Method of producing high T.sub.c superconducting NbN films

Thin films of niobium nitride with high superconducting temperature (T.sub.c) of 15.7.degree. K. are deposited on substrates held at room temperature (.about.90.degree. C.) by heat sink throughout the sputtering process. Films deposited at P.sub.Ar >12.9.+-.0.2 mTorr exhibit higher T.sub.c with increasing P.sub.N2,I, with the highest T.sub.c achieved at P.sub.N2,I =3.7.+-.0.2 mTorr and total sputtering pressure P.sub.tot =16.6.+-.0.4. Further increase of N.sub.2 injection starts decreasing T.sub.c.

BACKGROUND OF THE INVENTION 
This invention relates to a method of producing niobium nitride (NbN) films 
of high superconducting transition temperature (T.sub.c) by dc reactive 
magnetron sputtering, and more particularly for producing such films on a 
substrate maintained at room temperature. 
NbN thin films are important for a variety of applications due to their 
high superconducting transition temperature, and their robust refractory 
nature. They have been used as coatings on carbon fibers for 
superconducting cables, for superconducting magnets utilizing tape 
windings [R. T. Kampwirth, et al. ibid., 498 (1985)], and to form 
superconductor-insulator-superconductor (SIS) tunnel junctions for 
Josephson device applications [J. C. Villegier, et al., ibid., 498 (1985), 
E. J. Cukauskas, et al., ibid., 505 (1985), R. B. VanDover, et al., Appl. 
Phy. Letters 41, 764 (1982)]. Recently, 
superconductor-insulator-superconductor (SIS) junctions have also 
attracted a great deal of attention because of their potential use as 
quasi-particle tunneling based quantum mixers in submillimeter wave 
heterodyne receivers. [T. G. Phillips, et al., Ann. Rev. Astron. 
Astrophys., 20 285 (1982), J. R. Tucker, Appl. Phys. Lett. 36 477, (1980), 
K. H. Gundlach, et al., Appl Phys. Lett. 41, 294 (1982)] The presently 
used, mechanically soft, Pb-based SIS devices degrade on thermal cycling. 
Moreover, a low superconducting gap parameter (.about.1.5 meV) limits 
their use to frequency values .about.700 GHz. 
Niobium nitride, with its hard refractory nature, offers a potential of 
stability over repeated thermal cycling, and its high superconducting gap 
parameter (.about.3 meV) promises an extended frequency range of 
application up to .about.1500 GHz. [T. H. Geballe, et al., Physics 2, 293 
(1966)] predicted an upper limit of 18.degree. K. for stoichiometric NbN. 
Since then, numerous workers have deposited NbN thin films by a variety of 
techniques [J. R. Gavaler, et al., J. Vac. Sci. Tech. 6, 177 (1969), Y. M. 
Shy, et al., J. Appl. Phys. 44, 5539 (1973), Gin-ichiro Oya, et al., J. 
Vac. Sci. Tech. 7, 644 (1970), S. A. Wolf, et al., ibid, 17 411 (1980), K. 
S. Keskar, et al., J. Appl. Phys. 45, 3102 (1974), K. Takei, et al., Jpn. 
J. Appl. Physics 20, 993 (1981), R. T. Kampwirth, et al., IEEE Trans. 
Magn. Mag-17, 565 (1981)], and have obtained T.sub.c values in a range of 
15.degree.-17.degree. K. using high substrate temperatures 
(.gtoreq.500.degree. C.). However, for the ease of device fabrication, it 
is of extreme importance to be able to deposit NbN films on substrates 
held at room temperature. Such a film could then be deposited as a counter 
electrode in SIS junctions without causing any thermal or mechanical 
degradation to the underlying delicate tunneling barrier. Another 
advantage of room temperature deposition of NbN is that the substrate 
could also be polymeric or coated with photoresist making the films 
accessible for conventional lithographic techniques of patterning. 
The occurrence of superconductivity in transition metal nitrides depends 
sensitively on their stoichiometry and crystal structure. [L. E. Toth, 
Transition Metal Carbides and Nitrides, Academic, New York (1971), 217] 
Reactive magnetron sputtering is one of the most suitable techniques for 
deposition of such materials with stringent composition/structure 
requirements. Reactive magnetron sputtering offers an excellent control 
over the rates and pressure of reactants taking part in the reaction, and 
thereby the stoichiometry of the product. Moreover, the sputtering gas 
pressure used in the process enables one to control the film 
microstructure, purity, and stress density in the films; and absence of 
the secondary electron bombardment of the substrates allows independent 
control of the substrate temperature, a critical parameter in the 
deposition of such materials as NbN. 
