High field critical current densities and resistive transition of NbCN superconducting films

We studied the influence o f the carbon density on the critical current densities o f niobium carbonitride films in high magnetic fields. The samples were about 2 200 A thick and were obtained by reactive sputtering either from a Nb-C target in a residual A r + N 2 atmosphere or from a bulk Nb target in a residual A r + N 2 + CH 4 atmosphere. They all had sufficiently high critical temperature, around 15 K, which implies that the ratio o f carbon atoms over the total supplementary nitrogen and carbon atoms lay in the range 0.06 to 0.35. The samples were immersed in the liquid helium a t 4 . 2 K. The registered V(I) curves either showed an hysteretic feature or not, depending on the composition and/or the applied magnetic field. The best critical current densities, about several 10 l ° A m -2 in a zero field were still more than 3 x 108 in a 14 T field, irrespective of direction, transverse or perpendicular. In the former case, some samples were tested up to 18 7-, a value for which the best critical density was 107 A m -2. Heterogeneous samples which have periodic variations in carbon concentration, have still greater critical densities in high transverse fields. The Jc (B) curves show a plateau or a slight hump when the field is such that the flux line lattice parameter fits the period o f the carbon ratio variations. The resistivities o f the different films are also high enough, more than 10-6 ~rn. So these alloys might be of interest in a superconducting switchgear, using the resistive transition to the normal state in order to limit a short circuit current.

High field critical current densities and resistive transition of NbCN superconducting films Y. Brunet, J. Mazuer and M. Renard The pseudo-binary, NbCN, superconducting compounds have a B~ type structure. They were first prepared in the form of a bulk sintered specimen ~ and were shown to have critical temperatures as high as 18 K. I' 2 The critical magnetic field Bc2 is lower than 12 or 13 T and the critical current densities are less than that of technological NbTi, from around one order of magntiude. In order to improve the superconducting properties in high fields, techniques other than sintering were attempted, such as diffusion a or chemical vapour deposition. 4

10 -7 Torr in the sputtering room. The partial pressures of the reactive gases are controlled by mass-spectroscopy. The substrates are square (25 x 25 mm 2) polished glass which may be heated up to 800°C before and during deposition for cleaning. Two different targets were used : the first type of pure niobium sputtered in a residual atmosphere containing argon, nitrogen and methane; and the second type, a sintered niobium carbide disk, similar to the targets used by Gavaler et al. s The sputtering room contains only argon and nitrogen.

The best materials are now prepared by sputtering, s a method which was first improved with the binary niobium nitride compound.6, 7 NbCN sputtered films having Tc similar to those of bulk materials were deposited on quartz or glass substrates. The high value of the estimated critical magnetic field, more than 25 T, is directly connected to the high, normal state resistivity, according to the mixed state theory.S, 9

The films were deposited at a rate of 100 A per minute and have a thickness somewhat greater than 2000 A, a value necessary to get sufficient critical temperature. ~° This thickness is known to within 100 A. Influences of the fdm composition and substrate temperature on critical temperature and normal state resistivity were studied.12 Preliminary measurements show that the best critical temperatures were obtained when the ratio (C + N)/Nb of atoms (carbon + nitrogen over the niobium atoms) is around 1.3. All the samples we studied in high fields have a ratio (C + N)/Nb around this value, but a wide range of values for the ratio C/(C + N), a parameter to which the critical current density at zero field was shown to be more sensitive, 1° exhibiting a maximum value around C/(C + N) ~ 0.12.

The films we studied were sputtered at the Laboratoire d'Etude des Matdriaux Minces, D6partment de M6tallurgie, Centre d'Etudes Nucl6aires de Grenoble. The critical temperature of the films lay within the range 10 to 16 K, 1° slightly lower than those previously reported, s In this paper we have studied the influence of the carbon and nitrogen ratio on the high fields properties, specifically the critical current densities when the magnetic field is applied either parallel or perpendicular to the film.

