Ferroelectric SrBi2Ta2O9 Thin Films Research Articles

Ferroelectric SrBi2Ta2O9 Thin Film Studied by Micro-Raman Scattering and Atomic Force Microscopy

Ferroelectric SrBi2Ta2O9 thin films were prepared by metal-organic decomposition technique on Si wafers. The thickness dependence of the films was studied by micro-Raman scattering and atomic force microscopy (AFM). From the AFM observation, it is found that films were constructed of c-axis-oriented grains. The low frequency component of the Raman spectra of the films decreases as the temperature increases. The spectral analysis of the low frequency component has been carried out by using a damped harmonic-oscillator model and a relaxational model. The relaxation time has the similar value of that reported in KDP. It is suggested the physical origin of the low frequency component may be polarization fluctuation.

Structural and electrical properties of metal–ferroelectric–insulator–semiconductor structure of Al/SrBi2Ta2O9/HfO2/Si using HfO2 as buffer layer

Ferroelectric SrBi2Ta2O9 (SBT) thin films have been deposited by the radio-frequency magnetron sputtering technique on bare p-Si as well as on HfO2 insulating buffer p-Si. XRD patterns revealed the formation of a well-crystallized SBT perovskite thin film on the HfO2 buffer layer. The electrical properties of the metal–ferroelectric–insulator–semiconductor (MFIS) structure were characterized by varying thicknesses of the HfO2 layer. The MFIS structure exhibits a maximum clockwise C–V memory window of 1.60 V when the thickness of the HfO2 layer was 12 nm with a lower leakage current density of 6.20 × 10−7 A cm−2 at a positive applied voltage of 7 V. However, the memory window reaches a maximum value of 0.7 V at a bias voltage of ±5 and then decreases due to charge injection in the case of the insulating buffer layer thickness of 3 nm. The density of oxide trapped charges at/near the buffer layer–ferroelectric interface is studied by the voltage stress method. Capacitance–voltage (C–V) and leakage current density (J–V) characteristics of the Al/SBT/HfO2/Si(1 0 0) capacitor indicate that the introduction of the HfO2 buffer layer prevents interfacial diffusion between the SBT thin film and the Si substrate effectively and improves the interface quality. Furthermore, the Al/SBT/HfO2/Si structures exhibit excellent retention characteristics, the high and low capacitance values clearly distinguishable for over 1 h and 30 min. This shows that the proposed Al/SrBi2Ta2O9/HfO2/Si structure is ideally suitable for high performance ferroelectric memories.

Preparation of SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub> Ferroelectric Thin Films by Liquid Source Misted Chemical Vapor Deposition Method Using Inorganic Salt Solution

A liquid source misted chemical vapor deposition (LSMCVD) system was used to deposit SrBi2Ta2O9 ferroelectric thin films on Pt/TiO2/SiO2/Si substrates using strontium nitrate, bismuth nitrate and fluorine tantalum acid as the starting ingredients. Citric acid, ethylene glycol, and ethylenediaminetetraacctic acid were employed as chelating agents to form homogeneous and stable sol. A density, homogenous, crack-free and c-axis oriented SrBi2Ta2O9 thin films were prepared successfully and the resultant thin film exhibits the ferroelectric properties of 2Pr of 3.8 C/cm2, 2Ec of 60kV/cm at ±3.5 V, respectively.

Pulsed Laser Deposition of SrBi2Ta2O9 Thin Films on Si Substrate

AbstractSummary: Systematic study of pulsed laser deposition of non‐c‐axis‐oriented polycrystalline SrBi2Ta2O9 ferroelectric thin films on Pt‐coated silicon substrates has been performed. Both in situ and ex situ routes have been explored. Ex situ deposited films produced predominantly (115) and (200) orientations, while in situ deposition resulted in a mixture of (115) and (104) orientations. The growth was studied as a function of the ceramic targets Bi‐enrichment, target‐substrate distance, annealing temperature, and the utilization of an external electric field applied on the substrate during the post‐annealing procedure. Variation of those parameters appears to significantly affect the phase purity and crystalline quality of the films. The minimization of pyrochlore and fluorite phases and the related Bi2Pt formation have been achieved through the introduction of layered Bi‐enrichment gradient architecture and lowering the annealing temperature. SBT d33 piezoelectric coefficients of an order of 12–15 were measured. X‐ray diffraction was used to characterize structural perfection of the films. The nanoscale mechanism of piezoelectric behavior has been investigated by means of piezoresponse force microscopy.Electromechanical hysteresis loops of SBT films.imageElectromechanical hysteresis loops of SBT films.

