Characterization Of Ferroelectric Thin Films Research Articles

Toward accurate ferroelectric polarization estimation in nanoscopic systems

The nanoscopic characterization of ferroelectric thin films is crucial from their device application point of view. Standard characterization techniques are based on detecting the nanoscopic charge compensation current (switching current) caused by the polarization reversal in the ferroelectric. Owing to various surface and bulk limited mechanisms, leakage currents commonly appear during such measurements, which are frequently subtracted using the device I–V characteristic by employing a positive-up-negative-down measurement scheme. By performing nanoscopic switching current measurements on a commonly used ferroelectric, BiFeO3, we show that such characterization methods may be prone to large errors in polarization estimation on ferro-resistive samples due to current background subtraction issues. Specifically, when the resistance of the ferroelectric thin film is associated with polarization reversal, background current subtraction is not accurate due to the mismatch of the I–V characteristics for two polarization states. We show instead that removing the background current by an asymmetric least-squares subtraction method, though not perfect, gives a much better estimation of the ferroelectric properties of the sample under study.

Determination of the doping profile of lanthanum impurities in thin-film barium titanate structures

The paper presents a calculation of doping concentration profile in a two-layer barium titanate film, one of the layers of which contains lanthanum impurity. The theoretically predicted distributions were experimentally verified. Influence of the doping profile on the magnitude of the electrocaloric (EC) response of ferroelectric films is assessed by numerical simulation. It is demonstrated that at the same La concentration, the magnitude of the EC effect can change by 40% due to the distribution of the lanthanum concentration in the structure. The results obtained allow optimizing the technological process to ensure the required characteristics of ferroelectric thin films.

A method for estimating defects in ferroelectric thin film MOSCAPs

We propose a capacitance measurement scheme that enables quantitative characterization of ferroelectric thin films integrated on semiconductors. The film defect density is estimated by measurements of the CV hysteresis and frequency dispersion, whereas important device parameters such as memory window and endurance can be extracted by a unidirectional CV method. The simple measurement scheme and the usage of metal-oxide-semiconductor capacitors rather than MOSFETs make the proposed methods suitable for the future optimization of ferroelectric field effect transistor and negative capacitance field effect transistor gate stacks. Specifically, we present data for the narrow bandgap semiconductor InAs and show that low temperature characterization is critical to reduce the influence of the minority carrier response; however, the methods should be transferrable to room temperature for semiconductors with a wider bandgap. Our results clearly indicate that the defect density of the HfxZr1−xO2 (HZO) films increases at the crystallization temperature, but the increase is modest and remains independent of the annealing temperature at even more elevated temperatures. It is also shown that the shrinkage of the memory window caused by field cycling is not accompanied by an increase in defect density.

An interface charge model for ferroelectric field effect transistor

ABSTRACTAn interface charge model for ferroelectric-gate field-effect transistor (FeFET) is developed by combining the basic device equations of metal-oxide-semiconductor field-effect transistors with the polarization characteristics of ferroelectric thin films. This model presents the characteristics of FeFET considering interface charge between the ferroelectric thin film and the insulator layer. Simulations demonstrate that the interface charge will cause the surface potential of the semiconductor and the drain current left shift, and the memory windows are narrowed down, which are resulted from the space charge of the surface of the semiconductor. Meanwhile, the value of polarization almost does not change in FeFET. Furthermore, the simulation of FeCMOS indicates that the output voltage will left shift as the interface charge increases.

Nanosecond Characterization of Regional Domain Imprint from Fast Domain Switching Currents in Pb(Zr,Ti)O3 Thin Films**Supported by the National Key Basic Research Program of China under Grant No 2014CB921004, the National Natural Science Foundation of China under Grant Nos 61225020 and 61176121, and the Program for Professor of Special Appointment (Eastern Scholar) in Shanghai.

The traditional imprint characterization of ferroelectric thin films estimates imprint time dependence of the mean coercive voltage of all domains from a polarization-voltage hysteresis loop, which shows a semilogarithmic time dependence above an initial imprint time of τ0 > 1 μs at room temperature. Below τ0, the imprint effect is believed to be weak. In consideration of region-by-region domain reversal under a rising pulsed voltage with ordered coercive voltages increasing from zero up to the maximum applied voltage during capacitor charging time, we can estimate the imprinted coercive voltage of each domain from domain switching current transient separately with imprint time as short as 20 ns. In disagreement with the previous observations, all imprinted coercive voltages for the domains in Pt/Pb(Zr0.4Ti0.6)O3/Pt thin-film capacitors show step-like increases at two characteristic times of 300 ns and 0.27s. The imprint effect is surprisingly strong enough even at shortened time down to 20 ns without any evidence of weakening.

