A SIMPLE METHOD FOR PREPARING SUPERCONDUCTING H I G H Tc T H I N FILMS

P. Romano, A. Nigro, R. Vagl io Dipart imento d i F is ica, Un ive rs i t a ' d i Salerno

84100 Salerno, I t a l y

E. S igno re l l i ITM, C.N.R., C i n i s e l l o Balsamo,Italy

K.E. Gray Argonne National Laboratory, Argonne, I l l i n o i s

Abst ract

High q u a l i t y superconducting t h i n f i l m s o f the h igh Tc compound YBa2Cu307 have been prepared on sapphire us ing a new, simple method t h a t combines d.c. magnetron spu t te r i ng from a s ing le t a r g e t and evaporation from a heated source. The method should be expec ia l l y su i tab le f o r deposi t ing t h i n f i l m s on complex surfaces. (onset) were prepared us ing an YCu3 t a r g e t and evaporating barium by a s p e c i a l l y designed, feedback contro l led, molybdenum heater us ing both pure barium o r a barium-alluminum a l l oy . The f i l m s were character ized by X-rays, E lect ron Dispersion Spectroscopy and o the r techniques-

--

YBa2Cu307-a f i l m s w i t h Tc=92K

In t roduc t fon

The p o s s i b i l i t y o f preparing h igh q u a l i t y t h i n f i l m s o f the new oxide superconductors i s essent ia l f o r p r a c t i c a l uses i n superconducting e lec t ron i cs and good r e s u l t s have been obtained so f a r us ing d i f f e r e n t deposi t ion techniques, i nc lud ing sput ter ing, e lec t ron beam, M.B.E. ,laser evaporation and ~ t h e r s . l - ~ Thin f i l m s o f the new superconductors could a l so be considered, i n the next f u t u r e as a coat ing l aye r f o r r.f. superconducting c a v i t i e s and supercon- duct ing sh ie lds f o r various appl icat ions.

I n t h i s case the choice o f the deposi t ion technique i s determined by the need o f coat ing q u i t e l a rge o r complex surfaces.

simple and f l e x i b l e technique t h a t combines d.c. magnetron spu t te r i ng and thermal evaporation from m e t a l l i c sources t h a t a l lows h igh deposi t ion ra tes and o f fe rs , i n p r i n c i p l e , the p o s s i b i l i t y o f uniform coat ing o f r a t h e r complex surfaces.

Pre l iminary r e s u l t s obtained by t h i s method deposi t ing YBa2Cu3074 f i l m s on sapphire substrates are repor ted i n the fo l lowing.

With t h i s i n mind we have developed a new, very

Y-Ba-Cu-0 Thin F i lm DeDosition Technique

Our Y-Ba-Cu-0 f i l m s were r e a l i z e d i n an o i l - f r e e vacuum system based on a cryogenic pump w i t h an u l t ima te pressure i n the low 10-7 mbar range . con f igu ra t i on i s reported.

I n f i g . 1 a schematic drawing o f our codeposit ion

MOLYBDENUM HEATER

\

THERhIkL EVAPORATOR

\

THICKNESS MONITOR

F- VC",

--of- SPUTTERING MAGNETRON

N~oEIUM SCREEN

Figure 1. evaporation codeposit ion system.

Schematic drawing o f the d.c. magnetron-

A one inch, fused homogenous YCu3 t a r g e t was sput tered i n argon a t a pressure o f 4x10'3 mbar, by a d.c. magnetron t r i o d e type gun (SIMARD).

The deposi t ion r a t e was cont inuosly monitored by an o s c i l l a t i n g quar tz detector and was very stable.

i n our deposi t ion condi t ions on the substrates as c a r e f u l l y v e r i f i e d on t e s t f i l m s by E.D.S. m i croan 1 ys i s.

Barium has been codeposited i n the presence o f the argon athmosphere by two d i f f e r e n t methods.

I n the f i r s t pure m e t a l l i c barium was thermal ly evaporated by a molybdenum Knudsen c e l l , whereas i n the second a BaAl4 a l l o y enclosed i n U shaped s tee l wires ( SAES Getters ) was used.

i n the wire, a t 950 "C pure barium i s dispensed ( no traces o f A1 were detected by E.D.S. on t e s t f i lms ).

