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Out-of-substrate plane orientation control of thin YBa2Cu3Ox films on NdGaO3 tilted-axes substrates
Physica C 434 (2006) 105–114 www.elsevier.com/locate/physc
Out-of-substrate plane orientation control of thin YBa2Cu3Ox films on NdGaO3 tilted-axes substrates Peter B. Mozhaev a,b,*, Julia E. Mozhaeva a,b, Igor K. Bdikin c,d, Iosif M. Kotelyanskii e, Valery A. Lusanov e, Jørn Bindslev Hansen b, Claus S. Jacobsen b, Andrey L. Kholkin d b
a Institute of Physics and Technology RAS, Moscow 117218, Russia Department of Physics, Technical University of Denmark, Lyngby, DK-2800, Denmark c Institute of Solid State Physics RAS, Chernogolovka, Moscow distr., 142432, Russia d CICECO, University of Aveiro, Aveiro 3810-193, Portugal e Institute of Radio Engineering and Electronics RAS, Moscow 125009, Russia
Received 27 September 2005; received in revised form 1 December 2005; accepted 7 December 2005 Available online 24 January 2006
Abstract Epitaxial heterostructures YBa2Cu3Ox(YBCO)/CeO2/NdGaO3 were prepared on tilted-axes NdGaO3 substrates using laser ablation technique. Morphology, crystal structure and electrical properties of the obtained films were characterized. The seeding mechanisms are affected by the tilt angle, resulting in superior YBCO films on NdGaO3 substrates in an intermediate range of tilt angles of 6–14. The introduction of CeO2 layer leads to change of the YBCO film orientation: at low deposition rate c-oriented films are formed, while at high deposition rates the film grows with c-axis tilted along the [1 1 0] NdGaO3 direction. Bi-epitaxial films and structures were prepared by removal of part of the CeO2 layer using ion-beam milling. 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 74.78.Bz Keywords: Bi-epitaxial films; Seeding layer; Tilted-axes substrates
1. Introduction d-Wave order parameter symmetry and high gap energy make high-temperature superconductors (HTSC) a good choice for fabrication of qubits for quantum computations. Formation of Josephson junctions with equilibrium phase shift equal to p—the p-junctions—allows preparation of the ‘‘quiet’’ qubit [1], one of the most promising qubit types. p-junctions in all-HTSC circuits were demonstrated using a complicated four-crystal design [2], signs of p-junction formation in a more simple bi-crystal configuration were observed in [3,4]. Bi-epitaxial technique [5] eliminates *
Corresponding author. Address: Institute of Physics and Technology RAS, Moscow 117218, Russia. Tel.: +7095 3324894; fax: +7095 1293141. E-mail address: [email protected] (P.B. Mozhaev). 0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.12.003
geometrical limitations on the HTSC circuit design, but electrical properties of such junctions were shown to be inferior compared to the bi-crystal ones [6]. Out-of-substrate-plane tilt of the HTSC thin film crystallographic axes, often called [1 0 0]-tilt, significantly improves Josephson junction quality and was extensively studied over the last five years, both in bi-crystal [7] and in bi-epitaxial [8] design. For the latter technique the p-junction formation was demonstrated when CeO2 fluorite-type seeding layer was utilized [8]. The bi-epitaxial technique with out-of-substrate-plane tilt, developed in [8], utilizes the (1 1 0) SrTiO3 substrate, providing the single tilt angle of 45 and the only tilt direction around the {1 0 0} thin film axis. The possibility to prepare bi-epitaxial thin films and structures for other tilt angles was demonstrated in [9], while Kim and Youm
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[10] fabricated Josephson junctions for a tilt angle of 30 using miscut SrTiO3 substrate. In both cases the seeding layer of fluorite crystal structure (CeO2, Y:ZrO2) provided c-oriented growth of HTSC YBa2Cu3Ox (YBCO) independently of the substrate and seeding layer orientation. Similar effects were observed for other non-perovskite miscut substrates and seeding layers, for example, MgO [11]. The mechanism of this c-oriented growth, as far as we know, was not studied, and neither deposition parameters, nor limiting factors were determined. We studied the YBCO growth on miscut NdGaO3 substrates for the miscut angles 5–27, with and without implementation of a seeding CeO2 layer. We determined the deposition parameters for c-oriented growth and prepared bi-epitaxial thin films in this system. 2. Experimental NdGaO3 substrates and CeO2 seeding layer were chosen because their lattice constants show the best match to the (ab) plane of the YBCO thin film: lattice mismatch along the {1 1 0} YBCO directions is 0.32% and 0.64%, correspondingly. As a consequence, the growth mode is less affected by the strains caused by the lattice mismatch. We used (1 1 0)-oriented NdGaO3 substrates as standard, because the (1 1 0) plane of NdGaO3 provides more isotropic surface lattice (rectangular, translation distances ˚ and d[110] = 7.72(2) A ˚ ) compared with d[001] = 7.72(7) A another commonly used (0 0 1) plane of NdGaO3. The miscut substrates were formed by rotation of the substrate plane from the (1 1 0) crystallographic plane around the [0 0 1] axis of NdGaO3. This approach makes it possible to check the miscut angle c by X-ray diffraction from the (h k 0) planes of NdGaO3. The substrate surface was polished using a specially developed chemical–mechanical process. CeO2 seeding layer is known to provide additional in-substrate-plane rotation of the crystallographic axes by 45. Such mutual mis-orientation of the parts of a bi-epitaxial film is necessary to obtain p-junctions. The YBCO thin films and CeO2 seeding layers were deposited by laser ablation of stoichiometric ceramic targets. The details of the technique can be found elsewhere [12]. The parameters of the CeO2 deposition were optimized to obtain smooth thin film of single orientation (0 0 1) CeO2k(1 1 0) NGO on a standard (1 1 0) NdGaO3 substrate. Substrate temperature during deposition was held at 800 C, ablation was done at energy density of 1.75 J/cm2 in 0.5 mbar of a mixture of argon and oxygen (Ar/O2 = 7/3). The deposition rate at these conditions ˚ /pulse, thickness of the film varied from was about 3 A ˚ . No post-deposition annealing was per100 to 900 A formed; the film was cooled down to room temperature in working atmosphere at maximal possible rate. The YBCO thin films deposition parameters (700– 800 C, 1.3 J/cm2, oxygen partial pressure 0.15 mbar) were optimized to obtain the highest temperature of the superconducting transition Tc. To decrease the surface rough-
ness the deposition was performed in Ar/O2 = 8/2 mixture, the total pressure was 0.8 mbar. Typical deposi˚ /pulse, and typical tion rate for these conditions was 0.6 A ˚ film thickness was 1500 A. A pre-bake step (annealing of the substrate before deposition at 800 C in 5 mbar of oxygen) significantly decreased both size and density of the particles on the thin film surface. Post-deposition annealing was performed at 450 C in 800 mbar of oxygen for 1 h. All YBCO films showed Tc higher than 89 K and narrow superconducting transition, proving good uniformity of the film structure. The structural properties of thin films and multilayers were studied using X-ray diffraction technique, the surface morphology was observed by SEM and AFM. Electrical properties of the superconducting films were measured with non-contact techniques. The rocking curve in a wide angular range technique allows rapid determination of mutual orientation of the crystallographic planes of the thin film, substrate surface plane and substrate crystallographic planes ([9], Fig. 1). The 2h angle between the incident and reflected X-ray beam (characteristic radiation) is set equal to the Bragg diffraction value for one of the film diffraction peaks. For a certain x angle, corresponding to the angular position of the chosen film crystallographic plane, a broad peak on the scan will be observed. The X-ray broadband radiation is not eliminated by a monochromator or a filter, so for each crystallographic plane of the substrate X-ray radiation of some wavelength will satisfy the Bragg equation. At the corresponding x angles narrow substrate reflection
Fig. 1. Measurement of inclination angles between the substrate and film crystallographic planes using rocking curve in a wide angular range technique. Inset: geometrical setup of the measurement.
