- Email: [email protected]
Superconducting properties of CVD tantalum films
Volume 6. number
MATERIALS
3
SUPERCONDUCTING E.S. VLAKHOV ’ Institute qfSolld
PROPERTIES
‘, K.A. GESHEVA State Physrcs, Bulgarian
h Central
Laborator))
Received
26 October
qfSolar
December
LETTERS
OF CVD TANTALUM
’ and V.T. KOVACHEV Academy
qfScrences,
I987
FILMS
a
SoJia 1784, Bulgarra
Energy and New Energy Sources, Bulgarian
Academy
qfSciences,
Sofia 1784, Bulgaria
1987
Tantalum films on AlLO substrate are obtained by CVD of TaCI, at atmospheric pressure. The critical temperature of the transition to the superconductmg state, the critical magnetic field and the volt-ampere characteristics of the films are investigated. Significant increase in the critical magnetic field and the typical behaviour of type II superconductors was established.
1. Introduction The preparation of films by deposition from the gas phase (CVD) is a convenient technique for obtaining thin films from high-temperature transition metals and their compounds. Tantalum, together with its nitrides and carbides are interesting because of their superconducting properties. The present work describes an investigation of the superconducting properties of tantalum films deposited on Al2O3 substrates.
2. Preparation and experimental technique The preparation of tantalum films on quartz and steel substrates is described in detail in ref. [ 11. The chloride process proposed for the first time by Powell et al. [ 21 is used: TaCl, + $Hz - 71” Ta+5HCl
.
(1)
The reaction is carried out at atmospheric pressure. The samples are heated indirectly through a graphite susceptor coated with a Sic protective layer. A highfrequency generator allows temperature regulation by the anode voltage or by a disconnection of one of the feed phases. The temperature regulator with a Pt/Pt-Rh thermocouple controls the temperature of the graphite susceptor with an accuracy of III 10°C. The sublimator immersed in a silicon oil bath is kept 58
with an accuracy of + 1 “C within the temperature range up to 150°C. Ar and Hz gases, of initial purities of 99.95 and 99.99% respectively, are further purified by removal of oxygen and moisture with zeolite and palladium catalyst: WcaW, 2H~0 . 2Hz + O2 The sheet resistance of the obtained samples is measured by a VEECO-EPP-100; the thickness by a TALYSTEP protilometer. The critical temperature of the transition to the superconducting state and the resistance at low temperatures are measured using the resistance technique. The critical current Z, is determined at a voltage drop of 1 uV/cm and the critical magnetic field H, at 0.5 R, (where R, is the resistance in the normal state). The temperature of the sample is measured using a calibrated germanium thermometer of Scientific Instruments, with sensitivity not less than 0.02 K. In the presence of a magnetic field it is measured by the helium vapour pressure registered by a mercury differential manometer. Ta films of lowest resistance at room temperature are chosen from the available ones. For example, sample P-6, 1.1 urn thick has a specific resistance approximately equal to that of the bulk material, i.e. 12.65~ 10e6 Q cm. The resistance ratio R soo dR4.z K = 1.683. The transition to the superconducting state of the same sample is shown in fig. 1. Fig. 2 shows the critical magnetic field at
Volume
II. numb
3
MATEKI.ALS
LETTERS
Dcccmher
1487
1
+ Fig.
3. Volt-amp&
characterstlcs
of
(‘Cl)
tantalum
film.
I-=/(/). L-L 2 I?
i 3,OG
2,5C
3,so
4,00
4,50
ToK Fig. I. Crlttcal CVD
tcmperaturc
tantalum
film.
of the superconductlng
R/R,,=/1
T):
transition
R,,)=3.48 K;
7;(R=0.5
of A7;
T=constant. The magnetic field is created by a watercooled Bitter solenoid. The volt-ampere characteristics at different temperatures are presented in fig. 3.
= 0.06 K.
3. Experimental results and discussion It is convenient to analyse the properties of the films on the basis of bulk material properties. .-\ccording to ref. [ 31, ;’ = 16.6 g/cm’, pJilii h = I3 uR cm. T,=4.4 K, H,=830Oe;a,,=3.3 .p\ fora Ta bulksample. Three types of tantalum films [ 31 are found to exist: a-Ta film corresponding to the bee structure of the bulk sample, j3-Ta film of tetragonal structure and a third type of “low density”. The CVD tantalum films obtained by us have a critical temperature which is the same as that for a-Ta films. i.e. 7,=3.3 K (R= 0.5R,,). However, the film specific resistance at T= 300 K is very close to that of the bulk sample
R/Rn l.CO
p=R,d=(llSXlO-‘R/0)x1.1
H[Koe]
I 15.0 Fig. 2. Critical
magnetic
field of CVD tantalum
film:
K:(~)7~=2.08K:(3)T=2.24K.RIR,=_f(H),11,(1.98K)=11.2 kOe:
H,(2.08K)
= 10.4 kOe;SI,
(2.24 K) ~9.45
kOe.
