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An experimental study of the main involved parameters in the epitaxial growth of CeO2 buffer layers on nickel tapes
Physica C 366 (2002) 109±116
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An experimental study of the main involved parameters in the epitaxial growth of CeO2 buer layers on nickel tapes M.A. Arranz a,*, B. Holzapfel b, N. Reger b, J. Eickemeyer b, L. Schultz b a
Departamento de Fõsica Aplicada, Facultad de Ciencias Quõmicas, UCLM, 13071 Ciudad Real, Spain b Institute of Solid State and Materials Research, IFW 01069 Dresden, Germany Received 19 February 2001; received in revised form 17 April 2001; accepted 26 April 2001
Abstract In order to investigate the optimal conditions for highly oriented and epitaxial buer layers on nickel based tapes, we have grown several sequences of cerium oxide ®lms by pulsed laser deposition technique. Amongst the usual parameters governing this procedure and connected to obtain high quality buer layers, three of them have requested a special attention through this work: the target±substrate distance, the buer layer deposition temperature and the gas atmosphere inside the vacuum chamber during the growing process. Based on the following results, we have found a full crossed relation amongst them, which develops in obtaining textured layers of highly oriented CeO2 along the (0 0 1) ordered nickel substrate. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Cd; 85.25.Kx; 68.55.Jk Keywords: Buer layers; Biaxial texture; Rolled-Ni substrates; Application of high TC superconductors.
1. Introduction The employment of Ni-based tapes as metallic substrates for superconducting ®lm deposition is receiving a great deal of interest through the last years [1,2]. Such metallic tapes can easily develop a cubic textured structure by means of uniaxial rolling and ulterior annealing (rolling-assistedbiaxially-textured tapes, (RABIT) technique) [3± 5]. This feature provides them to be a cheap and ductile quasi-single crystal substrate, into which high quality crystalline ®lms can be deposited
* Corresponding author. Tel.: +34-926-295-300; fax: +34926-295-318. E-mail address: maarranz@®ap-cr.uclm.es (M.A. Arranz).
after. For instance, the growing of high TC superconducting ®lms on them is a point of great interest for industrial purposes, as these coated tapes are expected to be used in high power conduction lines or high ®eld magnets working eciently at liquid nitrogen temperature. Previous works concerning the growth of YBCO ®lms on buered nickel tapes have reported to support high critical current densities even comparable to the ones obtained on ceramic single crystal like SrTiO3 [6±9]. As it can be found in those references, in order to obtain high textured and oriented superconducting ®lms on metallic tapes, it is unavoidable to intercalate some kind of named buer layer, commonly ceramic ®lms, e.g. CeO2 or yttria stabilized zirconia. The role of this intermediate deposit is mainly to extend the textured and
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oriented structure of the metallic substrate onto the superconducting ®lm, gradually adjusting the initial lattice mismatch between Ni and YBCO. Also concerning that purpose, the buer layer provides a barrier to prevent either the metal diffusion into the copper oxide compound [9,10] or the metallic surface oxidation during the superconducting ®lm deposition. According to its mentioned task as a biaxial texture precursor, the buer layer must be grown on Ni tape showing the least number of misorientations either relative to the substrate normal axis or referred to the inplane alignment of the nickel surface (cube on cube architecture). Slight deviations in the crystallinity of the buer layer during its growth are straight transduced in a diminishment of the capability of the upper deposited superconducting ®lm to stand high values of the critical current density, due to the appeareance of additional large-angle grain boundaries or structural defects promoted by the buer layer. Mainly, the aim of this work has been to identify and correlate the eects on the buer layer deposition produced directly by the mentioned growing parameters, taking carefully into account of the high biaxially textured structure requirement.
electron diraction (RHEED) ensemble was also set in to test in situ the biaxial alignment either the nickel tape or the oxide buer layers grown on. Further details of the experimental setup have been previously reported [11]. Prior to the coating processes, a background pressure of 10 6 mbar was achieved in the deposition chamber with a turbomolecular vacuum system. After etching the substrates at room temperature by means of the Ar-ion gun, we annealed them at 500°C in forming gas (Ar H2 mixture; partial pressure of 1:3 10 3 mbar) during 30 min. The grade of quality in the (0 0 1) Ni cubic textured structure was examined through the diraction patterns obtained from the (1 1 0) and (1 0 0) directions of the incident electron beam on the tape surface (see Fig. 1). Once the cube textured tape orientation was clearly obtained, dierent types of buer layers were deposited after. To characterize all the grown samples, we have measured their out-of-plane orientation with an X-ray diractometer (PW3050, CoKa radiation) and the in-plane texture was quantitatively evaluated with a Philips four circle equipment (CuKa radiation).
