Decrease In Remanent Polarization Research Articles

Preparation and electrical properties of high-Curie temperature ferroelectrics

( $1-x$ )Pb(Fe 0.5 Nb 0.5 )O 3 - x Pb(Zr 0.2 Ti 0.8 )O 3 (PFN-PZT, x = 0.75 , 0.80, 0.85) ferroelectric ceramics were prepared by conventional solid-state reaction method via the wolframite precursor route. XRD measurements confirmed that the synthesized PFN-PZT ceramics are of pure tetragonal perovskite structure. With the increase of Pb(Zr 0.2 Ti 0.8 )O 3 (PZT) content, tetragonality (defined as the ratio of cell parameter c/a ) increases slightly accompanied by the variation of cell volume. At the optimized sintering condition of 1175 °C for 2 h, the 0.20PFN-0.80PZT ceramics exhibit the largest value of relative density (93.77%). The PFN-PZT ceramics exhibit first-order ferroelectric phase transition of typical normal ferroelectrics, where the dielectric response peaks are narrow, sharp and without frequency dispersion, and the dielectric constant above Curie temperature ( T C ) can be fitted well by Curie law. The sintered PFN-PZT ceramics exhibit high- T C , and with the increase of PZT content, T C increases and reaches 368, 394 and 401 °C, respectively. With the increase of PZT content, the P-E ferroelectric hysteresis loops of the PFN-PZT ceramics become narrower accompanied by the decrease of remanent polarization ( P r ) and coercive field ( E C ). Piezoelectric constant d 33 of all the PFN-PZT ceramics is small, less than 10 pC/N.

Room temperature ferroelectric–ferromagnetic Bi3.25Sm0.75Ti2.98V0.02O12/La0.67 Sr0.33 MnO3 double layer heterostructure

A ferroelectric Bi3.25Sm0.75Ti2.98V0.02O12 (BSVT) film and colossal magnetoresistive La0.67Sr0.33MnO3 (LSMO) film heterostructure on Si with excellent performances of both ferroelectric and ferromagnetic properties at room temperature was fabricated by the pulsed laser deposition method. Saturated magnetization–magnetic field hysteresis loops and polarization–electric field loops proved the materials compatibilities in keeping their own intrinsic properties of LSMO and BSVT in the heterostructure. The remanent polarization (2Pr) of the BSVT is 56 µC cm−2 at an applied electrical voltage of 11 V. Fatigue measurements show a 25% decrease in remanent polarization after 106 read/write cycles but no further changes afterwards.

Electric Fatigue in Ferroelectric Lead Zirconate Stannate Titanate Ceramics Prepared by Spark Plasma Sintering

Ferroelectric lead zirconate stannate titanate ceramics were prepared by spark plasma sintering (SPS). Compared with its counterpart densified by conventional sintering (CS), the SPS material shows a smaller remanent polarization and maximum strain as well as a higher coercive field. Electric fatigue in both materials was investigated. In contrast to CS samples, the SPS specimens show a lower resistance to bipolar electric cycling, characterized by a faster decrease in remanent polarization and maximum strain at cycle number below 106.5 and a subsequent slower reduction of the properties at high cycle numbers up to 108.

Structural, Dielectric and Ferroelectric Properties of La Doped PbTiO3 Sol Gel Derived Thin Films

Ferroelectric lead lanthanum titanate (Pb1−x La x Ti1−x/4O3) thin films (x = 0.04 & 0.08) have been fabricated by sol-gel spin coating process on ITO coated 7059 glass substrates. Characterization of these films has been done by x-ray diffraction and atomic force microscope images. Tetragonality decrease with addition of La content. While dielectric constant and pyrocoefficient increase, Curie temperature decreases with La content. Also, there is decrease in remanent polarization and coercive field with addition of La content.

