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Surface and in-plane characterization of YBa2Cu3O7 thin films grown by laser ablation
PHYSICA
Physica C 179 ( i 99 ! ) 262-268 North-Holland
Surface and in-plane characterization of YBa2Cu307 thin films grown by laser ablation M.G. Karkut
a,
M. Guilloux-Viry a b, A. Perrin
a,
j. Padiou
a
and M. Sergent
a
a t'niversitb de Rennes I. Laborawire de Chimie Minbrale B, URA C N R S 254. Avenue du Gbn~raJ Leclerc, 35042 Rennes C~dex, France b C.N.E.T. Lannion 8, Division OCM, BP 40, 22301Lannion C~dex, France
Received 10 June 1991
We have used reflection high energy electron diffraction (RHEED) and oscillating crystal X-ray diffraction to analyze the surface and in-plane structural characteristics of YBa~Cu30, (YBCO) thin films that were grown in-situ by laser ablation onto substrates of (100) MgO and (100) SrTiO3. We obtain RHEED streaks, characteristic of atomically smooth surfaces and indicative of epitaxial order, for these thin films. Oscillating crystal diffraction also indicates that the YBCO film axes are collinear with the substrate axes. The films grown on ( ! 00) substrates are solely c-axis oriented ( perpendicular to the plane of the film ) as determined by 0-20 diffractometry and the FWHM of rocking curves about the (005) reflection are 0.3 ° to 0.5 ° for the best films on SrTiO3 substrates and 0.4: to 0.6: for films grown on MgO. Weissenberg photography allo~'s us to determine the in-plane structure of some films grown on MgO that display anomalous surface features seen by RHEED and by scanning electron microscopy.
1. Introduction As technical applications of the high-To superconducting (HTS) thin films come closer to reality, increased importance will be attached to the in,.plane structure of the thin films in order to determine their compatibility with various substrates and buffer layers, as well as to characterize the in-plane structure o," the substrates and buffer layers themselves. He, .: we present some results of two complementary methods, reflection high energy electron diffraction (RHEED) and oscillating crystal X-ray diffraction, to investigate the in-plane structure of YBCO thin films we have grown by laser ablation onto MgO and SrTiO3 substrates. The RHEED method is an in-situ method used to study surface structure. It has been used recently in MBE systems to study the initial stages of YBCO film growth [1,2]. More recently RHEED intensity oscillations have been observed during the deposition of oxide thin films [ 3 ]. These oscillations, which are attributed to layer by layer growth, can shed light on the nucleation and growth processes of these complex structures. As for oscillating crystal diffractometry, we have recently shown
[ 4 ] that this method is effective in characterizing the in-plane structure of HTS thin films. Since these methods are not commonly found in the HTS literature, we will use this opportunity to demonstrate their utility in providing information on the surface and in-plane features of laser ablated "123" thin films and their MgO and SrTiO3 substrates.
2. Deposition, 0-20 X-ray diffractometry, and superconductivity The films were grown by laser ablation. Incident laser pulses of 40 as duration and wavelength 2 = 308 nm are focused onto a rotating ceramic target which is at 45 ° with respect to the incident laser pulses. The ~llh~lral~ it glued with silver paint onto a stainless steel substrate holder which is heated to 600°C to 750:C, depending on the substrate, during deposition. The substrate directly faces the ta,'get and its distance from the target is adjustable and is usually kept at a distance of 45 mm. The base pressure of the system before deposition is ~ 5 X 10 -.7 mbar and during deposition there is a flowing 02 pressure of
092 !-4534/9 !/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
263
M.G. Karkut et al. /Surface and in-plane characterization of YBCO
~ 0.3 mbar. We operate the laser at a frequency between 0.67 and 4 Hz. We fix the pulse energy between 80 and 200 mJ and the beam is focused onto a square ~ 2 × 2 mm 2. After deposition the films are slowly cooled to room temperature in one atmosphere of oxygen and no further processing is performed. Before discussing the in-plane analysis, we will first briefly describe the film characterization by the more standard methods of 0-20 diffractometry, rocking curves, resistance and susceptibility measurements. Figure l shows diffractograms of YBCO films grown on (100) MgO and (100) SrTiO3 and demonstrates that the film are solely c-axis oriented even over a wide range of substrate temperatures. The diffractometer used for this work had a large instrumental width of ~ 0.2 °, so it is not possible to give a reliable value for the crystalline coherence perpendicular to the film using the Scherrer formula. We performed rocking curves about the 005 film reflection and these are shown in fig. 2 along with the rocking curves about the 200 reflection for the corresponding substrates. Rocking curves provide information on the mosaicity of the material and the full width half maximum, F W H M , is taken as a measure of the film's crystalline quality. In general, our YBCO films
006 M, 0(200)
~3L9
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003
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- -
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b
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•
•
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005
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~'0
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2b
,b
DIFFRAC1"IONANGLE (20 Cu Ka)
Fig. I. X-ray diffractogram of a YBa2Cu307 film grown in-situ on a substrate of (a) ( 100 ) MgO, and (b) ( ! 00 ) SrTiO3,
MgO
(200)
] 1./
-i
o
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SrTiO3 (200)
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Fig. 2. Rocking curves about the 005 reflection of: (a) a YBa2Cu307 film (left) and about the 200 reflection of its corresponding MgO substrate (right), (b) a YBa2Cu307 film (left) and about the 200 reflection of its corresponding SrTiO3 substrate.
