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Applied Physics Letters

Superconducting a-WxSi1−x nanowire single-photon detector with saturated internal quantum efficiency from visible to 1850 nm

Burm Baek, Adriana E. Lita, Varun Boehm Verma, Sae Woo Nam

Published Jun 20, 2011• 159 citations
Publisher

Abstract

We have developed a single-photon detector based on superconducting amorphous tungsten–silicon alloy (a-WxSi1−x) nanowire. Our device made from a uniform a-WxSi1−x nanowire covers a practical detection area (16 μm×16 μm) and shows high sensitivity featuring a plateau of the internal quantum efficiencies, i.e., efficiencies of generating an electrical pulse per absorbed photon, over a broad wavelength and bias range. This material system for superconducting nanowire detector technology could overcome the limitations of the prevalent nanowire devices based on NbN and lead to more practical, ideal single-photon detectors having high efficiency, low noise, and high count rates.

DOI
10.1063/1.3600793

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We have developed a single-photon detector based on superconducting amorphous tungsten–silicon alloy (a-WxSi1−x) nanowire. Our device made from a uniform a-WxSi1−x nanowire covers a practical detection area (16 μm×16 μm) and shows high sensitivity featuring a plateau of the internal quantum efficiencies, i.e., efficiencies of generating an electrical pulse per absorbed photon, over a broad wavelength and bias range. This material system for superconducting nanowire detector technology could overcome the limitations of the prevalent nanowire devices based on NbN and lead to more practical, ideal single-photon detectors having high efficiency, low noise, and high count rates.
Superconducting nanowire single photon detectors (SNSPD) made from amorphous superconductors have showed great promise for achieving high fabrication yields, due to the highly uniform nature of the films. We present progress on the development of SNSPD based on amorphous MoSi with a critical temperature of around 5 K, which is ideal for detector operation at temperatures of 1 – 2.5 K, accessible with widely available cryogenic systems. First generation devices have achieved a saturated internal efficiency from visible to near-infrared wavelengths, which is the first requirement for high overall system efficiency. The broadband response has allowed us to make a robust study the energy-current relation in these devices, which defines the current required for a saturated internal detection efficiency for a given incident photon energy. Contrary to previous studies with other material systems, we find a nonlinear energy-current relation, which is an important insight into the detection mechanism in SNSPDs. The latest generation devices have been embedded into an micro-cavity structure in order to increase the system detection efficiency, which has increased to over 65% at 1550 nm. The efficiency is believed to be limited by fabrication imperfections and we present ongoing progress towards improving this characteristic as well as the yield of the devices. Efforts are also being made towards increasing the maximum operating temperature of the devices.
Superconducting nanowire single-photon detectors (SNSPDs) are among the most sensitive single-photon detectors available and have the potential to transform fields ranging from infrared astrophysics to molecular spectroscopy. However, extending their performance into the mid-infrared spectral region - crucial for applications such as exoplanet transit spectroscopy and vibrational fingerprinting of molecules - has remained a major challenge, primarily due to material limitations and scalability constraints. Here, we report on the development of SNSPDs based on tungsten germanide, a novel material system that combines high mid-infrared sensitivity with compatibility for large-scale fabrication. Our detectors exhibit saturated internal detection efficiency at wavelengths up to 29 μm, while using 2.7x thicker films (8 nm vs 3 nm) and up to 4.5x wider nanowires (360 nm vs 80 nm) compared to mid-infrared-optimized SNSPDs fabricated from tungsten silicide. This advance will enable scalable, high-performance single-photon detection in a spectral region that was previously inaccessible, opening new frontiers in remote sensing, thermal imaging, environmental monitoring, molecular physics, and astronomy.
Nbx_xx​Ti1−x_{1-x}1−x​N low timing jitter single-photon detectors with unity internal efficiency at 1550 nm and 2.5 K
The requirements in quantum optics experiments for high single photon detection efficiency, low timing jitter, low dark count rate and short dead time have been fulfilled with the development of superconducting nanowire single photon detectors. Although they offer a detection efficiency above 90%, achieving a high time resolution in devices made of amorphous materials is a challenge, particularly at temperatures above 0.8 K. Devices made from niobium nitride and niobium titanium nitride allow to reach the best timing jitter, but in turn have stronger requirements in terms of film quality to achieve a high efficiency. Here we take advantage of the flexibility of reactive co-sputter deposition to tailor the composition of NbxTi1-xN superconducting films, and show that a Nb fraction of x = 0.62 allows for the fabrication of detectors from films as thick as 9 nm and covering an active area of 20 μm, with a wide detection saturation plateau at telecom wavelengths and in particular at 1550 nm. This is a signature of an internal detection efficiency saturation, achieved while maintaining the high time resolution associated with NbTiN and operation at 2.5K. With our optimized recipe, we reliably fabricated detectors with high critical current densities reaching a saturation plateau at 1550 nm with 80% system detection efficiency, and with a FWHM timing jitter as low as 19.47 ps.
In this Letter, we characterized superconducting nanowire single-photon detectors (SNSPDs) based on amorphous tungsten germanide (W0.8Ge0.2) superconducting films, which exhibit superconducting properties comparable to those of amorphous WSi. The nanowire width ranges from 60 to 90 nm, with a fixed filling factor of 30%. All fabricated devices exhibit saturated intrinsic detection efficiency for infrared photons with wavelengths up to 2 μm, indicating that WGe is a promising candidate for SNSPDs designed for mid- and far-infrared photon detection. The timing jitter of these detectors, measured with a room-temperature amplifier, ranges from 127 to 234.5 ps. Further improvements can be achieved through device geometry optimization or the implementation of cryogenic amplifiers.
Amorphous materials are attractive candidates for fabricating the superconducting nanowire single-photon detectors (SNSPDs) due to their superior tolerance and scalability over crystalline niobium nitride. However, the reduced superconducting transition temperature degenerates both operating temperature and saturation efficiency. Herein, the SNSPD (6.5 nm thickness and 50 nm width) based on the amorphous Mo 0.8 Si 0.2 film with a high optical absorption coefficient demonstrates close-to-unity intrinsic detection efficiency for 1550 nm photons from 75 mK to 2.2 K. Further, a high-performance array SNSPD with optimized 90 nm-width wires is also demonstrated. As-fabricated uniform 4-pixel SNSPD exhibits a saturation plateau for the photon counts at 2.2 K, which overcomes the limitation of operation at low temperature ( < 1 K ) for traditional amorphous SNSPDs. Coupled with superior intrinsic quantum efficiency, highly efficient photon counts, and low dark count ratio, this detector paves a way for achieving high efficiency and superior yield for large array systems.
Abstract We demonstrate the fabrication of micron-wide tungsten silicide superconducting nanowire single-photon detectors on a silicon substrate using laser lithography. We show saturated internal detection efficiencies with wire widths ranging from 0.59 µm to 1.43 µm under illumination at 1550 nm. We demonstrate both straight wires, as well as meandered structures. Single-photon sensitivity is shown in devices up to 4 mm in length. Laser-lithographically written devices allow for fast and easy structuring of large areas while maintaining a saturated internal efficiency for wire widths around 1 µm.