2. Experimental
Fig. 2 shows the 2h scans for a set of CeO2 samples grown in dierent deposition atmospheres with a repetition rate of 3 Hz during 20 min, in all cases. The substrate temperature was 700°C and the target distance, d, was ®xed to 6 cm. The partial pressure was 2 10 3 mbar for both oxygen or argon environments. As it can be seen, the (1 1 1) CeO2 growing direction predominates when an oxidating atmosphere is present since the ®rst stages of deposition (Fig. 2a). This eect is due to the appearance of a rapid enhancing NiO layer over the substrate surface. It is already reported [12±14] that (1 1 1) orientation relative to the metallic tape normal axis is the preferred growing mode of nickel oxide at ambient temperature. In our samples, this fact allows immediately the (1 1 1) CeO2 direction to appear after. As the deposition begins in vacuum environment, both (1 1 1) and (0 0 1) CeO2 re¯ections emerge simultaneously, depending on the intensity of the last
A pulsed excimer laser (KrF, 248 nm wavelength) was used to deposit CeO2 oxide buer layers on Ni based tapes (10 10 0:1 mm3 , dimensions). The laser was focused into the vacuum chamber to reach an energy density around 3 J/ cm2 at the target. The metallic substrates were pasted with Ag epoxy onto the heater holder and their position could be varied with an XYZmanipulator. Before deposition, the as cold-rolled tapes were placed just in front of the polycrystalline targets, having therefore their normal axis parallel to the growing direction of the ablated particles. The vacuum chamber was equipped with an Ar-ion gun, which focused the beam at a 55° angle relative to the substrate normal axis. This experimental tool enabled us to clean the substrate surface from contaminants and some oxide layers in a preliminary stage. A re¯ection high energy
3. Results and discussion
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Fig. 1. Ni substrate RHEED patterns for a (0 0 1) cubic textured structure: (a) initial pattern at room temperature, (b) and (c) after the annealing at 500°C for the (1 1 0) and (1 0 0) directions of incidence, respectively.
one on the NiO ®lm formed during the oxygenated deposition period (Fig. 2b). The third diraction plot can enforce this previous assertion: the substitution of an inert gas (here argon) for oxygen during the complete deposition, inhibits strongly the Ni oxidation providing a much stronger (0 0 1) CeO2 re¯ection (Fig. 2c). Also the target substrate distance has a strong eect on the appearance of the (1 1 1) NiO ®lm orientation. Fig. 3 shows H±2H diraction plots obtained for CeO2 deposited on nickel tapes in argon or vacuum environment, when varying the substrate±target distance, d. The deposition tem-
perature was raised up to 800°C and the other growth parameters were maintained the same as in Fig. 2. As it can be seen, the shorter the distance is (d 4 cm in Fig. 3a), the stronger Ni oxidation occurs, translating the unlike (1 1 1) orientation into the CeO2 ®lm. On the other hand, slowing the speed of ablated particles through a longer trip in some inert gas (d 8 cm in Fig. 3b) eliminates such orientation while only the (0 0 1) CeO2 growing mode, parallel to the substrate normal axis, remains without any traces of NiO diraction peaks. Although the CeO2 (1 1 1) orientation appears in both cases, for longer substrate±target
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Fig. 2. 2h scans for CeO2 layers grown on Ni textured tape in dierent atmospheres: (a) full deposition in oxygen, (b) ®rst 5 min in vacuum and the rest of deposition in oxygen, (c) same procedure as (b) but with argon.
Fig. 3. 2h scans for CeO2 layers grown on Ni textured tape with dierent target±substrate distances, d. (a) Sample deposited in Ar atmosphere at 800°C with d 4 cm, (b) same as (a) but with d 8 cm, (c) same as (b) but in vacuum. Scales represented in the lower plot are the same to all of them. The indexed peaks correspond to CeO2 buer (vertical legends).