grown on SrTiO3 have narrower rocking curve widths t0.3 ° 1o 0.4 ° ) than those films grown on MgO (0.4 ° to 0.6 = ). This is probably because SrTiO3 is an intrinsically better substrate for "123" thin films than is MgO. Recall that the lattice parameter of SrTiO3 (a=3.905 ,~,) makes a better match with the "'123" materials than does the lattice parameter of MgO ( a = 4 . 2 ,i,). Even though we do show the rocking curves for the corresponding substrates in fig. 2 it is not evident that there is a very strong correlation of the rocking curve widths of the substrate with that of the film since we have observed the F W H M of the film can be larger, tl~e same, or even narrower than the rocking curve F W H M of the substrate on which it is grown. It is rather than surface quality of the substrate, whi,_h we characterize by RHEED, which is essential to ensure the growth of epitaxial thin films. The superconducting properties of YBCO films are of high quality with T¢ onsets of ~ 91 K and "~o" 8889 K and Jc "" 106 A / c m 2 at 77 K for E!m~ " , SrTiO3. The resistive transitions for films on MgO, although narrow, are a few degrees lower. We have observed
M.G. Karkut et al. / Surface and in-plane characterization o.f YBCO
264
this same difference in /'co for films sputtered onto SrTiO3 and MgO [ 5] although now the laser deposited films have Tcos a coupled of degrees higher than their sputter deposited counterparts. Figure 3 (a) is a normalized resistance curve for a film grown on SrTiO3 and (b) shows resistance curves for two different films on MgO in which the only difference in the deposition procedure has been an increase of the substrate temperature To. Further increase in Ts beyond 670°C results in a decrease in Tc and lowering Ts below 630°C results in a degradation of the crystalline properties of the film as well as lowering To. Superconducting transitions have also been detected
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by an AC susceptibility technique [ 6 ]. The best films on both MgO and SrTiO3 substrates have narrow ( < 1 K) AC transitions suggestive of excellent film homogeneity• Figure 4 shows the in-phase Z' and X" components as a function of temperature for a film grown on MgO.
3. In-plane characterization We use a RIBER model DER 410 10 keY RHEED system. The substrate holder allows for x, y, z and rotational motion, all of which facilitate the positioning of the electron beam onto the substrate surface. The RHEED is useful not only for studying the structure of the films but also for evaluating the quality of the substrates. The basics of RHEED are the following. The 10 keV electron beam angle of incidence is on the order of a degree, the transverse momentum is weak and diffraction takes place essentially from the su~ace layer of atoms. Since the reciprocal lattice of an ordered surface layer of atoms is an array of rods perpendicular to the surface, the condition for diffraction is met when there is intersection of a rod, or rods, with the Ewald sphere. Because the radius of the Ewald sphere is very large, 2=0.122 A for 10 keV electrons, the first row of rods will touch a relatively large section of the equatorial portion of the sphere. These are seen as streaks on the phosphor detecting screen. RHEED streaks are considered indicative of
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'
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i
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I 86
88
I 90
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I 96
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Fig. 3. (a) normalized resistance vs. temperature for a YBCO film grown on SrTiO3, (b) two different YBCO films grown on MgO substrates. T~ indicates the substrate temperature.
I
82
~
I
1
84
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86
1
I
88
t
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T E ~ ' d ~ E R A T U R E (K)
Fig. 4. The in-plane Z' and quadrature X" components of the AC susceptibility vs. temperature for a YBCO film grown on MgO.
