Article Microstructure and Properties of PZT Films with Different PbO Content—Ionic Mechanism of Built-In Fields Formation

Nikolay Mukhin 1,2,* , Dmitry Chigirev 2, Liubov Bakhchova 3 and Andrey Tumarkin 2 1 Institute for Micro and Sensor Systems, Otto-von-Guericke University, 39106 Magdeburg, Germany 2 Faculty of Electronics, Saint Petersburg Electrotechnical University “LETI”, Saint Petersburg 197376, Russia 3 Institute for Automation Engineering, Otto-von-Guericke University, 39106 Magdeburg, Germany * Correspondence: [email protected] or [email protected]

 Received: 8 August 2019; Accepted: 6 September 2019; Published: 10 September 2019 

Abstract: Experimental studies were conducted on the effects of oxide on the microstructure and the ferroelectric properties of lead zirconate-titanate (PZT) films obtained by the method of radio frequency (RF) magnetron sputtering of a ceramic PZT target and PbO2 powder with subsequent heat treatment. It is shown that the change in ferroelectric properties of polycrystalline PZT films is attributable to their heterophase structure with impurities of lead oxide. It is also shown that, even in the original stoichiometric PZT film, under certain conditions (temperature above 580 ◦C, duration greater than 70 min), impurities of lead oxide may be formed. The presence of a sublayer of lead oxide to a denser formation of crystallization centers of the perovskite phase, resulting in a reduction of the grain size as well as the emergence of a charge on the lower interface. The formation of the perovskite structure under high-temperature annealing is accompanied by the diffusion of lead into the surface of the film. Also shown is the effect of the lead segregation on the formation of the self-polarized state of thin PZT films.

Keywords: PZT; lead oxide; heat treatment; grain boundaries; self-polarization

1. Introduction Ferroelectric materials in both bulk and thin-film forms are widely used in electronics and technology. They are used in non-volatile memory, dynamic random-access memory, capacitors, devices designed for volume and surface acoustic waves, deflectors, radiation detectors, optical fibers and delay lines, frequency multipliers, and in photovoltaic and other devices [1–4]. At present, work aimed at obtaining and studying the properties of ferroelectric materials is being conducted in many countries in major state scientific centers [Sandia Lab (US), National Renewable Energy Lab (US), National Aeronautics and Space Administration (NASA) (US)] and with the participation of commercial companies [Intel (US), Toshiba (Japan), Gennum Inc. (Canada), Paratek Inc. (US)]. Solid solutions of lead zirconate-titanate [Pb(Zr,Ti)O3, abbreviated as PZT] are among the most widely available ferroelectric materials. Some oxides, e.g., PbO, TiO2, ZrO2, that are part of PZT have unique electrophysical and mechanical characteristics and a nanoscale structure in the case of the formation of thin layers [5–8]. PZT is interesting because it allows almost all applications of ferroelectrics to be covered due to the fact that, depending on the composition [9] and the alloying additives [10,11], it is possible to synthesize PZT with varied physical properties and material parameters over a very wide range while adapting them to the requirements of the specific practical application. Such properties are associated with high dielectric permeability and residual polarization, a large number of transition cycles, high breakdown voltage, pyroelectric and piezoelectric effects, electro-optical phenomena, photoelectricity, etc.

Materials 2019, 12, 2926; doi:10.3390/ma12182926 Materials 2019, 12, 2926 2 of 14

Because of the wide variety of processes involved in the formation of the microstructure and the properties of PZT, various research groups still continue to investigate the PZT material [11–15]. The variety of studies to control the properties and the structure of PZT based films can be divided into several branches. The first is to control the properties and the structure of PZT films by manipulating their stoichiometry and impurities [10,11]. The second is the production of epitaxial films [16]. The third is the study of interfaces and particularly the influence of mechanical stresses at the boundary of PZT films with the substrate and the sublayers [17,18]. Fourth is a study of size effects [19]. Fifth is the creation of ferroelectric composites [20], metamaterials, and superlattices such as in [21–23], where PZT can also be used. The latter branches are a field for obtaining new effects and new devices. In this paper, we focused only on the study of the effect of non-stoichiometry on the properties of PZT films. In studying how to control the properties of PZT by manipulation of the conditions in which it is synthesized, the lead behavior in PZT has become an important area of research [24–26]. The effects of formation and imperfection in PZT films on their ferroelectric polarization were explored in [27]. In [28,29], model-based representations were developed, according to which high-temperature annealing in an oxygen-containing medium of thin polycrystalline PZT films causes diffusion of lead to the grain boundary. The volatility of lead vapors during high-temperature synthesis of PZT complicates the control of the stoichiometric composition of the material [25]. The PZT exhibits a high sensitivity in its physical and its structural properties to the lead content [26,30,31]. In [25,29], it was reported that the diffusion of lead causes the formation of a lead oxide phase at the grain boundaries of the polycrystalline PZT films. It is interesting to note that, in other lead-containing materials, similar phenomena [32,33] were observed and demonstrated some commonality. The formation of a lead oxide phase may have both negative consequences in the form of deterioration of the ferroelectric properties of the PZT and positive ones in the form of new useful effects, such as the photovoltaic effect [34]. Therefore, research into the influence of lead oxide content on the kinetics of formation and the electrophysical properties of polycrystalline PZT films is very topical. The purpose of the present work is to study the influence of lead oxide and temperature-time heat treatment regimes on the microstructure and the properties of polycrystalline PZT films in order to create a scientific basis for the controlled synthesis of ferroelectric films with defined properties.

2. Materials and Methods To study the influence of lead oxide on the properties of PZT and the thin-film structures based on it, several series of samples were prepared. Thus, three series of samples of film structures were produced: a) Capacitor structures with polycrystalline PZT films with the stoichiometric composition: Pt/PZT/Pt/Ti/SiO2/Si; b) Capacitor structures of the same type as variant (a) but with PZT films with 10 mol.% of lead oxide excess; c) Multilayer structures with PZT films and a thin sublayer of lead oxide between the ferroelectric film and the lower structure-forming electrode Pt/PZT/PbO/Pt/Ti/SiO2/Si.

