Monocrystalline NbN nanofilms on a 3C-SiC∕Si substrate

Abstract

The authors have realized NbN ͑100͒ nanofilms on a 3C-SiC ͑100͒/Si͑100͒ substrate by dc reactive magnetron sputtering at 800°C. High-resolution transmission electron microscopy ͑HRTEM͒ is used to characterize the films, showing a monocrystalline structure and confirming epitaxial growth on the 3C-SiC layer. A film ranging in thickness from 3.4 to 4.1 nm shows a superconducting transition temperature of 11.8 K, which is the highest reported for NbN films of comparable thickness. The NbN nano-films on 3C-SiC offer a promising alternative to improve terahertz detectors. For comparison, NbN nanofilms grown directly on Si substrates are also studied by HRTEM. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2766963͔The ability to grow superconducting NbN films of several nanometer thick is of significant importance to the development of modern photon detector technology. Superconducting hot electron bolometer ͑HEB͒ mixers based on such nanofilms are the only sensitive heterodyne detectors for high-resolution spectroscopy at frequencies between 1.5 and 6 THz.1-4 These detectors will be used on the Herschel space telescope 5 and are required in various future conceptual space missions. 6 Another type of detector, the superconducting single photon detector ͑SSPD͒, 7 is based on similar films and is ultrafast and sensitive for the detection of both visible and infrared photons. SSPDs can perform high speed photon counting which has many applications, for example, optical communications and quantum information.To date, HEB mixers are based on ultrathin NbN films grown primarily on substrates such as Si with its native oxide, 2-4 MgO, 8 and Si with a buffering MgO film. 9 SSPDs are based on NbN films grown on sapphire substrates. For films with an intended thickness of 3.5 nm ͑not directly measured͒, the highest superconducting transition temperatures ͑T c ͒ are reported to be 9.5-11 K.9 Among them, NbN films on MgO, MgO buffer layers and sapphire substrates have higher T c than NbN films on Si substrates. These substrates allow for epitaxial growth of the NbN films, 8-10 resulting in a monocrystalline structure. For HEB mixers, Si is a preferred substrate because of its low loss at terahertz frequencies, well-established processing technology, and inherent reliability. However, the drawback to using Si in this case is the limited intermediate frequency bandwidth, which is set by the thermal time constant.In this letter, we demonstrate superconducting NbN nanofilms on a 3C-SiC buffered Si substrate. The films were characterized by high-resolution transmission electron microscopy ͑HRTEM͒. In addition, the superconducting properties were measured.The 3C-SiC buffer layers were heteroepitaxially grown on Si ͑100͒ substrates by atmospheric pressure chemical vapor deposition at 1280°C using a process described in detail elsewhere.11 To ensure reasonably good crystal quality near the top surface of the 3C-SiC layer given its lattice mismatch with Si, we choose a thickness of 1 m for 3C-SiC layer. As reported previously, 11 t...

Ultrathin NbN Films for Superconducting Single-Photon Detectors

“…The results confirm the cubic lattice of our NbN films, as well that it is relaxed. The a values are lower than reported in literature (a = 4.389 Å [8]) that may be caused by difference in stoichiometry. The problem requires an investigation to exclude residual gases being trapped during the film deposition.…”

“…SSPDs are typically made by patterning a meander-type nanostructure in an ultrathin (≈ 5 nm) film deposited on a sapphire [1], MgO [7], or, recently, 3C-SiC/Si [8] single-crystal substrates. Single-crystal quality NbN films are deposited by either DC or RF reactive magnetron sputtering on substrates maintained at the 700-850…”

Noise temperature and beam pattern of an NbN hot electron bolometer mixer at 5.25 THz

“…The details of the NbN film can be found in Ref. 13. The bridge is con-nected to the antenna by Nb ͑10 nm͒/Au ͑50 nm͒ superconducting bilayer contact pads.…”

NbN hot electron bolometer mixer at 5.3 THz

“…The details of the NbN film can be found in Ref. 8. The bridge is connected to the antenna by Nb (10 nm)/Au (50 nm) superconducting bilayer contact pads [8].…”