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Journal of Applied Physics

Use of granular NbN as a superconducting bolometer

Kurt Weiser, Ulrich Strom, S. A. Wolf, D. U. Gubser

Published Jul 1, 1981• 35 citations
Publisher

Abstract

An ultrathin film of granular NbN is used as a superconducting bolometer. The bolometer has a high operating temperature (∼12 K), is very sensitive (∼5000 Ω/deg), and exhibits a fast thermal response time, which is estimated to be less than 10−10 sec.
DOI
10.1063/1.329298

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The photoresponse of 10 nm thick superconducting NbN meander lines is measured using an amplitude modulated infrared semiconductor laser operating at a wavelength of 1300 nm. The response time of the film is found to be less than 1 ns with a measured responsivity of up to 1500 V/W of absorbed power at 100 kHz. Thermal properties of the film are extracted from current–voltage characteristics using a self-heating hot spot model. At temperatures well below the superconducting transition, the magnitude of the photoresponse is found to be an order of magnitude too large to be purely bolometric, even when electron heating and effects due to intergranular weak links are taken into account. The photoresponse is seen to be bolometric near Tc. Other contributions to photoresponse are discussed, including kinetic inductance and the photofluxonic effect.
In this paper, the superconducting NbN hot electron bolometer terahertz detector with an Nb5N6buffer layer was fabricated on the Si substrate. The Nb5N6buffer layer can significantly improve the superconductivity of ultra-thin NbN films. Preliminary results show that with a 30-nm-thick buffer, the zero resistance superconducting transition temperature of a 6-nm-thick NbN film device is 12.6 K, and the superconducting critical current of the device is increased by four times. The noise temperature, IF bandwidth and direct detection sensitivity of the device will be further characterized in following experiments.
A substrate step technique has been used to fabricate superconducting NbN weak links of length ∼750 Å, width ∼1.5 μm. The film in the weak-link region is less than 100 Å thick, and shows evidence of being granular. These devices have very high resistance (as much as 30 Ω), are quite rugged, and show almost ideal Josephson response to microwave radiation at 70 GHz. A vortex flow model is expected to apply because of the small coherence length in the weak-link material. However, the well-known resistively shunted junction model is found to be a significantly better predictor of device behavior.