Most of the characteristics of the Films are summarized in Table 1. The ratio (C + N)/Nb and C/(C + N) are deduced from measuring C, N and Nb using nuclear reactions, the resulting precision is around 15% on all the ratios.

Sample preparation and characteristics The sputtering apparatus has already been described. 1~ Briefly summarized, it is a triode type system in which primary and secondary pumps allow a residual pressure of The authors are at CRTBT-CNRS and Laboratoire d'Electrotechnique, ERA 534 BP 186, 38042 Grenoble, France. Received 3 November 1978.

CRYOGENICS. FEBRUARY 1 9 7 9

Experimental procedure and results The resistive transitions. After sputtering, the geometry of the the samples is defined by engraving the films as shown in Fig. 1. Electrical connections for the current, I, and voltage, 1I, are soldered with indium. We measured the variations of the voltage along the sample when varying the current

0011-2275/79/020107-06 $02.00 © 1979 IPC Business Press

107

Table 1. The temperature of the substrates was 600~C for all the samples except no 3 ( 5 0 0 ° C ) and 12 ( 8 0 0 ° C )

No

Composition Thibkness, A (C+N)/Nb C/C + N

Pn at Tc + AT, #~cm

Tc, K

1

2 300

1.4

0.06

140

15.0

2

2 400

1.2

0.08

120

15.1

3

2000

1.3

0.10

127

14.3

4

2000

1.2

0.12

130

15.0

5

2 200

1.2

0.14

95

15.6

6

2 000

1.4

0.20

100

15.5

7

2400

1.4

0.23

110

13.7

8

2 200

1.2

0.25

105

15.7

9

1 700

1.4

0.26

95

15.9

10

2 300

1.4

0.27

73

14.9

11

2 300

1.3

0.29

83

14.7

12

2 100

1.4

0.33

64

15.2

13

2400

1.3

0.36

82

15.1

Samples 7 and 10 to 13 were preparedfrom NbC target, others from Nb target.

IO

_

V

E

x"

/= i,f 0.10

0.15

0.20

0.25 /,A

0.60

0.65

(the curve on the right, Fig. 1). The step which reaches 0.2 to 0.5 mV in amplitude is around the middle value between Jc and Jt. For some samples, we did not observe such a step, and the slope of the curve regularly increases with the current up to the resistive transition (the curve on the left, Fig. 1). If.we decrease the current before the resistive transition is reached, the same curve is registered, except near the step where a slight hysteresis is observed. A third type of behaviour also appeared with samples 1 and 2 for which Jc = Jt in all B± (sample 1) or for B± ~< 8 T (sample 2). For most of our registrations, we used a current source which was protected against the high voltage appearing for J > J t in the normal state, and the generator was automatically disconnected. We were also able to use in some cases, for the lowest currents, a source which allowed us to observe the complete superconducting to normal and inversely normal to superconducting transition, by decreasing the current from J > Jt to zero. We always observed an hysteretic behaviour which reduced with increasing the field, in both the transverse and perpendicular geometry. Examples of such variations are given in Figs 2 and 3 and will be discussed later. The critical current densities. Comparison of the critical values for some samples is given in Fig. 4 as a function of the magnetic field. Samples 1 and 13 are at the edges of the composition range when considering the ratio C/(C + N). Three other samples are presented which give the best results either with the perpendicular or/and parallel geometry. The Je values at a given B for the other specimens were always limited by the extreme curves on Fig. 4. We soon noted 13 that the best Jc values are about four times less than those measured by Gavaler et al. s However, our scope here is not to discuss the absolute values OfJc, the comparison being always somewhat difficult for different experimental techniques, but the variations in Jc from one sample to another. So it appears from Fig. 4 that the best absolute results are obtained in the perpendicular geometry, with sample 3 which has a ratio C/(C + N) = 0.1. When decreasing or increasing C/(C + N), the behaviour is modified with respect of the field geometry, Jc remaining better only for the high B_Lvalues and decreasing less than Jc in the parallel direction at all fields for all samples where C/(C + N) > 0.25.