. 20.0
(I ) T= 1.98
urn
The width of the transition to the superconducting state determined within the limits of (0. I-0.9) R/R,, is AT,=O.O6K. This small value of A7: implies that the sample is highly single-phase. The critical magnetic fields H, vary between 11.2 and 9.45 kOe in the temperature range of (0.57-0.64)T/T,. The measured values exceed by more than an order of magnitude the critical magnetic field of a bulk sample (I-I, =830 Oe). In this
Volume 6. number
3
MATERIALS
______.
LETTERS
December
where &r= ((“1) I” is the effective coherence length. The coherence length of Ta at 0 K according to ref. [ 61 is to= 900 A. The length of the free path can be assessed with the correlation p,l=3,1xlO-‘Qcm .
10-A! 0,95
1.05
l,oo
1.10
JiJC Fig. 4. RIR,=f(J/J,). type II superconductor.
Curve
1: type I superconductor;
curve 2:
respect, the investigated films behave similarly to type II superconductors. This conclusion is confirmed by the volt-ampere characteristics in fig 3 which show a non-linear dependence. The same data are presented in fig. 4 in coordinates RIR, =f( J/J,). The line marked 1 denotes the behaviour of type I superconductors which, on passing to a transitional state, show a sharp increase in resistance and reach R, at negligible change of the transport current. Curve 2 for the Ta film shows that the resistance is a function of the current density as is found to be the case for type II superconductors. The initial non-linearity of V(Z) in fig. 3 is possibly due to a non-simultaneous separation of the different vortices from the pinning-centres in constrast to the region of high Z/I, where the motion of the vortex structure is entirely viscous and the dependence of I/ on I is linear. In order to estimate the influence of the size effect on the critical magnetic field the numerical solution of the Ginzburg-Landau equation for intermediate layer thickness will be used, assuming that K = a,.( 0)/j and the empirical relation 6(t) = 6( 0)( 1 - t “) - I” is valid, where t= T/T,. It is shown that the size effect appears at thicknesses smaller than
2dC=
60
4.5 (1_~4)1/2
1987
f&fir
(3)
found for Nb [ 71 which possesses almost the same resistance at room temperature as tantalum. Substitutingpo=PTa(4.2 K) in (3) we obtain 1=38.6 A, and &,= 186 A. The critical layer thickness in compliance with (2) at a temperature of 2.08 K is d,= 896 A. Thus, the size effect should appear at thicknesses smaller than 1000 A, while the thickness of the film investigated here is an order of magnitude higher. According to Hauser et al. [ 51, H, for a 300 8, thick tantalum film is 26 kOe at 1.3 K. The increase in critical magnetic field is much higher than the one expected from the size effect. The fact that the investigated tantalum films have thickness exceeding d, by an order of magnitude and H, higher compared to the one of the bulk material obviously needs another explanation. Similar behaviour is observed by the authors [ 5 ] for niobium films, 37000 8, thick. It is established that the reason for this is the decrease of the effective coherence length owing to the smallgrain structure of the film (grain size of 30- 100 A as determined by electron diffraction). Considering Gorkov’s correlation: H,, =H,(1.77-0.43t2
+0.07t4)K,
(4)
using our experimental H,(T) data from fig. 2 and H,=830 Oe we obtain KZ 12.2 and H,,(O)=17.9 kOe. On the other hand, for a “dirty” superconductor: K=0.7156L(0)//; substituting 6,(O)=
&
(5)
the obtained ~1=658
K
and 1 we obtain
.i .
This value is close to the experimentally measured values [4] for pure metals. The decrease of the free electron path in the investigated films and the increase of the magnetic field can be a result of impurities (metallic - Nb or gases - NZ, Hz, 0,) resulting from the method of preparation. The Nb addition is determined by the purity
\‘olumc
6. number
3
MATERIALS
of the imtial TaCl, (99.99%) and ought to be negligible: thus Nb cannot be the reason for so high an increase of the critical magnetic field. It is difficult to measure gaseous additions but, if present, they would strongly affect the specific resistance and the critical temperature, which is not observed. There are data for the superconducting properties of the tantalum hydrides that can be formed eventually, due to the fact that hydrogen is used in the CVD process. On the other hand, the Nb and Ta films obtained by physical vapor deposition [ 5 ] also show high critical fields, despite that this technology excludes the effect of the hydrogen admixtures. Thus, we can conclude that the presence of some quantity of impurities is not a determining factor for the specific behaviour of the tantalum films investigated here.
