Properties Of Ferroelectric Films Research Articles

Epitaxial Growth of BaTiO3 Thin Films by High Gas Pressure Sputtering

Heteroepitaxial BaTiO3 thin films with lattice-misfit strain were prepared on SrRuO3/SrTiO3 substrates by rf magnetron sputtering under high gas pressure, in order to realize large area deposi-tion with uniform ferroelectric properties. Lattice constants, composition and ferroelectric properties of the epitaxial BaTiO3 films were characterized as a function of pressure as well as the incident angle of sputtered particles, under the assumption that the sputtered particles were eradiated from the eroded area of the target surface. Although ferroelectric properties of films sputtered under low gas pressure had remarkable place dependability, they have been drastically improved by high gas pressure sputtering. X-ray diffraction analyses revealed that the BaTiO3 film deposited with a pressure of 6.3 Pa had a large lattice strain of 5%, even though the substrate was placed at a position directly facing the erosion area of the target, where a number of high-energy ions such as oxygen ions are thought to collide against the film surface if the pressure is low. The BaTiO3 film exhibited a ferroelectric hysteresis loop with remnant polarization of 43 µC/cm2. These results indicate that high gas pressure sputtering is effective for achieving large area uniformity of ferroelectric properties in heteroepitaxial BaTiO3 thin films.

The Properties of Ferroelectric Films at Small Dimensions

▪ Abstract This paper reviews the literature on size effects in ferroelectric materials, with an emphasis on thin film perovskite ferroelectrics. The roles of boundary conditions, defect chemistry, electrode interfaces, surface layers, and microstructure in controlling the measured properties of ferroelectric films, as well as the observed deviation from bulk properties are discussed. Examples of the manifestation of size effects in terms of the low and high field dielectric properties, the piezoelectric effect, and the leakage behavior of films are given.

Orientation Selection in Sol-Gel Derived PZT Thin Films

AbstractMotivated by the growing impact of PZT film orientation on ferroelectric film properties as film thickness is scaled down, we present basic studies on orientation selection in sol-gel derived PZT films, using pre-annealed Pt/Ti electrode layers as a model electrode system. FTIR was used to study, on a real temperature scale, chemical reactions in the films during the initial thermal steps prior to crystallization. We found that the chemical structure of the pyrolyzed film has a much larger impact on orientation selection than has previously been realized. In addition to pyrolysis conditions, the ambient used for the crystallization step was found to play a crucial role in orientation selection. As film thickness decreases, excessive oxygen incorporation in the films is seen to result in the loss of the preferential (111) texture when crystallization is performed in air. By performing crystallizations in N2, 40 nm thick PZT films with a strongly preferential (111) orientation could be obtained.

Thin Na0.5K0.5NbO3 Films for Varactor Applications

AbstractSingle phase Na0.5K0.5NbO3 (NKN) thin films have been pulsed laser deposited on SiO2/Si(C-01) wafers and LaAlO3(001) and MgO(001) single crystals. Radio frequency (up to 1 MHz) and microwave (up to 50 GHz) dielectric spectroscopy studies have been carried out to characterize thin NKN films for electrically tunable microwave device applications. Films on single crystal oxide substrates showed tunabilities as high as 30-40 % at 40 V bias and dissipation factor of 0.01-0.02 at 1 MHz. The films on Si substrates showed low dielectric losses of < 0.01, and low leakage currents. Dielectric properties of ferroelectric films on Si substrates at low frequencies are greatly influenced by the depletion capacitance and the resistance inserted by semiconductor substrate. Microwave frequency measurements for NKN film on Si wafers yield more than 10 % tunability at 50 GHz and loss tan σ <0.1 at 10 GHz.

Phase transition properties of ferroelectric thin films with diluted surfaces

From transverse Ising model, the phase transition properties of ferroelectric films with part of surface diluted with non-ferroelectric sites have been studied. It was found that the Curie temperature is reduced by the proportion of the diluted area, and the reduction is more sensitive when the film becomes thinner. The surface polarization is always reduced by surface dilution.

Compositional dependence of electrical characteristics of SrBi2(Ta1−xNbx)2O9 thin-film capacitors

Ferroelectric capacitors using SrBi2(Ta1−xNbx)2O9 (SBTN) were compositionally altered varying Nb concentration from 0 to 1, the corresponding I–V and P–E electrical characteristics evaluated from room temperature to 145 °C. These temperature evaluations reveal that the leakage current will increase with larger Nb concentration and the dominant conduction mechanism changes from Schottky to Frenkel–Poole emission. The ferroelectric hysteresis curve shifts in the direction of negative polarization as the temperature or the concentration of Nb increases. Concentration increases in Nb reduces the temperature dependence of remnant polarization and coercive field. Film resistance to imprint and degradation from elevated temperature improves. Substituting b-site Nb for Ta allows imprinted capacitors to recover by application of either bipolar fatigue pulses at room temperature (RT) or, cycling P–E measurement pulses at elevated temperature. Due to this asymmetrical tradeoff of film ferroelectric properties, there is an optimum identifiable range of Nb concentrations (0.25–0.5) capable of achieving memory performance. Optimized SBTN ferroelectric films will suitably perform in integrated circuit memory function applications provided that the leakage current incurred at higher Nb concentrations can be reduced.

