Physica C 235 240 (1994)679-680                                                                         PHYSI0A
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680                                           11. Stduble-PSmpin et al. 'PDv.vica C 235 240 (1994) 679 657)




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1994 nucleation and growth of c parallel grains in co-evaporated sm ba2cu3oy films

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1994 nucleation and growth of c parallel grains in co-evaporated sm ba2cu3oy films

  1. 1. Physica C 235 240 (1994)679-680 PHYSI0A North-Holland Nucleation and growth of c-parallel grains in co-evaporated SmBa2Cu3Oy films B. St~iuble-Piimpina, V.C. Matijasevic a, J.G. Wen b*, H.M. Appelboom a, B. Ilge a, J. Venvik a, J.E. Mooija, W.J.A.M. Petersea, P.M.L.O. Scholtea, F. Tuinstraa, H.W. Zandbergen b aDept, of Applied Physics, Solid State Physics/DIMES, TU Delft, 2628 CJ Delft, The Netherlands bCentre for HREM, Lab. of Material Science, TU Delft, 2628 AL Delft, The Netherlands Co-evaporated SmBa2Cu3Oy thin films were investigated by means of Scanning Tunnelling Microscopy (STM), x-ray diffraction and Transmission Electron Microscopy (TEM). All films were predominantly oriented with their c-axis perpendicular (c±) to the substrate, with small fractions of c//grains. Although certain films seem to display a full coverage with c// material when imaged by STM, our study does not support the existence of a critical thickness at which c± growth switches to c//growth. Furthermore, it was found that the volume fraction of c / / t o c_L material increases with increasing supersaturation, indicating that kinetic factors are important to understand the nucleation and growth of c// grains. In the study presented here, epitaxial SmBa2Cu3Oy (Sm123) films were grown on SrTiO 3 (001) substrates by co-evaporation in an ozone beam. The films were deposited under conditions where the crystallographic c-axis is expected to grow perpendicular to the substrate (c±) [1]. It is known t h a t this orientation corresponds to the one with the lowest surface energy. Nevertheless, a certain amount of grains growing with their crystallographic c-axis parallel to the substrate (c//) was present in all films. The question whether the nucleation of Figure 1. STM image of a Sm123 film. these c// grains has to be understood within a thermodynamic or a kinetic approach is still completely different surface morphology, as controversial. Hawley et al. [2] reported the shown by the STM image in Fig. 1. Thin and observation of a critical thickness at which c& elongated slabs corresponding to c// grains seem growth switches to c / / g r o w t h and suggested that to cover the whole surface of this film. On the the relaxation of strain due to substrate/film other hand, it is known from x-ray diffraction lattice mismatch was a possible reason for this (rocking curves) that, for this sample, the volume transition. On the other hand, Pennycook et al. ratio of c// to c& material is only 8.5(1)%. The argued that the nucleation of c// grains is combination of these two results might suggest favoured at high supersaturation because their that, in an initial stage of deposition, this film growth requires less surface diffusion [3]. grows with a c± orientation, followed by a cross- When investigated on a nanometer scale using over to c// growth after a certain critical STM, the surfaces of most films are characterised thickness. Such an interpretation, however, is by a superposition of terraces separated from not supported by cross-sectional TEM each other by steps of approximately 12,°~ measurements. As displayed in Fig. 2, TEM corresponding to the length of a unit cell along images do not show the start of c / / g r o w t h on a the c-axis. Certain films, however, display a specific distance from the substrate, although one * Present address: ISTEC, 10-13 Shinonome, l-theme, Koto-ku, Tokyo 135, Japan 0921-4534/94/S07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)00900-7
