Conditions for in-situ growth of BiSrCaCuO thin films by laser deposition

PhysicaC 214 (1993) 195-203 North-Holland

Conditions for in-situ growth of BiSrCaCuO thin films by laser deposition G. Poullain, R. Desfeux a n d H. M u r r a y Laboratoire de cristallographie et sciences des mat~riaux (URA CNRS 1318) ISMRA, boulevard Mar~chal duin, F14050 Caen, France

Received 19 May 1993 Revised manuscript received21 June 1993

Superconductingthin films of Bi 2212 have been depositedby laser ablation. The influenceof the densityof the target upon the 'compositionof the film is described. The substrate temperature is shown to be verycritical: the in-situ depositionof well crystallized films requires an accurate heating system control. The influence of the oxygenpressure during deposition is especiallydiscussed. Our work indicates that the end-of-depositionpart of the processingcannot sufficientlyimprove the oxidation of the films. Therefore we present preliminary results on a new way to increase the in-situ oxidation of the films: the plasma assisted laser deposition method. The best in-situ film showsa critical temperature (R = 0) of 83 K.

1. Introduction The achievement of superconducting devices such as SQUIDS requires much work on the preparation and characterization of high-To cuprate thin films. Excellent results have now been obtained for YBCO using either pulsed laser ablation or sputtering on various substrates. This is not the case for bismuth cuprates for which the state of the art is less advanced. The optimisation of the critical temperature of 2201 and 2212 in-situ thin films up to 83 K has been demonstrated only recently, using the sputtering technique [ 1, 2 ]. The authors have indeed shown that 2212 films deposited by this method on SrTiO3 or MgO substrates require a very rich oxygen atmosphere (more than 90% oxygen in the discharge) in order to reach critical temperatures routinely above 80 K. On the contrary, six years after the discovery of superconductivity in this material, many attempts to prepare in-situ laser ablated thin films of bismuth did not allow a Tc of 80 K to be exceeded [ 3-12 ]. Furthermore, two recently published works have deafly shown that the preparation of in-situ 2212 bismuth films by laser ablation is extremely difficult, mainly because of the poor oxidation of the film during the process [ 11, 12 ]. This situation is

particularly unfortunate if one takes into account the great potentiality of the bismuth cuprates for the elaboration of epitaxial heterostructures like 2212/ 2201/2212. We report in this paper our results on the elaboration ofin-situ 2212 thin films by laser ablation, with particular attention paid to the oxygen pressure during the process. We also present a way to enhance the oxidation of the film using a plasma assisted laser deposition method.

2. E x p e r i m e n t a l

A KrF pulsed excimer laser (wavelength 2 = 2 4 8 nm) was used for film deposition. The beam was focused on the rotating target to get an energy density close to 2.5 J / c m 2. The repetition rate was 1 Hz to 3 Hz, corresponding to a deposition rate ranging from 0.3/~/s to 1/~/s. The target-to-substrate distance was 5 cm. The target preparation will be described in the following section. The base pressure inside the chamber was 10 -6 mbar. The substrates, MgO single crystals, were cleaned in acetone and ethanol for l0 min and were fixed with Ag paste in the vacuum chamber. Before starting the process, the substrates

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G. Poullain et al. / Conditions for in-situ growth

were heated for 10 min under deposition conditions: typically an oxygen pressure of 0.2 mbar and a temperature around 800°C. A thermocouple mounted near the substrates indicated the heating system temperature, but the real surface temperature of the substrates remained unknown. The films were cooled in 30 min in an oxygen pressure of 800 mbar. Typical film thicknesses were 2000 A. Plasma assisted laser deposition (PLD) was first used by Witanachchi et al. for the deposition of YBCO thin films [ 13 ]. Their purpose was to find a low temperature way for the preparation of YBCO thin films, but the PLD technique has never been used in the case of bismuth thin films in order to enhance oxidation of the film. In our PLD system, a biased copper ring was set between the target and the substrate holder. Just at the beginning of the deposition, a DC voltage was applied to get a permanent oxygen plasma (the power could be varied between 1 to 10 W). Note that the laser plume is shortened and narrowed by the plasma presence. Four gold dots were evaporated on the film for electrical measurements (see ref. [ 8 ] for further experimental conditions). The composition of the film was determined by energy dispersive X-ray analysis ( E D X ) and the film crystalline structure by X-ray diffractometry using Cu K e radiation.