BaA14 a l l o y i s s tab le upon exposure a t the ambient athmosphere ( Ba on the contrary i s very react ive, expec ia l l y w i t h oxigen ).

Moreover the BaAl4 a l l o y can be produced i n various d i f f e r e n t shapes and barium can be a l so uni formly dispensed by l a rge surfaces, a l lowing,

The 1 t o 3 composition was we l l reproduced

When the a l l o y i s heated, by passing a cu r ren t

This l a s t method i s convenient because the

Manuscript received August 22, 1988 0018-9464/89/0300-2481$01 .0001989 IEEE

1 _.

" 1 2482

i n p r i n c i p l e , design conf igurat ions f o r coat ing w i t h Y-Ba-Cu-0 f i l m s ra the r complex surfaces.

The main drawback o f t h i s second method i s i n the d i f f i c u l t y o f s t a b i l i z i n g the deposi t ion ra te , so tha t , f o r the p re l im ina ry runs repor ted here, the Knudsen c e l l method has been preferred, that , by a feedback con t ro l1 through a thickness monitor, guarantees an adeguate s t a b i l i t y ( w i t h i n 5% ),

As shown i n f i g . 1 i n our system the substrates are mounted on a molybdenum heater t h a t can reach temperatures up t o 1100 " C y and oxigen can be spread c lose t o the substrates ( the maximum oxigen pressure i n the chamber dur ing spu t te r i ng i n the 10-3 mbar range).

However, f o r the runs discussed here, the heater was kept a t 250 "C and no oxigen was used, so t h a t an ex -s i t u oxigen annealing procedure was necessary a f t e r deposi t ion t o get the r i g h t f i l m s t ructure.

The d e t a i l s o f the annealing procedure w i l l be discussed i n the next section.

For a l l the deposi t ions repor ted here the samples-sources distance was 15 cm and the spu t te r i ng t a r g e t vo l tage was 250 V.

i n t h i s condi t ions was "< 10 A/sec, even much higher ra tes (-50 A/sec ) are i n p r i n c i p l e poss ib le w i t h our system.

top o f the f i l m t o reduce react ions dur ing the t r a n s f e r from the vacuum system t o the furnace.

i s

The ove ra l l growth r a t e 0: the YBaeCug f i l m though

A t h i n (-100 1) YCu3 over layer was sput tered on

Results and Discussion

About twenty YBa2Cu307-a were prepared on sap- ph i re substrates w i t h the method described above.

A f t e r c a l i b r a t i o n o f the two thichness monitors and a couple of t e s t f i l m deposit ions t o ad jus t the two sources r e l a t i v e rates, f i l m s w i t h Y:Ba:Cu=1:2:3 r a t i o s were r o u t i n e l y obtained ( i n s i d e few percent accuracy).

Spectroscopy analysis, t h a t a lso showed very good composition un i fo rm i t y a l l over the 5 x 20 m 2 depo- s i t e d f i l m surface.

meter, was about .6 r(m f o r the f i r s t deposi t ions and 1.2 um f o r the fo l l ow ing ones.

Various oxigen annealing procedures were tes ted expec ia l l y f o r the f i r s t f i l m s produced. The best r e s u l t s were obtained by r a p i d l y ( t h) b r i ng ing the samples a t 850°C i n f l ow ing oxigen, keeping them a t t h i s temperature f o r 10 minutes and f i n a l l y s lowly ( 4 h) cool ing a t room temperature, always i n oxigen f low.

Annealing o f the samples a t lower temperatures produced semiconducting f i lms , whereas higher tem- peratures o r longer times a t 850°C increased f i l m react ions w i t h the sapphire substrate, as c l e a r l y v i s i b l e by X-rays o r R.B.S. analys is , determining poor superconducting proper t ies.