Films of PZT and lead oxide were obtained by the method of reactive radio frequency (RF) magnetron sputtering on a specially designed piece of equipment inside the vacuum chamber of which were arranged two magnetrons with a diameter of 100 mm connected to a high-frequency generator operating at a frequency of 13.56 MHz and with a facility to vary the input power to the magnetron from 10 W to 1000 W. The target used for the sputtering of the lead oxide and the PZT films was a powder consisting of analytically pure PbO2 distributed in a uniform layer over the entire surface of the magnetron and industrial ferroceramic PZT with the composition Pb(Zr0.58Ti0.42)O3. The Zr/Ti content of PZT ceramic was selected due to the fact that its composition is near the morphotropic phase boundary and exhibits increased structural sensitivity of its properties, thus allowing the greatest Materials 2019, 12, 2926 3 of 14 possible expected changes in the material properties as a function of the lead content. As a rule, when sputtering oxygen-containing materials, a deviation in the stoichiometry of the oxygen is observed in the film obtained, and thus to compensate for the loss of oxygen in the growing films, the sputtering of the targets was performed in a gas mixture of argon and oxygen. The parameters of the deposition process are shown in Table1. PZT films of different thicknesses from 0.3 to 1.5 µm were formed using a two-stage method. In the first stage, the PZT films were applied by RF magnetron sputtering onto the cold substrate ( 130 C), allowing better control of their composition. The films were then annealed in air in a muffle ≈ ◦ furnace to induce the perovskite crystallization phase. Heat treatment was carried out using different temperature and time conditions (from 530 to 630 ◦C for 30–180 min). The films were heated in the furnace (ETALON, Omsk, Russia) at a rate of 10 ◦C/min, and the cooling was carried out at the same rate as the oven (ETALON, Omsk, Russia). To exclude various random factors, all the films were formed in a single technological deposition cycle. The lower structure-forming electrode was formed by platinum films with a thickness of 120 nm, which were deposited by the method of -plasma sputtering in a triple-electrode system on substrates coated with an adhesive titanium sublayer.

Table 1. Parameters of the deposition processes for PbO and lead zirconate-titanate (PZT) films.

Parameters of Technological Process PbO PZT Power of radio frequency (RF) discharge, watt 38 100 Diameter of target, mm 100 100 Composition of gaseous mixture 76%Ar + 24%O2 76%Ar + 24%O2 2 2 Working pressure of gaseous mixture, mm Hg 10− 10− Temperature of substrate, ◦C 130 or 400 130 Growth rate of films, nm/min 10 5.8

The magnitudes of the coercive fields and the residual polarization of the PZT films were determined from the dielectric hysteresis loops obtained by the Sawyer Tower method. The values of the dielectric constant and the tan delta angle or the dissipation factor were determined by means of an E7-12 immittance meter (Tetron, Moscow, Russia) at a frequency of 1 MHz. Investigation of the morphology of the surface layers was performed on an atomic-powered microscope (NT-MDT Spectrum Instruments, Zelenograd, Russia) with a resolution of 10 nm in the horizontal plane of the film under test and with no less than 1 nm in the vertical. The microstructure layers were investigated by SEM on a system supplied by the Helios Nanolab D449 FEI Company, Hillsboro, OR, USA. Investigations of the elemental composition were carried out on an ECO-3 Auger spectrometer (Burevestnik, St. Petersburg, Russia). The main technical parameters of the equipment were as follows: 9 vacuum in the measuring chamber, <10− mm Hg; energy of the primary electrons, 3 keV; probe diameter, 5 µm; electron probe current, 5 10 7 A; resolution factor (E/dE), 250. The phase × − composition of the samples was studied using X-ray diffraction on a Shimadzu XRD 6000 diffractometer, Shimadzu Corp., Kyoto, Japan. The identification of the phases was carried out using the X-ray analysis database Powder Diffraction File.

3. Results The measurement results of dielectric hysteresis loops and capacitance-voltage characteristics (CV) for PZT films, which were crystallized at 550 ◦C, 580 ◦C, and 600◦C, are shown in Figure1. These are the results of stoichiometric PZT films of relatively large thickness—around 1.5 µm—which was chosen in order to see the volumetric effects. The values of residual polarization (Pr), coercive field (Ec), control coefficient (C0/Cmin), and dielectric permittivity (ε) of PZT films were determined by experimental dependencies. The results are presented in Table2. Materials 2019, 12, x FOR PEER REVIEW 4 of 14

135 Materials 2019, 12, 2926 Table 2. Electro-physical parameters of PZT films. 4 of 14

Heat Treatment Conditions Pr, Ec, ε tgδ C0/Cmin T, °C t, Tablemin 2. Electro-physical parameters of PZTµC/cm films.2 kV/cm 530 Heat Treatment70 Conditions 120 0.085 - - 1 ε tgδ P ,µC/cm2 E ,kV/cm C /C T, C t, min r c 0 min 530 ◦ 120 70 0.019 - - 1 530 70 120 0.085 - - 1 550 70 400 0.176 18 87 1.9 530 120 70 0.019 - - 1 550 550120 70 350 400 0.1760.22 1814 8767 1.9 1.8 550 120 350 0.22 14 67 1.8 580 70 770 0.165 20 68 2.5 580 70 770 0.165 20 68 2.5 580 580120 120 730 730 0.1740.174 14.514.5 4242 2.6 2.6 600 70 750 0.09 22 162 3.1 600 70 750 0.09 22 162 3.1 600 120 570 0.085 17 152 2.6 600 630120 70 570 560 0.120.085 1917 171152 2.5 2.6 630 120 510 0.11 14 160 2.3 630 70 560 0.12 19 171 2.5 630 120 510 0.11 14 160 2.3 These PZT films were deposited by sputtering a stoichiometric target without any lead oxide excess. 136 These PZT films were deposited by sputtering a stoichiometric target without any lead oxide excess.