LETTERS
December
19x7
size factor which cannot explain the increase of the critical magnetic field is assessed. The observed specific features are caused mainly by the reduced coherence length, probably due to the small-grain structure of the obtained CVD films.
Acknowledgement The authors thank Professor St. Kanev for his permanent interest in this work, and the personnel of the International Laboratory for Strong Magnetic Fields and Low Temperatures. Wrociaw. Poland. for the opportunity to carry out magnetic measurements.
References [II 4. Conclusion
K. Gesheva Tantalum Annual
Tantalum films on A1203 substrate are obtained by CVD of TaCl, at atmospheric pressure. The critical temperature of the transition to the superconducting state, the critical magnetic field and the volt-ampkre characteristics of the obtained films are investigated. Significant increase in the critical magnetic field and the typical behaviour of type II superconductors was established. The influence of the
and E. Vlakhov. Thin
Fltms
Rrpons.
(‘VD
Method
on Dlffcrenr
TechnIcat
Physics.
f~)r Producing
Substrates.
I’niverslt!.
Vol. 23. Part 2
(1986)p.
139. [‘l [31
C‘.E. Powell.
I.E.
them.
(1948)158.
Sot. 85
D.A. McClean.
141 W. Buckel.
Campbell
and B.W.
J. Etectrochem.
Superleltung.
2nd Ed, ( Physik-Vcrtag.
(ionsrr.
Sot. Japan 34
Grundlagen Wemhelm.
J. Etectro-
( 1966 ) I.
und Auwendungen. 1977)
p 375.
[51 J.J. Hauser and H.C. Theuerer. Phys. Re\. I344 i 1964) (61 B.B. Goodman. IBM J. Res. Develop. 6 ( IO61163. and K.H. Bcrtet. [‘I N.E. ,Atekseevskii. V.I. Nijankobskll Metal.
I Metattorcd.
37
(I 974)
198. Fir.
68.
61
MATERIALS
3
SUPERCONDUCTING E.S. VLAKHOV ’ Institute qfSolld
PROPERTIES
‘, K.A. GESHEVA State Physrcs, Bulgarian
h Central
Laborator))
Received
26 October
qfSolar
December
LETTERS
OF CVD TANTALUM
’ and V.T. KOVACHEV Academy
qfScrences,
I987
FILMS
a
SoJia 1784, Bulgarra
Energy and New Energy Sources, Bulgarian
Academy
qfSciences,
Sofia 1784, Bulgaria
1987
Tantalum films on AlLO substrate are obtained by CVD of TaCI, at atmospheric pressure. The critical temperature of the transition to the superconductmg state, the critical magnetic field and the volt-ampere characteristics of the films are investigated. Significant increase in the critical magnetic field and the typical behaviour of type II superconductors was established.
1. Introduction The preparation of films by deposition from the gas phase (CVD) is a convenient technique for obtaining thin films from high-temperature transition metals and their compounds. Tantalum, together with its nitrides and carbides are interesting because of their superconducting properties. The present work describes an investigation of the superconducting properties of tantalum films deposited on Al2O3 substrates.
2. Preparation and experimental technique The preparation of tantalum films on quartz and steel substrates is described in detail in ref. [ 11. The chloride process proposed for the first time by Powell et al. [ 21 is used: TaCl, + $Hz - 71” Ta+5HCl
.