High Energy Oxygen Ion Induced Modifications in Ferroelectric SrBi2Ta2O9 Thin Films

The effects of high energy oxygen ion irradiation on the ferroelectric properties of SrBi 2 Ta 2 O 9 (SBT) thin films are reported. The films were grown using pulsed laser ablation. Fluence dependant degradation in the ferroelectric properties was observed. The dielectric constant did not show any frequency dispersion in both the unirradiated and irradiated films. After irradiation, the films showed only a decrease in the capacitance value retaining their ferroelectric nature. The switching voltages increased with broader peaks and showed a partial recovery after annealing of the irradiated film. A slim hysteresis loop with decreased P s , P r and E c values was observed. The current density increased to a larger extent. The decrease in the dielectric constant, capacitance and polarization values was to a lesser extent as compared to those of PZT thin films under identical irradiation conditions. The results presented in this paper are illustrative of the fact that the layered perovskites have a stable structure and are less sensitive to extrinsic effects. The relatively high resistance of bismuth layered perovskites such as SBT to the high energy radiation along with the greater fatigue endurance as compared to PZT make them better candidates for applications in memory devices.

Micropatterning of SrBi2Ta2O9 Ferroelectric Thin Films Using a Selective Deposition Technique Combined with Patterned Self-Assembled Monolayers and Liquid-Source Misted Chemical Deposition

We demonstrated a novel patterning method using a selective deposition technique of ferroelectric thin films without etching. Self-assembled monolayers (SAMs) of 1H,1H,2H,2H-perfluorodecyltriethoxysilane [CF3(CF2)7CH2CH2Si(OC2H5)3; FAS] were patterned on Pt/SiO2/Si substrates. The patterned SAMs on these surfaces defined the selective area on which the liquid-source misted chemical deposition (LSMCD) of SrBi2Ta2O9 (SBT) was performed. Pt top electrodes were then sputter-deposited on the patterned SBT films by the liftoff method. The remanent polarization (Pr) of the obtained SBT films was 7.5 µC/cm2 with good squareness of the hysteresis loops for 50 µm×50 µm square capacitors. By this method, we succeeded in fabricating uniform lines of ferroelectric thin films of 0.5 µm width. These results show that a selective deposition technique is promising for realizing high-density ferroelectric random access memories (FeRAMs).

Progressive Loss of Ferroelectricity Under Bipolar Pulsed Fields and Experimental Determination of Non-Switchable Polarization in Au/Ba0.5Sr0.5-TiO3/SrRuO3 Thin-Film Capacitors

175 nm-thick Ba0.5Sr0.5TiO3 (BST) thin film fabricated by pulsed laser deposition (PLD) technique is found to be a mixture of two distributions of material. We discuss whether these two components are nano-regions of paraelectric and ferroelectric phases, or a bimodal grain-size distribution, or an effect of oxygen vacancy gradient from the electrode interface. The fraction of switchable ferroelectric phase decreases under bipolar pulsed fields, but it recovers after removal of the external fields. The plot of capacitance in decreasing dc voltage (C(V ↓) versus that in increasing dc voltage C(V ↑) is a superposition of overlapping of two triangles, in contrast to one well-defined triangle for typical ferroelectric SrBi2Ta2O9thin films.

Influence of Low Frequency Dielectric Dispersion on the Phase Transition for the Pulsed Laser-Deposited SrBi2Ta2O9Thin Film