Influence of the annealing temperature on the ferroelectric properties of niobium-doped strontium–bismuth tantalate

Characteristics of ferroelectric thin films of niobium-doped strontium–bismuth tantalate (SBTN), which were deposited by magnetron sputtering on Pt/TiO2/SiO2/Si substrates, are investigated. To form the ferroelectric structure, deposited films were subjected to subsequent annealing at 700–800°C in an O2 atmosphere. The results of X-ray diffraction showed that the films immediately after the deposition have an amorphous structure. Annealing at 700–800°C results in the formation of the Aurivillius structure. The dependences of permittivity, residual polarization, and the coercitivity of SBTN films on the modes of subsequent annealing are established. Films with residual polarization 2Pr = 9.2 µC/cm2, coercitivity 2Ec = 157 kV/cm, and leakage current 10–6 A/cm2 are obtained at the annealing temperature of 800°C. The dielectric constant and loss tangent at frequency of 1.0 MHz were e = 152 and tan δ = 0.06. The ferroelectric characteristics allow us to use the SBTN films in the capacitor cell of high density ferroelectric random-access non-volatile memory (FeRAM).

Ferroelectric properties of niobium-doped strontium bismuth tantalate films

The characteristics of ferroelectric thin films of strontium bismuth tantalate (SBT) and niobium-doped strontium bismuth tantalate (SBTN) deposited by radio-frequency (RF) magnetron sputtering on Pt/TiO2/SiO2/Si substrates were investigated. For the formation of the structure of the ferroelectric material, the deposited films were subjected to a subsequent annealing at temperatures of 970–1070 K in an O2 atmosphere. The results of the X-ray diffraction analysis demonstrated that, in contrast to SBT films, in which the Aurivillius phase is formed only at annealing temperatures of 1050–1070 K, the formation of this phase in SBTN films is observed already at a temperature of 970 K. The dependences of the dielectric permittivity, remanent polarization, and coercive force of the SBT and SBTN films on the subsequent annealing conditions were determined. It was found that, upon doping of the SBT films with niobium, the remanent polarization increases by a factor of approximately three, the Curie temperature increases by 50 K, and the dielectric permittivity also increases. It was revealed that, in contrast to the SBT films, the polarization of the SBTN films is observed already at an annealing temperature of approximately 970 K. It was shown that the replacement of SBT films by SBTN films in the manufacture of high-density nonvolatile ferroelectric randomaccess memory (FeRAM) capacitor modules makes it possible to decrease the synthesis temperature from 1070 to 990–1000 K, which improves the compatibility with the planar technology of semiconductor devices. However, it turned out that an increase in the coercive field makes niobium-doped SBT films less attractive for the use in FeRAM.

Reliability Characteristics of Ferroelectric $ \hbox{Si:HfO}_{2}$ Thin Films for Memory Applications

Reliability characteristics of ferroelectric thin films (10 nm) based on Si-doped HfO2 have been investigated with focus on potential memory applications. Extensive retention, imprint, and endurance data for this new type of ferroelectric material are presented for the first time. The variability of reliability characteristics in terms of capacitor annealing temperatures as well as excitation amplitude effects is analyzed. Stable ferroelectric switching behavior can be observed in a wide temperature range from 80 K up to 470 K. Bake tests at 125 °C show almost no retention loss for saturated polarization states up to cumulative testing times of 1000 h. In addition to the same-state retention, opposite-state retention was observed to be equally stable. Traditional imprint behavior of the programmed state occurs after a few hours of baking time, and stable behavior could be verified until the end of the 1000-h retention test. Endurance characteristics of the ferroelectric thin films are shown to depend significantly on the annealing temperature of the capacitors and on the cycling voltage during testing. In thin films which had been annealed at 1000 °C, breakdown at 2 MV/cm limits endurance after 108 cycles. A lower annealing temperature of 650 °C could improve the breakdown-limited endurance to 1010 cycles.

Dielectric response of fully and partially depleted ferroelectric thin films and inversion of the thickness effect

We study the effect of full and partial depletion on the dielectric response characteristics of ferroelectric thin films with impurities via a computational approach. Using a thermodynamic approach along with the fundamental equations for semiconductors, we show that films with partial depletion display unique features and an enhanced dielectric response compared with those fully depleted. We find that the capacitance peak at switching can be significantly suppressed in the case of high impurity densities (>1025 m−3) with relatively low ionization energy, of the order of 0.5 eV. For conserved number of species in films, electromigration of ionized impurities at room temperature is negligible and has nearly no effect on the dielectric response. In films with high impurity density, the dielectric response at zero bias is enhanced with respect to charge-free films or those with relatively low impurity density (<1024 m−3). We demonstrate that partially depleted films should be expected to exhibit peculiar capacitance–voltage characteristics at low and high bias and that the thickness effect probed in experiments in ferroelectric thin films could be entirely inverted in thin films with depletion charges where a higher dielectric response can be measured in thicker films. Therefore, depletion charge densities in ferroelectric thin films should be estimated before size-effect-related studies. Finally, we noted that these findings are in good qualitative agreement with dielectric measurements carried out on PbZrxTi1−xO3.

