Characterization of Bi and Fe co-doped PZT capacitors for FeRAM

Abstract

Ferroelectric random access memory (FeRAM) has been in mass production for over 15 years. Higher polarization ferroelectric materials are needed for future devices which can operate above about 100 °C. With this goal in mind, co-doping of thin Pb(Zr₄₀,Ti₆₀)O₃ (PZT) films with 1 at.% Bi and 1 at.% Fe was examined in order to enhance the ferroelectric properties as well as characterize the doped material. The XRD patterns of PZT-5% BiFeO₃ (BF) and PZT 140-nm thick films showed (111) orientation on (111) platinized Si wafers and a 30 °C increase in the tetragonal to cubic phase transition temperature, often called the Curie temperature, from 350 to 380 °C with co-doping, indicating that Bi and Fe are substituting into the PZT lattice. Raman spectra revealed decreased band intensity with Bi and Fe co-doping of PZT compared to PZT. Polarization hysteresis loops show similar values of remanent polarization, but square-shaped voltage pulse-measured net polarization values of PZT-BF were higher and showed higher endurance to repeated cycling up to 10¹⁰ cycles. It is proposed that Bi and Fe are both in the +3 oxidation state and substituting into the perovskite A and B sites, respectively. Substitution of Bi and Fe into the PZT lattice likely creates defect dipoles, which increase the net polarization when measured by the short voltage pulse positive-up-negative-down (PUND) method.     

Microstructure, domain structures and ferroelectric properties in (Pb0.8,La0.1,Ca0.1)TiO3/Pb(Zr0.2,Ti0.8)O3 bilayered thin film

Ferroelectric (Pb_0.8,La_0.1,Ca_0.1)TiO₃/Pb(Zr_0.2,Ti_0.8)O₃ (PLCT/PZT) bilayered thin film was prepared on Pt(111)/Ti/SiO₂/Si(100) substrate by RF magnetron sputtering technique. Pure perovskite crystalline phase, determined by X-ray diffraction, was formed in the PLCT/PZT bilayer. The bilayered film exhibited a very dense and smooth surface morphology with a uniform grain size distribution. The ferroelectric domain structures were investigated by a combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM, respectively). It is demonstrated by both VPFM and LPFM observations that out-of-plane and in-plane lamellar ferroelectric domains coexist in the bilayered thin film. The PLCT/PZT bilayered film possesses good ferroelectric properties with relatively high spontaneous polarization (2P_s = 82 µC/cm²) and remnant polarization (2P_r = 26.2 µC/cm²).     

溶胶-凝胶法制备Bi0.975La0.025Fe0.975Ni0.025O3铁电薄膜的结构及物理性能

采用溶胶-凝胶法在Pt/Ti/SiO_2/Si(111)衬底上制备了Bi_(0.975)La_(0.025)Fe_(0.975)Ni_(0.025)O_3(BLFNO)铁电薄膜。利用X射线衍射(XRD)、原子力显微镜(AFM)及其压电模式(PFM)对薄膜的晶体结构、表面形貌以及铁电畴结构进行了研究。研究发现,BLFNO为结晶良好的钙钛矿结构多晶薄膜,且薄膜表面颗粒生长均匀。PFM测试图显示铁电薄膜在自发极化下的铁电畴结构清晰,铁电电容器具有良好的铁电性能。应用铁电测试仪对Pt/BLFNO/Pt电容器进行测量,得到了饱和性良好的电滞回线。在828kV/cm的外加电场下,Pt/BLFNO/Pt电容器的剩余极化强度为74.3μC/cm~2,表明La、Ni的共掺杂没有明显抑制铁电电容器的剩余极化强度,铁电电容器具有良好的铁电性能。漏电流研究结果表明,La、Ni元素的共掺杂有效降低了薄膜的漏电流密度,在277.8kV/cm外加电场下漏电流密度在10-4 A/cm2量级,明显小于纯BFO薄膜的漏电流密度。正半支漏电流曲线满足SCLC导电机制,对于负半支曲线,当电场强度大于22.2kV/cm时,同样遵循SCLC导电机制;但是,当电场强度小于22.2kV/cm时,曲线斜率约为4.8,表明参与导电贡献的电子数较多,归因于极浅陷阱俘获的电子在外加电场作用下参与了导电行为。室温下磁滞回线测试结果表明BLFNO薄膜具有反铁磁性质。     