Ferroelectric bi-layered perovskite SrBi2Ta2O9 (SBT) thin films are good candidates for fatigue-free FRAM applications. SBT has excellent resistance against fatigue even with simple Pt electrodes. The dielectric and electric properties of the SBT system were widely studied in bulk materials and thin films. It is know that the composition of SBT ceramics is generally different from a stoichometry, and reported that Sr-deficient and Bi-excess ceramic and thin films show improved ferroelectric properties and changed ferroelectric phase transition temperature (Tc). 8–13) Shimakawa et al. reported values of Tc of about 300 and 400 C for powder samples of stoichometric (SrBi2Ta2O9) and nonstoichometric (Sr0:85Bi2:2Ta2O9) compositions, respectively, invoking the structural distortion to justify the observed Tc temperature shift. Shoji et al. have found Tc value close to 322 C for stoichiometric ceramic samples. Onodera et al. have also reported thermal anomalies in the specific heat at 237 and 397 C for a ceramic sample of composition SBT, and no anomalies in the dielectric constant and spontaneous polarization were reported. Thin film capacitors of SBT, PZT, and (Ba,Sr)TiO3 (BST) show remarkable low frequency dielectric dispersion below Tc. It has been shown that this phenomenon greatly affects the electrical properties of the capacitors. The frequency dependence of the measured dielectric constant and dielectric loss reflects the intrinsic property of the bulk material, the effects from the electrodes, and any internal interfacial barriers. Since the above results were mostly for oxide ferroelectric materials, it will be very interesting to study the frequency and temperature dependence of the complex dielectric constant of ferroelectric compounds such as SBT in this system. Up to now, studies of the temperature behavior of SBT thin films are scarce and issues on the phase transitions and their mechanisms are still open. The point of interest here has been to visualize how ferroelectric SBT thin film behaves under the influence of its conductivity, with regard to its dielectric characteristic. In this paper, we aim to report the dielectric constant measurements carried out on SBT thin film, as a function of the frequency below and above ferroelectric phase transition temperature. The thin film of SBT ferroelectric were synthesized on Pt/ Ti/SiO2/Si substrates using the PLD method at 540 C in oxygen pressure of 200mTorr, and then post-annealed at 800 C in an oxygen atmosphere for 1 h. We confirmed that the annealed SBT films have a highly 1⁄2115 -oriented perovskite phase by x-ray diffraction analysis. The thickness of the film observed by scanning electron microscopy was d 1⁄4 270 nm. Top electrode was used a Pt with the area of 2:8 10 4 cm, which was formed using a DC sputtering method onto SBT thin films. The complex ac impedance, Z ð!Þ, was measured between 10 1 Hz and 10 Hz at room temperature by using an Impedance Analyzer (Solatron SI1260). Polarization vs electric field (P–E) curve was observed at room temperature using a RT66A ferroelectric tester from Radiant Technology. The temperature dependence of the real (0) dielectric constant for two frequencies is shown in Fig. 1. At high frequency (100 kHz), we notice that there is a maximum in 0 at about Tc 1⁄4 295 C, which we believe corresponds to the ferroelectric phase transition as shown in inset Fig. 1, the P– E hysteresis loop is saturated and symmetric below Tc (at room temperature). But, dielectric anomaly related to phase transition appears not to at low frequency (100Hz) due to the rapid increase of 0 caused by the electrode surface capacitance, which is much larger than the bulk capacitance. This may mean that there is a coupling between the space charge and ferroelectricity. The frequency dependence of the (a) real 0 and (b) imaginary 00 parts of the complex dielectric constant on log– log scale are shown in Figs. 2(a) and 2(b), respectively. Both 0 and 00 show a strong dispersion at low frequencies. Such a strong dispersion in both the components of the complex dielectric constant appears to be a common feature in ferroelectrics associated with ionic conductivity and is referred to as low frequency dielectric dispersion (LFDD). This is in complete contrast to the effect due to dc conductivity, where the 00 is proportional to 1=!. The dispersion in the 00 of the complex dielectric constant is stronger than that in the 0 as shown in Fig. 2(b). This is because of the influence of the dc conductivity on 00. The low frequency slope of the curve log 00 vs log f is close to 1 indicating the predominance of the dc conduction in this frequency region. According to Joncher’s universal law, the complex

Preparation of Si-Added SrBi2Ta2O9 Ferroelectric Thin Films by RF Magnetron Sputtering at High Temperature*

Si-added SrBi2Ta2O9 (SBT) ferroelectric thin films were formed by RF magnetron sputtering on a Pt/Ti/SiO2/Si structure. The substrate was heated from 400°C to 650°C to crystallize SBT films. Sputtering power and Ar/O2 pressure were 75 W and 3.0 Pa, respectively. It was found that the Si-added SBT films were crystallized at a deposition temperature between 500°C and 600°C, which was much lower than the conventional annealing temperature (800°C) of the film deposited at room temperature. * Originally presented at 10th European Meeting on Ferroelectricity, Cambridge, United Kingdom, August 3–8, 2003.

