Lowtemperature neutron irradiation of magnetronsputtered NbN filmsR. Herzog, H. W. Weber, R. T. Kampwirth, K. E. Gray, and H. Gerstenberg Citation: Journal of Applied Physics 69, 3172 (1991); doi: 10.1063/1.348557 View online: http://dx.doi.org/10.1063/1.348557 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/69/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Neutron irradiation effect on radio-frequency magnetron-sputtered GaN thin films and Au/GaN Schottkydiodes J. Vac. Sci. Technol. B 20, 1821 (2002); 10.1116/1.1498275 Reactive magnetron sputter-deposition of NbN and (Nb,Ti)N films related to sputtering sourcecharacterization and optimization J. Vac. Sci. Technol. A 19, 1840 (2001); 10.1116/1.1349189 The effects of highfluence neutron irradiation on the superconducting properties of magnetron sputteredNbN films J. Appl. Phys. 64, 1301 (1988); 10.1063/1.341850 Summary Abstract: Low temperature deposition and properties of superconducting NbN by reactive dcmagnetron sputtering J. Vac. Sci. Technol. A 1, 365 (1983); 10.1116/1.572137 Superconductivity transition temperatures of rf reactively sputtered NbN Films J. Appl. Phys. 44, 5069 (1973); 10.1063/1.1662091

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

142.150.190.39 On: Mon, 22 Dec 2014 16:59:09

Low-temperature neutron irradiation of magnetron-sputtered NbN films FL Herzog and..H. W. Weber Atominstitut der Osterreichischen Universitiiten, A-1020 Wien, Austria and Max Planck Institut ftir Festkijrperforschung, High-Field Magnet Laboratory,, F-38042 Grenoble, France

FL T. Kampwirth and K. E. Gray Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439

H. Gerstenberg Fakulttit ftir Physik E 21, Technische Universitiit Miinchen, D-8046 Garching, Germany

(Received 7 May 1990; accepted for publication 10 December 1990)

Four types of differently prepared NbN films were irradiated at 4.6 K with fast neutrons to a fluence of 5.3 X lo** m - * (E> 0.1 MeV). The critical current densities J, were measured in magnetic fields up to 23 T prior to irradiation, following low-temperature irradiation, and again after an annealing cycle to room temperature. In all films, J, was found to be completely unchanged by the radiation and annealing treatments in fields up to 15 T, but to increase at higher magnetic fields. At the same time, the upper critical fields B, increased by about 0.5 T ( -2%). Replotting J, versus reduced field B/B,, leads to identical field dependencies also in the high-field range. Hence, the observed increase of J, is equantitatively explained as a Bc2 effect.

I. INTRODUCTION

Neutron irradiation experiments carried out on a vari- ety of magnetron-sputtered NbN filmslT2 have demon- strated an extraordinarily high radiation tolerance of these materials even at very high neutron fluences (1O23 m-*, E > 0.1 MeV). Whereas T, dropped already at low fluences by - 5% and remained constant further on, J, was found to degrade by -20% in magnetic fields up to - 15 T, but to remain unchanged or even increase at higher fields due to an increase of the upper critical field Be2 and the appearance of peak effects near BC2. Critical current densities exceeding 1O8A m - * at 20 T have been reported at a fluence level of 1023m - *. Therefore, it has been suggested that sputtered NbN films should be consid- ered as promising candidate materials for applications in radiation environments, especially superconducting mag- nets for fusion reactors. However, these earlier results were obtained by exposing the superconductors to neutron irra- diation at ambient reactor temperature, in contrast to the actual operating conditions of a fusion magnet, where the damage will be introduced into the material at -5 K. In addition, occasional warm-ups of the magnet to room tem- perature have to be expected during the plant lifetime, which will lead to annealing processes in the material. Clearly, both of these effects have to be investigated exper- imentally in order to finally establish the suitability of the material for this application.

In the present paper we report on low-temperature (4.6-K) irradiation experiments on four different NbN films up to a fluence of 5.3 X 102* m-*(E > 0.1 MeV). After the irradiation the samples were warmed up to 77 K and transferred at this temperature into the measuring sys- tem. Having completed all the necessary measurements, the samples were annealed at room temperature and the measurements repeated. Hence, the requirements men- tioned above were met in all aspects except for the

warm-up to 77 K. Unfortunately, this step could not be avoided, because in situ testing of superconductors in mag- netic fields up to 23 T is not feasible at any of the existing low-temperature irradiation facilities. From the data to be presented in the following, we feel confident that the trans- fer at 77 K will not affect our major conclusion, namely, that NbN is indeed the most radiation resistant supercon- ductor known to the present day.

II. EXPERIMENT

A series of NbN films representing a range of prepa- ration conditions were prepared on polished sapphire sub- strates using magnetron reactive sputtering.3 Details of the four types of materials (A-D) selected for the irradiation program have been described previously,’ and the main differences between the materials may be summarized as follows. Low deposition rates ( 1.4 and 4.1 nm s - ’ for samples A and B, respectively) were chosen to optimize the transition temperatures (A: 14.21 K; B: 16.03 K) and

10” 00000 D . unirrodiated

0 5 10 8

d 5 ‘20

FIG. 1. Critical current densities vs field for films A, B, C, and D in the unirradiated state.