0.70

Fig. 1 The V~/~ curves for sample3 at 10.5 T (on the right) and 13 at 10.7 T (on the left). The vertical arrowspointing upwardsat the end of the curves correspond to the normal state transition. The two possible directions of the applied magnetic field are also defined

density in a constant magnetic field which,may be settled at values up to 18 T. The current density J is always perpendicular to the magnetic field, but the field may generally be applied in two possible directions with respect to the surface of the films, either perpendicular or parallel. We so define two geometries (Fig. 1) the perpendicular (B±) and the transverse one (Btr). For each chosen value of the magnetic field, we register the V(/) curve. The critical current density Jc is defined as the one for which a 1/aV cm -1 electrical field appears along the sample. It is often very different from what we call Jt, the transition current density, a value for which the tangent to the V(/) curve just before transition is crossing the current axis. Generally, when J > Jc, the voltage begins to increase slowly up to a step more or less high, depending upon the field and composition

108

to

m o

0.1

0.2

0.3

/,A

Fig. 2 T h e V(I) curves at constant field for sample n o 12. The values along t h e curves are in Tesla. - - - - - is for the transverse orientation, the three others for the perpendicular o r i e n t a t i o n

CRYOGENICS. FEBRUARY 1979

/

5 D

/

>-

%.

i0 e°

4--

-.. ~ a 2 \

3--

x

A

2--

I0 e I

"4

--

O

,,j o a

0.1

0.5

0.2 /,A

E <~

. -

. o I0 8

\

\

/ 10 7

7,

e

IO 0

O.I

0.2 /,A

b

0

5

I0 B,T

0.3

Fig. 3 a - is the V(I) curves labelled in Tesla f o r sample 2 at constant B t r ; b - is f o r sample 8 at B t r (16 and 18.2 T) and B± (12 and 13 T). The curves which end w i t h a vertical upwards a r r o w are those in B L, b u t w i t h an amplified vertical scale by a factor 10 2 (12 T) and 10 3 (13 T)

In order to study more precisely the influence of the composition, we draw the variation o f J c versus C/(C + N) for some values of Btr and B± (Figs 5 and 6). It is somewhat difficult to derive a general law, but it may be seen that in the transverse geometry the critical current density is not very sensitive to the composition; at 18 T the ratio between the highest and lowest Jc is around one order of magnitude; it is only 3 at 5 T. In contrast, Jc in B± varies much more with C/(C + N), especially in high magnetic fields. At B± = 10 T, the preceding ratio between the extreme Jc values is 30 and when B± t> 13 T, Jc is quite a fast decreasing function of C/(C + N).

15

20

Fig. 4 The critical current densities as a function of the applied field for sample 1 ( ) 3 (------) 13 ( . . . . ) 6 in Btr (e) and in B 1 (o) and 2 in B t r (/x) and B i (&)

itself would then fix the critical temperature which really is that of bulk NbN and not bulk NbCN. To verify the influence of the carbon precipitates, inhomogeneous samples were prepared the composition of which varies periodically along the thickness. Such a variation is obtained by modulating the nitrogen partial pressure of the sputtering room 14' 16 between two values Pmax and Pmin. So, depending on the time during which these pressures are settled, corresponding thicknesses lmax

Table 2.

Heterogeneous samples

Period, ~,

Pn at Tc+ AT, /zD, cm

Tc,

No

Thickness, A

A

2 200

120

-

13.0

B

3 800

250

--

12.1

C

2 000

80

286

10.3

D

2 300

130

109

12.8

E

2 500

200

82

12.8

K

Inhomogeneous samples. In previous studies of the samplesla, 14 we noted that the films did not exhibit such a columnar structure as that reported by Gavaler et al, ~sbut are made of very thin crystals around 8 0 A large. Furthermore, it is most probable that these crystals are not a NbCN compound, but a NbN matrix containing a carbon phase in the form of more or less dispersed precipitates. These precipitates have never been seen experimentally, using electronic spectroscopy for instance, but they might be C, NbC or CN or some other phase with C. This phase might constitute the principal pinning centres, the matrix

CRYOGENICS.

