. (a) corresponds to grown at and (b) shows grown at . Both films were grown for . grown films consist of nitrogen poor fcc δ-NbN, as indicated by the blue shaded region, whereas grown films exhibit nitrogen rich hexagonal ϵ-NbN, as indicated by the red shaded region. The corresponding crystal structures of δ-phase NbN in (a) and ϵ-phase NbN in (b) are shown as insets.
Schematic of opto-electrical SSPD characterisation setup. A meander-type detector (SEM pictures on the left) is operated just below the critical current by using a bias-tee (1.5 mH inductance and capacitance) connected to a 100 kΩ resistor and a constant voltage source . The SSPD can be modelled as an inductor and a time-dependent resistor . As depicted schematically in the leftmost SEM image, a normal conducting region is formed upon photon absorption. This leads to a redirection of the bias current onto the output-capacitor, producing a voltage pulse V(t) which is monitored using an oscilloscope.
(a) Temporal dependence of the hotspot resistance (red squares) and detector bias current (blue circles) for the thick (bottom panel) and thin (top panel) SSPD. (b) Schematic of a photon induced resistive region along the nanowire. (c) Histograms of the amplified voltage pulses resulting from hotspot detection events for different bias currents in units of . The leftmost (rightmost) data sets were obtained from SSPDs defined with thick films.
Number of detected photon events as a function of the top-incident photon number for a 4 nm thin SSPD (a) and a 22 nm thick SSPD (b), respectively. The data sets are shown in a double logarithmic plot and presented for different ratios. The corresponding dark count rates are indicated by the dashed lines on the left. The solid lines are linear fits with slopes of (thin device) and (thick device) revealing the single photon character of the detected events. A maximum detection efficiency of is reached for the 4 nm detector at , whereas the 22 nm SSPD shows detectivity for a current of . The inset shows the measured beam profile used for illumination in a contour plot. The grey line superimposed on the contour plot represents a horizontal cross-section of the beam profile.
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