ARTICLE IN PRESS                                              Journal of Crystal Growth 310 (2008) 1991–1998              ...
ARTICLE IN PRESS1992                                 H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998have ext...
ARTICLE IN PRESS                                       H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998      ...
ARTICLE IN PRESS1994                                                H. Du et al. / Journal of Crystal Growth 310 (2008) 19...
ARTICLE IN PRESS                                         H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998    ...
ARTICLE IN PRESS1996                                        H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998e...
ARTICLE IN PRESS                                      H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998       ...
ARTICLE IN PRESS1998                                    H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998[18] ...
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Growth and structural characterization of epitaxial Ba0.6Sr0.4TiO3 films deposited on REScO3(1 1 0) (RE ¼ Dy, Gd) substrates using pulsed laser deposition

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Growth and structural characterization of epitaxial Ba0.6Sr0.4TiO3 films deposited on REScO3(1 1 0) (RE ¼ Dy, Gd) substrates using pulsed laser deposition

  1. 1. ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 1991–1998 www.elsevier.com/locate/jcrysgroGrowth and structural characterization of epitaxial Ba0.6Sr0.4TiO3 films deposited on REScO3(1 1 0) (RE ¼ Dy, Gd) substrates using pulsed laser deposition Hui Dua, Patrick J. Fishera, Marek Skowronskia, Paul A. Salvadora,Ã, O. Maksimovb a Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA b Material Research Institute, Pennsylvania State University, University Park, PA 16802, USA Available online 19 November 2007Abstract Ba0.6Sr0.4TiO3 films were deposited by pulsed laser deposition on orthorhombic REScO3(1 1 0) (RE ¼ Dy, Gd) single-crystalsubstrates. Films were investigated for their growth mode, crystalline quality, and strain states. Substrates were treated prior to growth toproduce atomically flat surfaces having wide terraces (E200 nm) and clear unit-cell-high steps. Atomic force microscopy and reflectionhigh-energy electron diffraction indicated that the films grew epitaxially in a two-dimensional (2D) layer-by-layer mode. X-raydiffraction showed that all films (200 nm thick and less) were coherently/orthorhombically strained to the substrate according to the ¯epitaxial relationship: (0 0 1)filmJ(1 1 0)substrate; [1 0 0]filmJ[0 0 1]substrate (and [0 1 0]filmJ[1 1 0]substrate). (0 0 2) rocking curves were 17 and20 arcsec wide for films grown on RE ¼ Dy and Gd, respectively.These films have rocking curve widths and dislocation densities that areseveral orders of magnitude lower than a film grown on SrTiO3 (0 0 1).r 2007 Elsevier B.V. All rights reserved.PACS: 81.15.Fg; 61.05.jh; 77.55.+fKeywords: A1. Defects; A3. Pulsed laser deposition; B1. Barium strontium titanate; B1. Perovskites; B1. REScO3 (RE ¼ Dy; Gd); B2. Dielectric materials1. Introduction the cause of these diminished values, such as lower dielectric constants, lower tunabilities, and/or increased (Ba,Sr)TiO3 films have been investigated extensively dielectric losses.because they exhibit high dielectric constants that are Examples of such defects include so-called ‘‘dead layers’’tunable into the microwave frequencies [1] as well as near the interface between electrodes and the functionalferroelectric properties [2,3], rendering them useful as film [4,5], inhomogeneous stresses associated with the hightunable capacitors for RF communications [1] and as a density [6] of threading dislocations (41010 cmÀ2) andmedia for non-volatile memory applications [2]. For vacancies, and charge carriers introduced by high levels oftunable RF applications, Ba0.6Sr0.4TiO3 has been widely point defects [3,7,8]. Additionally, ‘‘dead layers’’ have beenstudied since its Curie temperature is just below the room suggested to be associated with other interfacial defects,temperature, leading to a