Ferroelectric Polarization Fatigue in PZT-Based RAINBOWs and Bulk Ceramics

The ferroelectric fatigue behavior of high-displacement reduced and internally biased oxide wafer (RAINBOW) actuators was investigated and compared with that of conventional bulk ceramics. Piezoelectric lead zirconate titanates (PZT-5H and PZT-5A) as well as electrostrictive lead lanthanum zirconate titanate (PLZT 9/65/35) materials were studied. Piezoelectric bulk ceramics experienced a 25% decrease in remanent polarization over 10–8 switching cycles, whereas RAINBOW polarization degraded by 33%–53%. Bulk ceramic and RAINBOW electrostrictors both fatigued only 8%. Bulk ceramics fatigued as a result of reorientation of the polar axes away from the electric-field direction. RAINBOW fatigue was believed to have been caused by degradation at the interface between the oxide and chemically reduced layers.

Dielectric properties of SrBi 2− xPr xNb 2O 9 ceramics ( x=0, 0.04 and 0.2)

SrBi 2− x Pr x Nb 2O 9 ( x=0, 0.04 and 0.2) ceramics were prepared by a solid state reaction method. X-ray diffraction analysis indicated that single-phase layered perovskite structure ferroelectrics were obtained. A relaxor behavior of frequency dispersion was observed among Pr-doped SrBi 2Nb 2O 9. The degree of frequency dispersion Δ T increased from 0 for x=0–7 °C for x=0.2, and the extent of relaxor behavior γ increased from 0.94 for x=0–1.45 for x=0.2. The substitution of Pr ions for Bi 3+ ions in the Bi 2O 2 layers resulted in a shift of the Curie point to lower temperatures and a decrease in remanent polarization. In addition, the coercive field 2 E c reduced from 110 kV/cm for an undoped specimen to 90 kV/cm for x=0.2.

Dielectric, pyroelectric and ferroelectric behavior of PZTN 65/35/ x ferroelectric ceramic system

Niobium doping lead zirconate titanate [PZTN 65/35/ x ( x = 1, 2, 4, 5)] ferroelectric ceramics show perovskite structure and a rombohedral phase. The average grain size, the maximum dielectric permittivity, the transition temperature and the remanent polarization decrease when the niobium concentration increases, while the coercive field does not show any particular behavior and the porosity increases. The results are discussed taking into account the incorporation of the niobium to the lattice and the variation of the grain size. Preliminary results of the pyroelectric behavior are shown.

Film Thickness Dependence of Electrical Properties for Pb(Zr,Ti)O3 Thin Films Prepared on (111)Ir/TiO2/SiO2/Si and (111)Pt/TiO2/SiO2/Si Substrates by Pulsed-Metalorganic Chemical Vapor Deposition

Polycrystalline Pb(Zr,Ti)O3 (PZT) films with various film thicknesses were deposited on (111)Ir/TiO2/SiO2/Si and (111)Pt/TiO2/SiO2/Si substrates at 540°C by source-gas-pulsed metalorganic chemical vapor deposition (pulsed-MOCVD). PZT films deposited on (111)Ir/TiO2/SiO2/Si substrates showed good surface flatness and lower leakage current density. The rate of the decrease of remanent polarization (P r ) and the rate of increase of coercive field (E c ) for the films with decreasing the film thickness smaller on (111)Ir/TiO2/SiO2/Si substrates than those of (111)Pt/TiO2/SiO2/Si substrates. In addition, P r and E c values saturated at low voltage when the film thickness was the same. As a result, good ferroelectricity with P r and E c values of 40 μC/cm2 and 140 kV/cm were obtained for 35 nm-thick films prepared on (111)Ir/TiO2/SiO2/Si substrates by pulsed-MOCVD.

Combined effect of thickness and stress on ferroelectric behavior of thin BaTiO3 films