FEBRUARY

1979

The temperature of the substrates was 6 0 0 ° C target is the sintered NbC disk.

for

all the samples. The

109

+

-4-

+

i0 I0

• ++

+

++

+

ii •

10 9

0

•

+

O0

•

•

0

0

/-

critical current densities in the transverse geometry given in Fig. 7. In a 5 T field, values greater than 2 x 101°A m -2 are reached. A more complete analysis is shown for sample A on Fig. 8, where Jc (B) for an homogeneous sample is given as reference. This sample is prepared in a residual nitrogen pressure, the value of which is fixed to an average p between Pmax and Pmin weighted by the corresponding term lmax and lmin of the period according to the relation : p

--

'I'

0

/max + lmin

O

E

It may be seen that critical densities are sensitive enough to the field orientation with respect to the surface of the film; so an error of 10 degrees in the parallelism results in a reduction by about a factor of two o f the current densities. As expected, no improvement of the Jc values was obtained in the perpendicular geometry in which the sample was also tested after being covered with 4 or 5 mm of silicon grease. Such a modification of heat transfer with the helium bath induced no reduction of the Jc values.

o

i0 8

--

•

10 7 --

0

O0

0

0

O0 0

I

I0 e

0

I

0.1

= Pmax x/max + Pmin x lmin

Our experiments were carried out at 4.2 K, the samples being immersed in the liquid helium. No experimental

I

0.2

C{C+N)

Discussions

0.5 -I I• • I1•

Fig. 5 V a r i a t i o n s o f Jc as a f u n c t i o n o f the c o m p o s i t i o n o f the f i l m s f o r B t r equal t o 0 (=) 1 T (+) 5 T (A) 10 T (n) 14 T (e) and 18 T (o)

•

i0 e° 0

0

0

0

0

•

O

0

A +

i 0 t° +

0 0

+

•

O

9

Io

+

+

O

+ +

•

0

+

0 0

13 13

10 9

+

D

•

E

0

Q

O ~

v

•

v

•

IO° E

V

%

O

•

.2O O 10 7 --

107

O

O A

io 6

O

I O.I

I 0.2

C(C+N)

I 0.3

-I

i0 a _

Fig. 6 V a r i a t i o n s o f Jc as a f u n c t i o n of the c o m p o s i t i o n f o r Bj.(~) are f o r B j_ = 13 T, o t h e r signs are the same as in Fig. 5 A

and lmin, with different carbon concentrations, are deposited, the period of the variations being/max + lminA number of samples were so prepared and characterized ~6 and some of them were tested in magnetic field up to 18 T. Their characteristics are summarized in Table 2, and their

110

0

I 5

I io B,T

I 15

20

Fig. 7 The critical current densities as a function of the applied field for the inhomogeneous samples A (o) B (m) C (o) D (e) E (~)

CRYOGENICS.

FEBRUARY

1979

attempts were made to determine the critical magnetic field, which might be deduced from the mixed state theory leading to the following relation 4 Bc2(0)

3.11 x 10aTPnTc

=

Unfortunately the heat capacity coefficient 7 is not known for all the entire NbCN systems. The value 0.21 kJ m -a K -2 given 17 for bulk NbCo.aNo.7 corresponds to a critical field equal to 11 T when pn = 100/1~2cm and Tc = 15 K. Such a low value may evidently not concern our samples which still exhibit a critical current density greater than 107A m -2 at 18 T. The spin paramagnetic limit, ~s Bp (0) = 1.84 Tc, when existing for these samples would be 28 T, probably around 8 T higher than the value deduced from a rough extrapolation of the Jc(B) curves. The resistive transitions which are shown in Figs 1 to 3 are characteristic of most of the samples. No modifications were observed when successive tests were performed. We have noted in a previous paper la the stability of the Jc(B) characteristics in time. Fig. 2 is an example of the stable behaviour in high field; the curve at 10.6 T was exactly the same when registered after 14 months during which the sample remained at ambient temperature and pressure