Effect of hydrogen on Pb(Zr,Ti)O3-based ferroelectric capacitors

The properties of ferroelectric films are known to degrade when subjected to hydrogen in forming gas anneals. Earlier studies have attributed this degradation to the loss of oxygen from these films during these anneals. In this study, we show that though oxygen is lost during forming gas annealing, hydrogen incorporation is the primary mechanism for the degradation of ferroelectric properties. Raman spectra obtained from the forming gas-annealed films show evidence of polar hydroxil [OH−] bonds in the films. The most probable site for hydrogen ions is discussed based on ionic radii, crystal structure, electrical properties, and Raman spectra. We propose that the hydrogen ion is bonded with one of the apical oxygen ions and prevents the Ti ion from switching. Pyroelectric measurements on forming gas-annealed capacitors confirm that the capacitors no longer possess spontaneous polarization.

Tunability and loss tangent of ferroelectric films in superconducting resonator

Using a high-temperature superconducting resonator, measurements of the dielectric properties of ferroelectric films at microwave frequency ( GHz) are performed. The variation in dielectric constant, , of film under the influence of an applied dc voltage, , at liquid-nitrogen temperature affects the value of the capacitance, C, and loss tangent, , of the ferroelectric capacitor. In order to evaluate the values of C and of the capacitor, we used the resonator which has a planar ferroelectric capacitor in the break of the microstrip line. The couplings of each microwave input and output port are designed in the resonator, including the bias circuits of the zero-field points. ferroelectric capacitors with values of the capacitance of about 0.5 pF and less than 0.01 successfully control the tunability of the resonance frequency in the resonator with a factor of .

Effect of bismuth on the ferroelectric properties of SrBi2Ta2O9 thin films deposited on Pt/SiO2/Si by a modified radio-frequency magnetron sputtering technique

Bi-layered ferroelectric SrBi2Ta2O9 (SBT) films were successfully deposited on Pt/SiO2/Si substrates using a sintered SrBi2Ta2O9 ceramic target, and Bi and Ta metal targets by a modified rf magnetron sputtering technique. The ferroelectric properties of the films were greatly dependent on the bismuth sputtering power. The SBT films deposited under the conditions of rf power of 100 W, Bi dc power of 25 W, and Ta dc power of 10 W showed a dense and uniform microstructure. A remanent polarization, Pr, of 7.51 μC/cm2 and a coercive field Ec, of 28 kV/cm were obtained for an excitation voltage of 5 V. The SBT films show practically no polarization fatigue after 7×1010 switching cycles. Their retention characteristics look very promising up to 106 sec.

Effects of Annealing in Ar Gas on Ferroelectric Properties of SrBi2Ta2O9 Thin Films

The effects of annealing processes at 700°C in Ar gas, added prior to the oxidizing processes of SrBi2Ta2O9 (SBT) films at the lowered temperature of 700°C, are investigated in terms of the ferroelectric properties of the films. SBT films 200 nm thick were spin-coated on Pt(200 nm)/Ti(30 nm)/SiO2(200 nm)/Si substrates by using the chemical liquid deposition method. The annealing was found to be effective in obtaining a large saturated polarization and a remanent polarization 2Pr of more than 10 µC/cm2 with leakage current less than 1×10-6 A/cm2 at the process temperature of 700°C. Structural and compositional analyses suggest that some Bi atoms in the films become free of oxygen and arrive at the lattice sites as a result of the annealing in non-oxidizing Ar gas for periods shorter than 100 s. Consequently, the crystallization of SBT layered structures may be promoted even at temperatures as low as 700°C.

Optical and electro-optical properties of sol-gel derived ferroelectric thin films

Ferroelectric (FE) thin films have been widely studied, especially for their electrical and ferroelectric properties, such as nonvolatile FE memory and ultrahigh density DRAMs. FE films also give rise to a variety of interesting and unique optical applications, such as electro-optic switches, passive and active waveguides, second harmonic generation, optical data storage, optical diodes and photovoltaic devices. Their dielectric properties have been extensively optimized but the optical and electro-optical aspects have not been satisfactorily explored. This paper will review the latest advances in the optical and electro-optical properties of FE films, highlighted by data obtained from our laboratories. Optical (refractive index and waveguiding loss) and electro-optical (linear (r 33) and quadratic (R 33) electro-optic coefficients) properties will be presented. Optimization of these properties by precise tailoring of chemistry, processing conditions and microstructure is discussed. Low optical loss, i.e., < 2dB/cm and an exceptionally high r 33 value of 315pm/V from PLZT films were obtained.

