Preparation and characterization of ferroelectric SrBi2Ta2O9 thin films on Si using Al2O3 buffer layers

SrBi₂Ta₂O₉ (SBT) thin films were prepared on p-type Si(100) substrates with Al₂O₃ buffer layers. Both the SBT films and the Al₂O₃ buffer layers were deposited by a pulsed laser deposition technique using a KrF excimer laser. An Al prelayer was used to prevent Si surface oxidization in the initial growth stage. It is shown that Al₂O₃ buffer layers effectively prevented interdiffusion between SBT and Si substrates. Furthermore, the capacitance–voltage (C-V) characteristics of the SBT/Al₂O₃/Si heterostructures show a hysteresis loop with a clockwise trace, demonstrating the ferroelectric switching properties of SBT films and showing a memory window of 1.6 V at 1 MHz. Received: 17 July 2000 / Accepted: 16 August 2000 / Published online: 30 November 2000     

Characteristics of SrBi2Ta2O9 ferroelectric films on Si using LaAlO3 thin film as an insulator

Metal–ferroelectric–insulator–semiconductor structures using LaAlO₃ (LAO) layers as an insulating barrier have been investigated. LAO films were deposited on n-Si substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). SrBi₂Ta₂O₉ (SBT) films were prepared as ferroelectric layers at a low processing temperature of 650 °C by a metalorganic decomposition method. The MOCVD-derived LAO buffer layer shows an amorphous structure, relatively high dielectric constant, and good electrical properties. Au/SBT/LAO/n-Si exhibits a larger counterclockwise C–V memory window of 3.7 V and a lower leakage-current density of 2.5×10^-8 A/cm² at an applied voltage of 10 V. It has been confirmed that the hysteresis loop is caused by ferroelectricity. The Auger electron spectrometry depth profile indicates that the introduction of the LAO buffer layer prevents the interfacial diffusion between SBT and the Si substrate effectively and improves the interface quality. PACS 77.84.Dy; 81.15.Fg     

The structural and electric behavior of SrBi2Ta2O9 ferroelectric thin films with H+ implantation

The structural and electrical characteristics of H⁺-implanted SrBi₂Ta₂O₉ (SBT) ferroelectric thin films were investigated by X-ray diffraction analysis and electrical measurements. 25 keV H⁺ with doses ranging from 1 × 10¹⁴/cm² to 3 × 10¹⁵/cm² were implanted into the Sol-Gel prepared SBT ferroelectric thin films. The X-ray diffraction patterns of SBT films show that no difference appears in the crystalline structure of H⁺-implanted SBT films compared with unimplanted films. Ferroelectric properties measurements indicate that both remnant polarization and the coercive electric field of H⁺-implanted SBT films decrease with increasing the implantation dose. The disappearance of ferroelectricity was found in the H⁺-implanted SBT films up to a dose of 3 × 10¹⁵/cm². The leakage current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the H⁺-implanted SBT films were also discussed before and after a recovery process.     

Nonlinear response and optical limiting in SrBi(2)Ta(2)O(9) thin films

SrBi(2)Ta(2)O(9) (SBT) thin films on quartz substrates were prepared by use of the pulsed-laser deposition technique. The nonlinear refractive indices, n(2) , of the SBT films were measured by use of z-scan techniques with picosecond pulses. Large negative nonlinear refractive indices of 3.84 and 3.58cm(2)/GW were obtained for the wavelengths 532 nm and 1.064mum , respectively. The two-photon absorption coefficient was determined to be 7.3 cm/GW for 532 nm. The limiting behavior of SBT thin film on a quartz substrate was investigated in an f/5 defocusing geometry by use of 38-ps-duration, 532-nm, 1.064mum laser excitation.     

