coatings Article Pulsed Laser Deposition of Epitaxial Non-Doped PbTiO3 Thin Films from PbO–TiO2 Mosaic Targets Joe Sakai * , José Manuel Caicedo Roque, Pablo Vales-Castro, Jessica Padilla-Pantoja, Guillaume Sauthier and José Santiso Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC and The Barcelona Institute of Nanoscience and Nanotechnology (BIST), UAB Campus, 08193 Bellaterra, Spain; jose.caicedo@icn2.cat (J.M.C.R.); pablo.vales@icn2.cat (P.V.-C.); jessica.padilla@icn2.cat (J.P.-P.); guillaume.sauthier@icn2.cat (G.S.); jose.santiso@icn2.cat (J.S.) * Correspondence: sakai.joe@gmail.com Abstract: PbTiO3 (PTO) suffers from difficulty in preparing high-density robust bulk ceramics, which in turn has been a bottleneck in thin films growth with physical vapor deposition (PVD) methods. In the present work, we prepared non-doped PTO thin films by a pulsed laser deposition (PLD) method with either a single PTO target or a mosaic target consisting of PbO and TiO2 pie-shaped pieces. On the PTO single target, laser irradiation caused selective ablation of Pb, resulting in Ti- rich cone-shaped pillar structure on the surface, whereas the irradiated surface of PbO and TiO2 pieces was smoother. Epitaxial PTO films deposited on SrTiO3 (001) substrates from the pie-chart targets with PbO:TiO2 areal ratio from 3:5 to 5:3 resulted in composition, crystallinity, flatness, and ferroelectric properties almost independent of the areal ratio. The averaged composition of each   film was close to stoichiometric, suggesting a compositional self-control mechanism. For growing Citation: Sakai, J.; Roque, J.M.C.; epitaxial and high-quality non-doped PTO films, a PbO–TiO2 pie-chart target is advantageous in Vales-Castro, P.; Padilla-Pantoja, J.; easiness of handling and stable surface structure. Sauthier, G.; Santiso, J. Pulsed Laser Deposition of Epitaxial Non-Doped Keywords: pulsed laser deposition; ferroelectric material; lead titanate; mosaic target; sequential PbTiO3 Thin Films from PbO–TiO2 ablation; compositional self-control mechanism Mosaic Targets. Coatings 2021, 11, 662. https://doi.org/10.3390/coatings 11060662 1. Introduction Academic Editor: Oleksandr Tkach PbTiO3 (PTO) is a pioneering and still important ferroelectric material with a perovskite- type structure [1,2]. The crystalline structure of PTO is cubic at temperatures above Received: 9 May 2021 Accepted: 29 May 2021 490 ◦ C, whereas it is tetragonal at lower temperatures with a large c/a ratio of 1.06 at room Published: 31 May 2021 temperature (RT) [1]. The significant structural phase transition often causes cracks in the whole volume of PTO bulk ceramics during cooling after sintering at a higher temperature. Publisher’s Note: MDPI stays neutral This makes the PTO bulk ceramics extremely fragile, which in turn is a difficulty for using with regard to jurisdictional claims in it as a target material typically needed in thin films growth processes by physical vapor published maps and institutional affil- deposition (PVD) techniques. To overcome this fragility, one popular solution is using iations. sintering aids or doping with other elements to form “modified PTO” [3,4]. Another factor that could make the growth of PTO thin films difficult is the high vapor pressure of lead oxide. In general, PLD method allows thin films growth with cation composition close to that of the target. However, when the material contains a volatile ele- Copyright: © 2021 by the authors. ment, it is often observed that film composition progressively deviates from stoichiometry Licensee MDPI, Basel, Switzerland. because of the volatile element depletion when depositing at elevated temperatures [5]. This article is an open access article Many groups growing PTO films by PVD techniques with a single PTO target attempt distributed under the terms and to prevent the Pb deficiency with strategies that can be categorized into two, namely, to conditions of the Creative Commons mitigate the re-evaporation of Pb from the growing film or to supply species with an excess Attribution (CC BY) license (https:// Pb/Ti ratio onto the substrate to compensate for the Pb loss. The practical deposition creativecommons.org/licenses/by/ conditions based on the former strategy include (a) low substrate temperature (550 ◦ C 4.0/). or lower) [6–8], and (b) high oxygen ambient pressure (100 mTorr or higher) [7,9–14]. A Coatings 2021, 11, 662. https://doi.org/10.3390/coatings11060662 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 662 2 of 11 common strategy for the latter category is (c) to use a Pb-rich target [6,7,10,15], although the exact amount of Pb in excess will depend on the deposition conditions. Supplying species from two targets has been reported to effectively function for preparing high-quality thin films containing a high vapor-pressure element. The alternative ablation of two (or more) targets has been regularly employed since the beginning of combinatorial PLD researches [16,17]. The species sequentially ablated from multiple targets are expected to mix on the growing surface to form a uniform film provided that the individual layer thickness for each cycle is at the monolayer level to allow for intermixing. In that way, the ratio of species supplied from multiple targets can be controlled by modifying the number of laser pulses on each target. However, this method typically results in a larger setup complexity due to the need for constant exchange of target positions (or laser redirecting to the different targets) and result in an overall reduced growth rate. This can be regarded as an advanced technique belonging to category (c). In this way, Maruyama et al. have succeeded in obtaining epitaxial LiCoO2 thin films by alternative laser ablation of Li2 CO3 and CoO targets [18]. A similar approach by using the simultaneous ablation of two targets (with two independent lasers or a split laser beam) can also be a powerful tool for this purpose, in this case avoiding continuous target exchange [19,20]. In this manner, Hussey and Gupta have prepared good quality YBa2 Cu3 O7−d (YBCO) films by