VNU- JOURNAL OF SCIENCE, Mathematics - Physics. T.xx, N03AP, 2004
E F F E C T O F S U B S T R A T E T E M P E R A T U R E S O N T H E P R O P E R T IE S O F Y B a2C u 30 7.fi T H IN FIL M S
N . D. M in h , T. D. H ien , N. K. M an International T raining In stitute for M aterials Science (ITIM S),
H anoi University o f Technology c . K. O ng
Centre for Superconducting and Magnetic Materials, N a tio n a l U niversity o f Singapore, Singapore 119260
A b s tr a c t: Y B a 2C u 30 7^ th in film s h a v e b een d e p o site d by p u lse d la s e r d e po sition (P L D ) o n s in g le c ry s ta llin e S r T i0 3 (00 1) sub strates. T h e d e p o sitio n w a s carried o u t at d iffe re n t su b s tra te te m p e ra tu re s T s= 6 6 0 -7 4 0 °c. T h e re s u lt s h o w s th a t th e film s a re s tro n g ly orie n te d w ith th e c-axis. T he op tim a l c ritica l te m p e ra tu re (Tc) a n d c ritica l c u rre n t d e n s ity (Jc) a re ob tain ed at T s=6 9 0 -7 0 0 °c , w ith v a lu e s of T C= 91 K a n d J c=1.1 M A /c m 2 at 7 7 K a n d in zero m a g n e tic field.
1. I n t r o d u c t io n
T he discovery of h ig h-tem perature superconductivity se t off an explosion of research on the developm ent of h ig h-tem perature superconducting (HTS) devices based on YBa,Cu:i0 7.i thin films, including microwave devices, Josephson junctions and tran sistors [1].
T he films em ployed in th is study were deposited by pulsed la s e r deposition (PLD).
The du ra tio n of a la s e r pulse can ra nge between nanoseconds an d femtoseconds, w hich can create extrem ely energetic pulses in very localized are a s [2]. A blated species from a target come into oxygen en v ironm ent an d tran sfer to the sub strate. A relatively high oxygen background pre ssu re can be used with th is technique. T his allows for the fabrication of oxide an d superconductor film s w ith a nearly perfect stoichiom etry.
2. E x p e r i m e n t p r o c e d u r e
T he YBa.Cu30 7.5 (YBCO) th in film s were prepared by PLD. Before deposition, the single cry stalline SrTiOa (001) s u b s tra te s were w ashed w ith 5% HNOa solution in an ultrasonic cleaner an d subsequently by deionized w ater, acetone and ethanol. The excimer laser ( \ = 248 nm) w as o p erated a t 250 m J/pulse for 30 m in for each deposition.
T h e s u b s tra te is m ounted on th e h e a te r in the cross-area of th e PLD plum e caused by the beam and its te m p e ra tu re (Tg) w as in the range of 660-740°C. T he am bient oxygen p ressure was kep t a t 0.2 m bar. T he films w ere annealed a t 400uc for ab out 1 h and cooled down to room te m p e ra tu re for about 2-3 h in the am bient oxygen p re ssu re of 1 atm . The films th ickness is ab o u t 260 nm . T he l x l cm2 sam ples w ere p a ttern ed , yielding bridges 50
|im long an d 20 fim wide.
T he m icrostructures of th e sam ples w ere analysed by m eans of X-ray diffraction with CuKo radiation, atom ic force microscopy (AFM). DC electrical re sistan c e m easurem ents were ca rrie d o u t usin g the sta n d a rd four-probe m ethod w ith silver alloy solders a s electrical
9 7
98 N . D. M in h , T. D. H ỉe n , c . K . O n g a n d N . K. M a n
contacts. The tra n s p o rt critical cu rren t densities (Jc) of the sam ples w e re m easured a t 77K using the four-probe m ethod w ith the 1 Ị.iV/cm criterion with m icrobridge of 20 um X 50 nm p atterned by photolithography.
F ig .l. AFM side-view images of YBCO/STO films at Ts-680°c (a) and Ts=700°c (b).
3. R e s u lts a n d d is c u s s io n
In AFM experim ents, we observed a surface morphology ch a rac teristic of island grow th, w ith occasional out-grow ths, see Fig. 1.
M ixed grow th consists of island after the first m onolayer forms successfully (Stranski- K ranstanov mode) [3]. T he grow th islands had a typical lateral size of about 50-60 nm. This is very sim ilar to th e re s u lts found by other authors under the sam e deposition conditions [4],
The presence of th e (ỚỠ/) peaks in the XRD p a tte rn of the film s corresponds to a well-
crystallized single orthorhom bic phase and c-axis-oriented film, as show n in Fig. 2. The c- axis of the film h a s a norm al value of 11.68Ả. The rocking curves o f th e (005) YBCO reflection showed a narrow shape with sim ilar FWHM of 0.2° for th e film. I t show s th a t the YBCO thin films of high stru c tu ra l quality have been epitaxially grow n on STO substrates.