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peaks will be seen. The angular distance between the peaks is the inclination angle between corresponding substrate and film crystallographic planes [9]. Utilization of this technique was made possible by the fact that [1 0 0] (or [0 1 0]) YBCO and [1 1 0] CeO2 axes were parallel to the substrate tilt direction [0 0 1] NdGaO3 for all studied samples. 3. Results and discussion To avoid misunderstanding we will use the following notations: • the substrate plane is the plane of substrate surface; • the crystallographic planes and directions are marked with indexes N, C and Y, corresponding to NdGaO3 substrate, CeO2 seeding layer and YBCO thin film; • the miscut angle c is the angle between the substrate plane and the (1 1 0)N; • the tilt angle is the angle between crystallographic plane of CeO2 seeding layer or YBCO thin film, usually (0 0 1)C and (0 0 1)Y, and the substrate plane; • the inclination angle is the angle between crystallographic planes of the substrate and a film, or between the film and the seeding layer; • the mis-orientation is the spread of orientations of individual grains of the film or the seeding layer around the average orientation.
3.1. YBCO thin films on miscut NdGaO3 substrates Detailed results of our study of YBCO thin film structure, growth mechanisms, and electrical properties on miscut NdGaO3 substrates can be found elsewhere [13]. Briefly, the YBCO thin films grow in agreement with the epitaxial relations h1 0 0iY ð0 0 1ÞY k½0 0 1N ð1 1 0ÞN
ð1Þ
for all studied angles, except for the special case of (1 2 0)N substrate (miscut angle 18.4). We did not observe the pseudo-a oriented domains ((0 0 1)Yk(1 1 0)N) dominance at miscut angles higher than 20, as it was in [9]. Probably, the careful optimization of the deposition processes on (1 1 0)N substrates, performed in this study before depositions on the miscut substrates, resulted in formation of thermodynamically more favorable pseudo-c oriented grains [14] for miscut angles up to 26. Formation of perpendicular-oriented domains on tilted-axes substrates [15] and on etched step edges [16] was also observed for miscut angles higher than 30. We have found three angular ranges of YBCO thin film morphology on miscut substrates. The AFM microphotographs of typical films are shown in Fig. 2. For low miscut angles (vicinal range) the structure and morphology of the films do not differ much from that of a film deposited on a standard (1 1 0)N-oriented substrate. Increase of the miscut angle leads to an increase of the grain size, with improved
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surface smoothness, better crystal quality, and improved electrical parameters [13]. The angular borders of this medium range depend on the deposition technique and for laser ablation are 6–11. Increase of the miscut angle over 11 results in formation of high steps on the film surface (‘‘step bunching’’), with corresponding inferior structural and electrical parameters of the films. The difference between the three angular ranges is best illustrated by dependence of the inclination angle between the (0 0 1)Y plane and (1 1 0)N plane on the miscut angle of the substrate (Fig. 3). Grain collisions in the beginning of the deposition and strain introduced by the lattice mismatch between the substrate and the film lead to slight inclination of the (0 0 1)Y plane from the (1 1 0)N plane of the substrate. Substrate miscut introduces steps of oneunit-cell height on the substrate surface with density proportional to tangent of the miscut angle. Such steps act as seeding centers for the thin film grains. At certain density of the steps all seeding occurs at the step edges, and grain collisions decrease due to regular distance between the seeds. As a result, the atomic planes of the film are finely aligned with the substrate planes (Fig. 3, medium part). ‘‘Step bunching’’, taking place at high miscut angles, again increase strain in the film and some of this strain is released as inclination of the (0 0 1)Y plane. Note, that no strain is introduced along [0 0 1]N direction, and the film remains well aligned with the substrate planes in this direction. Our conclusion that seeding center density is the main factor determining the YBCO thin film formation is corroborated by the fact that the ‘‘medium’’ angular range is different for laser ablation and DC sputtering deposition techniques (1.5–3). The average grain size in these two cases is 0.35 and 1.2 lm, respectively, for the films, deposited on standard (1 1 0) NdGaO3 substrates. This value can be considered to be a measure of the mean free path of the adatoms on the substrate surface. The four times greater grain size for DC sputtering can be caused by ion bombardment of the substrate surface, suppressing adatom incorporation into the growing film lattice and increasing the mobility of the adatoms along the surface. The average distance between the steps along the substrate surface for the medium angular range is 5 times longer for DC ˚ ) compared with the laser ablation sputtering (73–147 A ˚ ). One seed on a standard-oriented subtechnique (16–32 A strate corresponds to 100–150 one-unit-cell steps of the substrate. The actual grain size of the tilted-axes film of the medium range is greater than that of the standard-oriented film (Fig. 4), and corresponds to 300 one-unit-cell steps. The lattice mismatch along the c-axis of the film is 1/150 of the substrate lattice constant (considering pseudo-cubic perovskite lattice). This agreement leads us to the hypothesis that the lattice strain along the c-axis is the limiting factor of the grain size for an optimally seeded film. Further increase of the miscut angle to the high-angle range results in a decrease of step width (w, grain size across terraces in Fig. 4), in agreement with this hypothesis.
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Fig. 2. Typical AFM microprofilograms of the YBCO thin films on NdGaO3 miscut substrates. The miscut angles are 1.5, 5 and 18.4, the color scale is 40, 150 and 70 nm, respectively.
We plotted a fitting curve (step height h = const, dashed curve in Fig. 4), but the observed decrease of w is faster than the calculated curve. A possible reason is intergrowth of the grains in the medium angular range, increasing the observed average grain size. A correct approximation can be done only in the high-angle range, where we have too few points. The relation hw = const, observed in [9], decreases even more slowly. The epitaxial relations between the film and the substrate were the same for all studied orientations except the (1 2 0)N substrate. This orientation in orthorhombic notation corresponds to (3 1 0)N orientation in the pseudo-cubic notation; i.e., the surface of such substrate represents a set of one-unit-cell steps with average width
of three-unit-cells. The NdGaO3 lattice constant in the ˚ , hence, the step width corpseudo-cubic notation is 3.86 A ˚ ). Formation responds well to the c-axis of the film (11.7 A of a (1 0 9)Y-oriented YBCO film with c-axis oriented along the steps plane ((0 0 1)Yk(1 1 0)N) is promoted by the absence of anti-phase boundaries, typical for the films with standard epitaxial relations (1). This effect was observed and explained for the first time in [17]. We found that the orientation of the film on the (1 2 0)N substrate depends on the deposition rate. At low deposition rate (pulse repetition rate 2 Hz, giving deposition rate of ˚ /min) the film followed standard epitaxial relations 72 A ˚ /min) the (1), while at high deposition rate (10 Hz, 360 A resulting film was 90% (1 0 9)Y-oriented. This result dem-
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saturation values (high deposition rates), while at low deposition rate the film has enough time to relax to a thermodynamical equilibrium state, which is determined by the minimal surface energy of the film [14]. 3.2. CeO2 thin films on miscut NdGaO3 substrates The CeO2 seeding layer for all studied miscut angles and deposition parameters grows with a single orientation close to the standard epitaxial relations for CeO2 growth on perovskite substrates: ½1 1 0ð0 0 1ÞCeO2 k½0 0 1ð1 1 0ÞNdGaO3
Fig. 3. Orientational parameters of the YBCO thin films on the miscut NdGaO3 substrates. Bottom: miscut angle dependence of inclination angles between (1 1 0)N and (0 0 1)Y planes around the substrate tilt axis [0 0 1]N (dashed line) and around perpendicular in-substrate-plane direction (solid line). Top: width of the rocking curve of the (0 0 5)Y diffractional peak, measured along the substrate tilt axis [0 0 1]N.