simultaneously shedding laser beams to individual YBa2 Ox and CuO targets [21]. A much simpler approach for sequential ablation is based on using a mosaic target consisting of pieces of multiple materials, which requires only one rotating target [5]. Since PLD film growth rates produce sub-monolayer thickness per pulse (typically below 1 Å/pulse depending on the deposition conditions, namely laser fluence, gas pressure and target-to-substrate distance), the number of pulses in each part of the target at standard target rotation speeds produce a sequence of very short pulses allowing for homogenous cross plane composition. Besides, the possible composition inhomogeneities caused by slight variations in the sequence, or when ablating at the boundary regions between both compounds in the target, are averaged during film growth and expected to produce negligible effects. In this way, Lee et al. have demonstrated to compensate the typical Zn deficiency in the PLD growth of ZnGa2 O4 (ZGO) films at high temperature by using a mosaic target consisting of ZGO and ZnO [22]. The mosaic target strategy has been successfully adopted to deposit PTO films, since both PbO and TiO2 are robust ceramics unlike PTO. Funakoshi et al. have already performed PLD of PTO polycrystalline films on silicon (Si) substrates using a PbO–TiO2 mosaic target [23]. Nothing precludes the use of this method for growing PTO epitaxial films, except for the larger difficulty due to the need of higher deposition temperatures to achieve crystalline order. However, to our knowledge, there has been no previous report on the growth of epitaxial PTO films using a mosaic target. In the present study, we prepared epitaxial PTO films on perovskite-type oxide single- crystal substrates with PLD. Either a mosaic target consisting of PbO and TiO2 pie-shaped pieces or, for comparison, a single PTO target was employed. A cone-shaped pillar structure, accompanied by Pb-deficient composition, was observed on the surface of the single PTO target after laser irradiation and supposed to impede the reproducible Pb/Ti ablation during deposition. PTO epitaxial films were grown from a series of pie-chart targets resulting in composition, crystallinity, flatness, and ferroelectric properties comparable with the film obtained from a single PTO target. 2. Experiments 2.1. Fabrication and Characterization of Targets To prepare the PbTiO3 target, mixed powders of PbO and TiO2 were pressed in 30 mm- diameter dies, and calcinated in air at 800 ◦ C for 1 h. No sintering aids were used in this case. The product was ground and re-pressed and sintered in air at 1160 ◦ C for 2 h. The bulk was extremely fragile so that it easily breaks when picked up with tweezers. On the other hand, individual PbO and TiO2 targets were prepared by pressing the powder were then cut into quarter circle or one-eight circle shape with a diamond saw. The pie- shaped pieces were fixed on an aluminum disk with a double-sided carbon tape in various PbO:TiO2 areal ratios of 3:5, 4:4, and 5:3 (Figure 1a). A commercial (Nb,Mg)-modified Pb1.1TiO3, much denser, was also used from com- parison (Nb and Mg are sintering aids). The laser-induced evolution of surface morphol- Coatings 2021, 11, 662 3 of 11 ogy and composition was studied for the homemade PbTiO3 target, as well as for the com- mercial Pb1.1TiO3, and the homemade PbO/TiO2 targets. For this, the target surface was irradiated by a focused excimer laser (COMPex 102, KrF gas, wavelength of 248 nm, Lambda in the 30 Physik, Göttingen, mm-diameter dies Germany) and sinteringbeam in at at 750 ◦ Cfluence airdifferent for 1 h,inand 100 at mTorr ◦ C for 1200 oxygen 6pressure. The morphology h, respectively. Both PbOand andthe TiOcomposition 2 disks thusof the target obtained surface were wasThe robust. observed with a 3 mm-thick scanning disks wereelectron then cutmicroscopy into quarter(SEM, circle Quanta 650 FEG, or one-eight circleFEI, shapeHillsboro, OR, USA) with a diamond and saw. Thean pie-shaped pieces were energy-dispersive fixed X-ray on anspectroscopy (EDX) aluminum disk with a double-sided (X-Max, carbon tape Oxford Instruments, in High various Wycombe,PbO:TiO areal ratios of 3:5, 4:4, and 5:3 (Figure 1a). UK), 2respectively. Figure1.1. Schematics Figure SchematicsofofPLD PLDusing usingaaPbO–TiO PbO–TiO pie-charttarget. 2 2pie-chart target.(a) (a)Configuration Configurationofofpie-chart pie-charttarget, tar- get, laser spot, and substrate. The laser spot is off-centered with respect to rotation axis of the laser spot, and substrate. The laser spot is off-centered with respect to rotation axis of the target. tar- get.Sequence (b) (b) Sequence of PbO of PbO and TiO and TiO 2 ablation. 2 ablation. 2.2. Thin Films Growth A commercial (Nb,Mg)-modified Pb1.1 TiO3 , much denser, was also used from compari- son (Nb PTO and Mgfilms thin are sintering were grown aids).mainly The laser-induced evolution on single-crystal SrTiO of3 (STO; surface001) morphology substratesandus- composition ing either thewas PTOstudied for theorhomemade single target the PbO–TiO PbTiO 3 target, 2 pie-chart as well target. TheasSTO for the commercial substrates were Pb 1.1 TiO3 , and chemically andthethermally homemade PbO/TiO2totargets. pre-treated For this, theflat form atomically target surface surface withwas irradiated a step (~4 Å by a focused excimer laser (COMPex 102, KrF gas, wavelength of 248 height) and terrace structure [24]. The deposition conditions for PTO films were as follos: nm, Lambda Physik, Göttingen, Germany) beam substrate temperature of 550at°C, different ambient fluence oxygenin pressure 100 mTorr ofoxygen pressure. 100 mTorr, The mor- laser repetition phology and the composition of the target surface was observed with rate of 2 Hz, total pulse number of 1760, energy on the target surface of 50 mJ, fluence a scanning electronof microscopy 1.3 J/cm2, and (SEM, Quanta 650 distance target–substrate FEG, FEI, ofHillsboro, 50 mm. AsOR, USA)electrode a bottom and an energy-dispersive layer for electrical X-ray (EDX) spectroscopy characterization, (X-Max, a 15 nm-thick LaOxford Instruments, High Wycombe, UK), respectively. 