(1)
The reaction is carried out at atmospheric pressure. The samples are heated indirectly through a graphite susceptor coated with a Sic protective layer. A highfrequency generator allows temperature regulation by the anode voltage or by a disconnection of one of the feed phases. The temperature regulator with a Pt/Pt-Rh thermocouple controls the temperature of the graphite susceptor with an accuracy of III 10°C. The sublimator immersed in a silicon oil bath is kept 58
with an accuracy of + 1 “C within the temperature range up to 150°C. Ar and Hz gases, of initial purities of 99.95 and 99.99% respectively, are further purified by removal of oxygen and moisture with zeolite and palladium catalyst: WcaW, 2H~0 . 2Hz + O2 The sheet resistance of the obtained samples is measured by a VEECO-EPP-100; the thickness by a TALYSTEP protilometer. The critical temperature of the transition to the superconducting state and the resistance at low temperatures are measured using the resistance technique. The critical current Z, is determined at a voltage drop of 1 uV/cm and the critical magnetic field H, at 0.5 R, (where R, is the resistance in the normal state). The temperature of the sample is measured using a calibrated germanium thermometer of Scientific Instruments, with sensitivity not less than 0.02 K. In the presence of a magnetic field it is measured by the helium vapour pressure registered by a mercury differential manometer. Ta films of lowest resistance at room temperature are chosen from the available ones. For example, sample P-6, 1.1 urn thick has a specific resistance approximately equal to that of the bulk material, i.e. 12.65~ 10e6 Q cm. The resistance ratio R soo dR4.z K = 1.683. The transition to the superconducting state of the same sample is shown in fig. 1. Fig. 2 shows the critical magnetic field at
Volume
II. numb
3
MATEKI.ALS
LETTERS
Dcccmher
1487
1
+ Fig.
3. Volt-amp&
characterstlcs
of
(‘Cl)
tantalum
film.
I-=/(/). L-L 2 I?
i 3,OG
2,5C
3,so
4,00
4,50
ToK Fig. I. Crlttcal CVD
tcmperaturc
tantalum
film.
of the superconductlng
R/R,,=/1
T):
transition
R,,)=3.48 K;
7;(R=0.5
of A7;
T=constant. The magnetic field is created by a watercooled Bitter solenoid. The volt-ampere characteristics at different temperatures are presented in fig. 3.
= 0.06 K.
3. Experimental results and discussion It is convenient to analyse the properties of the films on the basis of bulk material properties. .-\ccording to ref. [ 31, ;’ = 16.6 g/cm’, pJilii h = I3 uR cm. T,=4.4 K, H,=830Oe;a,,=3.3 .p\ fora Ta bulksample. Three types of tantalum films [ 31 are found to exist: a-Ta film corresponding to the bee structure of the bulk sample, j3-Ta film of tetragonal structure and a third type of “low density”. The CVD tantalum films obtained by us have a critical temperature which is the same as that for a-Ta films. i.e. 7,=3.3 K (R= 0.5R,,). However, the film specific resistance at T= 300 K is very close to that of the bulk sample
R/Rn l.CO
p=R,d=(llSXlO-‘R/0)x1.1
H[Koe]
I 15.0 Fig. 2. Critical
magnetic
field of CVD tantalum
film:
K:(~)7~=2.08K:(3)T=2.24K.RIR,=_f(H),11,(1.98K)=11.2 kOe:
H,(2.08K)
= 10.4 kOe;SI,
(2.24 K) ~9.45
kOe.
. 20.0
(I ) T= 1.98
urn
The width of the transition to the superconducting state determined within the limits of (0. I-0.9) R/R,, is AT,=O.O6K. This small value of A7: implies that the sample is highly single-phase. The critical magnetic fields H, vary between 11.2 and 9.45 kOe in the temperature range of (0.57-0.64)T/T,. The measured values exceed by more than an order of magnitude the critical magnetic field of a bulk sample (I-I, =830 Oe). In this
Volume 6. number
3
MATERIALS
______.
LETTERS
December
where &r= ((“1) I” is the effective coherence length. The coherence length of Ta at 0 K according to ref. [ 61 is to= 900 A. The length of the free path can be assessed with the correlation p,l=3,1xlO-‘Qcm .
10-A! 0,95
1.05
l,oo
1.10
JiJC Fig. 4. RIR,=f(J/J,). type II superconductor.