Fig. 4. Dependence of grain size of the YBCO thin film on the miscut angle of the NdGaO3 substrate along (top line) and across (bottom line) the [0 0 1]N axis. The dashed line shows h = const fit of the experimental data. The vertical dotted lines mark the angular ranges of different growth modes.
onstrates the predominant effect of the film–substrate interface on the orientation of the growing film for high super-
ð2Þ
The actual growth mode can be characterized as a texture with the texture axis [1 1 0]Ck[0 0 1]N, and dominant orientation of the (0 0 1)C inclined by 3–6 from the (1 1 0)N plane from the substrate plane (Fig. 5a and b). Single orientation of the film was confirmed by measurements of the rocking curves in a wide angular range with Bragg reflection angle corresponding to other film crystallographic planes. With an increase of the substrate miscut angle the inclination angle between the film and substrate crystallographic planes increased, and showed a maximum at 14– 18 (Fig. 6). The reasons for such discrepancy from the standard epitaxial relation (0 0 1)Ck(1 1 0)N are not clear, as well as the mechanism of the inclination formation. A similar discrepancy between (0 0 1)C and (1 1 0)N planes with (0 0 1)C inclination from the substrate plane was observed in [9] (type II epitaxy). Our calculations of exact orientation of the CeO2 films gives a surprising result: the type IIa epitaxy for the (1 2 0)N substrate gives in fact a (1 1 3)C-oriented CeO2 film, while type IIb epitaxy for the (1 3 0)N substrate results in a (2 2 1)C-oriented CeO2 film. The small-index planes of (h h l) family are known to become stable for some etching processes, so we cannot exclude growth of a ‘‘strangely’’-oriented CeO2 thin film on a miscut substrate. The inclination of the (0 0 1)C plane from the crystallographic planes of a sapphire substrate was observed in [18], but the film crystallographic plane was tilted towards the substrate plane. This effect was explained as minimization of the surface energy when the surface of the film is close to the small-index crystallographic plane. In our case neither surface energy minimization, nor stabilization of ‘‘strange’’ small-index planes can explain the observed inclination of the (0 0 1)C planes. As a possible clue to solving the problem we note the proportionality between the substrate miscut angle c and the inclination angle between the crystallographic planes of the film and the substrate (see the straight fit in Fig. 5). After 6 inclination is achieved, the inclination angle abruptly drops to 2–3. The lattice constant of the CeO2 layer calculated from the position of the (0 0 2)C Bragg reflection peak was ˚ for all samples, suggesting high oxygen con5.41 ± 0.005 A tents in the film. The width of h/2h-peaks was close to the theoretical minimum due to thickness broadening. The width of the (0 0 2)C peak never exceeded 0.5, and the
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Fig. 6. Dependences of inclination between film and substrate crystallographic planes in the epitaxial heterostructures YBCO/CeO2/NdGaO3 on substrate miscut angle. The YBCO films were obtained at high deposition rates. The dashed lines are approximating first four points of each dependence by equation y = const Æ x.
rocking curve broadening is a spread of the orientation of individual grains from the main orientation. ˚ thick CeO2 seeding layer The roughness of the 300 A ˚ measured by AFM and profile meter was less than 20 A for all miscut angles. No specific features could be seen on the seeding layer surface, probably due to lack of accuracy of the measurement (the resolution of the used AFM ˚ ). mode was 15 A 3.3. YBCO thin films over CeO2 seeding layer on miscut NdGaO3 substrates
Fig. 5. Rocking curves in a wide angular range taken from epitaxial heterostructures YBCO/CeO2/NdGaO3 on miscut substrates. Zero x angle corresponds to the substrate plane. Top curve corresponds to (0 0 4) CeO2 Bragg diffraction angle (69.4), bottom curve corresponds to (0 0 7) YBCO Bragg diffraction angle (54.9). (a) Miscut angle 18.4 (the (1 2 0)N substrate), high YBCO deposition rate; (b) miscut angle 23.4, low YBCO deposition rate, the CeO2 seeding layer is partially removed by ion-beam milling.
˚ thick film is about theoretical broadening for the 300 A 0.3. The rocking curve width was 0.7–0.8 and it did not significantly change with 2h. Probably the reason for the
The orientation and crystal quality of the YBCO thin films over CeO2 seeding layer on miscut NdGaO3 substrates showed strong dependence on deposition rate, miscut angle, and seeding layer thickness. ˚ /min) the YBCO At high deposition rate (10 Hz, 360 A thin film showed textured growth with the texture axis h1 0 0iYk[1 1 0]Ck[0 0 1]N and small (1–5) inclination of the dominant (0 0 1)Y plane orientation both from the (1 1 0)N and (0 0 1)C planes (Fig. 5a). The orientation of the film was confirmed by measurement of the rocking curves in a wide angular range with Bragg reflection angle corresponding to other film crystallographic planes. For example, in Fig. 5a the (1 0 8)Y peak can be seen on the top curve (Bragg angle for (0 0 4)C reflection 2h = 69.4 is close to the Bragg angle for (1 0 8)Y reflection 2h = 68.9), corresponding to the intensive left YBCO peak on the bottom curve (angular distance between (0 0 1)Y and (1 0 8)Y planes
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is 20.6). This growth mode is very close to the standard epitaxial relations of the YBCO thin film growth on a bare NdGaO3 miscut substrate (1), the same way as the growth mode of CeO2 seeding layer is close to the standard epitaxial relations of CeO2 growth on perovskite substrates (2). The (0 0 1)Y plane was inclined from the substrate (1 1 0)N plane from the substrate surface plane, maximal inclination was observed at 20–24 (Fig. 6). Similar to CeO2 growth, the inclination angle is proportional to the miscut angle, until an inclination of 6 is achieved. No clear relation between CeO2 seeding layer and YBCO thin film can be seen. At low miscut angles the (0 0 1)Y plane was inclined from (0 0 1)C towards the substrate plane, but for miscut angles higher than 20 the (0 0 1)Y plane showed inclination from seeding layer crystallographic plane from the substrate plane. This result differs from the (0 0 1)Y tilt towards the substrate plane, observed in [11,19], minimizing the surface energy of the film similarly to CeO2 in [18]. We note that in all cases [11,18,19] the lattice mismatch between the film and substrate was about 10%. Relatively weak bonds at the film–substrate interface allow tilt of the film axes to decrease film surface energy. In this study the mismatch of all layers of heterostructure YBCO/ CeO2/NdGaO3 is within 1%, providing much stronger bonds at the interfaces. The width of the X-ray diffraction peaks on h/2h-scans (Fig. 7, upper curve on inset) was rather small (0.5–0.8), and close to that of YBCO thin films on bare substrates, confirming high crystal quality of the YBCO thin films obtained at high deposition rates. The rocking curve width is broader than that of the YBCO thin film on a bare substrate, in agreement with texture growth mode compared with epitaxial growth. Typical AFM microprofilogram (Fig. 8a) does not differ much from the ones of the YBCO film deposited on a bare NdGaO3 miscut substrate (Fig. 2c). The c-oriented films were obtained when the laser repe˚/ tition rate was decreased to 2 Hz (deposition rate 72 A min), and only for miscut angles higher than 14 (see right YBCO peak on Fig. 5b, bottom curve). No directional features or big steps can be seen on the surface of such film (Fig. 8b). The rocking curve width of the c-oriented films did not change much with increasing 2h angle, characterizing spread of the grain mis-orientation. For high miscut angles (c > 16) the rocking curve maximum corresponded to the substrate plane position within accuracy of the measurement (0.05), and the width of the rocking curve was 1.5– 5. For small miscut angles the rocking curve width increased to 7–12, and the (0 0 1)Y plane tilted from the substrate plane irregularly by 1–5. We can conclude that the observed films are texturised with the same texture axis as the tilted YBCO films at high deposition rate (h1 0 0iYk[1 1 0]Ck[0 0 1]N), but the dominant orientation of the (0 0 1)Y plane is turned from (1 1 0)N to the substrate plane position.
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Fig. 7. Rocking curve in a wide angular range for the (0 0 1 1)Y diffraction peak, taken from the epitaxial heterostructure YBCO/CeO2/NdGaO3 on a substrate with miscut angle 26.6 (the (1 3 0)N substrate). On inset: h/2hscan of the c-oriented part of the YBCO film (bottom curve) and of the tilted-axes YBCO film (upper curve).
