- Email: [email protected]
Superconducting properties of evaporated tantalum films
Thin Solid Films, 44 (1977) L l-L5 Q Elsevier Sequoia S.A.. Lausanne-Printed
Ll
in the Netherlands
Letter
Superconducting
properties of evaporated tantalum films
ROSA M. AGUADO
BOMBIN AND W. E. J. NEAL
Physics Department,
University of Aston, Gosra Green, Birmingham
(Received
May 27, 1977; accepted
B4 7ET (Ct. Britain)
June 14, 1977)
1. Introduction The superconducting properties of over 80 evaporated films of tantalum (99.99(6)x) formed part of an investigation into the influence of deposition parameters and structure on electrical and optical properties. Other properties of the films have been reported elsewhere ‘* 2. The films were prepared in a stainless steel chamber in which the base pressure was 1 x lo-” Torr. The pressure during deposition lay between 1 x 10e8 and 2 x 10m8 Torr and the temperatures of the substrates of fire polished borosilicate glass be individually from 423 773 K. to eight could deposited
; (b) films in which the b.c.c. and f.c.c. structures coexisted up to film thicknesses of several hundred nanometres. An estimate of the proportion of f.c.c. and b.c.c. structure in the second group was obtained by comparing integrated intensities of the strongest lines using Xradiation from iron, copper and chromium targets. 2. Gaseous impurities Gebhardt and Rothenbacker3 and De Sorbo4 have shown that the resistance for niobium films ratio Rresl(R300 - R,,,) varies linearly with oxygen concentration and Gebhardt and Rothenbacker3 have also shown that the change in residual resistivity p,,, for tantalum films due to interstitial nitrogen is proportional to changes in the concentration of nitrogen. Gaseous impurities also reduce the critical temperature T, of films below the value for bulk material. The measured reductions in T, observed in the films in this work were greater than reductions which could be attributed to gaseous impurities alone. Gaseous impurities were reduced by prolonged outgassing of the chamber, substrates and the tantalum source and by the gettering action of a large area of freshly evaporated tantalum both prior to and during the deposition of the films. 3. Experimental results All transitions from the normal
to the superconducting
states were measured
resistivelq. In agreement with Gcrstcnberg and Hall’ and Marcah MC found that the resistive transition was sharp for thick films but became broader as the film resistivity increased and T, decreased. The chanyc in tcmperaturc AT for the resistance to change from R,,, to zero was 0.0 I K at TL = 4.45 K. increasing to 0.5 K at T, = I .8 K (i.c,. for films less than 30 nm thick). The values quoted for in paper relate to the temperature \vhen film resistance cqual to R,,, Figure shows a plot of critical temperature T, against film resistivit) ratio ~~~~~~~~~~~~ for films with IOO”,, b.c.c. phase. The effect of substrate tcmperaturc during deposition on the critical temperature is shown in Fig. 2. Each curvt‘ corresponds to a diRerent set of ~mples prepared simultancousl>. It can be seen that TL increases with increasing substrate tcmperaturc and for set A the value of 7: approaches the bulk value of4.48 K.