Thin BaTiO3 (BTO) layers were deposited by magnetron sputtering on a Si substrate between two thin LaNiO3 (LNO) electrodes (LNO/BTO/LNO/Si structure). The thickness of the BTO layer was varied between 35 nm and 1 μm. The stress, dielectric permittivity, and ferroelectric hysteresis loop were measured after deposition and during heating. It was found that the BTO layers are under tensile stress at the Curie temperature. The tensile stress increases with decreasing layer thickness leading to a shift of the Curie point to a lower temperature, a decrease of remanent polarization, and an increase of the coercive field. The effect of tensile stress on the Curie temperature is dominant within a restricted range of stresses between 300 and 450 mega Pascals (MPa), where the Curie temperature decreases linearly by about 0.16 °C/MPa. A good correlation was found between stress and dielectric measurements with respect to the temperature of the Curie point. The ferroelectric hysteresis loops showed that the ferroelectric-paraelectric phase transition starts at the Curie temperature of the BTO layer and continues within a range of temperatures until about the Curie temperature of the bulk BTO material. The ferroelectric behavior indicates dominance of domains within the interior part of the BTO layer and minor contribution of surface domains. The results are compared with a theoretical approach of size-driven phase transition in stress-induced ferroelectric thin films.

Tetragonal distortion of c domains in fatigued Pb(Zr,Ti)O3 thin films determined by x-ray diffraction measurements with highly brilliant synchrotron radiation

We performed x-ray diffraction measurements by using highly brilliant synchrotron radiation on only a small region of 250-nm-thick Pb(Zr0.33Ti0.67)O3 polycrystalline films with gold top electrodes after applying various numbers of switching cycles of the electric field. The films were deposited on Pt/SiO2/Si substrates by low-temperature metalorganic chemical vapor deposition. The plane spacing and integrated intensity of 004 and 400 diffraction patterns were plotted against the number of switching cycles. We found a good correlation between the increase in 004-plane spacing and the decrease in remanent polarization. This correlation indicates that tetragonal distortion of c domains is closely related to the fatigue phenomenon.

Mechanical and electric fatigue of PZT(53/47) films on metallic substrates

We have investigated the fatigue of electromechanical and dielectric properties of sol-gel derived PZT(53/47) thin films deposited on metallic substrates by means of electric and mechanical cycling. For the mechanical cycling a two point bending method was used to apply transversal stress to the samples. During mechanical cycling the piezoelectric coefficient d31 remained constant up to about 105 cycles, for a higher number of cycles a strong decrease was observed. During electric cycling no significant changes in the ferroelectric and electromechanical hysteresis loops could be found up to about 3×105 cycles. Above this number the coercive field increases, the maximum strain and the remanent polarization decrease. Obviously each electric cycling of the investigated films is accompanied by a mechanical cycling. It is assumed, that microcracks induced by mechanical stress are the main reason for the deterioration of the physical properties films during electric and mechanical cycling both.

Ferroelectric characteristics of liquid source misted chemical deposition (LSMCD)-derived SrBi 2.4Ta 2O 9 thin films with thickness variation

SrBi 2.4Ta 2O 9 (SBT) thin films of 70–400 nm thickness were prepared on platinized Si substrates by liquid-source misted chemical deposition (LSMCD), and the thickness dependence of the ferroelectric characteristics was investigated. The grain size of the LSMCD-derived SBT films was approximately 150 nm and hardly varied with the film thickness. The 70-nm thick SBT film exhibited remanent polarization (2 P r) of 13.5 μC/cm 2 and a coercive field value (E c) of 63 kV/cm at ±3 V. Within the thickness range of 70–400 nm, the LSMCD-derived SBT films exhibited size effects, i.e. a decrease in remanent polarization and relative permittivity and an increase in the coercive field with a reduction in the film thickness. The LSMCD-derived SBT films with a thickness of 70–400 nm exhibited fatigue-free behavior for up to 10 12 switching cycles.

Effect of lead additive on the ferroelectric properties and microstructure of SrxPbyBi2zTa2O9 thin films