distances its absolute magnitude is almost vanishing. Finally, the results presented in Fig. 3c (the same target±substrate spacing of 8 cm but in turbomolecular vacuum) show again the very important role of the inert gas to slow and reduce the reactive and ionized particles arising from the target. The longer d spacing avoids the appearance of the (1 1 1) CeO2 re¯ection but, in absence of any inert atmosphere, is not eective enough to detain the arrival of free oxygen species at the nickel tape (see the (1 1 1) NiO peak). These previous experimental results can be explained in the framework of the following qualitative model. The laser plasma plume, which is formed after the ablation of target material and the interaction with the excimer laser pulse, consists of highly excited species (mainly atoms and ions) that move towards
the substrate. Their energy distribution inside the plasma plume at the substrate position depends strongly on deposition parameters like the deposition pressure. For laser ablation in vacuum the ablated particles have a high and constant kinetic energy on their way to the substrate. However, during laser ablation in background gas atmosphere, there is a strong interaction of the ablated species with the background gas by atomic collisions, resulting in a complex hydrodynamic expansion of the plasma plume. During this expansion shockwaves can be formed and inside the high density plasma cloud the energy of the species is thermalized and recombination processes appear because of the high collision rate with cool background gas. This increases the amount of mole cular species, e.g. Ce 4 , O2 , CeO2 , and reduces
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the amount of ablated free oxygen when the ablation is performed in an inert gas atmosphere like Ar. In our experimental situation we believe, that this unbound oxygen can react on the Ni-substrate surface to NiO, resulting in the unwanted polycrystalline (1 1 1) NiO orientation. Laser ablation in an Ar background atmosphere and using larger target±substrate distances reduces in our model the amount of unbound oxygen at the substrate position resulting in a reduced polycrystalline NiO formation as it is observed in our experiments. Another parameter involved in the buer layer growing process, which has also focused our interest in this work, has been the substrate temperature during the deposition. It is largely shown in Refs. [4,15] that special care has to be taken choosing the deposition temperature due to its strong eect not only on the growing direction of the buer layers [15] but also on the degree of texture and crystallinity of them [4], straight affected by the recrystallization processes of the nickel tape [16] and the kinetics of the epitaxial growth. In order to investigate such in¯uence, two series of CeO2 samples were grown on textured Ni and NiMo metallic tapes, respectively, at dierent substrate temperatures ranging from 400°C to 800°C. Depositions were made in Ar atmosphere (2 10 2 mbar) with a repetition rate of 10 Hz during 20 min for each sample. The substrate were placed at d 8 cm from the target in all cases. It has been reported that small doping of Ni matrix with Mo (i.e. 0.1 at.% Mo, as employed along this work) [16] can shift the bare nickel secondary recrystallization to higher temperatures, therefore translating the appearance of misorientations in the buer layers up to thresholds well above their usual deposition temperature interval. The H±2H diraction plots for both types of substrates are displayed in Figs. 4 and 5 for Ni and NiMo substrates, respectively. At low deposition temperatures it can be seen that CeO2 buer layers grow in a mixture of both (1 1 1) and (0 0 1) directions (see Figs. 4 or 5). This common result on both kind of metallic substrates, could be originated in the low diusion energy available for the deposited atoms on the textured metallic surface, resulting in poor epitaxial growth. When the CeO2 deposition temperature increases, the (1 1 1) re¯ection diminishes
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Fig. 4. Behavior of the 2h diraction plots with deposition temperature for CeO2 buered Ni tapes. The indexed peaks correspond to CeO2 re¯ections. Note that the (3 1 1), (2 2 2) and (2 0 0) diraction intensities have been magni®ed in a 10 factor for comparison.
gradually and almost vanishes over 700°C, where the (0 0 1) direction is clearly predominant (specially in the case of buered NiMo tape). From our results, this temperature interval (600±700°C) seems to be the best one for growing the ceramic buer layer, while a good degree of alignment to the textured tape has been achieved. That convenient temperature range, and specially, the upper value of 700°C for CeO2 deposition agree with reported results [9]. The temperature dependent eect of the epitaxial growth kinetics can be clearly seen in the corresponding CeO2 (1 1 1) pole-®gures for the buered NiMo tapes (Fig. 6). At 300°C only the polycrystalline (1 1 1) growth component is present (Fig. 6a). Increasing the deposition temperature to 400°C (Fig. 6b) results in the occurrence of the biaxially oriented (1 0 0) orientation,
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Fig. 6. Dependence of the (1 1 1) CeO2 pole ®gure on the deposition temperature: (a) 300°C, (b) 400°C, (c) 500°C and (d) 600°C. Fig. 5. Behavior of the 2h diraction plots with deposition temperature for CeO2 buered NiMo tapes. The indexed peaks correspond to CeO2 re¯ections. Note that the (3 1 1), (2 2 2) and (2 0 0) diraction intensities have been magni®ed in a 10 factor for comparison. The smaller (3 1 1) CeO2 re¯ection peak for NiMo tape suggests a weaker distortion of its textured structure than in the Ni case.
