Growth and Dielectric Properties of BaTiO 3 /SrTiO 3 Artificial Lattice with Asymmetric Stacking Sequence

BaTiO 3 /SrTiO 3 artificial lattices were deposited on (La,Sr)CoO 3 coated MgO(100) single crystal substrate by pulsed laser deposition (PLD). The stacking thickness ratio of consisting BTO and STO layers was varied while one of the both consisting layers was maintained at 2 unit cell layer thickness. The stacking periodicity of BTO/STO artificial lattice was, therefore in the range from asymmetric BTO 2 unit cell /STO 32 unit cell to BTO 32 unit cell /STO 2 unit cell structure with a total thickness of 100 nm. As the stacking thickness of the BTO layer increased with a fixed stacking thickness of the STO layer (i.e., 2 unit cell thickness), the lattice distortion of the STO layer increased due to mechanical constraint from the thick BTO layer while the lattice distortion of the BTO layer was relatively unchanged. The lattice distortion (c/a) of the STO layer reached at 1.10 for the stacking periodicity of BTO 32 unit cell /STO 2 unit cell . The multiplayer structure with variable stacking thickness of the STO layer and fixed stacking thickness of the BTO layer exhibited the large variation in the lattice distortion of the BTO layer. The dielectric constant of the BTO/STO artificial lattice decreased with the stacking thickness ratio of the BTO and STO layers. This was attributed to severe lattice distortion of the STO (or BTO) in the multiplayer structure of variable stacking thickness of the BTO (or STO) layer. The dielectric constant of the BTO/STO artificial lattice reached a maximum i.e., 1230 at a stacking periodicity of BTO 2 unit cell /STO 2 unit cell.     

Dielectric Behavior of BaTiO3/SrTiO3 Oxide Artificial Lattice by Strain Manipulation

BaTiO₃ (BTO)/SrTiO₃ (STO) artificial lattices have been deposited on (La,Sr)CoO₃/MgO (100) substrate by pulsed laser deposition (PLD). Strain manipulation in the oxide artificial lattice was carried out by varying stacking sequence and period of BTO and STO layers with layer-by-layer growth technique. A wide variation of the lattice distortion for both BTO and STO layers has been obtained from this strain manipulation. The dielectric constant of the BTO and STO lattices was sensitively influenced by the lattice distortion. Moreover, it is found that there exist a certain degree of lattice distortion of the strained BTO and STO lattices leading to a maximum value of their own dielectric constant of the BTO and STO lattices. Consequently, an appropriate combination of stacking period and sequence (resulting in a specific lattice distortion) led to the large dielectric constant (1230) and extremely large tunability (94%) in the BTO/STO oxide artificial lattice.     

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.     

Preparation of Textured Growth Pb(Zr,Ti)O3 Thin Films on Si Substrate Using SrTiO3 as Buffer Layers

PZT thin films are deposited on SiO₂/Si substrate by metallo-organic decomposition (MOD) process, using SrTiO₃ (STO) as buffer layer for textured growth. The STO layers deposited on SiO₂/Si substrate by pulsed laser deposition process show (100)/(200) preferred orientation, whereas the STO buffer layer deposited on silica substrate using spin-coating technique show random orientation behavior. The use of STO as buffer layers enhanced the crystallization and the preferred orientations of the PZT films. The PZT on STO buffered SiO₂/Si substrates thus obtained possess high refractive index, (n)_PZT/STO = 2.1159, and are of good enough quality for optical waveguide applications.     

Surfaces structure of bulk and thin film ferroelectrics

Abstract

Based on the experimental results of thickness dependences of breakdown voltage and dielectric permittivity for BaTiO₃ family ceramics, (Pb, La)TiO₃ thin films and commercial multilayer capacitors, surface layer structures are discussed. Surface layers inside ferroelectric materials are consisting of non-ferroelectric Kanzig layer with low dielectric permittivity and with higher concentrations of impurities due to the inter diffusion between substrates and bulk, internal stresses induced by lattice mismatch, cubic-tetragonal phase transition, electric field induced anisotropies and internal bias field (space charge field). The Voltage - Current characteristics (V-I curve), D-E loops of ferroelectric materials show asymmetric behaviors. Saturation phenomena of V-I curves are observed only ferroelectric temperature region. The breakdown voltage almost depends on the non-ferroelectric Kanzig layer and the dielectric permittivity depends on the volume fraction of non-ferroelectric parts. Log-log plots of dielectric permittivity and breakdown voltage suggest that the thickness of non-ferroelectric Känzig layer should be at least more than 10～20 nm and in the grain boundary thickness of bulk ferroelectric materials should be more, especially in the liquid phase sintering, the thickness of grain boundary is a order of 0.1～0.2 μm.     

