Defects in Strained Epitaxial SrRuO 3 Films on SrTiO 3 Substrates

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1 Materials Transations, Vol. 48, No. 10 (2007) pp to 2562 Speial Issue on Advanes in Eletron Mirosopy for Materials Charaterization #2007 The Japan Institute of Metals Defets in Strained Epitaxial Films on Substrates Sang Ho Oh*, Ju Hyung Suh and Chan Gyung Park Department of Materials Siene and Engineering, Pohang University of Siene and Tehnology, Pohang , Korea Transmission eletron mirosopy (TEM) analyses of the defets formed in epitaxial films on (001) substrates are reported. With preparing three different forms of TEM speimens, i.e. plan-view, ross-setional and free-standing speimens, various TEM tehniques were implemented with plaing emphasis on the effet of misfit strain on the defet formation. With in-situ TEM heating observations, the present TEM results provide insights into the formation mehanism of misfit disloations, the ourrene of anti-phase boundary ribbons near the misfit disloations, and the strutural phase transitions of epitaxial perovskite films. [doi: /matertrans.md200713] (Reeived April 3, 2007; Aepted May 21, 2007; Published July 19, 2007) Keywords: transmission eletron mirosopy, strontium ruthenium oxide, misfit disloation, anti-phase boundary, orthorhombi domain 1. Introdution is a metalli ondutive oxide that rystallizes in an orthorhombi distorted perovskite-type struture with the lattie parameters of a ¼ 0:5538 nm, b ¼ 0:5573 nm, and ¼ 0:7856 nm at room temperature. 1) Beause of the small distortions in RuO 6 otahedra arrangement, the rystal an be regarded as a pseudo-ubi struture with a lattie parameter of a 0 ¼ 0:393 nm. has a lattie parameter of nm and is a standard substrate used for the epitaxial growth of perovskite films. The / film/substrate system shows the smallest known lattie misfit (0.64%) in perovskite oxide ombinations. The heteroepitaxial / films have been extensively explored reently as bottom eletrodes or buffer layers for growing high-quality ferroeletri and superonduting oxides beause the grown films are eletrially ondutive, struturally ompatible, and hemially stable when in ontat with most of the perovskite oxides. 2 4) Understanding and ontrol of the defets forming in an epitaxial film are important beause they an growin to subsequent ative overlayers or modify their growth modes. Although the defets suh as misfit disloations and orientation domains in films are regarded as being energetially stable, the assoiated defets an ause strutural imperfetions at the film surfae or the interfae with substrate. Misfit disloations relax the aumulating misfit strain over a ertain film thikness. 5) Misfit disloations are, however, inevitably assoiated with threading disloations whih extend and deposit interfaial segments. The threading disloations an degrade the flatness of film at the intersetion with film surfae. 6) In addition, threading disloations are also known to deteriorate eletrial properties of epitaxial films. The orthorhombi struture of may result in the formation of orientation domains on the substrate. The orthorhombi orientation domains are linked by orthogonal twin relationships at their boundaries. 7) The resulting twin boundaries in a film an indue a roughness at *Present address: Materials Siene and Tehnology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. shoh@ornl.gov the intersetion with film surfae. In this artile, we present transmission eletron mirosopy (TEM) analyses of the misfit disloations, the anti-phase boundary (APB) ribbons and the orthorhombi orientation domains generated in strained films grown on (001) substrates. The harateristis of defets are investigated by using various TEM tehniques and the effet of misfit strain on the generation of these defets is disussed. 