BENTIVEGNA G.

GIARDINI GUIDONI A.

MAROTTA V.

MORONE A.

PARISI G. P.

PETTITI, Ida

SNELS M.

English

Archivio della ricerca- Università di Roma La Sapienza

Applied Surface Science 69 (1993) 365-369 North-Holland applied surface science Laser deposition of thin films of high T, superconductors In situ analysis of the transient species formed in the plume and surface diagnostics of the deposited material A. Giardini Guidoni, I. Pettiti Dipartimento di Chimica, Universitri La Sapienza, P.le A. More 5, 00185 Rome, Italy A. Morone, V. Marotta, M. Snels, G.P. Parisi and G. Bentivenga Istituto Materiali Speciali, Area della Ricerca di Potenza, CNR, V?a S. Loja lot. Zona Industriale, 85050 Tito (PZ), Italy Received 2 June 1992; accepted for publication 30 October 1992 The present work reports some results on the laser deposition technique of high T, superconducting films. In situ analysis of transient species formed in the plume and surface diagnostic of the deposited Bi-Sr-Ca-Cu-0 (BSCCO) thin films has been performed. It has been ascertained that mixed oxide cluster ions are produced in the plume together with neutral and ionized atoms of constituent material. The effect of annealing parameters on the structure and quality of deposited thin films of BSCCO is reported and discussed. 1. Introduction In the last few years many research groups applied different techniques (molecular beam epitaxy, sputtering, etc.) for the production of high T, superconducting thin films with the aim to optimize the deposition processes and the thin film quality 111. In our laboratory a systematic study of thin film production by laser ablation is in progress [21. The interaction of intense laser radiation with solid matter causes several pro- cesses to occur: energy transfer, evaporation of solid material, generation of a dense plasma, formation of charged and neutral particles. The study of all these phenomena is rather compli- cated and theoretical models based on local ther- modynamic equilibrium and hydrodynamical equations of motion can give only a qualitative explanation of the processes taking place during laser deposition. Measurements of space and time dependent emission spectra of the species pre- sent in the plume provide a key to understand the dynamics and reactivity of the ablated materials in gas phase. Evidence of chemical reactions in the plume has been previously ascertained. In particular it has been shown that the addition of copper oxide to second group elements, Ca, Ba, Sr, oxides leads to the formation of mixed ions at the expenses of pure oxide clusters [3]. The aim of this work is to throw some light on the role played by the transient species in the aggregation process and by the annealing parameters on the structure and quality of films. 2. Experimental The laser ablation of BSCCO superconducting and nonsuperconducting targets has been per- formed using a frequency doubled (A = 532 nm, pulse duration 10 ns) or quadrupled (A = 266 nm) Nd-YAG laser (Quantel 581). The vacuum chamber for ablation and deposition, already de- scribed [2], is equipped with a quartz laser en- trance window and with rotatable support for target and substrate holder. The pressure in the 0169-4332/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved A. Giardini Guidoni et al. / Laser deposition of thin films of high T, superconductors Fig. 1. Scheme of the experimental apparatus for laser abla- tion and deposition. chamber is kept at 1.5 x 1O-6 mbar during the ablation process. The laser energy is about 2.5-3 J/cm*, the incidence angle with respect to the target surface is 45” (fig. 1). The ablated material originates a luminous plume which extends about 2 cm from the surface. Time-of-flight mass spec- troscopy [3] and luminescence measurements [41 were utilized to obtain information on the inter- mediate species, neutral and ionized, present in the plume. Emission spectra of the species pre- sent have been measured through an optical mul- tichannel analyzer (EG&G OMA 1460) mounted on a small (30 cm) monocromator (Jarrel Ash). The spectral resolution of this