first step to achieve a stoichiometric target-to-substrate transfer using a high density target. The following procedure was employed: appropriate amounts of Bi203, SrCO3, CaCO3 and CuO were mixed, ground and pressed. The target was then heated l h at 1000°C with an increasing ramp of 100 ° C/h. After the complete melting of the powder, the temperature was decreased at 20°C/h to room temperature. The target was then annealed again at 840°C for 150 h leading to the almost pure 2212 cuprate (fig. l ). With this procedure, the target density was better than 95%. Unfortunately, this kind of target sintering did not allow us to get a stoichiometric target-to-substrate transfer: the average composition, analysed by EDX, was Bi2.3Srl.sCal.2CU2Ox. Moreover, a variation of the oxygen pressure during the deposition induced a slight change in the film composition, as pointed out by Lee [ 11 ]. In the same way, the film composition was also slightly altered when the substrate temperature was modified [ 8 ]. Thus, even with a high density target, the composition of the films appears to be sensitive to the deposition parameters, contrary to YBCO. It should be noted that the homogeneity range of the 2212 phase varies with the method of preparation and especially with the oxygen pressure, ranging, for instance, from Bi2Sr2CaCu2Os to Bi2.~Srl.9CaCu208 [ 14,15 ]. Consequently, a compositional fluctuation can lead to some variation of

3. Results and discussion ........

3.1. Film composition

Many papers have shown that the deposition of YBCO thin films is favoured by the use of a high density target: in particular, high densification allowed for a stoichiometric target-to-substrate transfer and led to smooth surfaces with a reduced amount of droplets. However, in the case of bismuth cuprates, most of the time a compensated target is used, i.e., the authors adjust the target composition to get stoichiometric films [ 3-6, 8, 12 ]. Furthermore, deposition performed with stoichiometric targets leads to a variation of the film composition [ 11 ] or to additional peaks in the X-ray diffraction patterns indicating undesirable phases [7, 9, 10]. For industrial purposes, attempts were made in a

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G. Poullain et al. I Conditions for in-situ growth

the physical properties of 2212 single phase films. In this work, two additional difficulties were encountered in the determination of the film compositions. First, the EDX analysis accuracy could be estimated to be around 5% and secondly, all parameters being the same, the film composition reproducibility was found also to be around 5%. Consequently, a 5% to 10% fluctuation of the composition might significantly change the physical properties of the 2212 films. For these reasons, we gave up the stoichiometric high density target, and we used a simplified sintering procedure: appropriate amounts of the powders were mixed, pressed and annealed at 800°C during 16 h. By trial and error methods, the target composition was adjusted to BiLgSr3.2Ca~.tCu2Ox to get 2212 films on the substrate. Target densities were routinely around 50%. It is worth to note the large correction on the strontium which is in accordance with the results reported in ref. [ 11 ]. Using compensated targets we were able to adjust the film composition to 2212 even when the oxygen pressure or substrate temperature were significantly changed.

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3.2. Substrate temperature As in many works, the exact substrate temperature is not known since it is nearly impossible to measure the real surface temperature during the deposition. Consequently, our reference was the surface temperature of the substrate holder, close to the substrate position. Though some crystallization could be obtained at temperatures as low as 550°C, especially when lithium was added to the target as previously reported [ 16 ], the quality of the films was progressively increased as the temperature was brought near the film melting (around 800°C). Xray diffraction patterns for different substrate temperatures and an oxygen pressure of 0.2 mbar are shown on fig. 2. One can notice that the or 0 0 12 peak becomes narrow only as the substrate temperature reaches the melting temperature of the film. In order to understand the origin of the peak broadening, the films were studied by electron microscopy (fig. 3 ). As long as the 0 0 12 peak is broad (fig. 4 ( b ) ) one observes 2201/22i2 intergrowths a n d / o r highly disordered structures. One example is shown on fig. 3(b), where the variation of the fringes

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2-THETA Fig. 2. X-ray diffraction patterns ofin-situ 2212 bismuth films: (a) Tsur~tmte= Tn~at- 40 °C, (b) Ts= Tin- 20"C, (c) T , = T m - 10°C, (d) T,=Tm-5*C. Note that the width of the peaks are continuously narrowed as the substrate temperature is brought near the melting of the film.