Effect of Fe and Fe-Ba substitution on the piezoelectric and dielectric properties of lead zirconate titanate ceramics

Polycrystalline samples of Fe and Fe-Ba doped lead zirconate titanate (PZT) ceramics near the morphotrophic phase boundary have been synthesized by a solid-state reaction technique. Preliminary X-ray analysis of the compound confirms that there is no change in the crystal structure of PZT on co-doping with Fe and Ba. The maximum mechanical quality factor Q_m was found to be 1000 for Fe doped material and 880 for Fe-Ba doped material. The electromechanical coupling factor for Fe and Fe-Ba doped samples were 0.535 and 0.495 respectively. The corresponding values for the piezoelectric charge constant d₃₃ were 135 and 250 pC/N respectively. These results are discussed in terms of position occupied by dopants in to the lattice and their corresponding microstructures. These Fe-Ba doped PZT materials could be likely candidates for high power ultrasonic and underwater SONAR transducer systems.     

(111)-Oriented Pb(Zr0.52Ti0.48)O3 thin film on Pt(111)/Si substrate using CoFe2O4 nano-seed layer by pulsed laser deposition

Perovskite Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin film with perfect (111)-orientation was achieved on CoFe₂O₄ seeded-Pt(111)/Ti/SiO₂/Si substrate by pulsed laser deposition technique using target with limited excess Pb. Pyrochlore phase formation was suppressed on Pt by CoFe₂O₄ nano-seed layer (~7 nm), and perovskite PZT was achieved at temperature as low as 550 °C. CoFe₂O₄ seed layer that has perfect (111)-orientation acts as a promoter for perfectly (111)-orientated growth of PZT. PZT film grown at 600 °C has higher degree of crystalline orientation, lower surface roughness, and compacted microstructure in comparison to the film grown at 550 °C. The PZT film has a nano-size grain-feature structure with grain size of about 40–60 nm. Perovskite formation was also confirmed by ferroelectric measurement. The ferroelectric properties of PZT film grown at 600 °C is higher than that grown at 550 °C which could be attributed to the enhancement of the crystalline orientation, crystallinity, and microstructure of the film.     

Structural evaluation of sol-gel derived lead strontium titanate diffusion barrier for integration in lead zirconate titanate transducer design

Sol-gel derived Pb₄₀Sr₆₀TiO₃ (PST) thin film has been investigated as a diffusion barrier for integrating in PbZr₃₀Ti₇₀O₃ (PZT) device structures on Si substrates. PST film was deposited on SiO₂/Si substrate and annealed at a relatively low temperature range of 550-600 °C producing a crack-free, smooth and textured surface. Following deposition on PST/SiO₂/Si template PZT thin film was crystallised exhibiting random grain orientations and an insertion of the bottom Pt/Ti electrode forming PZT/Pt/Ti/PST/SiO₂/Si stacks promoted the preferred PZT (111) perovskite phase. PZT (111) peak intensity gradually decreased along with slight increase of the PZT (110) peak with increasing annealing temperature of the buffer PST film. The dielectric and ferroelectric properties of the PZT with barrier PST deposited at 550 °C were assessed. The dielectric constant and loss factor were estimated as 390 and 0.034 at 100 kHz respectively and the remnant polarisation was 28 µC/cm² at 19 V. The performance of the PZT/PST device structures was compared to similar PZT transducer stacks having widely used barrier TiO₂ layer.     

Enhanced ferroelectric properties of epitaxial Pb(Zr,Ti)O3 film with LaAlO3 buffer

PbZr_0.52Ti_0.48O₃ films (PZT) have been grown epitaxially on SrRuO₃/LaAlO₃ (SRO/LAO) substrates using pulse laser deposition. In order to improve the ferroelectric properties of the PZT, one LAO buffer was introduced into the interface of PZT/SRO. The dependence of the electrical properties of the PZT films on the buffer thickness was studied. When a 10-nm-thick buffer was used, the remnant polarization (Pr) of the PZT film reached 58 ± 5 μC/cm², 2 times larger than the sample without any buffer layer. The leakage current was reduced 1-2 orders of magnitude. Besides, the PZT film with 10-nm-thick LAO buffer also exhibited good fatigue endurance after 10⁹ switching cycles. These results could propose one effective way to improve the properties of ferroelectric films deposited on oxide electrodes.     