0.6Sr0.4MnO3 (LSMO) layer was deposited with deposi- tion conditions of 650 °C, 100 mTorr, 2 Hz, 740 pulses, 50 mJ, 2.0 J/cm2, and 50 mm, prior 2.2. Thin Films Growth to the PTO layer growth. The target rotates with a speed of 3 rpm during laser irradiation. PTO thin Rastering, films as well as were grown rotation, weremainly used on onthe single-crystal single PTO and SrTiO 3 (STO; LSMO 001)for targets substrates uniform using either the PTO single target or the PbO–TiO pie-chart target. ablation [25], whereas only rotation (no rastering)2was used on the mosaic targets. After The STO substrates were chemicallythe the deposition, and thermally sample pre-treated was cooled to roomto form atomically temperature at a flat ratesurface with a in of −10 °C/min step O2 (~4 Å height) ambient of 10andTorr. terrace structure Hereafter, we label[24]. the ThePTO/LSMO/STO deposition conditions samples forprepared PTO films were with the as follos: substrate temperature of2 550 ◦ pie-chart target with the PbO:TiO areal C, ambient ratios of 3:5,oxygen 4:4, and pressure of 100 mTorr, 5:3 as samples laser PC35, PC44, repetition rate of 2 Hz, total pulse number of 1760, energy on the and PC53, respectively, and the one with the single PTO target as sample SP. The actual target surface of 50 mJ, fluence of 1.3 J/cm2 , and target–substrate distance of 50 mm. As a bottom electrode layer pulse numbers fed to PbO and TiO2 pieces during one cycle of rotation are shown in Table for electrical characterization, a 15 nm-thick La0.6 Sr0.4 MnO3 (LSMO) layer was deposited 1. The deviation of pulse numbers ratio from the central angle ratio between PbO and with deposition conditions of 650 ◦ C, 100 mTorr, 2 Hz, 740 pulses, 50 mJ, 2.0 J/cm2 , and TiO2, which should theoretically be identical (Figure 1b), is due to a slight offset between 50 mm, prior to the PTO layer growth. The target rotates with a speed of 3 rpm during the target center and the rotation axis. laser irradiation. Rastering, as well as rotation, were used on the single PTO and LSMO targets for uniform ablation [25], whereas only rotation (no rastering) was used on the mosaic targets. After the deposition, the sample was cooled to room temperature at a rate of −10 ◦ C/min in O2 ambient of 10 Torr. Hereafter, we label the PTO/LSMO/STO samples prepared with the pie-chart target with the PbO:TiO2 areal ratios of 3:5, 4:4, and 5:3 as samples PC35, PC44, and PC53, respectively, and the one with the single PTO target as sample SP. The actual pulse numbers fed to PbO and TiO2 pieces during one cycle of rotation are shown in Table 1. The deviation of pulse numbers ratio from the central angle ratio between PbO and TiO2 , which should theoretically be identical (Figure 1b), is due to a slight offset between the target center and the rotation axis. Coatings 2021, 11, 662 4 of 11 Table 1. Deposition parameters and characteristics of four samples. Number of Pulses PbO:TiO2 PTO Growth Rate PTO out-of-Plane PTO 002 Rocking Sample Name Irradiated on PbO:TiO2 Areal Ratio (10−3 nm/pulse) Lattice Spacing (Å) Curve FWHM (deg) per 1 Cycle SP - - 28 4.158 0.080 PC35 3:5 12:30 15 4.146 0.081 PC44 4:4 18:24 14 4.146 0.087 PC53 5:3 24:18 8.6 4.141 0.090 2.3. Characterization of Films Crystallinity of PTO films was evaluated with 2θ–ω scans, rocking curves and re- ciprocal space mapping (RSM) by X-ray diffraction (XRD) using a diffractometer with a four-circle goniometer (X’pert PRO-MRD, Malvern-PANalytical, Almelo, The Netherlands). Surface morphology was observed with atomic force microscopy (AFM). X-ray photoemis- sion spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS) were employed to characterize the composition of the PTO films. XPS measurements were performed at RT with a hemispherical analyzer (PHOIBOS 150, SPECS, Berlin, Germany). Here, calculation of the Pb ratio, defined as Pb/(Pb + Ti), was based on three different Pb lines, namely 4d (binding energies 413 and 435 eV), 4f (138 and 143 eV), and 5d (20 and 22 eV), and one Ti line 2p (455 and 461 eV). Due to the different binding energies, the three Pb lines give the signals originating from surface layers of different thicknesses. This was used to account for possible Pb composition gradients at the surface. The RBS measurement was performed with a tandem accelerator at Centro de Micro-Análisis de Materiales, Madrid, Spain. He+ ions of an energy of 2 MeV were directed to the sample at an angle of 2.8◦ from the normal direction, and the scattered particles were detected at an angle of 165◦ with respect to the incident angle. The ferroelectric properties of the films were evaluated through phase–bias curves and write–erase tests using a piezoresponse force microscopy (PFM) system. Both AFM and PFM were carried out using a scanning probe microscopy system (MFP-3D Classic, Asylum Research, Santa Barbara, CA, USA). See [20] for the detail of XRD, XPS, and PFM procedures. 3. Results 3.1. Morphology and Composition of the Target Surface Surface SEM images of a commercial (Nb,Mg)-doped Pb1.1 TiO3 target are shown in Figure 2a–c. The non-irradiated pristine region (Figure 2a) consists of random-shaped grains of several tens’ micrometers. Adjacent grains are weakly connected, resulting in an overall porous structure. Cone-shaped pillars appeared on the surface of the target immediately after irradiation of a few laser shots. The pillars pointed to the direction of laser incidence (Figure 2b). The surface after being used for several tens of depositions with fluence of 1 to 2 J/cm2 shows significant cone-shaped pillars at the level of several tens of micrometers (Figure 2c). EDX analysis revealed Ti-rich (Pb-poor) composition on the surface with such a pillar structure. The off-stoichiometry suggests that selective ablation of Pb has occurred due to its high vapor pressure, leaving Ti component on the surface. This cone-shaped pillar structure looks similar to the one generated on a YBCO target [26]. The pillars on the YBCO bulk were reported to be Cu-poor and Y-rich, and was