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Abstract
We report research on superconducting transition temperature (Tc) tuning and the antireflective layer of NbxSi1-x films that were used as the absorbing layer in inductive superconducting transition edge detectors (ISTED). The Tc of NbxSi1-x film absorber should be tuned to be to the operating temperature range of the readout nano superconducting quantum interference devices. The composition ratio of Nb/Si was controlled by the co-sputtering powers of the Nb and Si target to adjust the Tc. To improve the detection efficiency, a 30 nm Nb95.7Si4.3 film with Tc 6.3 K was chosen to demonstrate the effect of the antireflective layer SiNx made by low temperature plasma enhanced chemical vapor deposition (LT-PECVD). According to the spectral refractive indexes and extinction coefficients of the Nb95.7Si4.3 and SiNx films, the structure parameters for 633 nm incident light were designed and the optical properties were calculated. The reflectivity measurements showed that the reflectivity was effectively reduced, which was consistent with the calculation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Superconducting single-photon detectors (SSPD) are research hotspots in recent years for photon quantum communication, distance measurements and single-photon metrology because of the rapid response and high sensitivity. Among different types of SSPD such as nanowire single-photon detectors [1–3], transition edge sensors [4–6], superconducting tunnel junction detector [7–8], microwave kinetic inductance detectors [9] and inductive superconducting transition edge detectors (ISTED) [10–11], ISTED based on nano superconducting quantum interference readout detectors (nano-SQUID) which have a low minimum detectable energy even above 7 K has shown great potential.
ISTED is composed by a nano-SQUID and a thin-film superconducting absorber located in the loop of the nano-SQUID [10] and works at the absorber’s transition temperature. The working principle is shown in formula (1).
For the detection efficiency, the material and reflection of the absorber were the key factors. Superconducting NbxSi1-x films are amorphous alloys achieved by co-sputtering of Nb and Si [13], and the Tc, resistivity and optical properties can be easily adjusted with the Nb content x by controlling the sputtering rate of Nb and Si source separately. In addition to the applications as the barrier for Josephson-junction circuits [14–19], NbxSi1-x films are also used as the absorbing unit in TES system [20–21]. Here we chose thin NbxSi1-x films as the absorber of the ISTED.
The main methods to decrease reflection were a resonant cavity or an anti-reflection coating fabricated on the surface of the absorbing film. The resonant cavity consisted of Ta2O3/SiO2 multilayer films have been used in SNSPD [22–23], but the preparation process of such multilayer films is complex to control. The anti-reflection coating is easy to prepare and can effectively reduce the reflectivity. Monolayer SiO2, SiNx or a composite structure of the both were also used as anti-reflection layers on the metal films [24–25]. The structure can be realized by a PECVD process at a temperature below 80 ℃ to avoid the oxidation of superconducting materials.
In this paper we report the research on the tuning of Tc of the NbxSi1-x film with changing the Nb component x, and the anti-reflection SiNx layer for 633 nm on the Nb95.7Si4.3 film.
2. Experiments and results
2.1 Fabrication of NbxSi1-x and SiNx films
The superconducting NbxSi1-x film was deposited using a magnetron co-sputtering technique which had showed the advantages of good uniformity, low substrate temperature and strong film adhesion in an ultra-high vacuum system. Si and glass substrates were used for the characterization of the optical properties and evaluation of the characteristics of anti-reflection film. The sputtering Ar pressure was controlled at 5 mTorr, which was fine-tuned to make the stress of Nb film between −50 MPa to −100 MPa. The maximum power of Nb target was 500 W and that of Si was 250 W. The Nb/Si component ratio were controlled by the sputtering power of Nb and Si. A series 30 nm NbxSi1-x films were synthesized. The content of Nb x was determined by the ratio of the deposition rate of Nb to the sum of the two materials at the corresponding power. In ISTED, superconducting NbxSi1-x films should be fabricated on the Si substrates, so NbxSi1-x films were firstly deposited on the Si substrate to measure reflection characteristics, and then transparent glass substrates were used to evaluate the absorptivity of the NbxSi1-x films and SiNx /NbxSi1-x bilayers.
SiNx anti-reflection films were synthesized by LT-PECVD. Since the NbxSi1-x film was easily oxidized, the deposition temperature was only 60 ℃. The chamber pressure was 7 mTorr, and the flow rate of nitrogen and silane was 5 sccm and 50 sccm respectively. The deposition rate of SiNx films was about 1.80 nm/s.
