Comparative analysis for the crystalline and ferroelectric properties of Pb(Zr,Ti)O3 thin films deposited on metallic LaNiO3 and Pt electrodes

The crystalline quality and ferroelectricity of the Pb(Zr,Ti)O₃ (PZT) films deposited on the metallic LaNiO₃ (LNO) and Pt electrodes were comparatively analyzed to investigate the possibility for their application to non-volatile memory devices. LNO thin films were successfully deposited on various substrates by using r.f. magnetron sputtering even at a low temperature ranging from 250 to 500 °C, and the ferroelectric PZT thin films were spin-coated onto the LNO and Pt bottom electrodes. Metallic LNO thin films exhibited [100] orientation irrespective of the substrate species and PZT films coated onto LNO films had highly a- and c-axis orientations, while those with Pt bottom electrode were polycrystalline. PZT films with LNO bottom electrode had smaller grain size and larger dielectric constant compared to those grown on the Pt electrode. The ferroelectric thin films fabricated on LNO bottom electrode displayed an asymmetric D–E hysteresis loop, which was explained by the defect effects formed at the interface. Especially, the LNO/PZT/LNO capacitor was found to significantly improve the polarization fatigue and the effects of the LNO electrodes to the fatigue were discussed.     

The thickness effect of Bi3.25La0.75Ti3O12 buffer layer in PbZr0.58Ti0.42O3/Bi3.25La0.75Ti3O12 (PZT/BLT) multilayered ferroelectric thin films

A series of PbZr_0.58Ti_0.42O₃ (PZT) thin films with various Bi_3.25La_0.75Ti₃O₁₂ (BLT) buffer layer thicknesses were deposited on Pt/TiO₂/SiO₂/p-Si(100) substrates by RF magnetron sputtering. The X-ray diffraction measurements of PZT film and PZT/BLT multilayered films illustrate that the pure PZT film shows (111) preferential orientation, and the PZT/BLT films show (110) preferential orientation with increasing thickness of the BLT layer. There are no obvious diffraction peaks for the BLT buffer layer in the multilayered films, for interaction effect between the bottom BLT and top PZT films during annealing at the same time. From the surface images of field-emission scanning electron microscope, there are the maximum number of largest-size grains in PZT/BLT(30 nm) film among all the samples. The growth direction and grain size have significant effects on ferroelectric properties of the multilayered films. The fatigue characteristics of PZT and PZT/BLT films suggest that 30-nm-thick BLT is just an effective buffer layer enough to alleviate the accumulation of oxygen vacancies near the PZT/BLT interface. The comparison of these results with that of PZT/Pt/TiO₂/SiO₂/p-Si(100) basic structured film suggests that the buffer layer with an appropriate thickness can improve the ferroelectric properties of multilayered films greatly.     

Effect of annealing ambient on structure and ferroelectric properties of Pb(Zr0.4Ti0.6)O3 thin films on LaNiO3 coated Si substrates

Pb(Zr_0.4Ti_0.6)O₃ [PZT(40/60)] thin films were deposited onto LaNiO₃ (LNO) coated Si substrates by sol-gel technique. Three kinds of gases, air, O₂ and N₂, were used as the annealing ambient. The effect of the annealing ambient on their structure and ferroelectric properties was investigated. The results showed that both the films annealed in air and O₂ were the complete perovskite phase with (1 0 0) preferential orientation, while those annealed in N₂ were random orientation including some pyrochlore phases. As compared with the air ambient, either too much O₂ or too much N₂ was detrimental to the ferroelectric properties of PZT films. The difference in structure and ferroelectric properties was mainly associated with the intermediate phases and the concentration of domain pinning centers in the films.     

Large electrocaloric effect of highly (100)-oriented 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 thin films with a Pb(Zr0.3Ti0.7)O3/PbOx buffer layer

0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ (PMN-PT) thin films with a lead zirconate titanate Pb(Zr_0.3Ti_0.7)O₃ (PZT)/PbO_x buffer layer were deposited on Pt/TiO₂/SiO₂/Si substrates by radio frequency magnetron sputtering technique, and pure perovskite crystalline phase with highly (100)-preferred orientation was formed in the ferroelectric films. We found that the highly (100)-oriented thin films possess not only excellent dielectric and ferroelectric properties but also a large electrocaloric effect (13.4 K at 15 V, i.e., 0.89 K/V) which is attributed to the large electric field-induced polarization and entropy change during the ferroelectric-paraelectric phase transition. The experimental results indicate that the use of PZT/PbO_x buffer layer can induce the crystal orientation and phase purity of the PMN-PT thin films, and consequently enhance their electrical properties.     

