MOCVD of (Ba,Sr)TiO3: Nucleation and Growth

(Ba_0.7Sr_0.3)TiO₃ and SrTiO₃ thin films were deposited on Pt electrodes in a planetary multi-wafer MOCVD reactor. The nucleation behavior and the size of the stable nuclei were investigated by different SPM techniques. Characteristic differences were observed for different deposition temperatures, i.e. a homogeneous nucleation of small BST grains on the larger Pt grains at 565°C and a dominating nucleation at the grain boundaries at 655°C. The micro structural evolution after further film growth was investigated by HRTEM and revealed randomly oriented grains (typical inplane size 10–20 nm) with a high density of twins at 565°C and (100)-oriented defect free grains of only slightly increased size at 655°C. For SrTiO₃ the inplane grain size was increased, however, the (100) texture was less perfect. As the electrical properties like permittivity and also leakage current depend on film thickness the final discussions of the electrical properties are based on thickness series (5 nm–100 nm films) and evaluated within the phenomenological dead layer model.     

Leakage Current Measurements of STO and BST Thin Films Interpreted by the "Dead" Layer Model

Electrical properties of Strontium-Titanate (STO) and Barium-Strontium-Titanate (BST) thin films capacitors were investigated. The STO films were fabricated by chemical solution deposition (CSD) with thickness between 50 and 150 nm, while the BST films were deposited by metal organic chemical vapor deposition (MOCVD) with thickness between 20 and 110 nm. All films were grown on platinized and oxidized silicon wafers. As top electrodes platinum (Pt) was deposited on top of the ceramic film by sputtering. The electrode size varied between 8*10 m 3 to 1 mm 2 . The leakage current measurements were performed at different temperatures ranging from 15 to 200C and the applied voltage varied between 0 and - 4 V. Capacitance was measured at RT up to - 3 V bias at 1 kHz and 50 mV oscillation voltage. The main results are: The effective barrier heights extracted from the temperature dependence of leakage current are about 1.35 eV for STO and 0.94 eV for BST for the temperature region >100C. The field dependencies of the leakage current show almost perfect linear behavior in a "Schottky" plot for BST while STO reveals 2 separated Schottky regions. The permittivity extracted from the field dependence using the simple thermionic emission model with Schottky lowering results in rather improbable values of the effective Richardson constant A * and unphysical values of the relative optical permittivity, l r, opt <1. The use of a modified model with low permittivity interface layers ("dead layers"), as suggested by the thickness dependence of the inverse capacitance, solved these difficulties. The parameters extracted from fits of this model are discussed.     

Structure of Ferroelectric (Ba,Sr)TiO3 Thin Films for Tunable Microwave Devices

Ferroelectric (Ba,Sr)TiO₃ films have been deposited on (001) MgO single crystals by a pulsed laser deposition with oxygen background while heating the substrates. Deposited BST films exhibit epitaxial growth along (001), which are confirmed by x-ray diffraction measurement. Structure of (Ba,Sr)TiO₃ along in-plane and surface normal direction have been investigated and found to have a tetragonal distortion depend on the deposition conditions, such as oxygen pressure. Lattice parameter decreases with increasing oxygen pressure, and tetragonallity (c/a) changes from 1.005 to 0.997 as oxygen pressure increase. Interestingly, energy gap measured by FTIR decreases with decreasing oxygen pressure until it reach a certain oxygen pressure, then increases again with increasing oxygen pressure. Furthermore, microwave properties of devices measured by a HP 8510C vector network analyzer from 0.045–20 GHz suggest that the least distorted films exhibit a larger dielectric constant changes with dc bias field.     

Optimum Reflection-Type Phase Shifter Using (Ba,Sr)TiO3 Thin Film

In this paper, in order to obtain a large differential phase shift with a little change in applied voltage, a ferroelectric reflective load circuit has been designed on top of barium strontium titanate (Ba,Sr)TiO₃ [BST] thin film. The design of the ferroelectric reflection-type phase shifter is based on a reflection theory of terminating circuit, which has a reflection-type analogue phase shifter with two ports terminated in symmetric phase-controllable reflective networks. To achieve large amounts of phase shift in low bias-voltage range, the effects of change of capacitance and transmission line connected with two coupled ports of a 3-dB 90° branch-line hybrid coupler have been investigated. A large phase shift with a small capacitance change in the parallel terminating circuit has been demonstrated in the paper.     

