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Ion beam sputtering and properties of YBaCuO superconducting thin films
Physica C 153-155 (1988) 1449-1450
North-Holland, Amsterdam
ION BEAM SPUTYERING AND PROPERTIES OF YBaCuO SUPERCONDUCTING THIN FILMS D. Pavuna 1, W. Baer 1, H. Bergerl, H.J. Mathieu 2, A. Vogel2, M. Schmidt3, V. Gasparov3*, M. Affronte 1, F. Vasey 1, F.K. Reinhart 1 1 Ecole Polytechnique F6d6rale de Lausanne, D6partement de Physique, 1015 Lausanne 2 ibid, D6partement des Mat6riaux, ch. de Bellerive, Lausanne, Switzerland 3 Universit6 de Lausanne, Institut de Physique Exp~rimentale, 1015 Lausanne, Switzerland We have produced YBaCuO thin films by mono-target ion beam sputtering. Post annealing in 02 at 850°C results in superconducting films on (100) SrTiO3 with R = 0 at ~60 K. Preliminary results of chemical analysis of the films by X-ray photoelectron spectroscopy (XPS), as well as combined Secondary Ion Mass (SIMS) and Auger Electron S p e c t r o s c o p y (AES) indicate formation of hydroxides and carbonates in the surface of samples and somewhat lower concentration of oxygen than the optimal value of 6.9. In this brief paper we s u m m a r i z e the following : i) the production of a m o r p h o u s , nonconducting films by m o n o - t a r g e t ion b e a m sputtering ii) the subsequent t h e r m a l t r e a t m e n t and the properties of resulting YBaCuO films on sapphire and SrTiO 3 substrates; iii) the preliminary results on the chemical analysis of these films and the studies of the interface between YBaCuO and the substrate. The ion beam sputtering unit consists of a Balzers UMS 500 P ultra high v a c u u m chamber into which we mounted an Ion Tech 2.5"-1.5 kV ion b e a m gun, target holder, quartz monitor, oxygen inlet tube and the substrate holder. We used a "1:2:3" YBaCuO target which was made by a "standard" p r o c e d u r e of sintering the stoichiometric mixture of Y 2 0 3 , B a C O 3 and CuO and calcinating it at 950°C for 24 hours in oxygen. After regrounding and c o m p a c t i n g the p o w d e r s , we o b t a i n e d sputter targets of about 15 m m thickness and 50 m m in diameter. These were treated again at 950°C in oxygen for 24 hours and
then slowly cooled (<100°C/hour) to a m b i e n t temperature. Typical sputtering p a r a m e t e r s were as follows : Argon pressure ~ 10 -3 10 -4 "forr, ion beam current 40-60 m A , beam voltage 1000-1200 V; the d e p o s i t i o n rate was ~1 g m / h o u r with a partial oxygen pressure of <10 -3 Torr; typical film thickness was 2-3 g m . G o o d and reproducible results for films on S r T i O 3 (with R = 0 at T = 60 K) are obtained by an oxygen partial pressure of 5 x 10 -4 Torr and substrate t e m p e r a t u r e of 400°C. While higher t e m p e r a t u r e m a y be d e s i r a b l e in order to g r o w crystalline Y B a 2 C u 3 0 6 + x (say at ~800°C), it increases the interdiffusion inbetween the YBaCuO and the substrate elements (like Sr, Ti or A1). The "as sputtered" films were a l w a y s nonconductive and amorphous a s determined by XRD. The films deposited on s a p p h i r e or (100) S r T i O 3 crystallised predominantly into an o r t h o - r h o m b i c p h a s e after annealing in oxygen at 850°C for about 1 hour. It is essential to heat and cool the s a m p l e s s l o w l y (< 100°C/h); otherwise semiconducting films are formed.