Electron emission of silicon field emitter arrays coated with N-doped SrTiO3 film

Nitrogen doped SrTiO₃ (STO) thin films have been fabricated on Si field emitter arrays (FEAs) using reactive RF magnetron sputtering in Ar-N₂ mixture ambient for electron emission applications. The nitrogen incorporation in STO films was revealed both in Fourier transform infrared (FTIR) spectroscopy and in Auger electron spectroscopy (AES). Low dose incorporation of nitrogen in STO films shows enhanced crystallinity, whereas the overdosed films show the degraded perovskite structure. The results demonstrate that the threshold emission field is lowered tremendously from 36.24 V/μm for uncoated FEAs to 17.37 V/μm for 30-nm-thick STO coated FEAs deposited in 50% N₂ ambient. The enhanced electron emission characteristics are highly correlated with the nitrogen incorporation in STO and film thickness. The substitution of nitrogen for oxygen may result in the band-gap narrowing in STO with enhanced electron emission. The thickness dependence might be related to the formation of space-charge-induced band-bending interlayer at STO/Si interface.     

Dielectric Characteristics of In-Plane Polarized Lead Zirconate Titanate Thin Films on Oxide Layers

Sol-gel derived lead zirconate titanate (PZT) films have been prepared on STO-passivated silicon substrates. The STO buffer layer of thickness about 55 nm is prepared by rf-magnetron sputtering. XRD results reveal that the PZT film has well-crystallized perovskite phase, indicating that the thin STO layer can effectively prevent reaction and inter-diffusion between the PZT film and silicon substrate. Interdigitated electrodes (IDEs) have then been deposited on the PZT film by magnetron sputtering and patterned using the standard photolithography. With the IDEs, the dielectric and ferroelectric properties of the PZT film under transverse or in-plane electric fields have been investigated. By assuming a uniform distribution of electric field (in-plane electric field model), the estimated relative permittivity of the PZT film is about 2100, while the dielectric loss is less than 1%. Good in-plane polarization hysteresis loop is observed, showing an observed remanent polarization value of 21 μC/cm², which is comparable to that of a PZT film with both top and bottom electrodes. The in-plane polarized PZT/STO/SiO₂/Si film can be used to fabricate d₃₃-mode unimorph bending transducers, which will have much better performance than the conventional bending transducers driven electromechanically through the piezoelectric d₃₁ mode.     

Effect of seed layers on dielectric properties of Ba(Zr0.3Ti0.7)O3 thin films

The Barium zirconium titanate Ba(Zr_0.3Ti_0.7)O₃ thin films were prepared on Pt/Ti/SiO₂/Si substrates with seed layers at the BZT/Pt interface by sol–gel process. Microstructure and structure of thin films were examined. Dielectric properties of thin films with various seed layers thicknesses were investigated as a function of frequency and direct current electric field. The tunability and dielectric constant of BZT thin films increased with increasing seed layer thickness from 0 to 20 nm, while it decreased with a further increase in thickness above 20 nm, meanwhile, the leakage current showed the similar tendency at applied electric field of 250 kV/cm. The optimized seed layer thickness for BZT thin films plays an important role in maintaining the high tunability and low leakage current, which are suitable for microwave device applications.     

Structure Simulation on Twisted Boundaries in SrTiO<Subscript>3</Subscript> Bicrystals by Molecular Dynamics Calculation

Bicrystals of SrTiO₃ with twisted boundaries were prepared by a HIP method and were observed with a transmission electron microscope (TEM). The bicrystals exhibited different nonlinear current-voltage characteristics depending on twist angle. A Molecular dynamics calculation was applied in order to understand the detailed interface structure. The periodicity of the Σ5 twisted boundary was calculated to be 1.58 times longer than the lattice constant of SrTiO₃ along the (130) axis, and agreed with the periodicity observed by TEM, i.e., 1.57. Coincidence sites formed between Ti ions of the TiO₂ layer and O ions of the SrO layer at the interface and did not shift the positions predicted by a coincidence sites lattice model; such coincidence sites caused the structural periodicity along the interface.     

Bilayered Pb(Zr,Ti)O3/(Bi,Nd)4Ti3O12 thin films

Bilayered ferroelectric thin films consisting of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) have been successfully synthesized on Pt/Ti/SiO₂/Si substrates, via a combined sol–gel and rf-sputtering route. Their ferroelectric and dielectric properties are critically dependent on the phases present, film texture and in particular layer and film thicknesses. Due to the coupling of PZT and BNT bilayers, there requires an optimized thickness combination of the two ferroelectric layers, in order to give rise to the wanted ferroelectric and dielectric properties, while the phenomenon can not be accounted for by the simple series connection model.     