high and tunable dielectric such as misfit dislocations generated to accommodate theconstant and lower losses than other compositions. mismatch between the crystal structures of the film andUnfortunately, (Ba,Sr)TiO3 films exhibit properties that substrate [7–9]. Nevertheless, conclusive experimental dataare less attractive for these applications when compared to that links property degradation to a specific defect haveknown bulk values [1–3]. It is widely believed that defects been difficult to generate owing to the difficulties related to(that are largely absent in bulk crystals and ceramics) are controlling independently each of these defects, particu- larly in generating films having low dislocation densities ÃCorresponding author. Tel.: +1 412 268 2702; fax: +1 412 268 3113. and low strain states. In this work, we will show that high- E-mail address: paul7@andrew.cmu.edu (P.A. Salvador). quality epitaxial films of Ba0.6Sr0.4TiO3 can be realized to0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jcrysgro.2007.10.086
  2. 2. ARTICLE IN PRESS1992 H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998have extremely low dislocation densities; these will Thirdly, their lattice constants and thermal expansionultimately allow for one to decouple the influence of strain coefficients are very close to the bulk values for Ba0.6Sr0.4-and dislocation densities on the structure–property rela- TiO3 [16]. Lastly, both BaTiO3 [15] and SrTiO3 [17] filmstions of thin films. have been grown on these substrates and have been During thin-film growth, at least two sources of reported to have novel dielectric and ferroelectric proper-substrate-related strain energy accumulation occur in thin ties owing to the films’ high quality and coherently strainedfilms: a lattice parameter mismatch and a thermal natures. Compared to either BaTiO3 or SrTiO3, Ba0.6Sr0.4-expansion mismatch. The lattice mismatch, or the differ- TiO3 has a much lower mismatch with these substrates and,ence in the periodicity of bonding patterns in the interfacial as such, it should yield much better films having even fewerplanes between the substrate and film, causes the film to dislocations than the end-member compositions. In thisstrain in an effort to avoid a large interfacial energy report, we present the surface preparation of REScO3increase that would arise from broken bonds. This causes substrates and the subsequent 2D growth of Ba0.6Sr0.4TiO3strain energy to accumulate in the film as the thickness films that have structural qualities similar to those of theincreases [9,10]; when the thickness reaches a critical value, REScO3 substrates. Ba0.6Sr0.4TiO3 films were also depos-dislocations are generated to relax the strain energy. Such ited on SrTiO3 (0 0 1) for comparison.dislocations that form to accommodate the lattice mis-match are called misfit dislocations and, while they tend to 2. Experimental proceduremove ultimately to the interface and may participate in thedead layer property degradation, in their motion through A 2 in diameter cylindrical Ba0.6Sr0.4TiO3 target wasthe film they tend to leave dislocation segments that thread formed using standard ceramic processing methods [18] bythrough the film (i.e., threading dislocations). Threading reacting intimately mixed stoichiometric quantities ofdislocations result in residual inhomogeneous strains BaCO3, SrCO3, and TiO2 in air at 1100 1C for 12 h. Thethroughout the film [9–12] that can severely impact film powders were reground and then sintered in air at 1450 1Cproperties. for 6 h. X-ray diffraction (XRD) confirmed that the target Most epitaxial (single-crystal-like) perovskite dielectric was composed of single-phase Ba0.6Sr0.4TiO3 having athin films, such as (Ba,Sr)TiO3, have been grown on the ˚ lattice parameter 3.957 A, which is slightly smaller than thecommercially available perovskite single crystals, such as ˚ literature value of 3.964 A [1].SrTiO3 [13] or LaAlO3 [10], or rock salt single crystals, Commercial (CrysTec GmbH) REScO3(1 1 0) (RE ¼ Dysuch as MgO [14]. Although these substrates are single and Gd) Czochralski-method-grown [19] single-crystalcrystals, either they have high dislocation