Film structure and ferroelectric properties of in situ grown SrBi2Ta2O9 films

Ferroelectric SrBi₂Ta₂O₉ (SBT) films were grown by pulsed-laser deposition (PLD) at different substrate temperatures and fluences. A correlation between film structure and ferroelectric properties is established. The dielectric function ε^′ of thin SBT films shows a Curie–Weiss behavior well below the peak temperature T_max and relaxor-like behavior in the vicinity of T_max. Domain walls have a strong influence on the dielectric and ferroelectric properties and on the polarization fatigue of SBT films below 100 °C. The formation of ferroelectric phases is favored at lower substrate temperatures by incorporating Bi₂O₃ template layers into the structure. Received: 18 March 1999 / Accepted: 19 March 1999 / Published online: 5 May 1999     

SrBi2Ta2O9 ferroelectric thin film capacitors: degradation in a hydrogen ambient

The effects of annealing in forming gas 5% hydrogen, 95% nitrogen; FGA) are studied on spin-coated SrBi₂Ta₂O₉ (SBT) thin films. SBT films on a platinum bottom electrode are characterized with and without a platinum top electrode. Films are characterized by residual stress measurements, scanning electron microscopy (SEM), Auger electron spectroscopy (AES), high-temperature X-ray diffraction (HT-XRD) and secondary ion mass spectrometry (SIMS). To determine the degree of strain, lattice constants of Pt are measured by X-ray diffraction (XRD). HT-XRD of blanket SBT/Pt/Ti films in forming gas revealed that the bismuth-layered perovskite structure of SBT is stable up to approximately 500 °C. After formation of an intermediate phase between 550 °C and 700 °C, SBT changes its structure to an amorphous phase. SIMS analysis of Pt/SBT/Pt samples annealed in deuterated forming gas (5% D₂, 95% N₂) showed that hydrogen accumulates in the SBT layer and at the platinum interfaces next to the SBT. After FGA of blanket SBT films, tall platinum–bismuth whiskers are seen on the SBT surface. It is confirmed that these whiskers originate from the platinum bottom electrode and grow through the SBT layer. FGA of the entire Pt/SBT/Pt/Ti stack shows two different results. For the samples with a high-temperature annealing (HTA) step in oxygen after top electrode patterning, peeling of the top electrode is observed after FGA. For the samples without a HTA step, no peeling is observed after FGA. The residual stress at room temperature is measured for blanket platinum wafers deposited at different temperatures. It is found that an increase in tensile stress caused by the HTA step in oxygen is followed by a decrease in stress caused by the hydrogen in the forming gas. Without HTA, however, an increase of stress is observed after FGA. PACS 77.84.-s; 81.40.-z; 77.55.+f     

Calculation of electrocaloric properties of ferroelectric SrBi2Ta2O9

Theoretical work on the dependence of polarization on variation of temperature for ferroelectric SrBi₂Ta₂O₉ due to applied electric field shift ΔE was done, showing good agreement with experimental data of previous work. Phenomenological model and thermodynamic calculation predicted electrocaloric entropy changes, heat capacity changes, and temperature changes as a function of temperature under different electric field shifts. Maximum entropy change, relative cooling, and refrigerant capacity due to applied electric field shift were calculated.     

Characteristics of SrBi2Ta2O9 ferroelectric films on GaAs with a TiO2 buffer layer

Mainly [115]-oriented SrBi₂Ta₂O₉ (SBT) films were prepared on GaAs(100) substrates with TiO₂ buffer layers. Both the SBT films and the TiO₂ buffer layers were deposited by pulsed laser deposition (PLD) using a KrF excimer laser. The depth profile of the constituent elements observed by Auger electron spectrometry (AES) shows no remarkable diffusion at both the interfaces between SBT and TiO₂ and between TiO₂ and the GaAs substrate. The electrical characteristics of the Pt/SBT/TiO₂/GaAs(100) structures show a ferroelectric hysteresis loop with a small remanent polarization (∼0.5 μC/cm²). Received: 1 March 2002 / Accepted: 3 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: xhliu81@hotmail.com RID="**" ID="**"Present address: Data Storage Institute, DSI Building, 5, Engineering Drive 1 (off Kent Ridge Crescent, NUS) 117608 Singapore     

