J P Marchet

C Champeaux

D Damiani

P Marchet

A Catherinot

Interaction laser U.V (KrF 248 nm). Cible supraconductrice YBaCuO : -étude du panache par spectrométrie de masse -application à la réalisation de couches minces

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-Analyse du panache par spectrométrie de masse

Un laser à excimères KrF (248 nm, énergie de 20 à 300 mJ pour une impulsion de 14 ns) est focalisé avec une incidence de 35 °, sur un échantillon supraconducteur fritté placé à l'intérieur d'une enceinte à ultra-vide (10 -9 mbar) et muni d'un mouvement de rotation. Le panache plasma formé est analysé grâce à un spectromètre de masse quadrupolaire QMG 420 ALZERS équipé d'un pompage turbomoléculaire différentiel. Du fait du faible rapport signal/bruit lors de la détection d'espèces neutres, notre étude s'est limitée aux ions. Outre les ions des atomes élémentaires (Y + , Ba + , Cu + , O + ) et des molécules diatomiques oxydes (YO + , BaO + , CuO + ) observés par spectroscopie optique [1], des espèces plus complexes telles que YO2 + , Y2O2 + , BaO2 + , Ba2O + et Ba2O2 + ont été détectées. L'intensité des espèces ioniques mono et di-atomiques suit une loi quasi-exponentielle en fonction de la fluence alors que celle des ions plus lourds présente un maximum pour une fluence de l'ordre de 3 J/cm 2 (voir par exemple les figures 1 et 2). Notons que c'est à cette fluence que nous obtenons les meilleurs dépôts. Les orbes d'éjection des ions, déterminés en inclinant la cible, montrent que leurs distributions angulaires très spéculaires peuvent être approximées par une loi en cos n où n varie de 90 à 170 suivant l'espèce (exemple figure 3 pour O + ).

-Corrélation : panache de vapeur éjectée -dépôts

Afin de corréler les résultats de l'étude précédente à la distribution en épaisseur, un dépôt a été réalisé sur un substrat silicium auquel était accolée une grille. Les mesures d'épaisseur effectuées (figure 4) à l'aide d'un profilomètre DEKTAK IIA sont comparées à une fonction en cos p . p varie de 16 au centre (dans un disque d'environ 1 cm de diamètre) à 12 à la périphérie. A partir des approximations en cos n (orbe) et cos p (épaisseur), nous avons montré que les ions représentent 10 % des espèces contribuant à l'épaisseur du dépôt et qu'une couche de bonne qualité ne peut être obtenue que dans un cône d'angle 10° centré autour de la normale à la cible.

-Application à la réalisation de couches minces

Les analyses du panache par spectroscopie optique et par spectrométrie de masse permettent la détermination des conditions de transfert stoechiométrique des espèces cationiques (Y, Ba, Cu) de la cible vers le substrat. Mais l'élaboration de dépôts supraconducteurs de bonne qualité nécessite la maîtrise de divers paramètres tels par exemple les température et pression de dépôt, l'atmosphère et la pression de refroidissement. Une étude systématique [START_REF] Champeaux | Matériaux et microélectronique hybride[END_REF] de ceux-ci nous a conduit à l'élaboration de films supraconducteurs epitaxiés (001) (axe c perpendiculaire au substrat) présentant une température critique de 90 K. et une largeur de transition inférieure à 1 K sur substrats monocristallins MgO

  (fig 5) et LaA1O3 (fig 6) et ce, de manière reproductible.

Figures 1 et 2 :

 2 Figures 1 et 2 : Evolution de l'intensité avec la fluence

Figure 3 :Figure 4 :

 34 Figure 3 : Distribution angulaire pour O +


P Marchet

C Champeaux

J P Mercurio

B Frit

A Catherinot

Pulsed laser deposition of YBa2Cu3O7-x superconducting thin films: correlation between preparation conditions and structural and electrical properties

YBa2Cu3O7-x thin films were obtained on MgO single crystals using pulsed laser deposition with a KrF excimer laser (248 nm) operated at 10 Hz with a laser fluence of 3 J/cm 2 . Substrate temperature was monitored at 790°C with an halogen lamp and oxygen pressure was maintained at 0.3 mbar. Structural parameters were systematically investigated by X-ray diffraction. Electrical properties were characterized using a DC four probes method (25 to 300 K). In order to understand parameters which may lead to epitaxial thin films with high superconducting properties (i.e. transition temperature as high as 90 K, low transition width and critical current density of about 10 5 to 10 6 A/cm 2 ), the role of cooling process was studied for purely c-axis oriented films using different cooling gases (oxygen or argon) and different cooling pressures. The effects of cooling process were checked by systematic study of cell parameters, rocking curves, critical temperature and transition width. Optimum cooling pressure led to thin films with a transition width of about 0.5 K and critical current density of about 10 6 A/cm 2 at 80 K.