* Permanent address : Institute of Solid State Physics of USSR Academy of Sciences, 142432 Chernogolovka, USSR 0921-4534/88/$03.50© Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
1450
D. Paouna et al. / 1on beam sputtering and properties of thin films
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SPUTTER TINE (mini
Figure 1 : Resistivity of 3 Ixm thick YBaCuO films deposited on polished (100) SrTiO3 substrate a) Fresh sample, annealed in oxygen for 1 h at 850°C b) After a few days in humid atmosphere c) After a few weeks
The freshly annealed films on (100) SrTiO3 substrates are m e t a l l i c . T h e i r resistivity is < 1 m f l c m and the temperature c o e f f i c i e n t is p o s i t i v e . As presented in Figure 1, R=0 is measured around 60 K. This, together with preliminary structural studies, indicates that a fraction of the crystallites has its c-axis perpendicular to (100) SrTiO 3 substrate, p r o b a b l y in zones fairly close to the interface. These films are more stable than the ones on sapphire. XPS studies show that, if left in air, bom f i l m s and bulk s a m p l e s f o r m s u r f a c e hydroxides and carbonates, as identified by chemical shifts in the 01s and C l s spectra. The initial amount of t h e s e compounds depends strongly on the p r e p a r a t i o n method. We note that the bulk samples exhibit t h e s a m e t e n d e n c y ; n e v e r t h e l e s s , the i n i t i a l amount of OH is much higher. A d d i t i o n a l studies of CuO, BaO and Y 2 0 3 contaminated with water show that it is primarily BaO and somewhat less Y 2 0 3 that f o r m the h y d r o x i d e s and c a r b o n a t e s . Such e f f e c t s seem to lower the critical temperature and
Figure 2 : R e l a t i v e atomic c o n c e n t r a t i o n obtained using standard elemental s e n s i t i v i t y f a c t o r s for d i f f e r e n t e l e m e n t s across the - 9 0 0 /~ thin, as d e p o s i t e d , a m o r p h o u s Y B a C u O film on (100) SrTiO3 substrate. Sputter rate was ~ 100 /~/min.
gradually distroy unprotected films.
superconductivity
of
In a d d i t i o n to e l e c t r o n microprobe analysis of the stoichiometric composition of our films, we have also started a surface analysis of thin films to determine chemical composition as a function of depth as well as the interface YBaCuO-(100) SrTiO3, sapphire and G a A s . A c o m b i n e d P e r k i n E l m e r Atomika SIMS/AES unit with 5 keV A r + - i o n s as p r i m a r y ions was used. P r e l i m i n a r y studies were carried out on ~900 /~ thin, as deposited, a m o r p h o u s films. The results of Figure 2 indicate that we have fairly uniform deposition of all the constituent elements. Some interdiffusion occurs within ~200 /~ of the interface (see Figure 2). This analysis, t o g e t h e r with r e s i s t i v i t y and X - r a y data indicate that: i) the o x y g e n c o n t e n t in our f i l m s is somewhat lower than 6.9, most likely around 6.7 ii) the relative ratio between metallic e l e m e n t s still d e v i a t e s f r o m the o p t i m a l "1:2:3" ratio. These points need further quantitative studies which are currently in progress.
North-Holland, Amsterdam
ION BEAM SPUTYERING AND PROPERTIES OF YBaCuO SUPERCONDUCTING THIN FILMS D. Pavuna 1, W. Baer 1, H. Bergerl, H.J. Mathieu 2, A. Vogel2, M. Schmidt3, V. Gasparov3*, M. Affronte 1, F. Vasey 1, F.K. Reinhart 1 1 Ecole Polytechnique F6d6rale de Lausanne, D6partement de Physique, 1015 Lausanne 2 ibid, D6partement des Mat6riaux, ch. de Bellerive, Lausanne, Switzerland 3 Universit6 de Lausanne, Institut de Physique Exp~rimentale, 1015 Lausanne, Switzerland We have produced YBaCuO thin films by mono-target ion beam sputtering. Post annealing in 02 at 850°C results in superconducting films on (100) SrTiO3 with R = 0 at ~60 K. Preliminary results of chemical analysis of the films by X-ray photoelectron spectroscopy (XPS), as well as combined Secondary Ion Mass (SIMS) and Auger Electron S p e c t r o s c o p y (AES) indicate formation of hydroxides and carbonates in the surface of samples and somewhat lower concentration of oxygen than the optimal value of 6.9. In this brief paper we s u m m a r i z e the following : i) the production of a m o r p h o u s , nonconducting films by m o n o - t a r g e t ion b e a m sputtering ii) the subsequent t h e r m a l t r e a t m e n t and the properties of resulting YBaCuO films on sapphire and SrTiO 3 substrates; iii) the preliminary results on the chemical analysis of these films and the studies of the interface between YBaCuO and the substrate. The ion beam sputtering unit consists of a Balzers UMS 500 P ultra high v a c u u m chamber into which we mounted an Ion Tech 2.5"-1.5 kV ion b e a m gun, target holder, quartz monitor, oxygen inlet tube and the substrate holder. We used a "1:2:3" YBaCuO target which was made by a "standard" p r o c e d u r e of sintering the stoichiometric mixture of Y 2 0 3 , B a C O 3 and CuO and calcinating it at 950°C for 24 hours in oxygen. After regrounding and c o m p a c t i n g the p o w d e r s , we o b t a i n e d sputter targets of about 15 m m thickness and 50 m m in diameter. These were treated again at 950°C in oxygen for 24 hours and
then slowly cooled (<100°C/hour) to a m b i e n t temperature. Typical sputtering p a r a m e t e r s were as follows : Argon pressure ~ 10 -3 10 -4 "forr, ion beam current 40-60 m A , beam voltage 1000-1200 V; the d e p o s i t i o n rate was ~1 g m / h o u r with a partial oxygen pressure of <10 -3 Torr; typical film thickness was 2-3 g m . G o o d and reproducible results for films on S r T i O 3 (with R = 0 at T = 60 K) are obtained by an oxygen partial pressure of 5 x 10 -4 Torr and substrate t e m p e r a t u r e of 400°C. While higher t e m p e r a t u r e m a y be d e s i r a b l e in order to g r o w crystalline Y B a 2 C u 3 0 6 + x (say at ~800°C), it increases the interdiffusion inbetween the YBaCuO and the substrate elements (like Sr, Ti or A1). The "as sputtered" films were a l w a y s nonconductive and amorphous a s determined by XRD. The films deposited on s a p p h i r e or (100) S r T i O 3 crystallised predominantly into an o r t h o - r h o m b i c p h a s e after annealing in oxygen at 850°C for about 1 hour. It is essential to heat and cool the s a m p l e s s l o w l y (< 100°C/h); otherwise semiconducting films are formed.
