Ferroelectric Thin Film Capacitors Research Articles

Mechanism of high temperature hydrogen reduced polarization degradation in Pb(Zr1−xTix)O3 ferroelectric thin film capacitors

AbstractAnnealing Pb(Zr1−xTix)O3 (PZT) thin films in the hydrogen ambient can result in the unacceptable degradation of ferroelectric properties due to interaction of H2 forming gas with the ferroelectric materials. Since the understanding of this degradation mechanism is limited, this issue arouses a big problem for the application of PZT in high‐density non‐volatile ferroelectric random access memories (NVFRAM). In order to resolve this problem, we investigate Pt/PZT/Pt capacitors before and after H2 annealing at various temperatures. The capacitors lost their ferroelectric properties during hydrogen annealing. The measurements of polarization hysteresis loops and leakage currents suggest that there are different degradation mechanisms for the reduced PZT films. Although hydrogen induced degradation can be partially recovered by post‐annealing of the thin films in the O2 ambient, all the remanent polarizations after recovery are much smaller than their virgin values, which indicates the irreversible damage of the ferroelectricity during the forming gas annealing (FGA) process. In order to investigate the degradation mechanism, X‐ray diffraction was performed before and after FGA, and atomic force microscopy images were taken to observe the topograghy and microstructure changes. A conductive Pt‐coated piezoelectric force microscope probe was used as a mobile top electrode to check ferroelectric and piezoelectric properties within microregions of PZT films before and after FGA.

Epitaxial Ferroelectric Heterostructures Fabricated by Selective Area Epitaxy of SrRuO3 Using an MgO Mask

Illustration of a new high-temperature hard-mask process based on traditional lithography and selective wet-etching of MgO. The hard mask is compatible with standard nano-lithography techniques and heat treatments in excess of 1000 °C. Here, this technique is applied to produce temperature-stable contacts that give rise to low leakage, improved fatigue properties, and excellent high-temperature stability in ferroelectric thin-film capacitors.

Ultrafast electrical measurements of polarization dynamics in ferroelectric thin-film capacitors

We have developed a new approach to measure fast electrical signals during polarization switching in ferroelectric thin-film capacitors. This article describes a simple method for probing transient currents and voltages in a broad range of time scales from microseconds to hundreds of picoseconds. In order to test our approach, we probed polarization dynamics in commercial PbZr(0.2)Ti(0.8)O(3) ferroelectric capacitors in which the measured switching time was as fast as 1.7 ns.

Structural and ferroelectric behaviours in blends of vinylidene fluoride oligomer and poly(vinylidene fluoride-trifluoroethylene) copolymer thin film

The structure and ferroelectric properties of ultrathin blend films fabricated by the spin-coating process at room temperature from the mixed solution of vinylidene fluoride oligomers (OVDF) (n = 18) and poly(vinylidene fluoride-trifluoroethylene) copolymers (PVDF-TrFE) are investigated in detail. When OVDF content in mixture is between 40% and 60%, the remanent polarization and fatigue behaviour of the P(VDF-TrFE)/OVDF blend films are all improved. The ferroelectric thin film capacitor with poly(styrenesulfonate) (PSSH) interlayer exhibits a prominent performance with a remanent polarization of 120 mC m−2 even as the thickness of the blend film is 80 nm. Meanwhile, we demonstrate that the increase in the OVDF content from 0% to 100% cannot affect the molecular chain orientation of the P(VDF-TrFE)/OVDF blend films and the molecular chains are in parallel with the substrate.

Effect of the microwave signal power on the controllability of a ferroelectric phase shifter

We consider a microwave phase shifter based on thin-film ferroelectric parallel-plate capacitors operating as a part of a phased array at an elevated microwave power level and for a limited power in the control circuits. To eliminate capacitance modulation in parallel-plate capacitors by an ac field, we propose the separation of the effects of the dc control voltage and the ac voltage based on the symmetry of the capacitance-voltage characteristic of the unit including two capacitors parallel-connected in the ac voltage U ac and series-connected in dc voltage U dc. The experiment shows that for U dc > U ac, the phase of the wave passing through the phase shifter weakly depends on the microwave signal power. The reflection phase shifter is tested for a pulsed power of the microwave signal up to 6 W and a power in the control circuit on the order of 10 μW.