M.G. Karkut et al. /St#face and in-plane characterization o.f YBCO
atomically smooth surfaces, and the distance between the streaks is related to the distance between the surface rows of atoms which diffract the incident electron beam. I'he presence of diffraction spots suggests that there is scattering from the bulk of the material, i.e. the surface is no longer smooth but it is still oriented. Figure 5 shows R H E E D photographs taken of the SrTiO3 surface and then of the deposited film at three different azimuths. We emphasize here that we do not use the R H E E D during the deposition since, without differential pumping, the electron gun cannot support the high 02 pressures necessary to produce the film. The streaks in fig. 5 (a) demonstrate the high quality of the substrate surface. (The bright circular spot in the center of the film is only an artifact and is the light from the filament of the electron gun.) We have found that the substrate must present RHEED streaks before deposition. If a substrate has a R H E E D pattern com-
265
posed only of oriented spots then the film on which it is deposited is more than likely to be of inferior quality, i.e. not epitaxial and not superconducting down to 77 K. Thus we use the RHEED to screen the substrates before deposition. The streaks of fig. 5 (b) imply the surface of the film to be atomically smooth. The azimuth of the SrTiO3 substrate was exactly the same for fig. 5 (a) and (b) and this demonstrates that the surface film axes are parallel to the substrate axes. In (c) the substrate holder has been rotated by 45-', corresponding to the ( 1 1 0 ) azimuth. In fig. 5(d) the substrate has been rotated by 26.5 ¢, which corresponds to the ( 1 2 0 ) azimuth. The distance between the streaks becomes larger as we go from 5 (b) to (d) which is consistent with the decreasing distance between the rows of surface atoms. In fig. 5 (d) the reflections resulting from the intersection of the second row of rods with the Ewald sphere can also be seen. In figs. 6 ( a ) and (b) we present the RHEED
Fig. 5. RHEED patterns of (a) a (100) SrTiO3 subslrale, (b) after the deposilion ofa YBCO film and corresponding to the ( 100~ azimuth; (c) and (d) are the same as (b) but correspond to the ( II 0) and (120) azimulhs, respectively.
266
M.G. Karkut et al. / Stoface and in-plane characterization of YBCO
Fig. 6. RHEED patterns for (a) (lO0) MgO substrate, ( tO0> azimuth, (b) ( ! 10> azimuth of the YBCO film.
Fig. 7. (a) Scanning electron micrograph of a YBCO film. The film is smooth but with sub-micronic particles on the surface; (b) SEM micrograph ofa YBCO film on MgO which displays an anomalous surface structure which makes an angle ot"45 ° with the (100) substrate ax~s.
patterns of MgO alone corresponding to the ( 1 0 0 ) azimuth, and after the deposition of a YBCO film corresponding to the ( l l 0 ) azimuth. The streaks again demonstrate the epitaxial quality of the films. Our experience has been that the R H E E D streaks for films grown on MgO are. in general, less fine than those produced for films grown on SrTiO3 even though the RHEED streaks produced by the two substrafes alone can be comparable in quality. If we suppose that some of the width of the streaks is coming from grain misalignment, then it is probable that the ~,~idth ofthe RHEED streaks would be inversely proportional to the grain size. Since the lattice midmatch is smaller for YBCO on SrTiO~ than for YBCO
on MgO, we expect, from general physical considerations, the grain size for YBCO films on MgO to be smaller, and hence the RHEED streaks to be broader, than for YBCO films deposited on SrTiO3. We now turn to some anomalous growth behavior that we have occasionally observed on MgO substrates and which we believe was due to the substrates being inhomogeneously heated. We first observed this phenomenon in RHEED patterns which consisted of unevenly spaced streaks. We used a scanning electron microscope to further investigate this anomalous growth mode. Figure 7 ( a ) is a SEM photograph of an epitaxially grown YBCO film which does not display any anomalous structure. However,
M.G. Karkut et al. / Surface and in-plane characterization of YBCO
X o
-I.
4-b +
,, + +
X +X + x
÷ 4+
X X
+ + + +
X X X + ..F + X + X + ..l-
Fig. 8. Oscillating crystal diffraction patterns of two YBa2Cu307 films deposited on (100) MgO. Vertical arrows indicate YBCO diffraction rows and horizontal arrows indicate the MgO diffraco tion spots. In (a) the oscillation is about the ( 1 0 0 ) direction of the substrate and in (b) the oscillation is about the ( I l0 ) substrate axis. This thin film displays no anomalous growth since the rows of diffraction ~pots due to YBCO correspond only to the (100) interplanar film spacings (a), and only to the (110) interplanar film spacings (b). In (c) and (d) are diffraction patterns shown ofa YBCO film grown on (100) MgO that displays anomalous growth. The ost illations are about the ( 100 ) substrate axis and the ( 1 1 0 ) substrate axis, respectively. We indicate schematically the diffraction spots: ( + ) and ( x ) correspond Io (, I00) and ( 1 1 0 ) axis rotation patterns respectively, and ( o ) corresponds to diffraction from the substrate.