Role of bismuth precursor in crystallization of SrBi2Ta2O9 thin films

Crystallization of SrBi₂Ta₂O₉ (SBT) thin films was studied as a function of viscosity of bismuth precursor and baking temperature, in order to fabricate capacitors with improved ferroelectric properties. SBT thin films were deposited on to Pt substrates using a chemical solution deposition (CSD) technique. Post-deposition anneal at 750 °C for 1 h in oxygen atmosphere revealed a significant influence of baking temperature and the viscosity of bismuth precursor on the microstructure and the ferroelectric properties of SBT thin films. A high baking temperature (350 °C) and a low viscosity of bismuth precursor (8 cp) yielded larger amounts of Bi₂O₃ secondary phase, smaller SBT grains (104 nm), and lower remanent polarization (P_r=2.0 7c/cm²). Additionally, these films exhibited a very high rate of ageing (>45% reduction in P_r after 7 days). A modified CSD process is suggested, which could suppress the formation of Bi₂O₃ secondary phase. Films fabricated using modified CSD technique exhibited a much larger grain size of 165 nm, higher P_r of 7.2 7c/cm², and significantly improved ageing characteristics (<1% reduction in P_r after 7 days). A qualitative model to describe the ageing in SBT-based capacitors is also suggested.     

Voltage shift of hysteresis loops of SrBi2Ta2O9 thin films under unipolar stress

Voltage shifts of hysteresis loops of metalorganic decomposition (MOD)-derived SrBi₂Ta₂O₉ (SBT) thin films, known as imprint, have been observed after exposing the thin-film capacitors to unipolar pulses. The voltage shift changes with cumulative total time at maximum voltage, following a relationship with no pulse-width dependence. The origin of the voltage shift is briefly discussed in terms of an internal bias field induced by injected electrons trapped at positive polarity. The pulse-measurement responses are greatly affected by the internal bias field, even though no imprint failure was observed up to 10¹⁰ unipolar pulses. The voltage shift and asymmetric properties can be removed easily by applying bipolar pulses of saturation amplitude. Received: 27 June 2000 / Accepted: 16 August 2000 / Published online: 5 October 2000     

Photoacoustic spectroscopy of thin SiO2 films grown on (100) crystalline Si substrates

Photoacoustic Spectroscopy (PAS) has been used to measure the thickness of thin SiO₂ films grown on (100) Si wafers. The data are in reasonable agreement with a simple theoretical model. It suggests that photoacoustic Spectroscopy is complementary to optical interferometry, in that it is capable of giving quantitative estimates of thin transparent films on opaque substrates of low reflectivity via the transmitted fraction of the optical energy incident on the sample. Both theoretical and experimental results indicate that PAS can be very useful in the measurement of thin films on substrates of low reflectivity.     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

SrBi2Ta2O9/Bi4Ti3O12复合铁电薄膜的制备与特性研究

采用溶胶凝胶工艺在p-Si衬底上制备了SrBi2Ta2O9/Bi4Ti3O12复合铁电薄膜. 研究了SrBi2Ta2O9/Bi4Ti3O12复合薄膜的微观结构与生长行为、铁电性能和疲劳特性. 研究表明: Si衬底Bi4Ti3O12薄膜易于沿c轴择优生长,并有利于SrBi2Ta2O9/Bi4Ti3O12复合薄膜的生长.合理的SrBi2Ta2O9/Bi4Ti3O12厚度配比能获得较好的铁电性能和优良的抗疲劳特性,SrBi2Ta2O9/Bi4Ti3O12厚度配比为1∶3的复合薄膜的剩余极化强度和矫顽电场分别为8.1 μC/cm2 和 130 kV/cm,其无疲劳极化开关次数达1011以上.     

Ferroelectric domain structure of SrBi2Nb2O9 epitaxial thin films

The domain structure in a ferroelectric with well-defined crystallography and negligible ferroelastic distortion (<0.002%) is reported. In contrast to prototypical ferroelectrics in which long-range elastic strain dictates the domain structure, in SrBi2Nb2O9 the elastic term is insignificant, allowing dipole-dipole interactions and domain wall energies to dominate in determining the domain structure. Electron microscopy reveals ferroelectric domains that are irregularly shaped and highly curved. Out-of-phase boundary defects are shown to be weakly correlated with 90 degrees ferroelectric domain structure.     