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Fig. 3. (a) The electron microscopy image gives evidence of a regular stacking of the layers along the c-axis (2212) and the ED patterns exhibit sharp reflections. (b) A disordered grain of a 2212 film characterized by a broad 0 0 12 peak in the X-ray patterns. 2212 and 2201 members are aleatory stacked along the c-axis and highly disturbed areas (curved arrow) are observed; the corresponding ED patterns exhibit streaks along c*.

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latter under an oxygen pressure of 0.2 mbar, while a 0.7 mbar oxygen pressure was set for the former. Nevertheless, the film grown in a higher pressure has a critical temperature 10 K higher than the other one. This suggests that one has to optimise the film through the critical temperature but also through the film crystallization. These results are rather different from those obtained for YBCO thin f'rims; as a matter of fact the substrate temperature required for an excellent crystallization extended at least 10°C around 750°C. On the contrary, for bismuth films, it is now well established that the optimum temperature is very close below the melting of the film. As the substrate temperature was increased above the melting temperature, thin films were decomposed so that parts of the substrate remained uncovered, other parts exhibiting well crystallized 2212 and 2201 phases as shown in the X-ray diffraction patterns (fig. 5). However, the critical temperatures ( R = O ) for these films were often in the range 75 K to 80 K, but their superconducting volume was in fact very small, due to the large areas of uncovered substrate. Hence, well crystallized bismuth f'rims were only obtained just below the decomposition temperature, in a very narrow range. The regulation of the heating system should therefore be accurate, i.e. with a fluctuation of less than 1 ° C, more paniculady at the end of the deposition when oxygen pressure is quickly increased. One can indeed observe, in the few earlier seconds of the filling, a rapid overheating (of

Fig. 4. X-ray diffraction patterns of the films observed in TEM. Note the differentwidth of the 0 0 12 peak: (a) narrow, (b) broad correspondingto the disordered structure presented on fie, 3 (b).

periodicity along c is clearly observed, with mainly 2201 and 2212 members; in the same way, highly disturbed zones are common features (curved arrow). The corresponding electron diffraction (ED) patterns exhibit diffuse streaks along the c*-axis. On the contrary, when the 0 0 12 peak is narrow (fig. 4 ( a ) ) , the electron microscopy image shows very regular fringes which attest to the regular stacking of the layers along the c-axis according to a 2212 sequence (fig. 3 ( a ) ) ; the corresponding ED patterns exhibit sharp reflections without any streaks. It should be mentioned that both films were deposited at the same temperature ( T, = Tmelt - 4 0 o C ) , but the

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G. Poullain et al. / Conditions for in-situ growth

about 10°C) which has to be compensated by the heating regulation to prevent fdm melting.

3.3. Oxygen pressure The oxygen pressure is, together with t h e substrate temperature, the second key parameter for Bi-2212 film crystallization. As reported in the previous section, the study of this parameter is complicated by the simultaneously induced compositional change. Typically, as the oxygen pressure was increased, the bismuth content in the film was reduced, while the calcium amount was enhanced, in accordance with their respective masses. For a given substrate temperature, the critical temperature (resistively measured) can be related to the oxygen pressure as shown on fig. 6. Here the highest Tc is obtained for an oxygen pressure of 0.2 mbar. Unfortunately, one can conclude from table 1 that films were better crystallized for an oxygen pressure of only 0.1 mbar. This discrepancy between the two parameters, namely critical temperature and crystallization, increased as the substrate temperature was brought near film melting. This is illustrated on figs, 7 and 8 that reproduce the critical temperature versus oxygen pressure as the substrate temperature approached the melting point. On one hand, the oxygen pressure has to be brought to 0.7 mbar for the critical temperature criterion, but on the other hand 2201-2212 intergrowths and a disordered structure is obtained and 7O

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Fig. 6. Critical temperature (resistively measured) vs. oxygen pressure during the deposition for a 2212 film grown 40 ° C below the temperature o f the melting. The best critical temperature (around 67 K ) was obtained for an oxygen pressure o f 0.2 mbar.

