TiO3 Layers Research Articles

The effects of capping barrier layers on the compositional and structural variations of integrated Pb(Zr, Ti)O3 ferroelectric capacitor having the dimension 3 × 3 μm2

Structure and composition of the ferroelectric Pb(Zr, Ti)O3 layers in a capacitor of the ferroelectric random-access memory (FeRAM) device having a density of 64 k were investigated by transmission electron microscopy (TEM) together with the energy-dispersive spectroscopy (EDS) technique. The 250 nm thick PZT layer derived by the sol-gel route showed a 2–3% Pb-deficient, 3–4% Ti-deficient, and 5–7% Zr-excess composition at the top electrode interface compared to the bulk composition when they were as-fabricated. The local compositional nonuniformity became more critical as the integration process proceeded, which seriously degraded the ferroelectric hysteresis and the device yield. The major cause of the compositional variation was the outward diffusion of Pb through the capping barrier TiO2 layer during annealing at 650 °C. The AlN capping barrier layer was also not effective in suppressing the diffusion of Pb. However, the Al2O3/TiO2 double capping layer was very effective in suppressing the outward diffusion of Pb, and excellent ferroelectric characteristic was expected.

Permittivity and Microstructure of (Ba,Sr)TiO3 Films: Temperature and Electric Field Response

AbstractEpitaxial heterostructures (001)(Y,Nd)Ba2Cu3O7-δ∥(100)SrTiO3∥(001)(Nd,Y)Ba2Cu3O7-δ,(100)SrRuO3∥(100)Ba0.8Sr0.2TiO3∥(100)SrRuO3, (100)SrRuO3∥(100)SrTiO3∥(100)SrRuO3 and (100)SrTiO3∥(001)YBa2Cu3O7-δ have been grown by laser ablation. There was only a small difference of the dielectric permittivity, in the temperature range 180-300K, between a bulk single crystal and an epitaxial (100)SrTiO3 layer inserted between either high-Tc superconducting or SrRuO3 electrodes. At T<1 50K, on the other hand, the response of the dielectric permittivity of the SrTiO3 layer on temperature or electric field depended to a large extent upon the materials used as bottom and top electrodes in the heterostructures. The temperature dependence of the dielectric permittivity for the SrTiO3 layer in (100)SrRuO3∥(100)SrTiO3∥(100)SrRuO3 was well extrapolated by a Curie-Weiss relation in the range of T=80-300K, with about the same Curie constant (C0=7.5 × 104 K) and Curie temperature (TCurie=21K) as in a bulk single crystal. At temperatures higher the phase transition point (65 K), the electric field response of the permittivity of the SrTiO3 layer between high-TC superconducting or metallic oxide electrodes was well extrapolated by the same relation used for a bulk single crystal. The smallest loss factor, tanδ, was measured for the capacitance (100)SrRuO3∥(100)SrTiO3∥(100)SrRuO3 (T ≈ 50-300K, f=100kHz). The measured conductance G for the SrTiO3 layer in the (001)(Y,Nd)Ba2Cu3O7-δ heterostructure fitted well the relation InG∼-(ED/kT), with ED=0.08-0.09 eV in a temperature range close to 300K. Pronounced hysteresis was observed in the temperature dependence of the dielectric permittivity for the (100)Ba0.8Sr0.2TiO3 layer at temperatures close to the phase transition point, like in the case of a bulk single crystal. The permittivity of the (100)Ba0.8Sr0.2TiO3 layer decreased more than 50% when an electric field of 2.5×106V/m (T ≈ 300K, f=100 kHz ) was applied.

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 μm thick was formed on the MgO substrate. Capacitance and dissipation factor (tanδ) at 1 kHz–100 mV were 32 nF and 1.5% respectively. Capacitance per unit volume of 33 μF/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 MΩ-μF. MOCVD was proposed as one of the promising manufacturing technologies for multilayer ceramic capacitors of high performance with sub-micron thick dielectric layers.

Control of structural, electrical properties of (Ba,Sr)TiO3/RuO2 thin films by the application of amorphous (Ba,Sr)TiO3 layer

The structural, electrical properties of (Ba,Sr)TiO3[BSTO]/RuO2 thin films were controlled by the addition of amorphous BSTO layers between crystalline BSTO film and RuO2 substrate. We prepared BSTO films with double-layered structure, that is, amorphous layers deposited at 60 °C and crystalline films. Crystalline films were prepared at 550 °C on amorphous BSTO layers. The thickness of amorphous layers was varied from 0 to 170 nm. During the deposition of crystalline films, the crystallization of amorphous layers occurred and the structure was changed to circular while crystalline BSTO films showed columnar structure. Due to insufficient annealing effect, amorphous BSTO phase was observed when amorphous layers exceeded 30 nm. Amorphous BSTO layer could also prevent the formation of oxygen deficient region in RuO2 surface. Leakage current of total BSTO films decreased with increasing amorphous layer thickness due to structural modifications. Dielectric constant showed maximum value of 343 when amorphous layer thickness was 30 nm at which the improvement by grain growth and the degradation by amorphous phase were balanced.