densities substrates were obtained as 10 Â 10 Â 0.5 mm3 coupons.themselves [13], which the films inherit during growth The as-received substrates had their (1 0 0) surfacesand results in a lower bound for the film dislocation density polished to angstrom level roughness using chemical–me-that is high, or they have large lattice mismatches with the chanical polishing. These substrates were then treated tofilm, causing high threading dislocation contents to form produce high-quality-terraced surfaces in a manner similarduring the relaxation process. Films grown on these to that reported for SrTiO3 crystals [20]. Briefly, thesubstrates have a high density of dislocations [10,13,14]. substrates were ultrasonically cleaned with acetone and To minimize the concentration of dislocations in a film, methanol for 5 min each, and were then ultrasonicallythe substrate should have a low dislocation density and a cleaned in deionized water for 15 min. Next, the substratesclose lattice mismatch to the film, thereby preventing the were etched in a commercial semiconductor-grade buf-film from inheriting dislocations or generating new ones fered-HF solution (Sigma Aldrich, pH ¼ 4) for 40 s. Later,during relaxation of the mismatch [10–12]. Two additional they were annealed in air at 1000 1C for 1.5 h.features help minimize the dislocation content in thin films: The film growth was carried out in a hybrid systemminimizing the thermal expansion mismatch between the capable of depositing films using either pulsed laserfilm and substrate and controlling the growth mode to be a deposition (PLD) and/or molecular beam epitaxy (MBE)two-dimensional (2D) layer-by-layer mode [10]. Minimiz- (PVD Products, Inc.), although only the PLD techniqueing the thermal expansion mismatch means that the strain was carried out in this work. A schematic of the mainstate does not change significantly during cooling from the chamber is given in Fig. 1. Samples are introduced into thegrowth temperature to room temperature, preventing main chamber through a separately pumped introductionfurther introduction of dislocations. Also, 2D growth chamber. The base pressure of the main chamber isinhibits specific types of dislocation nucleation, which maintained at 10À8 Torr using 1260 l/s turbo pumpedhelps limit the number of dislocations. backed by a dry pump. The substrates are moved into In this work, orthorhombic REScO3 (RE ¼ Dy and Gd) the heater assembly and heated from their backside using asingle-crystal substrates were used. Firstly, they have been SiC heater that is not in direct contact with the substrates.reported to have low dislocation contents (or high crystal The temperature of the substrate reported here corre-qualities) [15]. Secondly, they have a perovskite crystal sponds to that of the heater coil; there is likely a 100 1Cstructure and their (1 1 0) planes have nearly square meshes drop at the substrate face. A differentially pumped electronthat are similar to (0 0 1) planes of (Ba,Sr)TiO3 [16]. gun (STAIB) is mounted on the chamber and used to carry
  3. 3. ARTICLE IN PRESS H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998 1993 gain E0.2, a proportional gain E0.4, and an amplitude set point around 1.5–2.0 nV. XRD and X-ray reflectivity (XRR) were carried out using a Phillips X’Pert system (Philips Analytical X-ray B.V., The Netherlands). Both out-of-plane and in-plane structural characterizations (yÀ2y, o, and f scans) were done using lens mode optics [22,23]. Reflectometry optics were used for the reflectivity measurements. High-resolution XRD was carried out using a hybrid optical module, in which an X-ray mirror and a 4 Â Ge(2 2 0) crystal monochromator were inserted in the incident beam path and a triple-axis Ge(2 2 0) 3-bounce channel-cut analyzer with a divergence of 12 arcsec was inserted in the diffracted beam path. HF etching (pH=4 for 1, 3, and 5 min) was used to determine dislocation densities in the substrates by count-Fig. 1. Schematic of the laser–MBE system used in this research. L ing the dislocation etch pits observed by optical micro-represents the target-to-substrate distance (75 mm). scopy and AFM. Transmission electron