(a) (b)

137 FigureFigure 1. CharacteristicsCharacteristics of of PZT PZT filmsof filmsof 1.5 µ m1.5 thickness µm thic formedkness atformed different at crystallization different crystallization temperatures: 138 temperatures:(a) hysteresis ( loops;a) hysteresis (b) the loops; dependences (b) the dependences of the reversible of the dielectric reversible permittivity dielectric on permittivity the electric on field. the 139 electric field. The studies showed a significant effect of heat treatment parameters on the structure and the 140 propertiesThe studies of PZT showed films. Filmsa significant annealed effect at 550 of ◦heatC had treatment low ε, P rparameters, and C0/Cmin onvalues. the structure This was and due the to 141 propertiesthe fact that, of PZT at this films. annealing Films temperature,annealed at 550°C the structure had low of ε the, Pr, films and remainedC0/Cmin values. heterophase This was (Figure due to2), 142 thei.e., fact along that, with at this perovskite annealing crystallites, temperature, the the inclusions structure of of pyrochlore the films remained phase were heterophase preserved. (Figure With 143 2),increasing i.e., along temperature, with perovskite the sharecrystallites, of pyrochlore the incl phaseusions decreased,of pyrochlore which phase led were to the preserved. growth of Withε and 144 increasingPr values temperature, at the temperature the share of580 of pyrochlore◦C. At further phase increase decreased, of annealing which led temperature, to the growth the of observedε and Pr 145 valuesdecrease at the of dielectric temperature permeability of 580 °C. values At further could beincrease connected of annealing with formation temperature, of thin the layers observed of lead 146 decreaseoxide. This of dielectric occurred withinpermeability the intergranular values could space be connected and at the interfacewith formation of the thin of thin film layers with platinum of lead 147 oxide.electrodes. This occurred The presence within of thethe heterophase intergranular structure space ofand PZT at filmsthe interface was confirmed of the bythin the film results with of 148 platinumgrazing incidenceelectrodes. X-ray The presence diffraction of (GXRD).the heteroph Afterase heat structure treatment, of PZT PZT films films was had confirmed a polycrystalline by the 149 resultsstructure of withgrazing a preferential incidence orientationX-ray diffraction in (110) direction,(GXRD). After as shown heat in treatment, Figure2. PZT films had a 150 polycrystallineWith an increase structure of with annealing a preferential temperature orientation up to 580 in (110)◦C, the direction, degree of as texturing shown in of Figure the films 2. also 151 grew.With With an furtherincrease increase of annealing in temperature temperature or up heat to treatment580 °C, the time, degree the of appearance texturing of of the a leadfilms oxidealso 152 grew.phase With was further observed, increase which in was temperature accompanied or heat by thetreatment decrease time, in intensitythe appearance of all the of reflexesa lead oxide from 153 phaseperovskite was observed, phase. It waswhich assumed was accompanied that the formation by the ofde leadcrease oxide in intensity occurred of near all the reflexes interfaces from and 154 perovskiteat the PZT phase. crystallites It was periphery. assumed that Similarly, the formation the structural of lead changesoxide occurred associated near with the interfaces a decrease and in theat 155 thedegree PZT ofcrystallites ordering causedperiphery. bythe Similarly, appearance the stru of leadctural oxide changes should associated have led with to a modificationa decrease in of the the films’ surfaces.

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156 degreeMaterials 2019of ordering, 12, x FOR PEERcaused REVIEW by the appearance of lead oxide should have led to a modification5 of 14 of the 157 Materialsfilms’2019 surfaces., 12, 2926 5 of 14 158156 degreeThe of resultsordering of caused the study by the of appearance the surface of morphologylead oxide should of PZT have films led to that a modification were exposed of the to the 159157 annealingfilms’ surfaces. in air in different conditions showed that the film that was treated at 530 °C was The results of the study of the surface morphology of PZT films that were exposed to the annealing 160158 distinguishedThe results by of a lowerthe study surface of the roughness surface asmorphology compared ofto thePZT films films formed that were at higher exposed temperatures. to the in air in different conditions showed that the film that was treated at 530 C was distinguished by a 161159 Atannealing the same in time,air in PZT different films formedconditions at 530 showed °C were that characterized the film that by was small◦ treated dielectric at 530 constant, °C was which 160 lower surface roughness as compared to the films formed at higher temperatures. At the same time, 162 indicateddistinguished that by the a filmslower were surface in roughnessthe pyrochlore as compar phasede, the to the formation films formed of which at higher begins temperatures. at temperatures 161 PZTAt filmsthe same formed time, at PZT 530 filmsC were formed characterized at 530 °C were by smallcharacterized dielectric by constant, small dielectric which constant, indicated which that the 163 above 450 °C. ◦ 162 filmsindicated were inthat the the pyrochlore films were phase, in the thepyrochlore formation phas ofe, which the formation begins atof temperatureswhich begins at above temperatures 450 ◦C. 163 above 450 °C.

164 Figure 2. X-ray diffraction patterns of PZT films of 1.5 µm thickness formed at different temperatures 164 FigureFigure 2. 2.X-ray X-ray di diffractionffraction patternspatterns ofof PZT films films of of 1.5 1.5 µmµm thickness thickness formed formed at atdifferent different temperatures temperatures 165 and times of treatment in an oxygen-containing medium. The inset shows the surface morphology of 165 andand times times of of treatment treatment in in an anoxygen-containing oxygen-containing medium. The The inset inset shows shows the the surface surface morphology morphology of of 166166 PZTPZT films filmsfilms after afterafter heat heat heat treatment treatment treatment in in the in the followingthe following following conditions: conditions: conditions: (a ()a 580) (580a)◦ C 580°C for for°C 70 70for min; min; 70( bmin; ()b 580) 580 (b◦)C °C580 for for 120°C 120 for min; 120 167167 (c)min; 600 ◦((cCc)) 600 for600 120°C °C for for min. 120 120 min. min.