Enhanced ferroelectric properties of Pb(Zr0.80Ti0.20)O3/PbO thin films prepared by radio frequency magnetron sputtering

The Pb(Zr_0.80Ti_0.20)O₃ (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by radio frequency (rf) magnetron sputtering method. The PbO buffer layer improves the microstructure and electrical properties of the PZT thin films. High phase purity and good microstructure of the PZT thin films with a PbO buffer layer were obtained. The effect of the PbO buffer layer on the ferroelectric properties of the PZT thin films was also investigated. The PZT thin films with a PbO buffer layer possess better ferroelectric properties with higher remnant polarization (P_r = 25.6 μC/cm²), and lower coercive field (E_c = 60.5 kV/cm) than that of the films without a PbO buffer layer (P_r = 9.4 μC/cm², E_c = 101.3 kV/cm). Enhanced ferroelectric properties of the PZT thin films with a PbO buffer layer is attributed to high phase purity and good microstructure.     

Ferroelectric and piezoelectric behavior of (111)-oriented Pb(ZrxTi1−x)O3 thin films on cobalt ferrite nano-seed layered Pt(111)/Si substrate

Perfect (111)-oriented Pb(Zr_xTi_1?x)O₃ (PZT) thin films were grown on cobalt ferrite buffered Pt(111)/Ti/SiO₂/Si substrate by pulsed laser deposition method using various targets with different Zr/Ti ratios ranging from 30/70 to 70/30. The results of X-ray diffraction analyses indicated that the composition of morphotropic phase boundary in the present PZT films is same as the bulk PZT (Zr/Ti = 52/48). The effect of Zr/Ti ratio of the PZT films was investigated by the ferroelectric domain structure and the piezoelectric characteristics of the films by piezoresponse force microscopy, as well as polarization measurement. The results revealed that the present tetragonal PZT film has a higher ferroelectric domain switching than rhombohedral one and the film with composition of Zr/Ti = 52/48 showed relatively high value of squareness of P–E loop and E_c as well as high piezoresponse.     

Structural and electrical characterizations of Bi(Zn0.5Ti0.5)O3 doped lead zirconate titanate ferroelectric films with enhanced ferroelectric properties

Bi(Zn_0.5,Ti_0.5)O₃ (BZT) doped Pb(Zr_0.4,Ti_0.6)O₃ (PZT) films were fabricated using a chemical solution deposition method and were characterized intensively in the present work. It was discovered that the room temperature remnant polarization and zero-field longitudinal piezoelectric constant of the BZT-doped PZT film were enhanced by 23% and 30%, respectively, as compared with those of the undoped PZT film prepared under the same experiment conditions. In order to explain the improved ferroelectric properties, the phase structures of the BZT-PZT and undoped PZT films were experimentally investigated in a broad temperature range (from 30 to 600 °C) by using the high temperature two-dimensional X-ray diffraction method. It was found that the improvement in ferroelectricity does not correspond to an elevated Curie temperature (T_C) or a substantially larger tetragonality (c/a). The difference on the change of T_C by doping of Bi-based perovskites in PZT solid solutions between this work and some previous investigations was explained on the basis of Zr/Ti ratio, and the necessity of an in-depth theoretical investigation was addressed.     

Composition dependent preferential orientation, dielectric and ferroelectric properties of Pb(ZrxTi1 − x)O3 thin films derived by sol-gel process

Pb(Zr_xTi_1 − x)O₃ (x = 0.35, 0.40, 0.60, 0.65) thin films were prepared by sol-gel spin on technique. From the X-ray diffraction analysis, PZT films with Zr-rich compositions (x = 0.60 and 0.65) had (111) preferential orientation and the preferential orientation changed to (100) for Ti-rich compositions (x = 0.35 and 0.40). The dielectric measurements on the above compositions at room temperature showed that the dielectric constant values were higher in Zr-rich compositions compared to Ti-rich compositions. The ferroelectric behavior measured in terms of the remnant polarization (P_r) and coercive field (E_c) up to an applied field of 260 kV/cm depicted that the Zr-rich PZT films with (111) preferential orientation had higher P_r and lower E_c values compared to the Ti-rich PZT films with (100) preferential orientation can be understood from the domain switching mechanism.     