2. Experimental Details Epitaxial films were grown on (001) substrates by ion beam sputtering. (001) substrates with the dimension of :5 mm 3 were used, of whih one side was polished in optial grade. The misut angle of the (001) surfae was about 0.2. The TiO 2 - terminated (001) surfae was prepared by treating the rystal with a ph-ontrolled NH 4 F-HF solution, following the method desribed by Kawasaki et al. 8) The ething seletively removes SrO terminated pathes on the surfae, resulting in a single TiO 2 -terminated surfae with a unit ell step height. films were grown on both non-ethed and ethed substrates. The films with various thiknesses were deposited: 5, 7, 10, 20, 50, 75,, 200, and 300 nm at 800 C. The deposition rate was maintained onstant at 0.04 nm s 1 for all films. Further details on the film growth are given elsewhere. 5,6) In the present artile, we onentrate on the three film thikness levels, i.e. 10 nm, 20 nm and 75 nm. In the presene of misfit disloations, all of the films showed semi-oherent interfaes with substrates. 5) A field emission mirosope operating at 200 kv (JEM- 2F, JEOL) was used for TEM. In-situ TEM heating was performed in a onventional TEM (JEM-200 CX, JEOL) by using a double-tilt hot stage holder. The plan-view and rosssetional TEM speimens were prepared following onventional methods onsisting of dimpling and ion milling for eletron transpareny. A free-standing film was prepared by dissolving the substrate in a HF: HNO 3 :H 2 O solution mixed in approximately 1:1:1ratio. The free-standing film was supported in a standard arbon support film on Cu grid. The Burgers vetor of misfit

2 Defets in Strained Epitaxial Films on Substrates 2557 disloations was determined by g b invisibility riteria, high resolution transmission eletron mirosopy (HRTEM), and large-angle onvergent eletron beam diffration (LAC- BED). In a bright-field (BF) LACBED, the substrate disloations onneted to misfit disloations showed pronouned g b interations with the refletion lines in the LACBED disk, whih results in the formation of n nodes at the rossing position aording to g b ¼ n (g = refletion plane indies; b ¼ Burgers vetor; n ¼ integer). We used the onvention desribed by Tanaka et al. for the determination of the relative sign of n. 9) In addition to the two-beam BF imaging of disloations, the onventional g=3g weak beam dark field (WBDF) imaging was used, espeially for the sample with a high density of misfit disloations. The evolution of rystal struture and orthorhombi orientation domains of the films were determined by seleted-area eletron diffration (SAED). The rystallographi diretion and plane indies of were indexed based on the pseudo-ubi struture, unless stated otherwise. 3. Results and Disussion 3.1 Misfit disloations Figure 1 and show ross-setional view of a set of misfit disloation array in two-beam BF and WBDF imaging modes, respetively. The 20 nm thik was partially relaxed with forming a network of misfit disloations along the two equivalent hi diretions (the orresponding planview TEM images of this sample are shown in Fig. 4 and 7). The measured disloation spaing was 120{150 nm in average. 5) At the partiular two-beam ondition of g ¼ 040, the array of misfit disloations lying along the [] viewing diretion is visible in a projetion whereas the other array along the [] diretion is invisible due to the g b ¼ 0.Itis observed that several misfit disloations (e.g. BC in Fig. 1) are onneted to pre-existing disloations in the substrate ( AB ). (see also the long threading disloation in Fig. 3, whih is also one of suh substrate disloations). This indiates that the substrate disloations are working as effetive soures for the nuleation of misfit disloations. A shemati drawing in Fig. 1() outlines the disloation onfiguration. The threading omponent, as indiated by CD, is not shown in Fig. 1 and. The Burgers vetor of misfit disloations was determined by g b reations between the high order Laue zone (HOLZ) lines in a BF LACBED pattern and a substrate disloation onneted to misfit disloation. The 10 nm thik film was used in plan-view [001] orientation. This film thikness was hosen as the density of misfit disloations was muh lower than that in 20 nm thik sample. At least three g b reations were made for the unambiguous determination of b, as displayed shematially in Fig. 2. Counting the number of nodes at the rossing point and simple alulation yield that b is ah011i type. 5,10) A typial pattern obtained with the g ¼ 240 is shown in Fig. 2. When we define the line diretion