Ag/Bi4Ti3O12/p-Si异质结的制备及其C-V特性研究

为制备符合铁电存储器件要求的高质量铁电薄膜,采用溶胶-凝胶(Sol-Gel)工艺,制备了Si基Bi4Ti3O12铁电薄膜及MFS结构的Ag/Bi4Ti3O12/P-Si异质结,对Bi4Ti3O12薄膜的相结构特征及异质结的C-V特性进行了测试与分析.XRD图谱显示,Si基Bi4Ti3O12薄膜具有沿c-轴择优取向生长的趋势,而Ag/Bi4Ti3O12/p-Si异质结顺时针回滞的C-V特性曲线则表明,该异质结可实现电极化存储.此外,对该异质结C-V特性曲线的非对称及向负偏压方向偏移的产生原因也进行了分析.在此基础上,为提高铁电薄膜的铁电性能及改善其C-V特性提出了合理的结构设想.     

结构设计对铁电薄膜系统电滞回线的影响

为制备符合Si集成铁电器件要求的高质量Si基铁电薄膜，采用溶胶—凝胶(sol—gel)工艺，制备了MFM及MFS结构的铁电薄膜系统，研究了不同结构及不同衬底对铁电薄膜系统铁电性能及电滞回线的影响，并对这些差异产生的主要影响因素进行了分析，在此基础上，提出并制备了Ag／Pb(Zr0.52Ti0.48)O3／Bi4Ti3O12／p—Si多层结构，该结构铁电薄膜系统的铁电性能及电滞回线的对称性有明显改善，有望应用于Si集成铁电器件。     

Chemically cross-linked thin poly(vinylidene fluoride-co-trifluoroethylene)films for nonvolatile ferroelectric polymer memory

Both chemically and electrically robust ferroelectric poly(vinylidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) films were developed by spin-coating and subsequent thermal annealing with the thermal cross-linking agent 2,4,4-trimethyl-1,6-hexanediamine (THDA). Well-defined ferroelectric β crystalline domains were developed with THDA up to approximately 50 wt %, with respect to polymer concentration, resulting in characteristic ferroelectric hysteresis polarization-voltage loops in metal/cross-linked ferroelectric layer/metal capacitors with remnant polarization of approximately 4 μC/cm(2). Our chemically networked film allowed for facile stacking of a solution-processable organic semiconductor on top of the film, leading to a bottom-gate ferroelectric field effect transistor (FeFET). A low-voltage operating FeFET was realized with a networked PVDF-TrFE film, which had significantly reduced gate leakage current between the drain and gate electrodes. A solution-processed single crystalline tri-isopropylsilylethynyl pentacene FeFET with a chemically cross-linked PVDF-TrFE film showed reliable I-V hysteresis with source-drain ON/OFF current bistablility of 1 × 10(3) at a sweeping gate voltage of ±20 V. Furthermore, both thermal micro/nanoimprinting and transfer printing techniques were conveniently combined for micro/nanopatterning of chemically resistant cross-linked PVDF-TrFE films.     

Integration of BiFeO3 thin films on Si wafer via a simple sol-gel method

Integration of BiFeO₃ (BFO) films on Si substrate is desirable from an application point of view. The growth of (110)-textured BFO thin films with high quality on Si(100) substrate was realized by a seeding technique via a simple sol-gel method. Obviously switchable ferroelectric domains were observed, and in the meantime, the BFO films also exhibited a weak magnetization which somewhat showed dependence on the film thickness. Such an integration of the BFO films on Si constructed a metal/ferroelectric/insulator/semiconductor structure, which presented a memory feature as evidenced by capacitance-voltage hysteresis.     

Dependence of ferroelectric and magnetic properties on measuring temperatures for polycrystalline BiFeO(3) films

A multiferroic BiFeO(3) film was fabricated on a Pt/Ti/SiO(3)/Si(100) substrate by a chemical solution deposition (CSD) method, and this was followed by postdeposition annealing at 923 K for 10 min in air. X-ray diffraction analysis indicated the formation of the polycrystalline single phase of the BiFeO(3) film. A high remanent polarization of 89 microC/cm(2) was observed at 90 K together with a relatively low electric coercive field of 0.32 MV/cm, although the ferroelectric hysteresis loops could not be observed at room temperature due to a high leakage current density. The temperature dependence of the ferroelectric hysteresis loops indicated that these hysteresis loops lose their shape above 165 K, and the nominal remanent polarization drastically increased due to the leakage current. Magnetic measurements indicated that the saturation magnetization was less than 1 emu/cm(3) at room temperature and increased to approximately 2 emu/cm(3) at 100 K, although the spontaneous magnetization could not appear. The magnetization curves of polycrystalline BiFeO3 film were nonlinear at both temperatures, which is different with BiFeO3 single crystal.     