A Retention Model for Ferroelectric-Gate Field-Effect Transistor

By combining the basic device equations of metal-oxide-semiconductor field-effect transistors with the polarization retention characteristics of ferroelectric thin films, a retention model for ferroelectric-gate field-effect transistor (FeFET) is developed based on the depolarization field of the ferroelectric. This model gives the retention characteristics of FeFET both in short and long time scales. Simulations demonstrate that large polarization will result in large depolarization field, which causes fast polarization loss in turn. The retention characteristics of the off-state is poorer than that of the on-state for p-FeFET, because the depolarization field in accumulation state is much larger than that in strong inversion state. Further analysis indicates that an appropriate holding voltage could be used to restrain the depolarization field in accumulation state to improve the retention characteristics of p-FeFET.

Coupling of Defect Fields to Domains and Phase Transition Characteristics of Ferroelectric Thin Films with Charged Defects

ABSTRACTWe analyze the effect of charged defects on the electrical domains, phase transition characteristics and electrical properties of ferroelectric thin films with thin dead layers using a non-linear thermodynamic model. Depending on their density and field strength, defects can pin and couple to electrical domains in the film. For ultrathin films, depolarizing effects dominate and the transition from the paraelectric state is into the multidomain ferroelectric state during cooling and is strongly smeared. The competition between defect induced extrinsic effects and the dead layer related limit is demonstrated.

Influence of space-charge on hysteresis loop characteristics of ferroelectric thin films

Hysteresis loops of Pb(Zr, Ti)O3 (PZT) thin films obtained by using a Sawyer-Tower (ST) circuit are affected by many factors. This paper investigated the influence of space charge on the hysteresis loop of thin-film ferroelectrics, based on the model that polarization consists of two parts: linear and switching polarization. It is found that the space charge affects both the shape and offset of the ideal hysteresis loop. Further investigation shows that the practical hysteresis loop has a close relationship with the equivalent ST circuit parameters: the leakage resistance of the FE film, the equivalent input impedance of the measurement equipment, the signal source, and other similar parameters. The normally assumed symmetric hysteresis loop without any offset is obtained with an ST circuit when the output becomes stable. The hysteresis loop obtained at the initial stage of applied signal depends on the initial status of the FE film, and remnant polarization causes an initial offset that gradually disappears.

A New Method for Electrical Characterization of Ferroelectric Thin Films at Microwave Frequencies

In this work, varactor shunt switches (VSS) are utilized as test structures for characterization of ferroelectric thin films. The varactor shunt switch consists of coplanar waveguide input and output feed lines, and a shunt parallel plate varactor. The simplicity of the device structure and the ease of modeling the device allow one to match the experimental scattering parameters (S parameters) to the modeled results, and hence characterize the dielectric properties of the ferroelectric thin-film accurately. This paper illustrates the precise characterization of the ferroelectric thin-films using the VSS as a test structure.

Study on the Ferroelectric Thin Films for Uncooled Infrared Detection

Some results on the microstructure controlling and characterization of ferroelectric thin films for uncooled infrared detections are presented. Several novel technologies have been developed to control the microstructures, e.g., grain size, grain shape, and the texture of BST, PZT thin films as well as to fabricate PZT thin films at low temperature. 256 × 1 pixels infrared detector has been prepared and used to realize infrared thermal image. A highly self-polarization in PMNT relaxor films has been observed. A novel readout method with infrared-visible transition mechanism is also discussed.

Interfacial varactor characteristics of ferroelectric thin films on high-resistivity Si substrate

Ferroelectric Ba(Zr0.25Ti0.75)O3 (BZT) thin films were deposited on high-resistivity Si substrate without or with inserting a high-k buffer layer of Ta2O5. The varactor characteristics of the BZT capacitors in metal-oxide-semiconductor structure were studied. At low frequency (1MHz), the capacitors exhibit a negatively tunable characteristic, i.e., [C(V)−C(0)]∕C(0)&amp;lt;0, against dc bias V, but opposite tunable characteristics were found at microwave frequencies (&amp;gt;1GHz). The change of voltage-dependent characteristic is attributed to the effect of low-resistivity interface induced by charged defects formed from interfacial oxidation of Si in screening the microwave from penetrating into the bulk of Si.