In addition, due to the high kinetic energy bombardment processes involved 
in sputter deposition, it is generally quite successful in obtaining 
metastable phases at relatively low substrate temperatures. D. Bacon, et 
al., J. Appl. Phys. 54 6509 (1983) have used reactive magnetron sputtering 
on ambient temperature (.about.90.degree. C.) substrates to yield T.sub.c 
values .about.14.2.degree. K. by optimizing the total sputtering pressure 
and Ar to N.sub.2 ratio. A totally different approach has been taken by 
some workers, of including methane [E. Cukauskas, J. Appl. Phys. 54 1013 
(1983) and U.S. Pat. No. 4,426,268; E. Cukauskas, et al., J. Appl. Phys. 
57, 2538 (1985)] or cyanogen [T. L. Francavilla, et al., IEEE Trans Magn. 
MAG-17, 569 (1981)], in the reactive gas mixture with the intent of 
introducing carbon to stabilize the desired B1 (fcc, NaCl type) crystal 
structure. The lowest substrate temperature used in such a process, as 
reported more recently by E. Cukauskas, et al., J. Appl. Phys. 57, 2538 
(1985), has been 200.degree. C. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the dc reactive magnetron 
sputtering of niobium in a reactive gas mixture of high purity Ar and 
N.sub.2 is optimized to yield stoichiometric NbN films, with high T.sub.c 
onto a substrate held at room temperature by a heat sink or a water cooled 
substrate holder. Room temperature is about 20 degrees C. as defined in 
Hackl's Chemical Dictionary, 4th Edition, McGraw-Hill Book Company. These 
films possess the B1 crystal structure with a (111) texture. In films 
deposited with argon partial pressure, P.sub.Ar, greater than 12.9.+-.0.2 
Torr, increasing nitrogen injection initially improves the orientation in 
the B1 crystal structure and the T.sub.c of the film. However, beyond a 
certain threshold the structure distorts into a tetragonal phase with a 
consequent reduction in T.sub.c. The most dominant factors governing the 
formation of the transition metal nitrides are the relative metal and 
nitrogen fluxes incident on the substrate. The background argon pressure 
plays a major role in determining the overall reactive sites and residence 
times available for the nitrogen. Key parameters for an optimum high 
T.sub.c (.about.15.7.degree. K.) NbN film deposited on a substrate held at 
room temperature are: argon partial pressure (P.sub.Ar) 12.9.+-.0.2 mTorr; 
nitrogen partial pressure (P.sub.N2,I) 3.7.+-.0.2 mTorr; and total 
sputtering pressure (P.sub.tot) 16.6.+-.0.4 mTorr. Control of these three 
parameters eliminates the need for methane or cyanogen to obtain high 
T.sub.c NbN films, and the advantage is that the substrate can be held at 
room temperature during the sputtering deposition process.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, a conventional planar magnetron sputtering system 
(manufactured by US Inc.) is shown with a 2" diameter, 0.125" thick, 
99.91% pure niobium target 10 for depositing NbN films onto a precleaned 
glass or sapphire substrate 11 fixed on a water cooled substrate holder 12 
located .about.7 cm away from the target. An oil-free ultra high vacuum 
system is used to evacuate a chamber 13 of the sputtering system to a 
background pressure of .about.8.times.10.sup.-8 Torr. The vacuum system is 
comprised of a turbo molecular pump 14a backed by a mechanical pump 14b 
with a copper trap 14c therebetween, which in operation is followed by a 
300 liter/sec ion pump 15a supported by a titanium cyrosublimation pump 
15b. A differentially pumped residual gas analyzer (RGA) 16 is provided to 
monitor the system during deposition. The pumping for the RGA is provided 
by a turbomolecular pump 17a backed by a mechanical pump 17b. 
