Discharge voltage characteristic of the plasma in the magnetron sputter chamber. The pure Argon plasma (empty circles) is stable for currents above 80 mA. When nitrogen is added to the atmosphere (filled circles), the dependence becomes non-monotonic with a similar but off-set dependence for low currents, illustrated by the two parallel dashed lines. In the transition region, superconducting NbN is deposited (grey area). The higher the sputter current I SP, the higher is the Nb content in the sputtered film. Red crosses mark the deposition conditions of the four selected films.
Parameters of 4 nm thick NbN films deposited at various sputter currents I SP covering the gray region of Figure 1. The critical temperature T C (filled circles, left axis) shows an optimum at 145 mA. The residual resistivity ratio RRR (empty circles, right axis) increases continually with increasing sputter current. Dashed lines are to guide the eyes.
Ratio of the measured critical current of each sample to the calculated depairing critical current at 4.2 K in relation to the sputter current I SP at which the NbN film, of which the detector was structured, was deposited (dashed line is to guide the eye). The inset shows the temperature-dependence of the critical current of one of the samples in direct comparison with the Bardeen critical current (solid line).
Typical DCR behavior on relative bias current for one selected detector per each film. The sputter current of the respective film is given below each line. Solid lines are exponential fits of the DCR with respect to I B/I C m. The grey horizontal bar marks the threshold of 100 s−1.
Normalized spectra of detection efficiency of I B = 0.85I C m to I B = 0.95I C m for four selected detectors made from different NbN chemical composition as indicated in the legend. The solid lines show the fitting function that was used to determine the cut-off wavelength λC according to Eq. (4).
Measured cut-off wavelengths of all samples over the respective ratio of bias current to depairing current at which the measurement was taken. The colors/symbol shapes mark detectors that were patterned from the same film. Solid lines show the dependence predicted by the hot spot model λC = λC,0 (1-I B/I C d)−1, where the λC,0 values have been calculated from the material parameters of each film (see Table I). The dashed lines mark the mean values of the I C m/I C d ratios for each film (see Figure 3).
Parameters of NbN thin films deposited at the sputter current I SP: Thickness d, sheet resistance R S at 20 K, critical temperature T C, critical current density j C at 4.2 K, electron diffusion coefficient D, coherence length ξ, energy gap 2Δ at 4.2 K, and calculated cut-off prefactor λC(0).
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