of disloations as [], the Burgers vetor of misfit disloation is perpendiular to the line (edge type) and, at the same time, it is inlined by 45 with respet to the interfae plane, due to the existene of the out-of-plane omponent ah001i. The in-plane edge omponent of Burgers vetor A () [001] [] B [] C g 040 g 040 B D 20 nm ahi works for the relaxation of misfit strain whereas the out-of-plane omponent vetor parallel to the growth diretion annot aommodate misfit strain. As a strained epitaxial film is growing thiker than the ritial thikness, the aumulating misfit strain begins being relaxed by introduing disloations. The potential soures for the nuleation of misfit disloations are either pre-existing disloations in the substrate or inhomogeneities at the film surfae or both. As a film is grown epitaxially, the preexisting disloations in a substrate are easily extended into the growing film. Above the ritial thikness these threading disloations are fored to bow and move (by glide or limb) in the film, thereby laying down an interfaial segment at (or near) the interfae. The disloation density in ommerially available (001) substrate is known to be an order of m 2. 11) In the plan-view TEM observations of A C 20 nm Fig. 1 Cross-setional two-beam BF and WBDF image of 20 nm thik film grown on an ethed substrate. The two-beam ondition was g ¼ 040. Only the misfit disloations lying along the [] diretion are visible in a projetion. () A shemati drawing of the observed disloation onfiguration. AB indiates a pre-existing disloation in the substrate, BC an interfaial segment, and CD a threading segment of misfit disloation.

3 2558 S. H. Oh, J. H. Suh and C. G. Park 220 [] g 220 [] g Fig. 2 A shemati illustration of a BF LACBED pattern. The refletion lines used for g b interations are shown with the indies. The g b reations between a substrate disloation and various refletion lines were obtained by moving the sample position, as exemplified by retangles. An example of BF LACBED pattern. The retangle in highlights the g b reation of a substrate disloation with ð240þ refletion line, yielding n ¼ nm and 20 nm thik films, the disloations onneted to the interfaial segments of misfit disloations are muh longer than the film thikness. Obviously, it indiates that suh long disloations are not the threading segments whih propagate in the film but the substrate disloations. The short threading segments (orresponding to CD in Fig. 1()) are not frequently observed, implying that one nuleated, threading disloations propagate aross the whole sample area. The nuleation of disloation half-loops at stress onentrations of the film surfae is a plausible mehanism but would be more diffiult than the extension of the substrate disloations. This might be true for the present films, onsidering that they grew by the flow of atomi steps, i.e. step flow growth mehanism. 6) If a three-dimensional island Fig. 3 Plan-view two-beam BF images of 20 nm thik film on a non-ethed substrate taken at g ¼ 220 and g ¼ 200 diffration onditions. APB ribbons make alternating diffration ontrast around the misfit disloations. growth mode prevailed for the growth of, the introdution of misfit disloation through the boundaries of islands would be an additional soure during the oalesene stage. For the present / system, we suggest that the most effetive disloation soure is the extension of pre-existing disloations in. It should be addressed though that the substrate disloations in a single rystal (an order of m 2 ) an not fully aount for the observed disloation density on the interfae plane, whih is estimated to be m 2. One may onsider a higher density of disloations near the edges of small square substrates of, presumably introdued during utting or grinding proesses APB ribbons near the misfit disloations The presene of APBs was observed adjaent to the misfit disloations. Figure 3 and show two-beam BF images of the 20 nm thik film grown on a non-ethed substrate. The APB ribbons exhibited an inverse diffration ontrast aross their outer boundaries ompared to the regular staking region, whih varied depending on the two-beam ondition due to their harateristi displaement vetors.