Ba0.6Sr0.4TiO3薄膜的微结构及铁电性能研究

采用射频磁控溅射法制备了Ba0.6Sr0.4TiO3(简称BST)薄膜材料,研究了BST薄膜的组成、晶体结构、表面形貌及介电性能.介电偏压特性曲线和电滞回线都表明其具有铁电性,厚度为500 nm、晶粒尺寸为30 nm的BST薄膜,介电系数电压变化率(介电调谐率)为29.4%,矫顽场强(EC)约为12.1 kV/cm,并讨论了介电偏压特性曲线和电滞回线之间的联系,解释了电滞回线不对称的原因.     

Preparation and characterization of highly (1 1 1)-oriented (Pb,La)(Zr,Ti)O3 thin films by sol-gel processing

Lanthanum modified lead zirconate titanate (Pb_0.91La_0.09)(Zr_0.65Ti_0.35)O₃ (PLZT) ferroelectric thin films were grown on Pt/Ti/SiO₂/Si(1 0 0) and fused quartz substrates using a sol-gel method with rapid thermal annealing processing. The results showed that the highly (1 1 1)-oriented pervoskite PLZT thin film growth on Pt/Ti/SiO₂/Si(1 0 0) substrates. The electrical measurements were conducted on PLZT films in metal-ferroelectric-metal capacitor configuration. The PLZT thin films annealed at 600 °C showed well-saturated hysteresis loops with remanent polarization and coercive electric field values were 10.3 μC/cm² and 36 kV/cm, respectively, at an applied field of 300 kV/cm. At 100 kHz, the dielectric constant and dielectric loss of the film are 682 and 0.021, respectively. The PLZT thin film on fused quartz substrate, annealed at 600 °C, exhibited good optical transmittance, the band gap of optical direct transitions is 3.89 eV.     

溶胶-凝胶法制备Bi0.975La0.025Fe0.975Ni0.025O3铁电薄膜的结构及物理性能

采用溶胶-凝胶法在Pt/Ti/SiO_2/Si(111)衬底上制备了Bi_(0.975)La_(0.025)Fe_(0.975)Ni_(0.025)O_3(BLFNO)铁电薄膜。利用X射线衍射(XRD)、原子力显微镜(AFM)及其压电模式(PFM)对薄膜的晶体结构、表面形貌以及铁电畴结构进行了研究。研究发现,BLFNO为结晶良好的钙钛矿结构多晶薄膜,且薄膜表面颗粒生长均匀。PFM测试图显示铁电薄膜在自发极化下的铁电畴结构清晰,铁电电容器具有良好的铁电性能。应用铁电测试仪对Pt/BLFNO/Pt电容器进行测量,得到了饱和性良好的电滞回线。在828kV/cm的外加电场下,Pt/BLFNO/Pt电容器的剩余极化强度为74.3μC/cm~2,表明La、Ni的共掺杂没有明显抑制铁电电容器的剩余极化强度,铁电电容器具有良好的铁电性能。漏电流研究结果表明,La、Ni元素的共掺杂有效降低了薄膜的漏电流密度,在277.8kV/cm外加电场下漏电流密度在10-4 A/cm2量级,明显小于纯BFO薄膜的漏电流密度。正半支漏电流曲线满足SCLC导电机制,对于负半支曲线,当电场强度大于22.2kV/cm时,同样遵循SCLC导电机制;但是,当电场强度小于22.2kV/cm时,曲线斜率约为4.8,表明参与导电贡献的电子数较多,归因于极浅陷阱俘获的电子在外加电场作用下参与了导电行为。室温下磁滞回线测试结果表明BLFNO薄膜具有反铁磁性质。     

Electrical and reliability properties of PZT thin films for ULSI DRAM applications

The electrical and reliability characteristics of ferroelectric capacitors fabricated using sol-gel derived 50/50 lead-zirconate-titanate (PZT) thin films have been examined for ULSI DRAM (dynamic random access memory) applications. Various electrode materials, film thicknesses (200 nm to 600 nm) and capacitor areas were used. A large stored-energy density (Q(c)) of 15 muC/cm(2) (at 125 kV/cm) was measured using different methods. The results indicate that PZT thin films exhibit material properties which might satisfy the requirements of ULSI DRAMs.     

Fabrication of one-transistor-capacitor structure of nonvolatile TFT ferroelectric RAM devices using Ba(Zr0.1Ti0.9)O3 gated oxide film.

In this study, the Ba(Zr0.1Ti0.9)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 microC/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 microm and channel length = 8 microm has been successfully fabricated and the ID-VG transfer characteristics also are investigated in this study.     

