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The sputtering of high Tc A15 Nb3Si and V3Ge
Solid State Communications, Vol. 24, pp. 733—737, 1977.
Pergamon Press.
Printed in Great Britain
THE SPUTTERING OF HIGH T~A15 Nb3 Si AND V3Ge R.E. Somekh and J.E. Evetts Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, England (Received 17 June 1977 by C. W. McCombie) We report some initial results on the preparation of A15 Nb3Si and V3Ge using a getter sputtering technique. Under sufficiently clean conditions we observe an increase in the superconducting transition temperature. DC onsets in excess of 14 and ilK have been observed for Nb3 Si and V3Ge respectively. In each case a positive identification of the A15 phase has been made. HIGH PRESSURE DC getter sputtering has been used substrate heater on the pressure is of order 2—4 x 10-8 with great success by Gavaler [1] and Testardi [2] as a torr. The MkII system is a Vacuum Generators Ltd. method for producing A15 Nb3Ge with superconducting UHV system with an ultimate pressure of 3 x lO~ torr transition temperatures in excess of 22.5 K. The techusing a 9 in. oil diffusion pump with a CTR trap, a small nique is attractive because of its versatility and potential ion pump and a well-housed sublimation pump. With the cleanliness and the relative ease with which different heater at 1470 K the pressure is typically 0.5—2 X 10- 10 alloys or intermetallics may be produced by simply torr. The heater on which the sapphire substrates rest is changing the target. The main disadvantage is the relmade of thin tantalum strip. In the MkII system the atively slow deposition rate, however the much faster inner hitrogen cooled chamber can be completely isolated chopper controlled evaporation systems of Sigsbee [3] from the rest of the system (60!) by means of an internal and Hallak et a!. [41 have not yet produced quite as high valve. In the MkI system pure argon sputtering gas is bled transition temperatures. through at the rate of 0.3 torr 1 sec’ while in the We report here preliminary results on Nb3 Si and MkII system it is admitted during the pre-sputtering V3Ge. They are part of a larger study of A15 compounds stage and the inner chamber sealed. The argon gas whose overall objective is to understand the role of reservoir is common to the two systems and consists of metastability, impurities and disorder in the attainment an argon cylinder, (2 ppm 02 impurity, better than 5 N of high Ta’s in Al5 materials. Initially Nb3Ge, V3 Si, overall), connected through a needle-valve to a 251 V3Ge and Nb3 Si are being studied in order to determine reservoir chamber. This chamber is separately connected the general patterns associated with this group of to both MkI and MkII and can be pumped to below materials. l0~torr. The same gas pressure and sputtering voltage It is clear from the literature that there are a large have been used throughout this study, namely 500— number of known and unknown parameters in a sputter. 550 V and a pressure of about 400 torr. The2 plasma and deing system, it is therefore vital to specify the exact current density is in the range 1—3 mAcm condition of any deposition being performed. We are position rates onto substrates about 2 cm away from the carrying out the investigation using two independent target have been in the range 0.5—1 Asec’. Each getter sputtering units, this has proved a very useful aid sputtering run of 3 hr was preceeded by a period of coolto eliminating “system” orientated parameters. The two ing to get the whole inner chamber to liquid nitrogen systems used (referred to as MkI and MkII) are basically temperatures and a preheat at some substrate temperathe same as that of Theuerer and Hauser [5]. The subture above that to be used in the run. This was followed strates and target are within a liquid nitrogen cooled by a “pre-sputter” of at least ~hr to give a final clean up. chamber in the main vacuum system, the walls of this A shutter was then opened for the actual deposition chamber continually getter impurities from the sputtersequence. After each run the shutter was closed and the ing gas during deposition. In the MkI version the inner heater switched off and sputtering carried on for a few can and outer vacuum system (201) are only loosely minutes to keep gettering the system as it cooled. The isolated. The ultimate pressure of 0.5—2 x 108 torr is substrate temperature (Td) was estimated from the achieved after bakeout with an in situ sublimation pump current density in the heater strip and a previous caliand a double trapped 2 in. mercury pump. With the bration run using an optical pyrometer with the nitrogen ,