Introduction

Since the discovery of superconductivity [1], numerous investigations have been devoted to the preparation of superconducting thin films. Comparatively to the different deposition techniques [2][3][4][5][6][7], pulsed laser deposition rapidly appeared to be the most suitable technique to obtain the correct cationic stoichiometry [8]. However, to produce reproducible high quality YBaCuO thin films suitable for practical applications, not only appropriate cationic stoichiometry but also oxygen stoichiometry control is necessary because YBaCuO is a non stoichiometric oxide [9,10]. In addition, physical properties of YBaCuO, particularly conductivity, are very anisotropic [11]. So it is very important to obtain oriented thin films and to try to obtain epitaxial material. However, crystallographic orientation, cationic and oxygen stoichiometry depend on simultaneous control of many parameters during deposition and cooling steps. The influence of preparation conditions have been reported in precedent papers [START_REF] Champeaux | Laser Ablation of Electronic Materials : Basic Mechanisms and Applications[END_REF][START_REF] Champeaux | E-MRS Spring Meeting[END_REF], so we report here the influence of the cooling step for samples prepared with parameters leading to purely c axis oriented films, because only few work has been devoted to the influence of cooling conditions for YBaCuO thin films prepared by this technique [START_REF] Ohkubo | [END_REF].

Thin films elaboration

YBaCuO thin films were obtained on MgO (100) single crystals using pulsed laser deposition, with a KrF excimer laser (λ = 248 nm) operated at 10 Hz with a laser fluence of about 3 J/cm 2 . During deposition process, substrate can be heated by an halogen lamp and its temperature is kept at 790°C. Pure oxygen is introduced in the vacuum chamber at a pressure of 0.3 mbar. Deposition process takes about 10 minutes to obtain 300 nm thick film. After deposition, the vacuum chamber is filled with the selected gas (see table 1) and samples are allowed to cool down without any control of cooling speed. We systematically studied the effects of different cooling atmospheres using oxygen or argon and different cooling pressures (10 -6 mbar to 1065 mbar). This process leads to black mirror like films purely c axis oriented.

Sample characterization

Immediatly after deposition, structural properties are investigated using a standard θ/2θ diffractometer (Cu Kα). Thin films obtained by this process are purely c axis oriented, so only (001) lines are detected by X-ray diffraction. Computation of these different lines gives the c parameter and the value of the oxygen content "x" [10,15]. Rocking curve of the (005) YBaCuO line gives an idea of the mosaicity of the film. The thermal evolution of electrical resistivity is measured by a standard DC four probes method in a closed cycle helium cryostat (25-300 K). Transition width ∆T is defined using the 90% -10% criterion. Critical current density is measured on a 0.5 mm x 25 µm etched line, using a 1 µV criterion. Previous studies [START_REF] Champeaux | Laser Ablation of Electronic Materials : Basic Mechanisms and Applications[END_REF][START_REF] Champeaux | E-MRS Spring Meeting[END_REF] gave evidence for the achievement of purely c axis oriented films for particular values of oxygen pressure and temperature during deposition, but oxygen deficiency, as measured by X-ray diffraction and superconducting properties (∆T = 6 K), were not optimum. Some authors [START_REF] Ohkubo | [END_REF] reported that deposited material was tetragonal with an oxygen content close to 6, and that oxygen incorporation was occuring by in-diffusion during cooling down under oxygen. They reported too that increasing oxygen pressure during cooling was leading to an improvement of YBaCuO oxygen content, as measured by decrease of c cell parameter. So we tried to improve oxygen content and superconducting properties by a systematic study of cooling process.

Eight samples were cooled under oxygen using different total pressures (samples D2 to D9, see table 1). In order to give evidence for the necessity of an oxygen atmosphere during cooling, one sample was cooled under vacuum (D1) and two samples were cooled down under argon (D10 and D11).

c lattice parameter

X-ray diffraction spectra show only (001) lines. For sample cooled under vacuum, the c cell parameter is high, giving an oxygen deficiency of about x ≈ 0.8 [10,15]. c decreases rapidly with increasing oxygen cooling pressure (fig. 1), indicating that oxygen deficiency is decreasing too, from x ≈ 0.8 to x ≈ 0.2 [10,15], according to Ohkubo values [START_REF] Ohkubo | [END_REF].