The h/2h-scans (Fig. 7, bottom curve on inset) of the coriented films showed very sharp peaks, close to the theoretical limit due to the thickness of the films. The c-oriented films probably consist of grains of very high crystal quality and low internal strain, but mis-oriented from the substrate plane over a relatively broad range. The c lattice constant, determined using the (0 0 l) family of peaks on the h/2hscans, is smaller than that of the films deposited on a bare ˚ and 11.7 ± 0.02 A ˚ , corresubstrate (11.675 ± 0.015 A spondingly). This decrease of the c lattice constant is typical for deposition on CeO2 and can be explained by tensile strain introduced into the film by the seeding layer [20]. The reason for the re-orientation of YBCO thin film grown on a non-perovskite substrate is minimization of surface energy when the surface of the film is terminated with the crystallographic planes of minimal energy ((0 0 1)Y for YBCO). This effect was described in many papers (for example, [9–11]), but the influence of the deposition rate was not studied. The c-orientation of the YBCO thin film grown on a CeO2 layer using pulsed laser deposition technique in [9] was attributed to the high deposition rate compared with DC sputtering deposition technique resulting in tilted-axes YBCO films. Our present results are contradictory to this conclusion: the c-oriented film is formed at low deposition rate, while increase of deposition
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Fig. 8. Typical AFM microprofilograms of the epitaxial heterostructures YBCO/CeO2/NdGaO3 on miscut substrates. (a) and (b) bi-epitaxial film on the 18.4-miscut substrate, the tilted-axes YBCO film on the bare substrate and the c-oriented YBCO film, correspondingly; (c) mixed-orientation thin film, miscut angle 26.6. The color scale is 50, 75, and 160 nm, correspondingly.
rate results in tilt of the YBCO film axes. Probably the reason lies in the difference between the continuous deposition during DC sputtering and the pulsed nature of laser deposition. Re-orientation of the film grains requires interpulse relaxation time, when the film is not exposed to ion bombardment. Insufficient relaxation time (high deposition rate during PLD), or absence of relaxation time (DC sputtering) does not allow recrystallisation of the film grains into the lowest energy orientation. These considerations are in good agreement with the deposition rate dependence of YBCO film orientation on
the bare (1 2 0)N substrate. Low deposition rate provides sufficient relaxation time to re-orient the film into pseudo-c orientation with lowest surface energy, but at high deposition rate the interfacial conditions between the film and the substrate become dominant and result in formation of pseudo-a-oriented films. The dependence of the film properties on the deposition temperature is usual for YBCO deposition on CeO2 layers. An increase of the deposition temperature from 700 C results in improvement of both crystal structure quality and electrical properties until at 750 C the first signs of
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chemical interaction between CeO2 and deposited material appear: the particle density on the film surface increases, and additional peaks can be seen on the X-ray h/2h-scans. Further increase of deposition temperature results in deterioration of the film properties. Deposition under non-optimal conditions may result in a film of mixed orientation, containing both c-oriented and tilted-axes grains (Figs. 5a, and 7). The part of c-oriented grains in the films increased with an increase of the miscut angle in constant deposition conditions. The morphology of mixed-orientation film is shown on Fig. 8c, both elongated and rounded grains can be seen simultaneously. The elongation direction corresponds to the tilt axis [0 0 1]N. The quality of the c-oriented film depends also on the thickness of the seeding layer. With decrease of CeO2 thickness the orientational quality (rocking curve width, orientation along substrate plane) of the c-oriented film decreases, and for seeding layer thicknesses less than ˚ the formation of c-oriented films was suppressed. 100 A The reason for such behavior is unclear: chemical interaction between seeding layer and growing YBCO film could explain it, but no signs of such interaction were detected by AFM or diffractional studies for deposition temperatures below 740 C. The CeO2 layer crystal structure can be significantly modified by strain, introduced by the substrate [21], and this re-structuring of the CeO2 layer towards the perovskite substrate could explain tilted YBCO growth on a thin CeO2 layer. Such modifications diminish with an increase of the layer thickness, so the ‘‘native’’ CeO2 lattice on the surface of a thick CeO2 layer can be the reason for c-oriented YBCO growth. Unfortunately, in our YBCO/ CeO2/NdGaO3 system the lattice mismatch is so small that no significant changes in the CeO2 layer lattice could be introduced by strain. 3.4. Bi-epitaxial YBCO thin films Bi-epitaxial YBCO thin films were fabricated on CeO2/ NdGaO3 heterostructures by removal of CeO2 seeding layer from part of the substrate by ion-milling. The milled depth was determined using the pre-calibrated milling rate and exceeded the thickness of CeO2 layer. The studies of step-edge junctions showed the possibility of growing epitaxial YBCO thin film growth on an ion-beam milled perovskite surface. The YBCO thin film was deposited at optimal conditions for c-oriented film formation. A rocking curve in a wide angular range, taken from such a sample, is shown in Fig. 5b. Both the c-oriented part peak (right) and the epitaxial tilted-axes film peak (left) can be seen. The tilted-axes part of the film is strictly oriented along the (1 1 0)N plane, in contrast with the mixed-orientation film (Figs. 5a and 7). The Bragg diffraction angle during this measurement was 2h = 54.9, corresponding to the ˚ ). c lattice constant of the tilted-axes YBCO film (11.7 A The Bragg diffraction angle for the c-oriented part is differ˚ ), so the intensity of the correent (2h = 55.15, c = 11.65 A
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sponding peak on the scan is smaller than at the optimal Bragg angle. The actual volume of the two parts of the film is equal. The surface morphology of the film is shown in Fig. 8a and b; two different patterns are clearly seen. No mixedgrain areas like Fig. 8c were observed. The bi-epitaxial films were patterned using photolithography and wet etching for electrical characterization of the films and the bi-epitaxial junctions. The results of the measurements will be published elsewhere. 4. Conclusions YBCO thin film deposition on miscut NdGaO3 substrates with and without seeding CeO2 layer was studied. c-oriented film formation was observed for low deposition rates and high miscut angles 14–26. YBCO films with differently oriented parts (out-of-plane bi-epitaxial films) were fabricated and characterized using X-ray diffractional techniques and AFM. Acknowledgment The work was partially supported by the INTAS grant No. 01-0249. One of the authors (I.B.) is grateful to the Foundation for Science and Technology of Portugal for financial support. References [1] G. Blatter, V.B. Geshkenbein, L.B. Ioffe, Phys. Rev. B 63 (17) (2001) 174511. [2] R.R. Schulz, B. Chesca, B. Goetz, et al., Physica C 341 (2000) 1651. [3] W.K. Neils, D.J. Van Harlingen, S. Oh, et al., Physica C 368 (2002) 261. [4] P.B. Mozhaev, I.V. Borisenko, E.G. Ovsyannikova, et al., Physica C 372–376 (2002) 150. [5] K. Char, M.S. Colclough, S.M. Garrison, et al., Appl. Phys. Lett. 59 (6) (1991) 733. [6] Yu. Boikov, Z.G. Ivanov, G. Brorsson, T. Claeson, Supercond. Sci. Technol. 7 (1994) 281. [7] U. Poppe, Y.Y. Divin, M.I. Faley, et al., in: Proceedings of ASC’2000, 2000. [8] F. Tafuri, F. Miletto Granocio, F. Carillo, et al., Physica C 326 (1999) 63. [9] I.K. Bdikin, P.B. Mozhaev, G.A. Ovsyannikov, et al., Physica C 377 (2002) 26. [10] J.H. Kim, D. Youm, Physica C 275 (1997) 273. [11] M.G. Norton, B. Moeckly, C.B. Carter, R.A. Buhrman, J. Cryst. Growth 114 (1991) 258. [12] P.B. Mozhaev, G.A. Ovsyannikov, J.L. Skov, Zhurnal Telhnicheskoi Fiziki 44 (2) (1999) 242 (in Russian). [13] P.B. Mozhaev, J.E. Mozhaeva, I.K. Bdikin, et al., in: Proceedings of ICMNE’2003, Proceedings of SPIE, vol. 5401, 2004, p. 597. [14] F. Miletto Granozio, U. Scotti di Uccio, J. Cryst. Growth 174 (1–4) (1997) 409. [15] Yu.Ya. Divin, U. Poppe, J.-W. Seo, B. Kabius, K. Urban, Physica C 235 (1994) 675. [16] K. Hermann, G. Kunkel, M. Siegel, J. Schubert, W. Zander, A.I. Braginski, C.L. Jia, B. Kabius, K. Urban, J. Appl. Phys. 78 (2) (1995) 1131. [17] Y.Y. Divin, U. Poppe, C.L. Jia, et al., in: Proceedings of EUCAS’99, 1999.
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[18] P.B. Mozhaev, J.E. Mozhaeva, I.K. Bdikin, O.G. Rybchenko, Crystallogr. Rep. 49 (Suppl. 1) (2004) S129. [19] E. Stepantsov, M. Tarasov, A. Kalabuhov, et al., J. Appl. Phys. 96 (6) (2004) 3357.