generated when the laser fluence was below a threshold of 1 J/cm2 [26]. In the case of PTO, unlike YBCO, the off-stoichiometry remained in any fluence examined in the experiment, from 0.3 to 4.7 J/cm2 . Figure 2d,e shows SEM images of the homemade PTO target. The pristine surface, even though flatter than the commercial Pb1.1 TiO3 target, is filled with microcracks that explain its fragility (Figure 2d). Appearance of cone-shaped pillars due to laser irradiation (Figure 2e) was not as remarkable as that on the commercial target shown in Figure 2b. Still, this target was not free from a compositional shift towards Pb-poor values. Figure 2f shows the relationship between the laser fluence and the Pb ratio in the commercial (Nb,Mg)- gs 2021, 11, Coatings 2021,REVIEW x FOR PEER 11, x FOR PEER REVIEW 5 of 11 5 of 11 Coatings 2021, 11, 662 5 of 11 shows thebetween shows the relationship relationship between the laser the laser fluence and thefluence and in Pb ratio thethe Pbcommercial ratio in the commercial (Nb,Mg)-doped (Nb,Mg)-doped Pb1.1TiO doped Pb and3 the Pb1.13 TiO 1.1 TiO andhomemade3 and the homemadethe homemade PbTiO 3 targets. PbTiO PbTiO 3 In bothtargets. cases, In both large cases, devia- large devia- 3 targets. In both cases, large deviations from tions tions from thethe from initial the initial composition initial composition composition were were observed observed were down observed downtotoabout down aboutPb/(Pb to about Pb/(Pb+ + Pb/(Pb Ti)Ti)= =0.4. 0.4.Alt- + Ti) Although= 0.4. Alt- devia- hough deviationhough tiontends deviation tendstotoreduce tends reduceasasthe to reduce thefluence as the fluenceincreases fluence increasesafter increases aftera aminimum minimumafter a minimum point, point, it it is is not point, it not completely is not completely completely suppressed. suppressed. suppressed. Similar Similarsurface surfaceSimilar surface morphology morphology morphology observations observations observations were werecarried carriedoutwere outfor carried forthe out for the bulk targets the bulk bulk targets ofof the targets thebinary of the binary binaryoxides oxides PbO PbOand oxides and TiO TiO2.PbO 2The . Theand PbO PbOTiO 2. The surface surface PbO was surface wasquite quite flatwas flat and quite andcrack- flat and crack-free crack- either at free either pristine state free either at pristine at pristine state (Figure (Figure 3a) state 3a) or (Figure or after after laser3a) or after laser irradiation laser irradiation irradiation (Figure (Figure 3b). TiO (Figure 3b). TiO22,, initially TiO , initially initially consisting of 2 consisting consisting of ~10 µm-sized µ m-sized of ~10 µ m-sized grains grains (Figuregrains (Figure 3c), 3c),was was(Figure deformed3c), was deformed by deformed bythe thelaser lasertotobyshow show thea 3dlaser a 3d to showwavy structure, a 3d structure, wavystructure, rather ratherthanwavy than rather pillarlike pillarlike than (Figure (Figurepillarlike 3d). 3d). (Figure 3d). Figure 2. SEM images Figure 2. SEM Figure of (a–c) 2. (a–c) SEM images a commercial images of of (a–c)(Nb,Mg)-doped (Nb,Mg)-doped a commercial a commercial (Nb,Mg)-doped Pb1.1 TiO3 target (d,e) aPb and3 target Pb1.1TiO 1.1TiO3 target homemade and and pure (d,e) a home- (d,e)3atarget. PbTiO home- made (a) Pristine. (b) pure PbTiO Irradiated made with 50 pure 3 target. pulses PbTiO (a) of 3 target. (a) Pristine. (b) Irradiated Pristine. laser (b) Irradiated with a fluence of with 0.3 50 J/cmpulses with 2 while of the50target laser pulses with of laser awas fluence with fixed.of a fluence 0.3After (c) usedof for0.3 J/cm 2 while the target was fixed. (c) After used for several tens of depositions with fluence of 1 to 2 several tens J/cm 2 while thewith of depositions target was fixed. fluence of 1 to(c)2 After J/cm2used for several . (d) Pristine. (e)tens of depositions Irradiated with fluence with 50 pulses of laserofwith 1 to 2a fluence of 2 J/cm J/cm 2. (d) Pristine. 2. (d) Pristine. (e) Irradiated with 50 pulses of laser with a fluence of 0.9 J/cm 2 while the target (e) Irradiated with 50 pulses of laser with a fluence of 0.9 J/cm 2 while the target 0.9 J/cm while the target was fixed. Scale bars correspond to 50 µm in (a–c) and 100 µm in (d,e). (f) Pb ratio of commercial was fixed. Scalewas barsfixed. Scale bars correspond to 50correspond µ m in (a–c) toand 50 µ100 m in µm(a–c) and 100 in (d,e). µ mratio (f) Pb in (d,e). (f) Pb ratio of commercial of commercial Pb1.1 TiO3 and homemade PbTiO Pb 3 target 1.1TiO surface after 50 pulses of irradiation as a function of laser fluence. Dashed lines 3 and homemade PbTiO3 target surface after 50 pulses of irradiation as a function of laser Pb1.1TiO3 and homemade PbTiO3 target surface after 50 pulses of irradiation as a function of laser represent the designed composition of each target. fluence. Dashedfluence. Dashed lines represent lines the represent designed the designed composition composition of each target. of each target. Figure of Figure Figure 3. SEM images 3. (a,b) 3. SEM PbO SEM images andof images (a,b) of(c,d) PbO (a,b)TiO PbO and and (c,d) 2 target(c,d) TiO TiO22 target surfaces, target (a,c) surfaces, surfaces, pristine (a,c) (a,c) and pristine pristine (b,d) and and (b,d) after used (b,d) after after used used for several for several depositions with for several depositions depositions with fluence of with fluence 1 to 2fluence of 1 J/cm . Scale 2 to 2 J/cm of 1 tobars 22. Scale bars correspond to 100 µ m for all panels. correspond 2 J/cm to 100 . Scale bars µ m for allto correspond panels. 