Reduction of film thickness for chemical solution deposited PbZr0.3Ti0.7O3 thin films revealing no size effects and maintaining high remanent polarization and low coercive field

Polycrystalline Pb(Zr_0.3,Ti_0.7)O₃ (PZT) thin films were prepared on platinized silicon wafers by chemical solution deposition (CSD) with thicknesses down to 30 nm. Electrical measurements with the superior ferroelectric properties of high remanent polarization (P_r) and low coercive field (E_c) will be presented for thicknesses down to 50 nm. In order to decrease the thickness of electrically dense PZT thin films by the CSD method experiments have been performed by using different degrees of dilutions of the precursor stock solutions, i.e. instead of diluting the PZT stock solution with 2-butoxyethanol in the standard ratio of 1:1 before the spin-on process, the dilution is increased stepwise to a ratio of 1:4. In addition the films have been annealed in nitrogen atmosphere instead of the typical oxygen atmosphere which has been shown to strongly improve a preferential (111) orientation of the PZT film [G. J. Norga, L. Fe, Mat. Res. Soc. Symp. Proc. Vol. 655, CC9.1.1 (2001)]. The approach of Norga et al. is confirmed and complemented by means of electrical hysteresis measurements.     

Crystallization behavior of Nd-doped SrBi2Ta2O9 thin films prepared by magnetron sputtering

Nd-doped SrBi₂Ta₂O₉ thin films are magnetron-sputtered on Pt/Ta/SiO₂/Si substrates. The effect of heating rate on crystallization behavior is investigated with conventional furnace annealing (CFA) and rapid thermal annealing (RTA). Grazing incidence X-ray diffraction and field emission scanning electron microscopy reveal that the crystallization goes through three stages (phases): amorphous, fluorite and finally Aurivillius. Under RTA, the fluorite-to-Aurivillius transformation starts around 100 °C lower than that under CFA. The reasons behind the transformation temperature drop are also discussed.     

Structure and electrical properties of (100)-oriented Pb(Zn1/3Nb2/3)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 thin films on La0.7Sr0.3MnO3 electrode by chemical solution deposition

(100)-oriented 0.462Pb(Zn_1/3Nb_2/3)O₃–0.308Pb(Mg_1/3Nb_2/3)O₃–0.23PbTiO₃ (PZN-PMN-PT) perovskite ferroelectric thin films were prepared on La_0.7Sr_0.3MnO₃/LaAlO₃ (LSMO/LAO) substrate via a chemical solution deposition route. The perovskite LSMO electrode was found to effectively suppress the pyrochlore phase while promote the growth of the perovskite phase in the PZN-PMN-PT film. The film annealed at 700 °C exhibited a high dielectric constant of 2130 at 1 kHz, a remnant polarization, 2P_r, of 29.8 μC/cm², and a low leakage current density of 7.2 × 10^− 7 A/cm² at an applied field of 200 kV/cm. The ferroelectric polarization was fatigue-free at least up to 10¹⁰ cycles. Piezoelectric coefficient, d₃₃, of 48 pm/V was also demonstrated. The results showed that much superior properties could be achieved with the PZN-PMN-PT thin films on the solution derived LSMO electrode than on Pt electrode by sputtering.     

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.     

Electrical properties of Ta2O5 thin films deposited on Cu

The electrical and dielectric properties of reactively sputtered Ta₂O₅ thin films with Cu as the top and bottom electrodes forming a simple metal insulator metal (MIM) structure, Cu/Ta₂O₅/Cu/n-Si, were studied. Ta₂O₅ films subjected to rapid thermal annealing (RTA) at 800°C for 30 s in N₂ ambient crystallized the film, decreased the leakage current density and resulted in reliable time-dependent dielectric breakdown characteristics. The conduction mechanism at low electric fields (<100 kV/cm) is due to Ohmic conduction; however, the Schottky mechanism becomes predominant at high fields (>100 kV/cm). Present studies demonstrate the use of Cu as a potential electrode material to replace the conventional precious metal electrodes for Ta₂O₅ storage capacitors.     