Ferroelectric thin films of bismuth-containing layered perovskite SrxPbyBi2zTa2O9 have been prepared using the metalorganic decomposition method. The effect of both Sr and Pb content on the crystal structure, microstructure, and ferroelectric properties of SrxPbyBi2.3Ta2O9 films was investigated. A maximum remanent polarization of 2Pr=19.2 μC/cm2 was obtained for the SrxPb0.2Bi2.3Ta2O9 film with 20 mol % Sr-deficient composition as prepared at 800 °C, which could be the compromising effects of Sr content on both grain growth and second phase formation of BiTaO4. The substitution of Pb for Bi is accompanied by the occurrence of oxygen vacancies to compensate the charge balance, which is responsible for grain growth mechanism in Sr0.8Pb0.2Bi2.3Ta2O9 films. Fatigue endurance of Sr0.8Bi2.3Ta2O9 films becomes problematic after 109 cycles with a decrease in remanent polarization to 85% of the original value. This phenomenon was related to electron injection and creation of electron traps due to the occupation of Sr vacancies by Bi cations. It is demonstrated that the fatigue endurance of Sr0.8Bi2.3Ta2O9 film can be improved by doping with 20 mol % PbO.

Acoustic emission in PZT under bipolar electric driving and uniaxial mechanical stress

The acoustic emission (AE) from ferroelectrics has at least two possible sources, switching and microcracking. Both components have to release a distinct amount of elastic energy in a brief time span to generate the burst like AE. Whether ferroelectric switching on its own can generate AE is not fully understood. Furthermore the internal discharges in pores or microcracks can be an AE source. An easy way to suppress the ferroelastic switching component is to apply uniaxial mechanical stress parallel to the switching direction. This publication deals with the acoustic emission behavior of commercial ferroelectric-ferroelastic PZT ceramics under constant uniaxial load and concurrent electric hysteresis measurements. Both the coercive field and the remanent polarization decrease under compressive mechanical stress. Mechanical loading causes a stronger decrease in the AE activity and the dissipated AE-energy than in the remanent polarization. The threshold polarization for the occurrence of AE decreases with the mechanical load. Discharges in pores and microcracks can be discarded as AE sources.

Plasma Etching Damage to Ferroelectric SrBi2Ta2O9 (SBT) Thin Films and Capacitors

AbstractThe effects of reactive ion etching damage on the electrical properties of Pt/SBT/Pt capacitors have been investigated. The plasma treated SBT/Pt layers showed a significant decrease in remanent polarization compared with that of the reference sample. The remanent polarization of the plasma treated layers varied with the gas ratios of the Cl2/Ar plasma. XPS analysis of the plasma treated SBT/Pt samples showed that the surface composition was significantly changed as the gas ratios were varied, which resulted in a polarization decrease in the plasma treated samples. Plasma treatment also caused a voltage shift of the hysteresis loops along the voltage axis. The magnitude of the voltage shift was increased for the chlorine-rich plasma. The results of surface analysis revealed that the voltage shift is caused by oxygen deficiency at the SBT surface. Based on our experimental results, reactive ion etching damage was explained in terms of physical and electrical effects of the plasma on the electrical properties of the ferroelectric Pt/SBT/Pt capacitors.

Imprint of Oriented Pb(Zr, Ti)O 3 Thin Films with Oxygen Atmosphere in Cooling Process

Imprint properties of reactively sputtered Pb(Zr, Ti)O3 [PZT] thin films were studied as a function of oxygen pressure during cooling after deposition. The well-oriented PZT films had a perovskite phase without any second phase regardless of the oxygen cooling ambient. The surface morphology of the PZT films became somewhat rough with decreasing oxygen pressure because of the evaporation from the surface of the PZT films. Grain size, however, did not change significantly. As the oxygen cooling pressure increased, the hysteresis property of the PZT thin films improved and attained good squareness. The decrease of remanent polarization, retained polarization and the coercive field was observed with decreasing oxygen pressure. It was observed that as the atmosphere in the cooling process of PZT films becomes a reducing one, the P–E hysteresis loop shifts to a positive voltage asymmetrically. The direction of internal bias field was from the bottom to top electrode, which was confirmed by a field-accelerated retention test. In addition, this field could be diminished by thermal treatment in oxygen atmosphere.