Curve
1: type I superconductor;
curve 2:
respect, the investigated films behave similarly to type II superconductors. This conclusion is confirmed by the volt-ampere characteristics in fig 3 which show a non-linear dependence. The same data are presented in fig. 4 in coordinates RIR, =f( J/J,). The line marked 1 denotes the behaviour of type I superconductors which, on passing to a transitional state, show a sharp increase in resistance and reach R, at negligible change of the transport current. Curve 2 for the Ta film shows that the resistance is a function of the current density as is found to be the case for type II superconductors. The initial non-linearity of V(Z) in fig. 3 is possibly due to a non-simultaneous separation of the different vortices from the pinning-centres in constrast to the region of high Z/I, where the motion of the vortex structure is entirely viscous and the dependence of I/ on I is linear. In order to estimate the influence of the size effect on the critical magnetic field the numerical solution of the Ginzburg-Landau equation for intermediate layer thickness will be used, assuming that K = a,.( 0)/j and the empirical relation 6(t) = 6( 0)( 1 - t “) - I” is valid, where t= T/T,. It is shown that the size effect appears at thicknesses smaller than
2dC=
60
4.5 (1_~4)1/2
1987
f&fir
(3)
found for Nb [ 71 which possesses almost the same resistance at room temperature as tantalum. Substitutingpo=PTa(4.2 K) in (3) we obtain 1=38.6 A, and &,= 186 A. The critical layer thickness in compliance with (2) at a temperature of 2.08 K is d,= 896 A. Thus, the size effect should appear at thicknesses smaller than 1000 A, while the thickness of the film investigated here is an order of magnitude higher. According to Hauser et al. [ 51, H, for a 300 8, thick tantalum film is 26 kOe at 1.3 K. The increase in critical magnetic field is much higher than the one expected from the size effect. The fact that the investigated tantalum films have thickness exceeding d, by an order of magnitude and H, higher compared to the one of the bulk material obviously needs another explanation. Similar behaviour is observed by the authors [ 5 ] for niobium films, 37000 8, thick. It is established that the reason for this is the decrease of the effective coherence length owing to the smallgrain structure of the film (grain size of 30- 100 A as determined by electron diffraction). Considering Gorkov’s correlation: H,, =H,(1.77-0.43t2
+0.07t4)K,
(4)
using our experimental H,(T) data from fig. 2 and H,=830 Oe we obtain KZ 12.2 and H,,(O)=17.9 kOe. On the other hand, for a “dirty” superconductor: K=0.7156L(0)//; substituting 6,(O)=
&
(5)
the obtained ~1=658
K
and 1 we obtain
.i .
This value is close to the experimentally measured values [4] for pure metals. The decrease of the free electron path in the investigated films and the increase of the magnetic field can be a result of impurities (metallic - Nb or gases - NZ, Hz, 0,) resulting from the method of preparation. The Nb addition is determined by the purity
\‘olumc
6. number
3
MATERIALS
of the imtial TaCl, (99.99%) and ought to be negligible: thus Nb cannot be the reason for so high an increase of the critical magnetic field. It is difficult to measure gaseous additions but, if present, they would strongly affect the specific resistance and the critical temperature, which is not observed. There are data for the superconducting properties of the tantalum hydrides that can be formed eventually, due to the fact that hydrogen is used in the CVD process. On the other hand, the Nb and Ta films obtained by physical vapor deposition [ 5 ] also show high critical fields, despite that this technology excludes the effect of the hydrogen admixtures. Thus, we can conclude that the presence of some quantity of impurities is not a determining factor for the specific behaviour of the tantalum films investigated here.
LETTERS
December
19x7
size factor which cannot explain the increase of the critical magnetic field is assessed. The observed specific features are caused mainly by the reduced coherence length, probably due to the small-grain structure of the obtained CVD films.
Acknowledgement The authors thank Professor St. Kanev for his permanent interest in this work, and the personnel of the International Laboratory for Strong Magnetic Fields and Low Temperatures. Wrociaw. Poland. for the opportunity to carry out magnetic measurements.
References [II 4. Conclusion
K. Gesheva Tantalum Annual
Tantalum films on A1203 substrate are obtained by CVD of TaCl, at atmospheric pressure. The critical temperature of the transition to the superconducting state, the critical magnetic field and the volt-ampkre characteristics of the obtained films are investigated. Significant increase in the critical magnetic field and the typical behaviour of type II superconductors was established. The influence of the
and E. Vlakhov. Thin
Fltms
Rrpons.
(‘VD
Method
on Dlffcrenr
TechnIcat
Physics.
f~)r Producing
Substrates.
I’niverslt!.
Vol. 23. Part 2
(1986)p.
139. [‘l [31
C‘.E. Powell.
I.E.
them.
(1948)158.
Sot. 85
D.A. McClean.
141 W. Buckel.
Campbell
and B.W.
J. Etectrochem.
Superleltung.
2nd Ed, ( Physik-Vcrtag.
(ionsrr.
Sot. Japan 34
Grundlagen Wemhelm.
J. Etectro-
( 1966 ) I.
und Auwendungen. 1977)
p 375.
[51 J.J. Hauser and H.C. Theuerer. Phys. Re\. I344 i 1964) (61 B.B. Goodman. IBM J. Res. Develop. 6 ( IO61163. and K.H. Bcrtet. [‘I N.E. ,Atekseevskii. V.I. Nijankobskll Metal.
I Metattorcd.
37
(I 974)
198. Fir.
68.
61