Haase, E.L.

Ruzicka, J.

English

Haase, E. L.

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KfK 4409 September 1988 lmprovements in the Properlies of NbN Layers Through the Presence of Small Amounts of Aluminum During Spultering E. L. Haase, J. Ruzicka Institut für Nukleare Festkörperphysik Kernforschungszentrum Karlsruhe KERNFORSCHUNGSZENTRUM KARLSRUHE Institut für Nukleare Festkörperphysik KfK 4409 Improvements in the Properties of NbN Layers Through the Presence of Small Amounts of Aluminum During Sputtering E.L. Haase and J. Ruzicka* Kernforschungszentrum Karlsruhe GmbH, Karlsruhe *On leave of absence frorn the Physical Institute, Czechoslovak Acaderny of Sciences, Prague, CSSR Als Manuskript vervielfältigt Für diesen Bericht behalten wir uns alle Rechte vor Kernforschungszentrum Karlsruhe GmbH Postfach 3640, 7500 Karlsruhe 1 ISSN 0303-4003 IMPROVEMENTS IN THE PROPERTIES OF NbN LAYERS THROUGH THE PRESENCE OF SMALL AMOUNTS OF ALUMINUM DURING SPUTTERING Pure phase B1 NbN layers could be sputtered in our labora-tory only with a maximum Tc of 15.9 K and a residual resistance ratio RRR of <0.9. Samples with a Tc of up to 16.7 K invariably had a large fraction of hexagonal foreign phase. If under simi-lar conditions one sputters layers in the presence of small amounts of Al, one obtains pure B1 phase NbN layers with a maximum Tc of 16.9 K. The RRR rises to values of up to 1 .16. The specific resistance drops from typical values of araund 400 u~cm to values as low as 73 u~cm. The width of the X-ray lines drops occasionally, indicating increased grain size. Al acts like a catalyst, being substituted with less than 0.03 at% for a high T layer. When Al is substituted for Nb in detectable c quantities both Tc and the lattice parameter drop. To verify the above results the experiment was repeated in a magnetron rather than a RF sputter system with similar outcome. VERBESSERUNG IN DEN EIGENSCHAFTEN VON NbN SCHICHTEN DURCH GEGEN-WART VON KLEINEN MENGEN VON ALUMINIUM WÄHREND DES SPUTTERNS Phasenreine B1 NbN Schichten konnten in unserem Labor nur mit maximalem Tc von 15.9 K und einem Restwiderstandsverhältnis von <0.9 hergestellt werden. Proben mit einem Tc bis zu 16.7K hatten immer einen großen Anteil von hexagonaler Fremdphase. Wenn man unter gleichen Bedingungen Schichten in Gegenwart kleiner Mengen von Aluminium durch Sputtern herstellt, erhält man phasenreine Schichten mit einem Tc bis 16.9 K. Das Restwiderstandsverhältnis steigt auf Werte bis zu 1.16. Der spezifische Widerstand fällt von typischen Werten um 400 u~cm arf Werte bis zu 73 u~cm. Die Breite der Röntgenlinien nimmt gelegentlich ab 1 was auf erhöhte Korngrößen hinweist. Das Aluminium wirkt wie ein Katalysator, da es für eine hoch-Tc Schicht mit weniger als 0.03 at% substituiert wird. Wenn Niob durch Aluminium in meßbaren Mengen ersetzt wird, fällt sowohl Tc als auch der Gitterparameter. Um obige Resultate zu reproduzieren, wurde das Experiment in einer Magnetron- statt HF-Anlage mit ähnlichen Resultaten wiederholt. - 1 -1. INTRODUCTION In the last few years numerous publications have appeared on NbN (1-8) and extensive literature cited therein. For very recent work see the proceedings of the LT18 (8). The interest lies partly in the development of supercon-ducting junctions (2). On the other hand the critical currents of up to 10 7 A/cm2 are very high and hence the development of this material for high current conductors (3,4,8) will continue, as long as the new oxidic superconductors do not reach compar-able high current densities in bulk form. Furthermore it will continue to be needed for applications where radiation resistance is important, such as accelerators and fusion reactors. One should be aware that NbN is about a factor of 10 4 more resistant to radiation darnage (9). A brief version of this paper has been presented at the International Conference on High-Temperature Superconductors and Materials and Mechanisms of Superconductivity (10). 