NbN Superconducting Devices

The introduction of hard and refractory materials such as Nb and NbN into Josephson junctions improved the durability in thermal cycles. This improvement has promoted rapid development of superconducting devices. 

Nb/oxide/Nb, Nb/a-Si/Nb, and NbN/oxide/NbN junction processes were proposed by IBM (1), Sperry (2), and ETL (3), respectively. Some preliminary work on an integrated device succeeded with these processes. After that, an Nb/AlO /Nb junction process was proposed by AT T (4). This process is widely used in the field of superconducting electronics. Many researchers have obtained fine results in application work on a superconducting device. 

RT Kampwirth, KE Gray. NbN materials development for practical superconducting devices. IEEE Trans Magn 17 565, 1981. 

NbN/BN granular films are sensitive, high-speed detectors for pulsed far infrared radiation. These films are deposited on sapphire or quartz substrates by reactive radiofrequency sputtering of a split Nb/BN target in an Ar/N2 atmosphere. These devices can also be operated in the superconducting or resistive region critical temperatures range between 1.6 to 15 K,... 

Ceramic superconducting films are divided into three classes, Bl-type compounds, ternary compounds, and high-temperature oxide superconductors. The Bl-type (NaCl-type structure) compound superconductors consist of nitrides and carbides with 5A, 6A, and 7A transition metals, such as TiN, ZrN, HfN, VN, NbN TaN, MoN, WN, TiC, ZrC, HfC, VC, NbC, TaC, MoC, WC, NbNi tC t, hex-MoN, and hex-MoC. Regarding the thin-film material, it is notable that NbN and NbN] (C ( (x = 0.08 and 0.15) have superconducting critical temperature, T, values of 17.3 and 17.8 K, respectively. The deposition method used is almost always sputtering or CVD. The properties of films deposited by the former method are superior. A highly reliable Josephson device was realized with an NbN film. 

Nb SIS junctions are very sensitive mixer devices, but only for frequencies below the gap energy of Nb( 700 GHz). Superconducting hot electron bolometers (HEBs) use the electron temperature-dependent resistance in superconducting narrow film strips. The mixer performance of the Nb HEB mixer is promising but the bandwidth, determined by the electron-phonon interaction time, is very narrow (—90 MHz). NbN has a short electron-phonon interaction time, so it is possible to obtain a larger bandwidth of 1 GHz. NbN has a larger gap energy than Nb, so the NbN HEB mixer can be operated over 1 THz. In some studies, preliminary experimental results were achieved (91-94). 

Integrated DC superconducting quantum interference device (DC-SQUID) magnetometers were fabricated using NbN/NbO t/NbN or NbN/MgO/NbN junctions (95-97). By using NbN/MgO/ NbN junctions, 10 K operation was achieved (96). 

Some microwave devices have been reported (96-102). A tunable superconducting phase shifter has been demonstrated using NbN microstrips (99). The tuning mechanism is based on inductance modulation by injecting excess quasi-particles into the NbN microstrips. Flux-flow oscillators have been demonstrated using NbCN/MgO/NbCN junctions (101). Radiation in a frequency range from 580 to 710 GHz was confirmed experimentally. 

N Suzuki, K Yamamoto, S Matsumoto, I Kurosawa, M Aoyagi, S Takada. Fabrication of superconducting NbN gate MOSFET for hybrid circuit applications. Extended Abstracts of 1994, International Conference on Solid State Devices and Materials, Yokohama, Japan, 1994, pp 547-549. 

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