* Permanent address : Institute of Solid State Physics of USSR Academy of Sciences, 142432 Chernogolovka, USSR 0921-4534/88/$03.50© Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
1450
D. Paouna et al. / 1on beam sputtering and properties of thin films
10(
S
m
.,e
b
~
~
o
~o
,~,o
,~o
~,o
2~o
tz,~,.uK~n
~tt
.L
~
~
~
"
"
2
:-
" a~
~::
~o
T(K)
u 0
z
,
,
'
10 r
.
,,
,* 20
SPUTTER TINE (mini
Figure 1 : Resistivity of 3 Ixm thick YBaCuO films deposited on polished (100) SrTiO3 substrate a) Fresh sample, annealed in oxygen for 1 h at 850°C b) After a few days in humid atmosphere c) After a few weeks
The freshly annealed films on (100) SrTiO3 substrates are m e t a l l i c . T h e i r resistivity is < 1 m f l c m and the temperature c o e f f i c i e n t is p o s i t i v e . As presented in Figure 1, R=0 is measured around 60 K. This, together with preliminary structural studies, indicates that a fraction of the crystallites has its c-axis perpendicular to (100) SrTiO 3 substrate, p r o b a b l y in zones fairly close to the interface. These films are more stable than the ones on sapphire. XPS studies show that, if left in air, bom f i l m s and bulk s a m p l e s f o r m s u r f a c e hydroxides and carbonates, as identified by chemical shifts in the 01s and C l s spectra. The initial amount of t h e s e compounds depends strongly on the p r e p a r a t i o n method. We note that the bulk samples exhibit t h e s a m e t e n d e n c y ; n e v e r t h e l e s s , the i n i t i a l amount of OH is much higher. A d d i t i o n a l studies of CuO, BaO and Y 2 0 3 contaminated with water show that it is primarily BaO and somewhat less Y 2 0 3 that f o r m the h y d r o x i d e s and c a r b o n a t e s . Such e f f e c t s seem to lower the critical temperature and
Figure 2 : R e l a t i v e atomic c o n c e n t r a t i o n obtained using standard elemental s e n s i t i v i t y f a c t o r s for d i f f e r e n t e l e m e n t s across the - 9 0 0 /~ thin, as d e p o s i t e d , a m o r p h o u s Y B a C u O film on (100) SrTiO3 substrate. Sputter rate was ~ 100 /~/min.
gradually distroy unprotected films.
superconductivity
of
In a d d i t i o n to e l e c t r o n microprobe analysis of the stoichiometric composition of our films, we have also started a surface analysis of thin films to determine chemical composition as a function of depth as well as the interface YBaCuO-(100) SrTiO3, sapphire and G a A s . A c o m b i n e d P e r k i n E l m e r Atomika SIMS/AES unit with 5 keV A r + - i o n s as p r i m a r y ions was used. P r e l i m i n a r y studies were carried out on ~900 /~ thin, as deposited, a m o r p h o u s films. The results of Figure 2 indicate that we have fairly uniform deposition of all the constituent elements. Some interdiffusion occurs within ~200 /~ of the interface (see Figure 2). This analysis, t o g e t h e r with r e s i s t i v i t y and X - r a y data indicate that: i) the o x y g e n c o n t e n t in our f i l m s is somewhat lower than 6.9, most likely around 6.7 ii) the relative ratio between metallic e l e m e n t s still d e v i a t e s f r o m the o p t i m a l "1:2:3" ratio. These points need further quantitative studies which are currently in progress.