Texture control and its impact on the properties of SrBi2Ta2O9 thin films prepared by pulsed laser deposition

SrBi₂Ta₂O₉ (SBT) ferroelectric thin films with different preferred orientations were deposited by pulsed laser deposition (PLD). Several methods have been developed to control the preferred orientation of SBT thin films. For SBT films deposited directly on Pt/TiO₂/SiO₂/Si substrates and in situ crystallized at the deposition temperature, the substrate temperature has a significant impact on the orientation and the remnant polarization (Pr) of the films; a higher substrate temperature benefits the formation of (115) texture and larger grain size. The films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C are (115)-oriented and exhibit 2Pr of 6 μC/cm². (115)- and (200)-predominant films can be formed by using a La_0.85Sr_0.15CoO₃ (LSCO) buffer layer or by annealing amorphous SBT films deposited on Pt/TiO₂/SiO₂/Si substrates at 450 °C using rapid thermal annealing (RTA). These films exhibit good electric properties; 2Pr of the films are up to 12 μC/cm² and 17 μC/cm², respectively. The much larger 2Pr of the films deposited on the LSCO buffer layer and of the films obtained by RTA than 2Pr of the films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C is attributed to a stronger (200) texture. Received: 30 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Growth and characterization of ferroelectric SrBi2Ta2O9 single crystals via high-temperature self-flux solution method

SrBi₂Ta₂O₉ (SBT) single crystals were produced by the high-temperature self-flux solution method using a Bi₂O₃ flux modified with B₂O₃. The processing conditions were optimized to obtain large and translucent SBT crystals with a layered habit and typical dimensions of approximately 7 × 5 × 0.2 mm. X-ray diffraction and x-ray topography measurements revealed that the major faces of the crystals with natural rectangular platelet morphology are perfectly (001)-oriented with edges directed along the [110] directions. The high quality of the crystals was confirmed by rocking curves (half-width of 0.04° for the (0018) reflection) and by ferroelectric measurements. The anisotropy in the dielectric and ferroelectric properties was investigated both along the [110] (ab plane) and the [001] (c axis) directions. The growth mechanism, morphology, and dielectric anisotropy of the SBT crystal platelets are discussed based on its crystallographic structure. This article was submitted by the authors in English     

A leed-aes study of thin Pd films on Si(111) and (100) substrates

A system Pd (deposit)-Si (substrate) has been studied by LEED and AES. Pd₂Si formed on Si(111) became epitaxial after a short time of annealing at a temperature between 300 and 700°C, while the Pd₂Si formed on Si(100) did not, in both cases the surfaces of the Pd₂Si being covered with a very thin Si layer. A sequence of superstructures (3√3 × 3√3), (1 × 1), and (2√3 × 2√3) was observed successively in Pd/Si(111) as the annealing temperature was increased. A (√3 × √3) structure was obtained by sputtering the 3√3 surface slightly. It was found that the √3 structure corresponds to Pd²Si(0001)-(1 × 1) grown epitaxially on Si(111), and that the 3√3 structure comes from the thin Si layer accumulated over the silicide surface, while the 2√3 and 1 structures arise from a submonolayer of Pd adsorbed on Si(111). Superstructures observed on a Pd/Si(100) system are also studied.     

铁电体SrBi2Nb2O2电子能带结构的第一性原理研究

采用第一性原理的方法计算了SrBi₂Nb_2O.9（SBN）的顺电相、铁电相的电子结构.顺电相是间接带隙, 铁电相是直接带隙，它们的大小分别为1.57和2.23 eV.顺电相和铁电相的价带顶均主要来自于O₂p态的贡献.而顺电相和铁电相的导带底则分别来自Nb₄d态和Bi₆p态的贡献.计算表明SBN铁电相的低的漏电流与Bi 6p轨道有关.由顺电相到铁电相时，Nb₄d和O_{2 关键词： 顺电相 铁电相 态密度 电子能带结构}