2. EXPERIMENTAL The sputtering conditions for the two systems are summa-rized in Table 1. The substrate temperature for optimum Tc was in the range of 820-880°C. The optimum N2 partial pressure for magnetronsputtering was 1.4 Pa. The special method to determine the lattice parameters a0 of stressed films is described in ref. 11 andin particular the simplification pointed out in 12 is used. Not applying this method may cause an error of as large as 1% in a . 0 The substrates were mounted on a resistively heated Ta-strip. For the RF-runs four substrates were used, while for the magnetron-runs single substrates were employed. The geometry was chosen such that there was an Al-gradient along the substrates. The composition of the samples was partly characterized by an electron microprobe. For the Tc measurements the resistive four point method was used. The 1% onset points are given. Table 1 - 2 -Preparation Conditions for Nb1 Al N -x x RF-Sputtering Magn.-Sputtering Substrate temperature/°C 790-910 820-1100 N2 partial pressure/Pa 0.4 0.3-2.1 Ar+N 2 pressure/Pa 2.0 5.2-15.5 Power/W 500(RF) 300 (DC) Voltage/V :::1800 :::570 Distance anode/cathode/mm :::4 0 ~45 area fraction Al-Nb 0-3% 0-5% diameter cathode/mm 150 75 3. RESULTS AND DISCUSSION Reuschenbach (13) analyzed by Rutherford backscattering and X-ray analysis a Nb layer on Al 2o3 nitrided at 1450°C by Remeika (6). See sample C4113M in Table 2 and its remarkably high RRR. Fig. 1 (Fig. 14 in ref. 13) shows the Rutherford backscattering spectrum of the nitrided Nb layer. A few at% of Nb appear to be missing near the rearside of the layer extending until the middle. A corresponding excess of Al appears to be visible above the Al edge of the substrate. This makes it plausible that a Substrate reaction has taken place and Al has diffused into the rearhalf of the layer. This is not surprising as Al is soluble in Nb up to 23 at% and substrate reactions of Nb on Al 2o3 are known to take place at temperatures above 900°C - 3 -20,_---------------------------------------------. 16 1<\0 12 rl w ~ 0::: 1-s 8 0 u Fig. 1 4 NBN 2 MEV ALPHA ................. I Nb ENERGY rKEVJ Rutherford backscattering spectrum of the nidrided Nb layer, sample C4113M. Fig. 2 (Fig. 13 in ref. 13) shows the X-ray diffraction diagram of the same layer. Essentially only the B1-phase is visible. The top half of the picture shows the same diagram with an expanded counts/channel scale. A few very weak peaks of a foreign phase are visible, which however do not stem from Nb 3Al. From the intensities the fraction of foreign phase is estimated to be less than about 0.2 at.%. The information from both figures makes it plausible that Al is substitutional on Nb sites in the B1 phase. Fig. 2 8 7 6 tri s 0 .-:: w ~lf er: f-z: 68 u 2 :1 120 100 80 w f- 60 ;:2 f-z :::::> 0 u Lf0 20 8 0 8 I I 16 2Lf ~\....)-16 2Lf - 4 -NBN I I I I I I I 82 Lf0 Lf8 S6 6lf 72 80 ANGLE THETA mEGREESl NBN t j 1 82 Lf0 Lf8 S6 6Lf 72 80 ANGLE THETA [DEGREESl X-ray diffraction diagram of sample C4113M (bottom half). The top half shows the same diagram with an expanded cfs scale. Note the weak foreign phase peak near 8 = 26°. Fia. 3 14 [J 12 [J 0.435 D - 5 -D D D D 0.437 B 1--()--1 D m m m D m <D NbN o Nb1_xAlxN m NbN o Nb1_xAlxN ()) } RF } Magnetron 0.439 ao /nm Plot of T vs. lattice Parameter a for samples c - 0 prepared with and without Al in a RF and a magnetron sputtering system. We now deliberately added Al by preparing layers on single crystal sapphire substrates by reactive sputtering with composite Nb-Al cathodes. For optimum Tc the substrate temperatures were around 