The sputtering is carried out in a reactive gas mixture of high purity 
(99.999%) argon(Ar) and nitrogen (N.sub.2). The flow rates of Ar and 
N.sub.2 into a mixing chamber 18 are independently controlled by their 
respective flow meters 19 and 20. The pressure in the chamber is monitored 
by a capacitance manometer 21 (MKS, Type 270 A). Since the primary 
objective of this invention is to deposit NbN films on substrates 
maintained at room temperature, a chromelalumel thermocouple 22 is mounted 
on the substrate 11, or on an adjacent control substrate, to monitor the 
substrate temperature during sputter deposition. The substrate or 
substrates are thermally coupled to the substrate holder 12 which is held 
at room temperature by water cooling during the sputtering process. In 
that manner, the substrate or substrates are maintained at room 
temperature. It should be noted that the sputter gun 23 is also water 
cooled in the customary way. 
Typically, an initial background pressure of argon (P.sub.Ar) is set. A 
flow of nitrogen is added to obtain the total pressure (P.sub.tot). The 
difference between P.sub.tot and P.sub.Ar gives a measure of the pressure 
of nitrogen injected (P.sub.N2,I) under the dynamic equilibrium 
conditions. The flow of nitrogen is then eliminated and the niobium target 
10 is pre-sputtered onto a mechanical shutter 24 for at least two minutes 
in each run to expose a fresh niobium surface. The variation in P.sub.Ar 
during pre-sputtering is insignificant, indicating that the better pumping 
speed for Ar due to the Nb flux is negligible as compared to the chamber 
pumping speed. Subsequently, the pre-set nitrogen flow is reintroduced and 
a stable plasma is set. In each run, the dc voltage from a power supply 25 
is adjusted (320-350 volts) to obtain a constant discharge current 
(.about.1 Amp). The total pressure of the gases during sputtering 
(P.sub.sp) is reduced, indicating the consumption of N.sub.2 in the 
reactive process of NbN formation. After allowing a few minutes for 
equilibrium to attain, deposition of the film on the substrate is 
initiated by removing the shutter. 
Water cooling of the substrate holder 12 maintains the substrate 11 at room 
temperature during the entire deposition run. Even in the absence of water 
cooling, the substrate temperature shows only a small initial increase 
with time, reaching an equilibrium value, not more than .about.90.degree. 
C. in a complete deposition run. Both the flow rates, (varying in the 
range 2-4 sccm for Ar and 0.3-1 sccm for N.sub.2 for the complete set of 
runs) and chamber pressure were closely monitored during each entire run 
along with the power applied to the target. Typical values of the voltage, 
current, and the deposition rate are -325 V, 1.0A, and 13.5 .ANG./sec 
respectively. Typical film thickness as measured by a surface profilometer 
is .about.5000 .ANG.. 
Following the above procedure, systematic sets of NbN films have been 
deposited under various partial pressures of argon, ranging from .about.5 
mTorr to .about.17 mTorr, mixed in each case with different partial 
pressures of nitrogen in the range of 2 mTorr to 6 mTorr. The surface of 
each film was examined with a scanning electron microscope (SEM). 
The film resistance as a function of temperature was measured using a 
four-probe arrangement. A calibrated silicon diode (LakeShore Cryotronics, 
Westerville, Ohio) was used as a temperature sensor, with a measurement 
accuracy of .+-.0.1.degree. K. in the 4.degree.-20.degree. K. range. The 
study of the crystal structure was performed in reflection mode using 
CuK.alpha. radiation on a Siemens Allis D-500 x-ray diffractometer. 
FIG. 2 shows the dependence of superconducting transition temperature 
(T.sub.c) of the NbN films on the total pressure (P.sub.tot) for several 
different partial pressures of argon (P.sub.Ar) with nitrogen injection 
pressure (P.sub.N2,I) varying from 2-5 mTorr in each case. For films 
deposited at P.sub.Ar .gtoreq.12.9 mTorr, the T.sub.c increases with 
increasing N.sub.2 injection, reaches a maximum, and starts decreasing 
again. The films deposited at 12.9 mTorr argon pressure with 3.7 mTorr 
N.sub.2 pressure exhibit the largest T.sub.c =15.7.degree. K. Films 
deposited with P.sub.Ar .ltoreq.10 mTorr and P.sub.N2,I .gtoreq.3 mTorr 
possessed compressive stresses leading to delamination. SEM study of such 
films revealed a gradual eruption of stress relief (blister) patterns. 