4 Defets in Strained Epitaxial Films on Substrates 2559 g nm 1 nm Fig. 4 A plan-view WBDF image taken at g ¼ 020. A 20 nm thik film grown on an ethed substrate was used. APB ribbons make bright ontrast around the misfit disloations. The onfiguration of APBs with respet to the misfit disloations indiates that they were formed during the propagation stage of the threading segments of misfit disloations. Muh narrower widths of APBs were observed for the films grown on the ethed substrates, where the density of misfit disloations was also higher than that observed on the non-ethed substrate. Figure 4 shows a planview WBDF image obtained from the 20 nm thik film grown on an ethed substrate. The APB ribbons bounding misfit disloations appeared in bright ontrast, of whih the width is a few 10 nm. In general, an APB is haraterized by a rystallographi shear vetor R desribing the relative displaement of the two parts of the rystal on either side of the boundary. 12) The rystallographi shear vetor of an APB is analogous to the Burgers vetor of the disloation that produes a staking fault on glide. The perturbation in atomi staking indued by APB was observed using HRTEM in ross-setional hi diretion. Figure 5 shows a h110i Bragg-filtered HRTEM image of a misfit disloation loated at the interfae in an end-on projetion (the raw image an be found in Ref. 5)). The misfit disloation is displayed as the two partial edge disloations with Burgers vetors of a=2h011i type with the same sense, whih makes a sum of ah011i (denoted by the disloation symbol? in blak olor). The presene of additional partial disloations with (projeted) Burgers vetor of a=2h011i type an be identified along the interfae (the disloation symbol? in white olor). In eah pair at both sides of the misfit disloation, a partial disloation is followed by another partial disloation whih has Burgers vetor with the opposite sense. Plan-view TEM observation showed that the APB ribbons were very narrow in the 75 nm thik film grown on an ethed substrate, only a few nm in width. The extra half planes of partial disloations are indiated by white lines. Sine the Burgers vetor of a=2h011i is not a lattie translation vetor in the perovskite struture, a partial disloation makes the fault in staking sequene. 12) The following partial disloation with the opposite sense restores the original staking sequene. Thus, Fig. 5 A h110i Bragg-filtered HRTEM image of the / interfae in ross-setional [] orientation. A misfit disloation is loated at the enter of the interfae, whih is displayed as the two neighboring partial disloations with b ¼ a=2h011i in the same sense (disloation symbol? in blak olor). In both sides of misfit disloation, APB ribbons onsist of the two partial disloations with b ¼ a=2h011i in the opposite sense. the staking fault bounded by two partial disloations with Burgers vetors with the opposite sense results in the formation of an APB domain. It should be addressed that there exists ambiguity in the determination of full Burgers vetor of the partial disloations in the HRTEM image (i.e. whether it is a=2h011i or a=2h111i) due to the projetive nature of HRTEM image along the [] diretion. Referring to the onfiguration, the formation of APB is losely related to the propagation of misfit disloation suh that the strain field around the disloation ore may failitate the lattie shearing of APB. Although the partial disloations in APB arry the in-plane edge omponents, i.e. a=2hi, their ontributions to the misfit relaxation are not signifiant sine they are aneled eah other in a pair. Instead, it an effetively aommodate loal non-stoihiometry in the film. Oh and Park reported that a small amount of exess Ru in an be aommodated as a planar defet struture onsisting of edge-shared RuO 6 otahedra with a displaement vetor a=2h110i. 12) Further exess of Ru above the solubility limit in, however, resulted in the formation of Ru preipitates. 12) Figure 6 shows the Ru preipitates formed in a Ru-exess film. The Ru preipitates were grown in rod-like shapes and were oriented preferentially along the h110i diretions of the substrate. The interfae between Ru preipitate and substrate was inoherent due to the large lattie misfit (5.3%). Near the preipitates the lattie ohereny between the film and the substrate was loally disturbed. The experimental findings that the formation of APB was more pronouned on a non-ethed (001) surfae further supports the role of APB. The SrO surfae pathes with half unit ell height present on the non-ethed (001) surfae might attrat RuO 2 preferentially, leading to the formation of APB with respet to the film growing on neighboring TiO 2 terminations.