LETTERS
L3
For films with mixed f.c.c. and b.c.c. structures, the f.c.c. phase could be considered as a temperature-dependent impurity as far as resistivity was concerned. In spite of the fact that the presence of f.c.c. structure raises the resistivity and lowers the resistivity ratio, both of which lead to the lowering of T, in tantalum films, the transition temperatures of films with mixed phases were found to be equivalent to the critical temperature for films of pure b.c.c. structure of the same thickness within the limits of experimental error, even for thick films with up to 50 % f.c.c. phase. It was also found in this work that films prepared on substrates at temperatures below about 450 K did not exhibit superconducting properties (at least above 1.5 K). The lattice parameter a, for such films departed from the bulk value by up to 2.5 x. Similarly films prepared at low deposition rates (approximately 4 nm min ‘) also had increased values of a, and were not superconducting above 1.5 K. The decrease of T, with increase in lattice parameter for niobium has been mentioned by Neugebauer and Ekwal17. 4. Discussion A decrease in the critical temperature of tantalum films with decreasing thickness has been reported by several authors *-ll. In general, decreasing the thickness of a film results in a decrease of the resistivity ratio which in turn, as seen in Fig. 1, leads to a reduction in T,. The fact that in this work T, was found to increase with increasing deposition temperature (Fig. 2) is also consistent with observations by other authors in that an increase in deposition temperature reduces the concentration of gaseous impurities, structural defects, stresses and the departure from the bulk lattice constant, as has been shown by Neal and Bombin’. By contrast, Schrey et al. l2 found that for sputtered films a decrease in the substrate temperature increased T,. a fact which is difficult to explain in the light of present observations. Empirical relationships have been found between the critical temperature and the normal electrical properties which indicate that some form of microscopic defect which affects electrical properties is probably one of the most significant factors affecting the transition temperature. A relationship between T, and the room . temperature resistivity /)300 ( u s1 cm) for the tantalum films in this investigation can be given as T,r = Ki C, exp( -a~&
(1)
where T,, and Tcb are the film and bulk critical temperatures respectively. K, and a are constants given as 1.15 and 0.011 respectively and Tcb = 4.48 K. Between 4.45 and 2.3 K the agreement with experiment is within 3% (the room temperature resistivity range being 13-75 usZ cm) and below 2.3 K within 15 “6. a has dimensions of conductivity and the corresponding resistivity pa = 90.9 uR cm. Most samples in this investigation satisfied Matthiessen’s rule2, so that p3,,0-p1,, = constant (i.e. for films greater than 40 nm thick). A relationship between the film transition temperature and the temperature coefficient of resistivity can be given as T,. = A exp( - UK, Tcb) whereA=2andK2is4.17x10-3.
(2)
L4
I I TTI
Two relationships between T, and the rcsistivit!, already been given by Bombin and Neal I3 :
ratio
for tantalum
KS
have
with the value of X , = 2.0. Table I compares the experimental rc‘sults with values calculated on the basis of eqn. (I ). Table II compares experimental and calculated values of 7, based on eqns. (3) and (4). TABLE
I
TABLE
II
(1) Correlations have been found between the critical temperature 7, of thin evaporated tantalum films and other electrical properties in the thickness ranye I I .5-~950 nm. (2) Gaseous impurities (oxygen. nitrogen) do not account for the large fall in Tc from the bulk value as the tilm thickness decreases.
L5
LETTERS
(3) Films with a mixture of f.c.c. and b.c.c. phases had of the same value within experimental error as films with presence of f.c.c. does, however, reduce the resistivity ratio concluded that the critical temperature of the f.c.c. structure structure. I 2 3 4 5 6 7 8 9 IO I1
12 I3
transition temperatures a pure b.c.c. phase. The of the films’ and so it is is close to that for b.c.c.
R. M. Aguado Bombm and W. E. J. Neal. %n Solic/Film.r, 42( 1977) 91. W. E. J. Neal and R. M. Aguado Bombin, Thirr SolirlFii’lm.~, 44 (1977) 169. E. Gebhardt and R. Rothenbacker. Z. MrrullX-d.. 54 (1963) 623. W. De Sorbo, Pkys. Rer.. /32 (1963) 107. D. Gerstenberg and M. Hall. J. Elrcrrochem. Sot., I I I ( 1964) 936. R. B. Marcus, J. Appl. P/IJ.s., 37 (1966) 8. C. A. Neugebauer and R. A. Ekwall. J. Appl. Ph,~s.. 35 (1964) 547. I. J. Budnick. Phtx Rec.. IIY (1959) 1578. C. Sulkowski and J. Mazur. Acre PhyA. PO/.. 2Y (1966) 107. J. J. Hauser and H. C. Theuerer. Rec. Mod. Ph>,s.. 36 (1964) 80. E. A. Lynton. B. Serin and M. Zucker. J. Phj,s. Chrm. Solid.s, 3 (1957) 165. F. Schrey. R. D. Mathis. R. T. Payne and L. E. Murr. Thin Solid Films. 5 (1970) 29. R. M. Aguado Bombin and W. E. J. Neal, Appl. Phj,s. Left., 28 (1976) 410.