fig. 7 (b) reveals a structure which is rotated to the crystalline axes of the cleaved MgO substrate. This lighter needle-like network, as seen in fig. 7(b), co-
267
exist with the darker " n o r m a l " growth mode and is also c-axis oriented as determined by 0-20 diffractometry. To garner more in-plane information about these curiosities, we subjected them to oscillating crystal diffraction in a Weissenberg camera. A description of this method for studying the in-plane structure of thin films is given in ref. [4]. Briefly, a single crystal substrate axis is aligned parallel to the camera axis. A monochromatic X-ray beam impinges onto the rotating substrate. A (central) row of reflections will be recorded which correspond to the scattering vector perpendicular to the plane of the substrate and also parallel rows to the left and right of this center row if the substrate is single crystal. The distance between these rows gives the inplane lattice parameter of the substrate. If now a thin film is grown on the substrate, additional reflections will be recorded due to the film. If a film is epitaxially grown onto the substrate, then additional rows of spots will appear and will be parallel to the substrate rows. If, however, the film is only textured, with the grains of the films randomly oriented in the film plane, then rings will be observed instead of well-defined spots. With this Weissenberg method the inplane lattice parameter, a, of the substrate or the thin film can be determined from lhe formula: a = n2/sin [arctan ( d / 2 R
)] ,
where n is the order of the row of streaks (for the central row u = 0 ), R is tile radius of the camera (for this work 2 R = 57.295 mm), d is the distance between two rows symmetric about the central row, and 2 is the wavelength o f t h e X-radiation ( 2 = 1.542 A). It can at times be difficult to distinguish the thin film reflections from those of the substrate. If the in-plane lattice parameter of the film and the substrate are about equal, as is the case for c-axis oriented YBCO and SrTiO3, then the two sets of points will occupy the same row. If the in-plane lattice parameters are substantially different, as is the case with c-axis orie_n_!ed YRCO and MgO. then the film points will be displaced relative to those of the substrate. In figs. 8 ( a ) and (b) we show diffraction patterns of a YBCO film/MgO substrate which shows no anomalous gro~vth. The substrate was oscillated about the ( 1 0 0 ) MgO axis in fig. 8(a) and about the ( l l 0 ) MgO axis in fig. 8(b). We mark the spots due to MgO with the horizontally pointing arrows. The rows of
268
M.G. Karkut et ai. / Surface and in-plane characterization o,f YBCO
spots centered about the center row are due to the YBCO film and are marked by the vertically pointing arrows. In fig. 8 (a) the rows correspond to the (100) interplanar spacing of the YBCO film and in fig. 8 (b) to the (110) interplanar spacing of the YBCO film. (The streaks radiating out from the bottom center of the photographs are due to the continuous background X-radiation and are not significant.) By contrast, the photograph taken of the anomalous YBCO film about the ( 1 0 0 ) substrate axis as shown in fig. 8(c) shows not only the expected YBCO reflections which correspond to the aaxis lattice parameter (marked by + in the schematic below fig. 8 ( c ) ) but also additional points ( marked by x in the schematic) which correspond to (v/2/2)a. This means that the a- and b-axes of a portion of the YBCO film are oriented at 45 ° to the axes of the MgO substrate. For a direct comparison with fig. 8 (b) we show the same anomalous YBCO film but now rotated about the ( I l 0) substrate direction as shown in fig. 8(d). Here we see the expected points corresponding to the (110) interplanar spacing (marked by x in the schematic below fig. 8(d)) as in fig. 8(a) but also reflections (marked by + ) corresponding to the 45 ° rotation with respect to the substrate axes. This demonstrates that the anomalous growth is epitaxial in the sense that it is strictly colinear with the (110) direction of the MgO substrate. This study further demonstrates the utility of the Weissenberg method in exploring unusual surface features of thin films.
4. Conclusion We have deposited by laser ablation high quality YBa2Cu307 films on substrates of (100) MgO and (100) SrTiO3. Films on (100) substrates are c-axis
oriented and have narrow (FWHM~0.30 ° for YBCO on SrTiO3) rocking curves. The superconducting properties are good with Tc0s> 87 K and Jc (77 K ) ~ 106 A/cm 2. In addition, we have demonstrated the epitaxial order of these ablated films by two methods, RHEED and oscillating crystal X-ray photography, as yet not commonly employed in the HTS field. Films grown on (100) substrates display well-defined RHEED streaks which are indicative of an atomically flat surface. The distance between the streaks for different azimuths corresponds to expected interrow spacings. Oscillating crystal (Weissenberg) photographs show that the films are aligned with the substrate axes even in the case of the "anomalous" 45 ° crystal growth.
Acknowledgements This work was supported in part by the Centre National d'Etudes des Telecommunications, CNET Lannion B, under contract No. 89 8B054, by the Minist6re de la Recherche et de la Technologic under contract No. 89 H0556, and by the Fondation Langlois.
References [ 1 ] T. Terashima, K. lijima, K. Yamamoto, K. Hirata, Y. Bando and T. Takada, Jpn. J. Appl. Phys. 28 (1989) L987. [2 ] F. Baudenbacher, H. Karl, P. Berberich and H. Kinder, J. Less-Comm. Met. 164-165 (1990) 269. [ 3 ] T. Terashima, Y. Bando, K. lijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Lett. 65 (1990) 2684. [ 4 ] A. Perrin, M.G. Karkut, M. Guilloux-Viry and M. Sergent, Appl. Phys. Lett. 58 ( 1991 ) 412. [ 5 ] M. Guilloux-Viry, M.G. Karkut, A. Pert'in, O. Pena, J. Padiou and M. Sergent, Physica C 166 ( ! 990) 105. [6] O. Pena, Meas. Sci. Techn. 5 ( 1991 ) 470.
