Enhanced Superconducting Nanowire Single Photon Detector Performances using Silicon Capping

Abstract

Niobium Titanium nitride (NbTiN) based superconducting nanowire single photon detectors (SNSPDs) are known for their high performance across a wide spectral range, from the X-ray to the mid-infrared. Nonetheless, fabrication challenges and performance degradation attributable to surface oxidation and lack of uniformity in films thinner than 5 nm remain a significant barrier for achieving high-quality detectors. In this work, we study the influence of a Silicon capping layer on film properties and on the performance of SNSPDs. A Silicon capping layer effectively suppresses oxidation and increases the superconducting transition temperature. This enables superconductivity in films as thin as 3 nm at 3 K, increases critical current in patterned nanowires and significantly extends the saturation plateau from the visible to the near infrared (up to 2050 nm): These detectors maintain sub-50 ps timing jitter, even for nanowires as wide as 250 nm and with detection areas of 20x20μm2. Our results establish that thinner films protected by a capping layer allow for the fabrication of wider wires, decreasing nanofabrication challenges and extending the operating temperature range for efficient single photon detection.

Figures (4)

• Figure 1: (a) SEM image of a meander-shaped superconducting nanowire detector made from a 4 nm thick NbTiN film capped with 5 nm Silicon layer (b) AFM image of a 3 nm NbTiN thin film capped with 5nm Si, showing the surface morphology and extracted roughness of the NbTiN layer. (c) BF-STEM image of a 4 nm NbTiN film capped with a 5 nm Silicon layer, displaying the layer interfaces and showing the multilayer stack. (d) EDX elemental maps acquired from the region shown in Fig 1(c), confirming the spatial distribution and chemical separation of Nb (white), Ti (green), and Si(red).
• Figure 2: (a) Critical temperature $T_{c}(K)$ versus $R_{sh} (\Omega/\Box)$ as a function of thickness (3–9 nm) for samples without Si capping. The inset shows data for superconducting samples (4–9 nm) within a narrower temperature range near their superconducting transitions. (b) Critical temperature $T_{C}(K)$ Versus $R_{sh} (\Omega/\Box)$ as a function of thickness (3-9 nm) for samples with a 5 nm Si capping layer. The inset shows the superconducting samples (3–9 nm) within a narrower temperature range near their superconducting transitions. (c) Comparison of $R_{sh}$ ($\Omega/\Box$) as a function of NbTiN layer thickness (3–9 nm) without (Black) and with 5 nm Si capping (red). (d) Comparison of $T_{c}(K)$ as a function of film thickness (3–9 nm) without (Black) and with 5 nm Si capping layer (red). (e) RRR as a function of thickness (3–9 nm) without (Black) and with Si capping (red).
• Figure 3: (a-c) Normalized counts rate of SNSPDs fabricated from 9 nm NbTiN (a-c) and 4 nm NbTiN (d-f) with a 5nm Silicon capping layer (orange) and without a capping layer (grey). All nanowires were patterned with a 100 nm wire width, a fill factor of 50% and a detector size of 20x20$\mu m^{2}$. Three more sets of devices fabricated in the same fashion exhibiting similar behavior can be found in supplementary (Fig.3) .
• Figure 4: (a) Normalized count rate of SNSPDs fabricated from 4 nm-thick NbTiN films with 250 nm nanowire width and 5 nm Si capping layer, measured at 178 mK. (b) Timing jitter (main panel) measured at 1550nm, using an APE picoEmerald pulsed laser. The numerical value of the timing jitter was measured by fitting an exGaussian curve (red curve) to the time-correlated single-photon counting histogram (blue curve), resulting in a full width at half maximum (FWHM) jitter of 43 ps. Inset of Fig.4(b) displays a detection pulse with a reset time constant of 14.6 ns