[20] Y.A. Boikov, T. Claeson, D. Erts, F. Bridges, Z. Kvitky, Phys. Rev. B 56 (17) (1997) 11312. [21] G. Linker, R. Smithey, J. Geerk, F. Ratzel, R. Schneider, A. Zaitsev, Thin Solid Films 471 (2005) 320.
Out-of-substrate plane orientation control of thin YBa2Cu3Ox films on NdGaO3 tilted-axes substrates Peter B. Mozhaev a,b,*, Julia E. Mozhaeva a,b, Igor K. Bdikin c,d, Iosif M. Kotelyanskii e, Valery A. Lusanov e, Jørn Bindslev Hansen b, Claus S. Jacobsen b, Andrey L. Kholkin d b
a Institute of Physics and Technology RAS, Moscow 117218, Russia Department of Physics, Technical University of Denmark, Lyngby, DK-2800, Denmark c Institute of Solid State Physics RAS, Chernogolovka, Moscow distr., 142432, Russia d CICECO, University of Aveiro, Aveiro 3810-193, Portugal e Institute of Radio Engineering and Electronics RAS, Moscow 125009, Russia
Received 27 September 2005; received in revised form 1 December 2005; accepted 7 December 2005 Available online 24 January 2006
Abstract Epitaxial heterostructures YBa2Cu3Ox(YBCO)/CeO2/NdGaO3 were prepared on tilted-axes NdGaO3 substrates using laser ablation technique. Morphology, crystal structure and electrical properties of the obtained films were characterized. The seeding mechanisms are affected by the tilt angle, resulting in superior YBCO films on NdGaO3 substrates in an intermediate range of tilt angles of 6–14. The introduction of CeO2 layer leads to change of the YBCO film orientation: at low deposition rate c-oriented films are formed, while at high deposition rates the film grows with c-axis tilted along the [1 1 0] NdGaO3 direction. Bi-epitaxial films and structures were prepared by removal of part of the CeO2 layer using ion-beam milling. 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 74.78.Bz Keywords: Bi-epitaxial films; Seeding layer; Tilted-axes substrates
1. Introduction d-Wave order parameter symmetry and high gap energy make high-temperature superconductors (HTSC) a good choice for fabrication of qubits for quantum computations. Formation of Josephson junctions with equilibrium phase shift equal to p—the p-junctions—allows preparation of the ‘‘quiet’’ qubit [1], one of the most promising qubit types. p-junctions in all-HTSC circuits were demonstrated using a complicated four-crystal design [2], signs of p-junction formation in a more simple bi-crystal configuration were observed in [3,4]. Bi-epitaxial technique [5] eliminates *
Corresponding author. Address: Institute of Physics and Technology RAS, Moscow 117218, Russia. Tel.: +7095 3324894; fax: +7095 1293141. E-mail address: [email protected] (P.B. Mozhaev). 0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.12.003
geometrical limitations on the HTSC circuit design, but electrical properties of such junctions were shown to be inferior compared to the bi-crystal ones [6]. Out-of-substrate-plane tilt of the HTSC thin film crystallographic axes, often called [1 0 0]-tilt, significantly improves Josephson junction quality and was extensively studied over the last five years, both in bi-crystal [7] and in bi-epitaxial [8] design. For the latter technique the p-junction formation was demonstrated when CeO2 fluorite-type seeding layer was utilized [8]. The bi-epitaxial technique with out-of-substrate-plane tilt, developed in [8], utilizes the (1 1 0) SrTiO3 substrate, providing the single tilt angle of 45 and the only tilt direction around the {1 0 0} thin film axis. The possibility to prepare bi-epitaxial thin films and structures for other tilt angles was demonstrated in [9], while Kim and Youm
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[10] fabricated Josephson junctions for a tilt angle of 30 using miscut SrTiO3 substrate. In both cases the seeding layer of fluorite crystal structure (CeO2, Y:ZrO2) provided c-oriented growth of HTSC YBa2Cu3Ox (YBCO) independently of the substrate and seeding layer orientation. Similar effects were observed for other non-perovskite miscut substrates and seeding layers, for example, MgO [11]. The mechanism of this c-oriented growth, as far as we know, was not studied, and neither deposition parameters, nor limiting factors were determined. We studied the YBCO growth on miscut NdGaO3 substrates for the miscut angles 5–27, with and without implementation of a seeding CeO2 layer. We determined the deposition parameters for c-oriented growth and prepared bi-epitaxial thin films in this system. 2. Experimental NdGaO3 substrates and CeO2 seeding layer were chosen because their lattice constants show the best match to the (ab) plane of the YBCO thin film: lattice mismatch along the {1 1 0} YBCO directions is 0.32% and 0.64%, correspondingly. As a consequence, the growth mode is less affected by the strains caused by the lattice mismatch. We used (1 1 0)-oriented NdGaO3 substrates as standard, because the (1 1 0) plane of NdGaO3 provides more isotropic surface lattice (rectangular, translation distances ˚ and d[110] = 7.72(2) A ˚ ) compared with d[001] = 7.72(7) A another commonly used (0 0 1) plane of NdGaO3. The miscut substrates were formed by rotation of the substrate plane from the (1 1 0) crystallographic plane around the [0 0 1] axis of NdGaO3. This approach makes it possible to check the miscut angle c by X-ray diffraction from the (h k 0) planes of NdGaO3. The substrate surface was polished using a specially developed chemical–mechanical process. CeO2 seeding layer is known to provide additional in-substrate-plane rotation of the crystallographic axes by 45. Such mutual mis-orientation of the parts of a bi-epitaxial film is necessary to obtain p-junctions. The YBCO thin films and CeO2 seeding layers were deposited by laser ablation of stoichiometric ceramic targets. The details of the technique can be found elsewhere [12]. The parameters of the CeO2 deposition were optimized to obtain smooth thin film of single orientation (0 0 1) CeO2k(1 1 0) NGO on a standard (1 1 0) NdGaO3 substrate. Substrate temperature during deposition was held at 800 C, ablation was done at energy density of 1.75 J/cm2 in 0.5 mbar of a mixture of argon and oxygen (Ar/O2 = 7/3). The deposition rate at these conditions ˚ /pulse, thickness of the film varied from was about 3 A ˚ . No post-deposition annealing was per100 to 900 A formed; the film was cooled down to room temperature in working atmosphere at maximal possible rate. The YBCO thin films deposition parameters (700– 800 C, 1.3 J/cm2, oxygen partial pressure 0.15 mbar) were optimized to obtain the highest temperature of the superconducting transition Tc. To decrease the surface rough-
ness the deposition was performed in Ar/O2 = 8/2 mixture, the total pressure was 0.8 mbar. Typical deposi˚ /pulse, and typical tion rate for these conditions was 0.6 A ˚ film thickness was 1500 A. A pre-bake step (annealing of the substrate before deposition at 800 C in 5 mbar of oxygen) significantly decreased both size and density of the particles on the thin film surface. Post-deposition annealing was performed at 450 C in 800 mbar of oxygen for 1 h. All YBCO films showed Tc higher than 89 K and narrow superconducting transition, proving good uniformity of the film structure. The structural properties of thin films and multilayers were studied using X-ray diffraction technique, the surface morphology was observed by SEM and AFM. Electrical properties of the superconducting films were measured with non-contact techniques. The rocking curve in a wide angular range technique allows rapid determination of mutual orientation of the crystallographic planes of the thin film, substrate surface plane and substrate crystallographic planes ([9], Fig. 1). The 2h angle between the incident and reflected X-ray beam (characteristic radiation) is set equal to the Bragg diffraction value for one of the film diffraction peaks. For a certain x angle, corresponding to the angular position of the chosen film crystallographic plane, a broad peak on the scan will be observed. The X-ray broadband radiation is not eliminated by a monochromator or a filter, so for each crystallographic plane of the substrate X-ray radiation of some wavelength will satisfy the Bragg equation. At the corresponding x angles narrow substrate reflection
Fig. 1. Measurement of inclination angles between the substrate and film crystallographic planes using rocking curve in a wide angular range technique. Inset: geometrical setup of the measurement.