Comparative investigating the properties of Pb-based multilayer ferroelectric thin films for FeRAM

The PbZr_0.3Ti_0.7O₃(PZT) thin film and multilayer PbZr_0.3Ti_0.7O₃/PbTiO₃(PZT/PT), PbTiO₃/PbZr_0.3Ti_0.7O₃/ PbTiO₃(PT/PZT/PT) thin films were prepared by a Sol-Gel method on the Pt(111)/Ti/SiO₂/Si(100) substrate for FeRAM application. The microstructure, ferroelectric, fatigue, dielectric, and leakage current properties of these thin films were investigated. The results indicate that the multilayer PT/PZT/PT thin film have a better ferroelectric, fatigue, dielectric and leakage current density properties. Its remanent polarization P_r reaches a maximum value of 21.2 μC/cm² and the coercive field E_c gets to a minimum value of 64.2kV/cm. After 10¹⁰ cycles, it still has more than 80% remnant polarization. The PT/PZT/PT thin film exhibits lower dielectric constant and lower dielectric loss, which is beneficial for FeRAM application. It also has the lowest leakage current density. The leakage current mechanism of the PZT, PZT/PT and PT/PZT/PT thin films is correlated to the microstructure and can be modeled in terms of GBLC and SCLC theory. The microstructure and electric properties of these films are correlated. The double-sided PT seed layers enhance not only the microstructure but also the electric properties. It is evident that the PT/PZT/PT multilayer thin film is a promising candidate for FeRAM application.     

Orientation control and electrical properties of sputtered Pb(Zr,Ti)O3 films

Relationship between the crystallographic orientation and the electrical properties of the Pb(Zr,Ti)O₃, (PZT) thin films prepared by rf magnetron sputtering was investigated. The PZT films were deposited at 150, 250 or 340°C and, followed by rapid thermal annealing (RTA). It was found that the crystallographic orientation of the PZT films could be controlled only by the deposition temperature and the ferroelectric properties were dependent upon the orientation of the films. It was suggested that the difference in the atomic mobility at the substrate surface during deposition was closely related to the film orientation. The films with (111) orientation showed relatively high capacitance and the remanant polarization values.     

Compositionally graded Pb(Zr,Ti)O3 thin films with different crystallographic orientations

Compositionally graded ferroelectric PbZr_xTi_1−xO₃ (PZT) films were deposited using a sputtering method and crystallized in situ at 500 °C. The films showed purely (100) or (111) crystallographic orientation when grown on Si/SiO₂/TiO₂/Pt substrates, while they exhibited c-axis epitaxial microstructure when prepared on MgO/Pt substrates. Their crystallographic orientation was controlled owing to a thin TiO_x layer sputtered on substrates prior to PZT deposition. Analysis performed by Auger depth profile clearly confirmed the variation of composition in the films. Coercive fields from 80 kV/cm to 200 kV/cm and remnant polarization as large as 45 μC/cm² were obtained. However, no typical offset was observed on hysteresis loops, unlike previous works related to graded PZT films.     

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100)_B3, (110)_B3, and (111)_B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100)_B3 substrates, Fe(100)_bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110)_bcc and Fe(111)_bcc single-crystal films are respectively obtained on GaAs(110)_B3 and GaAs(111)_B3 substrates at RT-300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0-9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc-Fe crystal.     

Role of strain and lattice distortion on ferroelectric and piezoelectric properties of bismuth magnesium zirconate substituted sodium bismuth titanate ceramics

Effect of weak ferroelectric perovskite, bismuth magnesium zirconate [Bi(Mg_0.5Zr_0.5)O₃] substitution in lead-free sodium bismuth titanate [(Na_0.5Bi_0.5)TiO₃] ceramics is studied. Influence of substitution on intrinsic and extrinsic contribution and impact on ferroelectric and piezoelectric properties are investigated. Improved spontaneous polarization (P_s), increased remnant polarization (P_r), decreased coercive field (E_c) and high piezoelectric coefficient (d₃₃) are obtained for x = 0.01 mole fraction of Bi(Mg_0.5Zr_0.5)O₃ substitution due to decrease in rhombohedral lattice distortion and homogeneous strain. Small rhombohedral lattice distortion (δ_r) and minimum homogeneous strain (δ) are the primary intrinsic parameters which favours the extrinsic parameters such as mobility of non-180° domain reorientation, domain switching and domain wall motion. Enhanced mobility softens the coercive field and increases remnant polarization to maximum. Reduced rhombohedral lattice distortion, low strain and enhanced mobility are the key factors for enhanced piezoelectric constant, highest remnant polarization and decreased coercive field in non-MPB (1 ? x)(Na_0.5Bi_0.5)TiO₃–xBi(Mg_0.5Zr_0.5)O₃ solid solutions.     