733
734
THE SPUTFERING OF HIGH T~Al5 Nb3 Si AND V3Ge
Vol. 24, No. 10
Table 1. Data for samples from Run 62 in MkII system and Run 68 in MkI system using an argon arc melted V3 Ge target. Transition temperatures from 4 terminal DC resistance measurements are given at onset (1%) andfor 90%, 50% and 10% ofthe normal state resistance. Also given is the room temperature resistance ratio (RRR) for each sample and the substrate temperature during deposition, (Td) Run/Sample number
T~(onset) (K)
T~(0.9) (K)
T~(0.5) (K)
T~(0.1) (K)
Td (K)
RRR
62/1 62/2 62/3 62/4 62/5
6.2 11.2 10.5 7.0 6.8
5.7 10.5 9.7 6.7 6.7
5.5 8.6 7.3 6.6 6.7
5.4 7.3 6.6 6.5 6.5
1010 1060 1150 1240 1370
6.5 10.0 14.9 22.0 13.6
68/1 68/2 68/3 68/4
6.2 6.9 6.7 6.7
6.1 6.6 6.7 6.7
6.0 6.4 6.6 6.6
5.9 6.25 6.5 6.5
1120 1200 1300 1410
2.8 13.0 29.0 18.4
Table 2. Data for samples from Runs 63, 67, 71 and 76 using an argon arc melted Nb3 Si target and Run 92 using a composite target. Transition temperatures from 4 terminal resistance measurements are given at onset (1%) and for 90%, 50%, and 10% of the normal state resistance. Also given is the room temperature resistance ratio (RRR) for each sample and the substrate temperature during deposition, (Td) Run/Sample number
T~(onset) (K)
T~(0.9) (K)
63/1 63/2 63/3 63/4
5.8 9.0 13.7 14.0
4.5 7.3 11.8 12.8
<4.2 5.2 8.5 8.6
67/1 67/2 67/3
6.9 10.5 8.5
5.7 8.5 8.5
5.1 7.5 8.2
71/i 71/2 71/3 71/4
7.5 8.7 8.7 8.9
5.1 8.6 8.65 8.8
76/i 76/2 76/3 76/4+
13.5 9.5 9.4 12.0
9.7 9.45 9.3 10.0
92/5 92/6 92/7 92/8 92/9 92/10
<4.2 10.4 12.4 11.2 8.1 7.0
<4.2 9.0 10.0 8.8 7.8 6.9
+
—
*
—
Substrate at end of heater Td uncertain. Only two terminal measurements made.
T~(0.5) (K)
Td (K)
R.R.R.
<4.2 <4.2 <4.2 <4.2
1060 1150 1250 1370
1.75 1.74 2.4 2.6
4.7 7.2 8.1
1270 1360 1470
1.64 2.28 4.05
4.4 8.55 8.6 8.7
<4.2 8.5 8.5 8.5
1270 1360 1470 1610
1.6 6.1 5.0 4.5
7.7 9.45 9.0 8.4
7.0 9.4 9.0 7.2
1370 1470 1570 1720
2.4 4.9 4.0
<4.2 <4.2 8.0 7.8 7.2 6.8
1340 1340 1340 1340 1340 1340
2.96 2.50 2.20 2.25 2.88 2.17
<4.2 7.0 8.3 7.9 7.6 6.8
T~(0.1) (K)
—
System
MkIl
Mkl
MkII
*
MkII
Vol. 24, No. 10
THE SPUTTERING OF HIGH T~A15 Nb3Si AND V3Ge
735
Sample No T~onset
92/6 T~ 10 4K
I~UJ ~92/7
I
44°
(211) I
42°
400 —
2e,
(210)
I
38°
36°
11
Tc 92/8 11.2Kj~ 1 92/9 Ic 81Kj~ 124K
(200) 34° T~.- 7K
Bragg Angle
Fig. 1. X-Ray traces in the low angle region for a series of samples prepared on a constant temperature substrate (Run 92, Td = 1350 K). The Nb—Si composite target gives a gradient of silicon concentration; the samples towards the bottom of the figure being poorer in silicon. Values for the T~onset (1%) are given for each sample. Positions of the prominent AlS reflections 200, 210 and 211 are indicated by the dashed lines and can be clearly identified for samples with a high T~onset. (Radiation: Cu Ka, X = 1.542 A). can be removed so that the heater, with substrates, could be seen through a window on the side of the vacuum system. We have found that there are many indicators of the general cleanliness of the two systems and the common argon gas reservoir arrangement. As we have managed to lower the impurity concentration, over a
in the sputtering gas. DC resistance measured onsets of 22.7 K with midpoints of 21.5 K have been regularly obtained by careful control of the oxygen level. However we are not at present sure of the role that the oxygen plays. 1. THE PREPARATION OF V 3 Ge
