Pulsed laser deposition of lead-zirconate-titanate thin films and multilayered heterostructures

There is an increasing interest in lead-zirconate-titanate (PZT) based ferroelectric thin film and devices in recent years. Pulsed laser deposition (PLD) technique has been demonstrated to be a versatile and successful tool for the deposition of epitaxial multi-component metal oxide films and heterostructures. This review presents a reasonable understanding of the relationship between PLD processing and composition, crystal structure and orientation of PZT ferroelectric thin films, and heterostructures. Processing-related issues from PLD of PZT thin films and material-integration strategies developed to fabrication of highly oriented or epitaxial PZT thin film based capacitors with excellent ferroelectric properties are discussed in detail. PACS 81.15.Fg; 68.55.Jk; 77.22.Ej     

Electric and ferroelectric properties of PZT/BLT multilayer films prepared by photochemical metal-organic deposition

The electric and ferroelectric properties of lead zirconate titanate (PZT) and lanthanum-substituted bismuth titanate (BLT) multilayer films prepared using photosensitive precursors were characterized. The electric and ferroelectric properties were investigated by studying the effect of the stacking order of four ferroelectric layers of PZT or BLT in 4-PZT, PZT/2-BLT/PZT, BLT/2-PZT/BLT, and 4-BLT multilayer films. The remnant polarization values of the 4-BLT and BLT/2-PZT/BLT multilayer films were 12 and 17 μC/cm², respectively. Improved ferroelectric properties of the PZT/BLT multilayer films were obtained by using a PZT intermediate layer. The films which contained a BLT layer on the Pt substrate had improved leakage currents of approximately two orders of magnitude and enhanced fatigue resistances compared to the films with a PZT layer on the Pt substrate. These improvements are due to the reduced number of defects and space charges near the Pt electrodes. The PZT/BLT multilayer films prepared by photochemical metal-organic deposition (PMOD) possessed enhanced electric and ferroelectric properties, and allow direct patterning to fabricate micro-patterned systems without dry etching.     

Effect of Pb2Ru2O7-x (PRO) conductive interfacial layers on ferroelectric properties of Pt/Pb(Zr0.35Ti0.65)O3/Pt capacitors for nonvolatile memory applications

A conductive material, Pb₂Ru₂O_7-x (PRO), containing Pb in a cubic structure was introduced into a Pt/PZT interface in an attempt to improve the ferroelectric properties of PZT films. PRO and PZT films were prepared by rf magnetron sputtering and chemical solution deposition, respectively. The resistivity of PRO thin films in a hybrid-type electrode (PRO/Pt) structure was approximately 35–45 μΩ·cm and the surface roughness remained constant with increasing annealing temperature. The PRO interlayers suppressed the loss of Pb in PZT layers by diffusion to the Pt/PZT interface. The increase in remanent polarization was largely dependent on the PRO interlayers inserted at the bottom-Pt/PZT interface rather than at the top-Pt/PZT interface. In addition, the leakage-current behavior of PZT films in a sandwich structure was improved substantially compared to the case of PRO interlayers only at the bottom-Pt/PZT interface. Thus, the PRO interlayers play an important role in improving the ferroelectric properties of PZT thin films for use in nonvolatile memory device applications. PACS 68.55.-a; 73.40.Rw; 73.61.Ng; 77.55.+f; 81.15.Cd     

Au/PZT/BIT/p-Si异质结的制备与性能研究

采用脉冲激光沉积(PLD)工艺,制备了以Bi₄Ti₃O₁₂(BIT)为过渡阻挡层的Au/PZT/BIT/p-Si异质结.研究了BIT铁电层对Pb(Zr_0.52Ti_0.48)O₃(PZT)薄膜晶相结构、铁电及介电性能的影响,对Au/PZT/BIT/p-Si异质结的导电机制进行了讨论.氧气氛530℃淀积的PZT为多晶铁电薄膜,与直接淀积在Si基片上相比,加入BIT铁电层后PZT铁 关键词： 铁电薄膜 异质结构 脉冲激光沉积(PLD)     

Polarization instability in polydomain ferroelectric epitaxial thin films and the formation of heterophase structures

A thermodynamic theory is developed for dense laminar domain structures in epitaxial ferroelectric films. It is found that, at some critical misfit strain between the film and substrate, the 90 degrees c/a/c/a domain structure becomes unstable with respect to the appearance of the polarization component parallel to domain walls, which results in the formation of a heterophase structure. For PbTiO3 and BaTiO3 films, the stability ranges of polydomain and heterophase states are determined. Dielectric anomalies accompanying misfit-strain-driven structural transformations are described.     