Table I Full width half m a x i m u m ( F W H M ) of the main (0 0 I) peaks of c-axis oriented in-situ 2212 films grown at a substrate temperature of Tme~t-40°C. Well crystallizedfilms were obtained with an oxygen pressure of 0.I mbar during the deposition Oxygen

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a 0.25 mbar oxygen pressure should be used for optimum crystallization. One can also notice, as has been pointed out by Perriere for instance [ 17 ], that the surface of the film becomes always rougher as the oxygen pressure is increased above 0.2-0.3 mbar. This suggests that the unsuccessful results previously reported on the deposition of laser ablated 2212 thin fdms come from this particular difficulty, that either films are well crystallized, but critical temperatures are low because of oxygen deficiency, or the oxygen content is sufficient to get a high critical temperature but in fact films are poorly qrystallized, major parts consisting of disordered structures. For this reason, we have finally oriented our work toward the deposition of well crystallized films and tried by different ways to enhance the film oxidation, but keeping the oxygen pressure to 0.2 mbar during deposition.

G. Poullain et aL / Conditions for in-situ growth 0.9 77K

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Fig. 8. O p t i m u m range o f the oxygen pressure during the deposition vs. substrate temperature. Critical temperatures are indicated on the figure. Note that as soon as the substrate temperature reaches the melting point, the oxygen pressure range becomes larger.

film after annealing at 350°C in an atmospheric pressure of oxygenduring 30 min, (c) the same film after annealing at 350°C in 100 bar of oxygenduring 100 h. (X-raydiffractionpatterns of this film are given in the inset. )

In a first step, we have focused our study on the oxygen introduction at the end of the deposition. The best results were systematically obtained when the oxygen pressure was increased up to 800 mbar in the five minutes following the end of the laser shots. We have also established that a too slow temperature decrease (below 300 ° C / h ) was detrimental to the film oxidation. As a matter of fact, the films systematically lost oxygen under a 800 mbar oxygen pressure when the temperature exceeded 250°C as we reported earlier [ 18 ]. We have indeed shown that low annealing temperatures can quickly (in a few minutes) modify the oxygen content of the film. In this study, we have tried a post-annealing treatment at 350°C under increasing oxygen pressures. Damages were observed when an oxygen pressure below 20 bar was applied: the critical temperature were rapidly reduced, but the X-ray diffraction patterns remained unchanged (fig. 9). On the other hand, higher oxygen pressures (up to 100 bar) allowed one to enhance the critical temperatures. But very long annealing times (100 h) were required and only increases of a few K were obtained. Moreover, after such long annealing times, film surfaces became rougher and the benefit of in-situ processing was partially lost. We concluded that the end-of-deposition part of processing could not improve sufficiently the oxidation of the film. Thus the only remaining way to increase film ox-

idation was to use a more efficient oxidizing gas, like N20 or 03 instead of oxygen. Some attempts were made using N20. Well crystallized films were obtained, but the critical temperature was not increased. This feature can probably be explained by the lower oxygen pressure that was obtained in spite of the more oxidizing power of N20. Therefore, considering the results recently obtained by two groups with sputtering under high oxygen pressure [ 1,2 ], we have tried to use plasma assisted laser deposition to enhance oxidation of our 2212 films. The idea is that the oxidation state of oxygen in the plasma (O + or O*, i.e. excited neutral oxygen, in most cases) can favour the film oxidation while keeping a low pressure required for a good crystallization. A copper electrode was employed to prevent film contamination by sputtered species, and a DC power of 1 to 10 W was applied. All the parameters being the same, we clearly obtained a significant increase of To, around 5 K, when plasma was added during deposition. Figure 10 shows as an example the resistive versus temperature measurements of two films deposited with the same procedure except the plasma presence for film B. Nevertheless, except for one sample with a critical temperature of 83 K (fig. 11 ), Tc remained generally in the range 75 K to 80 K. It should be emphasized that some difficulties connected with the PLD technique are to be over-