Spatially resolved mapping of disorder type and distribution in random systems using artificial neural network recognition

The spatial variability of the polarization dynamics in thin film ferroelectric capacitors was probed by recognition analysis of spatially resolved spectroscopic data. Switching spectroscopy piezoresponse force microscopy (SSPFM) was used to measure local hysteresis loops and map them on a two dimensional (2D) random-bond, random-field Ising model. A neural-network based recognition approach was utilized to analyze the hysteresis loops and their spatial variability. Strong variability is observed in the polarization dynamics around macroscopic cracks because of the modified local-elastic and electric-boundary conditions, with the most pronounced effect on the length scale of \ensuremath{\sim}100 nm away from the crack. The recognition approach developed here is universal and can potentially be applied for arbitrary macroscopic and spatially resolved data, including temperature- and field-dependent hysteresis, I-V curve mapping, electron microscopy electron energy loss spectroscopy (EELS) imaging, and many others.

Effect of UV radiation on slow relaxation processes in ferroelectric thin-film capacitors

The influence of UV radiation with wavelengths of about λ = 340 nm and power within 0.001–0.15 mW on the suppression of slow relaxation of residual polarization in ferroelectric capacitors based on thin (Ba,Sr)TiO3 (BSTO) films in a paraelectric state has been studied. Experimental data on the behavior of capacitance relaxation time and conduction current in ferroelectric capacitors under the action of UV radiation are presented. A mechanism of relaxation with a characteristic time on the order of 1 s is found that is not affected by UV radiation.

Fatigue characteristics of poly(vinylidene fluoride-trifluoroethylene) copolymer ferroelectric thin film capacitors for flexible electronics memory applications

The electrical fatigue characteristics of 100 nm thick, solution processed poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer thin film Metal-Ferroelectric-Metal (MFM) capacitors have been studied. After one million stress cycles at an electric field of 1.4 MV/cm and 500 Hz switching frequency, 87% of the initial switching polarization is maintained, with the non-switching polarization remaining low. The kinetics of the switching polarization were studied before and after electrical stress. Fatigue increases the required activation field for switching polarization and suppresses switchable polarization. The activation field increases after electrical stress and higher activation fields were required for higher electric field stressing. The reduction in switchable polarization is approximately 10% of the initial switching polarization for all electric fields and frequencies studied. We demonstrated that under operating conditions compatible with flexible electronic applications, the optimized ferroelectric thin film capacitor reported shows stable switching polarization and acceptable fatigue after electrical stress.

Polarization behavior of poly(vinylidene fluoride-trifluoroethylene) copolymer ferroelectric thin film capacitors for nonvolatile memory application in flexible electronics

The time domain and electric field dependence of the polarization switching kinetics of poly(vinylidene fluoride-trifluoroethylene) copolymer based thin film metal-ferroelectric-metal capacitors have been characterized. At room temperature, the time required for complete switching polarization decreases from >1 s to <50 μs as the voltage is increased from 6 to 12 V, while low nonswitching polarization is maintained. In the time domain, the ferroelectric switching polarization reversal behavior for devices biased above the coercive field follows the nucleation-limited-switching model. The exponential relationship between switching time and applied electric field indicates nucleation dominated switching kinetics. Switching behavior as a function of temperature was also characterized from −60 to 100 °C in the voltage range of 6–12 V. Higher temperatures induce larger dc conductance leakage at low frequencies and increases nonswitching polarization for all the voltages studied. It is demonstrated that for certain frequencies, by controlling the switching voltage, our optimized ferroelectric thin film capacitor shows stable switching polarization in a temperature range compatible with flexible electronics applications.

Bipolar and unipolar electrical fatigue in ferroelectric lead zirconate titanate thin films: An experimental comparison study

By performing standard positive-up-negative-down, hysteresis-loop and dielectric measurements on the ferroelectric lead zirconate titanate thin-film capacitors subject to bipolar/unipolar electrical cycling, we show that unipolar fatigue is evident though still less severe than bipolar fatigue conducted at the same voltage. That has been attributed to a series of periodic events of polarization backswitching (driven by the residual depolarization field) and switching (driven by the residual applied field) during unipolar electrical cycling, and explained using the LPD-SICI model (LPD-SICI stands for local phase decomposition caused by switching-induced charge injection). The dielectric results have been used to estimate the effective thickness di of the fatigue-induced degraded (pyrochlorelike) interfacial layer after bipolar/unipolar fatigue, which has not been done so far to our best knowledge. The fact that di is still much less than the film thickness even after the most severe bipolar fatigue strongly suggests that polarization fatigue in ferroelectrics is an interface effect, not a bulk one.