detection system is about 0.3 nm, the temporal resolution about 100 ns. An MgO or SrTiO, substrate, glued by silver paint to a mechanical support, was positioned in front of the target at a distance of about 2.5 cm and kept at room temperature during the abla- tion experiment. Thin films of Bi,Sr,CaCu,O, material were deposited on SrTiO, (100) or MgO (200) substrates by laser ablation of supercon- ducting target of BSCCO synthesized by calcining mixtures of Bi,O,, CaCO, and CuO at 840°C. Other deposits were produced by laser ablation of stoichiometric 2 : 2 : 1: 2 nonsuperconducting pellets. A deposition rate of 0.19 rim/s has been measured. The thickness of thin films, measured with an alfa-step profilometer was typically 1 pm after 2 hours of irradiation. The deposited films were annealed in air in the temperature range between 840 and 88o”C, for different duration time. X-ray analysis of the target and deposited films was performed with a Philips diffractometer model PW 1710. Analysis 1 WAVELENGTH WAVELENGTH Fig. 2. Optical spectra of laser produced plume from BSCCO target at pressure of 10m6 Torr. A. Giardini Guidoni et al. / Laser deposition of thin films of high T, superconductors 367 of standard pellets and of the produced thin film either “as deposited” and after annealing was performed by SEM and XPS. XPS spectra were collected by an LH XI Leybold spectrometer using unmonochromatized Mg Ka radiation (1253.6 eV>. A standard four probe technique was used to measure the resistance of the an- nealed samples. 3. Results and discussion Time-of-flight mass analysis of the ionic species produced by laser near the surface (1 mm) in superconducting BSCCO targets have been al- ready reported [4] and the results will be here only summarized. Sr, Bi, Ca, Cu and simple oxide ions predominate in the mass spectrum and masses of simple and mixed positive cluster oxide ions are present. The mass spectra of nonsuper- conducting specimens are similar in type of ions and relative intensity. This feature is characteris- tic also of the other metal oxide mixtures and of oxide mixtures of superconductor precursors [3]. The mass spectrometric plume analysis was per- formed in the region near to the laser spot on the surface but it cannot be excluded that heavier aggregates are formed at a larger distance from the target. They could eventually reach the sub- strate and initiate the film growth. The emission spectrum from the laser produced plume in a superconducting target has been already reported [4] and does not essentially differ from that of a superconducting specimen. The total emission of the plume at the dis- tance of 5 mm from the target is shown in fig. 2. Atom and ion emissions are strongly predomi- nant. Very weak emission features attributable to CuO, CaO and SrO molecules could be present. Time resolved spectra measurements for most of the emission lines show a prompt and delayed emission which have already been attributed to laser and collisional excitation of ablated species, respectively [5]. For example the optical emission as a function of time, of the Sr3+ ion (A = 346.6 nm> at an observation distance of 5 mm from the target surface, is reported in fig. 3. The delayed emission disappears at 1 ps, is peaked at around Fig. 3. Time resolved optical emission of Sr2+ ions (h = 346.4 nm) at 0.5 cm distance from a BSCCO target. 0.2 KS. The low time resolution allowed only an estimation of time-of-flight velocity which is of the order of lo6 cm/s in qualitative agreement with previous measurements [5]. XPS surface analysis of films obtained from a superconducting target confirmed the presence of all the con- stituent elements. XPS analysis of the Cu2p re- gion of the target and deposited film before and after annealing showed that the copper valence is restored after the annealing treatment in the presence of oxygen [61. Analogous results are obtained also in the case of a nonsuperconduct- ing target. Annealing duration and temperature affect the quality of the deposited thin films. In table 1 the results obtained on some samples originated from a superconducting and nonsuper- conducting target in different annealing condi- tions are reported. The resistivity versus tempera- ture characteristics of the samples A to F of the Table 1 Results obtained on some examples originated from a super- conducting and nonsuperconducting target in different an- nealing conditions Sample Target type Dep. Dep. Annealing T, Major time temperature time (K) phase fh) (K) (h) A Superc. 