Improvement of critical temperature of niobium nitride deposited on 8-inch silicon wafers thanks to an AlN buffer layer

Abstract

In this paper, we study the crystalline properties and superconducting critical temperature of ultra-thin (5–9 nm) NbN films deposited on 8-inch silicon wafers by reactive sputtering. We show that the deposition of NbN on a thin (10–20 nm) AlN buffer layer, also synthesized by reactive sputtering, improves the critical temperature by several Kelvin, up to 10 K for 9 nm NbN on 20 nm AlN. We correlate this improvement to the higher-crystalline quality of NbN on AlN. While NbN deposited directly on silicon is polycrystalline with randomly oriented grains, NbN on AlN(0001) is textured along (111), due to the close lattice match. The superconducting properties of the NbN/AlN stack are validated by the demonstration of fibre-coupled normal-incidence superconducting nanowire single photon detectors. The whole fabrication process is CMOS compatible, with a thermal budget compatible with the integration of other passive and active components on silicon. These results pave the way for the integration of a large number of surface or waveguide-integrated detectors on large-scale silicon wafers. Furthermore, as AlN is transparent over a broad wavelength range from the visible to the near-infrared, the optimized superconducting NbN/AlN stack can be used for a wide variety of applications, from imaging to quantum communications and quantum computing.

Superconducting niobium nitride: a perspective from processing, microstructure, and superconducting property for single photon detectors

“…NbN thin films with (111) preferential texture and a T c = 14.3 K were achieved in this case. The introduction of an AlN buffer layer has also been effectively used to enhance the T c of the NbN films on Si substrates [43][44][45]. Figure 2 shows the thickness dependence of transition temperature and transition width of the NbN films on Si with and without an AlN buffer layer [43].…”

CMOS-Compatible Ultrathin Superconducting NbN Thin Films Deposited by Reactive Ion Sputtering on 300 mm Si Wafer

“…These T C values were higher compared with the previous reports on NbN films deposited on glass or GaAs substrates via sputtering, but lower than the epitaxial NbN grown on AlN/sapphire via MOVPE and sputtering, which implies a direct correlation between T C and NbN crystalline quality [22][23][24]26]. The T C values of our NbN films with an AlN buffer layer on 300 mm Si wafer grown at a substrate temperature of 400 • C were comparable with the Tc values of NbN films with a similar thickness and an AlN buffer layer on 200 mm Si wafer [17]. It is noted that the suppression of T C in thinner NbN films is widely reported, where several physical mechanisms including proximity effect, weak localization, interface, and quantum size effect were proposed to be the possible origins [27][28][29].…”

“…[13][14][15]. Recently, there has been a growing interest in leveraging the 300 mm (12 inch) advanced CMOS fabrication environment for scaling up quantum devices due to the availability of state-of-the-art semiconductor manufacturing tools and high-precision process control [16][17][18][19]. Giewont et al proposed the development of a large-scale photonic quantum chip using advanced CMOS and siliconon-insulator (SOI) technologies, in which NbN SNSPDs could be integrated with other photonic components, e.g., waveguides, couplers, phase shifters, etc., for photonic quantum computing [18].…”

“…It is well known that the performance of NbN-based quantum devices is very sensitive to their crystallinity and superconducting properties [20,21]. Previously, large-scale high-quality NbN has only been demonstrated on an 8 inch (200 mm) Si wafer, with a focus on SNSPD device performance [17]. There is still a lack of fundamental study on the structural and superconducting properties of NbN thin films deposited on a full 300 mm Si wafer.…”

Reduction of Twin Boundary in NbN Films Grown on Annealed AlN

“…Employing single-crystal NbN films could reduce the variation in device performance of SNSPDs. Recently, AlN was proposed as a substrate for the epitaxial growth of NbN films. − The distance between Nb (or N) atoms on the (111) plane of rock-salt-type NbN is 0.310 nm, which is close to the a -axis lattice constant of AlN (0.3112 nm). This small lattice mismatch facilitates the growth of high-quality NbN films on AlN substrates.…”