  2. 2. 680 11. Stduble-PSmpin et al. 'PDv.vica C 235 240 (1994) 679 657) 0 .£ 7 p = 5. ] It-:" m b a r 5 % 1 > -1 ~9(i 731} 77(~ ~ lU ,~,,5~~ Figmre 3. Volume ratio of c/I to c 2 m a t e r i a l as ~ fhnction Tsu b. The line is a gmide to t h e eye. b) X-ray dill?action frocking curves) was a p p l i e d to d e t e r m i n e t h e volume r a t i o of c//to c± m a t e r l a / as a function of s u b s t r a t e t e m p e r a t u r e Tsu b and c h a m b e r p r e s s u r e p (Fig° 3). The full dots r e p r e s e n t a series of films grown at a c h a m b e r p r e s s u r e of 5.10 .6 mbar, which c o r r e s p o n d s i.,~ an ozone incidence r a t e at t h e s a m p l e ,,f a p p r o x i m a t e l y 1016 molecules/cm2sec. The open dot refers to a film grown a t a s o m e w h a t higher p r e s s u r e of 3.10 .5 m b a r . The e r r o r b a r s for t h e s e m e a s u r e m e n t s a r e of the size of t h e dots or F i g u r e 2. (a) T E M cross-sectional view of t h e smaller. Fig. 3 clearly shows t h a t t h e r e l a t i v e Sm123 film whose surface is shown in Fig. 1. (b) a m o u n t of c// m a t e r i a l p r e s e n t in a film r a p i d l y E n l a r g e d view of (a). The d a r k lines c o r r e s p o n d i n c r e a s e s fbr d e c r e a s i n g Tsu b and for i n c r e a s i n g to p l a n e s p e r p e n d i c u l a r to t h e c-axis of Sm123. p, i.e. far i n c r e a s i n g s u p e r s a t u r a t i o n . B a s e d on t h e s e r e s u l t s , it s e e m s likely t h a t t h e reduced should k e e p in m i n d t h a t with this t e c h n i q u e mobility of a d s o r b e d species on t h e surface of' t h e only a thin slice of a 3D s a m p l e is i n v e s t i g a t e d . fihn d u r i n g deposition is r e s p o n s i b l e fbr the D u e to an a n i s o t r o p y in t h e growth speeds nucleation of c// g r a i n s . In addition, due to the p a r a l l e l a n d p e r p e n d i c u l a r to t h e c-axis, c//grams a n i s o t r o p y of growth s p e e d s p a r a l l e l and a r e not only thin a n d long, b u t also r a t h e r tall, as p e r p e n d i c u l a r to t h e c-axis, once a c//-nucleus is c o m p a r e d to t h e a v e r a g e t h i c k n e s s of t h e film fbrmed, conditions of large s u p e r s a t u r a t i o n will (i.e. 150 nm). This o b s e r v a t i o n is e m p h a s i s e d by a fhvour the r a p i d growth of" such grains, at tile "shadow-effect" t a k i n g place d u r i n g film growth e x p e n s e of t h e f u r t h e r growth of c ± - g r a l n s a n d r e s u l t i n g in a film t h i c k n e s s of less t h a n 150 p r e s e n t on t h e film. nm in b e t w e e n n a r r o w l y s p a c e d c// g r a i n s (see In conclusion, t h e r e s u l t s p r e s e n t e d here Fig. 2a). W h i l e t h e STM i m a g e s e e m s to i n d i c a t e indicate that kinetic factors, such as a full coverage of t h e film s u r f a c e by c// grains, s u p e r s a t u r a t i o n , play an i m p o r t a n t role in the the TEM r e s u l t s clearly show t h a t c± regnons nucleation and g r o w t h of" c// g r a i n s in Sm123 exist in b e t w e e n t h e slabs. This d i s c r e p a n c y can films. No evidence for a cross-over from c± to c// be e x p l a i n e d by t h e fact t h a t due to its l a t e r a l growth was fbund. d i m e n s i o n s , a S T M - t i p will u s u a l l y not be able to scan t h e regions in b e t w e e n two c// slabs. The REFERENCES: s a m e g r a i n s of c// material will most probably be 1. H.M. A p p e l b o o m et al., P h y s i c a (] 2 1 4 (1993) :423. i m a g e d by v a r i o u s l a t e r a l l y and v e r t i c a l l y shifted 2. M.E. H a w l e y et al., U ] t r a m i c r o s c o p y 42 (1992) 71}5. mini-tips. 3. S.J. P e n n y c o o k et al., P h y s i c a C 202 ( 1 9 9 2 ) 1.

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