Crystal texture selection in epitaxies of orthorhombic antiferromagnetic YMnO3 films

The orthorhombic phase of YMnO₃ has been epitaxially stabilized on SrTiO₃ single crystal substrates. Changing the substrate orientation, the out-of-plane orientation of the epitaxial films can be tuned to (001), (101), or (100). Depending on the orientation, the films present varied crystal domain structures and lattice strains. Single domain YMnO₃(100) films have been grown on SrTiO₃(110) substrates, whereas they present two and three crystal variants on SrTiO₃(001) and SrTiO₃(111), respectively. Epitaxial stress in each domain is anisotropic, tensile in one direction and compressive in the other. It has consequences in the texture selection when domains with different out-of-plane orientation can grow epitaxially. Antiferromagnetism of YMnO₃ films has been confirmed by measuring the exchange bias field induced in an underlying ferromagnetic SrRuO₃ layer.     

BaPbO3 conductive coating layers on platinized Si substrate for the growth of PbZr1−xTixO3 thin films

Barium metaplumbate (BaPbO₃, BPO) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol–gel method and a rapid thermal annealing (RTA) process. X-ray diffraction (XRD) was used to characterize the crystalline structure of the resultant films. It was shown that the formation of perovskite BPO greatly depends on the lead concentration and the final annealing temperature. In terms of the semi-quantitative energy dispersion spectrum (EDS) analysis, the ratio of Pb/Ba in the BPO ceramic films increases as the final heating temperature increases. Using BPO as buffer layers, PZT thin films with a pure perovskite structure were grown at a very low temperature of 500 °C by the sol–gel technique and the RTA process. The remanent polarization of Pt/PZT/BPO/Pt ferroelectric capacitors is about 17 μC/cm² at an applied voltage of 3 V.     

Structure evolution and ferroelectric properties of Bi3.4Yb0.6Ti3O12 thin films crystallized under a moderate temperature

Ytterbium-doped Bi₄Ti₃O₁₂ (Bi_3.4Yb_0.6Ti₃O₁₂, BYT) ferroelectric thin films were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by chemical solution deposition (CSD). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to identify the crystal structure, the surface and cross-section morphology of the deposited ferroelectric films. Structure evolution and ferroelectric properties of the as-prepared thin films annealed under different temperatures (600 °C-750 °C) were studied in detail. Additionally, the mechanism concerning the dependence of electrical properties of the BYT ferroelectric thin films on the annealing temperature was discussed.     

Preparation, dielectric and ferroelectric properties of Pb5Ge3O11 and Pb5Ge2.85Si0.15O11 thin films fabricated by sol-gel process

Lead germanate-silicate (Pb₅Ge_2.85Si_0.15O₁₁) ferroelectric thin films were successfully fabricated on Pt/Ti/SiO₂/(100)Si substrates by the sol-gel process. The thin films were fabricated by multi-coating at preheating temperatures of 350 and 450 °C. After annealing the thin films at 600 °C, the films exhibited c-axis preferred orientation. The degree of c-axis preferred orientation of the thin films preheated at 350 °C was higher than that of films preheated at 450 °C. Grain growth was influenced by the annealing time. The thin films exhibited a well-saturated ferroelectric P-E hysteresis loop when preheated at 350 °C and annealed at 600 °C for 1.5 h. The values of the remanent polarization (Pr) and the coercive field (Ec) were approximately 2.1 μC/cm² and 100 kV/cm, respectively.     

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.     

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).     

Thickness dependence of structural and ferroelectric properties of sol-gel Pb(Zr0.56Ti0.44)0.90(Mg1/3Nb2/3)0.10O3 films

Sol-gel Pb(Zr_0.56Ti_0.44)_0.90(Mg_1/3Nb_2/3)_0.10O₃ (PZT-PMN) films were prepared onto the Ti/Pt/Ti bottom electrode by multilayer spin coating. The film thickness ranged from 0.22 to 0.88 μm. The Pt top electrodes were deposited on the PZT-PMN films by DC sputtering. The structural and ferroelectric properties of PZT-PMN films were investigated as a function of film thickness by X-ray diffraction (XRD), scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and by measuring the relative permittivity. The film retains the tetragonal perovskite structure with the [111] and [100] preferred orientations perpendicular to the film surface independent of film thickness. The [100] texture increases with increasing film thickness although the [111] texture is always predominant. The film consists of columnar grains. The average grain size is nearly independent of film thickness. The surface layer containing fine grains about 30 nm in diameter is induced on the top of the film. As the film thickness exceeds 0.44 μm, the number of the fine grains decreases remarkably. The crystalline interface layer about 10 nm thick is formed between the film and the bottom electrode. This interface layer is composed of Pt, Pb, Zr, Ti and O, while it is rich in Ti and deficient in Pb and O as compared with the inside of the film. The measured relative permittivity of the film increases with increasing film thickness, following the low permittivity interface model. On the basis of this model, the relative permittivity is estimated to be 3200 for the intrinsic PZT-PMN film, 750 for the surface layer and 50 for the interface layer.     

