Properties of NbN thin films deposited on ambient temperature substrates

Abstract

The preparation of cubic NbN films by reactive dc magnetron sputtering is described. These superconductive films are deposited at a sufficiently low temperature (<90 °C) that photoresist liftoff techniques and can be used in fabricating Josephson junctions. The superconducting transition temperature has been measured as a function of gas composition and pressure. It reaches a maximum of 14.2 K at 15% N2–85% Ar and 1.06 Pa total pressure. The resistivity ratio of these films is close to unity. Structural studies by transmission electron microscopy and electron diffraction show that films 100-nm thick or less are randomly oriented, and noncolumnar with a crystallite diameter of 5 nm and a lattice parameter of 4.46 Å, which is significantly higher than the bulk value for cubic NbN. The films are dense with void diameters no larger than 0.7 nm. Films 300-400 nm thick show a small degree of texturing in x-ray studies with a Read camera. Auger analysis shows a monotonic increase in the N/Nb ratio with increase in the N2/Ar ratio in the sputtering ambient up to 30% N2. From 30% to 50% N2 in the sputtering mixture the N/Nb ratio is constant. Small amounts of carbon impurity are found in all films.

A comparative study of transport properties in polycrystalline and epitaxial chromium nitride films

“…The simultaneous occurrence of negative chemical shift of N 1s peak and positive chemical shift of Cr 2p peak with the increase of f N 2 can be interpreted as increasing charge transfer from Cr to N with increasing nitrogen, 18 indicating a decrease of N-vacancy concentration with the increase of f N 2 . resistivity increases from 1.44 Â 10 À4 to 0.25 X cm with the increase of f N 2 , which is consistent with nitrogen-dependent resistivity of other transition metal nitrides such as ZrN, 19,20 TiN, [19][20][21] or NbN, 22 and should be attributed to the increase of nitrogen composition. [19][20][21][22] Figure 5(c) is a semilog plot of the normalized resistivity vs T…”

“…resistivity increases from 1.44 Â 10 À4 to 0.25 X cm with the increase of f N 2 , which is consistent with nitrogen-dependent resistivity of other transition metal nitrides such as ZrN, 19,20 TiN, [19][20][21] or NbN, 22 and should be attributed to the increase of nitrogen composition. [19][20][21][22] Figure 5(c) is a semilog plot of the normalized resistivity vs T…”

High efficiency NbN nanowire superconducting single photon detectors fabricated on MgO substrates from a low temperature process

“…1), but the thinnest film (t=3nm) still exhibited T C =8.6 K, ΔT C =0.9 K and RRR = 0.6, proof of the excellent quality of our low-temperature deposition process. The crystallinity of the best films was characterized using X-ray diffraction: they showed a fcc, NaCl-type crystal structure, with a lattice constant a 0 =4.45Å, which agrees with reported results [7,8]. From cross-sectional transmission electron microscopy (TEM) investigation it was determined that NbN grows on MgO substrates without any initial amorphous layers.…”

“…Low total pressures P tot (in the few mtorr range) were used, resulting in power densities of the order of 10W/cm 2 . The high sputtering energy allowed to promote the growth of high quality NbN films at a substrate temperature as low as T S = 400 • C. Following a well established procedure [7,8], the superconducting properties of NbN films were optimized studying the effects of P tot and composition of reactive gas on film structural and electrical properties. For every P tot the superconducting critical temperature T C showed a maximum as a function of nitrogen partial pressure P N 2 , which was varied within the limits determined by the requirement to produce the NbN superconducting δ phase [9].…”

Superconducting NbN microstrip detectors

“…In high-resistivity materials, pulses up to hundred mV could be expected, as opposed to (relatively) pure metals in where the pulse height tends to stay below one mV. As an example, NbN may have a resistivity [8] above 200 µΩcm and critical current up to 10 7 A/cm 2 [9]. With 200 nm normal zone the resulting voltage pulse is 80 mV using the above numbers.…”