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come. First, our experiments showed clearly that a stronger power should be employed to still improve oxidation. But in this case, sputtered copper would be additionaly deposited on the substrate and we would be obliged to compensate the target compositio n . Secondly, the laser plume is shortened by the plasma presence and the deposition rate is correspondingly reduced. However, the PLD technique appears as a potenti~ tool to overcome difficulties encountered in laser deposition of bismuth cuprates, owing to the stronger oxidation possibility in spite of a lower oxygen pressure during the deposition.

4. Conclusion Several years after the discovery of superconductivity in the bismuth cuprates; thin films deposition by laser ablation is yet not optimised. Among the dif-

ficulties encountered, our work was particularly focused on film composition, substrate temperature and oxygen pressure. First, the film composition was found to slightly depend on the parameters of the deposition, whatever the density of the target. For YBaCuO, a slight change of the average film composition can be masked in the physical characterizations, because quite a large part of the whole film remains composed of the Y1Ba2Cu307 well crystallized phase in addition to a small amount of undesirable phases. On the contrary, for bismuth compounds, a slight variation of the composition implies a cationic evolution of the main phase, that is not necessarily observable in the X-ray diffraction patterns, but that could rapidly modify the physical properties. Secondly, the substrate temperature has to he carefully controlled, particularly at the end of the deposition during the oxygen filling of the chamber. We estimate that the regulation of the heating system should have an accuracy of better than 1 °C, and that the optimum substrate temperature is just below the melting of the film. To conclude, and this is certainly the most difficult point, the oxygen pressure during the deposition is the key parameter for crystallization and also for the electrical properties, and we found a dramatic discrepancy between the optimised values of the oxygen pressure for these two parameters. Our conclusion is that one should keep a relatively low oxygen pressure during the deposition to get a well crystallized film, hut in that case a way to increase the film oxidation should be found. We proposed the use of a plasma assisted laser deposition method. This technique enabled us to get an improvement of the critical temperatures of well crystallized films, though the critical temperature were not routinely above 80 K because of the insufficient optimisation of the oxygen content in the films.

Acknowledgements The authors would like to thank M. Hervieu and B. Mercey for the TEM observations, A. Maignan for the magnetical characterization and B. Raveau

G. Poullain et al. I Conditions for in-situ growth

and J.F. Hamet for helpful discussions.

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[9] J.S. Moodera, A.M. Rao, A. Kussmaul and P.M. Tedmw, Appl. Phys. Lett. 57 (1990) 2498. [10] H.U. Krebs, M. Kehlenbeck, M. Steins and V. Kupcik, J. Appl. Phys. 69 ( 1991 ) 2405. [ 11 ] J. Lee, E. Narumi, C. Li, S, Patel and D.T. Shaw, Physica C 200 (1992) 235~ [12] M.M. Bedekar, A. Safari and W. Wilber, Physica C 202 (1992) 42. [ 13] S. Witanachchi, H.S, Kwok, X.W. Wang and D.T. Shaw, Appl. Phys. Len. 53 (1988) 234. [ 14] A.Q. Pham, M. Hervieu, A. Mailnan, C. Michel, J. Provost and B. Raveau, Physica C 194 (1992) 243. [ 15 ] R. Mfiller, T. Sehweitzer, P. Bohac, R.O. Suzl~ki and L.J. Gancher, Physica C 203 (1992) 299. [ 16 ] G. Poullain, T. Brousse, V. Chauvel-Kokabi, J.F. Hamet and H. Murray, Physica C 182 ( 1991 ) 143. [ 17 ] J. Perriere, in Laser ablation of electronic materials, eds. E. Fogarassy and S. Lacaze (North Holland, Amsterdam, 1992) p. 293. [ 18 ] T. Brousse, G. Poullain, J.F. Hamet and B. Blanc-Guilhon, Mater. Sci. Eng. B 13 (19923 35.