Ll
in the Netherlands
Letter
Superconducting
properties of evaporated tantalum films
ROSA M. AGUADO
BOMBIN AND W. E. J. NEAL
Physics Department,
University of Aston, Gosra Green, Birmingham
(Received
May 27, 1977; accepted
B4 7ET (Ct. Britain)
June 14, 1977)
1. Introduction The superconducting properties of over 80 evaporated films of tantalum (99.99(6)x) formed part of an investigation into the influence of deposition parameters and structure on electrical and optical properties. Other properties of the films have been reported elsewhere ‘* 2. The films were prepared in a stainless steel chamber in which the base pressure was 1 x lo-” Torr. The pressure during deposition lay between 1 x 10e8 and 2 x 10m8 Torr and the temperatures of the substrates of fire polished borosilicate glass be individually from 423 773 K. to eight could deposited
; (b) films in which the b.c.c. and f.c.c. structures coexisted up to film thicknesses of several hundred nanometres. An estimate of the proportion of f.c.c. and b.c.c. structure in the second group was obtained by comparing integrated intensities of the strongest lines using Xradiation from iron, copper and chromium targets. 2. Gaseous impurities Gebhardt and Rothenbacker3 and De Sorbo4 have shown that the resistance for niobium films ratio Rresl(R300 - R,,,) varies linearly with oxygen concentration and Gebhardt and Rothenbacker3 have also shown that the change in residual resistivity p,,, for tantalum films due to interstitial nitrogen is proportional to changes in the concentration of nitrogen. Gaseous impurities also reduce the critical temperature T, of films below the value for bulk material. The measured reductions in T, observed in the films in this work were greater than reductions which could be attributed to gaseous impurities alone. Gaseous impurities were reduced by prolonged outgassing of the chamber, substrates and the tantalum source and by the gettering action of a large area of freshly evaporated tantalum both prior to and during the deposition of the films. 3. Experimental results All transitions from the normal
to the superconducting
states were measured
resistivelq. In agreement with Gcrstcnberg and Hall’ and Marcah MC found that the resistive transition was sharp for thick films but became broader as the film resistivity increased and T, decreased. The chanyc in tcmperaturc AT for the resistance to change from R,,, to zero was 0.0 I K at TL = 4.45 K. increasing to 0.5 K at T, = I .8 K (i.c,. for films less than 30 nm thick). The values quoted for in paper relate to the temperature \vhen film resistance cqual to R,,, Figure shows a plot of critical temperature T, against film resistivit) ratio ~~~~~~~~~~~~ for films with IOO”,, b.c.c. phase. The effect of substrate tcmperaturc during deposition on the critical temperature is shown in Fig. 2. Each curvt‘ corresponds to a diRerent set of ~mples prepared simultancousl>. It can be seen that TL increases with increasing substrate tcmperaturc and for set A the value of 7: approaches the bulk value of4.48 K.