Interface depolarization field as common denominator of fatigue and size effect in Pb(Zr0.54Ti0.46)O3 ferroelectric thin film capacitors

Dielectric, hysteresis and fatigue measurements are performed on Pb(Zr0.54Ti0.46)O3 (PZT) thin film capacitors with different thicknesses and different electrode configurations, using platinum and LaNiO3 conducting oxide. The data are compared with those collected in a previous work devoted to study of size effect by R. Bouregba et al., [J. Appl. Phys. 106, 044101 (2009)]. Deterioration of the ferroelectric properties, consecutive to fatigue cycling and thickness downscaling, presents very similar characteristics and allows drawing up a direct correlation between the two phenomena. Namely, interface depolarization field (Edep) resulting from interface chemistry is found to be the common denominator, fatigue phenomena is manifestation of strengthen of Edep in the course of time. Change in dielectric permittivity, in remnant and coercive values as well as in the shape of hysteresis loops are mediated by competition between degradation of dielectric properties of the interfaces and possible accumulation of interface space charge. It is proposed that presence in the band gap of trap energy levels with large time constant due to defects in small nonferroelectric regions at the electrode—PZT film interfaces ultimately governs the aging process. Size effect and aging process may be seen as two facets of the same underlying mechanism, the only difference lies in the observation time of the phenomena.

Ferroelectric and piezoelectric thin films and their applications for integrated capacitors, piezoelectric ultrasound transducers and piezoelectric switches

Ferroelectric and piezoelectric thin films are gaining more and more importance for the integration of high performance devices in small modules. High-K 'Integrated Discretes' devices have been developed, which are based on thin film ferroelectric capacitors integrated together with resistors and ESD protection diodes in a small Si-based chip-scale package. Making use of ferroelectric thin films with relative permittivity of 950–1600 and stacking processes of capacitors, extremely high capacitance densities of 20–520 nF/mm2, high breakdown voltages up to 140 V and lifetimes of more than 10 years at operating voltages of 5 V and 85°C are achieved. Thin film high-density capacitors play also an important role as tunable capacitors for applications such as tuneable matching circuits for RF sections of mobile phones. The performance of thin film tuneable capacitors at frequencies between 1 MHz and 1 GHz is investigated. Finally thin film piezoelectric ultrasound transducers, processed in Si– related processes, are attractive for medical imaging, since they enable large bandwidth (>100%), high frequency operation and have the potential to integrate electronics. With these piezoelectric thin film ultrasound transducers real time ultrasound images have been realized. Finally, piezoelectric thin films are used to manufacture galvanic MEMS switches. A model for the quasi-static mechanical behaviour is presented and compared with measurements.

Nanoscale insight into the statics and dynamics of polarization behavior in thin film ferroelectric capacitors

In this study, we review recent advances in PFM studies of micrometer scale ferroelectric capacitors, summarize the experimental PFM-based approach to investigation of fast switching processes, illustrate what information can be obtained from PFM experiments on domains kinetics, and delineate the scaling effect on polarization reversal mechanism. Particular attention is given to PFM studies of mechanical stress effect on polarization stability.

Ferroelectric Thin-Film Capacitors and Piezoelectric Switches for Mobile Communication Applications

Thin-film ferroelectric capacitors have been integrated with resistors and active functions such as ESD protection into small, miniaturized modules, which enable a board space saving of up to 80%. With the optimum materials and processes, integrated capacitors with capacitance densities of up to 100 nF/mm2 for stacked capacitors combined with breakdown voltages of 90 V have been achieved. The integration of these high-density capacitors with extremely high breakdown voltage is a major accomplishment in the world of passive components and has not yet been reported for any other passive integration technology. Furthermore, thin-film tunable capacitors based on barium strontium titanate with high tuning range and high quality factor at 1 GHz have been demonstrated. Finally, piezoelectric thin films for piezoelectric switches with high switching speed have been realized.

Influence of metal-ferroelectric contacts on the space charge formation in ferroelectric thin film capacitors

We have studied the response speed characteristics of capacitor structures based on thin Ba0.3Sr0.7TiO3 (BSTO) films with metal-ferroelectric contacts of various types formed under different conditions. It is established that threshold voltages exist for the appearance of a residual space charge in some structures. A comparative analysis of the technological features of contact formation and experimental data on the characteristics of samples leads to the conclusion that, by forming Pt-BSTO contacts in an oxygen-containing atmosphere, it is possible to suppress the injection of carriers into the ferroelectric film in the capacitor structure.