period of time, from about 50 to 2 ppm we have seen a gradual rise in the transition temperature for V3 Si. Our initial attempts gave T~values in the range 13—15.5 K close to the values previously reported by Schmidt Ct al. [6]. Now we can prepare material with a T~,of 16.8 K in our MkH system, (room temperature resistance ratios in excess of 20). An indicator of both the absolute and relative cleanliness of the MkI system is seen in the fact that it is only with difficulty that a T~,of 16.6 K is obtained. The background 2 ppm of 02 in the continuous flow sputtering gas is probably responsible for the difference between the MkI and MklI systems. In the MkI system we have also been making Nb3 Ge and find 1230K), significantly that adding only 5 ppmchanges.the of oxygen,transition (with a Tdtemperature of obtained. This again indicates the low levels of impurity
The transition temperature of A15 V3Ge is generally quoted as 6.7K [7] ,however, there are two features of this material that are a little unusual; it has a relatively high B~2in comparison with other A15 materials of similar T~,and it displays an uncharacteristic temperature dependence ofonsets its velocity of sound [8]. Furthermore Gavaler found T~ of about 8 K when preparing it in an oxygen free and probably hydrogen rich environment [9]. Using our “clean” system we have prepared V3Ge with an onset of 11 .2 K (Table 1, Run 62). The target was an argon arc melted billet and Td ranged from 7’da around 1150K broad 1100 to 1400K across set of substrates on a transitions heater of variable width. With were observed with onsets well above 6.7 K; for higher
736
THE SPUTTERING OF HIGH T~AiS Nb3 Si AND V3Ge
values of Td the transition temperature returned to the standard bulk valve and the reduced resistivity ratio showed an increasing trend. In the MkI system, using the same target, T~was close to 6.7 K for all samples. X-ray scans of the samples showed that the Al 5 lattice parameter remained constant at 4.778 A, very close to that usually quoted for the bulk material [101. The material with the highest T~showed sharp Al 5 lines only; for higher substrate temperatures, and hence lower T~,the Al5 lines became broader and additional lines appeared which could not be correlated with Al 5 reflections. These results do not allow a simple interpretation of the resistivity ratio trends. The temperature dependence of the resistivity was similar to that of other A15 materials and characteristic of a metal with interband scattering and a peaked density of5, states [11]. In at higher the low temperature regime p(T) T~ temperatures the variation became exponential with p exp (T/Tr) and Tr about 85 K.
A..
1%.L,1ttP,.JtiA~.JI~
‘.1
1
U3~3l
In the literature Nb3 Si is reported as having a T~of about 9K when made as a film [12, 13] ; however Testardi [14] has recently achieved an onset as high as 12K by flash annealing an amorphous film. Also Pan etal. [15] have prepared Nb3Si with an onset of 19K using an explosive technique. We have made material with T~onsets as high as 14K. Two types of target were used in the experiments, either an argon arc melted target to give a deposition of uniform composition on a set of substrates at different temperatures, or a composite target to give a composition gradient across a set of substrates held at a fixed temperature. Experiments were carried out in both systems. Using the MkI system the maximum onsets observed were about 11 K, quite significantly less than the highest onsets observed using MkII. The initial inference was that a very low impurity level is required for the production of a high 7~material, However, the situation is not as simple as this, for attempts to reduce the impurity level by extending the pre-sputtering time or using a narrower heater (to reduce outgassing of the heater clamps) resulted in the transition temperature returning to 9K. Thus, while a relatively clean environment appears to be necessary for the production of a high T,,, it is evidently not sufficient; exactly what agency is producing the highest T~is not at present clear. Representative results are given in Table 2 which summanses four experiments using the argon arc melted target and one experiment using a variable composition target. The observation that the high values of T, corresponded with rather high substrate temperatures (1350 K) raised the question as to