microscopy (TEM) was used to characterize the dislocation density in theout reflection high-energy electron diffraction (RHEED) at reference films grown on SrTiO3(0 0 1).pressures up to 1 mTorr. The off-center position of the PLD target carousel is 3. Results and discussiondesigned to allow for the PLD plasma and beams from allsix effusion cell ports to impinge directly on the substrate. It is well known that the nature of the substrate surface,One of the effusion cell ports was fitted with a gas injector including both the number and type of surface steps andcapable of supplying a reactive gas such that it directly the chemical termination, can affect film growth and filmfaces the substrate surface. To improve the homogeneity of quality. In this research, treatment conditions previouslythe film, the substrate holder rotates during growth and a applied to SrTiO3 (0 0 1) surfaces [20] (described above) ledrastering mirror (located after the focusing lens in the to well-defined terraces on the REScO3(1 1 0) substrateoptical train) scans the focused laser beam over the entire surfaces. Fig. 2a shows the AFM topographic image from alength of the substrate surface (i.e., it crosses over the 4 Â 4 mm2 area of the GdScO3 substrate (similar resultstarget center line in the chamber). The rastering feature (in were found for DyScO3). The terraces have wavy step edgescombination with the target rotation feature) also improves but uniform widths of E200 nm. The step heights arethe ablation characteristics from the target by preventing ˚ between 2 and 4 A, as shown in Fig. 2d, corresponding tocone formation as in Ref. [21] and references therein. 1 Â or 2 Â the distance between neighboring planes along PLD was carried out using a KrF laser (l ¼ 248 nm) the [1 1 0] direction of the distorted perovskite cell.focused to an areal laser energy of 1 J/cm2 at the target Generally, this implies that the surface has mixed GdOsurface and pulsed at a rate of 1 Hz. The substrate-to-target and ScO2 chemical terminations, with respect to the surfacedistance (L) was maintained at 75 mm. Samples were terminations of an ideal crystal, though more detailedheated to the deposition temperature at 30 1C/min in the experiments need to be carried out to pinpoint the surfacebackground pressure. Once at the deposition temperature, chemical information. Overall, the surface steps corre-the dynamic deposition atmosphere was established by spond to an atomically flat (1 1 0) surface with stepsthrottling the turbo pump to 66% of its maximum rotation accommodating a miscut angle of E0.11.speed and then feeding molecular O2 gas into the chamber Importantly, wide flat terraces are ideal for 2D growth ofthrough the gas injector at 27.3 sccm, using a mass flow Ba0.6Sr0.4TiO3 and the substrate step-heights correspond tocontroller. This procedure resulted in a chamber pressure 1 2 or 1 Â the unit cell height of cubic Ba0.6Sr0.4TiO3. Theof 7 Â 10À4 Torr. RHEED was carried out with the average growth rate of films was determined using X-rayfilament operating at 15 kV and 1.5 A current with an ˚ reflectrometry to be 0.24 A/pulse. Films were grown toapproximate incident angle of E31. Films were deposited three thicknesses (25, 60, and 200 nm) and then character-to obtain a specific thickness and then cooled down to ized for their surface morphology. Figs. 2b and c show theroom temperature at 30 1C/min in the deposition atmo- ex-situ AFM topographs of 25 and 200-nm-thicksphere. Ba0.6Sr0.4TiO3 films deposited on GdScO3, respectively. Atomic force microcsopy (AFM) was carried out on a These figures indicate that the films grow in a manner thatVeeco NanoScope Dimension 3100 (Veeco Instruments continually replicates the original terrace structure, sinceInc.) using the NanoScope Software (version 6.13). A the film surfaces are similar in nature to the substratetapping mode tip—1–10 O cm phosphorous (n)-doped Si surfaces, with nearly identical terrace widths. A fewtip (Model-RTESP, Veeco Instruments Inc.)—was islands are observable on the terraces in both figures,mounted and operated at a scan rate ¼ 1 Hz, an integral although they are more evident in the 200-nm-thick film.