Investigation of PbZr0.4Ti0.6O3 capacitors with room temperature as-grown LaNiO3 electrodes

LaNiO₃ (LNO) film grown at room temperature (RT) by RF magnetron sputtering is used as the electrode for integrating LaNiO₃/PbZr_0.4Ti_0.6O₃/LaNiO₃ (LNO/PZT/LNO) capacitor on SrTiO₃ (STO) substrate. For comparison, LNO film grown at 250 °C is also used as the electrode of PZT capacitor. Reflection high energy electron diffraction (RHEED) technique is used to characterize the LNO film, it is found that LNO film prepared at 250 °C is epitaxial although no diffraction pattern is found for RT as-grown LNO. Ferroelectric properties of PZT films strongly depend on the LNO bottom electrodes. The remanent polarization (P _r) and coercive voltage (V _c), measured at 5 V, for the capacitors with LNO bottom electrodes prepared at RT and 250 °C, are 20 and 37 μC/cm², 1.67 and 1.95 V, respectively. No obvious degradation of polarization for PZT capacitors with RT as-grown LNO electrodes can be found. Room temperature as-grown LNO as both bottom and top electrodes to fabricate ferroelectric capacitors can save 2/3 thermal budgets, which may pay a way to decrease the potential challenges of devices resulting from the oxidation, interdiffusion or reactions during integrating ferroelectric capacitors with Si technologies.     

Enhanced ferro- and piezoelectric properties in (100)-textured Nb-doped Pb(ZrxTi1 − x)O3 films with compositions at morphotropic phase boundary

To develop high-performance piezoelectric films on conventional Pt(111)/Ti/SiO₂/Si(100) substrates, sol-gel-derived highly [100]-textured Nb-doped Pb(Zr_xTi_1 − x)O₃ (PNZT) thin films with different Zr/Ti ratios ranging from 20/80 to 80/20 were prepared and characterized. The phase structure, ferroelectric and piezoelectric properties of the PZNT films were investigated as a function of Zr/Ti ratios, and it was confirmed that there was distinct phase transition of the PNZT system from tetragonal to rhombohedral when the Zr/Ti ratio varied across the morphotropic phase boundary (MPB). The Nb-doped PZT films showed enhanced remanent polarization but reduced coercive field, whose best values reached 75 μC/cm² and 82 kV/cm, respectively at the composition close to MPB. In addition, the [100]-textured PNZT film at MPB also shows a high piezoelectric coefficient up to 161 pm/V. All these properties are superior to those for undoped PZT films.     

Electric characteristics of organic thin-film transistors and logic circuits with a ferroelectric gate insulator

Organic electronic devices using a pentacene have improved importantly in the last several years. We fabricated pentacene organic thin-film transistors (OTFTs) with dielectric SiO₂ and ferroelectric Pb(Zr_0.3,Ti_0.7)O₃ (PZT) gate insulators. The organic devices using SiO₂ and PZT films had the field-effect mobility of approximately 0.1 and 0.004 cm²/V s, respectively. The drain current in the transfer curve of pentacene/PZT transistors showed a hysteresis behavior originated in a ferroelectric polarization switching. In order to investigate the polarization effect of PZT gate dielectrics in a logic circuit, the simple voltage inverter using SiO₂ and PZT films was fabricated and measured by an output-input measurement. The gain of inverter at the poling-down state was approximately 7.2 and it was three times larger than the value measured at the poling-up state.     

Improvements of the ferroelectric properties of high-valence Tb-doped Bi4Ti3O12 thin film grown by sol-gel method

High-valence Tb-doped bismuth titanate (Bi_3.6Tb_0.4Ti₃O₁₂) (BTT) ferroelectric film was fabricated on Pt/TiO₂/SiO₂/Si (100) substrate by sol-gel technique. The BTT film had a polycrystalline perovskite structure with uniform and dense surface morphology. At a maximum applied electric field of 540 kV/cm, a remnant polarization of 59.8 μC/cm² and coercive field of 298 kV/cm were observed through ferroelectric measurements. The measured dielectric constant and loss of the film were 490 and 0.047 at a frequency of 1 MHz. The film showed excellent anti-fatigue characteristics with less than 2% degradation in switchable polarization after 1.0 × 10¹⁰ switching cycles. These improved ferroelectric properties may be attributed to the structural distortion and the low concentration of oxygen vacancy associated with Tb substitution for Bi.