Occurrence of such compressive stresses in thin polycrystalline films [D. 
W. Hoffman, et al., Thin Solid Films, 45, 387 (1977)] deposited by 
magnetron sputtering at low carrier gas pressure is not unusual. Further 
enhancement in these stresses prohibited analysis of films deposited at 
higher nitrogen pressures for these lower P.sub.Ar values. 
FIG. 3 presents the x-ray diffraction results. For a fixed high argon 
partial pressure (P.sub.Ar :12-17 mTorr), patterns (a) through (d) in FIG. 
3 are characteristic of films deposited with P.sub.N2,I values of 3, 3.5, 
4, and 4.5 mTorr, respectively, whereas pattern (e) is characteristic of a 
film made at low P.sub.Ar values (&lt;10 mTorr) with high nitrogen injection 
(&gt;3 mTorr). Table I summarizes the observed (d) spacing values and 
relative line intensities (normalized for the 111 line) for the 
diffraction patterns shown in FIG. 3. Also listed in the table are the 
standard values of the B1(fcc, NaC1 type) [D. Bacon, et al., supra and G. 
Brauer and H. Kirner, Z. Anorg. Allg. Chemie. 328, 34 (1964)] and 
tetragonal [N. Terao, Jpn. J. Appl. Phy. 4, 353 (1965)] crystal structure 
of NbN, for comparison. The patterns (a) through (d) of FIG. 3 correspond 
predominantly to the B1 crystal structure. Moreover, a clear trend of 
increasing intensity ratio of the line (111) with respect to the line 
(200) is observed as P.sub.N2,I is increased (decreasing intensity of line 
(200) in the plots of patterns a-d which are normalized for the 111 line 
intensity). To a lesser extent, the intensity ratio of the line (222), 
second harmonic of (111), to (200) also exhibits a similar increase. In 
addition, patterns (c) and (d) shows a few more weak lines that correspond 
to the tetragonal phase of NbN with a progressive increase in their 
intensities from pattern (c) to (d). Finally pattern (e), which shows 
maximum noise presumably due to the immense strain in those films, is the 
most tetragonal rich phase with no specific orientation present. 
The films made on amorphous glass substrates and on single crystal sapphire 
substrates showed identical properties. Irrespective of the substrate 
used, the crystal structure and texture followed the same behavior which 
were exclusively dependent on the reactive gas composition and the total 
pressure. 
FIGS. 2 and 3 shows a distinct correlation between the improvement of 
T.sub.c value and the enhancement in the (111) line intensity, 
corresponding to the initial increase in nitrogen injection at constant 
P.sub.Ar value. Increase in nitrogen injection pressure beyond a certain 
threshold however leads to a gradual symmetry modification from the cubic 
to a tetragonal phase. This is accompanied by a consequent decrease in 
T.sub.c. This shows that the distortion of the B1 structure into the 
tetragonal phase is detrimental to a high transition temperature. It is 
known that T.sub.c of the B1 transition metal nitrides is very sensitive 
to the nitrogen/metal ratio [L. E. Toth, supra]. The B1 phase of NbN.sub.x 
has been reported [G. Brauer, et al., supra] over a significant 
stoichiometry range (x=0.85 to x=1.06), with the lattice parameter, a, 
showing a systematic variation from a=4.375 .ANG. to a=4.395 .ANG.. The 
highest T.sub.c films indeed exhibit only those diffraction lines which 
correspond to the B1 structure (e.g., FIG. 3b) and yield a lattice 
parameter value of 4.38.+-.0.01 .ANG.. The large uncertainty in the 
lattice parameter is primarily due to the large full width at half maxima 
(.gtoreq.0.3.degree.) of the diffraction lines, presumably caused by the 
grain size distribution and structural imperfections frozen in the films. 
The enhancement in the (111) line intensity with deposition conditions is 
primarily due to a strong orientation present in the films with the (111) 
direction perpendicular to the substrate plane. Variations in the film 
stoichiometry may also have contributed to some extent to the changes in 
the relative intensities of the diffraction lines. 