Temperature effects on charge retention characteristics of integrated SrBi2(Ta,Nb)2O9 capacitors

Temperature effects on charge retention characteristics of integrated SrBi2(Ta,Nb)2O9 thin film capacitors were examined in the temperature range of 27–150°C. The decay in remanent polarization at 27°C was linear in logarithmic time from 10−3 to 105 s with a decay rate of 0.24 μC/cm2 per decade. The elevation of storage temperature resulted in an instantaneous decrease in remanent polarization, while the decay rate at elevated temperatures after the instantaneous decrease was as small as that at 27°C. The instantaneous decrease in remanent polarization caused by elevating the temperature was explained by the temperature dependence of spontaneous polarization in the vicinity of the second order transition temperature. The development of asymmetry in the hysteresis loop during high temperature storing indicates that the logarithmic time dependence of the decay in remanent polarization is due to redistribution of space charges rather than polarization reversal.

Electrode Dependences of Switching Endurance Properties of Lead-Zirconate-Titanate Thin-Film Capacitors

Switching endurance properties of lead-zirconate-titanate thin-film capacitors with gold, platinum and iridium top electrodes were investigated using a pulse switching characterization technique. Lead-zirconate-titanate capacitor structure formed with Ir top and bottom electrodes exhibited superior switching endurance to Au/PZT/Ir and Pt/PZT/Ir capacitors. The difference between switched and nonswitched polarizations of an Ir/PZT/Ir capacitor reversed by bipolar pulses was more than 38 µ C/ cm2 after 2×109 switching cycles. Nonswitched polarization of this capacitor for negative read-pulses decreased gradually with increase in the number of switching cycles. When positive and negative unipolar pulses, and DC biases were applied to the top electrodes of Pt/PZT/Ir and Ir/PZT/Ir capacitors, remanent polarizations of each capacitor were not changed significantly. However, nonswitched polarizations for negative read-pulses decreased with increase in the number of negative unipolar pulses applied or DC bias application time. Drastic decrease in remanent polarization of a Pt/PZT/Ir capacitor was caused only by bipolar pulse application. The reduction of nonswitched polarization for negative read-pulses suggested the formation of depletion-layer capacitances at the interfaces between top electrodes and PZT layers.

Characteristic Change due to Argon Ion Etching and Heat Treatment of (Pb, La)TiO3 Thin Films Fabricated by Multiple Cathode Sputtering

The ferroelectric properties of (Pb, La)TiO3 thin films fabricated by multiple cathode rf-magnetron sputtering were investigated. After argon ion etching of the (Pb, La)TiO3 thin films, the reduction of Pb-O bonds, shift of hysteresis loop on the E-axis and the decrease of remanent polarization was induced. After heat treatment at 700° C in N2, squareness and saturation of the loop worsened, and remanent polarization decreased. After reheat treatment of the films at 400° C for 1 h, the coercive field decreased, and the squareness of the loop was improved. These results were discussed from the viewpoint of the behavior of space charge.

Electronic and Ionic Trapping at Domain Walls in BaTiO3

We have coupled electron paramagnetic resonance (EPR) and polarization–voltage measurements to understand the effects of reducing ambients on the remanent polarization and density of paramagnetic centers in BaTiO3, single crystals. Two types of reducing ambients were explored; one was done under vacuum (slightly reducing) and the second was performed in forming gas (very reducing). It is found that the vacuum anneal caused a reduction in the remanent polarization and a concomitant decrease in the isolated Fe3+ EPR resonance. The Fe3+–Vo complex EPR signal was relatively unaffected by this vacuum anneal. By injecting charge using an ultraviolet (UV) light and an applied bias combination, the polarization and the isolated Fe3+ signal intensity were restored, thereby suggesting that the suppression of the remanent polarization is due to trapping of electronic charge at the domain walls. For the forming gas anneal, we observe a much larger decrease in remanent polarization with an accompanying decrease in both the isolated Fe3+ and Fe3+–Vo complex EPR signals. For this anneal, charge injection by the UV light/bias combination did not restore the polarization nor the EPR densities. The remanent polarization, the isolated Fe3+, and the Fe3+–VoVo complex could be restored only by a reoxidizing anneal, suggesting that ionic defects {oxygen vacancies) are now responsible for pinning the domain walls. Collectively, these results suggest reducing anneals can suppress the amount of switchable polarization in BaTiO3 by either electronic or ionic trapping mechanisms.