850°. Considering the low melting point of Al of 660°C the Al-sticking coefficient is likely tobe low. Fig. 3 shows T vs. a for layers with the two c 0 sputtering systems with and without Al. With rising a0 and falling Al-content Tc rises. The error bar indicates the error in a0• Predominantly points with high Tc were evaluated and are shown. Table 2 gives the data for selected samples. The highest T values are obtained phase pure for RF-prepared samples with c Al, from the point at 15.3 K/0.4377 nm to sample NAN3-4R with 16.9 K/0.4395 nm and less than 0.03 at% Al. RF-prepared samples - 6 -without Al have lower Tc and contain large fractions of the hexagonal phase, except for the sample at 15.9 K/0.4392 nm with a residual resistance ratio of 0.87. The magnetron pre-pared samples with Al extend from 13.5 K/0.4343 nm via sample NAN15R with 1.4 at% Al to sample NAN26RR with 16.5 K/0.4394 nm and 1.2 at% Al. The corresponding samples prepared without Al have all lower Tc extending from 13.8 K/0.4373 nm to 15.5 K/0.4384 nm. The sample NAN26RR shows an additional almost single crystalline line with a d-value of 1.0863 nm, to the large angle side of the 400 line with the same texture. Attempts to account for this extra line as superstructure line by doubling or trippling the unit cell were not convincing. However, the line does coincide with a line of a less common NbN phase, probably the s-phase. Sample NAN3-4R has less Al than the detection limit of 0.03 at%. This makes it probable, that it is not the Al-content but rather the role of Al during the film growth, which leads to phase purity and enhanced Tc. Possibly Al acts similar to a catalyst. More probably Al acts as a flux medium, which is not built into the NbN structure,as often used in the preparation of single crystals. Building Al into the B1-structure leads to a decrease in both Tc and a0, as evidenced by sample NAN15R with 1.4 at% Aland sample NAN17R with 1.6 at% Al. The a0-value of layer NAN17R is smaller by 1.2%, that of NAN15R is smaller by 0.7% than stoichiometric samples. Unfortunately, the metal film of the latter has a black layer on top, which is probably carbon, up to 6.2 at% totally. The residual resis-tance ratio of layer NAN26RR is 1.16, which is the highest which we prepared. Only films containing foreign phase have a larger RRR. The high p-value is probably not an inherent property of NbN films, but rather an artifact associated with the typical columnar growth and the possible existence of voids or oxide layers between the columns. Samples NAN26RR and NAN17R have a large excess of oxygen, correlated with a high p(RT). Sub-stoichiometric bulk NbN 0 . 9 has p(RT) = 44.4 ~Qcm and a RRR of Table 2 Properties of selected samples. RRR means residual resistance ratio Property/Sample C4113M NAN3-4R NAN15R NAN26RR NAN3-3L NAN17R Tc/K (Onset) 16.9 16.9 14.4 16.5 16.4 13.5 a0/nm 0.4411 0.4395 0.4363 0.4394 0.4386 0.4343 T ;oc s T0=1450 830 830 900 780 830 PNziPa - 0.40 1.4 1.4 0.40 2.0 PAr+Nl~ - 2.0 16 7.6 2.0 15.5 at% Nb 47.7 'l 50. 1 47.7 48.5 }49 .9 45.3 51.3 45.1 at% Al <2 .4> J <0.03 1.4 1.2 0.9 1.6 at% N 43.4 l 46.4 l 43.6 l 45.1 42.7 46.1 at% 0 49.9 49.4 5.9 3.6 6.0 2.2 J 3. 0 J 3.8 J49.9 at% C 4.3 3.0 2.5(<6.2) 2.5 1.5 1.2 -.J Layer thickness/nm 0.48 :::1.0 :::1.0 ~1 . 0 ~1.0 ::::1.0 p(RT)/1Jr2Crn 76 220 1940 1030 90 2050 RRR 1.27 0.993 0.735 1 . 160 1 . 