It may be mentioned here that films deposited at higher substrate 
temperatures (.about.450.degree. C.) reportedly show an enhancement in 
intensity of the (200) line. A similar observation has been reported by 
other workers in high temperature deposition with methane or cyanogen 
inclusion. [T. L. Francavilla, et. al., supra and E. J. Cukauskas, et al., 
supra]. However, the high temperature deposited films with which the 
present inventors experimented did not show any improvement in the 
superconducting transition temperature, and thus the modified reaction and 
film growth kinetics at higher substrate temperatures is excluded from the 
scope of this invention. 
The electrical resistivity and resistance ratio, R.sub.300.degree. 
K./R.sub.20.degree. K., for the films deposited under P.sub.Ar &gt;12mTorr 
and P.sub.N2,I &gt;3mTorr which possess predominantly B1 structure are 
summarized in Table II. The values are comparable to those observed by 
other workers. [D. Bacon, et al., supra and T. L. Francavilla, et al., 
supra]. The slight negative temperature coefficient observed in an 
indication of the degree of disorder in both metal and nonmetal vacancies. 
[L. E. Toth, supra page 190]. 
FIG. 4 presents nitrogen consumption, P.sub.N2,C, (as estimated from the 
difference of P.sub.tot and the pressure during sputtering, P.sub.sp, 
i.e., P.sub.N2,C =P.sub.tot -P.sub.sp) as a function of P.sub.N2,I for 
P.sub.Ar ranging from 5 to 17 mTorr. The dashed line represents P.sub.N2,C 
=P.sub.N2,I, the hypothetical case of complete consumption of injected 
nitrogen. Nitridation of the target commences at P.sub.N2,I .about.3 
mTorr, for P.sub.Ar =5 mTorr or 8 mTorr, as is indicated by a relatively 
sharp decrease in consumption with increasing injection. This transition 
point (deviation from the straight line plot) in the consumption versus 
injection characteristic is somewhat less abrupt than that commonly 
observed in the formation of reactively sputter deposited oxides. [Tetsuya 
Abe, and Toshiro Yamashina, Thin Solid Films, 30, 19 (1975); J. L. Vossen, 
W. Kern, (Ed.) Thin Film Processes, Academic Press (1978); and J. Heller, 
Thin Solid Films, 17, 163 (1973] Two general trends are noticed with 
increasing P.sub.Ar for a fixed P.sub. N2,I. First, there is a reduction 
in consumption (P.sub.N2,C) with increasing P.sub.Ar. Second, the 
consumption characteristic at and beyond the transition point becomes a 
weaker function of P.sub.N2,I with increasing P.sub.Ar. 
Clearly, it is possible to deposit the desired B1 structure of NbN 
substrates maintained at room temperature using magnetron sputtering. Its 
occurrence as well as crystalline texture evidently depend critically on 
the flux of reactants, namely Nb and N.sub.2, rather than the substrate 
temperature. In the absence of a direct measure of film stoichiometry or 
its indirect estimate from the lattice parameter (which has a limited 
accuracy), only the increasing T.sub.c with initial increase of P.sub.N2,I 
could be taken as an indication of the stoichiometry. Thus, for a film 
deposited at P.sub.Ar =12.7 mTorr and P.sub.N2 =3.7 mTorr, exhibiting 
T.sub.c .about.15.7.degree. K.; NbN.sub.x stoichiometry may have 
approached to x=1. In addition, the apparent preferred (111) orientation 
also may be a direct effect of improving stoichiometry (i.e., reduction in 
non-metal vacancies, increase in nitrogen consumption, P.sub.N2,C) and a 
contributory factor to high T.sub.c. 
The observed behavior at and beyond the transition region in FIG. 3, where 
the tetragonal phase of NbN makes its appearance, can be understood 
primarily in terms of the variations in P.sub.N2,C, since P.sub.N2,C may 
be considered as a direct measure of the amount of N.sub.2 actually taking 
part in the chemical reaction to form NbN.sub.x. Such a chemical reaction 
is influenced by the type and amount of reactant fluxes, as well as the 
presence of nonreacting species. In the pre-transition region, virtually 
all of the compound systhesis occurs at the substrate and chamber walls 
and the stoichiometry of the film depends on the relative rates of arrival 
of niobium vapor and reactive nitrogen gas onto the deposition surfaces. 