Polarization retention on short, intermediate, and long time scales in ferroelectric thin films

We developed a model with no adjustable parameter for retention loss at short and long time scales in ferroelectric thin-film capacitors. We found that the predictions of this model are in good agreement with the experimental observations in the literature. In particular, it explains why a power-law function shows better fitting than a linear-log relation on a short time scale (10−7 s to 1 s) and why a stretched exponential relation gives more precise description than a linear-log plot on a long time scale (>100 s), as reported by many researchers in the past. More severe retention losses at higher temperatures and in thinner films have also been correctly predicted by the present theory.

Polarization change in ferroelectric thin film capacitors under external stress

The changes in polarization and coercive voltage of lead zirconate titanate (PZT) thin film of 130 nm in thickness were measured under hydrostatic pressure up to 1.5 GPa. By converting the isotropic loading to an equivalent out-of-plane uniaxial compressive loading, it was found that these results were consistent with our previous results of uniform in-plane tensile stress, showing that the polarization changes under these two different loading conditions are of the same nature. The combined results demonstrate that the intrinsic polarization decreased linearly with the level of stress at a rate of −0.013 μC cm−2 MPa−1 with a critical stress of 1.6 GPa to paraelectric phase. Further comparison of our result with that of a thick PZT film and bulk PZT materials suggests that ferroelastic switching (90° switching) is dominant in the bulk material, and the polarization reduction in our thin film is due to material intrinsic lattice distortion under pressure, while the thick film response is intermediate. The effect of substrate clamping on ferroelastic domains is also shown.

LOW VOLTAGE ELECTRON EMISSION FROM BaTiO3 FERROELECTRIC THIN FILMS

ABSTRACT Electron emission from sol-gel derived thin films is demonstrated in this paper. The electron emission measurements have been performed using 1 μ m ferroelectric cathodes based on BaTiO3 spin coated thin films. Ferroelectric thin film capacitors were obtained onto a nichrome substrate using a very thin Au layer like top electrode. Then, an a.c. voltage is applied to the bottom electrode and electron emission is observed using a parallel cathode – positively biased anode arrangement in a vacuum chamber at 10− 4 Pa. Stable electron emission was obtained using both sine or square-wave voltage in a wide range of frequencies with the turn-on voltage of 50 V.

Ferroelectric and dielectric properties of SrBi4Ti4O15 thin films grown on Bi4Ti3O12 film layer

Ferroelectric and dielectric properties of bilayered ferroelectric thin films, SrBi4Ti4O15 grown on Bi4Ti3O12, were investigated. The thin films were annealed at 700°C under oxygen atmosphere. The bilayered thin films were prepared on a Pt(111)/Ti/SiO2/Si substrate by a chemical solution deposition method. The dielectric constant and dielectric loss of the bilayered thin films were 645 and 0.09, respectively, at 100 kHz. The value of remnant polarization (2Pr) measured from the ferroelectric thin film capacitors was 60.5 μC/cm2 at electric field of 200 kV/cm. The remnant polarization was reduced by 22% of the initial value after 1010 switching cycles. The results showed that the ferroelectric and dielectric properties of the SrBi4Ti4O15 on Bi4Ti3O12 ferroelectric thin films were better than those of the SrBi4Ti4O15 grown on a Pt-coated Si substrate suggesting that the improved properties may be due to the different nucleation and growth kinetics of SrBi4Ti4O15 on the c-axis-oriented Bi4Ti3O12 layer or on the Pt-coated Si substrate.

Reliability study of tunable ferroelectric capacitors

In this paper, the reliability of tunable ferroelectric thin film capacitors has been addressed. The long term stability of tunable capacitors based on Ba0.3Sr0.7TiO3 films deposited by chemical solution deposition onto an alumina substrate was investigated by monitoring their tunable performance as a function of the number of ON-OFF switching cycles and as a function of time. The conditions of civil aeronautic applications were chosen to plan the lifetime experiments. More than 300×106 of switching cycles with an amplitude of 150 V and frequency of 1 kHz were successfully applied to the investigated capacitors without damaging them. Moreover, no degradation in the C-V characteristic was observed after the application of 150 V dc voltage during 15 h. Additionally, the effect of illumination of the ferroelectric film by ultraviolet light on the tunable performance of the capacitors has been investigated. A positive effect of such illumination on the C-V characteristics of ferroelectric capacitor has been found.