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.     

Effect of epitaxial strain on the spontaneous polarization of thin film ferroelectrics

Epitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this Letter we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics , , and , and the multiferroic material . We show that the epitaxial strain dependence of the polarization varies considerably for the different systems, and in some cases is, in fact, very small. We discuss possible reasons for this different behavior and show that the effect of epitaxial strain can easily be understood in terms of the piezoelectric and elastic constants of the unstrained materials. Our results provide a computational tool for the quantitative prediction of strain behavior in ferroelectric thin films.     

无铅四方相钙钛矿短周期超晶格压电效应机理研究

超晶格压电行为与内部正离子之间的内在联系尚缺乏相关的研究.本文基于密度泛函理论的第一性原理方法,研究了三种无铅四方相钙钛矿铁电超晶格(BaTiO_3/SrTiO_3,KNbO_3/KTaO_3和BaTiO_3/KNbO_3)中A,B位正离子对整体的极化和压电贡献.通过计算超晶格不同轴向应变条件下原子结构和Born有效电荷,获得了超晶格和各个正离子的极化值和压电系数.结果表明,在轴向压缩应变条件下(-0.15—0 A),无铅超晶格中的正离子位移D(A)和D(B)受到抑制,在拉应变时位移才显著增大,因此极化和压电行为不明显.在轴向拉伸应变作用下(0—0.15 A),无铅超晶格中各原子的极化贡献显著增大,特别是B位原子Ti,Nb和Ta的极化贡献使得总的极化强度也显著提高,并当拉应变达到一定值,超晶格才会出现明显的压电行为.无铅超晶格的极化和压电行为主要由B位正离子贡献.     

Effect of out-of-plane misfit strain on phase diagrams and ferroelectric properties of ferroelectric films in vertical nanocomposite structures

A phenomenological thermodynamic Landau–Devonshire theory is developed to investigate phase diagrams of epitaxial ferroelectric films with out-of-plane misfit strain induced by vertical nanocomposites. The thermodynamic potential of ferroelectric films is obtained based on the boundary conditions of three-dimensional clamping induced by the vertical nanocomposites. Our calculated results indicate that the out-of-plane misfit strain modulates the transition temperature and spontaneous polarization of ferroelectric films in a wide range even the substrate does not provide an effective in-plane misfit strain control. An enhanced critical transition temperature up to 803 °C in BaTiO₃ films under a tensile out-of-plane misfit strain is predicted, which is consistent with the experimental result very well. The polarization properties of BaTiO₃ films can also be effectively modulated by the out-of-plane misfit strain which is controlled by the volume fraction of nanopillars in the vertical nanocomposites. Our method provides a theoretical guide for the out-of-plane strain engineering of ferroelectric films.     

外加电场对铁电薄膜相变的影响

采用热力学非线性理论,研究了外加电场对立方基底Pb(Zr_0.3Ti_0.7)O₃(PZT)铁电薄膜相变的影响.通过数值计算,得到了"失配应变-外加电场"相图,及外加电场与极化强度的关系.当外加电场达到186 kV/cm时,能使生长在SrTiO₃ 基底上PZT铁电薄膜从单斜r相转变为c相.在实验上,采用扫描探针显微镜通过对PZT薄膜施加不同的极化电场来研究了它的电畴翻转.从得到的压电响应相图可以看出,绝大多数的电畴是清晰可 关键词： 铁电薄膜 相变 扫描探针显微镜 失配应变     