i 0 t°

o 0

;-t°o

0

0

0

0

0

•

•

•

•

• o

o o lO 9

o

O+ ",~ % o÷ +

o %

\ \

E

o

\

\ +

\

\ \

o +

\

As far as the heterogeneous samples are concerned, the slight hump in the curve Jc(B) appears as a characteristic feature. It is related to the periodic variation of the composition, being shifted to higher fields when the period is decreasing. Much greater peak effects were previously observed 22 on Pb-Bi alloys. The situation of the hump on the Jc(B) curve is directly related to the vortex lattice parameter

\ \ \ I

i0 a

d-

io 7 0

I 5

I I0

I 15

B,T Fig. 8 The Jc(B) curves f o r samples A when the magnetic field is applied in the transverse geometry (o), when the angle between field and sample surface is 5 degrees (o) and 10 degrees (v). The other signs (+ and o) correspond to the perpendicular geometry, the sample being covered w i t h silicon grease in one case (o). The dashed line is for an homogeneous sample of same mean composition in the transverse direction

CRYOGENICS.

FEBRUARY

1979

without any protection. The general form of the transitions, shown in Fig. 3 is observed as well in the transverse as in the perpendicular geometry. When increasing the field, the transition is less abrupt and the hysteresis reduced; the straight line after transition corresponds to the normal state resistance of the sample. At first sight, we might think it a thermal hysteresis, the sample being heated by the transport current. We have seen on the inhomogeneous samples (Fig. 8) that the Jc(B) curve was not modified when the sample was covered by silicon grease. It was not the same with the Jt values which were lower in that case. The V(/) curves are partly the same when increasing I, but with grease the sudden resistive transition occurs at a lower voltage and a lower power since the thermal exchange is worse. However, some features do not agree with such a thermal explanation. First, the sudden resistive transition for a given sample at different fields does not occur for the same value of the dissipated power, as would be the case in a thermal transition. Secondly, the return to the mixed state always occurs at a much higher value of the power than that which is dissipated when the sample turns to the normal state. Thirdly, this power is sometimes very low, less than 40 mW cm -2 as appearing on the curve registered at 14 T (Fig. 3a). This value is more than one order of magnitude less than the pool boiling of liquid helium, 19 and may not explain an increase in the temperature of the film up to its critical value. So, the thermal transition is not a fully satisfying explanation and a theoretical analysis of our results perhaps might take into account the very high velocity of the flux lines when accelerated by the Lorentz's forces, just before the sudden resistive transition. For example, the electric field settled along sample 8 at 13 T (Fig. 3b) was 150 V m -~. The resulting vortice velocity exceeds 1 1 m s -1, corresponding to fluctuations of the superconducting parameter at frequencies greater than 1 GHz. The high electric field observed at high applied magnetic fields results from the high normal state resistivity of our samples. Much higher velocities of the flux lines (more than two orders of magnitude) were yet already observed on type II thin indium 2° or tin 21 films. These films exhibit voltage-current characteristics which are qualitatively very similar to those we observed, the resistive transition being also very smooth as the magnetic field increased. No attempts were made to look at some possible hysteretic behaviour when decreasing the transport current after transition.

It may be thought that when d is equal to the period l =/max + lmin of modulation of the carbon precipitates, the mean pinning force is increased and subsequently Jc as a result. For example, the maximum of the hump is obtained for B ~ 6 T with sample A. This corresponds to a lattice parameter d ~ 170 ,~, to be compared with a period equal to 120 A. In the case of sample C, no increase is obtained, but at 15 T (d ~ 100,8,) there is a decrease in the slope of the curve. This corresponds to a fit of the flux lines lattice with the position of the presumed carbon pinning centres.