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peaks will be seen. The angular distance between the peaks is the inclination angle between corresponding substrate and film crystallographic planes [9]. Utilization of this technique was made possible by the fact that [1 0 0] (or [0 1 0]) YBCO and [1 1 0] CeO2 axes were parallel to the substrate tilt direction [0 0 1] NdGaO3 for all studied samples. 3. Results and discussion To avoid misunderstanding we will use the following notations: • the substrate plane is the plane of substrate surface; • the crystallographic planes and directions are marked with indexes N, C and Y, corresponding to NdGaO3 substrate, CeO2 seeding layer and YBCO thin film; • the miscut angle c is the angle between the substrate plane and the (1 1 0)N; • the tilt angle is the angle between crystallographic plane of CeO2 seeding layer or YBCO thin film, usually (0 0 1)C and (0 0 1)Y, and the substrate plane; • the inclination angle is the angle between crystallographic planes of the substrate and a film, or between the film and the seeding layer; • the mis-orientation is the spread of orientations of individual grains of the film or the seeding layer around the average orientation.
3.1. YBCO thin films on miscut NdGaO3 substrates Detailed results of our study of YBCO thin film structure, growth mechanisms, and electrical properties on miscut NdGaO3 substrates can be found elsewhere [13]. Briefly, the YBCO thin films grow in agreement with the epitaxial relations h1 0 0iY ð0 0 1ÞY k½0 0 1N ð1 1 0ÞN
ð1Þ
for all studied angles, except for the special case of (1 2 0)N substrate (miscut angle 18.4). We did not observe the pseudo-a oriented domains ((0 0 1)Yk(1 1 0)N) dominance at miscut angles higher than 20, as it was in [9]. Probably, the careful optimization of the deposition processes on (1 1 0)N substrates, performed in this study before depositions on the miscut substrates, resulted in formation of thermodynamically more favorable pseudo-c oriented grains [14] for miscut angles up to 26. Formation of perpendicular-oriented domains on tilted-axes substrates [15] and on etched step edges [16] was also observed for miscut angles higher than 30. We have found three angular ranges of YBCO thin film morphology on miscut substrates. The AFM microphotographs of typical films are shown in Fig. 2. For low miscut angles (vicinal range) the structure and morphology of the films do not differ much from that of a film deposited on a standard (1 1 0)N-oriented substrate. Increase of the miscut angle leads to an increase of the grain size, with improved
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surface smoothness, better crystal quality, and improved electrical parameters [13]. The angular borders of this medium range depend on the deposition technique and for laser ablation are 6–11. Increase of the miscut angle over 11 results in formation of high steps on the film surface (‘‘step bunching’’), with corresponding inferior structural and electrical parameters of the films. The difference between the three angular ranges is best illustrated by dependence of the inclination angle between the (0 0 1)Y plane and (1 1 0)N plane on the miscut angle of the substrate (Fig. 3). Grain collisions in the beginning of the deposition and strain introduced by the lattice mismatch between the substrate and the film lead to slight inclination of the (0 0 1)Y plane from the (1 1 0)N plane of the substrate. Substrate miscut introduces steps of oneunit-cell height on the substrate surface with density proportional to tangent of the miscut angle. Such steps act as seeding centers for the thin film grains. At certain density of the steps all seeding occurs at the step edges, and grain collisions decrease due to regular distance between the seeds. As a result, the atomic planes of the film are finely aligned with the substrate planes (Fig. 3, medium part). ‘‘Step bunching’’, taking place at high miscut angles, again increase strain in the film and some of this strain is released as inclination of the (0 0 1)Y plane. Note, that no strain is introduced along [0 0 1]N direction, and the film remains well aligned with the substrate planes in this direction. Our conclusion that seeding center density is the main factor determining the YBCO thin film formation is corroborated by the fact that the ‘‘medium’’ angular range is different for laser ablation and DC sputtering deposition techniques (1.5–3). The average grain size in these two cases is 0.35 and 1.2 lm, respectively, for the films, deposited on standard (1 1 0) NdGaO3 substrates. This value can be considered to be a measure of the mean free path of the adatoms on the substrate surface. The four times greater grain size for DC sputtering can be caused by ion bombardment of the substrate surface, suppressing adatom incorporation into the growing film lattice and increasing the mobility of the adatoms along the surface. The average distance between the steps along the substrate surface for the medium angular range is 5 times longer for DC ˚ ) compared with the laser ablation sputtering (73–147 A ˚ ). One seed on a standard-oriented subtechnique (16–32 A strate corresponds to 100–150 one-unit-cell steps of the substrate. The actual grain size of the tilted-axes film of the medium range is greater than that of the standard-oriented film (Fig. 4), and corresponds to 300 one-unit-cell steps. The lattice mismatch along the c-axis of the film is 1/150 of the substrate lattice constant (considering pseudo-cubic perovskite lattice). This agreement leads us to the hypothesis that the lattice strain along the c-axis is the limiting factor of the grain size for an optimally seeded film. Further increase of the miscut angle to the high-angle range results in a decrease of step width (w, grain size across terraces in Fig. 4), in agreement with this hypothesis.
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Fig. 2. Typical AFM microprofilograms of the YBCO thin films on NdGaO3 miscut substrates. The miscut angles are 1.5, 5 and 18.4, the color scale is 40, 150 and 70 nm, respectively.
We plotted a fitting curve (step height h = const, dashed curve in Fig. 4), but the observed decrease of w is faster than the calculated curve. A possible reason is intergrowth of the grains in the medium angular range, increasing the observed average grain size. A correct approximation can be done only in the high-angle range, where we have too few points. The relation hw = const, observed in [9], decreases even more slowly. The epitaxial relations between the film and the substrate were the same for all studied orientations except the (1 2 0)N substrate. This orientation in orthorhombic notation corresponds to (3 1 0)N orientation in the pseudo-cubic notation; i.e., the surface of such substrate represents a set of one-unit-cell steps with average width
of three-unit-cells. The NdGaO3 lattice constant in the ˚ , hence, the step width corpseudo-cubic notation is 3.86 A ˚ ). Formation responds well to the c-axis of the film (11.7 A of a (1 0 9)Y-oriented YBCO film with c-axis oriented along the steps plane ((0 0 1)Yk(1 1 0)N) is promoted by the absence of anti-phase boundaries, typical for the films with standard epitaxial relations (1). This effect was observed and explained for the first time in [17]. We found that the orientation of the film on the (1 2 0)N substrate depends on the deposition rate. At low deposition rate (pulse repetition rate 2 Hz, giving deposition rate of ˚ /min) the film followed standard epitaxial relations 72 A ˚ /min) the (1), while at high deposition rate (10 Hz, 360 A resulting film was 90% (1 0 9)Y-oriented. This result dem-
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saturation values (high deposition rates), while at low deposition rate the film has enough time to relax to a thermodynamical equilibrium state, which is determined by the minimal surface energy of the film [14]. 3.2. CeO2 thin films on miscut NdGaO3 substrates The CeO2 seeding layer for all studied miscut angles and deposition parameters grows with a single orientation close to the standard epitaxial relations for CeO2 growth on perovskite substrates: ½1 1 0ð0 0 1ÞCeO2 k½0 0 1ð1 1 0ÞNdGaO3
Fig. 3. Orientational parameters of the YBCO thin films on the miscut NdGaO3 substrates. Bottom: miscut angle dependence of inclination angles between (1 1 0)N and (0 0 1)Y planes around the substrate tilt axis [0 0 1]N (dashed line) and around perpendicular in-substrate-plane direction (solid line). Top: width of the rocking curve of the (0 0 5)Y diffractional peak, measured along the substrate tilt axis [0 0 1]N.