In this region, the target erosion rate and therefore the Nb incident flux 
on the substrates and walls is essentially unchanged. P.sub.N2,C is 
therefore largely controlled by P.sub.N2,I. However, for a fixed 
P.sub.N2,I, as P.sub.Ar is increased, the ratio P.sub.Ar to P.sub.N2,I is 
increased resulting in an increase in the surface impingement rate of 
argon. These additional Ar atoms on the surface of the substrates and 
walls can lower the adatom mobility of the nitrogen atoms impinging there, 
which can reduce both, available reaction sites and the residence time of 
nitrogen as an adatom, resulting in the small deviation in P.sub.N2,C from 
the straight line (P.sub.N2,C =P.sub.N2,I) behavior. 
In the post-transition region, the reduction in P.sub.N2,C becomes even 
more pronounced. Here, the reactions occurring on the target as well as 
the substrate and the wall surface not only influence P.sub.N2,C, but also 
change the gas composition significantly. For a fixed Ar pressure, as 
N.sub.2 injection is increased to the point that impingement rate of 
nitrogen exceeds the flux of sputtered Nb atoms, there is a possibility of 
target surface nitridation. NbN is conducting in nature (.about. 150.mu. 
.OMEGA.), hence no appreciable change in power (V or I or deposition rate 
as seen in FIG. 5) occurs at the transition. Thus, the incident Nb flux 
remains largely unchanged, though the sputtered species may include a 
small fraction of molecular NbN. However, a relatively large reduction in 
nitrogen consumption does occur at this point. Associated with this 
reduction in P.sub.N2,C is a rise in the partial pressure of N.sub.2 and 
thus its impingement rate. It is this increase in excess N.sub.2 which 
greatly influences the structure in the post-transition region. 
During film growth, structural order is produced largely by the surface 
mobility of the adatoms. In the pre-transition region, the adatom mobility 
of niobium and nitrogen is large enough, even on the room temperature 
substrates, to form a well-oriented B1 structure. In the post-transition 
region, the adatom mobility is reduced by the presence of excess nitrogen 
incident on the substrate. With increasing P.sub.N2,I, beyond the 
transition, continuing the same trend of increasing stoichiometry, the 
material should be towards super stoichiometry, i.e., NbN.sub.1+.DELTA.. 
It is known that super stoichiometric NbN.sub.1+.DELTA., where .DELTA. can 
be as high as 0.06, could be formed but requires a presence of very high 
pressure (.about.240 atm.) nitrogen ambient and extremely high 
(.about.1200.degree.) temperatures. [G. Brauer and H. Kirner, Z. Anorg. 
Allg. Chemie. 328, 34 (1964)] Such a B1 structure then effectively 
incorporates more nitrogen than Nb atoms, contains metallic vacancies, and 
exhibits a distinct reduction in lattice constant compared to the 
stoichiometric NbN. The low pressuer sputtering environment is not 
expected to be conducive to such a process. 
Since the structure cannot incorporate any more nitrogen, an increase in 
the partial pressure of N.sub.2 occurs in the chamber. The resulting 
increase in the impingement rate of N.sub.2 leads to a further reduction 
in the adatom mobility on the substrate. Vacancy formation then becomes 
more probable as the structure tends to modify (reduction in lattice 
constant), and in that process distorts to the tetragonal phase in some 
regions rejecting a lot more nitrogen. Such vacancy formation may also be 
promoted by incident molecular fractions which are less free to orient 
themselves in the direction of a reactive site. Films at this stage 
exhibit a mixture of B1 and tetragonal phase (Table I). It may also be 
mentioned here that the metastable B1 phase of NbN is known [N. Terao, 
Jpn. J. Appl. Phy. 4, 353 (1965)] to distort into tetragonal phase on 
vacuum annealing. 
It is a common observation in any reactive magnetron sputtering, 
particularly with small size targets, that the target erosion rate during 
one deposition of film itself significantly modifies the overall 
conditions of dynamic equilibrium. [M. Gurvitch, J. Vac. Sci. Tech. A2, 
1550 (1984)] In the present study also, changes in sputtering conditions 
with time were observed. FIG. 6 shows the variation in nitrogen 
consumption as a function of injection at the start and at the finish of 
the deposition, for two different values of P.sub.Ar, thus giving a 
complete evolution in the rate of nitrogen consumption during the 
deposition. Therefore, the present films may have some gradient in their 
stoichiometry, microstructure, crystal structure, and thereby in the 
transition temperature, along the thickness. This would manifest itself in 
the form of a somewhat wide superconducting transition. The high T.sub.c 
films made at P.sub.Ar =12.9 mTorr have widths .about.0.2.degree. K., and 
the maximum transition widths seen for other films are .about.0.5.degree. 