but due to the low substrate temperature the epitaxial growth component shows is quite broad in-plane orientation. A further substrate temperature increase (500°C, Fig. 6c) both sharpens the epitaxial growth and reduces the (1 1 1) orientation. At 600°C ®nally the (1 1 1) orientation disappears and the in-plane orientation of the epitaxial (1 0 0) orientation reaches a value comparable to the cube-textured substrate (Fig. 6d). For higher temperatures, beside the (0 0 1) orientation other dierent CeO2 growth modes appear again: the (1 1 1) diraction peak in the case of NiMo substrates or the (1 1 1) and (3 1 1) ones for Ni substrates, respectively. This is a straight consequence of the Ni secondary recrystallization
process, which gradually results in the formation of great crystallites randomly oriented, and consequently disorientates the ordered sequence of deposited or growing buer layers. The ®nal result is a CeO2 thin ®lm with a great number of misorientations relative to the epitaxial alignment, which is traduced in the experimentally observed additional growing directions. As shown in the
Fig. 7. Two dimensional plot of the U-scans corresponding to the substrate (1 1 1) re¯ection after deposition of CeO2 at 800°C in Ar atmosphere. (a) Ni tape and (b) NiMo tape.
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experimental results, the eect is much stronger in the bare Ni buered tape (Fig. 4) than the NiMo case (Fig. 5), due to the mentioned shifting of the secondary recrystallization temperature to higher values. An additional experimental evidence of this eect can be found on correlating the texture Uscans of the substrate (1 1 1) re¯ection and the corresponding CeO2 (1 1 1) re¯ection of the oxide
115
buer grown at 800°C. As it is followed from the Fig. 7, some intense deviations from the cube±cube architecture turn up in the case of Ni substrate, on the contrary to NiMo tape. Such distorsions in the textured structure are due to the proximity of the secondary Ni recrystallization, which causes a much stronger eect in the undoped tape. It is readily seen in Fig. 8 how those imperfections are
Fig. 8. Three dimensional plot of the U-scans corresponding to the CeO2 (1 1 1) re¯ection for buer layers grown at 800°C and in Ar atmosphere. (a) CeO2 on Ni and (b) CeO2 on NiMo. Note the appearance of randomly oriented re¯ections only in the buered Ni tape, due to the strong eect of the Ni secondary recrystallization at high temperatures.
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straightly translated to the buer layer. For the NiMo buered substrate, the in-plane aligned structure of the buer layer is still held even at high temperature while for the Ni tape, randomly strong re¯ections show a drastic loss of biaxial alignment. In summary and according to our experimental results shown in this work, there is a strong crossed relation amongst the mentioned growing parameters, crucial to obtain high quality epitaxial buer layers on Ni-based tapes. First, the buer layer deposition temperature is constrained to the temperature interval between both Ni structural transitions, over the fcc cube texture development (well above 500°C) but below the appearance of the secondary recrystallization (under 800°C for Ni or higher for NiMo tapes). Once the deposition temperature is ®xed, the ordered growth of biaxially aligned buer layers along the substrate orientation appears to be free of unliked diraction peaks, when the deposition takes place in Ar environment and longer substrate±target distances. Our proposed explanation to this behavior is based on the scattering and thermalization of the ablated species by the inert gas, resulting in an enhanced formation of CeO molecules, and therefore in a reduced ability of the expanding plasma plume to oxidize the Ni substrate surface.
Acknowledgements We are indebted to L. Fern andez for her assistance in the preparation of this manuscript. M.A. Arranz gratefully acknowledges ®nancial support from the IFW-Dresden and Universidad de Castilla-La Mancha (UCLM) during this research.
References [1] A. Goyal, D.P. Norton, D.M. Kroeger, D.K. Christen, M. Paranthaman, E.D. Specht, J.D. Budai, Q. He, B. Saan, F.A. List, D.F. Lee, E. Hart®eld, P.M. Martin, C.E. Klabunde, J. Mathis, C. Park, J. Mater. Res. 12 (1997) 2924. [2] D.P. Norton, A. Goyal, J.D. Budai, D.K. Christen, D.M. Kroeger, E.D. Specht, Q. He, B. Saan, M. Paranthaman, C.E. Klabunde, D.F. Lee, B.C. Sales, F.A. List, Science 274 (1996) 755. [3] H.-L. Suo, M.-L. Zhou, L.-H. Zhai, Z. Sun, X.-F. Xu, W. Han, T.