100 µm for all panels. Coatings 2021,11, Coatings2021, 11,662 x FOR PEER REVIEW 66 of of 11 11 3.2. Properties 3.2. Properties of of PTO PTO Films Films XRD 2θ–ω XRD 2θ–ω scans scans around around STOSTO 002 002 reflection reflection of of samples samples SP,SP,PC35, PC35,PC44, PC44,andand PC53 PC53 are are shown in Figure 4a. In all the samples, the PTO 002 diffraction peak shown in Figure 4a. In all the samples, the PTO 002 diffraction peak was accompanied was accompanied by Laue by fringes, Laue which fringes, indicate which highhigh indicate crystallinity of the crystallinity of PTO the PTOlayer. Thickness layer. Thicknessof theofPTOthe layerlayer PTO was estimated was estimatedfrom thefromfringe interval the fringe to be 32.7, interval to be26.8, 24.1, 32.7, 26.8,and 15.2and 24.1, nm 15.2 for samples nm for SP, PC35, samples SP,PC44, PC35,and PC53, PC44, andrespectively. The growth PC53, respectively. rate ofrate The growth theof PTOthe layer (Table PTO layer 1) was (Table 1) highest was in sample highest SP and in sample SPlowest in sample and lowest PC53. The in sample PC53.slight Thedifference in the cell slight difference in parame- the cell ters (Table 1) parameters can 1) (Table qualitatively be explained can qualitatively through be explained the difference through of degree the difference of strain of degree of originating strain in the thickness originating variation. in the thickness The rocking variation. The curves rockingaround curvesthe PTO 002 around the reflection PTO 002 in the four reflection in samples showed showed the four samples FWHM FWHM smaller smaller than 0.1°, the0.1 than ◦ , the ones ofones SP and of SPPC35 being and PC35 slightly being smallersmaller slightly than thethanothers the (Figure 4b, Table others (Figure 4b,1). The 1). Table epitaxial relationship The epitaxial between relationship between films andfilms and substrate substrate was confirmed was confirmed with RSM,withasRSM, shown as shown in Figure in Figure 4c. The4c. The corre- spots spots corresponding sponding with with PTO, PTO, LSMO, LSMO, andare and STO STO are aligned aligned vertically, vertically, indicatingindicating the in-plane the in-plane lattice lattice of thefully of the films filmscoherent fully coherent to the to the substrate. substrate. Figure 4. (a) XRD 2θ–ω profiles around STO 002 Figure 4. 002 diffraction diffraction of of samples samplesSP, SP, PC35, PC35,PC44, PC44,andandPC53. PC53. The profiles are offset for The for clarity. clarity. (b) (b)XRD XRDrocking rockingcurves curvesaround aroundPTOPTO002 002diffraction of of diffraction thethe four four samples. (c) RSM of samples. of sample sample PC35 PC35 around around ((−103) −103) diffraction diffraction of of STO, STO, PTO, PTO, and LSMO. AFM AFM topographic topographic images of the the four four samples samples are are shown shown in in Figure Figure 5. 5. All All the the images images show show lines lines corresponding corresponding toto the the terraces terraces on on the the STO STO substrates. substrates. The The peak-to-valley peak-to-valley values values were within 1 nm in the 5 × 5 µm 2 area for all the samples. were within 1 nm in the 5 × 5 µ m area for all the samples. 2 Coatings 2021, 11, x FOR PEER REVIEW 7 of 11 gs 2021, 11,Coatings 11, 662 2021, REVIEW x FOR PEER 7 of 11 7 of 11 Figure 5. AFM topographicFigure images 5. of AFM topographic samples images (a) SP, (b) PC35, of(c) samples PC44, (a) andSP, (d)(b) PC35,Scale PC53. (c) PC44, and (d) PC53. bars correspond to Scale 1 µm.bars Figure 5. AFM correspond topographictoimages 1 µ m. of samples Color scale (a) SP, (b) PC35, (right-hand side (c) of PC44, d) is and (d) PC53. common for all Scale four bars images. Color scale (right-hand side of d) is common for all four images. correspond to 1 µ m. Color scale (right-hand side of d) is common for all four images. The results of XPS analysis of four samples are reported in Figure 6. Unexpectedly, The results the PbXPS of ratioanalysis of four samples are reported in Figure 6. Unexpectedly, in the PTO layers was found to be quasi-consistent among the four samples. samples. the Pb ratio inMeanwhile, the PTO layers Meanwhile, the was foundPb thecalculated calculated tocomposition Pb be quasi-consistent composition strongly strongly among depended the four depended onon samples. the Pb Pb the lines used lines for the used for Meanwhile, the thecalculated evaluation. evaluation. ThePb Pb composition Thelines withstrongly Pb lines higher depended binding with higher onenergy energy binding the Pb lines resulted in used the Pb resulted infor the ratio the Pbcloser ratioto stoi- closer evaluation. Theto Pb lines with stoichiometry. chiometry. higher Generally, binding Generally, the theenergy higher higher resulted in theenergy, the binding the binding energy, Pb thicker the ratiothe closer thicker the to stoi- surfacethe layer surface layer probed. chiometry. Generally, probed. Therefore, the higher Therefore, the binding the dependence the dependence energy, the thicker of Pbsuggests of Pb ratio the ratio suggests surface layer the possibility the possibility probed. of Pb of Pb segregation segregation in PTOin Therefore, thelayers dependence PTO layers of towards Pb ratio the suggests surface. ThethePbpossibility ratio larger of Pb than segregation 50% even towards the surface. The Pb ratio larger than 50% even for the highest binding en-for in PTO the highest binding layers towards the line energy ergy surface. line (Pb (PbThe Pbimplies ratio that 4d)implies 4d) larger thatthethan the 50% even thickness thickness for ofofPb the highest binding Pbsegregation segregation was larger was en- than larger than the XPS ergy line (Pb observation 4d) implies observation depth that the depth(a(afewthickness of fewnanometers). Pb nanometers). segregation The The was composition composition larger than difference difference the among XPS amongthe samples was the samples observation depth was (a few negligible negligible nanometers). compared compared Theevolution withwith the composition the alongdifference evolution the depth. along among the samples the depth. was negligible compared with the evolution along the depth. Pbratio Figure 6. Pb ratioofoffour foursamples samplesdeduced deducedusing using XPS XPS signals signals of of various various Pb Pb lines lines as functions as functions of of Figure 6. Pb ratio of fourenergy binding samples deduced of the using Pb lines. XPS Inset signals shows of various Pb a wide-range XPSlines as functions profile sampleofPC35. of sample PC35. binding energy of the Pb lines. Inset shows a wide-range XPS profile of sample PC35. We also We