5 2560 S. H. Oh, J. H. Suh and C. G. Park g nm Ru g nm 200 nm Fig. 6 A plan-view BF image showing the formation of Ru preipitates in a Ru-exess film. The preipitates were oriented along the h110i diretions of substrate. A ross-setional HREM image of one of the Ru preipitates in an end-on projetion. Fig. 7 A plan-view [001] zone-axis SAED pattern of 20 nm thik film grown on an ethed substrate. Enlarged view of a small setion in is presented with the orthorhombi -axis marked. A two-beam BF image taken at g ¼ 440 diffration ondition. (d) () () (d) Fig. 8 A plan-view [001] zone-axis SAED pattern of a 20 nm thik free-standing film. The diffration spots used for DF images in, () and (d) are indiated., () and (d) The orresponding DF images indiated in. The orientation of the orthorhombi -axis is marked in the exited domains in eah DF image.

6 Defets in Strained Epitaxial Films on Substrates Orthorhombi orientation domains The SAED pattern of a partially relaxed 20 nm thik film is shown in Fig. 7. The refletions originating from the orthorhombi symmetry of are notieable in the diffration pattern (marked by the orthorhombi -axis in the enlarged view). The dark-field (DF) images formed by eah orthorhombi refletion, however, yield diffuse orientation maps (the images are not shown here). The asymmetri distribution of misfit disloations with non-uniform spaings (Fig. 7) reflets that loal strain is not uniform in the film, whih leads to a weak orrelation in the tilting of RuO 6 otahedra over only a short distane. One obvious evidene of the orthorhombi symmetry is the generation of srew disloations running along the h110i diretions. These disloations aommodate non-orthogonal symmetry of the film lattie on the ubi lattie of substrate. 5,13) In ontrast, the free-standing film showed distintive domain patterns with forming well-defined domain boundaries. Figures 8 (d) show a SAED pattern and the DF images formed by eah orthorhombi refletion marked in. In the absene of misfit strain, the whole area of film beame stabilized in the orthorhombi struture with forming a salient orientation domain pattern. In-situ TEM heating of a free-standing film diretly shows that the strain-free undergoes a series of strutural phase transitions. Figure 9 shows a BF image and a SAED pattern taken at room temperature. The region of interest was bent down in a onave shape, showing a zoneaxis refletion pattern in the real spae image. The diffration pattern indiates that all possible orientation domains exist. At around 450 C, the 1= refletions disappeared in the diffration pattern while the growth of orthogonal twins is observed in the BF image (blak arrows). The diffration spots whih are indiative of the a and/or the b axes of the orthorhombi struture are still visible (white arrow). These results imply that the orthorhombi struture was transformed to a tetragonal struture (P4=mm) with a 0:557 nm and 0:395 nm. The tilting of RuO 6 otahedra along the - axis ourred suh that the out-of-phase arrangement of orner-shared otahedra beame in-phase, whih results in the redution of unit ell volume by half. The tetragonal phase nuleated with a twin relationship to the orthorhombi matrix. At 600 C, the tetragonal refletions disappeared ompletely, as a result of the seond phase transition to the ubi phase (Pm3m) witha 0:397 nm. These results reflet that at the growth temperature (800 C) the stable phase of is the ubi phase and, thus, the film is grown in pseudomorphi to the ubi lattie of. The tilting of RuO 6 otahedra that leads to the strutural phase transitions ours during the subsequent ooling stage. The tilting of RuO 6 otahedra depends ritially on the residual misfit stain in the film so does the evolution of orthorhombi orientation domains. 