-Y. Zuo, Physica C 282±287 (1997) 567. [4] Q. He, D.K. Christen, J.D. Budai, E.D. Specht, D.F. Lee, A. Goyal, D.P. Norton, M. Paranthaman, F.A. List, D.M. Kroeger, Physica C 275 (1997) 155. [5] M. Paranthaman, A. Goyal, F.A. List, E.D. Specht, D.F. Lee, P.M. Martin, Q. He, D.K. Christen, D.P. Norton, J.D. Budai, D.M. Kroeger, Physica C 275 (1997) 266. [6] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saan, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hart®eld, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [7] Y. Iijima, M. Hosaka, N. Tanabe, N. Sadakata, T. Saitoh, O. Kohno, K. Takeda, J. Mater. Res. 12 (1997) 2913. [8] D.K. Christen, D.P. Norton, A. Goyal, J.D. Budai, Q. He, C.E. Klabunde, D.M. Kroeger, M. Paranthaman, B. Saan, E.D. Specht, 21st International Conference on Low Temperature Physics Proceedings, 1996, p. 1531. [9] R.P. Wang, Y.L. Zhou, S.H. Pan, M. He, H.B. Lu, Z.H. Chen, G.Z. Yang, C.F. Liu, X. Wu, F.Y. Wang, Y. Feng, P.X. Zhang, X.Z. Wu, L. Zhou, Physica C 337 (2000) 87. [10] R.P. Wang, Y.L. Zhou, S.H. Pan, M. He, Z.H. Chen, G.Z. Yang, Physica C 328 (1999) 37. [11] V. Betz, B. Holzapfel, L. Schultz, Thin Solid Films 301 (1997) 28. [12] N.N. Khoi, W.W. Smeltzer, J.D. Embury, J. Electrochem. Soc. 122 (1975) 1495. [13] X. Zhang, G. Chen, Thin Solid Films 298 (1997) 53. [14] H. Bialas, K. Heneka, Vacuum 45 (1994) 79. [15] S. Zhu, D.H. Lowndes, J.D. Budai, D.P. Norton, Appl. Phys. Lett. 65 (1994) 2012. [16] B. de Boer, J. Eickemeyer, N. Reger, R. Opitz, J. Richter, H. Klauss, B. Holzapfel, L. Schultz, W. Prusseit, P. Berberich, in press.
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An experimental study of the main involved parameters in the epitaxial growth of CeO2 buer layers on nickel tapes M.A. Arranz a,*, B. Holzapfel b, N. Reger b, J. Eickemeyer b, L. Schultz b a
Departamento de Fõsica Aplicada, Facultad de Ciencias Quõmicas, UCLM, 13071 Ciudad Real, Spain b Institute of Solid State and Materials Research, IFW 01069 Dresden, Germany Received 19 February 2001; received in revised form 17 April 2001; accepted 26 April 2001
Abstract In order to investigate the optimal conditions for highly oriented and epitaxial buer layers on nickel based tapes, we have grown several sequences of cerium oxide ®lms by pulsed laser deposition technique. Amongst the usual parameters governing this procedure and connected to obtain high quality buer layers, three of them have requested a special attention through this work: the target±substrate distance, the buer layer deposition temperature and the gas atmosphere inside the vacuum chamber during the growing process. Based on the following results, we have found a full crossed relation amongst them, which develops in obtaining textured layers of highly oriented CeO2 along the (0 0 1) ordered nickel substrate. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Cd; 85.25.Kx; 68.55.Jk Keywords: Buer layers; Biaxial texture; Rolled-Ni substrates; Application of high TC superconductors.
1. Introduction The employment of Ni-based tapes as metallic substrates for superconducting ®lm deposition is receiving a great deal of interest through the last years [1,2]. Such metallic tapes can easily develop a cubic textured structure by means of uniaxial rolling and ulterior annealing (rolling-assistedbiaxially-textured tapes, (RABIT) technique) [3± 5]. This feature provides them to be a cheap and ductile quasi-single crystal substrate, into which high quality crystalline ®lms can be deposited
* Corresponding author. Tel.: +34-926-295-300; fax: +34926-295-318. E-mail address: maarranz@®ap-cr.uclm.es (M.A. Arranz).
after. For instance, the growing of high TC superconducting ®lms on them is a point of great interest for industrial purposes, as these coated tapes are expected to be used in high power conduction lines or high ®eld magnets working eciently at liquid nitrogen temperature. Previous works concerning the growth of YBCO ®lms on buered nickel tapes have reported to support high critical current densities even comparable to the ones obtained on ceramic single crystal like SrTiO3 [6±9]. As it can be found in those references, in order to obtain high textured and oriented superconducting ®lms on metallic tapes, it is unavoidable to intercalate some kind of named buer layer, commonly ceramic ®lms, e.g. CeO2 or yttria stabilized zirconia. The role of this intermediate deposit is mainly to extend the textured and
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oriented structure of the metallic substrate onto the superconducting ®lm, gradually adjusting the initial lattice mismatch between Ni and YBCO. Also concerning that purpose, the buer layer provides a barrier to prevent either the metal diffusion into the copper oxide compound [9,10] or the metallic surface oxidation during the superconducting ®lm deposition. According to its mentioned task as a biaxial texture precursor, the buer layer must be grown on Ni tape showing the least number of misorientations either relative to the substrate normal axis or referred to the inplane alignment of the nickel surface (cube on cube architecture). Slight deviations in the crystallinity of the buer layer during its growth are straight transduced in a diminishment of the capability of the upper deposited superconducting ®lm to stand high values of the critical current density, due to the appeareance of additional large-angle grain boundaries or structural defects promoted by the buer layer. Mainly, the aim of this work has been to identify and correlate the eects on the buer layer deposition produced directly by the mentioned growing parameters, taking carefully into account of the high biaxially textured structure requirement.