Vol. 24, No. 10
whether or not selective evaporation of silicon was occuring. This prompted us to use the composite target with a silicon gradient across it to produce a range of compositions at fixed substrate temperature. Although the preliminary results on the role of silicon evaporation are not clear-cut, T~onsets up to 12.4K were obtained and we were able to make a clear identification of the Al 5 phase. By comparing X-ray traces and T~ measurements for specimens with different compositions (Fig. 1) the X-ray reflections for an A15 phase were unequivocably identified. The lattice parameter was 5.18 A independent of T~,this is larger than the value inferred by Pan et al. [15] from high angle reflections, and considerably larger than that predicted by the Geller radii [16]. The figure shows the appearance of Al 5 lines in amongst X-ray studies on bulklines Nb of an Nb5 Si3 phase. Previous 3 Si have shown that A3B structures of the types Ti3P [17] and Au3Cu [18], can occur; we have also observed these phases, sometimes concurrently with the A15 phase in the same sample. It is interesting to note, however, that material with a T~ of 9 K showed no clear A15 phase. The complexity of the X-ray data and the breadth of the resistive transition suggests possibly that the high T~Al5 material consists of only a thin residual layer on the surface of the film. Further evidence for this is seen in the fact that although the films were prepared at 1350 K, if annealed for l2hr at 1040 K the A15 phase disappeared and T~fell back to an onset of 9.3 K (midpoint 8.0 K). The breadth of the resistive transition may therefore be explained in terms of a proximity effect of the low T~material affecting a thin layer of high T~material. Experiments are in progress to ion-etch away the surface layer in order to test this hypothesis. The resistivity measurements show a rather weakly characteristic “A15” shape which is also consistent with the above ideas. To conclude, we have found that V3Ge can be made with T~onsets much greater than the usually quoted bulk value. The precise conditions required to stabilize high or low T~,material are elusive, with repeat runs under near identical conditions still producing widely different results. We believe that V3Ge remains a material not fully understood and suggest that the general acceptance of a bulk T~value of 6.7 K must be doubted. A T~,near 11 K would be consistent with the predictions of lzyumov eta!. [19]. In the case of Nb3Si we have obtained a 1% onset in excess of 14K and have made a positive identification of an Al5 phase, with a rather expanded lattice parameter. The results are more reproducible than for V3 Ge but there are still features not fully understood in its preparation, in particular the stability with respect to trace impurities and in situ annealing at the deposition temperature.
Vol.24, No. 10
THE SPUTTERING OF HIGH Ti,, A15 Nb3Si AND V3Ge
737
REFERENCES 1. 2.
GAVALER J.R., App!. Phys. Lett. 23, 480 (1973). TESTARDI L.R., WERNICK J.H. & ROYER W.A., Solid State Commun. 15, 1(1974).
3.
SIGSBEE R.A., IEEE Trans. Mag. MAG13, 307 (1977).
4. 5.
HALLAK A.B., HAMMOND R.H., GEBALLE T.H. & ZUBECK R.B., IEEE Trans. Mag. MAG13, 311(1977). THEUERER H.C. & HAUSER J.J., Trans. Met. Soc. AIME 233, 588 (1965).
6. 7. 8.
SCHMIDT P.H., BACON D.D., BARZ H. & COOPER A.S., J. App!. Phys. 46, 2237 (1975). HARDY G.F. & HULM J.K., Phys. Rev. 89, 884 (1953). TESTARDI L.R. & BATEMAN T.B.,Phys. Rev. 154,402(1967).
9.
GAVALER J.R., MILLER J.W. & APPLETON B.R.,App!. Phys. Lett. 28,237(1976).
10.
JOHNSON G.R. & DOUGLASS D.H.,J. Low Temp. Phys. 14,565(1974).
11.
WOODARD D.W. & CODY G.D.,Phys. Rev. 136, A166 (1964).
12.
TESTARDI L.R., WERNICK J.H., ROYER W.A., BACON D.D. & STORM A.R., J. App!. Phys. 45,446 (1974). HAMMOND R.H., IEEE Trans. Mag. 11, 201 (1975).
13. 14. 15.
TESTARDI L.R., WAKIYAMA T. & ROYER W.A.,J. Appi. Phys. 48,2055(1977). PAN V.M., ALEKSEEVSKII V.P., POPOV A.G., BELETSKU YU. I., YUPKO L.M. & YAROSH V.V.,JETP Lett. 21, 228 (1975).