Physica C 179 ( i 99 ! ) 262-268 North-Holland
Surface and in-plane characterization of YBa2Cu307 thin films grown by laser ablation M.G. Karkut
a,
M. Guilloux-Viry a b, A. Perrin
a,
j. Padiou
a
and M. Sergent
a
a t'niversitb de Rennes I. Laborawire de Chimie Minbrale B, URA C N R S 254. Avenue du Gbn~raJ Leclerc, 35042 Rennes C~dex, France b C.N.E.T. Lannion 8, Division OCM, BP 40, 22301Lannion C~dex, France
Received 10 June 1991
We have used reflection high energy electron diffraction (RHEED) and oscillating crystal X-ray diffraction to analyze the surface and in-plane structural characteristics of YBa~Cu30, (YBCO) thin films that were grown in-situ by laser ablation onto substrates of (100) MgO and (100) SrTiO3. We obtain RHEED streaks, characteristic of atomically smooth surfaces and indicative of epitaxial order, for these thin films. Oscillating crystal diffraction also indicates that the YBCO film axes are collinear with the substrate axes. The films grown on ( ! 00) substrates are solely c-axis oriented ( perpendicular to the plane of the film ) as determined by 0-20 diffractometry and the FWHM of rocking curves about the (005) reflection are 0.3 ° to 0.5 ° for the best films on SrTiO3 substrates and 0.4: to 0.6: for films grown on MgO. Weissenberg photography allo~'s us to determine the in-plane structure of some films grown on MgO that display anomalous surface features seen by RHEED and by scanning electron microscopy.
1. Introduction As technical applications of the high-To superconducting (HTS) thin films come closer to reality, increased importance will be attached to the in,.plane structure of the thin films in order to determine their compatibility with various substrates and buffer layers, as well as to characterize the in-plane structure o," the substrates and buffer layers themselves. He, .: we present some results of two complementary methods, reflection high energy electron diffraction (RHEED) and oscillating crystal X-ray diffraction, to investigate the in-plane structure of YBCO thin films we have grown by laser ablation onto MgO and SrTiO3 substrates. The RHEED method is an in-situ method used to study surface structure. It has been used recently in MBE systems to study the initial stages of YBCO film growth [1,2]. More recently RHEED intensity oscillations have been observed during the deposition of oxide thin films [ 3 ]. These oscillations, which are attributed to layer by layer growth, can shed light on the nucleation and growth processes of these complex structures. As for oscillating crystal diffractometry, we have recently shown
[ 4 ] that this method is effective in characterizing the in-plane structure of HTS thin films. Since these methods are not commonly found in the HTS literature, we will use this opportunity to demonstrate their utility in providing information on the surface and in-plane features of laser ablated "123" thin films and their MgO and SrTiO3 substrates.
2. Deposition, 0-20 X-ray diffractometry, and superconductivity The films were grown by laser ablation. Incident laser pulses of 40 as duration and wavelength 2 = 308 nm are focused onto a rotating ceramic target which is at 45 ° with respect to the incident laser pulses. The ~llh~lral~ it glued with silver paint onto a stainless steel substrate holder which is heated to 600°C to 750:C, depending on the substrate, during deposition. The substrate directly faces the ta,'get and its distance from the target is adjustable and is usually kept at a distance of 45 mm. The base pressure of the system before deposition is ~ 5 X 10 -.7 mbar and during deposition there is a flowing 02 pressure of
092 !-4534/9 !/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
263
M.G. Karkut et al. /Surface and in-plane characterization of YBCO
~ 0.3 mbar. We operate the laser at a frequency between 0.67 and 4 Hz. We fix the pulse energy between 80 and 200 mJ and the beam is focused onto a square ~ 2 × 2 mm 2. After deposition the films are slowly cooled to room temperature in one atmosphere of oxygen and no further processing is performed. Before discussing the in-plane analysis, we will first briefly describe the film characterization by the more standard methods of 0-20 diffractometry, rocking curves, resistance and susceptibility measurements. Figure l shows diffractograms of YBCO films grown on (100) MgO and (100) SrTiO3 and demonstrates that the film are solely c-axis oriented even over a wide range of substrate temperatures. The diffractometer used for this work had a large instrumental width of ~ 0.2 °, so it is not possible to give a reliable value for the crystalline coherence perpendicular to the film using the Scherrer formula. We performed rocking curves about the 005 film reflection and these are shown in fig. 2 along with the rocking curves about the 200 reflection for the corresponding substrates. Rocking curves provide information on the mosaicity of the material and the full width half maximum, F W H M , is taken as a measure of the film's crystalline quality. In general, our YBCO films
006 M, 0(200)
~3L9
OO5 ,.__.j~
003
007 j
002 ]L
- -
~o
lo'
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b
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•
•
L~3
005
L 0:2 s8
~'0
3b
2b
,b
DIFFRAC1"IONANGLE (20 Cu Ka)
Fig. I. X-ray diffractogram of a YBa2Cu307 film grown in-situ on a substrate of (a) ( 100 ) MgO, and (b) ( ! 00 ) SrTiO3,
MgO
(200)
] 1./
-i
o
0.40 I
I
~.