Thickness dependence of ferroelectricity in Langmuir-Blodgett multilayer films of hemicyanine dyes

The thickness dependence of ferroelectricity in hemicyanine Langmuir-Boldgett multilayer films was reported in this paper. By the observed ferroelectric hysteresis loop, it was found that the coercive field decreased with increasing of film thickness monotonously and may be approximated by a power law Ec infinity N(-4/3) in the range from 30 to 200 nm, which is consistent with other conventional ferroelectric materials. The measurement of dielectric properties give the optimum thickness about 60 nm of hemicyanine LB films and their optimum value as ferroelectric storage-devices has the same order of magnitude as copolymer's P(VDF-TrFE) (70:30 mol%).     

Electric characterization of HfO2 thin films prepared by chemical solution deposition

Hafnium dioxide (HfO₂) thin films were prepared on Si substrates using the chemical solution deposition (CSD) method. The Au/HfO₂/n-Si/Ag structures were characterized by X-ray diffraction (XRD), C–V curves and leakage current measurements. A relative dielectric constant of about 13.5 was obtained for the 65 nm HfO₂ film. Atomic force microscopy (AFM) measurements show uniform surfaces of the films. C–V hysteresis was found for the metal-oxide-semiconductor (MOS) structures with HfO₂ films of 52 and 65 nm thick. It is found that the width of C–V windows is related with the thickness of the HfO₂ films. Furthermore, the C–V hysteresis reveals the possibility of stress-effect, suggesting that it is possible to use HfO₂ to build an MOS structure with controllable C–V windows for memory devices. The leakage current decreases as the film thickness increases and a relatively low leakage current density has been achieved with the HfO₂ film of 65 nm.     

Effect of the microwave oven on structural, morphological and electrical properties of SrBi4Ti4O15 thin films grown on Pt/Ti/SiO2/Si substrates by a soft chemical method

Thin films of SrBi₄Ti₄O₁₅ (SBTi), a prototype of the Bi-layered-ferroelectric oxide family, were obtained by a soft chemical method and crystallized in a domestic microwave oven. For comparison, films were also crystallized in a conventional method at 700 °C for 2 h. Structural and morphological characterization of the SBTi thin films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. Using platinum coated silicon substrates, the ferroelectric properties of the films were determined. Remanent polarization P_r and a coercive field E_c values of 5.1 μC/cm² and 135 kV/cm for the film thermally treated in the microwave oven and 5.4 μC/cm² and 85 kV/cm for the film thermally treated in conventional furnace were found. The films thermally treated in the conventional furnace exhibited excellent fatigue-free characteristics up to 10¹⁰ switching cycles indicating that SBTi thin films are a promising material for use in non-volatile memories.     

Theoretical investigation of the SAW properties of ferroelectric film composite structures

The characteristics of surface acoustic waves (SAW) propagating on a three-layered structure consisting of a perovskite-type ferroelectric film, a buffer layer and a semiconductor substrate have been studied theoretically. Large coupling coefficients (K(2)) can be obtained when the interdigital transducer (IDT) is on top of the perovskite-type ferroelectric film, with (type 4) and without (type 3) the floating-plane electrode at the perovskite-type ferroelectric film-buffer layer interface. In the above cases, the peak values of K (2) Of the Pb(Zr,Ti)O(3) (PZT) films (3.2%-3.8%) are higher than those of the BaTiO(3) (BT) and PbTiO(3) (PT) films. In the IDT configuration of type 4, there exists a minor peak of the coupling coefficients for the PZT and BT films, but not for the PT films when the normalized thickness (hK) of the perovskite-type ferroelectric film is about 0.3. The minor peak values of the coupling coefficients (0.62%-0.93%) for different layered structures (PZT/STO/Si, PZT/MgO/Si, and PZT/MgO/GaAs) all decrease when we increase hK value from 0 to 0.25. The results could be useful in the integration of ferroelectric devices, semiconductor devices, and SAW devices on the same substrate.     

Ferroelectricity in highly ordered arrays of ultra-thin-walled Pb(Zr,Ti)O3 nanotubes composed of nanometer-sized perovskite crystallites

We report the first unambiguous ferroelectric properties of ultra-thin-walled Pb(Zr,Ti)O 3 (PZT) nanotube arrays, each with 5 nm thick walls and outer diameters of 50 nm. Ferroelectric switching behavior with well-saturated hysteresis loops is observed in these ferroelectric PZT nanotubes with P r and E c values of about 1.5 microC cm (-2) and 86 kV cm (-1), respectively, for a maximum applied electric field of 400 kV cm (-1). These PZT nanotube arrays (10 (12) nanotubes cm (-2)) might provide a competitive approach toward the development of three-dimensional capacitors for the terabyte ferroelectric random access memory.     