Fig. 4. Dependence of grain size of the YBCO thin film on the miscut angle of the NdGaO3 substrate along (top line) and across (bottom line) the [0 0 1]N axis. The dashed line shows h = const fit of the experimental data. The vertical dotted lines mark the angular ranges of different growth modes.
onstrates the predominant effect of the film–substrate interface on the orientation of the growing film for high super-
ð2Þ
The actual growth mode can be characterized as a texture with the texture axis [1 1 0]Ck[0 0 1]N, and dominant orientation of the (0 0 1)C inclined by 3–6 from the (1 1 0)N plane from the substrate plane (Fig. 5a and b). Single orientation of the film was confirmed by measurements of the rocking curves in a wide angular range with Bragg reflection angle corresponding to other film crystallographic planes. With an increase of the substrate miscut angle the inclination angle between the film and substrate crystallographic planes increased, and showed a maximum at 14– 18 (Fig. 6). The reasons for such discrepancy from the standard epitaxial relation (0 0 1)Ck(1 1 0)N are not clear, as well as the mechanism of the inclination formation. A similar discrepancy between (0 0 1)C and (1 1 0)N planes with (0 0 1)C inclination from the substrate plane was observed in [9] (type II epitaxy). Our calculations of exact orientation of the CeO2 films gives a surprising result: the type IIa epitaxy for the (1 2 0)N substrate gives in fact a (1 1 3)C-oriented CeO2 film, while type IIb epitaxy for the (1 3 0)N substrate results in a (2 2 1)C-oriented CeO2 film. The small-index planes of (h h l) family are known to become stable for some etching processes, so we cannot exclude growth of a ‘‘strangely’’-oriented CeO2 thin film on a miscut substrate. The inclination of the (0 0 1)C plane from the crystallographic planes of a sapphire substrate was observed in [18], but the film crystallographic plane was tilted towards the substrate plane. This effect was explained as minimization of the surface energy when the surface of the film is close to the small-index crystallographic plane. In our case neither surface energy minimization, nor stabilization of ‘‘strange’’ small-index planes can explain the observed inclination of the (0 0 1)C planes. As a possible clue to solving the problem we note the proportionality between the substrate miscut angle c and the inclination angle between the crystallographic planes of the film and the substrate (see the straight fit in Fig. 5). After 6 inclination is achieved, the inclination angle abruptly drops to 2–3. The lattice constant of the CeO2 layer calculated from the position of the (0 0 2)C Bragg reflection peak was ˚ for all samples, suggesting high oxygen con5.41 ± 0.005 A tents in the film. The width of h/2h-peaks was close to the theoretical minimum due to thickness broadening. The width of the (0 0 2)C peak never exceeded 0.5, and the
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Fig. 6. Dependences of inclination between film and substrate crystallographic planes in the epitaxial heterostructures YBCO/CeO2/NdGaO3 on substrate miscut angle. The YBCO films were obtained at high deposition rates. The dashed lines are approximating first four points of each dependence by equation y = const Æ x.
rocking curve broadening is a spread of the orientation of individual grains from the main orientation. ˚ thick CeO2 seeding layer The roughness of the 300 A ˚ measured by AFM and profile meter was less than 20 A for all miscut angles. No specific features could be seen on the seeding layer surface, probably due to lack of accuracy of the measurement (the resolution of the used AFM ˚ ). mode was 15 A 3.3. YBCO thin films over CeO2 seeding layer on miscut NdGaO3 substrates
Fig. 5. Rocking curves in a wide angular range taken from epitaxial heterostructures YBCO/CeO2/NdGaO3 on miscut substrates. Zero x angle corresponds to the substrate plane. Top curve corresponds to (0 0 4) CeO2 Bragg diffraction angle (69.4), bottom curve corresponds to (0 0 7) YBCO Bragg diffraction angle (54.9). (a) Miscut angle 18.4 (the (1 2 0)N substrate), high YBCO deposition rate; (b) miscut angle 23.4, low YBCO deposition rate, the CeO2 seeding layer is partially removed by ion-beam milling.
˚ thick film is about theoretical broadening for the 300 A 0.3. The rocking curve width was 0.7–0.8 and it did not significantly change with 2h. Probably the reason for the
The orientation and crystal quality of the YBCO thin films over CeO2 seeding layer on miscut NdGaO3 substrates showed strong dependence on deposition rate, miscut angle, and seeding layer thickness. ˚ /min) the YBCO At high deposition rate (10 Hz, 360 A thin film showed textured growth with the texture axis h1 0 0iYk[1 1 0]Ck[0 0 1]N and small (1–5) inclination of the dominant (0 0 1)Y plane orientation both from the (1 1 0)N and (0 0 1)C planes (Fig. 5a). The orientation of the film was confirmed by measurement of the rocking curves in a wide angular range with Bragg reflection angle corresponding to other film crystallographic planes. For example, in Fig. 5a the (1 0 8)Y peak can be seen on the top curve (Bragg angle for (0 0 4)C reflection 2h = 69.4 is close to the Bragg angle for (1 0 8)Y reflection 2h = 68.9), corresponding to the intensive left YBCO peak on the bottom curve (angular distance between (0 0 1)Y and (1 0 8)Y planes
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is 20.6). This growth mode is very close to the standard epitaxial relations of the YBCO thin film growth on a bare NdGaO3 miscut substrate (1), the same way as the growth mode of CeO2 seeding layer is close to the standard epitaxial relations of CeO2 growth on perovskite substrates (2). The (0 0 1)Y plane was inclined from the substrate (1 1 0)N plane from the substrate surface plane, maximal inclination was observed at 20–24 (Fig. 6). Similar to CeO2 growth, the inclination angle is proportional to the miscut angle, until an inclination of 6 is achieved. No clear relation between CeO2 seeding layer and YBCO thin film can be seen. At low miscut angles the (0 0 1)Y plane was inclined from (0 0 1)C towards the substrate plane, but for miscut angles higher than 20 the (0 0 1)Y plane showed inclination from seeding layer crystallographic plane from the substrate plane. This result differs from the (0 0 1)Y tilt towards the substrate plane, observed in [11,19], minimizing the surface energy of the film similarly to CeO2 in [18]. We note that in all cases [11,18,19] the lattice mismatch between the film and substrate was about 10%. Relatively weak bonds at the film–substrate interface allow tilt of the film axes to decrease film surface energy. In this study the mismatch of all layers of heterostructure YBCO/ CeO2/NdGaO3 is within 1%, providing much stronger bonds at the interfaces. The width of the X-ray diffraction peaks on h/2h-scans (Fig. 7, upper curve on inset) was rather small (0.5–0.8), and close to that of YBCO thin films on bare substrates, confirming high crystal quality of the YBCO thin films obtained at high deposition rates. The rocking curve width is broader than that of the YBCO thin film on a bare substrate, in agreement with texture growth mode compared with epitaxial growth. Typical AFM microprofilogram (Fig. 8a) does not differ much from the ones of the YBCO film deposited on a bare NdGaO3 miscut substrate (Fig. 2c). The c-oriented films were obtained when the laser repe˚/ tition rate was decreased to 2 Hz (deposition rate 72 A min), and only for miscut angles higher than 14 (see right YBCO peak on Fig. 5b, bottom curve). No directional features or big steps can be seen on the surface of such film (Fig. 8b). The rocking curve width of the c-oriented films did not change much with increasing 2h angle, characterizing spread of the grain mis-orientation. For high miscut angles (c > 16) the rocking curve maximum corresponded to the substrate plane position within accuracy of the measurement (0.05), and the width of the rocking curve was 1.5– 5. For small miscut angles the rocking curve width increased to 7–12, and the (0 0 1)Y plane tilted from the substrate plane irregularly by 1–5. We can conclude that the observed films are texturised with the same texture axis as the tilted YBCO films at high deposition rate (h1 0 0iYk[1 1 0]Ck[0 0 1]N), but the dominant orientation of the (0 0 1)Y plane is turned from (1 1 0)N to the substrate plane position.
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Fig. 7. Rocking curve in a wide angular range for the (0 0 1 1)Y diffraction peak, taken from the epitaxial heterostructure YBCO/CeO2/NdGaO3 on a substrate with miscut angle 26.6 (the (1 3 0)N substrate). On inset: h/2hscan of the c-oriented part of the YBCO film (bottom curve) and of the tilted-axes YBCO film (upper curve).