Our th inne r f i l m s (0.6 um), annealed i n oxigen as described above, showed a room temperature r e s i s t i - v i t y o f about 10 uacm, a s l i g h t l y "non m e t a l l i c "

The composition was checked by Elect ron Dispersion

The f i l m thickness, measured by a step p r o f i l o -

temperature dependence o f the resistance, a sharp res is tance drop around 92K and a ra the r broad super- conducting t r a n s i t i o n present i ng a c l ear lis houl der" a t 60K and zero res is tance a t 10+15K, w i t h a c r i - t i c a l cu r ren t o f about 10 A/cm2 a t 4.2K.

S l i g h t l y b e t t e r r e s u l t s were obtained, i n a few cases, deposi t ing a 300 1 s i l v e r " b u f f e r l aye r " on the sapphire subst rate .7

higher zero res is tance temperatures (25t35K). For these f i l m s the e f f e c t o f the Ag b u f f e r l aye r was no t re levant .

s i stance ( r e s i s t i v e superconducting t rans i ti on) f o r a 1.2 um t h i c k f i l m on sapphire i s repor ted (dots) .

Thicker f i l m s (1.2 um) had a s i m i l a r behavior bu t

I n Fig. 2 the temperature dependence o f the re -

Figure 2. a 1.2 us t h i c k ph i re (dots) , and data from the same f i l m measured a f t e r three months o f ambient athmosphere storage (crosses).

Res is t i ve superconducting t r a n s i t i o n f o r f i l m deposited on sap-

Crosses represent data from the same sample re - measured a f t e r three months o f ambient athmosphere exposure ("desk storage").

The very good t ime s t a b i l i t y o f t h i s f i l m can be r e l a t e d t o i t s very compact s t ructure, as was ob- served by e lec t ron scanning microscopy (average g r a i n s i ze - 1 ym). Moreover f o r t h i s f i l m X-ray 8 - 28 d i f - f r a c t i o n showed e s s e n t i a l l y the r i g h t 1-2-3 s t ruc - t u r e w i t h very l i t t l e presence o f secondary phases.

The ra the r l a rge superconducting t r a n s i t i o n width has again t o be ascr ibed t o f i lm-subs t ra te react ions producing A1 contaminations and t o the non optimum oxigen stoichiometry.

I n conclusion good r e s u l t s have been obtained producing YBa2Cu30;~afilms on sapphire by a new code- p o s i t i o n method t h a t combines spu t te r i ng by an YCu3 t a r g e t and thermal Ba evaporation.

c h a r a c t e r i s t i c s and long term s t a b i l i t y a t room a i r exposure.

o f the same method t o o the r superconducting oxides, the use o f b e t t e r substrates (monoc r i s ta l l i ne MgO,

The best f i l m s have good s t r u c t u r a l and e l e c t r i c a l

Fur ther developments should inc lude the extension

SrTi03 ecc.) ,high rate depositions on larger sur- faces by the use of the described BaAl4 alloy and in-situ film growth.

Aknowledgments

The authors wish to thank B. Grozea, G. Perna

The work has been partially supported by C.N.R. and F. Vicinanza for technical assistance.

Comi tat0 Tecnologico.

References

(1) J. H. Kang ,R.T. Kampwi rth and K. E. Gray, Physics Letters, in press.

(2) S. J. Lee ,E. D. Ri ppert ,B. Y . Ji n ,S. N. Song ,S. J . Hwu K. Poeppelmeier and J.B. Ketterson, Appl . Phys. Lett. ,1194 (1987).

(3) R.B.Lai bowi tz,R.H.Koch,P.Chaudari and R.J. Gambino, Phys. Rev. B 2 ,8821 (1987).

I

2483

(4) 0. Di j kkamp ,T. Venkatesan ,X. D. Wu ,S. A. Shaheen N.Jisrawi,Y.H.Min-Lee,W.L.McLean and M.Croft Appl. Phys. Lett. 51, 619 (1987).

(5) J. Kwo ,T. C. Hs i eh, R. M. F1 emi ng ,M. Hong ,S. H. L i ou B.A.Davidson and L.C.Feldman, Phys. Rev. B - 36, 4039 (1987).

(6) D.K.Lathrop,S.E.Russek and R.A.Buhrman, Appl. Phys. Lett. 51, 1554 (1987).

(7) M.Gurwitch and A.T.Fiory, Appl. Phys. Lett. - 51, 1027 (1987).