o
)
I
i
-1
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0
1
A ® (degrees)
SrTiO3 (200)
•- ~
-I
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-1
<-- 0.25 °
0 -1 ~® (degrees)
Fig. 2. Rocking curves about the 005 reflection of: (a) a YBa2Cu307 film (left) and about the 200 reflection of its corresponding MgO substrate (right), (b) a YBa2Cu307 film (left) and about the 200 reflection of its corresponding SrTiO3 substrate.
grown on SrTiO3 have narrower rocking curve widths t0.3 ° 1o 0.4 ° ) than those films grown on MgO (0.4 ° to 0.6 = ). This is probably because SrTiO3 is an intrinsically better substrate for "123" thin films than is MgO. Recall that the lattice parameter of SrTiO3 (a=3.905 ,~,) makes a better match with the "'123" materials than does the lattice parameter of MgO ( a = 4 . 2 ,i,). Even though we do show the rocking curves for the corresponding substrates in fig. 2 it is not evident that there is a very strong correlation of the rocking curve widths of the substrate with that of the film since we have observed the F W H M of the film can be larger, tl~e same, or even narrower than the rocking curve F W H M of the substrate on which it is grown. It is rather than surface quality of the substrate, whi,_h we characterize by RHEED, which is essential to ensure the growth of epitaxial thin films. The superconducting properties of YBCO films are of high quality with T¢ onsets of ~ 91 K and "~o" 8889 K and Jc "" 106 A / c m 2 at 77 K for E!m~ " , SrTiO3. The resistive transitions for films on MgO, although narrow, are a few degrees lower. We have observed
M.G. Karkut et al. / Surface and in-plane characterization o.f YBCO
264
this same difference in /'co for films sputtered onto SrTiO3 and MgO [ 5] although now the laser deposited films have Tcos a coupled of degrees higher than their sputter deposited counterparts. Figure 3 (a) is a normalized resistance curve for a film grown on SrTiO3 and (b) shows resistance curves for two different films on MgO in which the only difference in the deposition procedure has been an increase of the substrate temperature To. Further increase in Ts beyond 670°C results in a decrease in Tc and lowering Ts below 630°C results in a degradation of the crystalline properties of the film as well as lowering To. Superconducting transitions have also been detected
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rr
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30
60
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C20 150 180 210 240 270 300 TEI4PERATURE K
by an AC susceptibility technique [ 6 ]. The best films on both MgO and SrTiO3 substrates have narrow ( < 1 K) AC transitions suggestive of excellent film homogeneity• Figure 4 shows the in-phase Z' and X" components as a function of temperature for a film grown on MgO.
3. In-plane characterization We use a RIBER model DER 410 10 keY RHEED system. The substrate holder allows for x, y, z and rotational motion, all of which facilitate the positioning of the electron beam onto the substrate surface. The RHEED is useful not only for studying the structure of the films but also for evaluating the quality of the substrates. The basics of RHEED are the following. The 10 keV electron beam angle of incidence is on the order of a degree, the transverse momentum is weak and diffraction takes place essentially from the su~ace layer of atoms. Since the reciprocal lattice of an ordered surface layer of atoms is an array of rods perpendicular to the surface, the condition for diffraction is met when there is intersection of a rod, or rods, with the Ewald sphere. Because the radius of the Ewald sphere is very large, 2=0.122 A for 10 keV electrons, the first row of rods will touch a relatively large section of the equatorial portion of the sphere. These are seen as streaks on the phosphor detecting screen. RHEED streaks are considered indicative of
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Fig. 3. (a) normalized resistance vs. temperature for a YBCO film grown on SrTiO3, (b) two different YBCO films grown on MgO substrates. T~ indicates the substrate temperature.