Effect of compositional variations on electrical properties in phase switching (Pb,La)(Zr,Ti,Sn)O3 thin and thick films

The composition-dependent electrical properties in (Pb,La)(Zr,Ti,Sn)O₃ antiferroelectric-ferroelectric phase switching thin and thick films have been systematically studied and compared with bulk ceramics. The films were deposited on Pt-buffered silicon substrates by a sol-gel method. The results show that the dependence of low-field dielectric properties on compositions in the films is similar to that in bulk ceramics but the variation of high field properties (polarization or hysteresis loops) is quite different, which may be attributed to the special mechanical boundary condition of the films. While all the films with compositions in the antiferroelectric tetragonal region in the phase diagram demonstrate the existence of remanent polarization in the hysteresis loops, the films with zero remanent polarization can be obtained in the antiferroelectric orthorhombic region. This is because the films are under high tensile stress due to the thermal mismatch between the film and substrate, which tends to stabilize the ferroelectric phase and causes the retention of ferroelectric phase for the films in the antiferroelectric tetragonal region because of their relatively small free energy difference between the antiferroelectric phase and ferroelectric phase.     

Characterization and electrical properties of chemical vapor deposited ferroelectric lead titanate films on titanium

Lead titanate thin films were deposited on titanium substrates by a chemical vapor deposition (CVD) process involving the application of vapor mixtures of Pb, ethyl titanate (Ti(C(2)H(5)O) (4)), and oxygen. Auger electron spectroscopy (AES) analyses were performed to determine the chemical composition of lead titanate films. AES analysis revealed that TiO(2) and TiO interlayers formed between the PbTiO(3) and titanium substrate. AES also showed that stoichiometry was obtained in the lead titanate film deposited at 750 degrees C, Ti(C(2)H(2)O)(4) with 0.152, an O(2) partial pressure of 0.06 atm, and a gas flow rate of 800 sccm. The lead titanate with a stoichiometric composition has a DC conductivity of 3.2x10(-12) Omega(-1)-cm(-1) at room temperature. The nonsaturating loops observed in the present investigation may be caused by TiO(2) and TiO layers between the conductive substrate and the PbTiO(3) ferroelectric film. The ferroelectric properties of the stoichiometric PbTiO(3) film included a remanent polarization of 14.1 muC/cm(3) and a coercive field of 20.16 kV/cm.     

The domain switching and structural characteristics of PLZT bulk ceramics and thin films chemically prepared from the same acetate precursor solutions

Lead lanthanum zirconate titanate (PLZT) ferroelectrics were produced in bulk ceramic and thin-film form from the same acetate precursor solutions in order to compare their electrical and physical properties. Bulk ceramics were hot pressed from chemically coprecipitated powders, and thin films were fabricated by spin coating on silver foil and platinum-coated silicon wafer substrates. A number of PLZT compositions were investigated, including ferroelectric memory materials near the morphotropic phase boundary with 2% La, memory and slim-loop ferroelectric x/65/35 (La/Zr/Ti) compositions with up to 12% La, as well as some antiferroelectric thin-film materials. Internal film stress from thermal expansion mismatch between films and substrates was found to contribute to differences in electrical properties and Curie temperatures between the thin film and bulk materials, as were interface layers between the films and substrates, mechanical clamping from the substrates and grain size.     

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.     

Magnetoelectric properties of Ba0.8Sr0.2TiO3–CoFe2O4 multilayered composite film via sol–gel method

Ba_0.8Sr_0.2TiO₃–CoFe₂O₄ (BST–CFO) multilayered composite films were prepared on Pt/Ti/SiO₂/Si substrates via a sol–gel method and spin-coating technique. Microstructures, electric property, magnetic property and magnetoelectric (ME) property of the composites were studied. Results show that the composite films calcined at 750 °C have BST and CFO phases and no obvious impurity phases were detected. Further, the composite films exhibit layered structures and a transition layer which is composed of interfacial delamination exists at the interface between BST and CFO layers. Ferroelectric and ferromagnetic properties were simultaneously observed in the films, evidencing the coexistence of the ferroelectric and ferromagnetic properties. Furthermore, the saturation magnetization value of the composite film is lower than that of the pure CFO film derived by the same processing as a result of the effect of the nonferromagnetic BST layers. Also, ferroelectric hysteresis loops reveal that the saturated polarization and remanent polarization of the composite film are lower than those of the pure BST films. In addition, the composite film exhibits a strong ME effect, which makes the composite film attractive for technological applications as devices.