16. 17.
JOHNSON G.R. & DOUGLASS D.H.,J. Low Temp. Phys. 14, 575 (1974). DEARDORFF D.K., SIEMENS R.E., ROMANS P.A. & MCCURE R.A.,J. Less Common Metals 18, 11(1969).
18.
GALASSO T. & PYTE J., Acta Cryst. 16, 228 (1963).
19.
IZYUMOV YU. A. & KURMAEV E.E., Usp. Fix. Nauk 118,53 (1976).
Pergamon Press.
Printed in Great Britain
THE SPUTTERING OF HIGH T~A15 Nb3 Si AND V3Ge R.E. Somekh and J.E. Evetts Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, England (Received 17 June 1977 by C. W. McCombie) We report some initial results on the preparation of A15 Nb3Si and V3Ge using a getter sputtering technique. Under sufficiently clean conditions we observe an increase in the superconducting transition temperature. DC onsets in excess of 14 and ilK have been observed for Nb3 Si and V3Ge respectively. In each case a positive identification of the A15 phase has been made. HIGH PRESSURE DC getter sputtering has been used substrate heater on the pressure is of order 2—4 x 10-8 with great success by Gavaler [1] and Testardi [2] as a torr. The MkII system is a Vacuum Generators Ltd. method for producing A15 Nb3Ge with superconducting UHV system with an ultimate pressure of 3 x lO~ torr transition temperatures in excess of 22.5 K. The techusing a 9 in. oil diffusion pump with a CTR trap, a small nique is attractive because of its versatility and potential ion pump and a well-housed sublimation pump. With the cleanliness and the relative ease with which different heater at 1470 K the pressure is typically 0.5—2 X 10- 10 alloys or intermetallics may be produced by simply torr. The heater on which the sapphire substrates rest is changing the target. The main disadvantage is the relmade of thin tantalum strip. In the MkII system the atively slow deposition rate, however the much faster inner hitrogen cooled chamber can be completely isolated chopper controlled evaporation systems of Sigsbee [3] from the rest of the system (60!) by means of an internal and Hallak et a!. [41 have not yet produced quite as high valve. In the MkI system pure argon sputtering gas is bled transition temperatures. through at the rate of 0.3 torr 1 sec’ while in the We report here preliminary results on Nb3 Si and MkII system it is admitted during the pre-sputtering V3Ge. They are part of a larger study of A15 compounds stage and the inner chamber sealed. The argon gas whose overall objective is to understand the role of reservoir is common to the two systems and consists of metastability, impurities and disorder in the attainment an argon cylinder, (2 ppm 02 impurity, better than 5 N of high Ta’s in Al5 materials. Initially Nb3Ge, V3 Si, overall), connected through a needle-valve to a 251 V3Ge and Nb3 Si are being studied in order to determine reservoir chamber. This chamber is separately connected the general patterns associated with this group of to both MkI and MkII and can be pumped to below materials. l0~torr. The same gas pressure and sputtering voltage It is clear from the literature that there are a large have been used throughout this study, namely 500— number of known and unknown parameters in a sputter. 