168168 WithWith an anan increase increaseincrease in in thein the the annealing annealing annealing temperature, temperature, temperature, the the surfacethe surface surface of of the theof films thefilms films became became became rougher rougher rougher due due to todue the to 169169 crystallizationthe crystallization process process process of the of of perovskitethe the perovskite perovskite phase phase phase and and the and the formation the formation formation of of a a columnarof columnar a columnar structure structure structure (Figure(Figure (Figure 3). 170170 The3). surface The surfacesurface of PZT ofof films PZTPZT formedfilms films formed formed at 580 ◦at C at580 for 580 70°C °C minfor for 70 was 70min characterizedmin was was characterized characterized by the formationby theby formationthe of formation individual of of 171171 150–250individual nm crystallites. 150–250150–250 nm nm Longercrystallites. crystallites. heat Longer treatmentLonger heat heat attreatment 580treatment◦C ledat 580at to 580 a°C decrease led°C ledto a to decrease in a the decrease size in ofthe in the size the crystallite ofsize the of the 172172 spacingcrystallite and thespacingspacing surface andand roughness, the the surface surface and roughness, roughness, at 600 ◦C, and they and at were at600 600 decomposed°C, °C,they they were intowere decomposed fully decomposed formed into crystallites intofully fully 173173 withformed a diameter crystallitescrystallites of 250–350 withwith a adiameter nm.diameter Based of of 250–350 on 250–350 thestudied nm. nm. Based Based series on ofonthe samples, thestudied studied theseries maximumseries of samples, of samples, dielectric the the 174174 constantmaximum value dielectricdielectric (770) was constantconstant observed value value with (770) the(770) filmswas was ob annealed servedobserved atwith 580with the◦ C. thefilms Apparently, films annealed annealed such at low 580at values580°C. °C. 175175 wereApparently, due to incorrect suchsuch low low stoichiometry values values were were due ofdue theto to incorrect films, incorrect since stoichiometry stoichiometry the formation of the of of thefilms, PZT films, layerssince since the occurred formationthe formation without of of 176176 compensationPZT layers occurredoccurred for the losswithout without of lead compensati compensati oxide. onon for for the the loss loss of leadof lead oxide. oxide.

(a) (b) (a) (b) Figure 3. SEM images of PZT film: (a) side view of PZT film on Pt electrode; (b) PZT surface. Materials 2019, 12, 2926 6 of 14

The decrease in ε with increased temperature above 580◦C or heat treatment times above 120 min (at 580 ◦C) was associated with pinning of polarization processes by charges located in the formed oxide layers of lead. It was assumed that the formation of lead oxide interlayers occurred via lead atoms located in the perovskite crystal grid. Their exit to the intergranular and the interface boundaries led to a decrease in the switching volume of the ferroelectric material, which was then accompanied by a decrease in ε. Also, during longer heat treatment, the thickness of PbO interlayers increased; consequently, the number of pinning charges at the interfaces also went up, which led to the removal of an even larger volume of ferroelectrics from the polarization processes. The latter was confirmed by the growth of switching fields for films annealed at 600 ◦C (Figure1). The observed decrease in the switching fields and the change in the shape of the dielectric hysteresis loops with increasing heat treatment time could be associated with the formation of depolarizing fields, symmetrically located with respect to the upper and the lower electrodes. Assuming that the dielectric constant of PbO was about 10 and the capacity of lead oxide located in the intergranular space could be neglected, the main contribution was made by PbO interface layers arranged in series with the perovskite phase with ε 800–900, where the total thickness of those layers ≈ was estimated at ~ 9–12 nm for films annealed at 600 ◦C for 120 min. The strong pinning of polarization processes by charges in the lead oxide layers led to the presence of a rather narrow capacitive hysteresis in the CV characteristics (Figure1b), which created conditions for the use of such films in electric field-controlled capacitors. The capacitance reproducibility for increasing and decreasing bias voltages to the capacitor structure was studied by applying a constant bias voltage from 5 to 35 V and back to 5 V for several cycles. It was found that the control ratio for PZT films annealed at 580 ◦C and 600 ◦C was 2.18, and the maximum capacity bias from the nominal value did not exceed 6%. We have also studied the effect of lead oxide sublayer on PZT films and PZT-based thin-film structures properties (Figure4). Figure4a shows that PZT film formed at 600 ◦C, without a sublayer, is characterised by an average grain size of 90 nm and a surface roughness of 50 nm. A film with 10 nm sublayer has a grain size of 70 nm and roughness of 60 nm, and with 20 nm sublayer, the size of grain is 30 nm and the roughness is 10 nm. Thus, the presence of a lead oxide sublayer results in a denser formation of crystallization centers of the perovskite phase, thereby decreasing the grain size and increasing their number. This prevents the formation of grain conglomerates, which leads to a significant decrease in the surface roughness of the PZT film. Materials 2019, 12, 2926 7 of 14 Materials 2019, 12, x FOR PEER REVIEW 7 of 14

(a) (b) (c)

209 Figure 4. Atomic Force Microscopy (AFM) imageimage of the PZT filmfilm surface: ( a) without a PbO sublayer; 210 ((bb)) withwith 1010 nmnm PbOPbO thickthick sublayer;sublayer; ((cc)) withwith 2020 nmnm thickthick PbOPbO sublayer.sublayer.

211 The studiesstudies ofof thethe electrophysicalelectrophysical characteristicscharacteristics (Figure5 5)) showed showed thatthat the the dielectric dielectric constant constant 212 of the sample without without a a sublayer sublayer was was much much higher higher (ε( =ε 760)= 760) thanthan that that of a of sample a sample with with a sublayer a sublayer (ε = 213 (410).ε = 410). The TheCV CVcharacteristic characteristic of the of the sample sample without without the the sublayer sublayer was was symmetrical symmetrical and and had had a control 214 ratioratio of 2.8, 2.8, while while the the CV CV characteristic characteristic of of the the sample sample with with the the sublayer sublayer was was not notsymmetrical symmetrical and andwas 215 wasshifted shifted by approximately by approximately 0.7 0.7V towards V towards positive positive voltages; voltages; the the control control ratio ratio was was 2.4. PZT filmsfilms 2 216 withoutwithout aa sublayersublayer and and with with PbO PbO were were characterized characterized by by residual residual polarization polarization of 13 of and 13 and 11 µ C11/cm µC/cmand2 217 aand coercive a coercive field of field 110 andof 110 185 kVand/cm, 185 respectively. kV/cm, respectively. It should be It noted should that be the noted dielectric that hysteresis the dielectric loop 218 ofhysteresis the PZT-PbO loop of sample the PZT-PbO shifted tosample the left, shifted which to spokethe left, to which the presence spoke to of the a built-in presence electric of a built-in field of 219 aboutelectric 15 field kV/cm. of Theabout results 15 kV/cm. of the studyThe results showed of that the high-temperature study showed that heat high-temperature treatment of PZT filmsheat 220 ledtreatment to the loss of PZT of lead films oxide. led to We the attempted loss of lead to experimentallyoxide. We attempted determine to experimentally such losses with determine the following such 221 experiment,losses with wherethe following 10 nm thick experiment, PZT films where samples 10 with nm a thick formed PZT perovskite films samples structure with (the formationa formed 222 tookperovskite place in structure air for 30 (the min formation at a temperature took place of 580 in◦C) air were for further30 min heatat a treatedtemperature for 180 of min. 580 The °C) Auger were 223 spectrafurther ofheat PZT treated films samplesfor 180 (Figuremin. The6) wereAuger taken spec immediatelytra of PZT films after RFsamples magnetron (Figure sputtering, 6) were taken after 224 theimmediately formation after of the RF perovskite magnetron phase, sputtering, and after after additionalthe formation annealing. of the perovskite Comparing phase, the spectra and after of 225 samples,additional the annealing. results of whichComparing are summarized the spectra in of Table samples,3, it can the be results noted of that, which during are the summarized formation ofin 226 theTable perovskite 3, it can be phase, noted the that, amount during of the lead formation oxide on of the the film perovskite surface decreased phase, the more amount intensively of lead oxide than 227 duringon the film additional surface heat decreased treatment. more This intensively proved that, than during during the additional formation heat of the treatment. perovskite This structure, proved 228 thethat, di duringffusion the of lead formation oxide toof thethe filmperovskite surface structure, occurred morethe diffusion intensively of lead than oxide after itsto the formation. film surface 229 occurred more intensively than after its formation.