029 ::::0.75 Method Diffusion RF Magnetron Magnetron RF Magnetron - 8 -0.98 (14). Sample NAN3-3L (Table 2) with a residual resis-tance ratio RRR of 1.029 and 0.9 at% Al is representative of RF-films grown with Al where RRR ranges from 0.92 to 1.13 with specific resistance values in the range from 73 to 220 ~Dem, typically 100 ~Dem. The maximum of our Tc vs. a0 data extrapolates to the data of ref. 7. Samples prepared in the RF-system generally have superior properties. That the Tc of the samples in the present study is limited to 16.9 K and not slightly higher is presumably due to. the presence of oxygen. The high Tc of up to 17.8 K of the samples of ref. 15 is presumably due to small amounts of Al that diffused into the Nbc 1 N layers -x x during preparation at 1100°C. The authors observed that at 1100°C sapphire yielded higher Tc than quartz substrates. 4. CONCLUSIONS The presence of small amounts of Al during sputtering improves the properties of NbN layers markedly: (i) Tc rises somewhat (0.2-1.0 K), (ii) the layers are pure B1-phases, (iii) the residual resistance ratio reaches up to values of 1 .16, (iv) the specific resistance drops to values as low as 73 ~Dem, (v) occassionally the x-ray line width decreases, pointing to the formation of larger grains. For a high Tc layer the Al is not incorporated in detectable amounts into the film. Possibly Al acts similar to a catalyst. When Al is incorporated, T and a fall. An alternative explana-c 0 tion is that Al behaves like a flux medium. The small electro-negativity difference Nb-Al of 0.1 and the smaller Al-radius probably play a role. The situation is reminiscent of the role of oxygen during the growth of Nb 3Ge films, except that the evi-dence is more clear-cut. - 9 -A similar situation exists when C is substituted for N, or Ti for Nb, even in small amounts (5,16) except that the mechanism is related to the fact that the number of electrons/ atom is approaching 4.85, where the highest Tc is observed. ACKNOWLEDGEMENTS The authors want to thank o. Meyer for a critical reading of the manuscript and H. Späte for the electron miereprobe analyses. They are grateful to L. Reuschenbach for permission to show figs. 1 and 2. REFERENCES (1) J.R. Gavaler, J.K. Hulm, M.A. Janocko and C.K. Jones, J. Vac. Sei. and Techn. ~ (1968) 177 (2) E.J. Cukauskas, W.L. Carter, S.B. Quadri and E.F. Skelton, IEEE Trans. Magn. MAG21 (1985) 505 (3) S. Ohshima, M. Dietrich, G. Linker, J. Appl. Phys. 57 (1985) 890 (4) R.T. Kampwirth, D.W. Capone, K.E. Gray, and A. Vincens, IEEE Trans. Magn. MAG21 (1985) 459 (5) E.J. Cukauskas, W.L. Carter, and S.B. Qadri, J. Appl. Phys. 57 (1985) 2538 (6) M. Gurvitch, J.P. Remeika, J.M. Rowell, J. Geerk, and W.P. Lowe, IEEE Trans. Magn. MAG21 (1985) 509 (7) T.H. Geballe, B.T. Matthias, J.P. Remeika, A.M. Clogston, V.B. Compton, J.P. Maita, and H.J. Williams, Physics 2 (1966) 293 (8) Proc. 18th Int. Conf. Low Temperature Physics, Jpn. J. Appl. Phys. ~ (1987) Suppl. 26-3, pp. 947-954 (9) B. Egner, J. Geerk, H.C. Li, G. Linker, 0. Meyer, and B. Strehlau, Jpn. J. Appl. Phys. ~ (1987) Suppl. 26-3, 2141 (10) E.L. Haase in Proc. Int. Conf. High-Temperature Supercon-ductors and Materials and Mechanisms of Superconductivity, Feb.-March 1988, Interlaken, Switzerland, in press (11) E.L. Haase, Thin Solid Films 124 (1985) 283 (12) E.L. Haase, in Proc. of the Int. Conf. on Advanced Methods in X-ray and Neutron Structure Analysis of Materials, Karlovy Vary, Oct. 1987, J. Hasek ed., Plenum Press, N.Y. (1988), in print (13) L. Reuschenbach, Diplama Thesis, Karlsruhe University (1985) p. 63 (14) L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, N.Y. (1971) p. 188 (15) T.L. Francavilla, S.A. Wolf, and E.F. Skelton, IEEE Trans. Magn. MAG17 (1981) 569 (16) E.L. Haase and J. Ruzicka, in Novel Superconductivity, S.A. Wolf and v.z. Kresin eds., Plenum Press, N.Y. (1987) 39 

Improvements in the properties of NbN layers through the presence of small amounts of aluminum during sputtering

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Improvements in the properties of NbN layers through the presence of small amounts of aluminum during sputtering

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