550 V and a pressure of about 400 torr. The2 plasma and deing system, it is therefore vital to specify the exact current density is in the range 1—3 mAcm condition of any deposition being performed. We are position rates onto substrates about 2 cm away from the carrying out the investigation using two independent target have been in the range 0.5—1 Asec’. Each getter sputtering units, this has proved a very useful aid sputtering run of 3 hr was preceeded by a period of coolto eliminating “system” orientated parameters. The two ing to get the whole inner chamber to liquid nitrogen systems used (referred to as MkI and MkII) are basically temperatures and a preheat at some substrate temperathe same as that of Theuerer and Hauser [5]. The subture above that to be used in the run. This was followed strates and target are within a liquid nitrogen cooled by a “pre-sputter” of at least ~hr to give a final clean up. chamber in the main vacuum system, the walls of this A shutter was then opened for the actual deposition chamber continually getter impurities from the sputtersequence. After each run the shutter was closed and the ing gas during deposition. In the MkI version the inner heater switched off and sputtering carried on for a few can and outer vacuum system (201) are only loosely minutes to keep gettering the system as it cooled. The isolated. The ultimate pressure of 0.5—2 x 108 torr is substrate temperature (Td) was estimated from the achieved after bakeout with an in situ sublimation pump current density in the heater strip and a previous caliand a double trapped 2 in. mercury pump. With the bration run using an optical pyrometer with the nitrogen ,
733
734
THE SPUTFERING OF HIGH T~Al5 Nb3 Si AND V3Ge
Vol. 24, No. 10
Table 1. Data for samples from Run 62 in MkII system and Run 68 in MkI system using an argon arc melted V3 Ge target. Transition temperatures from 4 terminal DC resistance measurements are given at onset (1%) andfor 90%, 50% and 10% ofthe normal state resistance. Also given is the room temperature resistance ratio (RRR) for each sample and the substrate temperature during deposition, (Td) Run/Sample number
T~(onset) (K)
T~(0.9) (K)
T~(0.5) (K)
T~(0.1) (K)
Td (K)
RRR
62/1 62/2 62/3 62/4 62/5
6.2 11.2 10.5 7.0 6.8
5.7 10.5 9.7 6.7 6.7
5.5 8.6 7.3 6.6 6.7
5.4 7.3 6.6 6.5 6.5
1010 1060 1150 1240 1370
6.5 10.0 14.9 22.0 13.6
68/1 68/2 68/3 68/4
6.2 6.9 6.7 6.7
6.1 6.6 6.7 6.7
6.0 6.4 6.6 6.6
5.9 6.25 6.5 6.5
1120 1200 1300 1410
2.8 13.0 29.0 18.4
Table 2. Data for samples from Runs 63, 67, 71 and 76 using an argon arc melted Nb3 Si target and Run 92 using a composite target. Transition temperatures from 4 terminal resistance measurements are given at onset (1%) and for 90%, 50%, and 10% of the normal state resistance. Also given is the room temperature resistance ratio (RRR) for each sample and the substrate temperature during deposition, (Td) Run/Sample number
T~(onset) (K)
T~(0.9) (K)
63/1 63/2 63/3 63/4
5.8 9.0 13.7 14.0
4.5 7.3 11.8 12.8
<4.2 5.2 8.5 8.6
67/1 67/2 67/3
6.9 10.5 8.5
5.7 8.5 8.5
5.1 7.5 8.2
71/i 71/2 71/3 71/4
7.5 8.7 8.7 8.9
5.1 8.6 8.65 8.8
76/i 76/2 76/3 76/4+
13.5 9.5 9.4 12.0
9.7 9.45 9.3 10.0
92/5 92/6 92/7 92/8 92/9 92/10
<4.2 10.4 12.4 11.2 8.1 7.0
<4.2 9.0 10.0 8.8 7.8 6.9
+
—
*
—
Substrate at end of heater Td uncertain. Only two terminal measurements made.
T~(0.5) (K)
Td (K)
R.R.R.
<4.2 <4.2 <4.2 <4.2
1060 1150 1250 1370
1.75 1.74 2.4 2.6
4.7 7.2 8.1
1270 1360 1470
1.64 2.28 4.05
4.4 8.55 8.6 8.7
<4.2 8.5 8.5 8.5
1270 1360 1470 1610
1.6 6.1 5.0 4.5
7.7 9.45 9.0 8.4
7.0 9.4 9.0 7.2
1370 1470 1570 1720
2.4 4.9 4.0
<4.2 <4.2 8.0 7.8 7.2 6.8
1340 1340 1340 1340 1340 1340
2.96 2.50 2.20 2.25 2.88 2.17
<4.2 7.0 8.3 7.9 7.6 6.8
T~(0.1) (K)
—
System
MkIl
Mkl
MkII
*
MkII
Vol. 24, No. 10
THE SPUTTERING OF HIGH T~A15 Nb3Si AND V3Ge
735
Sample No T~onset
92/6 T~ 10 4K
I~UJ ~92/7
I
44°
(211) I
42°
400 —
2e,
(210)
I
38°
36°
11
Tc 92/8 11.2Kj~ 1 92/9 Ic 81Kj~ 124K
(200) 34° T~.- 7K
Bragg Angle