alsocarried carriedout outRBSRBScharacterization characterization forfor two twoPTO PTOfilms grown films grownfor this purpose, for this one purpose, We also carried directly out on RBS a STO characterization substrate using for thetwo PTO single PTOfilms grown target and one directly on a STO substrate using the single PTO target and the other on a CeO for thethis purpose, other on a CeO 2 -buffered 2-buff- one directly on a STO r-plane ered substrate sapphire r-plane using (Al sapphire 2O (Al the O3single (11̅02)) 3 2(1102)) PTO target substrate substrate and using using the the other pie-chart the on atarget pie-chart CeO 2-buff- target ofofPbO:TiO PbO:TiO22 == 4:4. ered r-plane sapphire The (Al 2O3 (11 ̅ 02)) substrate chosenusing the pie-chart target fromofthe PbO:TiO 2 = 4:4.be completely The AlAl2OO substrate 33 substrate was was chosen so that so that the the Ti signals Ti signals from the film would film would be completely The Al2O3 substrate was separated from separated chosen from signals so that signals from the fromthe Ti signals thesubstrate. from substrate.The the Thefilm film filmon would onCeOCeO2 /Al be 2/Al2 completely O33 (11 2O ̅ 02) was (1102) was regarded regarded toto separated from besignals from thesince quasi-epitaxial substrate. the Thelayer film was on CeO 2/Al2O3to (11̅ 02) wasoriented. regarded to be quasi-epitaxial since thePTOPTO layer was confirmed confirmed tobebe(h00) (h00) oriented.The TheRBS RBS analysis analy- be quasi-epitaxial sis since the suggested suggested PTO Pb ratio layer of 49% Pb ratio was of and 49% confirmed 50% andfor to for the 50% be (h00) whole oriented. thethickness whole ofThe the RBS thickness PTOofanaly- layers the PTOin PTO/STO layers in sis suggestedPTO/STO Pb PTO/CeO and ratio and of 49% /Aland 2 O3 50% 2PTO/CeO samples,for the whole respectively thickness (Figure of 7). AsthethePTO layers composition 2/Al2O3 samples, respectively (Figure 7). As the composition ana- inanalyzed with PTO/STO andXPS PTO/CeO was 2/Al2O3 samples, consistent among respectively the four samples(Figure grown 7). As on the composition LSMO/STO, lyzed with XPS was consistent among the four samples grown on LSMO/STO, we specu- we ana- speculate that the lyzed with XPS was averaged consistent Pb ratio among of ~50% theis four samples adopted for grown all the on LSMO/STO, samples. late that the averaged Pb ratio of ~50% is adopted for all the samples. we specu- late that the averaged Pb ratio of ~50% is adopted for all the samples. Coatings 2021, 11, x FOR PEER REVIEW 8 of 11 Coatings 2021, 11, Coatings 2021, 11, 662 x FOR PEER REVIEW 88 of of 11 11 Figure 7. RBS profiles of PTO films deposited on (a) a STO substrate using a single PTO target, and (c)7. Figure Figure a CeO 7. RBS -buffered RBS 2profiles profiles of r-Al of PTO2Ofilms PTO 3 substrate films using deposited deposited ona(a) on 4:4aapie-chart (a) STO target.using STO substrate substrate Experimental using aa single datatarget, single PTO PTO and simu- target, and lated (c) curves anda (c) CeO are overlapped. -buffered a 2CeO r-Al2r-Al 2-buffered Depth O3 substrate profiles using 2O3 substrate deduced from a 4:4apie-chart using the RBS target. 4:4 pie-chart simulation Experimental target. results of datadata Experimental (b) and and simulated simu- PTO/STO lated curves are and curves are(d) PTO/CeO overlapped. overlapped. Depth2/Al2O3 samples. The depth profiles of Pb and Ti are completely over- Depth profiles profiles deduceddeduced from the from RBSthe RBS simulation simulation results results of (b) of (b) PTO/STO and lapped in and PTO/STO (d). (d) PTO/CeO2/Al2O3 samples. The depth profiles of Pb and Ti are completely over- (d) PTO/CeO2 /Al 2 O3 samples. The depth profiles of Pb and Ti are completely overlapped in (d). lapped in (d). In order In order to toconfirm confirmthe theferroelectric ferroelectricproperties propertiesofof the the four four samples, samples, write–erase write–erase testtest in in a few micrometers’ a fewInmicrometers’ square squarethe order to confirm region region and phase–bias and phase–bias ferroelectric measurement measurement properties of the fourwas was performed performed samples, using using PFM. write–erase test PFM. As in Asmicrometers’ ashown few shown in Figure in Figure 8, all 8, allregion samples square samples andshowed showed goodand good write phase–bias write andcharacteristics erase measurement erase wascharacteristics and and typical performed using typicalAs PFM. phase–bias phase–bias shown in curves, curves, which 8,which Figure prove prove good all samples good ferroelectricity ferroelectricity showed goodfor forPTO all the write and all the erase PTO films. films. characteristics and typical phase–bias curves, which prove good ferroelectricity for all the PTO films. Figure 8. Figure 8. PFM write–erase write–erase images images (top) (top) and and phase–bias phase–biascurves curves(bottom) (bottom)of ofsamples samples(a) (a)SP, SP,(b) (b)PC35, Figure 8. PFM write–erase images (top) and phase–bias curves (bottom) of samples (a) SP, (b)im- PC35, (c) (c) PC44, PC44, and and (d) (d) PC53. PC53. Color Color scale scale (right-hand (right-hand side of side d) isof d) commonis common for the for the four four PFM PFM images. ages. (c) PC44, and (d) PC53. Color scale (right-hand side of d) is common for the four PFM im- PC35, 4. Discussion ages. 4. Discussion In a binary oxide (AOx ) target the only possible composition deviation among the ablated In aspecies binaryisoxide 4. Discussion (AOxx) of the ratio O tothe target A, which can be compensated only possible through the composition deviation oxygen among the ambient ablated pressure. In aspecies Furthermore, binaryisoxide the ratio (AOxx) of deposition O tothe target A, only whichusing PbO and TiO can be composition possible compensated bulks, free from through the 2 deviation cone- oxygen among the shaped ambientpillars, ablated species is pressure.is desirable the ratio xinofterms Furthermore, to of A,reproducibility Odeposition using which can PbO in and film TiO be compensated growth. 