Effects of Conductive Oxide Electrode by Wet Process on Film Properties of Ferroelectric Films

Effects of Conductive Oxide Electrode by Wet Process on Film Properties of Ferroelectric Films

The size effect on the switching properties of ferroelectric films: a one-dimensional lattice model

A one-dimensional lattice model with non-ferroelectric boundary sites has been used to study the effect of size on the polarization-reversal process in ferroelectric films. We assume non-ferroelectric imperfections at the boundary sites which act as nuclei for the polarization reversal, and include them explicitly in the free energy. The dependences of the switching time on film thickness and on electric field strength have been obtained, and fitted with empirical formulae. The time evolution of the polarization under a step field has also been simulated, from which we can clearly see domain wall formation and motion. The polarization and current under a bipolar pulse signal are also obtained to demonstrate the different memory state responses, in which a back-switching phenomena can be observed clearly.

Pulse switching characterization of organometallic chemical vapor deposited PbZrxTi1−xO3 thin films for high-density memory applications

Abstract This paper addresses the electrical characterization by fast pulse measurements of organometallic chemical vapor deposited PbZrxTi1−xO3 (PZT) thin films with platinum electrodes on oxidized silicon wafers. Investigations of the switched and non-switched polarization with the pulse amplitude, number of switched cycles (fatigue), pulse width, time (ageing), and the sensitivity to disturbing pulses were made. Different processing conditions (i.e., annealing in oxygen after the deposition and structuring of the top electrode), compositions, thicknesses and operating temperatures (0–100°C) can affect the thin film's ferroelectric properties. Understanding these effects and trends aids in the optimization of PZT for high-density non-volatile random access memories and possibly other device applications. To supplement the understanding of the ferroelectric behavior in the films, transmission electron microscope investigations were made on two different compositions and unfatigued/fatigued capacitors.

Dielectric and Ferroelectric Properties of Sol-Gel Derived PLZT Films

ABSTRACTThere has not been much exploration of PLZT film properties as a function of compostion reported in the literature. A survey of numerous PLZT films covering a wide spectrum of the PLZT phase diagram was undertaken to explore the dependence of film properties on composition. A series of sol-gel derived PLZT films were prepared on platinized Si wafers and fired to 700C to obtain the perovskite phase. The film compositions include PLZT x/65/35, x/20/80, x/53/47 for x = 0, 2, 4, 6, 8, 10 and 12 and 7.5/x/y where x/y = 70/30, 53/47, 20/80 and 0/100. These films were characterized for their dielectric and ferroelectric properties. The films definitely showed a strong dependence of final film properties on composition, providing a valuable tool for the material engineering of ferroelectric film properties.

Properties of ferroelectric PbTiO3 films grown in an ionized cluster beam system

Two ionized cluster beam (ICB) sources were used to deposit PbTiO3 films 300 nm thick onto Pt-metallized, oxidized Si wafers 100 mm in diameter. Pb was deposited from one ICB source and Ti from the other. The deposition was carried out in a mixture of O2 and 6% O3 at a total pressure of 5×10−5 Torr. No substrate heating was used during deposition. However, thermal radiation from the ICB sources caused the substrate temperature to reach approximately 100 °C soon after the start of deposition. The as-deposited films were amorphous according to x-ray diffraction patterns. Annealing the films at 550 °C in 1 atm pressure O2 for 1 h led to the formation of a mixture of a perovskite-type phase and possibly some pyrochlore- and PbO-type structures. However, an anneal at 750 °C for 1 h in 1 atm pressure O2 produced the perovskite phase PbTiO3 with no other detectable phases. It was found that a slightly Pb-rich or stoichiometric Pb/Ti ratio in as-deposited films gave the best ferroelectric film properties. A Pb/Ti ratio much higher than stoichiometric (Pb/Ti ratio≳2) led to the formation of pits and pinholes in the films after annealing at 750 °C, whereas a Pb/Ti ratio lower than stoichiometric did not produce the perovskite phase after annealing at 750 °C. The pits and pinholes associated with Pb/Ti ratio larger than stoichiometric were probably caused by the evaporation of excess PbO during annealing. Films were deposited with and without the use of ionization and acceleration to 2 keV. In both cases, the as-deposited films were amorphous. There were no substantial differences between annealed films deposited with or without ionization and acceleration of the Pb and Ti beams. Ferroelectric domains in the annealed films have been observed by transmission electron microscopy. Ferroelectric switching (hysteresis loops) was recorded in annealed PbTiO3 films with stoichiometric Pb/Ti ratios after the films were capped with Pt to form small capacitors.