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M.G. Karkut et al. /St#face and in-plane characterization o.f YBCO
atomically smooth surfaces, and the distance between the streaks is related to the distance between the surface rows of atoms which diffract the incident electron beam. I'he presence of diffraction spots suggests that there is scattering from the bulk of the material, i.e. the surface is no longer smooth but it is still oriented. Figure 5 shows R H E E D photographs taken of the SrTiO3 surface and then of the deposited film at three different azimuths. We emphasize here that we do not use the R H E E D during the deposition since, without differential pumping, the electron gun cannot support the high 02 pressures necessary to produce the film. The streaks in fig. 5 (a) demonstrate the high quality of the substrate surface. (The bright circular spot in the center of the film is only an artifact and is the light from the filament of the electron gun.) We have found that the substrate must present RHEED streaks before deposition. If a substrate has a R H E E D pattern com-
265
posed only of oriented spots then the film on which it is deposited is more than likely to be of inferior quality, i.e. not epitaxial and not superconducting down to 77 K. Thus we use the RHEED to screen the substrates before deposition. The streaks of fig. 5 (b) imply the surface of the film to be atomically smooth. The azimuth of the SrTiO3 substrate was exactly the same for fig. 5 (a) and (b) and this demonstrates that the surface film axes are parallel to the substrate axes. In (c) the substrate holder has been rotated by 45-', corresponding to the ( 1 1 0 ) azimuth. In fig. 5(d) the substrate has been rotated by 26.5 ¢, which corresponds to the ( 1 2 0 ) azimuth. The distance between the streaks becomes larger as we go from 5 (b) to (d) which is consistent with the decreasing distance between the rows of surface atoms. In fig. 5 (d) the reflections resulting from the intersection of the second row of rods with the Ewald sphere can also be seen. In figs. 6 ( a ) and (b) we present the RHEED
Fig. 5. RHEED patterns of (a) a (100) SrTiO3 subslrale, (b) after the deposilion ofa YBCO film and corresponding to the ( 100~ azimuth; (c) and (d) are the same as (b) but correspond to the ( II 0) and (120) azimulhs, respectively.
266
M.G. Karkut et al. / Stoface and in-plane characterization of YBCO
Fig. 6. RHEED patterns for (a) (lO0) MgO substrate, ( tO0> azimuth, (b) ( ! 10> azimuth of the YBCO film.
Fig. 7. (a) Scanning electron micrograph of a YBCO film. The film is smooth but with sub-micronic particles on the surface; (b) SEM micrograph ofa YBCO film on MgO which displays an anomalous surface structure which makes an angle ot"45 ° with the (100) substrate ax~s.
patterns of MgO alone corresponding to the ( 1 0 0 ) azimuth, and after the deposition of a YBCO film corresponding to the ( l l 0 ) azimuth. The streaks again demonstrate the epitaxial quality of the films. Our experience has been that the R H E E D streaks for films grown on MgO are. in general, less fine than those produced for films grown on SrTiO3 even though the RHEED streaks produced by the two substrafes alone can be comparable in quality. If we suppose that some of the width of the streaks is coming from grain misalignment, then it is probable that the ~,~idth ofthe RHEED streaks would be inversely proportional to the grain size. Since the lattice midmatch is smaller for YBCO on SrTiO~ than for YBCO
on MgO, we expect, from general physical considerations, the grain size for YBCO films on MgO to be smaller, and hence the RHEED streaks to be broader, than for YBCO films deposited on SrTiO3. We now turn to some anomalous growth behavior that we have occasionally observed on MgO substrates and which we believe was due to the substrates being inhomogeneously heated. We first observed this phenomenon in RHEED patterns which consisted of unevenly spaced streaks. We used a scanning electron microscope to further investigate this anomalous growth mode. Figure 7 ( a ) is a SEM photograph of an epitaxially grown YBCO film which does not display any anomalous structure. However,
M.G. Karkut et al. / Surface and in-plane characterization of YBCO
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Fig. 8. Oscillating crystal diffraction patterns of two YBa2Cu307 films deposited on (100) MgO. Vertical arrows indicate YBCO diffraction rows and horizontal arrows indicate the MgO diffraco tion spots. In (a) the oscillation is about the ( 1 0 0 ) direction of the substrate and in (b) the oscillation is about the ( I l0 ) substrate axis. This thin film displays no anomalous growth since the rows of diffraction ~pots due to YBCO correspond only to the (100) interplanar film spacings (a), and only to the (110) interplanar film spacings (b). In (c) and (d) are diffraction patterns shown ofa YBCO film grown on (100) MgO that displays anomalous growth. The ost illations are about the ( 100 ) substrate axis and the ( 1 1 0 ) substrate axis, respectively. We indicate schematically the diffraction spots: ( + ) and ( x ) correspond Io (, I00) and ( 1 1 0 ) axis rotation patterns respectively, and ( o ) corresponds to diffraction from the substrate.