  4. 4. ARTICLE IN PRESS1994 H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998 Substrate 25nm film 200nm film 2nm µ 1µm 1.2 0.4 0.4 0.8Height (nm) Height (nm) Height (nm) 0.2 0.2 0.4 0.0 0.0 0.0 -0.2 -0.2 -0.4 -0.4 -0.4 -0.8 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Distance X (micron) Distance X (micron) Distance X (micron)Fig. 2. Atomic force microscopy results from treated substrates and deposited films. Topographic images and corresponding line scans (normal to thesteps) are shown in (a) and (d) for the GdScO3(1 1 0) surface, in (b) and (e) for a 25-nm-thick Ba0.6Sr0.4TiO3(0 0 1) film and in (c) and (f) for a 200 nm thickBa0.6Sr0.4TiO3(0 0 1) film.The root-mean-square (RMS) roughness of the films did Fig. 3c shows the RHEED pattern from the 60-nm-thickincrease slightly with the film thickness, although the films Ba0.6Sr0.4TiO3 (0 0 1) film along the [1 0 0] azimuth (theare still remarkably flat: the RMS values were 0.13, 0.23, [0 1 0] had a similar pattern), which is parallel to theand 0.29 nm for films 25, 60, and 200 nm thick, respectively. substrate [0 0 1] azimuth. The film’s RHEED pattern is bestFigs. 2e and f show that the number of steps is increasing described as a streaky pattern having diffuse spots lying onwith growth time and that the terrace heights range from 1 2 a semicircle with the Kikuchi lines in the background,to 4 unit cells in the 200-nm-thick film. consistent with a fairly flat surface that has an increased These AFM results indicate that the films grow in a 2D number of steps compared to the substrate. The RHEEDlayer-by-layer mode, with a small probability that a second pattern became more streaky as the growth proceeded andlayer nucleates before the first layer is completed. By the the streaks were still observed for the 200-nm-thick films.time the film is 200 nm, the RMS roughness is still less than These results are consistent with the AFM study.12% of the total thickness and the accumulated maximum The RHEED patterns shown above are consistentterrace height is 4 unit cells, although the vast majority are with an epitaxial relationship between the film andlower than this. The 2D layer-by-layer growth mode results substrate, where the [1 0 0]filmJ[0 0 1]substrate (and [0 1 0]filmJin all of the steps advancing at the same speed and the ¯ [1 1 0]substrate). Comparing the RHEED patterns, oneterraces and steps can be replicated to the surface of the notices that the spacing between the spots in Fig. 3a is 1 2200-nm-thick film. that of the spacing between the streaks in Fig. 3c. This arises Fig. 3a shows the RHEED pattern collected from the from the fact that the substrate has a doubled periodicitytreated GdScO3 substrate (the DyScO3 substrate was when compared to the periodicities in cubic Ba0.6Sr0.4TiO3. ¯similar) along the [1 1 0] azimuth. The pattern is best The REScO3 substrates adopt an orthorhombically dis-described as sharp diffraction spots lying on a semicircle torted perovskite structure [18] whose unit cell has thewith the Kikuchi lines in the background, consistent with dimensions of O2ap, O2ap, 2ap (where ap is a cubican atomically flat surface having wide terraces as observed perovskite lattice parameter), while bulk Ba0.6Sr0.4TiO3in the AFM topographs. The image (Fig. 3b) along the adopts a cubic unit cell of dimensions ap, ap, ap. The[0 0 1] azimuth was similar to this one. Both surfaces RHEED patterns indicate that the surfaces of each materialexhibited diffraction spots that were consistent with an have a 1 Â 1 unreconstructed surface cell when compared tounreconstructed (1 Â 1) surface of an orthorhombic crystal the bulk materials. Moreover, the substrate does not inducehaving bulk-like surface periodicities. an apparent change in the surface unit cell of the film.