2 855 2 73 2212 B Superc. 1 855 12 45 2212 C Superc. 3 840 1 65 2212 D Superc. 3 855 6 65 2212 E Superc. 3 880 12 65 2201 F Nonsuperc. 2 850 4 - 2212 368 A. Giardini Guidoni et al. / Laser deposition of thin films of high T, superconductors Fig. 4. Thin time R-T curves at different annealing condi- tions: (a) sample A; (b) sample B; (c) sample D; (e) sample E; (f) sample F. table 1 are plotted in fig. 4. It can be seen that a long annealing time (sample B) appears to deteri- orate the quality of the film. A higher annealing temperature produces a similar behaviour (sample E) as can be observed in the resistance measurements and in the X-ray diffraction patterns reported in fig. 5. The diffraction patterns of sample A and D show a series of strong peaks at 28 = 5.47”, 29.3” and 35.3”. These peaks are considered to correspond to (001) series of a unit cell with its long axis perpendicular to the substrate. The observed peaks are related to the 2 : 2: 1: 2 phase (T, 85 K). However, the spectra show that the film is polycrystalline with only a slight preferential c-axis orientation of the grains. When the BSCCO thin film was annealed near the melting point at 880°C the presence of the 20 = 7.2” and 21.9” peaks indicates a phase change from 2: 2 : 1 : 2 to 2 : 2 : 0 : 1. A similar behaviour was previously dis- cussed [7]. The attempt to obtain high T, super- conducting film from a nonsuperconducting tar- get of 2 : 2 : 1: 2 phase gave poor results probably due to the not-optimized heating conditions. Fig. 5. X-ray diffraction patterns for BSCCO thin film and target at different annealing conditions: (A) sample A, (D) sample D; (E) sample E; (G) nonsuperconducting target; (F) sample F. A P 8 8-28 A. Giardini Guidoni et al. / Laser deposition of thin films of high T, superconductors 369 In conclusion the present investigation clearly demonstrates that the plume of evaporated mate- rial is mainly formed by atoms, ions and few oxide clusters. Its composition appears to be not always representative of the composition of the target, however, the net result of deposition is stoichiometric. To obtain high quality thin films not only a suitable target but also a good choice of post annealing treatment is needed. References [l] D. Bauerle, Laser processing and diagnostic (Springer, Berlin, 1984) p. 166. [2] L. Correra, Ed., High temperature superconductor thin film (Elsevier, Amsterdam, 1991) p. 451. [3] A. Mele, A. Giardini Guidoni, D. Consalvo, D. Stranges and R. Teghil, Int. J. Mass Spectrom. Ion Processes 91 (1989) 319. [4] A. Giardini Guidoni, E. Desimoni, A.M. Salvi, R. Teghil, M. Ambrico, S. Piccirillo and M. Snels, Proc. Int. Conf. Laser 90, San Diego (1990) p. 331. [S] K.L. Saenger, J. Appl. Phys. 66 (1989) 4435; C. Girault, D. Damiani, J. Aubreton and A. Catherinot, Appl. Phys. Lett. 55 (1989) 2; H. Fucumera, H. Nakaminami, S. Eura and H. Masuhara, Appl. Phys. Lett. 58 (1991) 22; J.P. Zheng, Z.Q. Huang, D.T. Shaw and H.S. Kwak, Appl. Phys. Lett. 54 (1989) 3. [6] A. Giardini Guidoni, A. Morone, M. Snels, E. Desimoni, A.M. Salvi, R. Fantoni, W.C.M. Berden and M. Giorgi, Appl. Surf. Sci. 46 (1990) 321. [7] M. Knai, T. Kawai, M. Kawai and C. Kawai, Jpn. Appl. Phys. 27 (1988) L1293. 

LASER DEPOSITION OF THIN-FILMS OF HIGH T(C) SUPERCONDUCTORS - INSITU ANALYSIS OF THE TRANSIENT SPECIES FORMED IN THE PLUME AND SURFACE DIAGNOSTICS OF THE DEPOSITED MATERIAL

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LASER DEPOSITION OF THIN-FILMS OF HIGH T(C) SUPERCONDUCTORS - INSITU ANALYSIS OF THE TRANSIENT SPECIES FORMED IN THE PLUME AND SURFACE DIAGNOSTICS OF THE DEPOSITED MATERIAL

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