6) The previous studies reported that the ideal ubi phase (spae group, Pm3m; a > 0:393 nm) is transformed to the orthorhombi phase (Pbnm; a ¼ 0:5538 nm, b ¼ 0:5573, and ¼ 0:7856 nm) via a transient tetragonal phase with the spae group of either P4=mbm 14) or I4=mm. 15) The transformation is asribed to the result of rigid tilting of RuO 6 otahedra about all three axes. The out-of-phase tilting of the () RT otahedra results in a multipliation of the unit ell; the unit ell of the orthorhombi struture is four times that of the ideal ubi perovskite struture and ontains 20 atoms in four formula units. From a mirostrutural point of view, the phase transition an result in the formation of orientation domains, whih are related by orthogonal twins. In a highly strained film, however, the phase transitions an be suppressed, whih results in the stabilization of film into a pseudomorphi (ubi) struture even down to room temperature. 6) The evolution of orientation domains is, thus, in turn, losely related to the density and symmetry of disloation network. 6) 4. Conlusion ~450 o C ~600 o C Fig. 9 A plan-view BF image and the orresponding SAED pattern of a 75 nm thik free-standing film at room temperature, at 450 C and () at 600 C. The orthorhombi refletions orresponding to the a, b, and axes are indiated in. The twins nuleated upon the transition from orthorhombi to tetragonal struture are indiated by blak arrows in. The tetragonal refletion orresponding to 1=a periodiity, a 0:557 nm, is marked by white arrow in. We investigated the strutural imperfetions enountered in the growth of epitaxial films on (001) substrates by TEM. A ross-grid pattern of misfit disloations with ah011i Burgers vetor was formed at the interfae along the two equivalent hi diretions. The effetive Burgers vetor for the relaxation of misfit strain is the in-plane edge omponent of ahi. TEM observations revealed that the a a/b

7 2562 S. H. Oh, J. H. Suh and C. G. Park pre-existing disloations in the substrate at as effetive soures for the generation of misfit disloations. The formation of APB ribbons was observed along the misfit disloations. Wider APB ribbons were observed when film was grown on a non-ethed substrate. The formation of APB ribbons is assoiated with the aommodation of inhomogeneities in the surfae termination of substrate and/or in the loal omposition of growing film. During the ooling stage a (partially) relaxed film undergoes the strutural phase transitions whih are aounted by the tilting of RuO 6 otahedra. In-situ TEM heating of a free-standing film verifies that is deposited as a high-temperature stable ubi phase at the growth temperature, whih is also pseudomorphi to the ubi lattie of. The evolution of orientation domains is influened by loal residual strain in the film, suh that it tends to suppress the tilting of RuO 6 otahedra to the same orientation over a ertain distane. The strain effet is manifested, upon the removal of substrate, as a free-standing film develops a stable domain pattern with well-defined boundaries. REFERENCES 1) C. B. Eom, R. J. Cava, R. M. Fleming, J. M. Philips, R. B. van Dover, J. H. Marshall, J. W. P. Hsu, J. J. Krajewski and W. F. Pek, Jr.: Siene 258 (1992) ) Q. X. Jia, X. D. Wu, S. R. Foltyn and P. Tiwari: Appl. Phys. Lett. 66 (1995) ) S. C. Gausepohl, M. Lee, L. Antogonazza and K. Char: Appl. Phys. Lett. 67 (1995) ) J. Yin, Z. G. Liu and Z. C. Wu: Appl. Phys. Lett. 75 (1999) ) S. H. Oh and C. G. Park: J. Appl. Phys. 95 (2004) ) S. H. Oh and C. G. Park: J. Mater. Res. 21 (2006) ) J. C. Jiang, X. Q. Pan and C. L. Chen: Appl. Phys. Lett. 72 (1998) ) M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuhiya, M. Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto and H. Koinuma: Siene 266 (1994) ) M. Tanaka, R. Saito, K. Ueno and Y. Harada: J. Eletron. Miros. 29 (1980) ) N. D. Zakharov, K. M. Satyalaksmi, G. Koren and D. Hesse: J. Mater. Res. 14 (1999) ) J. Yamanaka: Mater. Trans., JIM 40 (1999) ) S. H. Oh and C. G. Park: Phil. Mag 83 (2003) ) J. W. Matthews: Phil. Mag. 29 (1974) ) J.-P. Maria, H. L. MKinstry and S. Trolier-MKinstry: Appl. Phys. Lett. 76 (2000) ) J. C. Jiang and X. Q. Pan: Phil. Mag. Lett. 80 (2000)