Fig. 1. X-Ray traces in the low angle region for a series of samples prepared on a constant temperature substrate (Run 92, Td = 1350 K). The Nb—Si composite target gives a gradient of silicon concentration; the samples towards the bottom of the figure being poorer in silicon. Values for the T~onset (1%) are given for each sample. Positions of the prominent AlS reflections 200, 210 and 211 are indicated by the dashed lines and can be clearly identified for samples with a high T~onset. (Radiation: Cu Ka, X = 1.542 A). can be removed so that the heater, with substrates, could be seen through a window on the side of the vacuum system. We have found that there are many indicators of the general cleanliness of the two systems and the common argon gas reservoir arrangement. As we have managed to lower the impurity concentration, over a
in the sputtering gas. DC resistance measured onsets of 22.7 K with midpoints of 21.5 K have been regularly obtained by careful control of the oxygen level. However we are not at present sure of the role that the oxygen plays. 1. THE PREPARATION OF V 3 Ge
period of time, from about 50 to 2 ppm we have seen a gradual rise in the transition temperature for V3 Si. Our initial attempts gave T~values in the range 13—15.5 K close to the values previously reported by Schmidt Ct al. [6]. Now we can prepare material with a T~,of 16.8 K in our MkH system, (room temperature resistance ratios in excess of 20). An indicator of both the absolute and relative cleanliness of the MkI system is seen in the fact that it is only with difficulty that a T~,of 16.6 K is obtained. The background 2 ppm of 02 in the continuous flow sputtering gas is probably responsible for the difference between the MkI and MklI systems. In the MkI system we have also been making Nb3 Ge and find 1230K), significantly that adding only 5 ppmchanges.the of oxygen,transition (with a Tdtemperature of obtained. This again indicates the low levels of impurity
The transition temperature of A15 V3Ge is generally quoted as 6.7K [7] ,however, there are two features of this material that are a little unusual; it has a relatively high B~2in comparison with other A15 materials of similar T~,and it displays an uncharacteristic temperature dependence ofonsets its velocity of sound [8]. Furthermore Gavaler found T~ of about 8 K when preparing it in an oxygen free and probably hydrogen rich environment [9]. Using our “clean” system we have prepared V3Ge with an onset of 11 .2 K (Table 1, Run 62). The target was an argon arc melted billet and Td ranged from 7’da around 1150K broad 1100 to 1400K across set of substrates on a transitions heater of variable width. With were observed with onsets well above 6.7 K; for higher
736
THE SPUTTERING OF HIGH T~AiS Nb3 Si AND V3Ge
values of Td the transition temperature returned to the standard bulk valve and the reduced resistivity ratio showed an increasing trend. In the MkI system, using the same target, T~was close to 6.7 K for all samples. X-ray scans of the samples showed that the Al 5 lattice parameter remained constant at 4.778 A, very close to that usually quoted for the bulk material [101. The material with the highest T~showed sharp Al 5 lines only; for higher substrate temperatures, and hence lower T~,the Al5 lines became broader and additional lines appeared which could not be correlated with Al 5 reflections. These results do not allow a simple interpretation of the resistivity ratio trends. The temperature dependence of the resistivity was similar to that of other A15 materials and characteristic of a metal with interband scattering and a peaked density of5, states [11]. In at higher the low temperature regime p(T) T~ temperatures the variation became exponential with p exp (T/Tr) and Tr about 85 K.
A..