  5. 5. ARTICLE IN PRESS H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998 1995Fig. 3. (a, b) The RHEED patterns of treated GdScO3 along the azimuth ¯of [1 1 0] and [0 0 1] before growth, respectively and (c) the RHEEDpattern of the 60 nm film along the azimuth of Ba0.6Sr0.4TiO3 [1 0 0]. Fig. 4. X-ray diffraction results from a 200-nm-thick film of Ba0.6Sr0.4- TiO3 film deposited on DyScO3(1 1 0): (a) the out-of-plane y–2y scan with the (0 0 l) (l ¼ 1, 2) peaks from the film marked F and the (h h 0) (h ¼ 1, 2) XRD was carried out to determine the structural peaks of the DyScO3 substrate marked S. The inset in (a) is one of the in- plane yÀ2y scans around one of the film’s (1 0 1) reflections, which alsoparameters of the films in more detail. The out-of-plane shows a substrate’s (1 1 2) reflection and (b) F scans registered from theyÀ2y scan is shown in Fig. 4a and demonstrates that the DyScO3 {2 0 2} reflections and the films {1 1 1} reflections.Ba0.6Sr0.4TiO3 film is (0 0 1) oriented; only the (0 0 l) (l ¼ 1,2, 3) film peaks (marked F) and the (h h 0) substrate peaks(marked S) are observable. For the 200-nm-thick films F scan registered around film {1 1 1} peaks and substrategrown on GdScO3(1 1 0), the out-of-plane lattice constant {2 0 2} peaks. The epitaxial relationship was determinedwas determined to be c ¼ 4.01270.002 A, which is˚ to (0 0 1)filmJ(1 1 0)substrate; [1 0 0]filmJ[0 0 1]substrate (andconsiderably larger than the cubic lattice parameter of ¯ [0 1 0]filmJ[1 1 0]substrate), which are also consistent with the ˚the target (3.957 A) or the literature value of bulk RHEED patterns collected before and after growth (see ˚Ba06.Sr0.4TiO3 (3.964 A). Fig. 3). For the 200-nm-thick film grown on GdScO3(1 1 0), F scans and yÀ2y scans were registered for the the in-plane lattice parameters were calculated to befilms {1 0 1} (C ¼ 451) and {1 1 1} (C ¼ 54.71) reflec- ˚ ˚ a100 ¼ 3.96470.002 A, a010 ¼ 3.97670.002 A, which aretions to determine the in-plane lattice parameters and the essentially the same as the in-plane lattice parametersepitaxial relationship. The inset in Fig. 4a gives the measured for the substrate. In other words, the 200-nm-yÀ2y scan recorded for a (1 0 1) reflection, which includes thick film is orthorhombically distorted and coherentlythe substrate’s (1 1 2) reflection. Fig. 4b shows the strained to the substrate, within the error limits of the
  6. 6. ARTICLE IN PRESS1996 H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998experiments. Similar results were obtained on the 100-nm- although the lattice is strained, it appears that the Ti–Othick films grown on DyScO3 (1 1 0) (meaning the films bond network does not directly match the Sc–O bondwere coherently strained), with the lattice parameters network. More details are required to fully understand thisof the Ba0.6Sr0.4TiO3 film being c001 ¼ 3.99970.002 A, ˚ observation.a100 ¼ 3.94770.002 A ˚ , and a010 ¼ 3.95070.002 A. ˚ High-resolution XRD rocking curves (o scans) were The lattice parameters of the substrates have been used to characterize the quality of both the substrates and ˚reported to be: for GdScO3—a ¼ 5.746 A, b ¼ 5.488 A, ˚ the films. Fig. 5 shows these scans for (a) the (2 2 0) peak ofc ¼ 7.934 A ˚ , and for DyScO3—a ¼ 5.720 A, b ¼ 5.442 A, ˚ ˚ GdScO3, (b) the (0 0 2) peak of the Ba0.6Sr0.4TiO3 film on ˚c ¼ 7.890 A [16,18]. Although the (1 1 0) planes have nearly GdScO3 (1 1 0), (c) the (2 2 0) peak of DyScO3, and (d) the ¯square meshes formed by the orthogonal [1 1 0] and [0 0 1] (0 0 2) peak of the Ba0.6Sr0.4TiO3 film on DyGdScO3(1 1 0).directions, there remains a slight orthorhombic distortion All of these peaks had full-width at half-maximumand a doubling of the periodicities along these directions (FWHM) below 20 arcsec (the optics had a resolution ofcompared to the basic perovskite structure. The theoretical E12 arcsec), which are much lower than either typicalmismatch between Ba0.6Sr0.4TiO3 and these two