For argon cooled samples, oxygen deficiency is about 0.6-0.7, less than the value obtained for vacuum cooled sample. So we can consider, as Ohkubo did, that during cooling, oxygen diffusion occurs from gazeous phase into the film. Equilibria conditions determine final oxygen content of samples.

Argon cooled samples could not increase their oxygen content during cooling because there was no oxygen in gazeous phase. So we can consider that before cooling, they were oxygen deficient, but oxygen deficiency is lower than reported by Ohkubo [START_REF] Ohkubo | [END_REF]. These results show clearly that during deposition, YBaCuO grows epitaxially with an oxygen deficiency of about 0.6-0.7, indicating a structure close to the orthorhombic to tetragonal transition [10,15].

Mosaicity of the films

X-ray diffraction rocking curves were systematically measured for (005) YBaCuO line. Evolution of full width at half maximum (FWHM) of the measured diagram clearly shows an evolution of the mosaicity of the film with cooling pressure (fig. 2). First, mosaicity decreases with increasing pressure, from 0.65° for 10 -6 mbar to about 0.5° for 100-150 mbar. Then, it increases to reach a constant value of 0.7°above about 200 mbar.

This evolution could be due to an increase of lattice mismatch between YBaCuO and MgO with increasing oxygen content, and consequently, an increase of interfacial stress accomodated by structural defects like dislocations.

Electrical properties

Electrical properties were measured between 25 and 300 K. Vacuum cooled sample is perfectly semiconducting (Fig. 3 a). Samples cooled under oxygen are all superconducting (fig. 3 a andb). Transition temperatures increase and transition width decreases with increasing cooling pressure (fig. 4). The curve suggests that an optimum cooling pressure exists, with a value between 100 and 300 mbar of oxygen.

Argon cooled samples are superconducting with lower transition temperatures (fig. 3 a) and very large transitions. This clearly indicates that for these films, YBaCuO is not tetragonal but orthorhombic because only orthorhombic YBaCuO is superconducting [10,15].

Critical current density

Three samples only were characterized (see table 1). Critical current densities of two samples are slightly lower than those reported before (10 6 A/cm 2 ). The abnormaly low value obtained for sample D9 is up to now unexplained. Further measurements are in progress in order to confirm both these values and the possible occurence of an optimum oxygen cooling pressure.

Conclusion

In situ growth of superconducting thin films depends on many interdependant parameters. For particular values of substrate temperature and oxygen pressure, deposited material is purely c axis oriented (X-ray diffraction pole figures, which are not reported here, clearly showed epitaxy of YBaCuO on MgO substrate). During deposition, superconducting YBaCuO grows epitaxially on MgO, with an oxygen deficiency of about x = 0.6, indicating a structure close to the orthorhombic to tetragonal transition. So we can considere that oxygen incorporation during deposition is not sufficient.

During cooling step, oxygen incorporation depends on oxygen partial pressure. If partial pressure exceeds equilibrium value, oxygen can diffuse into the film and so the oxygen content increases. This evolution leads to improvement of electrical properties.

Table 1 :

 1 Elaboration conditions and cooling parameters of the YBaCuO thin films.

Fig. 1 :

 1 Fig. 1 : Evolution of c cell parameter with cooling pressure for samples cooled under oxygen or argon.

Fig. 2 :

 2 Fig.2 : Evolution of (005) YBaCuO rocking curve FWHM. with cooling pressure.

Fig. 3 :

 3 Fig. 3 : Evolution of resistivity versus temperature characteristics : (a) D1 (vacuum cooled), D10 and D 11 (Argon cooled) and D2 (2mbar oxygen) (b) samples D4 to D9 cooled under increasing oxygen pressure.

Fig. 4 :

 4 Fig. 4 : Evolution of superconducting transition width with cooling pressure.

Acknowledgement

This work is partly supported by D.R.E.T. under contract No 88 -084.