2.2 Characterization of NbxSi1-x and SiNx films
The Tc of the NbxSi1-x films was measured by a four-terminal method. The relation between Tc of the 30 nm NbxSi1-x film and Nb contents x is shown in Fig. 1. Tc decreases from 8.2 K to 5.2 K with the Nb content from 100% to 91.8%. The large tuning range of Tc shows the superiority of NbxSi1-x. As nano-SQUIDs typically worked at 4.2 K to 8.2 K, the Nb95.7Si4.3 film with a Tc of 6.3 K was chosen to investigate the effect of anti-reflection SiNx layer.
The spectral complex refractive index n’ = n + ik of the NbxSi1-x films with x = 1, 95.7% and 91.8% from 400 nm to 2000nm measured by a spectral ellipsometer (Horiba UVISELTM) are shown in Fig. 2(a) and (b), n is the refractive index and k is the extinction coefficient. From the visible to infrared light, the refractive index n and extinction coefficient k increases with the Si content x. The Nb film has the lowest refractive index compared with the others at the same wavelength. As the wavelength of the incident light increases, the n and k of the NbSi films increases.
Fig. 2. The refractive index (a) and extinction coefficient (b) of Nb95.7Si4.3 films with different ratios of Nb/Si. (c) The refractive index of SiNx films.
The deposition conditions such as the deposition temperature, the chamber pressure, the flow rate and ratio of nitrogen/silane will affect the index of SiNx films. The refractive index of SiNx films deposited with conditions shown in Section A were characterized and shown in Fig. 2(c). The n decreases with the wavelength of the incident light.
2.3 Design, measurements, and discussions
Based on the results shown in section B, the simulated reflectivity (R) with thickness of SiNx film and wavelength on Nb95.7Si4.3/Si is shown in Fig. 3. As the thickness of the SiNx film decreases, the wavelength with the lowest R becomes smaller, and the lowest R also decreases. We focused on the ISTED for 633 nm light. At 633 nm, the n’ of Nb95.7Si4.3 is 2.68 + i 3.71, and the n of SiNx films, Si and glass are 1.89, 3.87 and 1.515, respectively. According to Fig. 3, the thickness of the SiNx film was 71 nm for the lowest R.
The spectral R of SiNx (71 nm)/Nb95.7Si4.3/Si calculated and measured using a Lambda 950 system are shown in Fig. 4. The calculated and measured spectral R, T and A of SiNx/Nb95.7Si4.3/glass are shown in Fig. 5. The measured R shows the same trends as calculated results and both R were reduced a lot at 633 nm. The deviation for the long wavelength range between the calculated and measured results was due to the strong absorption of the NbSi material, it may affect the sensitivity of film parameters measured by the ellipsometry method.
Fig. 5. The calculated and measured R (a), T(b), A(c) of Nb95.7Si4.3 film on glass wafer without and with SiNx films.
The data at 633 nm were extracted from Fig. 4 and Fig. 5, and summarized in Table 1. With the SiNx anti-reflection films, the R decreases from 65.4% to 19.6% and was in good consistency with calculation results from 61.3% to 20.0% which have shown excellent anti-reflection effect. According to theoretical calculation, the A is obviously improved at the same time. Because silicon is non-transparent for the 633 nm light, it is impossible to verify the theoretical results by experiments. So the same calculations and experiments have been done on the transparent glass substrates. As shown in Fig. 4 and Fig. 5, the R on the silicon and glass substrates shows the same trends. The measured A increases from 35.8% to 76.3% which in good agreement with theoretical calculation. The anti-reflection SiNx film can greatly increase the absorption of the Nb95.7Si4.3 film. The research of the anti-reflection layer on the glass substrates is universal for the application on the Si substrates. This will greatly benefit the efficiency of ISTED.
Table 1. The calculated and measured reflectance, transmittance and absorption coefficient of Nb95.7Si4.3 film without and with 68 nm SiNx films at 633 nm.
3. Conclusion
The Tc of NbxSi1-x films were tuned from 8.2 K to 5.2 K by adjusting the Nb content for the application in ISTED. The Nb95.7Si4.3 film with Tc 6.3 K was selected to verify the designed anti-reflection SiNx layer at 633 nm. The reflectance was significantly decreased from 65.4% to 19.6% which was coincident with the calculation and indicated that the SiNx layer was effective for improving the detection efficiency of ISTED.
Funding
National Key R&D Program of China (2017YFF0206105); National Natural Science Foundation of China (NSFC) (61701470).
Acknowledgments
This work was supported by the National Key R&D Program of China (2017YFF0206105) and National Natural Science Foundation of China (61701470).
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