Control of Interface Migration in Melt‐Infiltrated Niobium‐Doped Strontium Titanate by Solute Species and Atmosphere

The solid/liquid interface migration of niobium‐doped strontium titanate (SrTiO3) has been investigated by using barium and calcium solute sources in the form of a Cu‐Ba‐Ca‐O liquid. The solubilities of barium and calcium in SrTiO3 that is in equilibrium with Cu‐Ba‐Ca‐O melts can be determined by measuring the solute concentration in the migrated Sr(Ba,Ca)TiO3 regions of SrTiO3 grains. No interface migration is observed for a certain composition range of Cu‐Ba‐Ca‐O infiltrant. For SrTiO3 specimens that have been sintered in air, the zero driving force for the interface migration is found at a Ba:Ca ratio of ∼0.75 in Sr(Ba,Ca)TiO3 solid solution. This value is in good agreement with a predicted value of zero coherency strain in a thin diffusional Sr(Ba,Ca)TiO3 layer on bulk SrTiO3. For the specimens that have been sintered in 5H2‐95N2, however, a zero driving force results when the ratio is ∼0.84. The difference between the value for the specimens sintered in air and that for the specimens sintered in 5H2‐95N2 is attributed to the change in defect concentration via the change in atmosphere. The estimated difference in the lattice parameters of SrTiO3 sintered in air and those of SrTiO3 sintered in 5H2‐95N2 is ∼1.8 10−3Å (∼1.8 10−2 nm). The difference in lattice parameters has been further confirmed by using synchrotron X‐ray scattering experiment, which has revealed the lattice parameter to be 3.9053 Å (0.39053 nm) in air and 3.9070 Å (0.39070 nm) in 5H2‐95N2.

Dielectric properties of strained (Sr,Ca)TiO3/(Ba,Sr)TiO3 artificial lattices

Dielectric superlattices of (Sr,Ca)TiO3/(Ba,Sr)TiO3 have been formed by a pulsed laser ablation technique. The crystal structure is controlled with atomic order accuracy and a large lattice stress of 0.5–1 GPa can be introduced periodically at the interfaces owing to the lattice mismatch of the constituent layers. The (Sr0.3Ba0.7)TiO3/(Sr0.48Ca0.52)TiO3 superlattice shows dramatically large dielectric constant of 900 even at film thickness of 500 Å. The optimum pressure for inducing tetragonality in the (Ba,Sr)TiO3 layers and enhanced dielectric properties occurs at a lattice mismatch of 2.5–3%.

Patterning of dielectric oxide thin layers by microcontact printing of self-assembled monolayers

This communication describes a technique used to pattern oxide thin layers using microcontact printing (μCP) and sol-gel deposition. The technique involves μCP of self-assembled monolayers (SAM's) of alkylsiloxane on various substrates (SiO2/Si, sapphire, ITO, and glass), followed by deposition of oxide thin layers from sol-gel precursors. Delamination of oxide layers from SAM-derivatized regions allows selective deposition of crystalline dielectric oxide layers on underivatized regions. To demonstrate the viability of this technique for integrated microelectronics and optics applications, patterned (Pb,La)TiO3 (PLT) and LiNbO3 layers were deposited on sapphire, silicon, and indium tin oxide (ITO) substrates. Use of lattice-matched substrates allows lithography-free deposition of patterned heteroepitaxial oxide layers. Strip waveguides of heteroepitaxial LiNbO3 with 4 μm lateral dimensions were fabricated on sapphire. Dielectric measurements for patterned PLT thin layers on ITO are also reported.

Oxide superconductors and ferroelectrics?Materials for a new generation of tunable microwave devices

We describe materials deposition and characterization for a broad class of tunable microwave devices using high-temperature oxide superconductor and voltage-tunable oxide ferroelectric thin-film multilayer structures. Tl-Ba-Ca-Cu-O thin films deposited by sputtering ore-beam evaporation were patterned into microwave resonators, each consisting of two colinear microstrip line sections separated by a 5–20μm gap. A Ba0.1Sr0.9TiO3 (BST) layer was then over-coated to fill the gap. The relative dielectric constant of the BST films deposited by physical vapor or chemical techniques was measured at 77 K in the 1–10 MHz range and found to vary by up to a factor of 2 or more with voltage bias in test capacitor structures using Pt electrodes. In the BST (variable capacitor)-HTSC microwave resonator structures, the change of the relative dielectric constant of the BST under voltage bias has allowed variation of the fundamental frequency of up to 80 MHz in the 5–10 GHz range at 4 K. Film deposition by various techniques and associated structural, morphological, and electronic properties, as well as materials compatibility issues, are discussed.