柔性Pb(Zr_(0.53)Ti_(0.47))O_3薄膜的高温铁电特性

随着柔性电子产品的迅速发展,具有优异铁电和压电性的Pb(Zr_(0.53)Ti_(0.47))O_3 (PZT)薄膜在柔性的非易失性存储器、传感器和制动器等器件中有广泛的应用前景.同时,由于外部环境越来越复杂,具有高温稳定特性的材料和器件受到越来越多的关注.本文在耐高温的二维层状氟晶云母衬底上,用脉冲激光沉积技术制备出外延的PZT薄膜,并通过机械剥离的方法,得到柔性的外延PZT薄膜.研究了Pt/PZT/SRO异质结的铁电和压电性及其高温特性,发现样品表现出优越的铁电性,剩余极化强度(P_r)高达65μC/cm~2,在弯曲104次后其铁电性基本保持不变,且样品在275℃高温时仍然保持良好的铁电性.本文为柔性PZT薄膜在航空航天器件中的应用提供了实验基础.     

Normal and anomalous conductivity hystereses in the channel of a transparent ferroelectric transistor

Structures of the transparent ferroelectric field-effect transistor PZT/SnO₂/Al₂O₃ with “normal” and “anomalous” conductivity hystereses of the channel are prepared and investigated. The “normal” modulation loop for these structures is obtained for the first time. Antimony-doped SnO₂/Al₂O₃ epitaxial films evaporated by a YAG laser from a metal target are used as a channel of the field-effect transistor. Ferroelectric PZT films are deposited using magnetron sputtering. The distribution of deep levels at the PZT/SnO₂ interface is measured by the modified relaxation current method. It is established that the ratio between the polarization charge and the trap charge at the PZT/SnO₂ interface critically affects the direction of the conductivity hysteresis of the channel.     

Effect of excess Pb on crystallinity and ferroelectric properties of PZT(40/60) films on LaNiO3 coated Si substrates by MOD technique

Pb(Zr_0.4Ti_0.6)O₃ [PZT(40/60)] films were deposited onto LaNiO₃ (LNO) coated Si substrates by metal-organic decomposition (MOD) technique. Excess Pb was incorporated in the film by using excess Pb (2%–15%) in the solution. The crystallinity and ferroelectric properties of PZT films were investigated by using X-ray diffraction (XRD), RT66A test system and HP4194 impedance analyzer, respectively. Rayleigh law was employed to analyze the defect concentration in the films. The results show that all the PZT films show the (1 0 0) preferential orientation with complete perovskite structure except for the 2% film displaying some pyrochlore phase. The (1 0 0) preferential orientation is mainly attributed to LNO bottom electrode, which has the highly (1 0 0) preferential orientation. The 10% film shows the best polarization and dielectric properties. The remnant polarization and coercive field are about 10.1 μC/cm² and 73 kV/cm under an electric field around 330 kV/cm, respectively. And the dielectric constant and dissipation factor are about 656 and 0.022 at a frequency of 1 kHz, respectively. The good ferroelectric properties of the 10% film are mainly attributed to the low defect concentration in the film.     

Influence of the substrate on ferroelectric properties of 〈1 1 1〉 oriented rhombohedral Pb(Zr0.6Ti0.4)O3 thin films

〈1 1 1〉-oriented Pb(Zr_0.6Ti_0.4)O₃ thin films were elaborated in the same run by RF multitarget sputtering on Si/SiO₂/TiO₂/Pt(1 1 1) and LaAlO₃/Pt(1 1 1) substrates. PZT thin films were textured, exhibiting 〈1 1 1〉 fibre texture on silicon substrates whereas epitaxial relationships were found when grown on LaAlO₃/Pt(1 1 1). On the latter substrate, values of spontaneous polarization and of dielectric permittivity were measured close to that calculated previously along the 〈1 1 1〉 direction of PZT rhombohedral single crystal. On the contrary, spontaneous polarization and dielectric permittivity measured on PZT thin films deposited on platinized silicon were found deviating from calculated values. These different electrical results are attributed to different ferroelectric domain configurations.     