111

Conclusion The sputtered NbCN films show a particular behaviour near the resistive transition : when we recorded the transition curves at different constant fields, the hysteresis behaved as a decreasing function of the field, and it dissappeared in the highest field, where we observed a continuous and stable evolution up to the normal state. As we have seen, a simple thermal explanation is not very satisfying and a theoretical approach probably would have to take into account the high velocity of flux lines just before transition. When looking at the critical current densities, we observed that they are comparable and often higher than those of technological superconductors, even at fields as high as 15 T. The normal state resistivity ion, is such, that the quantity pn J2 is unusually large, around 10 is W m -a. This is of prime interest in switching devices, which may use the superconducting to normal transition, the volume of the device being inversely proportional to pn J r . The high value o f Pn may be used, in low temperature choppers, 23 allowing the measurements of continuous voltage as low as 10 -13 Volts. The authors wish to thank MM. J. Spitz and A. Aubert (Laboratoire d'Etude des Matdriaux Minces - Centre d'Etudes Nucl~aires de Grenoble) who have prepared and characterized the samples. Preliminary measurements of Tc and Pn were performed by the Service des Basses Temperatures (CENG). The present measurements were carried out at the Service National des Champs Intenses - CNRS - Grenoble. We also thank M.D. Tomasik (Laboratoire d'Electrotechnique, ERA 534, CNRS) for technical assistance in preparing the experimental set up and with the experiments.

112

References 1 2 3 4 5

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Matthias, B.T.PhysRev 92 (1953) 874 Pesall, N., Jones, C.K., Johansen, H.A., Hulm, J.K. Appl Phys Letters 7 (1965) 38 Hechler, K., Saur, E., Wizgall, H. Z Physiks 205 (1967) 400 Ohmer, M.C., Frederick, W.G.D.JApplPhys45 (1974) 1382 Gavaler, J.R., Janocko, M.A., Jones, C.K. Appl Phys Letters 8 (1971) 305 Gavaler,J.R., Hulm, J.K., Janocko, M.A., Jones, C.K. J Vac Sci Technol 6 (1969) 177 Spitzet, HJ. J Vac Sci Technol 9 (1972) 333 Maki, K. Physics 1 (1964) 21 Kim, Y.B., Hempstead, C.F., Stmad, A.R. Phys Rev 139 A (1965) 1163 Spitz, J., Aubert, A.,Rapport CEA-CENG no 61/74 Grenoble (1974) Spitz, J., Ghevallier, J., Aubert, A.J. Less Corn Met 35 (1974) 181 Spitz, J., Aubert, A. Proc 5th Int Conf on Chemical Vapor Deposition London (1975) 258 Mazuer, J., Aubert, A, Spitz, J. Communication at the meeting of the Soci6td des Electriciens et Electroniciens Grenoble, May 78. To be published Aubert, A. Memoire CNAM, Grenoble (1975) Gavaler, J.R., Janocko, M.A., Jones, C.K. J Vac Sci Technol 10 (1973) 17 Spitz, J., Aubert, A. Rapport DGRST, 73-7-1579 Grenoble (1976) Drew-Hughes, D. Practical Superconducting material, in Superconducting machines and devices, Plenum Press, New York (1974) Clogston, A.M. Phys Rev Letters 9 (1962) 266 Jergel, M., Stevenson, R. Cryogenics 12 (1972) 431 Takayama, T. JLow Ternp Phys 27 (1977) 359 Gubankov, V.N. Fizika Tverdogo Tela 14 (1972) 2618 USSR; Soviet Physics-Solid State 14 (1973) 2264 USA Raffy, M., Renard, J.C. Solid State Comrn 11 (1972) 1679 Berton, A., Bret, J.L., Chaussy, J., Odin, J. French Patent ANVAR no 734093, US Patent no 4074 343

CRYOGENICS. FEBRUARY 1979