substrates substrates, such as SrTiO3, or films grown on otheris about 70.2–0.4%, depending on the particular crystal substrates. Moreover, the films had only a marginaland direction. Nevertheless, each of these small mismatches increase in the FWHM values up to 200 nm. For example,would yield a strain relaxation critical thickness of about the FWHM of the 100-nm-thick film grown on DyS-150 nm (based on critical thickness calculation models cO3(1 1 0) was 17 arcsec (Fig. 5d), which was only 2 arcsec[11,12]). That the 200-nm-thick films appear to be fully larger than the 15 arcsec FWHM of the (2 2 0) substratestrained is not too surprising since the calculations are only peak (Fig. 5c). Similarly, the FWHM of the 200-nm-thickestimates and oxide films often remain strained to larger film grown on GdScO3(1 1 0) was 20 arcsec (Fig. 5b), whichvalues than these estimates predict. It should be noted that, was only 3 arcsec larger than the 17 arcsec FWHM of thealthough the XRD results imply that the films are (2 2 0) substrate peak (Fig. 5a). For comparison, a 200-nm-orthorhombically strained, the RHEED patterns do not thick (0 0 1)-oriented epitaxial film of Ba0.6Sr0.4TiO3 wasindicate that the lattice distortions that are present in the deposited on a single-crystal SrTiO3 (1 0 0) substrate. Thesubstrate (and that cause the lattice vectors to rotate and/ FWHM of the rocking curve was 364 arcsec, which isor double from the cubic perovskite protrotype) are not generally more typical of oxide films. Using the REScO3present at the surface of the growing film. In other words, substrates that have close lattice and thermal expansionsFig. 5. High-resolution XRD rocking curves of (a) GdScO3(2 2 0), (b) (0 0 2) peak of the Ba0.6Sr0.4TiO3 films on GdScO3, (c) DyScO3(2 2 0), and (d) (0 0 2)peak of the Ba0.6Sr0.4TiO3 films on DyScO3. The symbols are the data and the curves are the Gaussian fits to the data.
  7. 7. ARTICLE IN PRESS H. Du et al. / Journal of Crystal Growth 310 (2008) 1991–1998 1997matches to the Ba0.6Sr0.4TiO3 film allows for the growth of substrates, as determined using AFM and RHEED.substrate quality single-crystal films that, by implication, Extraordinarily high-quality films were grown on bothhave extremely low dislocation densities (Fig. 5). substrates, as observed in their 15–17 arcsec FWHMs in In an effort to approximate the dislocation densities, the rocking curves. The substrates were determined to havesubstrates were etched in the BHF acid for 5 min to pit the dislocation density of about 105–6/cm2, which is manydislocations. Dislocation etch pits were counted by both orders of magnitude lower than the dislocation density inAFM and optical microscopy. The dislocation densities in the films grown on conventional substrate SrTiO3 (0 0 1),the REScO3 substrates were about 105À6/cm2. The high- which had a FWHM of 364 arcsec. XRD showedresolution X-ray rocking curves are closely related to the that the films were epitaxial according to the relation-dislocation contents, which means that the films have ship: (0 0 1)filmJ(1 1 0)substrate; [1 0 0]filmJ[0 0 1]substrate (andabout the same density of dislocations as the substrates— ¯ [0 1 0]filmJ[1 1 0]substrate. All of the films were coherentlyabout 105–6/cm2 (which is a lower limit). The dislocations strained to the substrate (even those 200 nm thick) and hadin the films are largely inherited from substrate, since they an orthorhombic distortion, although the c-axis latticeare coherently strained (although this is a small strain). parameter was larger than expected for the in-plane strain,Plan view TEM was used to determine the dislocation implying that the films may have significant population ofdensity in Ba0.6Sr0.4TiO3 films grown on the SrTiO3(0 0 1) oxygen vacancies owing to the low activity of oxygen usedsubstrate. The dislocation density was determined by TEM during growth.to be 1012/cm2 in these films. Based on these initialestimates, there is a several