Quasi-static electric field–temperature diagrams in epitaxial relaxor ferroelectric films

Quasi-static polarization induced by dc electric field was studied in a broad range of temperatures in epitaxial films of relaxor ferroelectric PbMg_1/3Nb_2/3O₃ and PbSc_1/2Nb_1/2O₃. The electric field was applied and the response was measured along the out-of-plane <100> crystal direction of the epitaxial perovskite-structure (001) oriented films. The films remained in the relaxor state in zero-field cooling. A new polar state can be induced by electric field at a critical temperature below 100 K. The critical field and the induced polarization increased on cooling and reached the bulk-like values at 20 K. The orientational anisotropy of thin-film dipolar systems is discussed as a possible reason for the observed stable relaxor state.     

Highly (100) oriented Pb(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript>/LaNiO<Subscript>3</Subscript> films grown on amorphous substrates by pulsed laser deposition

Pb(Zr_0.52Ti_0.48)O₃ (PZT)/LaNiO₃ (LNO) thin films with highly (100) out of plane orientation were produced on SiO₂/Si(100) and alkaline earth aluminosilicate glass substrates by pulsed laser deposition (PLD). Orientations of both PZT and LNO films were evaluated using X-ray diffraction. The pure (100)-oriented PZT/LNO films were obtained under optimized deposition conditions. Time of flight-secondary ion mass spectrometry analysis showed that LNO could effectively block interdiffusion between the PZT films and the substrates. Fairly smooth surfaces of the PZT films with roughness of about 4 nm were observed using an atomic force microscope. Cross sectional examination revealed that the films grew in columnar grains. The PZT films grown on both SiO₂/Si and glass substrates demonstrated very good ferroelectric characteristic at room temperature with remnant polarization of up to 26 μC/cm². PACS 79.20.DS; 77.84.DY; 78.70.Ck     

不同晶化工艺对非晶PZT纳米薄膜形核取向生长机理的影响

射频磁控溅射法室温下在Pt/Ti/SiO2/Si上制备非晶Pb(Zr048Ti052)O3薄膜，非晶PZT薄膜分别经常规炉退火(CFA)处理和快速热退火(RTA)处理晶化为（100），（111）不同择优取向的多晶薄膜. 采用x射线衍射测定了薄膜相组分、择优取向度；用原子力显微镜和压电响应力显微镜观察了薄膜表面形貌，以及对应区域由自发极化形成的铁电畴像，观察了不同取向薄膜的电畴分布特征. 结果表明，RTA晶化过程钙钛矿结构PZT结晶主要以PZT/Pt界面处的PtPb化合物为成核点异质形核并类似外延的结晶生长，沿界面结晶速率远大于垂直膜面结晶速率，而CFA晶化样品成核发生在膜内杂质缺陷处，以同质成核为主. 不同的成核机理导致了不同晶面择优取向生长. 关键词： PZT薄膜 结晶 形核 力显微技术     

Nanoscale domain switching mechanism in Pb(Zr,Ti)O<Subscript>3</Subscript> thin film

Pb(Zr,Ti)O₃ (PZT) ferroelectric thin film was prepared by the sol-gel technique and crystallized with a (111) preferred orientation. The domain structure and polarization reversal behavior were investigated by using scanning force microscopy (SFM) piezoresponse mode at the nanometer scale. A step structure of approximately 30 nm in width was directly observed, which was formed during the polarization reversal process. The presence of the step structure reveals that the forward domain-growth mechanism is the dominant domain-switching process in our PZT thin films. Received: 6 August 2002 / Accepted: 9 August 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/5241-3122, E-mail: huarongzeng@163.net     

Interface control of emergent ferroic order in Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1)

We discovered from first principles an unusual polar state in the low n Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low-dimensional RP materials as is well known to occur in SrTiO(3). These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface.     

Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO₃ (BFO) thin films on SrRuO₃ (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2P_r = 59.6 μC/cm² and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO₃ substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO₃ substrates.