electron diraction (RHEED) ensemble was also set in to test in situ the biaxial alignment either the nickel tape or the oxide buer layers grown on. Further details of the experimental setup have been previously reported [11]. Prior to the coating processes, a background pressure of 10 6 mbar was achieved in the deposition chamber with a turbomolecular vacuum system. After etching the substrates at room temperature by means of the Ar-ion gun, we annealed them at 500°C in forming gas (Ar H2 mixture; partial pressure of 1:3 10 3 mbar) during 30 min. The grade of quality in the (0 0 1) Ni cubic textured structure was examined through the diraction patterns obtained from the (1 1 0) and (1 0 0) directions of the incident electron beam on the tape surface (see Fig. 1). Once the cube textured tape orientation was clearly obtained, dierent types of buer layers were deposited after. To characterize all the grown samples, we have measured their out-of-plane orientation with an X-ray diractometer (PW3050, CoKa radiation) and the in-plane texture was quantitatively evaluated with a Philips four circle equipment (CuKa radiation).
2. Experimental
Fig. 2 shows the 2h scans for a set of CeO2 samples grown in dierent deposition atmospheres with a repetition rate of 3 Hz during 20 min, in all cases. The substrate temperature was 700°C and the target distance, d, was ®xed to 6 cm. The partial pressure was 2 10 3 mbar for both oxygen or argon environments. As it can be seen, the (1 1 1) CeO2 growing direction predominates when an oxidating atmosphere is present since the ®rst stages of deposition (Fig. 2a). This eect is due to the appearance of a rapid enhancing NiO layer over the substrate surface. It is already reported [12±14] that (1 1 1) orientation relative to the metallic tape normal axis is the preferred growing mode of nickel oxide at ambient temperature. In our samples, this fact allows immediately the (1 1 1) CeO2 direction to appear after. As the deposition begins in vacuum environment, both (1 1 1) and (0 0 1) CeO2 re¯ections emerge simultaneously, depending on the intensity of the last
A pulsed excimer laser (KrF, 248 nm wavelength) was used to deposit CeO2 oxide buer layers on Ni based tapes (10 10 0:1 mm3 , dimensions). The laser was focused into the vacuum chamber to reach an energy density around 3 J/ cm2 at the target. The metallic substrates were pasted with Ag epoxy onto the heater holder and their position could be varied with an XYZmanipulator. Before deposition, the as cold-rolled tapes were placed just in front of the polycrystalline targets, having therefore their normal axis parallel to the growing direction of the ablated particles. The vacuum chamber was equipped with an Ar-ion gun, which focused the beam at a 55° angle relative to the substrate normal axis. This experimental tool enabled us to clean the substrate surface from contaminants and some oxide layers in a preliminary stage. A re¯ection high energy
3. Results and discussion
M.A. Arranz et al. / Physica C 366 (2002) 109±116
111
Fig. 1. Ni substrate RHEED patterns for a (0 0 1) cubic textured structure: (a) initial pattern at room temperature, (b) and (c) after the annealing at 500°C for the (1 1 0) and (1 0 0) directions of incidence, respectively.
one on the NiO ®lm formed during the oxygenated deposition period (Fig. 2b). The third diraction plot can enforce this previous assertion: the substitution of an inert gas (here argon) for oxygen during the complete deposition, inhibits strongly the Ni oxidation providing a much stronger (0 0 1) CeO2 re¯ection (Fig. 2c). Also the target substrate distance has a strong eect on the appearance of the (1 1 1) NiO ®lm orientation. Fig. 3 shows H±2H diraction plots obtained for CeO2 deposited on nickel tapes in argon or vacuum environment, when varying the substrate±target distance, d. The deposition tem-
perature was raised up to 800°C and the other growth parameters were maintained the same as in Fig. 2. As it can be seen, the shorter the distance is (d 4 cm in Fig. 3a), the stronger Ni oxidation occurs, translating the unlike (1 1 1) orientation into the CeO2 ®lm. On the other hand, slowing the speed of ablated particles through a longer trip in some inert gas (d 8 cm in Fig. 3b) eliminates such orientation while only the (0 0 1) CeO2 growing mode, parallel to the substrate normal axis, remains without any traces of NiO diraction peaks. Although the CeO2 (1 1 1) orientation appears in both cases, for longer substrate±target
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Fig. 2. 2h scans for CeO2 layers grown on Ni textured tape in dierent atmospheres: (a) full deposition in oxygen, (b) ®rst 5 min in vacuum and the rest of deposition in oxygen, (c) same procedure as (b) but with argon.