The h/2h-scans (Fig. 7, bottom curve on inset) of the coriented films showed very sharp peaks, close to the theoretical limit due to the thickness of the films. The c-oriented films probably consist of grains of very high crystal quality and low internal strain, but mis-oriented from the substrate plane over a relatively broad range. The c lattice constant, determined using the (0 0 l) family of peaks on the h/2hscans, is smaller than that of the films deposited on a bare ˚ and 11.7 ± 0.02 A ˚ , corresubstrate (11.675 ± 0.015 A spondingly). This decrease of the c lattice constant is typical for deposition on CeO2 and can be explained by tensile strain introduced into the film by the seeding layer [20]. The reason for the re-orientation of YBCO thin film grown on a non-perovskite substrate is minimization of surface energy when the surface of the film is terminated with the crystallographic planes of minimal energy ((0 0 1)Y for YBCO). This effect was described in many papers (for example, [9–11]), but the influence of the deposition rate was not studied. The c-orientation of the YBCO thin film grown on a CeO2 layer using pulsed laser deposition technique in [9] was attributed to the high deposition rate compared with DC sputtering deposition technique resulting in tilted-axes YBCO films. Our present results are contradictory to this conclusion: the c-oriented film is formed at low deposition rate, while increase of deposition
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Fig. 8. Typical AFM microprofilograms of the epitaxial heterostructures YBCO/CeO2/NdGaO3 on miscut substrates. (a) and (b) bi-epitaxial film on the 18.4-miscut substrate, the tilted-axes YBCO film on the bare substrate and the c-oriented YBCO film, correspondingly; (c) mixed-orientation thin film, miscut angle 26.6. The color scale is 50, 75, and 160 nm, correspondingly.
rate results in tilt of the YBCO film axes. Probably the reason lies in the difference between the continuous deposition during DC sputtering and the pulsed nature of laser deposition. Re-orientation of the film grains requires interpulse relaxation time, when the film is not exposed to ion bombardment. Insufficient relaxation time (high deposition rate during PLD), or absence of relaxation time (DC sputtering) does not allow recrystallisation of the film grains into the lowest energy orientation. These considerations are in good agreement with the deposition rate dependence of YBCO film orientation on
the bare (1 2 0)N substrate. Low deposition rate provides sufficient relaxation time to re-orient the film into pseudo-c orientation with lowest surface energy, but at high deposition rate the interfacial conditions between the film and the substrate become dominant and result in formation of pseudo-a-oriented films. The dependence of the film properties on the deposition temperature is usual for YBCO deposition on CeO2 layers. An increase of the deposition temperature from 700 C results in improvement of both crystal structure quality and electrical properties until at 750 C the first signs of
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chemical interaction between CeO2 and deposited material appear: the particle density on the film surface increases, and additional peaks can be seen on the X-ray h/2h-scans. Further increase of deposition temperature results in deterioration of the film properties. Deposition under non-optimal conditions may result in a film of mixed orientation, containing both c-oriented and tilted-axes grains (Figs. 5a, and 7). The part of c-oriented grains in the films increased with an increase of the miscut angle in constant deposition conditions. The morphology of mixed-orientation film is shown on Fig. 8c, both elongated and rounded grains can be seen simultaneously. The elongation direction corresponds to the tilt axis [0 0 1]N. The quality of the c-oriented film depends also on the thickness of the seeding layer. With decrease of CeO2 thickness the orientational quality (rocking curve width, orientation along substrate plane) of the c-oriented film decreases, and for seeding layer thicknesses less than ˚ the formation of c-oriented films was suppressed. 100 A The reason for such behavior is unclear: chemical interaction between seeding layer and growing YBCO film could explain it, but no signs of such interaction were detected by AFM or diffractional studies for deposition temperatures below 740 C. The CeO2 layer crystal structure can be significantly modified by strain, introduced by the substrate [21], and this re-structuring of the CeO2 layer towards the perovskite substrate could explain tilted YBCO growth on a thin CeO2 layer. Such modifications diminish with an increase of the layer thickness, so the ‘‘native’’ CeO2 lattice on the surface of a thick CeO2 layer can be the reason for c-oriented YBCO growth. Unfortunately, in our YBCO/ CeO2/NdGaO3 system the lattice mismatch is so small that no significant changes in the CeO2 layer lattice could be introduced by strain. 3.4. Bi-epitaxial YBCO thin films Bi-epitaxial YBCO thin films were fabricated on CeO2/ NdGaO3 heterostructures by removal of CeO2 seeding layer from part of the substrate by ion-milling. The milled depth was determined using the pre-calibrated milling rate and exceeded the thickness of CeO2 layer. The studies of step-edge junctions showed the possibility of growing epitaxial YBCO thin film growth on an ion-beam milled perovskite surface. The YBCO thin film was deposited at optimal conditions for c-oriented film formation. A rocking curve in a wide angular range, taken from such a sample, is shown in Fig. 5b. Both the c-oriented part peak (right) and the epitaxial tilted-axes film peak (left) can be seen. The tilted-axes part of the film is strictly oriented along the (1 1 0)N plane, in contrast with the mixed-orientation film (Figs. 5a and 7). The Bragg diffraction angle during this measurement was 2h = 54.9, corresponding to the ˚ ). c lattice constant of the tilted-axes YBCO film (11.7 A The Bragg diffraction angle for the c-oriented part is differ˚ ), so the intensity of the correent (2h = 55.15, c = 11.65 A
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sponding peak on the scan is smaller than at the optimal Bragg angle. The actual volume of the two parts of the film is equal. The surface morphology of the film is shown in Fig. 8a and b; two different patterns are clearly seen. No mixedgrain areas like Fig. 8c were observed. The bi-epitaxial films were patterned using photolithography and wet etching for electrical characterization of the films and the bi-epitaxial junctions. The results of the measurements will be published elsewhere. 4. Conclusions YBCO thin film deposition on miscut NdGaO3 substrates with and without seeding CeO2 layer was studied. c-oriented film formation was observed for low deposition rates and high miscut angles 14–26. YBCO films with differently oriented parts (out-of-plane bi-epitaxial films) were fabricated and characterized using X-ray diffractional techniques and AFM. Acknowledgment The work was partially supported by the INTAS grant No. 01-0249. One of the authors (I.B.) is grateful to the Foundation for Science and Technology of Portugal for financial support. References [1] G. Blatter, V.B. Geshkenbein, L.B. Ioffe, Phys. Rev. B 63 (17) (2001) 174511. [2] R.R. Schulz, B. Chesca, B. Goetz, et al., Physica C 341 (2000) 1651. [3] W.K. Neils, D.J. Van Harlingen, S. Oh, et al., Physica C 368 (2002) 261. [4] P.B. Mozhaev, I.V. Borisenko, E.G. Ovsyannikova, et al., Physica C 372–376 (2002) 150. [5] K. Char, M.S. Colclough, S.M. Garrison, et al., Appl. Phys. Lett. 59 (6) (1991) 733. [6] Yu. Boikov, Z.G. Ivanov, G. Brorsson, T. Claeson, Supercond. Sci. Technol. 7 (1994) 281. [7] U. Poppe, Y.Y. Divin, M.I. Faley, et al., in: Proceedings of ASC’2000, 2000. [8] F. Tafuri, F. Miletto Granocio, F. Carillo, et al., Physica C 326 (1999) 63. [9] I.K. Bdikin, P.B. Mozhaev, G.A. Ovsyannikov, et al., Physica C 377 (2002) 26. [10] J.H. Kim, D. Youm, Physica C 275 (1997) 273. [11] M.G. Norton, B. Moeckly, C.B. Carter, R.A. Buhrman, J. Cryst. Growth 114 (1991) 258. [12] P.B. Mozhaev, G.A. Ovsyannikov, J.L. Skov, Zhurnal Telhnicheskoi Fiziki 44 (2) (1999) 242 (in Russian). [13] P.B. Mozhaev, J.E. Mozhaeva, I.K. Bdikin, et al., in: Proceedings of ICMNE’2003, Proceedings of SPIE, vol. 5401, 2004, p. 597. [14] F. Miletto Granozio, U. Scotti di Uccio, J. Cryst. Growth 174 (1–4) (1997) 409. [15] Yu.Ya. Divin, U. Poppe, J.-W. Seo, B. Kabius, K. Urban, Physica C 235 (1994) 675. [16] K. Hermann, G. Kunkel, M. Siegel, J. Schubert, W. Zander, A.I. Braginski, C.L. Jia, B. Kabius, K. Urban, J. Appl. Phys. 78 (2) (1995) 1131. [17] Y.Y. Divin, U. Poppe, C.L. Jia, et al., in: Proceedings of EUCAS’99, 1999.
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