Thickness dependent ferroelectric properties of BSTO thin films deposited by RF magnetron sputtering

The thickness dependence of ferroelectric permittivity of (Ba, Sr)TiO₃ has been investigated. The BST films could be obtained to have a simple cubic perovskite structure, space group Pm3m, and practically c-axis epitaxial structure deposited at 800^∘C. Through post-annealing process, we have improved the dielectric properties; dielectric permittivity, dielectric loss, and tunability. The change in dielectric properties before and after annealing is attributed to the change in film strain and the contraction in film lattice. As the thickness of BST films increases from 55 nm to 350 nm, the dielectric constant of BST films increases from about 100 to above 670 due to the reduction of interfacial dead layers with low dielectric constant between films and top electrodes. The dielectric loss of BST thin films decreased as the thickness increases. The existence of interfacial dead layers in a thinner film had a larger effect on the effective dielectric constant than tensile strain between the BST films and MgO substrate.     

Low loss flexible SrTiO<Subscript>3</Subscript>/POE dielectric composites for microwave application

Flexible, high dielectric constant and low dielectric loss composites for microwave application fabricated with SrTiO₃ (STO) ceramic filler dispersed inside a thermoplastic polyolefin elastomer (POE) polymer matrix have been studied in this paper. The dielectric property and the mechanical property of STO/POE composites filled with different volume fraction of ceramic filler were investigated. The results indicated that with the increase volume fraction of ceramic filler, both the permittivity and the dielectric loss of composites increased. Good frequency stability within a wide range was observed in all the samples. For the composites containing 40 vol% STO, the composites has a tensile strength of 2.75 MPa with an elongation of about 90% at break value. The permittivity and the dielectric loss of the composites were 11.0 and 0.01 in microwave frequency, respectively. A microstrip transmission line on the composites containing 40 vol% STO as a microwave substrate is designed and measured after bending at different angles, meanwhile the transmission coefficients of the microstrip transmission line were unchanged when bending angle is less than 60°. This indicates that the STO/POE composites have the promising characteristics for potential applications in microwave substrate, flexible dielectric waveguide and related flexible microwave devices.     

Structural and dielectric properties of BaTiO3SrTiO3-multilayers deposited by PLD

Abstract

Dielectric superlattices are characterized by the structural variability of the film periodicity additional to the stochiometry. They have interesting dielectric properties different from the solid solutions of the same overall composition.

In this paper the structural and dielectric properties of BaTiO₃/SrTiO₃-superlattices prepared by pulsed laser deposition on single crystalline substrates are characterized in comparison to a (Ba_0.5Sr_0.5)TiO₃ solid solution. The films were characterized using X-ray diffraction, SEM and dielectric measurements.

The BaTiO₃/SrTiO₃-superlattices have a different frequency and a weaker bias and temperature dependence of the dielectric properties than the solid solution. The mechanical stresses caused by the lattice mismatch between the different layers could be one reason for this behaviour.     

Pulsed laser deposition (PLD) of oriented bismuth titanate films for integrated electronic applications

Abstract

In this paper we describe recent successes of growth of epitaxial bismuth titanate (BTO) films by pulsed laser deposition (PLD) suitable for electro-optic and electrical switching device structures, and fabrication of an improved gate structure for a ferroelectric memory FET (FEMFET). TEM and x-ray results indicate that excellent crystalline quality BTO films were achieved on LaAlO₃. Polarization switching was demonstrated for BTO capacitors with epitaxial superconducting YBa₂Cu₃O₇ as the lower electrode. Using an SiO₂ buffer layer, a BTO/Si structure was fabricated and direct charge modulation in the Si by polarization reversal in the BTO was demonstrated.     

Investigation of dielectric/metal bilayer sidewall diffusion barrier for Cu/porous ultra-low-k interconnects