3172 J. Appt. Phys. 69 (5), 1 March 1991 0021-8979/91/053172-04$03.00 0 1991 American Institute of Physics 3172

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

142.150.190.39 On: Mon, 22 Dec 2014 16:59:09

the critical current densities at low (B) and high (A) magnetic fields, respectively. Samples C and D were made under sputtering conditions (5.6 and 6.1 nm s - ‘>, which might be applicable in a commercial conductor production, and optimized for the best high-field properties. The tran- sition temperatures of samples C and D were 12.80 and 13.53 K, respectively, and the critical current densities at 19 T about 9 X 1O’A m - *. All samples were patterned to their final dimensions (l-mm-wide strips) using standard photolithographic techniques. Each strip contained sepa- rate voltage tabs (7.2 mm apart) and large current tabs (8.1 mm apart). Electrical contact was made by In solder-

Sample A a..** unirrodiotsd 00000 irr., cold transfer

10 +++++ irr., after warm-up

f’. ‘, (, *, ,‘,“‘I I (a) ‘B (Tj

TV”““““““““““’ .

. .

1O9i l

*a .

Orn

“;I05 *9 \ : 0

5 m .

10’: -Y

Q Sample C .

*mm** unirmdiated OOOOO iv., cold transfer

106; +++++ irr., after worm-up .:,

5 0 5 20 (b)

0 ‘B (T$

b’ “” “’ ” ” ‘( ” “I” . . .

l .

1 ogi l .

@ *

w- *.

E 108;

L .

\

c

10’1 l o 7” 1 Sample D

%

..a.* unirradiotsd 00000 irr., cold transfer

10’. +++++ iv., after warm-up

0 5 5 20 (c)

FIG. 2. Critical current densities vs field in the unirradiated state (0 0 l ), following low-temperature irradiation (0 0 0) and a subse- quent annealing cycle to room temperature ( + + + ). (a) Film A, (b) film C; (c) film D.

Sample C C+eeeC irr., cold transfer ++I++ iv., after warm-up

5 10 B 0;

5 20 d

FIG. 3. Enhancement factors JJJ& vs field. Irradiated cold transfer (0 0 0) and irradiated/warmed up to room temperature ( + + + ).

ing long pieces ( - 15 cm) of insulated Cu wire onto these tabs. The thickness of the films varied between 2.5 and 3.66 Pm.

The preirradiation characterization of the films has been discussed in much detail previously.‘S2 Transition temperatures T, and normal state resistivities p,, of those films selected for the present experiments were found to fall well within the range reported previously and were, there- fore, not investigated further. The critical current densities, however, were investigated carefully up to magnetic fields of 23 T in the High-Field Magnet Laboratory, Grenoble. In all cases a voltage criterion of - 1.4 PV cm - ’ ( 1 PV across the gauge length) was used to define the critical currents. A summary of these measurements is shown in Fig. 1, and reflects the characteristic preparation effects as discussed above.

The low-temperature irradiation was made at the low- temperature irradiation facility (TTB) of the FRM reactor in Munich. The neutron flux density at this position amounts to 3 X 1017m-2s-’ (E>O.l MeV); the irradi- ation was made up to a fluence of 5.3 X lo** m - * (E> 0.1 MeV). During irradiation, a temperature of 4.6 K was maintained at the sample holder. After the end of the

irr., cold transfer

FIG. 4. Kramer plot J, “2 B”4 as a function of field for film C. Unirradi- ated (OOO), cold transfer ( 0 0 0 ), and after warm-up ( + + + ).

3173 J. Appt. Phys., Vol. 69, No. 5, 1 March 1991 Herzog et al. 3173 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

142.150.190.39 On: Mon, 22 Dec 2014 16:59:09

TABLE I. Upper critical fields and critical current densities of NbN films A, C, and D.

J, ( lo8 Am-‘) &J(T), A&/B, ;(%) 16 T 19 T

Unirradiated Cold transfer Warm-up Unirradiated Cold transfer Warm-up Unirradiated Cold transfer Warm-up

A 23.41 23.90 + 2.1 23.92 + 2.1 2.48 2.3 1 2.08 0.75 0.88 1.10 C 19.71 20.20 + 2.5 20.22 + 2.5 0.90 1.14 1.21 0.04 0.09 0.08’ D 23.34 23.80 + 2.0 23.60 + 1.1 2.95 2.67 2.45 0.89 0.94 0.94

irradiation, the samples were kept at this temperature for several days, then warmed up to 77 K and transferred into a liquid-nitrogen transport container. Prior to the J, mea- surements, the samples were transferred into a flat Styro- foam container for mounting onto the measuring rig. To do this, the current and voltage leads were unrolled and the ends soldered to the contacts. (Unfortunately, one current lead of film B broke during irradiation and the attempts to achieve reasonable contact resistivities by clamping the current lead onto the film were unsuccessful.) In the fol- lowing, the whole measuring system was transferred quickly into the cryostat, which was kept at 4.2 K. This final transfer did not increase the sample temperature by more than 5 ’ above 77 K. Having completed the entire measuring cycle (to be denoted by “cold transfer” in the following), the samples were pulled out of the cryostat and kept at room temperature for 1 h. Then the J, measure- ments were repeated (“after warm-up”).