Electronic and structural properties of ferroelectric SrBi2Ta2O9 thin films and its schottky junction by photoelectron spectroscopies

Electronic and structural properties of ferroelectric SrBi2Ta2O9 thin films and its schottky junction by photoelectron spectroscopies

Electronic and structural properties of ferroelectric SrBi2Ta2O9 thin films and its schottky junction by photoelectron spectroscopies

Electronic and structural properties of ferroelectric SrBi2Ta2O9 thin films and its schottky junction by photoelectron spectroscopies

Dynamic response and hysteresis dispersion scaling of ferroelectric SrBi2Ta2O9 thin films

The dynamic hysteresis response of ferroelectric SrBi2Ta2O9 thin films versus periodically varying electric field over a frequency range of f=10−1–106 Hz and amplitude range of E0=15–158 kV/cm is measured utilizing the Sawyer–Tower method. The dynamic order parameter Q shows anomalous behavior against the field amplitude, and a single-peaked hysteresis dispersion is identified. The field response of hysteresis area 〈A〉 in the form of 〈A〉∝f2/3E02/3 over the low frequency range is evaluated, while the response over the high frequency range takes the form of 〈A〉∝f−1/3E02. We demonstrate that the hysteresis dispersion spectrum exhibits single-parameter scaling, and predicts a characteristic time for domain reversal that is inversely correlated to the field amplitude.

Scaling behavior of ferroelectric hysteresis loop in pulsed-laser-deposited SrBi2Ta2O9 thin film

Ferroelectric SrBi2Ta2O9 thin films were grown on a highly oriented Pt/Ti/SiO2/Si substrates using pulsed laser ablation. The hysteresis loop of ferroelectric SrBi2Ta2O9 was studied as a function of applied field amplitude. A scaling analysis of ferroelectric hysteresis loop area showed A∝Eα. The value of scaling exponent, α=0.40, is not similar to the reported theoretical and experimental values. This result shows the possibility that both ferroelectric bulk and thin-film systems may have different universal behaviors. Influence of potential in the surface of SrBi2Ta2O9 thin film was measured in the dc applied field range from 0 to 8 V by using electro force microscopy. Roughness of surface potential of SrBi2Ta2O9 thin film changed rapidly around the coercive voltage, Vc∼1.5 V. It is believed that the switching effect of SrBi2Ta2O9 thin film includes surface polarization at the surface of the thin film as well as pure spontaneous polarization in the bulk.

Domain structure study of SrBi2Ta2O9 ferroelectric thin films by scanning capacitance microscopy

Scanning capacitance microscopy was used to image the polarization-induced microstructural patterns of sol-gel derivative SrBi2Ta2O9 (SBT) thin films. A sharp image contrast was induced between the nanosized domains owing to the various polarities, so that the domain structure in the SBT thin film was clearly revealed. As a result, the switched and unswitched regions could be unequivocally identified. This investigation also confirms that the reversal polarization process of a ferroelectric domain is much easier inside a large grain than in a small grain.

A Significant Improvement in Memory Retention of Metal-Ferroelectric-Insulator-Semiconductor Structure for One Transistor-Type Ferroelectric Memory by Rapid Thermal Annealing

A rapid thermal annealing (RTA) process has improved the crystallinity of the ferroelectric SrBi2Ta2O9 (SBT) thin film in metal-ferroelectric-insulator-semiconductor (MFIS) structures. The RTA temperature and time were in the ranges from 600 to 1000°C and 30 s to 1 min, respectively. X-ray diffraction (XRD) patterns showed clear preferential perovskite SBT(115) peaks in the temperature range. The RTA increases grain size to approximately 180 nm but markedly decreases surface roughness to approximately 5 nm due to having a shorter annealing time than the furnace annealing. Leakage current density through the MFIS is below 5×10-8 A/cm2, and is fairly reduced especially at the low electric field region as a result of the shorter RTA time. Finally, a very long retention time of 6×105 s (i.e., one week) was obtained by the RTA at 1000°C for 30 s. The extrapolated retention time from the data was about 3×107 s (i.e., one year).

Preparation of Si-Added SrBi2Ta2O9 Ferroelectric Thin Films by RF Magnetron Sputtering

Si-added SrBi2Ta2O9 (SBT) ferroelectric films were prepared by RF magnetron sputtering on a Pt/Ti/SiO2/Si (100) structure. The films were deposited at temperatures below 100°C for surpressing Bi evaporation, and crystallized at 800°C in air. A typical composition was Sr0.79Bi2.37Ta2.00Si0.2Ox. The remanent polarization value (2Pr) of the Si-added SBT film was 16 μC/cm2. The Si atom addition was found to be effective in improvement of the fatigue and leakage current of SBT ferroelectric films. The leakage current density was further improved by annealing in the high-pressure oxygen ambient at 7 atms.