K. Although it is very difficult to estimate the extent of structural or 
chemical gradients in the films from the observed transition widths, one 
may expect sharper transitions if films can be made by suitably locking 
the deposition conditions at the optimum values. 
Finally, FIG. 2 suggests another possibility of a further improvement in 
the film T.sub.c. For the same nitrogen injection pressure (.about.3.5 
mTorr), FIG. 2 shows that T.sub.c improves with reducing Ar partial 
pressure. This is, of course, due only to the optimized incidence ratios 
of Nb, N.sub.2, and Ar on the substrate surface as other sputtering 
conditions are unchanged. The compressive stresses developed in the films, 
however, do not allow experimental verification of this fact to low Ar 
pressures (&lt;10 mTorr). If the compressive stresses in the film at lower Ar 
pressure could be suppressed by introduction of an additional suitable 
deposition parameter such as an application of appropriate substrate bias, 
T.sub.c may improve even further approaching the predicted value of 
18.degree. K. [T. H. Geballe, et al., supra] 
SIS tunnel junctions using these high T.sub.c films have been made. The 
native oxide obtained by room air oxidation or plasma oxidation is used as 
the tunnel barrier and thermally evaporated pure lead is used as a counter 
electrode. A typical current versus voltage (I-V) characteristic is shown 
in FIG. 7. It clearly indicates a uniform, single phase, large 
superconducting gap NbN material. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art. Consequently, it is intended 
that the claims be interpreted to cover such modifications and variations. 
TABLE I 
__________________________________________________________________________ 
Comparison of standard and observed d spacing values and relative 
intensity 
(normalized for 111 line) for the fcc and tetragonal phases of NbN 
Standard d values (.ANG.) 
Observed d values (.ANG.) and relative intensity (I) 
FCC Tetragonal 
Sample "a" 
Sample "b" 
Sample "c" 
Sample "d" 
Sample "e" 
d,(hkl) 
I d,(hkl) 
I d I d I d I d I d I 
__________________________________________________________________________ 
2.534(111) 
VS 2.53 
100 2.53 
100 
2.518(111) 
100 2.51 
100 2.52 
100 
2.51 
100 
2.194(200) 
VS 
2.19 
60 2.19 
15 2.19 
14 2.19 
21 2.19 
84 
2.191(200) 
100 
2.158(002) 
60 2.15 
7 2.15 
18 2.16 
78 
1.551(220) 
S 1.551(220) 
40 1.55 
28 1.55 
7 1.55 
6 1.55 
16 1.55 
52 
1.538(202) 
60 1.53 
5.5 1.53 
13 1.54 
54 
1.323(311) 
M 
1.32 
24 1.32 
5 1.32 
5 1.32 
18 1.32 
42 
1.321(311) 
20 
1.303(113) 
20 1.31 
4.5 1.31 
17 1.31 
47 
1.267(222) 
VW 1.26 
18 1.26 
6 1.26 
7 1.26 
14 1.26 
36 
1.097(400) 
VS 1.09 
14 1.09 
3 1.10 
4 1.10 
9 1.10 
35 
1.007(331) 
W 1.00 
12.5 
1.01 
3 1.00 
4 1.00 
9 -- -- 
__________________________________________________________________________ 
TABLE II 
______________________________________ 
##STR1## 
for NbN films with predominatly B1 structure deposited 
under varying mixtures of P.sub.Ar and P.sub.N2,I conditions. 
Reactive Gas Mixture 
(mTorr)P.sub.Ar 
(mTorr)P.sub.N2,I 
(.mu. .OMEGA.cm)Resistivity 
##STR2## 
______________________________________ 
17.0 3.0 83 0.87 
17.0 3.3 90 0.93 
17.0 3.6 114 0.81 
17.0 4.0 138 0.73 
12.9 3.4 188 0.95 
12.9 3.7 227 0.90 
12.9 4.1 292 0.85 
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