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(a) (b) (a) (b) 230 Figure 5. Comparison of the characteristics of PZT films with and without a PbO sublayer: (a) 230 FigureFigure 5. ComparisonComparison of of the the characteristics characteristics of PZT of PZT films films with andwith without and without a PbO sublayer: a PbO sublayer: (a) Hysteresis (a) 231 Hysteresis loops; (b) The dependences of the reversible dielectric permittivity on the electric field. 231 Hysteresisloops; (b) Theloops; dependences (b) The dependences of the reversible of the dielectric reversible permittivity dielectric onpermittivity the electric on field. the Theelectric thickness field. 232 The thickness of PZT film was 400 nm. 232 Theof PZT thickness film was of PZT 400 was 400 nm.

(a) (b) (c) (d) (a) (b) (c) (d) 233 FigureFigure 6. 6.Auger Auger spectra spectra of of PZT PZT films: films: ( a()a) after after deposition; deposition; ( b(b)) after after the the formation formation of of the the perovskite perovskite 233 Figure 6. Auger spectra of PZT films: (a) after deposition; (b) after the formation of the perovskite 234 phase;phase; (c ()c after) after annealing annealing in in air; air; (d ()d after) after vacuum vacuum annealing. annealing. 234 phase; (c) after annealing in air; (d) after vacuum annealing. Table 3. Intensities of PZT films components signal determined on the basis of Auger spectra. 235 Table 3. Intensities of PZT films components signal determined on the basis of Auger spectra. 235 Table 3. Intensities of PZT films components signal determined on the basis of Auger spectra. PZT Films Pb O Ti Zr PZT Films Pb O Ti Zr After depositionPZT Films on the “cold” substrate 3 Pb 9.75 O 2.25 Ti 1 Zr AfterAfter deposition the formation on the of the“cold” perovskite substrate phase 2.5 3 10.5 9.75 3 1 2.25 1 After deposition on the “cold” substrate 3 9.75 2.25 1 After theAfter formation repeated of the annealing perovskite in air phase 1.5 2.5 8 10.5 3.25 1 3 1 After the formation of the perovskite phase 2.5 10.5 3 1 After Afterrepeated vacuum annealing annealing in air 1.5 1.5 7 8 3 1 3.25 1 After repeated annealing in air 1.5 8 3.25 1 After vacuum annealing 1.5 7 3 1 After vacuum annealing 1.5 7 3 1 The results of the experiment agree with the concept of the formation of electrostatic charge on 236 The results of the experiment agree with the concept of the formation of electrostatic charge on 236 the freeThe surface results ofof thethe PZT experiment film associated agree with with the the concept surface segregationof the formation of lead of ions. electrostatic When the charge film was on 237 the free surface of the PZT film associated with the surface segregation of lead ions. When the film 237 thecooled free belowsurface the of Curie the PZT temperature, film associated this ionic with charge the surface formed segregat a self-polarizedion of lead PZT ions. state When (Figure the 7film). 238 was cooled below the Curie temperature, this ionic charge formed a self-polarized PZT state (Figure 238 was cooledAfter below deposition the Curie of the temperature, upper platinum this ionic electrodes charge formed (at ~150 a self-polarized◦C), the capacitance-voltage PZT state (Figure 239 7). 239 7).characteristics of the structure had a maximum internal field. With an increase in the annealing 240 After deposition of the upper platinum electrodes (at ~150°C), the capacitance-voltage 240 temperatureAfter deposition to the Curie of value the (300upper◦C), platinum the effect ofelec self-polarizationtrodes (at ~150°C), decreased, the sincecapacitance-voltage the conditions at 241 characteristics of the structure had a maximum internal field. With an increase in the annealing 241 characteristicsthe upper boundary of the ofstructure the PZT filmhad hada maximum already changed internal (thefield. metal With shields an increase the surface in the ionic annealing charge), 242 temperature to the Curie value (300 °C), the effect of self-polarization decreased, since the conditions 242 temperatureand the domain to the structure Curie value of the (300 film °C), after the cooling effect of formed self-polarization in a much decreased, smaller field since of thethe pre-surfaceconditions 243 at the upper boundary of the PZT film had already changed (the metal shields the surface ionic 243 atlayer. the Withupper an boundary increase inof the the annealing PZT film temperature had already up changed to 600 ◦ C,(the the metal capacitance shields increased,the surface and ionic the 244 charge), and the domain structure of the film after cooling formed in a much smaller field of the 244 charge), and the domain structure of the film after cooling formed in a much smaller field of the 245 pre-surface layer. With an increase in the annealing temperature up to 600 °C, the capacitance 245 pre-surface layer. With an increase in the annealing temperature up to 600 °C, the capacitance

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246 internalincreased, field and practically the internal disappeared field practically due to disappe the diffaredusive due equalization to the diffusive of the equalization PZT composition of the PZT in the 247 pre-electrodecomposition layerin the andpre-electrode its partial layer transition and its to partial the perovskite transition the perovskite phase.