Processing and characterization of ferroelectric thin films obtained by pulsed laser deposition

Ferroelectric thin films with compositions Pb 0.67La 0.22(Zr 0.2Ti 0.8)O 3 (PLZT) and Pb 0.988 (Zr 0.52Ti 0.48) 0.976Nb 0.024O 3 (PZTN) have been processed by radiofrequency assisted-pulsed laser deposition. The first set of films have relaxor properties and the second are classical ferroelectrics. The obtained films are polycrystalline, with perovskite structure and almost random orientation. The surface morphology has been investigated by atomic force microscopy. The ferroelectric properties have been obtained by hysteresis loop measurements. From measurements of the dielectric properties as a function of the frequency of driving signal, the amplitude and the rate of change of the bias field, the following characteristics have been found: (i) a linear decrease of the capacitance with the frequency logarithm which was attributed to a superposition of Debye-type relaxations with wide distribution of relaxation times; (ii) a strong nonlinear decreasing of the capacitance with the increasing of the bias field amplitude for PLZT films; (iii) a hysteresis-like dependence, for PZTN films, with maxima corresponding to polarization switching; the separation between these maxima decreases with the decreasing of the rate of change of the bias field. This has been attributed to the accumulation of mobile charged defects (oxygen vacancies) near electrodes which facilitates the nucleation of domains and polarization switching.

Imprint Characteristics of Ferroelectric Thin Films at Elevated Storage and Operation Temperatures

The imprint characteristics of SrBi2Ta2O9 (SBT) and Bi4-xLaxTi3O12 (BLT) thin films have been studied at elevated storage and operation temperatures for the application to ferroelectric random access memories (FeRAMs). After the storage at 125°C, a significant charge shift occurs as a function of the logarithmic time due to the thermally induced voltage shift. At an elevated operation temperature, the voltage shift divided by coercive voltage increases leading to the enhanced imprint degradation of polarization. Since the BLT thin film has larger remanent polarization and smaller voltage shift divided by coercive voltage at an elevated temperature than the SBT thin film, it has higher imprint endurance. In order to investigate the imprint characteristics on a device level, the sensing margin of the FeRAM cells using 0.8 µm complementary metal-oxide-semiconductor (CMOS) technology with SBT and BLT capacitors is also detected.

Chemical processing and characterization of ferroelectric thin films of bismuth-based layer-structured perovsktte cabi4ti4o15 with the octahedron number of 4

CaBi4Ti4O15 (CBTi144) thin films were prepared by spin-coating a precursor solution of metal alkoxides. As-deposited thin films began crystallization below 550°C and reached full crystallinity of a single phase of layered-perovskite at 650°C via rapid thermal annealing in oxygen. 650°C-annealed CBTi144 thin film showed random orientation and had a columnar structure on Pt-passivated Si substrate. The dielectric constant and loss factor were 300 and 0.033, respectively, at 100 kHz. The thin film exhibited P-E hysteresis loops. The remanent polarization (Pr) and coercive electric field (Ec) were 6.0 μC/cm2 and 78.7 kV/cm, respectively, at 9 V. The thin film showed good endurance properties against number of switching cycles.

Calculation of band bending in ferroelectric semiconductor

In order to investigate the electronic characteristics of ferroelectric thin films that behave as semiconductors, we derive an equation to calculate the band bending in ferroelectrics, to replace the Poisson equation. We find that the space-charge density term, , in the Poisson equation should be replaced by - Pr, where Pr is the remanent polarization. In order to simplify the band bending calculation, only one dimension is considered here. For example, in the case of the 3 V band bending in a metal-semiconductor Schottky barrier junction with a 1 µm thick ferroelectric semiconductor, the depletion region extends over the whole semiconductor, even with a dielectric constant of 10 and a donor density of 1 × 1017 cm-3 due to dPr /dx. However, the depletion layer width in a semiconductor without remanent polarization is only about 0.18 µm under the same conditions.

Electrical Characteristics of Chemical Solution Deposited Nd3+ Doped PZT (53/47) Thin Films in Planar Electrode Configuration

AbstractElectrical characteristics of ferroelectric thin films in planar electrode configuration are important to characterize these materials for their applications in micro electro mechanical (MEM) and tunable microwave devices. In the present work we have prepared polycrystalline Pb1划3x/2Ndx(Zr0.53Ti0.47)O3 (x = 0.0 to 10.0 at %) thin films on platinized silicon substrate by chemical solution deposition (CSD) technique. The films were characterized in terms of their dielectric and ferroelectric properties by depositing planar interdigital finger electrodes on the surface of the films by electron beam lithography. The capacitance and loss tangent of undoped and 4 at % Nd doped PZT films measured at 100 kHz were found to be 138 pF, 0.033 and 95 pF, 0.019 respectively. Saturated hysteresis loops were obtained in undoped PZT film by applying 100 V across 10 μm electrode separation. Nd doped PZT films on the other hand, electrically shorted at comparatively lower voltage. The electrical characteristics of these films are correlated with their phase formation behavior and microstructural features.