1%.L,1ttP,.JtiA~.JI~
‘.1
1
U3~3l
In the literature Nb3 Si is reported as having a T~of about 9K when made as a film [12, 13] ; however Testardi [14] has recently achieved an onset as high as 12K by flash annealing an amorphous film. Also Pan etal. [15] have prepared Nb3Si with an onset of 19K using an explosive technique. We have made material with T~onsets as high as 14K. Two types of target were used in the experiments, either an argon arc melted target to give a deposition of uniform composition on a set of substrates at different temperatures, or a composite target to give a composition gradient across a set of substrates held at a fixed temperature. Experiments were carried out in both systems. Using the MkI system the maximum onsets observed were about 11 K, quite significantly less than the highest onsets observed using MkII. The initial inference was that a very low impurity level is required for the production of a high 7~material, However, the situation is not as simple as this, for attempts to reduce the impurity level by extending the pre-sputtering time or using a narrower heater (to reduce outgassing of the heater clamps) resulted in the transition temperature returning to 9K. Thus, while a relatively clean environment appears to be necessary for the production of a high T,,, it is evidently not sufficient; exactly what agency is producing the highest T~is not at present clear. Representative results are given in Table 2 which summanses four experiments using the argon arc melted target and one experiment using a variable composition target. The observation that the high values of T, corresponded with rather high substrate temperatures (1350 K) raised the question as to
Vol. 24, No. 10
whether or not selective evaporation of silicon was occuring. This prompted us to use the composite target with a silicon gradient across it to produce a range of compositions at fixed substrate temperature. Although the preliminary results on the role of silicon evaporation are not clear-cut, T~onsets up to 12.4K were obtained and we were able to make a clear identification of the Al 5 phase. By comparing X-ray traces and T~ measurements for specimens with different compositions (Fig. 1) the X-ray reflections for an A15 phase were unequivocably identified. The lattice parameter was 5.18 A independent of T~,this is larger than the value inferred by Pan et al. [15] from high angle reflections, and considerably larger than that predicted by the Geller radii [16]. The figure shows the appearance of Al 5 lines in amongst X-ray studies on bulklines Nb of an Nb5 Si3 phase. Previous 3 Si have shown that A3B structures of the types Ti3P [17] and Au3Cu [18], can occur; we have also observed these phases, sometimes concurrently with the A15 phase in the same sample. It is interesting to note, however, that material with a T~ of 9 K showed no clear A15 phase. The complexity of the X-ray data and the breadth of the resistive transition suggests possibly that the high T~Al5 material consists of only a thin residual layer on the surface of the film. Further evidence for this is seen in the fact that although the films were prepared at 1350 K, if annealed for l2hr at 1040 K the A15 phase disappeared and T~fell back to an onset of 9.3 K (midpoint 8.0 K). The breadth of the resistive transition may therefore be explained in terms of a proximity effect of the low T~material affecting a thin layer of high T~material. Experiments are in progress to ion-etch away the surface layer in order to test this hypothesis. The resistivity measurements show a rather weakly characteristic “A15” shape which is also consistent with the above ideas. To conclude, we have found that V3Ge can be made with T~onsets much greater than the usually quoted bulk value. The precise conditions required to stabilize high or low T~,material are elusive, with repeat runs under near identical conditions still producing widely different results. We believe that V3Ge remains a material not fully understood and suggest that the general acceptance of a bulk T~value of 6.7 K must be doubted. A T~,near 11 K would be consistent with the predictions of lzyumov eta!. [19]. In the case of Nb3Si we have obtained a 1% onset in excess of 14K and have made a positive identification of an Al5 phase, with a rather expanded lattice parameter. The results are more reproducible than for V3 Ge but there are still features not fully understood in its preparation, in particular the stability with respect to trace impurities and in situ annealing at the deposition temperature.
Vol.24, No. 10
THE SPUTTERING OF HIGH Ti,, A15 Nb3Si AND V3Ge
737
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GAVALER J.R., App!. Phys. Lett. 23, 480 (1973). TESTARDI L.R., WERNICK J.H. & ROYER W.A., Solid State Commun. 15, 1(1974).
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TESTARDI L.R., WERNICK J.H., ROYER W.A., BACON D.D. & STORM A.R., J. App!. Phys. 45,446 (1974). HAMMOND R.H., IEEE Trans. Mag. 11, 201 (1975).
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TESTARDI L.R., WAKIYAMA T. & ROYER W.A.,J. Appi. Phys. 48,2055(1977). PAN V.M., ALEKSEEVSKII V.P., POPOV A.G., BELETSKU YU. I., YUPKO L.M. & YAROSH V.V.,JETP Lett. 21, 228 (1975).
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JOHNSON G.R. & DOUGLASS D.H.,J. Low Temp. Phys. 14, 575 (1974). DEARDORFF D.K., SIEMENS R.E., ROMANS P.A. & MCCURE R.A.,J. Less Common Metals 18, 11(1969).
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GALASSO T. & PYTE J., Acta Cryst. 16, 228 (1963).
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IZYUMOV YU. A. & KURMAEV E.E., Usp. Fix. Nauk 118,53 (1976).