Calculation of the intrinsic dead layers thicknesses from Au/Ba0.5Sr0.5TiO3/Pt thin film capacitors

Ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) films were prepared on Pt/Ti/SiO₂/Si substrates by the sol-gel process. The films were spin-coated at 2000 rpm for 30 secs and then pyrolysed for 5 mins at the temperature of 350^∘C. This coating procedure was repeated for 3, 4, 5 and 6 times to obtain BST films with different thicknesses. After coating the films with the desired repetition times, the films were finally annealed in a conventional furnace at temperatures ranging from 600^∘C to 800^∘C with a 50^∘C interval in between. The films obtained with an annealing procedure of 750^∘C were polycrystalline with the presence of an impurity BaCO₃ phase. The capacitance and leakage current were measured and used to extract information on the metal-BST interface. With the series capacitance model and modified Schottky emission equation, the thickness of the dead layers for Au/BST and Pt/BST interfaces were calculated to be less than 6 nm and 5 nm, respectively.     

Liquid source chemical vapor deposition of high-dielectric-constant (Ba,Sr)TiO3 films

High-dielectric-constant (Ba, Sr)TiO₃ [BST] films were deposited by the liquid source chemical vapor deposition (CVD) method. The system consisted of a single-wafer, low-pressure thermal CVD reactor, a vaporizer for liquid source materials, and a shower-type gas nozzle head, giving stable BST film deposition on a 6-in. diam. substrate with uniform thickness and uniform chemical composition ratio. The source materials employed were Ba(DPM)₂, Sr(DPM)₂, and TiO(DPM)₂ dissolved in tetrahydrofuran (THF), resulting in conformal step coverage of BST films at lowered substrate temperatures, where DPM denotes dipivaloylmethanate. Moreover, the two-step deposition technique was developed to restart protrusions formed on BST film surfaces at low temperatures, where the BST films consisted of a buffer layer and a main layer; the buffer layer was a layer about 60 Å thick of CVD-BST film annealed in N₂. Thus, the two-step CVD deposition of BST films on Pt and Ru electrodes achieved an equivalent SiO₂ thickness of t_eq ∼ 0.5 nm, a leakage current of J_L ∼ 1.0 × 10⁻⁸ A/cm² (at +1.1 V), and a dielectric loss of tan δ ∼ 0.01 at a total film thickness of 250 Å, along with conformal coverage of 80% for a trench with an aspect ratio of 0.65. Then, for BST films deposited on patterned electrodes 0.24 μm wide, 0.60 μm deep, and 0.15 μm high (each spaced by 0.14 μm), the capacitance was demonstrated to be increased without significant deterioration of the leakage current: the capacitance was increased in comparison with that for films on flat electrodes, by a factor corresponding to the increase in surface area due to sidewalls of storage-node-like pattern features. This capacitance increase reflects the most characteristic advantage of CVD, an excellent step coverage on microscopic pattern features. These electrical properties satisfy the specifications for capacitors for Gb-scale dynamic random access memories (DRAMs), giving a storage capacitance of more than 25 fF/cell for a stacked capacitor having a storage node 0.2 to 0.3 μm high. © 1998 Scripta Technica, Electr Eng Jpn, 125(1): 47–54, 1998     

The Effect of Stress on the Microwave Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films

Single phase, (1 0 0) epitaxial Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited onto (1 0 0) LaAlO₃ and MgO substrates by pulsed laser deposition (PLD). The capacitance and dielectric losses of as-deposited and annealed films have been measured from 1–20 GHz as a function of electric field (0–80 kV/cm) at room temperature. The dielectric properties are strongly affected by the substrate type, post-deposition annealing time (6 h) and temperature (1200°C). For epitaxial BST films deposited onto MgO, it is observed that, after a post-deposition anneal the dielectric constant and the dielectric loss decreases. For epitaxial BST films deposited onto LAO, a post-deposition anneal (1000°C) results in an increase in the dielectric constant and an increase in the dielectric loss. The dc electric field induced change in the dielectric constant tends to increase with the dielectric constant and is largest for as-deposited films on MgO and post-deposited annealed films on LAO. In general, for epitaxial BST films, a large electric field effect is observed in films that have a large dielectric loss and a small electric field effect in films that have a low dielectric loss. High resolution X-ray diffraction measurements indicate that deposited film exhibit a significant tetragonal distortion which is strongly affected by a by a post deposition anneal. The observed differences in dielectric properties of the epitaxial BST films on MgO and LAO are attributed to the differences in film stress which arise as a consequence of the lattice mismatch between the film and the substrate and the differences in the thermal coefficient of expansion between the film and the substrate. A thin amorphous buffer layer of BST has been used to relieve stress induced by the lattice mismatch between the film and the substrate. Unlike epitaxial films, stress relieved films do not show an inverse relationship between dielectric tuning and Q (1/tan) and may be superior materials for tunable microwave devices.     