fig. 7 (b) reveals a structure which is rotated to the crystalline axes of the cleaved MgO substrate. This lighter needle-like network, as seen in fig. 7(b), co-
267
exist with the darker " n o r m a l " growth mode and is also c-axis oriented as determined by 0-20 diffractometry. To garner more in-plane information about these curiosities, we subjected them to oscillating crystal diffraction in a Weissenberg camera. A description of this method for studying the in-plane structure of thin films is given in ref. [4]. Briefly, a single crystal substrate axis is aligned parallel to the camera axis. A monochromatic X-ray beam impinges onto the rotating substrate. A (central) row of reflections will be recorded which correspond to the scattering vector perpendicular to the plane of the substrate and also parallel rows to the left and right of this center row if the substrate is single crystal. The distance between these rows gives the inplane lattice parameter of the substrate. If now a thin film is grown on the substrate, additional reflections will be recorded due to the film. If a film is epitaxially grown onto the substrate, then additional rows of spots will appear and will be parallel to the substrate rows. If, however, the film is only textured, with the grains of the films randomly oriented in the film plane, then rings will be observed instead of well-defined spots. With this Weissenberg method the inplane lattice parameter, a, of the substrate or the thin film can be determined from lhe formula: a = n2/sin [arctan ( d / 2 R
)] ,
where n is the order of the row of streaks (for the central row u = 0 ), R is tile radius of the camera (for this work 2 R = 57.295 mm), d is the distance between two rows symmetric about the central row, and 2 is the wavelength o f t h e X-radiation ( 2 = 1.542 A). It can at times be difficult to distinguish the thin film reflections from those of the substrate. If the in-plane lattice parameter of the film and the substrate are about equal, as is the case for c-axis oriented YBCO and SrTiO3, then the two sets of points will occupy the same row. If the in-plane lattice parameters are substantially different, as is the case with c-axis orie_n_!ed YRCO and MgO. then the film points will be displaced relative to those of the substrate. In figs. 8 ( a ) and (b) we show diffraction patterns of a YBCO film/MgO substrate which shows no anomalous gro~vth. The substrate was oscillated about the ( 1 0 0 ) MgO axis in fig. 8(a) and about the ( l l 0 ) MgO axis in fig. 8(b). We mark the spots due to MgO with the horizontally pointing arrows. The rows of
268
M.G. Karkut et ai. / Surface and in-plane characterization o,f YBCO
spots centered about the center row are due to the YBCO film and are marked by the vertically pointing arrows. In fig. 8 (a) the rows correspond to the (100) interplanar spacing of the YBCO film and in fig. 8 (b) to the (110) interplanar spacing of the YBCO film. (The streaks radiating out from the bottom center of the photographs are due to the continuous background X-radiation and are not significant.) By contrast, the photograph taken of the anomalous YBCO film about the ( 1 0 0 ) substrate axis as shown in fig. 8(c) shows not only the expected YBCO reflections which correspond to the aaxis lattice parameter (marked by + in the schematic below fig. 8 ( c ) ) but also additional points ( marked by x in the schematic) which correspond to (v/2/2)a. This means that the a- and b-axes of a portion of the YBCO film are oriented at 45 ° to the axes of the MgO substrate. For a direct comparison with fig. 8 (b) we show the same anomalous YBCO film but now rotated about the ( I l 0) substrate direction as shown in fig. 8(d). Here we see the expected points corresponding to the (110) interplanar spacing (marked by x in the schematic below fig. 8(d)) as in fig. 8(a) but also reflections (marked by + ) corresponding to the 45 ° rotation with respect to the substrate axes. This demonstrates that the anomalous growth is epitaxial in the sense that it is strictly colinear with the (110) direction of the MgO substrate. This study further demonstrates the utility of the Weissenberg method in exploring unusual surface features of thin films.
4. Conclusion We have deposited by laser ablation high quality YBa2Cu307 films on substrates of (100) MgO and (100) SrTiO3. Films on (100) substrates are c-axis
oriented and have narrow (FWHM~0.30 ° for YBCO on SrTiO3) rocking curves. The superconducting properties are good with Tc0s> 87 K and Jc (77 K ) ~ 106 A/cm 2. In addition, we have demonstrated the epitaxial order of these ablated films by two methods, RHEED and oscillating crystal X-ray photography, as yet not commonly employed in the HTS field. Films grown on (100) substrates display well-defined RHEED streaks which are indicative of an atomically flat surface. The distance between the streaks for different azimuths corresponds to expected interrow spacings. Oscillating crystal (Weissenberg) photographs show that the films are aligned with the substrate axes even in the case of the "anomalous" 45 ° crystal growth.
Acknowledgements This work was supported in part by the Centre National d'Etudes des Telecommunications, CNET Lannion B, under contract No. 89 8B054, by the Minist6re de la Recherche et de la Technologic under contract No. 89 H0556, and by the Fondation Langlois.
References [ 1 ] T. Terashima, K. lijima, K. Yamamoto, K. Hirata, Y. Bando and T. Takada, Jpn. J. Appl. Phys. 28 (1989) L987. [2 ] F. Baudenbacher, H. Karl, P. Berberich and H. Kinder, J. Less-Comm. Met. 164-165 (1990) 269. [ 3 ] T. Terashima, Y. Bando, K. lijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Lett. 65 (1990) 2684. [ 4 ] A. Perrin, M.G. Karkut, M. Guilloux-Viry and M. Sergent, Appl. Phys. Lett. 58 ( 1991 ) 412. [ 5 ] M. Guilloux-Viry, M.G. Karkut, A. Pert'in, O. Pena, J. Padiou and M. Sergent, Physica C 166 ( ! 990) 105. [6] O. Pena, Meas. Sci. Techn. 5 ( 1991 ) 470.