Spectroscopic ellipsometry studies on ion beam sputter deposited Pb(Zr, Ti)O 3 films on sapphire and Pt-coated silicon substrates

Spectroscopic ellipsometry has been utilized to non-destructively depth profile multi-ion beam reactively sputtered lead zirconate titanate films. Some degree of inhomogeneity (in the form of low density layers or surface roughness) was found in all of the films examined. The evolution in the structure and microstructure of such films during post-deposition annealing was investigated with in situ spectroscopic ellipsometry. It was found that the onset of microstructural inhomogeneities was associated with the crystallization of the perovskite phase, and that the final film microstructure was dependent on the details of the annealing process. A model was developed to approximate the effect that local density variations play in determining the net electrical properties of ferroelectric films. Depending on the configuration of the embedded porosity, it was demonstrated that microstructural inhomogeneities can significantly change the net dielectric constant, coercive field, and remanent polarization of ferroelectric films. It has also been shown that low density regions near the film/substrate interface can result in apparent size effects in ferroelectric films.

Influence of grain boundary, defect and internal stress states on properties of ferroelectric materials

Abstract Performance characteristics in ferroelectric based components, sensors and thin films for memory and storage applications are significantly influenced by aging instability in these perovskite materials. The aging changes can be attributed both to dc electrochemical effects, where migration of ionic charges lead to degradation in permittivity, loss and resistance, and to changes in domain structure and mobility. This latter reflects the presence within the microstructure of defect states, diffuse grain boundary structures and internal stresses. Aging in these materials is also dependent on microstructure, impurities, dopants and stoichiometry, as well as on ambient temperature and applied dc field stresses. Significant improvements in aging and performance characteristics have been achieved with selected dopant additions to BaTiO3, such as ZrO2, which control grain morphology, resulting in structural modifications and decreased domain mobility. Permittivity has been found to be highly dependent on internal stress and this dependence has been mathematically modeled and related to the aging instability. Similarly, a model has been developed which explains the changes in grain morphology, resistivity, aging and PTC effects in terms of site occupancy of the dopants in the BaTiO3 lattice. Properties of ferroelectric films are also significantly influenced by structure, dopant incorporation and grain size, but understanding of these interrelationships is less advanced.

Sol-Gel processing of Nb-doped Pb(Zr, Ti)O3 thin films for ferroelectric memory applications

The ferroelectric properties of Nb-doped PZT thin films prepared by a sol-gel method were evaluated relative to memory device application requirements. Within the range of 0 to 4 mol %, Nb-doping of PZT compositions near the morphotropic phase boundary region (i.e. PZT 53/47) enhanced overall ferroelectric properties by reducing the te-tragonal distortion of the unit cell. A 4 mol % Nb-doped PZT 53/47 thin film (0.26 μm) had a coercivity of 8 V/ μm, a remanence ratio of 0.54, a switchable polarization of 45 μC/cm2, and a specific resistivity of 3 x 109 Ω-cm. Nb-doping levels in excess of 5 mol had a detrimental effect on the resulting thin film ferroelectric properties. X-ray diffraction (XRD) analysis of highly doped films showed development of a significant PbO phase accompanied by diffraction line broadening of the perovskite phase. As such, it was postulated that the creation of excessive lead vacancies in the PNZT lattice resulted in PbO accumulation at the grain boundaries which impeded grain growth, and hence, adversely affected ferroelectric switching performance. The fatigue performance of the sol-gel derived thin film capacitor system was a function of switching voltage. At switching fields sufficient to saturate the polarization, the endurance of the thin film capacitor was greater than 109 cycles. Cycling with lower fields reduced endurance values, but in all cases, the switchable polarization decreased linearly with the logarithm of cycles. Nb-doping did not have a significant effect on the fatigue performance.

Ferroelectric Properties of Lead Titanate Films Deposited by Low Pressure Chemical Vapor Deposition

Lead titanate films were deposited onto titanium substrate by low pressure chemical vapor deposition using reactant gas mixtures of Pb, , and oxygen. X‐ray diffraction and scanning electron microscopy analyses show that the second layer between film and titanium substrate is titanium dioxide rutile phase. Rutherford backscattering spectroscopy analysis reveals that the composition of the deposition surface is that of within the error and the deposition layers show the change of concentration with depth of each element of film. Measurement of hysteresis loop in order to obtain ferroelectric properties of unique films was performed. Hysteresis loop with parallel electrode pattern on film gives a saturation even if is in series with relatively high impedance of the linear dielectric intermediate layer . The ferroelectric properties of the film included a remanent polarization of 10 μC/cm2 and a coercive field of 25 kV/cm.