Fig.2. XRD 0-20 patterns of the YBCO thin film on STO substrate
Fig.3. Transition temperature of YBCO films vs. substrate temperature.
S ubstrate tem perature, Tq (”C) Fig.4. Critical current density of YBCO
filma (77K & 0T) V temperature
E ffe c t o f s u b s t r a te te m p e r a tu r e s o n th e p r o p e r tie s .. 9 9
T he dependence of s u b s tra te tem p eratu res on the tran sitio n te m p e ra tu re (Tc) and critical c u rre n t density (Jf.) of YBCO th in film s is shown in Fig. 3 & Fig. 4. I t is shown th a t the optim al tran sitio n tem p eratu re (TC=91K) was obtained a t a T s of around 700°c, while th e highest J r (1.1 MA/cm2 a t 77K) w as achieved a t 690°c. It is shown th a t th e su b strate te m p eratu res of Ts=690-700°c are enough high to allow sufficient surface m igration and interdiffusion of th e atom s for th e form ation of th e desired orthorhom bic YBCO crystal stru ctu re . In application of HTS th in films, critical c u rre n t density is m ore im portant th an tran sitio n te m p e ra tu re in coated conductors, a deposition te m p e ra tu re of 690°c w as chosen.
The field-dependent J c m easured a t liquid nitrogen tem p e ra tu re is plotted in Fig. 5.
In th is configuration, th e Lorenz force is perpendicular to th e a6 -plane an d vortices induced by th e applied m agnetic field have to cross th e superconducting C u 0 2 p lanes for flux flow to occur. It is shown th a t J r decreases rapidly w hen th e field is applied p arallel to th e c-axis of the film. The sudden drop in J c a s soon a s a m agnetic field is applied (B<0.25 T) is due to th e effect of Josephson weak links a t th e grain boundaries [5].
The macroscopic pinning force F,,=J(-xB for th in film is show n in sm all picture (Fig. 5). In the Fp vs. B curve, the broad peak in a ra th e r low-field region (B is about 0.35 T) and th e long ta il in th e high- field region su ggest a n unconventional pinning behavior. T he fact t h a t the pinning force peak ap p e ars in a quite low field region indicates th a t th e pinning of the pinning ce nters in th is film is not so weak.
The low macroscopic pinning force is due to th e w eak elastic properties of the flux-line lattice th a t is possibly re la te d to th e highly anisotropy layered s tru c tu re of th is m aterial.
4. C o n c lu s io n s
The successful p re p ara tio n of high quality YBCO superconducting thin films by pulsed laser deposition is shown. F ilm s deposition on single crystalline S rT i0 3 (001) su b stra te a t deposition tem p eratu re of 690°c pre sen t tran sitio n , tem p eratu re, TC=90K and critical c u rre n t density, J c= l . l MA/cm2 a t 77K. T hese film s have a sufficiently good quality to m ake th e high-perform ance microwave components.
Acknowledgements: T his work w as p artially supported by the S ta te Program on F undam ental R esearch of V ietnam u n d er th e G ra n t No. 421204 an d th e MOET Program u n d er th e Project No. B2004-59-13.
R e fe re n c e s
1. u . Brinknrann. Lambda Highlights No.19/20 (1989).
2. Q. Y. Ying, H. s . Kim, D. T. Shaw and H. s . Kwok, Appl. Phys. Lett. 55 (1989) 1041-1043.
3. H. -U. Habermeier, Journal o f Superconductivity: Incorporating Novel Magnetism 13 (2000) 871-875.
4. c . J. van der Beek, M. Konczykowski, A. Abaĩoshev, I. Abal'osheva, P.Gierlowski, s . J.
Lewandowski, M .v. Indenbom and S.Barbanera, arXiv:cond-mat/0202243 v2-22Aug (2002) M l .
5. L. Hua, J. Yoo. H, Kim, H. Chung, G. Qiao and G. Yuan, Physica c 337 (2000) 165-168.
\ ‘ĩ z
\ 1 E/
\
! » T.77KMagnetic Odd (Tesla) B//C
T=77K
--- o!o 0.2 0.4 0.6 0 8 1.0 M agnetic field (Tesla) Fig.5. J c value as a function of applied magnetic
field at 77 K for a 20 nm wide bridge patterned into the YBCO film at Ts=690 c.