The effect of rapid thermal annealing on structural and electrical properties of TiB2 thin films

This work reports on the effect of post-deposition rapid thermal annealing on the structural and electrical properties of deposited TiB₂ thin films. The TiB₂ thin films, thicknesses from 9 to 450 nm, were deposited by e-beam evaporation on high resistivity and thermally oxidized silicon wafers. The resistivity of as-deposited films varied from 1820 μΩ cm for the thinnest film to 267 μΩ cm for thicknesses greater than 100 nm. In the thickness range from 100 to 450 nm, the resistivity of TiB₂ films has a constant value of 267 μΩ cm.

A rapid thermal annealing (RTA) technique has been used to reduce the resistivity of deposited films. During vacuum annealing at 7 × 10⁻³ Pa, the film resistivity decreases from 267 μΩ cm at 200 °C to 16 μΩ cm at 1200 °C. Heating cycles during RTA were a sequence of 10 s. According to scanning tunneling microscopy analysis, the decrease in resistivity may be attributed to a grain growth through polycrystalline recrystallization, as well as to an increase in film density.

The grain size and mean surface roughness of annealed films increase with annealing temperature. At the same time, the conductivity of the annealed samples increases linearly with grain size. The obtained results show that RTA technique has a great potential for low resistivity TiB₂ formation.     

Fabrication of One-Transistor-Capacitor Structure of Nonvolatile TFT Ferroelectric RAM Devices Using Ba(Zr 0.1Ti 0.9)O 3 Gated Oxide Film

In this study, the Ba(Zr_0.1Ti_0.9)O₃ (BZ1T9) thin films have been well deposited on the Pt/Ti/SiO₂/Si substrate. The optimum radio frequency (RF) deposition parameters are developed, and the BZ1T9 thin films deposition at the optimum parameters have the maximum capacitance and dielectric constant of 4.4 nF and 190. As the applied voltage is increased to 8 V, the remnant polarization and coercive field of BZ1T9 thin films are about 4.5 muC/cm² and 80 kV/cm. The counterclockwise current hysteresis and memory window of n-channel thin-film transistor property are observed, and that can be used to indicate the switching of ferroelectric polarization of BZ1T9 thin films. One-transistor-capacitor (1TC) structure of BZ1T9 ferroelectric random access memory device using bottom-gate amorphous silicon thin-film transistor was desirable because of the smaller size and better sensitivity. The BZ1T9 ferroelectric RAM devices with channel width = 40 mum and channel length = 8 mum has been successfully fabricated and the I_D-V_G transfer characteristics also are investigated in this study.     

Fabrication and characterization of (1−x)SrBi2Ta2O9–xBi3TaTiO9 layered structure solid solution thin films for ferroelectric random access memory (FRAM) applications

We report on the properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ solid solution thin films for ferroelectric non-volatile memory applications. The solid solution thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi₂Ta₂O₉. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600°C and grain size was found to be considerably increased for the solid solution compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the solid solution thin films was in the range 180–225 for films with 10–50% of Bi₃TaTiO₉ content in the solid solution. Ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films were significantly improved compared to SrBi₂Ta₂O₉. For example, the observed remanent polarization (2P_r) and coercive field (E_c) values for films with 0.7SrBi₂Ta₂O₉–0.3Bi₃TaTiO₉ composition, annealed at 650°C, were 12.4 μC/cm² and 80 kV/cm, respectively. The solid solution thin films showed less than 5% decay of the polarization charge after 10¹⁰ switching cycles and good memory retention characteristics after about 10⁶ s of memory retention. The improved microstructural and ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films compared to SrBi₂Ta₂O₉, especially at lower annealing temperatures, suggest their suitability for high density FRAM applications.     

Deposition and dielectric properties of CaCu3Ti4O12 thin films on Pt/Ti/SiO2/Si substrates using pulsed-laser deposition

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully deposited on Pt/Ti/SiO₂/Si(1 0 0) substrates using pulsed-laser deposition technique. The crystalline structure and the surface morphology of the CCTO thin films were greatly affected by the substrate temperature and oxygen pressure. Thin films with a (2 2 0) preferential orientation were obtained at the substrate temperature above 700 °C and oxygen pressure above 13.3 Pa. The 480-nm thin films deposited under 720 °C and 26.6 Pa have a fairly high dielectric constant of near 2000 at 10 kHz and room temperature. The values of the dielectric constant and loss and their temperature-dependence under different frequency are comparable with those obtained in the epitaxial CCTO films grown on oxide substrates.