2 bulks, The freeappearance through from the cone- oxygen of cone-shaped shaped pillars, pillars is could desirable in perturb terms the of distribution of reproducibility in local film laser fluence, growth. The which could appearance of ambient pressure. Furthermore, deposition using PbO and TiO2 bulks, free from cone- drastically affect not only the ablation rate, and therefore the film growth rate, but also the shaped pillars, is desirable in terms of reproducibility in film growth. The appearance of quality of the film. The composition of the PTO layers, independent of the areal ratio of the pie-type PbO and TiO2 pieces, indicates that a compositional self-control (CSC) mechanism occurs Coatings 2021, 11, 662 9 of 11 during film growth. The CSC mechanism has been reported in PZT thin films grown on Pt/Ti/SiO2 /Si substrates with a sputtering method [27]. Provided that the deposition rate is not too high, as in the present case, the adatoms are allowed to migrate over the substrate or growing film surface. They will be crystallized to form a stoichiometric crystal with Pb:(Ti + Zr) equal to 1:1 or, if they are in excess, will be re-evaporated. The present result confirmed that this should have occurred for the A cations excess (Pb). Not incorporated into the perovskite structure, the excess Pb segregated towards the surface and ultimately was re-evaporated. Most probably, composition of the adatoms fed to the substrate was the closest to stoichiometric in PC35, resulting in the highest growth rate and the smallest FWHM, whereas it was the farthest in PC53 with the lowest growth rate. The growth rate variation among samples is quite consistent with the TiO2 supply rate (ratio in growth rate between PC35 and PC53 was 1.7, exactly the same as the ratio in number of TiO2 pulses/target turn between them; see Table 1). This suggests that Pb was always in excess, and is expelled from the film in the CSC mechanism. Similarly, It was also the CSC mechanism that resulted into stoichiometric films when deposited from the single PTO target on which Pb is selectively ablated (see Section 3.1). It would be informative to compare the present results with those by Funakoshi et al. [23]. In their PTO films grown on Si substrates, Pb/Ti was 2.59 (i.e., Pb ratio 72%) when PbO and TiO2 pieces were irradiated with the same number of laser pulses, whereas the pulse numbers ratio of 29:71 to PbO:TiO2 resulted in a stoichiometric PTO film. The Ti deficiency in the former film could be due to the difference of the deposition rate between PbO and TiO2 pieces. In the present study on epitaxial films, stoichiometric films were obtained when the pulse number ratio of PbO:TiO2 was at least in the range of 12:30 (=29:71) to 24:18 (=57:43). The difference clearly originated in the difference between perovskite-type single crystal substrates that promote epitaxial growth and Si substrates that do not. The best film in terms of crystallinity (PC35) was obtained with a pulse numbers ratio of 29:71, which was in agreement with the result of Funakoshi et al. It should be noted that PTO films deposited on Si substrates in the present study, from either PTO or pie-chart target during the same runs as those on STO, have always resulted in Pb-rich composition in EDX analysis (not shown). We attribute this difference to whether the CSC mechanism functioned or not depending on STO or Si substrates, rather than to the difference of the analysis techniques. The composition of a PTO film prepared by PLD would be affected by the balance among multiple factors such as: (i) selective ablation at the target (which would drive the film composition to Pb-rich); (ii) re-evaporation from the substrate (to Pb-poor); (iii) the CSC mechanism (to stoichiometry). In the case of epitaxial films grown in O2 of 100 mTorr at 550 ◦ C, the effect of (iii) was revealed to be more significant than (i) and (ii), meaning that the laser-induced off- stoichiometry on the single PTO target is not a severe issue in the composition viewpoint. However, the pie-chart target, which can compensate for the composition deviation due to the effect of (i), is useful when one attempts to deposit PTO films in a non-epitaxial manner. The robustness and pillar-free surface are also the advantages of the pie-chart target over the PTO target. The pie-chart target strategy would be generally applicable to materials containing elements with high vapor pressure, and probably to other physical vapor deposition techniques such as sputtering. 5. Conclusions Irradiation of a PTO target with a pulsed laser beam of any fluence causes selective ablation of Pb, which induces a Ti-rich cone-shaped pillar structure, affecting the stability and composition control of the thin film deposition process. On the other hand, the PbO and TiO2 targets are robust and free from cone-shaped pillars after laser irradiation. This allows for a better control of the deposition process. Coatings 2021, 11, 662 10 of 11 We prepared PTO thin films by the PLD method using either a PbO–TiO2 pie-chart targets or PTO single target. On STO (001) substrates, which allow the epitaxial growth, the PbO–TiO2 pie-chart target with the areal ratio from 3:5 to 5:3 provided films with composition, crystallinity, flatness, and ferroelectric properties almost independent of the areal ratio. Even the PTO single target produced films with properties similar to those from pie-chart targets, despite the surface off-stoichiometry caused by the laser irradiation history. To explain this coincidence a compositional self-control (CSC) mechanism was suggested to occur during the growth of epitaxial PTO films. In conclusion, the use of PbO–TiO2 pie-chart target produces high-quality epitaxial films comparable to the ones from a PTO single target. The advantages of the pie-chart target include the robustness of the target and the long-term controllability of ablated compositions. In the preparation of films containing volatile species (Pb, Bi, Zn, Sn, Li, . . . ) by PVD methods, the mosaic-target scheme is a realistic solution. Author Contributions: Conceptualization, J.S. (Joe Sakai); funding acquisition and reviewing draft, J.S. (José Santiso); development of methodology, J.M.C.R.; performing films deposition, EDX and SEM analysis, and writing draft, J.S. (Joe Sakai); performing XRD analysis, J.P.