A dielectric/metal bilayer structure of a-SiC:H/Ta was integrated and investigated for application as a sidewall diffusion barrier in a Cu/porous ultra-low-k interconnect structure. Different dielectric/metal bilayer thicknesses were investigated. The electrical tests and physical analyses indicate that this a-SiC:H/Ta bilayer structure is a more efficient sidewall diffusion barrier than the conventional physical vapor deposited (PVD) multistack TaN/Ta metal barrier. With a similar total sidewall barrier thickness, an even better barrier integrity and reliability can be achieved by using a thicker a-SiC:H layer and a correspondingly thinner Ta barrier. This achievement is mostly attributed to the surface modification and sealing of the porous ultra-low-k surface by the a-SiC:H layer. Thus, the scenario of the barrier failure due to defects in the Ta (or TaN) barrier layer directly deposited on rough porous ultra-low-k material is avoided. Bias-temperature stress (BTS) and time-to-failure (TTF) studies indicate the Copper penetration through the sidewall barrier into the porous dielectric was the dominating failure mechanism in the conventional PVD TaN/Ta barrier and bilayer barrier of thinner a-SiC:H (14 /spl Aring/) and thicker Ta (71 /spl Aring/) layers, although the latter enhanced the lifetime of interconnect structures considerably. The bilayer barrier consisting of a thicker a-SiC:H (60 /spl Aring/) and correspondingly thinner Ta (53 /spl Aring/) layers was more robust to protect the sidewall region so that this sidewall Cu diffusion induced failure mechanism was no longer found in Cu/porous ultra-low-k interconnect structures even after thermal stress at 200/spl deg/C for 120 h.     

Temperature dependence of mechanical deformations in strontium titanate thin film on the sapphire substrate

Abstract

The temperature dependence of the dielectric constant (ε) of SrTiO₃ films on the various dielectric substrates differs radically from ε(T) dependence for bulk single crystal. This difference may be connected with temperature dependence of mechanical stress in the film. XRD measurements suggest the high mechanical deformation (u) of STO film unit cells at room temperature. But there is no experimental data about dependence u(T) nowadays. Present work is devoted to investigation of temperature and electric field dependencies of capacitance (C(U,T), C(Q,T)) of capacitor on the structure STO film/sapphire substrate. It is shown that derivative of inverse capacitance on the temperature is not constant and has a maximum at temperature about 200 K. Position of maximum is not influenced by charge (Q_i) on the capacitor. The mechanical deformations play the dominant role in the formation of maximum. The temperature behavior of macroscopic mechanical deformations in STO thin film on sapphire substrate is discussed.     

Characterization of hetero-epitaxial PZT capacitor

Abstract

A hetero-epitaxial Au/PbZr_0.48Ti_0.52O₃(PZT)/SrRuO₃(SRO) capacitor was fabricated on a single crystal SrTiO₃ (STO) substrate by pulsed laser deposition. An SRO buffer layer (a nucleation layer) was formed at the SRO/STO interface to ensure the highly epitaxial growth of the PZT and SRO films. An X-ray diffraction measurement revealed that the (00l) planes of the PZT and SRO grew parallel to the substrate surface. A transition layer of ～ 5 nm thickness was observed at the SRO/STO interface by high-resolution transmission electron microscopy (HR-TEM). This transition layer corresponds to the nucleation layer intentionally grown at the interface. Remanent polarization of the capacitor was 32.1 μC/cm² due to the good epitaxial growth of the films.     

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.     

Dielectric properties of the surface layer in ultra-thin films of a VDF/TrFE copolymer

We measured the temperature and frequency dispersion of the permittivity of vinylidene fluoride and trifluoroethylene (VDF/TrFE) thin films spin-coated on glass substrates with evaporated aluminium electrodes. Assuming the existence of two surface layers with different dielectric properties from the bulk material, a thickness of 2 nm was deduced for each surface layer. The temperature dependence of the frequency dispersion of the permittivity of the surface layers was obtained. Each surface layer has a dielectric relaxation which is essentially the same as the segment mode of the amorphous region of the bulk. It shows no anomaly at the temperature of the paraelectric-ferroelectric phase transition for thick samples. The surface layers appear to be amorphous.     

PATTERNED HIGH FREQUENCY THICK FILM MEMS TRANSDUCER

ABSTRACT

Ferroelectric properties of BiFeO₃ (BFO) thin films epitaxially grown on SrRuO₃ (SRO)/(001)SrTiO₃ (STO) structure were investigated. First, bottom SRO electrodes were deposited on STO substrates by metalorganic chemical vapor deposition (MOCVD) or by sputtering. Then, BFO thin films were deposited on SRO/STO structures by chemical solution deposition. X-ray diffraction analysis showed that the SRO films deposited by both methods grew epitaxially on STO substrates as a single phase perovskite structure, but the out-of-plane lattice parameters of SRO were different, that is, they were 0.396 nm in MOCVD and 0.399 nm in sputtering. The leakage current densities were higher than 1 A/cm² at room temperature in BFO films on both MOCVD-and sputtered-SRO, but the current density in the film on sputtered SRO decreased to 2 × 10^?4 A/cm² at 80 K. The remanent polarization of approximately 50 μC/cm² was observed at 80 K in the BFO thin film on sputtered-SRO/STO.