Frequency Dependence of Polarization Switiching in SrBi2Ta2O9 Ferroelectric Thin Films

Frequency Dependence of Polarization Switiching in SrBi2Ta2O9 Ferroelectric Thin Films

Preparation of Si-Added SrBi2Ta2O9 Ferroelectric Thin Films by RF Magnetron Sputtering

Preparation of Si-Added SrBi2Ta2O9 Ferroelectric Thin Films by RF Magnetron Sputtering

AC conduction mechanism of SrBi2Ta2O9 thin films below room temperature

Ferroelectric thin films have tremendous application potential for data storage, sensors, uncooling infrared detectors, classes of tunable microwave devices, and microelectromechanical system technologies [1–4]. Carrier transport plays an essential role in determining some characteristics of the ferroelectric thin films such as fatigue and leakage to meet commercial requirements. In recent years, different transport mechanisms in ferroelectric materials, such as space charge limited conduction (SCL), thermionic emission limited conduction (TEL), and Poole-Frenkel emission limited conduction (PFL) have been reported [5–10]. Further, current-voltage (IV) characteristics may contain contributions from SCL and PFL conductions, and even TEL conduction at high direct current (dc). The latter TEL part can usually be distinguished by its dependence on the selection of electrode material. To date, most of the studies on the transport properties of ferroelectric films are based on dc measurements, such as dc IV measurements. In direct current conductivity measurements, all types of carrier contributions to conductance are superposed. Thus, the various transport mechanisms cannot be easily distinguished. It is well known, however, that each type of carrier (or mechanism) possesses an independent relaxation time. Based on this feature, it is easy to distinguish the contributions of various mechanisms to electrical transport from frequency dependent conductivity measurements. However, there has been little experimental work on alternating current (ac) transport properties of the thin films below room temperature, although data collected at a broader temperature range would be helpful in understanding the conduction mechanisms of these films. The electrical properties of ferroelectrics are known to be closely associated with their crystallographic structure. An important example is SrBi2Ta2O9 (SBT), a representative of a new class of ferroelectrics in which, the structure is based on perovskite octahedron layers (SrTa2O7)2− sandwiched between (Bi2O2)2+ layers. SBT exhibits almost no fatigue and low dc leakage on Pt electrodes at room temperature (up to 1012 cycles at saturation), accompanied by other superior properties [11–17]. These unusual properties have generated tremendous interests in the fundamental conduction mechanisms of SBT. In this letter, we report the ac transport properties from 100 Hz to 15 MHz of SBT ferroelectric thin films in the temperature range of 10 to 300 K. The dominant conduction mechanisms at different frequency ranges were analyzed. Most importantly, results indicated the existence of conduction through the perovskite octahedron layers of the SBT structure. SBT thin films were prepared by pulsed laser deposition (PLD) on heated Pt/Ti/SiO2/Si substrates (400 ◦C), followed by a post-annealing process at 750 ◦C for 1.5 h in oxygen [16] 30 nm thick Pt electrodes with an area of 2.0 × 10−4 cm2 were deposited on the top surface of the films through a shadow mask by a UHV electron beam evaporator (UMS 500p made by Balzers), giving the capacitive multilayered structure; Pt/SBT/Pt. The crystallographic structure of the SBT films was characterized by X-ray diffraction along the (008) and (115) direction. The microstructure of the films was observed by transmission electron microscopy, with the mean grain size of about 200 nm, thickness of about 200 nm, dense film morphology and abrupt interfaces. Ferroelectric properties of the thin films were determined with an RT66A ferroelectric tester from Radiant Technologies. A typical hysteresis loop showed that the remanent polarization and coercive field were 10 μC/cm2 and 57 kV/cm at room temperature, respectively. The electrical conductivity of the SBT thin films was measured by a HP4194A Impedance/Gain-Phase Analyzer in the temperature range of 10 to 300 K. These measured values of conductivity in the range between 100 Hz and 15 MHz were simulated by an equivalent circuit using an ideal capacitor shunted with an ideal resistor. The frequency dependence of the electrical conductivity is illustrated in Fig. 1 with temperature as a

Electrical properties of SrBi2Ta2O9 ferroelectric thin films at low temperature

The temperature dependence of electrical properties for SrBi2Ta2O9 thin film capacitors with platinum electrodes (Pt/SBT/Pt) on silicon wafers was studied from 10 to 300 K. With a decrease in temperature from 300 to 200 K, the remanent polarization of the thin films shows about an 11% reduction from its 300 K value; however, it is reduced by about 87% reduction from its 200 K value when the temperature drops from 200 to 100 K. With a decrease to 200 K, the polarization fatigue was significant, and the capacitor shows an approximate 29% reduction in polarization from its initial value following 1010 cycles. The dielectric response and leakage current of the thin films were also studied over the same lower temperature region. These results are helpful in the understanding of the fatigue-free behavior observed in SrBi2Ta2O9 thin films at room temperature and provide additional insight into their use for ferroelectric memory applications.