-P.; performing XPS analysis, G.S.; performing AFM and PFM observation, P.V.-C. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by Spanish Ministry of Competitiveness (projects MAT2016- 77100-C2-1-P and PID2019-108573GB-C21) and the AGAUR agency (project 2017SGR). ICN2 is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. SEV-2017-0706). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors are grateful to Gustau Catalan, Ying Liu, Alba Fuentes, Francisco Javier Belarre Triviño, and Marcos Rosado Iglesias for their support on experiments. Conflicts of Interest: The authors declare no conflict of interest. References 1. Shirane, G.; Hoshino, S.; Suzuki, K. X-ray study of the phase transition in lead titanate. Phys. Rev. 1950, 80, 1105–1106. [CrossRef] 2. Jaffe, B.; Cook, W.R.; Jaffe, H. Piezoelectric Ceramics; Academic Press: London, UK, 1971. 3. Ueda, I.; Ikegami, S. Piezoelectric properties of modified PbTiO3 ceramics. Jpn. J. Appl. Phys. 1968, 7, 236–242. [CrossRef] 4. Carl, K. Ferroelectric properties and fatiguing effects of modified PbTiO3 ceramics. Ferroelectrics 1975, 9, 23–32. [CrossRef] 5. Norton, D.P. Pulsed laser deposition of complex materials: Progress toward applications. In Pulsed Laser Deposition of Thin Films —Applications-led Growth of Functional Materials; Eason, R., Ed.; A John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2007; pp. 8–9. 6. Martín, M.J.; Mendiola, J.; Zaldo, C. Pulsed laser deposition of Sm modified PbTiO3 ferroelectric thin films. Ferroelectr. Lett. 1997, 22, 101–105. [CrossRef] 7. Choi, T.; Lee, J. Structural and dielectric properties of artificial PbZrO3 /PbTiO3 superlattices grown by pulsed laser deposition. Thin Solid Films 2005, 475, 283–286. [CrossRef] 8. Nishino, R.; Kozuka, Y.; Uchida, M.; Kagawa, F.; Kawasaki, M. Electrical conduction on the surface of ferroelectric PbTiO3 thin film inducedby electrolyte gating. Appl. Phys. Lett. 2018, 112, 051602. [CrossRef] 9. Palkar, V.R.; Purandare, S.C.; Pai, S.P.; Chattopadhyay, S.; Apte, P.R.; Pinto, R.; Multani, M.S. c-axis oriented ferroelectric thin films of PbTiO3 on Si by pulsed laser ablation. Appl. Phys. Lett. 1996, 68, 1582–1584. [CrossRef] 10. Wu, Y.M.; Lo, J.T. Growth of PbTiO3 thin film on Si(100) with Y2 O3 and CeO2 buffer layer. Jpn. J. Appl. Phys. 1998, 37, 4943–4948. [CrossRef] 11. Seo, J.; Ahn, Y.; Kim, W.H.; Son, J.Y. Local ferroelectric responses of epitaxial PbTiO3 thin films to heated atomic force microscopy. Mater. Lett. 2016, 168, 134–137. [CrossRef] 12. Li, S.; Zhu, Y.L.; Tang, Y.L.; Liu, Y.; Zhang, S.R.; Wang, Y.J.; Ma, X.L. Thickness-dependent a1/a2 domain evolution in ferroelectric PbTiO3 films. Acta Mater. 2017, 131, 123–130. [CrossRef] 13. Lin, J.L.; Wang, Z.J.; Zhao, X.; Liu, W.; Zhang, Z.D. Microstructures and ferroelectric properties of PbTiO3 /PbZrO3 superlattices deposited by pulse laser deposition. Ceram. Int. 2018, 44, 20664–20670. [CrossRef] Coatings 2021, 11, 662 11 of 11 14. Shin, H.W.; Son, J.Y. Nonvolatile ferroelectric memory based on PbTiO3 gated single-layer MoS2 field-effect transistor. Electron. Mater. Lett. 2018, 14, 59–63. [CrossRef] 15. Feng, Y.; Tang, Y.; Ma, D.; Zhu, Y.; Zou, M.; Han, M.; Ma, J.; Ma, X. Thickness-dependent evolution of piezoresponses and stripe 90◦ domains in (101)-oriented ferroelectric PbTiO3 thin Films. ACS Appl. Mater. Interfaces 2018, 10, 24627–24637. [CrossRef] 16. Christen, H.M.; Silliman, S.D.; Harshavardhan, K.S. Continuous compositional-spread technique based on pulsed-laser deposition and applied to the growth of epitaxial films. Rev. Sci. Instrum. 2001, 72, 2673–2678. [CrossRef] 17. Koinuma, H.; Takeuchi, I. Combinatorial solid-state chemistry of inorganic materials. Nat. Mater. 2004, 3, 429–438. [CrossRef] 18. Maruyama, S.; Kubokawa, O.; Nanbu, K.; Fujimoto, K.; Matsumoto, Y. Combinatorial synthesis of epitaxial LiCoO2 thin films on SrTiO3 (001) via on-substrate sintering of Li2 CO3 and CoO by pulsed laser deposition. ACS Comb. Sci. 2016, 18, 343–348. [CrossRef] 19. Sakai, J.; Autret-Lambert, C.; Sauvage, T.; Courtois, B.; Wolfman, J.; Gervais, F. Epitaxial composition-graded perovskite films grown by a dual-beam pulsed laser deposition method. J. Cryst. Growth 2013, 380, 106–110. [CrossRef] 20. Sakai, J.; Roque, J.M.C.; Vales-Castro, P.; Padilla-Pantoja, J.; Sauthier, G.; Catalan, G.; Santiso, J. Control of lateral composition distribution in graded films of soluble solid systems A1−x Bx by partitioned dual-beam pulsed laser deposition. Coatings 2020, 10, 540. [CrossRef] 21. Hussey, B.W.; Gupta, A. Synthesis of YBa2 Cu3 O7−d films from separate oxide targets. J. Appl. Phys. 1992, 72, 287–289. [CrossRef] 22. Lee, Y.E.; Norton, D.P.; Budai, J.D. Enhanced photoluminescence in epitaxial ZnGa2 O4 : Mn thin-film phosphors using pulsed-laser deposition. Appl. Phys. Lett. 1999, 74, 3155–3157. [CrossRef] 23. Funakoshi, H.; Fumoto, K.; Okuyama, M.; Hamakawa, Y. Fabrication of lead titanate thin film by laser ablation with alternate deposition of lead oxide and titanium oxide precursors. Jpn. J. Appl. Phys. 1994, 33, 5262–5264. [CrossRef] 24. Koster, G.; Kropman, B.L.; Rijnders, G.J.H.M.; Blank, D.H.A.; Rogalla, H. Quasi-ideal strontium titanate crystal surfaces through formation of strontium hydroxide. Appl. Phys. Lett. 1998, 73, 2920–2922. [CrossRef] 25. Doughty, C.; Findikoglu, A.T.; Venkatesan, T. Steady state pulsed laser deposition target scanning for improved plume stability and reduced particle density. Appl. Phys. Lett. 1995, 66, 1276–1278. [CrossRef] 26. Dam, B.; Rector, J.; Chang, M.F.; Kars, S.; de Groot, D.G.; Griessen, R. Laser ablation threshold of YBa2 Cu3 O6+x . Appl. Phys. Lett. 1994, 65, 1581–1583. [CrossRef] 27. Hase, T.; Hirata, K.; Amanuma, K.; Hosokawa, N.; Miyasaka, Y. Preparation of Pb(Zr, Ti)O3 thin films by multitarget sputtering. Jpn. J. Appl. Phys. 1994, 33, 5244–5248. [CrossRef]