Fig. 3. 2h scans for CeO2 layers grown on Ni textured tape with dierent target±substrate distances, d. (a) Sample deposited in Ar atmosphere at 800°C with d 4 cm, (b) same as (a) but with d 8 cm, (c) same as (b) but in vacuum. Scales represented in the lower plot are the same to all of them. The indexed peaks correspond to CeO2 buer (vertical legends).
distances its absolute magnitude is almost vanishing. Finally, the results presented in Fig. 3c (the same target±substrate spacing of 8 cm but in turbomolecular vacuum) show again the very important role of the inert gas to slow and reduce the reactive and ionized particles arising from the target. The longer d spacing avoids the appearance of the (1 1 1) CeO2 re¯ection but, in absence of any inert atmosphere, is not eective enough to detain the arrival of free oxygen species at the nickel tape (see the (1 1 1) NiO peak). These previous experimental results can be explained in the framework of the following qualitative model. The laser plasma plume, which is formed after the ablation of target material and the interaction with the excimer laser pulse, consists of highly excited species (mainly atoms and ions) that move towards
the substrate. Their energy distribution inside the plasma plume at the substrate position depends strongly on deposition parameters like the deposition pressure. For laser ablation in vacuum the ablated particles have a high and constant kinetic energy on their way to the substrate. However, during laser ablation in background gas atmosphere, there is a strong interaction of the ablated species with the background gas by atomic collisions, resulting in a complex hydrodynamic expansion of the plasma plume. During this expansion shockwaves can be formed and inside the high density plasma cloud the energy of the species is thermalized and recombination processes appear because of the high collision rate with cool background gas. This increases the amount of mole cular species, e.g. Ce 4 , O2 , CeO2 , and reduces
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the amount of ablated free oxygen when the ablation is performed in an inert gas atmosphere like Ar. In our experimental situation we believe, that this unbound oxygen can react on the Ni-substrate surface to NiO, resulting in the unwanted polycrystalline (1 1 1) NiO orientation. Laser ablation in an Ar background atmosphere and using larger target±substrate distances reduces in our model the amount of unbound oxygen at the substrate position resulting in a reduced polycrystalline NiO formation as it is observed in our experiments. Another parameter involved in the buer layer growing process, which has also focused our interest in this work, has been the substrate temperature during the deposition. It is largely shown in Refs. [4,15] that special care has to be taken choosing the deposition temperature due to its strong eect not only on the growing direction of the buer layers [15] but also on the degree of texture and crystallinity of them [4], straight affected by the recrystallization processes of the nickel tape [16] and the kinetics of the epitaxial growth. In order to investigate such in¯uence, two series of CeO2 samples were grown on textured Ni and NiMo metallic tapes, respectively, at dierent substrate temperatures ranging from 400°C to 800°C. Depositions were made in Ar atmosphere (2 10 2 mbar) with a repetition rate of 10 Hz during 20 min for each sample. The substrate were placed at d 8 cm from the target in all cases. It has been reported that small doping of Ni matrix with Mo (i.e. 0.1 at.% Mo, as employed along this work) [16] can shift the bare nickel secondary recrystallization to higher temperatures, therefore translating the appearance of misorientations in the buer layers up to thresholds well above their usual deposition temperature interval. The H±2H diraction plots for both types of substrates are displayed in Figs. 4 and 5 for Ni and NiMo substrates, respectively. At low deposition temperatures it can be seen that CeO2 buer layers grow in a mixture of both (1 1 1) and (0 0 1) directions (see Figs. 4 or 5). This common result on both kind of metallic substrates, could be originated in the low diusion energy available for the deposited atoms on the textured metallic surface, resulting in poor epitaxial growth. When the CeO2 deposition temperature increases, the (1 1 1) re¯ection diminishes
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Fig. 4. Behavior of the 2h diraction plots with deposition temperature for CeO2 buered Ni tapes. The indexed peaks correspond to CeO2 re¯ections. Note that the (3 1 1), (2 2 2) and (2 0 0) diraction intensities have been magni®ed in a 10 factor for comparison.
