Fast neutron damage and the current carrying capacity of niobium nitride

Sadagopan, V.; Gatos, H. C.; Hechler, K.; Saur, E.

doi:10.1007/bf01392217

Cited by 14 publications

(1 citation statement)

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“…The superconducting and normal-state properties of NbN films are affected by the stochiometry, 13 crystal phase, 14 impurity content, 15 disorder, 16 thickness, 17 and carrier concentration, 18 all of which can be influenced by the deposition method. Ultrathin NbN films are commonly prepared by reactive sputtering of a niobium target in a mixture of argon and nitrogen (N 2 ) gases.…”

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Bias sputtered NbN and superconducting nanowire devices

Dane

McCaughan

Zhu

et al. 2017

Applied Physics Letters

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Superconducting nanowire single photon detectors (SNSPDs) promise to combine near-unity quantum efficiency with >100 megacounts per second rates, picosecond timing jitter, and sensitivity ranging from x-ray to mid-infrared wavelengths. However, this promise is not yet fulfilled, as superior performance in all metrics is yet to be combined into one device. The highest single-pixel detection efficiency and the widest bias windows for saturated quantum efficiency have been achieved in SNSPDs based on amorphous materials, while the lowest timing jitter and highest counting rates were demonstrated in devices made from polycrystalline materials. Broadly speaking, the amorphous superconductors that have been used to make SNSPDs have higher resistivities and lower critical temperature (T c ) values than typical polycrystalline materials. Here, we demonstrate a method of preparing niobium nitride (NbN) that has lower-than-typical superconducting transition temperature and higher-than-typical resistivity. As we will show, NbN deposited onto unheated SiO 2 has a low T c and high resistivity but is too rough for fabricating unconstricted nanowires, and T c is too low to yield SNSPDs that can operate well at liquid helium temperatures. By adding a 50 W RF bias to the substrate holder during sputtering, the T c of the unheated NbN films was increased by up to 73%, and the roughness was substantially reduced. After optimizing the deposition for nitrogen flow rates, we obtained 5 nm thick NbN films with a T c of 7.8 K and a resistivity of 253 lX cm. We used this bias sputtered room temperature NbN to fabricate SNSPDs. Measurements were performed at 2.5 K using 1550 nm light. Photon count rates appeared to saturate at bias currents approaching the critical current, indicating that the device's quantum efficiency was approaching unity. We measured a single-ended timing jitter of 38 ps. The optical coupling to these devices was not optimized; however, integration with front-side optical structures to improve absorption should be straightforward. This material preparation was further used to fabricate nanocryotrons and a large-area imager device, reported elsewhere. The simplicity of the preparation and promising device performance should enable future high-performance devices.

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