MSnO3(M=Ca,Sr,Ba)的溶胶-凝胶法合成及性能

采用溶胶-凝胶法制备了钙钛矿结构的MSnO3(M=Ca,Sr,Ba)晶体.将其作为锂离子电池负极材料的活性物质,利用恒电流电池测试仪研究其电化学性能.结果表明:CaSnO3、SrSnO3和BaSnO3的首次放电容量分别为894.3 mAh/g、703.8 mAh/g和673.6 mAh/g,首次充电容量分别为418.3 mAh/g、257.8 mAh/g和224.8 mAh/g,不可逆容量损失分别达53.2%、63.3%和66.6%.     

Multilayer Ceramic Capacitors with Thin (Ba,Sr)TiO3 Layers by MOCVD

Barium strontium titanate ((Ba,Sr)TiO₃; BST) thin films were prepared on platinum-coated MgO substrates at 650°C by metalorganic chemical vapor deposition (MOCVD). Perovskite single phased BST thin films were obtained. Dielectric constant at 1 kHz–100 mV was 1000. Multilayer ceramic capacitor with twelve BST dielectric layers of 0.26 m thick was formed on the MgO substrate. Capacitance and dissipation factor (tan) at 1 kHz–100 mV were 32 nF and 1.5% respectively. Capacitance per unit volume of 33 F/mm³ provided 10 to 20 times larger volumetric efficiency than the conventional multilayer ceramic capacitors. Temperature coefficient of capacitance was –4000 ppm/°C. The leakage current at 1 V was 2.3×10^-9 A that yielded an acceptable CR product of 12.8 M-F. MOCVD was proposed as one of the promising manufacturing technologies for multilayer ceramic capacitors of high performance with sub-micron thick dielectric layers.     

Interface potential barrier height and leakage current behavior of Pt/(Ba,Sr)TiO3/Pt capacitors fabricated by sputtering process

Abstract

Variations of the leakage current behaviors and interface potential barrier height (φ _B ) of rf-sputter deposited (Ba, Sr)TiO₃ (BST) thin films, with thickness ranging from 20nm to 150 nm are investigated as a function of the thickness and bias voltages. The top and bottom electrodes are dc-sputter-deposited Pt films. φ _B critically depends on the BST film deposition temperature, postannealing atmosphere and time after the annealing. The postannealing under N₂ atmosphere results in a high interface potential barrier height and low leakage current. Maintaining the BST capacitor in air for a long time reduces the φ _B from about 2.4 eV to 1.6eV due to the oxidation. φ _B is not so dependent on the film thickness in this experimental range. The leakage conduction mechanism is very dependent on the BST film thickness; the 20nm thick film shows tunneling current, 30 and 40 nm thick films show Schottky emission current and the thicker films show a mixed characteristics and bulk and interface limited currents although the mechanism is not clearly understood at this moment.     

Preparation and electrical properties of Ni/(Ba,Sr)TiO3 PTC composite with low resistivity

Ni/ (Ba,Sr)TiO₃ PTC composite of low resistivity was fabricated by a solid state route. A mildly reducing sintering atmosphere was employed to avoid the oxidation of nickel. Metallic nickel is the main chemical state after sintering in the mildly reducing sintering atmosphere. With the increase in nickel amount, the room-temperature resistivity declines and the PTC effect worsens. The quantum mechanical tunneling effect at the Ni–(Ba_,Sr)TiO₃ interface is presumably the prime factor in the deterioration of the PTC effect. PbO–B₂O₃–ZnO–SiO₂ glass was added to modify the interface between nickel and (Ba,Sr)TiO₃ ceramics. The intergranular phase introduced by the glass has an amorphous structure and exists at the interfaces and triple junctions of (Ba,Sr)TiO₃ grains and nickel grains. No obvious diffusion occurs at the interface between crystalline (Ba,Sr)TiO₃ grain and the intergranular phase. Also the added-glass improves the distribution of metal phase. The proper glass addition screens interfacial electron tunneling effect and improves the composite electrical properties. An abundance of the intergranular phase due to excess glass will, however, result in high room-temperature resistivity. The influences of nickel amount and glass amount on the microstructure evolution and electrical properties were analyzed.     