gradually and almost vanishes over 700°C, where the (0 0 1) direction is clearly predominant (specially in the case of buered NiMo tape). From our results, this temperature interval (600±700°C) seems to be the best one for growing the ceramic buer layer, while a good degree of alignment to the textured tape has been achieved. That convenient temperature range, and specially, the upper value of 700°C for CeO2 deposition agree with reported results [9]. The temperature dependent eect of the epitaxial growth kinetics can be clearly seen in the corresponding CeO2 (1 1 1) pole-®gures for the buered NiMo tapes (Fig. 6). At 300°C only the polycrystalline (1 1 1) growth component is present (Fig. 6a). Increasing the deposition temperature to 400°C (Fig. 6b) results in the occurrence of the biaxially oriented (1 0 0) orientation,
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Fig. 6. Dependence of the (1 1 1) CeO2 pole ®gure on the deposition temperature: (a) 300°C, (b) 400°C, (c) 500°C and (d) 600°C. Fig. 5. Behavior of the 2h diraction plots with deposition temperature for CeO2 buered NiMo tapes. The indexed peaks correspond to CeO2 re¯ections. Note that the (3 1 1), (2 2 2) and (2 0 0) diraction intensities have been magni®ed in a 10 factor for comparison. The smaller (3 1 1) CeO2 re¯ection peak for NiMo tape suggests a weaker distortion of its textured structure than in the Ni case.
but due to the low substrate temperature the epitaxial growth component shows is quite broad in-plane orientation. A further substrate temperature increase (500°C, Fig. 6c) both sharpens the epitaxial growth and reduces the (1 1 1) orientation. At 600°C ®nally the (1 1 1) orientation disappears and the in-plane orientation of the epitaxial (1 0 0) orientation reaches a value comparable to the cube-textured substrate (Fig. 6d). For higher temperatures, beside the (0 0 1) orientation other dierent CeO2 growth modes appear again: the (1 1 1) diraction peak in the case of NiMo substrates or the (1 1 1) and (3 1 1) ones for Ni substrates, respectively. This is a straight consequence of the Ni secondary recrystallization
process, which gradually results in the formation of great crystallites randomly oriented, and consequently disorientates the ordered sequence of deposited or growing buer layers. The ®nal result is a CeO2 thin ®lm with a great number of misorientations relative to the epitaxial alignment, which is traduced in the experimentally observed additional growing directions. As shown in the
Fig. 7. Two dimensional plot of the U-scans corresponding to the substrate (1 1 1) re¯ection after deposition of CeO2 at 800°C in Ar atmosphere. (a) Ni tape and (b) NiMo tape.
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experimental results, the eect is much stronger in the bare Ni buered tape (Fig. 4) than the NiMo case (Fig. 5), due to the mentioned shifting of the secondary recrystallization temperature to higher values. An additional experimental evidence of this eect can be found on correlating the texture Uscans of the substrate (1 1 1) re¯ection and the corresponding CeO2 (1 1 1) re¯ection of the oxide
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buer grown at 800°C. As it is followed from the Fig. 7, some intense deviations from the cube±cube architecture turn up in the case of Ni substrate, on the contrary to NiMo tape. Such distorsions in the textured structure are due to the proximity of the secondary Ni recrystallization, which causes a much stronger eect in the undoped tape. It is readily seen in Fig. 8 how those imperfections are
Fig. 8. Three dimensional plot of the U-scans corresponding to the CeO2 (1 1 1) re¯ection for buer layers grown at 800°C and in Ar atmosphere. (a) CeO2 on Ni and (b) CeO2 on NiMo. Note the appearance of randomly oriented re¯ections only in the buered Ni tape, due to the strong eect of the Ni secondary recrystallization at high temperatures.
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straightly translated to the buer layer. For the NiMo buered substrate, the in-plane aligned structure of the buer layer is still held even at high temperature while for the Ni tape, randomly strong re¯ections show a drastic loss of biaxial alignment. In summary and according to our experimental results shown in this work, there is a strong crossed relation amongst the mentioned growing parameters, crucial to obtain high quality epitaxial buer layers on Ni-based tapes. First, the buer layer deposition temperature is constrained to the temperature interval between both Ni structural transitions, over the fcc cube texture development (well above 500°C) but below the appearance of the secondary recrystallization (under 800°C for Ni or higher for NiMo tapes). Once the deposition temperature is ®xed, the ordered growth of biaxially aligned buer layers along the substrate orientation appears to be free of unliked diraction peaks, when the deposition takes place in Ar environment and longer substrate±target distances. Our proposed explanation to this behavior is based on the scattering and thermalization of the ablated species by the inert gas, resulting in an enhanced formation of CeO molecules, and therefore in a reduced ability of the expanding plasma plume to oxidize the Ni substrate surface.
Acknowledgements We are indebted to L. Fern andez for her assistance in the preparation of this manuscript. M.A. Arranz gratefully acknowledges ®nancial support from the IFW-Dresden and Universidad de Castilla-La Mancha (UCLM) during this research.
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