(a) (b)

248 FigureFigure 7. 7.The The dependences dependences of of parameters parameters ofof PtPt/PZT/Pt/PZT/Pt structurestructure onon thetheannealing annealing temperature:temperature: ((aa)) the 249 dependencethe dependence of the of dielectric the dielectric constant cons ontant external on external electric electric field; (field;b) the (b dependence) the dependence of the of dielectric the 250 constantdielectric and constant the internal and the electric intern fieldal electric for the Ptfield/PZT for/Pt the structures Pt/PZT/Pt on thestructures annealing on temperature.the annealing PZT 251 filmstemperature.PZT of 300 nm thickness films of were 300 nm fabricated thickness by were magnetron fabricated sputtering by magnetron of PZT targetsputtering with of 10 PZT mol.% target of PbO 252 excesswith 10 with mol.% following of PbO crystallization.excess with following crystallization.

253 4.4. Discussion Discussion 254 TheThe variety variety of of processes processes duringduring thethe formationformation ofof PZT filmsfilms at the annealing stage stage are are depicted depicted in 255 Figurein Figure8a–c. 8a–c. Figure Figure8a shows 8a shows nucleation nucleation of the perovskiteof the perovskite (Pe) phase (Pe) in phase a pyrochlore in a pyrochlore (Py) or amorphous (Py) or 256 matrixamorphous (depending matrix on (depending the film deposition on the film conditions) deposition on variousconditions) inhomogeneities on various inhomogeneities (substrate, defects, 257 PbO(substrate,λ inclusions).Figure defects, PbO8λb showsinclusions).Figure growth of perovskite8b shows crystallitesgrowth of inperovskite the pyrochlore crystallites matrix inlimited the 258 bypyrochlore segregation matrix of excess limited PbO byλ ,segregation by other defects, of excess and PbO by theλ, by growth other defects, of neighboring and by the grains. growth Figure of 8c 259 showsneighboring the final grains. stage Figure of crystallization 8c shows the of final the stage film, theof crystallizatio grain boundaryn of the segregation film, the grain of PbO boundaryλ, and the 260 formationsegregation of intergranularof PbOλ, and the precipitates formation of of PbO intergranularλ. The formed precipitates film was of an PbO openλ. The system formed and film exchanged was 261 oxygenan open with system the and vapor-gas exchanged medium; oxygen in with addition, the vapor-gas partial removalmedium; ofin addition, the most partial volatile removal component, of 262 the most volatile component, PbOλ molecules, from the film was possible. Figure 8 also illustrates PbOλ molecules, from the film was possible. Figure8 also illustrates the e ffect of local stoichiometry 263 disturbancesthe effect of local on the stoichiometry polarization disturbances pinning in the on resultingthe polarization PZT films. pinning in the resulting PZT films. 264 TheThe study study showedshowed aa significantsignificant effect effect of of heat heat treatment treatment parameters parameters on on the the structure structure and and the the 265 properties of PZT films. It was shown (Figure 2) that, even in the original stoichiometric PZT film, properties of PZT films. It was shown (Figure2) that, even in the original stoichiometric PZT film, 266 under certain conditions (temperature above 580 °C, duration greater than 70 min), impurities of under certain conditions (temperature above 580 C, duration greater than 70 min), impurities of lead 267 lead oxide could be formed. This was because the◦ thermal treatment led to partial decomposition of oxide could be formed. This was because the thermal treatment led to partial decomposition of the 268 the PZT grain with the removal of the most active oxide (i.e., lead oxide) based on the following PZT grain with the removal of the most active oxide (i.e., lead oxide) based on the following reaction: 269 reaction:

{Pb(Zr,{Pb(Zr, Ti)O 3Ti)O}N 3}PbN→NPbmN(Zr,−m(Zr, Ti) Ti)NONO3N3N−mmλ ++ mmPbOPbOλ λ+ m+VmPbV +Pb m+λVmOλ VO (1) (1) → − − 270 where PbOλ is a lead oxide molecule that enters the PZT grain boundary; N is the total number of where PbOλ is a lead oxide molecule that enters the PZT grain boundary; N is the total number of 271 particles in the grain; m is the number of particles removed from the grain; VPb and VO are lead and m 272 particlesoxygen invacancies the grain; (at crystallizationis the number temperatures of particles of removed PZT, they from are theionized). grain; The VPb presenceand VO ofare an lead 273 andincreased oxygen number vacancies of ionized (at crystallization vacancies in temperaturesPZT grains could ofPZT, lead to they domain are ionized). wall pinning The presence(Figure 8d), of an 274 increasedwhich could number result of in ionized a decreased vacancies volume in of PZT the grains ferroelectric could film lead switched to domain by wallthe field. pinning This (Figureaffected8 d), 275 whichthe decrease could result of the in nonlinearity a decreased in volume the CV of characteristic the ferroelectric and filmthe decrease switched of by the the rectangularity field. This aff ectedof 276 thehysteresis decrease loops, of the as nonlinearityshown in Figure in the1. Equation CV characteristic (1) could be and reversed the decrease if the formed of the material rectangularity had an of 277 hysteresisexcess of loops,lead. However, as shown an in increase Figure1 .in Equation lead oxide (1) inclusions could be reversedalso led to if a the decrease formed in material the effective had an 278 excessswitching of lead. volume However, of the PZT an increasegrains (Figure in lead 8e), oxide also due inclusions to the increased also led conductivity to a decrease of in lead the oxide. effective switching volume of the PZT grains (Figure8e), also due to the increased conductivity of lead oxide. Materials 2019, 12, 2926 10 of 14 Materials 2019, 12, x FOR PEER REVIEW 10 of 14

279 Figure 8. TheThe processes processes during during the the formation formation of of PZT PZT films films at at the annealing and the effect effect of local 280 stoichiometry disturbancesdisturbances onon the the polarization polarization pinning pinning in in the the resulting resulting PZT PZT films: films: (a) ( nucleationa) nucleation of the of 281 perovskitethe perovskite (Pe) (Pe) phase phase in a pyrochlorein a pyrochlore (Py) or (Py) amorphous or amorphous matrix matrix on various on various inhomogeneities; inhomogeneities; (b) growth (b) 282 growthof perovskite of perovskite crystallites; crystallites; (c) grain (c) boundary grain boundary segregation segregation of PbO ofλ ;(PbOd) domainλ; (d) domain wall pinningwall pinning in PZT in 283 PZTgrains grains by ionized by ionized vacancies; vacancies; (e) PbO (e)λ PbOinclusionsλ inclusions and polarization and polarization pinning; pinning; (f) self-polarized (f) self-polarized film due film to 284 duethe doubleto the double charge charge defect layer.defect layer.

