Saturating Intrinsic Detection Efficiency of Superconducting Nanowire Single-Photon Detectors via Defect Engineering

A superconducting nanowire single-photon detector (SNSPD) has played a significant role in numerous applications for visible and near-infrared photon detection. SNSPDs with high system detection efficiency (SDE greater than 90%) would enable remarkable experiments in quantum information processing. Currently, niobium nitride- ($\mathrm{Nb}\mathrm{N}$) based SNSPDs are widely used in practical applications since they can operate in an inexpensive compact closed-cycle cryostat. However, it is a challenge to realize a $\mathrm{Nb}\mathrm{N}$ SNSPD with saturated intrinsic detection efficiency (IDE) while maintaining its high SDE at near-infrared wavelengths. We develop a postprocessing method to enhance the IDE of $\mathrm{Nb}\mathrm{N}$ SNSPDs to saturation without sacrificing SDE through defect engineering by a 20-keV helium ion irradiation. The enhancement of IDE is achieved because of the irradiation-induced reduction of the superconducting energy gap and the electron density of states at the Fermi level as determined by the electrical transport measurements. The change in the optical absorptance of the irradiated SNSPD is insignificant at the resonant wavelength as confirmed by the measured optical reflectance and SDE. By taking advantage of the IDE enhancement, the SDE of an irradiated device is significantly increased from 49% to 92% at 2.2 K for a 1550-nm wavelength.

Figure 1

(a) Enlarged optical photo of a SNSPD chip. The purple regions are covered with $\mathrm{Nb}\mathrm{N}$ film while the green regions are the DBR substrate. Golden marks are used for optical alignments. The active area is indicated with a red dot in the front of the dashed line. (b) SEM image of a nanowire with an 80-nm width and 160-nm pitch. (c) Schematic of the $\mathrm{He}$ ion irradiation process on SNSPD (not to scale). (d) Possible $\mathrm{Nb}\mathrm{N}$ crystal structures before (top panel) and after (bottom panel) $\mathrm{He}$ ion irradiation. In the bottom panel, dashed circles indicate the possible vacancies created by irradiation. (e) Simulated vacancy and ion distributions as a function of depth by using the SRIM method. The background is embedded with a TEM photo of a 9-nm-thick $\mathrm{Nb}\mathrm{N}$ film on the DBR substrate (only the first ${\mathrm{Si}\mathrm{O}}_2/{\mathrm{Ta}}_2{\mathrm{O}}_5$ bilayer is shown).

Figure 2

Left axis: Fluence dependence of the superconducting critical temperature (T_c, solid scatters) for $\mathrm{Nb}\mathrm{N}$ films with varying thicknesses. Right axis: Fluence dependence of the resistivity measured at 20 K (ρ_20K, open circles) for the 7-nm-thick $\mathrm{Nb}\mathrm{N}$ film.

Figure 3

Temperature dependence of resistivity for a 7-nm-thick $\mathrm{Nb}\mathrm{N}$ film at various irradiated fluences. Inset: normalized resistivity as a function of temperature in an enlarged scale.

Figure 4

(a) Temperature dependence of the resistivity for 7-nm-thick $\mathrm{Nb}\mathrm{N}$ films irradiated with F_i = 0 and 1 × 10¹⁶ ion cm⁻², recorded at different magnetic fields as indicated by the labels. (b) Temperature dependence of the upper critical field (B_c2) for the 7-nm-thick $\mathrm{Nb}\mathrm{N}$ film at different fluences. The experimental data and fitting results are indicated by solid scatters and dashed lines, respectively.

Figure 5

(a) Measured wavelength dependence of the reflectance (r) for the 7-nm-thick $\mathrm{Nb}\mathrm{N}$ film on the DBR substrate, irradiated at different fluences. (b) Fluence dependence of minimum reflectance (r_min) and its wavelength position (solid squares). As fluence increases, r_min slightly redshifts.

Figure 6

SDEs as a function of I_b for the SNSPDs before (open scatters) and after (solid scatters) $\mathrm{He}$ ion irradiation. The dotted and dashed lines are the sigmoid function fittings for the experimental data before and after irradiation, respectively.

Figure 7

SDEs (solid scatters) and DCRs (open scatters) of device d2 as a function of I_b, measured at different temperatures for the device d2 irradiated at F_i = 5 × 10¹⁵ ion cm⁻². The green dashed line is the sigmoid function fitting.

Figure 8

Physical properties of the same irradiated device d3 measured at different periods, that is, first time (1st, open scatters) and >1.5 years after (2nd, solid scatters). (a) Temperature dependence of resistivity (inset, temperature dependence of the normalized resistivity in an enlarged scale). (b) Bias current dependence of SDE.

Figure 9

Schematic of the experimental setup used for the SNSPD characterization.

Figure 10

Comparisons of detector performance before and after ion irradiation for the same device d4. (a) SDE as a function of bias current (I_b). (b) Timing jitter (TJ) vs I_b. (c) Single shot waveforms measured with different values of series resistor (R_s). (d) Normalized photon-response count rate (PCR) dependence of input photon rate, recorded for different cases. The correction factor for each case is shown in the labels. Inset, the raw data of PCR vs input photon rate. The corresponding symbols are the same as the ones shown in the main figure.

Figure 11

Intrinsic dark count rate (iDCR) before and after irradiation as a function of the normalized bias current (I_b/I_sw), measured at several operating temperatures (as shown in the labels). The red and black dashed lines are drawn to guide the eye.

Figure 12

(a) Refractive index of ${\mathrm{Si}\mathrm{O}}_2$ thin films measured before and after irradiation versus wavelength. Dashed and dashed-dotted lines are the reference data reported for unirradiated ${\mathrm{Si}\mathrm{O}}_2$ thin films. (b),(c) Measured refractive index (n of $\mathrm{Nb}\mathrm{N}$) and extinction coefficient (k of $\mathrm{Nb}\mathrm{N}$) of a 30-nm-thick $\mathrm{Nb}\mathrm{N}$ thin film deposited on a thermal oxidized $\mathrm{Si}$ wafer versus wavelength; (d) Comparison of the simulated optical absorptance (Abs) of the $\mathrm{Nb}\mathrm{N}$ nanowire on the DBR substrate versus wavelength by varying the n value of the top ${\mathrm{Si}\mathrm{O}}_2$ layer, as well as the optical parameters of the $\mathrm{Nb}\mathrm{N}$ thin film before and after irradiation.