Measurement of Microwave Dielectric Properties of (Sr,Ba)Nb2O6 Thin Films

(001) oriented (Sr,Ba)Nb₂O₆ (SBN) thin films were deposited on MgO (001) single crystal substrates by the pulsed laser deposition method. Structural properties of SBN films were investigated using X-ray diffractometer. The microwave dielectric properties of SBN films were examined by calculating the scattering parameter obtained using a HP 8510C vector network analyzer with the frequency range 0.5–20 GHz at room temperature under the dc bias field of 0–80 kV/cm for interdigital capacitors (IDT) and coplanar waveguide (CPW) device based on SBN/MgO layer structure. Thick metal electrode patterns were fabricated by dc sputtering deposition, photolithography and etching process. The IDT device based on (001) oriented SBN films exhibited about 40% capacitance tunability with an electric field change of 80 kV/cm at room temperature, and the dielectric quality factor was about 20 at 12 GHz with no dc bias.     

Orientation Control of (Bi,La) 4 Ti 3 O 12 Thin Films by Addition of Silicates and Germanates

Orientation of crystallites in (Bi,La) 4 Ti 3 O 12 (BLT) thin films were successfully controlled by adding such silicates and germanates as Bi 2 SiO 5 , ZrSiO 4 and La 2 GeO 5 . The ferroelectric films were formed by spin-coating mixed sol-gel solutions of BLT and silicate or germanate on a Pt/Ti/SiO 2 /Si structure. It was found from XRD analysis that the preferred orientation of crystallites in the BLT films was changed by the additives. That is, the orientation of crystallites in a Bi 2 SiO 5 -added BLT film was random, while it was almost perfectly c-axis-oriented in La 2 GeO 5 -added BLT and it was strongly (117)-oriented when ZrSiO 4 was added to BLT. It was also found that the remnant polarization and coercive field changed reflecting these different orientations.     

Interface Structures and Epitaxial Behavior of Ferroelectric (Ba,Sr)TiO 3 Thin Films

Perovskite Ba 0.6 Sr 0.4 TiO 3 thin films were epitaxially grown on (001) LaAlO 3 and (001) MgO by using pulsed laser ablation. The films are single crystal cubic structure with interface relationship of (001) BST // (001) MgO and <100> BST // <100> MgO for the films on (001) MgO and (001) BST // (001) LAO and <100> BST // <100> LAO . High-resolution transmission electron microscopy studies indicated that the initially grown layer of the film on both (001) MgO and (001) LAO is TiO 2 monolayer. Equally spaced misfit dislocations were found to form at the interface to release the lattice mismatch energy. Physical property measurements show that the films exhibit excellent room temperature dielectric behavior with very low dielectric loss of 0.007 and very large tenability of 45% at 1 MHz. The figure of merit of 60°/dB has been achieved for the microwave phase shifter operated at 24 GHz and room temperature.     

Microwave Tunable Properties of Ni-Doped (Ba0.5Sr0.5)TiO3 Thin Films Grown by Pulsed Laser Deposition

(Ba_0.5Sr_0.5)TiO₃ (BST) thin films were deposited by pulsed laser deposition (PLD) and investigated as a function of Ni dopant concentration in low and high frequency regions. In low frequency region (<10 MHz), the Ni-dopant concentration in BST films has a strong influence on the material properties including dielectric and tunable properties. Ni-doped (≤3 mol%) BST films showed denser, smoother morphologies and smaller grain sizes than those with 6 and 12 mol% Ni. Dielectric constant and loss of 3 mol% Ni-doped BST films were about 980 and 0.3%, respectively. In addition, tunability and figure of merit of 3 mol% doped BST films showed maximum values of approximately 39% and 108, respectively. In high frequency region (>1 GHz), the frequency tunability range at center frequency of undoped BST and 3 mol% Ni-doped BST coplanar waveguide (CPW) resonators showed 102 and 152 MHz, respectively at 30 V dc bias. The Ni-doped BST thin films are possible in applications of microwave tunable capacitors.     

Microwave Performance of Distributed Analog Phase Shifter Using Ferroelectric (Ba,Sr)TiO3 Thin Films

This work presents the design, fabrication and microwave performance of distributed analog phase shifter (DAPS) fabricated on (Ba,Sr)TiO₃ (BST) thin films for X-band applications. Ferroelectric BST thin films were deposited on MgO substrates by pulsed laser deposition. The DAPS consists of high impedance coplanar waveguide (CPW) and periodically loaded tunable BST interdigitated capacitors (IDC). In order to reduce the insertion loss of DAPS and to remove the alteration of unloaded CPW properties according to an applied dc bias voltage, BST layer under transmission lines were removed by photolithography and RF-ion milling. The measured results are in good agreement with the simulated results at the frequencies of interest. The measured differential phase shift based on BST thin films was 24° and the insertion loss decreased from ?1.1 dB to ?0.7 dB with increasing the bias voltage from 0 to 40 V at 10 GHz.     