III. RESULTS AND DISCUSSION

A survey of results obtained on films A, C, and D is shown in Fig. 2. The general features are, firstly, that data pertaining to the unirradiated and the irradiated state are hardly distinguishable in magnetic fields up to - 15 T, and secondly, that clear enhancements of J, occur in the irra- diated state at higher magnetic fields, in particular near B,,. In order to emphasize this effect, the enhancement factors JdJ& (where Jr0 refers to the unirradiated state) are plotted versus field for film C in Fig. 3. The differences between the annealed and the cold state are close to the limits of our experimental resolution and presumably in- significant.

The occurrence of this effect close to BcZ strongly sug- gests that radiation-induced enhancements of the normal state resistivity and consequently of the upper critical field might be responsible for the observed enhancement of J,. In order to check this possibility, evaluations of Br2 based on the Kramer model of flux lattice shearing4 have been made. According to this model, Jk’2B”4 should vary lin- early with induction B. However, the particular choice of this functional dependence is only of secondary impor- tance, as has been shown in Fig. 4, and a complete sum- mary of data is presented in Table I. In close agreement with our previous results,“* we note again that the data fit the function J”2B”4 very well and over a surprisingly large field range. The corresponding extrapolations of the upper critical fields indeed show radiation-induced enhancements of Bc2 by about 2%.

3174 J. Appl. Phys., Vol. 69, No. 5, 1 March 1991

Normalizing the data of Fig. 2 by the appropriate Bc2 value and replotting as a function of reduced induction B/B, leads to a complete overlap of the data. This clearly suggests that the radiation-induced increase of J, at high magnetic fields is indeed quantitatively explained by the radiation-induced increase of the upper critical field.

IV. SUMMARY AND CONCLUSIONS

In the present contribution we have reported on the first low-temperature neutron irradiation experiments on sputtered NbN films. Critical current densities have been measured in magnetic fields up to 23 T prior to irradiation, following 4.6-K irradiation (transfer at 77 K) up to a fluence of 5.3 X 10” rnw2 (E> 0.1 MeV), and following an annealing cycle to room temperature. J, in magnetic fields up to 15 T was found to remain completely unaf- fected, whereas at higher fields J, enhancements by up to a factor of -2 were observed, which could be explained quantitatively by the radiation-induced increase of Bc2.

These results support our previous conclusions, which were drawn on the basis of high-fluence ( 1O23 m - 2, neu- tron irradiations of the same films performed at ambient reactor temperature, namely, the NbN films show the highest radiation tolerance of all superconductors known at present. Based on our new results, which confirm this property also for low-temperature irradiation and thermal cycling conditions, this material indeed seems to be very well suited for applications in fusion reactor magnets. In fact, the lifetime fluences of current reactor designs vary between 1.6 and 5.7 x 1O22 m - 2 (E> 0.1 MeV) and, therefore, fall within the fluence range tested in the present experiments. The complete absence of J, degradations, even under conditions which are close to the actual magnet operating conditions (i.e., damage production at low tem- peratures and occasional annealing cycles to room temper- ature), should encourage the technological development of practical conductor fabrication processes for the construc- tion of radiation-resistant high-field magnets.

ACKNOWLEDGMENTS

We wish to thank H. Niedermaier (Vienna) and G. Maret and A. Klaschka (Grenoble) for their support and expert technical help with the experiments. This work was supported in part by Bundesministerium fur Wissenschaft und Forschung, Wien, and by the U. S. Department of Energy, Basic Energy Sciences, Materials Sciences, under Contract No. W-31-109-ENG-38.

Herzog et al. 3174 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

142.150.190.39 On: Mon, 22 Dec 2014 16:59:09

‘P. Gregshammer, H. W. Weber, R. T. Kampwirth, and K. E. Gray, J. 4E. J. Kramer, J. Appl. Phys. 44, 1360 (1973). Appl. Phys. 64, 1301 (1988). ‘H. W. Weber, W. Khier, M. Wacenovsky, and H. Hoch, Adv. Cryog.

‘H. W. Weber, P. Gregshammer, K. E. Gray, and R. T. Kampwirth, Eng. 34, 1033 (1988). IEEE Trans. Magn. MAG-25, 2080 (1989). ‘K. E. Gray, R. T. Kampwirth, D. W. Capone II, and J. M. Murdock,

‘R. T. Kampwirth, D. W. Capone II, K. E. Gray, and A. Vicens, IEEE IEEE Trans. Magn. MAC-25, 2060 (1989). Trans. Magn. MAG-21, 459 (1985). ‘H. W. Weber, Kerntechnik 53, 189 (1989).

3175 J. Appl. Phys., Vol. 69, No. 5, 1 March 1991 Herzog et al. 3175 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

142.150.190.39 On: Mon, 22 Dec 2014 16:59:09