order of magnitude difference Acknowledgmentsin the dislocation contents between these films andconventional films. The authors are grateful for the support from the ONR These experiments demonstrate that high-quality funding under Contract N00014-05-1-0238. The authorsBa0.6Sr0.4TiO3 films can be deposited onto substrates that are also thankful to Dr. R. Uecker from the Institute ofallow for the dislocation content to be minimized. Such Crystal Growth and Mr. Peters from the CrysTech GmbH,films can be used to unravel how the physical properties are Germany for providing the substrates.related to the different defects. It should be pointed outthat the c-axis lattice parameter is fairly large for the natureof the in-plane strain. This is likely a result of the low- Referencespressure deposition resulting in a high population of point [1] A.K. Tagantsev, V.O. Sherman, K.F. Astafiev, J. Venkatesh, N.defects (oxygen vacancies) [24]. Similar films have been Setter, J. Electroceram. 11 (2003) 5.grown on MgO under the same conditions and character- [2] S.P. Alpay, I.B. Misirlioglu, V. Nagarajan, R. Ramesh, Appl. Phys.ized with the Rutherford Backscattering Spectroscopy Lett. 85 (2004) 2044.(RBS); it was found that the film composition was [3] D. Balzar, P.A. Ramakrishnan, P. Spagnol, S. Mani, A.M. Hermann, M.A. Matin, Jpn. J. Appl. Phys. 41 (2002) 6628.Ba:Sr ¼ 59.6:40.4(71%) and (Ba+Sr):Ti ¼ 50.2:49.8(71%), [4] X.L. Li, B. Chen, H.Y. Jing, H.B. Lu, B.R. Zhao, Z.H. Mai, Q.J. Jia,in which the cation compositions did not deviate Appl. Phys. Lett. 87 (2005) 222905.much from nominal stoichiometry. The lattice parameter [5] C.B. Parker, J.P. Maria, A.I. Kingon, Appl. Phys. Lett. 81 (2002)of relaxed films on MgO(0 0 1) were c ¼ 4.005A and ˚ 340.a ¼ 3.978A ˚ , implicating oxygen vacancies as the [6] D. Balzar, P.A. Ramakrishnan, A.M. Hermann, Phys. Rev. B 70source of the large lattice parameters. Nevertheless, more (2004) 092103. [7] A.A. Sirenko, C. Bernhard, A. Golnik, M. Anna, J. Clark, W. Hao,detailed structural studies are required to determine the X. Si, X. Xi, Nature 404 (2000) 373.oxygen content and physical properties of these films. [8] M. Stenge, N.A. Spaldin, Nature 443 (2006) 679.Regardless, the films have extremely low dislocation [9] B. Misirlioglu, A.L. Vasiliev, M. Aindow, S.P. Alpay, R. Ramesh,contents and this should stay true for films of different Appl. Phys. Lett. 84 (2004) 1742. [10] I.B. Misirlioglu, A.L. Vasiliev, M. Aindow, S.P. Alpay, Integr.oxygen contents. Ferroelectr. 71 (2005) 67. [11] J.W. Matthews, A.E. Blakeslee, J. Crystal Growth 27 (1974) 118.4. Conclusions [12] R. People, J.C. Bean, Appl. Phys. Lett. 47 (1985) 322. [13] D.Y. Wang, J. Wang, H.L.W. Chan, C.L. Choy, J. Appl. Phys. 101 Ba0.6Sr0.4TiO3 films were grown on REScO3(1 1 0) (2007) 043515.(RE ¼ Dy, Gd) substrates that have high-crystal quality [14] J.C. Jiang, Y. Lin, C.L. Chen, C.W. Chu, E.I. Meletis, J. Appl. Phys. 91 (2002) 3188.(as observed in their 15–17 arcsec FWHMs in rocking [15] K.J. Choi, M. Biegalski, Y.L. Li, A. Sharan, J. Schubert, R. Uecker,curves) and good lattice and thermal expansion mismatch P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.Q. Chen, D.G.to the film. Films were deposited using a high-vacuum Schlom, C.B. Eom, Science 306 (2004) 1005.pulsed laser deposition system equipped with differentially [16] M.D. Biegalski, J.H. Haeni, S. Trolier-McKinstry, D.G. Schlom,pumped RHEED. The substrates were surface treated to C.D. Brandle, A.J. Ven Graitis, J. Mater. Res. 20 (2005) 952. [17] J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S.produce atomically flat, wide terraces having clear 1 and 1 2 Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q.unit-cell high steps. Epitaxial Ba0.6Sr0.4TiO3 films were Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, D.G.grown in 2D layer-by-layer mode on these treated Schlom, Nature 430 (2004) 758.
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