285 The choice ofof thethe optimal optimal value value of of the the lead lead concentration concentration was was not not so trivial so trivial and and was associatedwas associated with 286 withthe tasks the tasks being being solved. solved. The di Theffusion diffusion of lead of ions lead in ions the in film the led film to theled formationto the formation of a positive of a positive charge 287 chargeon the freeon the surface free ofsurface the PZT of the film PZT (Figure film8 f).(Figure The release 8f). The of leadrelease ions of tolead the ions surface to the was surface associated was 288 withassociated the formation with the offormation negatively of chargednegatively vacancies charge ind vacancies the bulk of in PZT. the Thus,bulk of a doublePZT. Thus, charged a double layer 289 wascharged formed. layer Oxygen was formed. ions and Oxygen oxygen ions vacancies and oxygen partially vacancies compensated partially for compensated these charges, for but these not 290 charges,completely, but sincenot completely, the formation since energies the formation of oxygen ener andgies leadof oxygen vacancies and lead in PZT vacancies were very in PZT diff erent.were 291 Thevery enthalpydifferent. of The formation enthalpy of anof formation ionized vacancy of an ionized in oxygen vacancy was 5.4 in eV oxygen and in was lead 5.4 was eV 3.3 and eV. in lead 292 was 3.3Let eV. us try to estimate the surface potential associated with the segregation of lead ions. Consider 293 the caseLet ofus a quasi-chemicaltry to estimate equilibrium the surface “PZT potential film surface-gas” associated (at with a crystallization the segregation temperature of lead of ions. PZT) 294 Considerin which thethe atomscase of in a the quasi-chemical film are mobile equilibrium and where “PZT both electronsfilm surface-gas” and atoms (at can a becrystallization rearranged. 2+ 2 295 Thistemperature leads to of achange PZT) in in which the number the atoms of all in defects, the film particularly are mobile toand the where release both of Pbelectronsand Oand− ionsatoms to 296 canthe PZTbe rearranged. film surface. This The leads following to a change also occur: in the surfacenumber charge of all defects, compensated particularly by the to space the release charge of in 297 Pbthe2+ volume and O2− of ions the to film the causes PZT film the appearancesurface. The of followin an electricg also potential, occur: surface the concentration charge compensated of individual by 298 thedefects space varies charge according in the tovolume the Boltzmann of the film ratio, causes and the the space appearance charge density of an andelectric the fieldpotential, potential the 299 concentrationare related by theof individual Poisson equation. defects Invaries this case,according it is necessary to the Boltzmann to take into ratio, account and thethe equilibrium space charge of 300 densityatomic and and electronic the field potential defects. Thus, are related for a PZT by the film, Poisso in then case equation. of Schottky In this volume case, it disordering, is necessary we to take may h i h i 1/2 00 = •• = 301 intoderive account as follows: the equilibriumVPb V Oof atomicKS . and Hereinafter electronic in thedefects. formulas Thus, Kreger’s for a PZT notation film, in is usedthe case [35]. of 302 SchottkyThe surfacevolume charge disordering, arises due we to may the factderive that theas follows: energy of [V the ] formation= [V··] =𝐾 of vacancies⁄ . Hereinafter by removing in the 2+ 2 303 formulasions from Kreger’s the crystal notation to the surfaceis used [35]. varies for both types of ions. The concentrations of Pb and O − 304 ions onThe the surface surface charge of the crystalarises aredue caused to the byfact the that diff erencethe energy in the of vacancy the formation concentrations of vacancies inside PZT. by 305 Theremoving formulation ions from of the the equations crystal to ofthe reactions, surface varies according for both to which types individual of ions. The vacancies concentrations are formed, of Pb is2+ 306 andcomplicated O2− ions on by the surface fact that of the the idea crystal of the are e causedffective by charge the difference and vacancies, in the tovacancy some extent,concentrations loses its 307 insidemeaning PZT. near The the formulation surface. In this of the regard, equations we use of these reactions, concepts according formally to to which explain individual surface phenomena. vacancies 2+ 2 308 are formed,Let’s assume is complicated that in certain by the places fact that of thethe surfaceidea of therethe effective are Pb chargeions andand invacancies, others, O to− someions. 309 extent,Denote loses the first its positionsmeaning bynear SPb,S the, thesurface. second In by this SO,S regard,. As a rule,we use these these are concepts not any places formally on theto explain surface, 310 surfacebut places phenomena. on the edges and corners of the growing face (i.e., so-called places of self-generation). 2 311 Let’sIt should assume be noted that that,in certain likewise places inside of PZT,the surface SPb,S are there vacancies are Pb near2+ ions oxygen and ions in others, O − and O S2−O,S ions.are vacancies near lead ions Pb2+. As a result, the capture by the O2 ion of the unoccupied space of S 312 Denote the first positions by SPb,S, the second by SO,S. As a rule,− these are not any places on theO,S 313 surface, but places on the edges and corners of the growing face (i.e., so-called places of 314 self-generation). 315 It should be noted that, likewise inside PZT, SPb,S are vacancies near oxygen ions O2− and SO,S are 316 vacancies near lead ions Pb2+. As a result, the capture by the O2− ion of the unoccupied space of SO,S

Materials 2019, 12, 2926 11 of 14

contributes to the formation of a new unoccupied space SPb,S and vice versa. Therefore, the transfer of 2+ 2 Pb and O − from the PZT volume to the surface can be put down as follows:

x x 2+ x x x 2 x Pb + S Pb + V00 + S ;O + S O − + V•• + S (2) Pb Pb,S → Pb,S Pb O,S O O,S → O,S O Pb,S

The electrochemical potentials ηi for structural elements with effective chemical potentials ξi and ziq charges are determined by the equation ηi = ξi + ziqφ, where φ is the electric potential. For any P reaction in which structural elements with νi number take part, at equilibrium: viηi = 0. From (2): i