(La1-xTbx)2/3Sr1/3MnO3薄膜的巨磁电阻效应

利用脉冲激光沉积法制备了（La1-xTbx)2/3Sr1/3MnO3系列薄膜样品。在77－300K的温度范围内,对样品的微观结构、磁化强度、电阻率和巨磁电阻效应等物理性质进行了系统研究。随着Tb含量x的增大,铁磁－顺磁转变温度随之下降,在x=0. 33样品中,最大磁化场μ0H=1.2T时,观察到34％的巨磁电阻。本文对其机理进行了研究和讨论。     

STRESS CONTROLLED FERROELECTRIC (BA0.5Sr0.5)TiO3 THIN FILMS PREPARED ON GD2O3/SRTiO3(001) BY A PULSED LASER DEPOSITION

ABSTRACT

Stress controlled epitaxial ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited on Gd₂O₃/SrTiO₃ by pulsed laser deposition with oxygen background pressure of 200 mTorr at the deposition temperature of 750°C. In order to control the stress in BST films, oxygen pressures for Gd₂O₃ buffer layers have been varied from 0.1 to 100 mTorr, while that of BST films have been fixed at 200 mTorr. It has been found that the lattice parameters of the BST films deposited on Gd₂O₃ were changed. Furthermore, microwave properties of co-planar waveguide (CPW) fabricated on BST films were investigated by a HP 8510C vector network analyzer from 1–20 GHz. Large dielectric tunabilities were observed from the CPW's fabricated on BST films deposited on Gd₂O₃ layers deposited at low and high oxygen pressures, 0.1 and 100 mTorr, respectively.     

Chemical Processing and Properties of (Sr,Ca) 2 (Nb,Ta) 2 O 7 Thin Films

Ca-doped Sr 2 (Nb,Ta) 2 O 7 thin films have been synthesized by the chemical solution deposition. Homogeneous and stable (Sr,Ca) 2 (Nb,Ta) 2 O 7 precursor solutions were prepared by optimizing the reaction of starting metal alkoxides in ethanol with a key additive of 2-ethoxyethanol. The improvement of ferroelectric properties of the Sr 2 (Nb,Ta) 2 O 7 based films were achieved through the Ca substitution into Sr 2 (Nb 0.3 Ta 0.7 ) 2 O 7 as well as the optimization of heating conditions. The crystallization temperature of the layered perovskite (Sr 0.9 Ca 0.1 ) 2 (Nb 0.3 Ta 0.7 ) 2 O 7 thin films on Pt/Ir/Ti/SiO 2 /Si substrates was found to be above 750C. (Sr 0.9 Ca 0.1 ) 2 (Nb 0.3 Ta 0.7 ) 2 O 7 thin films crystallized at 750C exhibited P r of 0.51 w C/cm 2 and E c of 69 kV/cm.     

Growth and Dielectric Properties of Ferroelectric BaTiO3 Thin Films for Cantilever-Type Microsensors

ABSTRACT

Ferroelectric BaTiO₃ (BTO) thin films were deposited on Si, silicon-on-insulator (SOI) and MgO substrates by pulsed laser deposition. The orientations of the films, polycrystalline and epitaxial phase, were controlled by the lattice mismatch between the BTO film and substrates. The structural properties and surface morphologies were examined using X-ray diffractometer and atomic force microscope. The dielectric properties of BTO films were investigated using metal-ferroelectric-metal (MFM) and interdigital co-planar capacitors. Conductive oxide layers, SrRuO₃(SRO) and La_0.5Sr_0.5CoO₃ (LSCO), were grown on Si and SOI substrates as bottom electrodes. For MFM capacitors based on Au/BTO/SRO/Si and Au/BTO/LSCO/SOI layer structures, a little asymmetric capacitance-voltage curves were obtained with about 36% capacitance tunability. The remanent polarizations were about 21 μC/cm² and the coercive fields were about 71 kV/cm. For an interdigital capacitor based on Au/BTO/MgO layer structure, a little lossy capacitance-voltage curve was obtained with about 64% capacitance tunability.