Marina Licheron

Isabelle Reynaud

Ricardo P S M Lobo

François Gervais

Corinne Champeaux

Pascal Marchet

Alain Catherinot

Layered Ba

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INTRODUCTION

A very interesting feature in the Ba-K-Pb-Bi-O superconductor family is the presence of superconductivity in both 2D [(Ba,K)2(Pb,Bi)O4] [l] and 3D [(Ba,K)(Pb,Bi)O3] structures [2,3]. A property which is not present in any other system. Moreover, superconductivity is found to be twice higher in 2D structures, comparing with 3D ones. The main problem related to the production of 2D compounds is the incorporation of large Bi concentrations. More than 15% of Bi leads to the formation of a insulator double-perovskite structure. Unfortunately, rising bismuth contents in the 2D structure increases the critical temperature. Based on the 3D structure (Ba,K)BiO3 data, one could expect the highest Tc for the layered structure in (Ba, K)2BiO4.

It seems that one of the best process in the production of superconducting films is the laser ablation. Excellent results have already been reported for YBaCuO with films presenting a high degree of orientation, high critical currents and sharp transitions [4].

In this work, we have tried to force the tetragonal (2D) structure in compounds with high Bi concentration with laser ablation. We present the first results of infrared characterisation of Ba2BiO4 films. A first analysis show that for smaller thickness, our films have the 2D structure but for thicker films it seems that a coexistence of 2D and 3D phases is developed.

EXPERIMENTAL

Films have been deposited using pulsed laser ablation. As a source, we have used an excimer laser operating at 248 nm (KrF) firing on sintered Ba2BiO4 pellets. In these pellets, we have only the double perovskite structure. In order to force the growth of the tetragonal structure rather than the cubic one we have used as substrate a MgO single crystal. The unit cell parameter of MgO is smaller than the one observed for the double perovskite and close to the unit cell parameter of the tetragonal phase. Anyway as the laser ablation process supposes that bonds between atoms are broken we can expect that the structure observed in targets will not necessarily be transposed to the films. Thus we shall expect that arriving at the substrate, the atoms will be disposed more favourably in the tetragonal layered phase.

Infrared reflection characterisation using a new Bruker IFS 307 spectrometer composed of two complementary interferometers covering a spectral range from far infrared up to ultraviolet. For our films we have measured room temperature reflection between 40 and 5000 cm -l .

RESULTS AND DISCUSSION

We show in Fig. I typical infrared reflection spectra obtained for such films. Film thickness has been determined from interference fringes in the near infrared region. For thicker films we can see that the global level of reflection is lower than for thinner ones in the region of 400 -800 cm -1 . Within this region lies a very strong reflection band of the MgO substrate. Around 500 cm -1 we find characteristic bands of the ab plane of the tetragonal phase in (Ba.K)2(Pb.Bi)O4 system. Thus the decrease of the reflection level at this region is just an effect of screening of the MgO band by the excitations observed in the film The thicker the film the more masked the MgO band is. The spectra can be quantitatively analysed in terms of a tour parameters oscillator model associated with simple interference calculations. The four-parameter model for the dielectric function (ε), based on transverse (TO) and longitudinal (LO) optical frequencies (Ω)) and their respective dampings (γ ) can be written as [5]

ε ε ∝ Ω ω iγ Ω ω iγ
Using Eq. 1 and the measured optical parameters of the MgO substrate we have calculated the oscillator parameters for the film through interference of normal incident light on the system film-substrate. Such a data analysis yields good fits for thinner films but does not fit well thicker ones. For the films which we have been able to determine the oscillator parameters we have found frequencies compatible with the ones present in the ab plane of tetragonal Ba2PbO. A clue to understand why thicker films cannot be fitted using the above model is given by preliminary measurements of X-ray scattering. It seems that for the first layers of deposited film we do force the tetragonal structure. By the way, increasing too much the number of layers (and thus film thickness) the cubic double perovskite phase starts developing. This means that thick films show a mixed structure: for the first layers in contact with MgO we find the 2D structure and as get away from the substrate the more stable cubic 3D phase is formed. Recent results have shown that this layered structured can also be formed in films with Potassium and Lead doping.

CONCLUSION

Infrared reflection measurements have been performed on laser ablated films of Ba2BiO4. A comparison of infrared data obtained for films and for tetragonal Ba2PbO4 shows that for thin films we have succeeded in forcing the formation of the tetragonal Ba2BiO4 by laser ablation methods. On the other hand, completing refiectivity data with X-ray diffraction measurements have allowed us to conclude that the tetragonal phase is not stable for thicker films.

Acknowledgments

RPSML thanks Brazilian agency CNPq.