   2+   x   x   x  η V00 + η Pb η Pb + η S η S = 0 (3) Pb Pb,S − Pb O,S − Pb,S

Equation (3) looks the same for oxygen ions. For uncharged structural elements ηi = ξi. If you accept that inside the PZT film (x = ) φ = 0, and on the surface (x = 0) φ = φ0, then inside the film:      2+ ∞  x  η V00 = ξ V00 . Given that ξ Pb ξ Pb , the Equation (3) is simplified to (and the same is for Pb Pb Pb,S ≈ Pb oxygen ions):

   x   x     x   x  ξ 00 + ξ ξ = φ ξ •• ξ + ξ = φ VPb SO,S PbPb,S 2q 0; VO SO,S PbPb,S 2q 0 (4) ∞ − − ∞ − ξ = ξ0 + [ ] The dependence of the potential on the concentration can be represented as: i i kT ln i , where [i] corresponds to the proportion of places occupied by the structural element i; k and T are Boltzmann constant and temperature. Since ξ0 values are independent of φ, they are also independent of x. Then, we may derive as follows:   •• [ x ]      [VO ] SO,S      φ •• 00   •• 00 4q 0 = G VO G V + kTln ∞ + 2 ln x  H VO H V 2.1 eV (5) − Pb  [ 00 ] [S ]  ≈ − Pb ≈  VPb Pb,S  ∞ where G and H are the Gibbs energy and enthalpy of formation, respectively. Since the electrochemical potential of each particle is constant throughout the system,

η ξ0 [ ] φ ξ0 [ ] i = i + kT ln i + zqi = i + kT ln i (6) ∞ Hence: kT ln[i]/[i] = zq φ is applicable to lead and oxygen vacancies, then for the potential and the − i space charge distribution∞ we get as follows:

∂2φ 2qnh i h io 2qnh i h i o = V00 V•• = V00 exp( 2φq/kT) V•• exp(2φq/kT) (7) 2 Pb O Pb O ∂x ε − ε ∞ − − ∞

The estimates show that, with small stoichiometry bias, the electrostatic potential associated with the segregation of lead ions onto the free surface of PZT films can reach 0.5 V. This potential is quite enough to cause a self-polarization state of PZT film when it is cooled below Curie temperature (due to the small coercive field). Further, when the film is cooled to room temperature, the self-polarized state becomes stabilized and the migration of lead ions occurs. The destruction of the self-polarized state is possible by repeated thermal treatment of the film above the Curie temperature with the deposited upper electrode, as shown in Figure7. The formation of double charged layers in PZT films is possible not only at the PZT-air interface but also at the PZT grain boundaries in a polycrystalline film. The mechanisms of such formations are similar. We did not see any significant shifts in the hysteresis loops or the capacitance-voltage characteristics in Figure1, since these were the results of stoichiometric PZT films of relatively large thickness (around 1.5 µm). Such a thickness was chosen in order to see the volumetric effect of lead output on the grain periphery (Figure2) according to Equation (1) and to minimize the surface e ffect at the film–air Materials 2019, 12, 2926 12 of 14 interface. Considering that the grain size of the PZT was about 90 nm (Figure4), about ten grains of the PZT fit into the film thickness. Therefore, when lead ions went to PZT grain surfaces, the internal fields were oriented in different directions. The result of this was the absence of a shift in characteristics but the appearance of a narrow capacitance-voltage hysteresis. Figure7, on the contrary, shows the built-in field because the thickness of the PZT film was about 300 nm and contained 10 mol.% excess of lead. We observed the effect associated with the release of lead on the free surface (upper interface of the film). This led to the formation of a built-in field at the upper interface and appeared in a strong left shift of the capacitance-voltage characteristic (and we saw a similar shift on hysteresis loops). When a lead oxide sublayer was added at the bottom interface of the PZT film, we also observed a shift in the characteristics (Figure5), but this shift was already in the other direction. These effects could be positive and practically interesting for obtaining stable self-polarized films without applying an external field (in order to use in pyroelectric based sensors, microelectromechanical devices, and so on). Another application is the production of non-linear capacitors with very narrow CV hysteresis. The future development of self-polarized PZT films will be associated with the production of multilayer structures and the study of their properties. In particular, developments in the field of creating ferroelectric superlattices [21–23] where PZT can be used look very promising.

5. Conclusions This work showed that increasing the temperature or the duration of heat treatment of PZT films could lead to the appearance of impurities of lead oxide, even if the PZT film did not originally contain a super-stoichiometric excess of lead, which was a non-obvious finding. It was also shown that, even in the original stoichiometric PZT film under certain conditions (temperature above 580 ◦C, duration greater than 70 min), impurities of lead oxide could be formed. It was shown that the presence of a sublayer of lead oxide led to a denser formation of crystallization centers of the perovskite phase, resulting in a reduction of the grain size and the emergence of a charge on the lower interface, which gave rise to the presence of an embedded field, leading to the self-polarization of the PZT film. Pinning of the polarization processes by charges, which were formed at the stage of the formation of the film due to the diffusion of lead ions, caused a decrease in the values of the dielectric constant and the growth of transition fields and the presence of a narrow capacitance hysteresis (or lack of it) on the capacity-voltage characteristic. The formation of the perovskite structure under high-temperature annealing was accompanied by the diffusion of lead into the surface of the film. It was found that the diffusion of lead oxide to the surface of the film occurred more intensely during the formation of the perovskite structure than after its formation. The temperature and the time regimes of the formation of Pt/PZT/Pt structures influenced the formation of the self-polarized PZT film, which could be attributed to the accumulation and the relaxation of an electrostatic charge on the free surface of the ferroelectric film associated with surface segregation of the lead ions.

Author Contributions: Conceptualization, N.M. and D.C.; methodology, N.M.; validation, L.B. and A.T.; formal analysis, N.M.; investigation, N.M. and D.C.; resources, N.M. and A.T.; data curation, N.M.; writing—original draft preparation, N.M.; writing—review and editing, L.B.; visualization, N.M. and D.C.; supervision, N.M. and A.T.; project administration, A.T.; funding acquisition, N.M. and A.T. Funding: This research was funded by the grant of The Ministry of Education and Science (Minobrnauka) of the Russian Federation (project Goszadanie № 3.3990.2017/4.6). Acknowledgments: The authors thank Afanasyev V.P. for arousing interest in the problem of the article and useful discussions. His impact on our research activities, including this contribution, cannot be overestimated. The authors are also grateful to Petrov A.A. for PZT Auger spectra analysis. Conflicts of Interest: The authors declare no conflict of interest. Materials 2019, 12, 2926 13 of 14


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