LETTERS
L3
For films with mixed f.c.c. and b.c.c. structures, the f.c.c. phase could be considered as a temperature-dependent impurity as far as resistivity was concerned. In spite of the fact that the presence of f.c.c. structure raises the resistivity and lowers the resistivity ratio, both of which lead to the lowering of T, in tantalum films, the transition temperatures of films with mixed phases were found to be equivalent to the critical temperature for films of pure b.c.c. structure of the same thickness within the limits of experimental error, even for thick films with up to 50 % f.c.c. phase. It was also found in this work that films prepared on substrates at temperatures below about 450 K did not exhibit superconducting properties (at least above 1.5 K). The lattice parameter a, for such films departed from the bulk value by up to 2.5 x. Similarly films prepared at low deposition rates (approximately 4 nm min ‘) also had increased values of a, and were not superconducting above 1.5 K. The decrease of T, with increase in lattice parameter for niobium has been mentioned by Neugebauer and Ekwal17. 4. Discussion A decrease in the critical temperature of tantalum films with decreasing thickness has been reported by several authors *-ll. In general, decreasing the thickness of a film results in a decrease of the resistivity ratio which in turn, as seen in Fig. 1, leads to a reduction in T,. The fact that in this work T, was found to increase with increasing deposition temperature (Fig. 2) is also consistent with observations by other authors in that an increase in deposition temperature reduces the concentration of gaseous impurities, structural defects, stresses and the departure from the bulk lattice constant, as has been shown by Neal and Bombin’. By contrast, Schrey et al. l2 found that for sputtered films a decrease in the substrate temperature increased T,. a fact which is difficult to explain in the light of present observations. Empirical relationships have been found between the critical temperature and the normal electrical properties which indicate that some form of microscopic defect which affects electrical properties is probably one of the most significant factors affecting the transition temperature. A relationship between T, and the room . temperature resistivity /)300 ( u s1 cm) for the tantalum films in this investigation can be given as T,r = Ki C, exp( -a~&
(1)
where T,, and Tcb are the film and bulk critical temperatures respectively. K, and a are constants given as 1.15 and 0.011 respectively and Tcb = 4.48 K. Between 4.45 and 2.3 K the agreement with experiment is within 3% (the room temperature resistivity range being 13-75 usZ cm) and below 2.3 K within 15 “6. a has dimensions of conductivity and the corresponding resistivity pa = 90.9 uR cm. Most samples in this investigation satisfied Matthiessen’s rule2, so that p3,,0-p1,, = constant (i.e. for films greater than 40 nm thick). A relationship between the film transition temperature and the temperature coefficient of resistivity can be given as T,. = A exp( - UK, Tcb) whereA=2andK2is4.17x10-3.
(2)
L4
I I TTI
Two relationships between T, and the rcsistivit!, already been given by Bombin and Neal I3 :
ratio
for tantalum
KS
have
with the value of X , = 2.0. Table I compares the experimental rc‘sults with values calculated on the basis of eqn. (I ). Table II compares experimental and calculated values of 7, based on eqns. (3) and (4). TABLE
I
TABLE
II
(1) Correlations have been found between the critical temperature 7, of thin evaporated tantalum films and other electrical properties in the thickness ranye I I .5-~950 nm. (2) Gaseous impurities (oxygen. nitrogen) do not account for the large fall in Tc from the bulk value as the tilm thickness decreases.
L5
LETTERS
(3) Films with a mixture of f.c.c. and b.c.c. phases had of the same value within experimental error as films with presence of f.c.c. does, however, reduce the resistivity ratio concluded that the critical temperature of the f.c.c. structure structure. I 2 3 4 5 6 7 8 9 IO I1
12 I3
transition temperatures a pure b.c.c. phase. The of the films’ and so it is is close to that for b.c.c.
R. M. Aguado Bombm and W. E. J. Neal. %n Solic/Film.r, 42( 1977) 91. W. E. J. Neal and R. M. Aguado Bombin, Thirr SolirlFii’lm.~, 44 (1977) 169. E. Gebhardt and R. Rothenbacker. Z. MrrullX-d.. 54 (1963) 623. W. De Sorbo, Pkys. Rer.. /32 (1963) 107. D. Gerstenberg and M. Hall. J. Elrcrrochem. Sot., I I I ( 1964) 936. R. B. Marcus, J. Appl. P/IJ.s., 37 (1966) 8. C. A. Neugebauer and R. A. Ekwall. J. Appl. Ph,~s.. 35 (1964) 547. I. J. Budnick. Phtx Rec.. IIY (1959) 1578. C. Sulkowski and J. Mazur. Acre PhyA. PO/.. 2Y (1966) 107. J. J. Hauser and H. C. Theuerer. Rec. Mod. Ph>,s.. 36 (1964) 80. E. A. Lynton. B. Serin and M. Zucker. J. Phj,s. Chrm. Solid.s, 3 (1957) 165. F. Schrey. R. D. Mathis. R. T. Payne and L. E. Murr. Thin Solid Films. 5 (1970) 29. R. M. Aguado Bombin and W. E. J. Neal, Appl. Phj,s. Left., 28 (1976) 410.