Characterization of Ni/Ce Co-Doped BST Thin Film Grown on LSCO/Pt Electrodes Prepared by Pulsed Laser Deposition

Perovskite (Ba_0.6Sr_0.4)TiO₃ (BST) thin films doped with Ni, Ce and Ni/Ce codopants were prepared on LSCO/Pt/SiO₂/Si substrates by pulsed laser deposition method. In this study, La_0.5Sr_0.5CoO₃ (LSCO) bottom electrodes were used to improve the crystallinity and dielectric properties of BST films. Single ion doped(1 mol%Ni doped,1 mol%Ce doped) BST films showed more improved crystallinity, smoother surface, and smaller grain size than that with 1 mol%Ni/1 mol%Ce. The dielectric constant and loss of Ni/Ce co-doped BST films were about 298 and 1.8%, respectively. In addition, tunability and figure of merit of co-doped BST films showed minimum values of approximately 9.3% and 5, respectively. With 1% Ni-doped BST thin films, results gave a tunability of 54.2% and a loss tangent of 1.8% while a figure of merit was 30. Correlation of the material properties with dielectric and tunable properties suggests the 1 mol%Ni-doped BST films are effective potential candidate for tunable device applications.     

Frequency and Voltage Dependent Dielectric Properties of Ni-doped Ba0.6Sr0.4TiO3 Thin Films

Highly (100) preferred undoped and 1–5% Ni-doped Ba_1–xSr_xTiO₃ (BST) thin films were deposited onto MgO (100) single crystal substrate at 750°C using pulsed laser deposition. BST thin film-based interdigital capacitors (IDC) were prepared by standard photolithography process. The microwave properties of BST films were measured at 10 GHz. Ni-doped BST films showed better dielectric properties by exhibiting improved dielectric Q while retaining an appropriate capacitance tuning compared to undoped BST films. 1% Ni-doped BST film showed the maximum figure of merit of 2896.1. It is suggested that 1 mol% Ni doped BST film is an effective candidate for high performance tunable device applications.     

Comparative Characteristics of Na0.5K0.5NbO3 Films on Pt by Pulsed Laser Deposition and Magnetron Sputtering

Ferroelectric Na_0.5K_0.5NbO₃ (NKN) thin films were grown on the Pt₈₀Ir₂₀ polycrystalline substrates by pulsed laser deposition (PLD) and radio frequency-magnetron sputtering (RF) technique using the same stoichiometric Na_0.5K_0.5NbO₃ ceramic target. X-ray diffraction proved both PLD- and RF-made Na_0.5K_0.5NbO₃/Pt₈₀Ir₂₀ films are single phase and have preferential c-axis orientation. Temperature dependence of dielectric permittivity reveals the presence of two phase transitions around 210 and 410°C. Capacitance vs. applied voltage C-V @ 100 kHz, I-V, and P-E hysteresis characteristics recorded for the vertical capacitive structures yielded loss tanδ = 0.026 and 0.016, tunability about 44.5 and 30% @ 100 kV/cm, Ohmic resistivity 6.7 × 10¹² Ω·cm and 0.2 × 10¹² Ω·cm, remnant polarization 11.7 and 9.7 μC/cm², coercive field 28.0 and 94.6 kV/cm for PLD- and RF-films, respectively. Piezoelectric test carried out in hydrostatic conditions showed piezoelectric coefficient d _H = 21 for PLD-NKN and 15 pC/N for RF-NKN film.     

Epitaxial Ba0.5Sr0.5TiO3 thin films for microwave phase shifters

Abstract

A tunable phase shifter was fabricated with epitaxial Ba_0.5Sr_0.5TiO₃ (BST) thin film and gold coplanar waveguide. BST thin film of the thickness ～0.5 μm was deposited by laser ablation on the MgO(OOl) single crystalline substrate. Gold electrode of the thickness ～2 μm was prepared by the sequence of thermal evaporation, electroplating, and wet etching. Epitaxial quality of the BST thin film was confirmed by X-ray diffraction. The microwave performance of phase shifter was measured at room temperature in the frequency range of 8–12 GHz, and with applied bias voltage of up to 30 V. Effect of Mn dopant in the epitaxial films was also considered.     

Design and implementation of combline bandpass tunable filter using barium-strontium-titanate thin-film capacitors

The design, fabrication and microwave properties of tunable fifth-order combline bandpass filter using etched barium-strontium-titanate (BST) thin films on sapphire (0001) substrates were investigated. At 1 MHz and 1000 kV/cm electric field, the dielectric tunability, the remanent polarization (2P_r) and the coercive electric field (2E_C) of BST films were 45.96%, 2.26 µC/cm² and 81.83 kV/cm, respectively. The loss tangent was 1.36% at zero electric field. After the BST parallel plate capacitors characterization, BST capacitors were loaded at the end of parallel coupled resonators in the design of the tunable filter. With the application of 20 V DC voltage, the center frequency of the filter varied from 1.17 GHz to 1.34 GHz which corresponds to a relative shift of 13.5%.     

Rf Sputtered Na0.5K0.5NbO3 Films on Oxide Substrates as Optical Waveguiding Material

Highly crystalline Na_0.5K_0.5NbO₃ (NKN) thin films of 1–2 μm thickness were deposited by rf-magnetron sputtering of a stoichiometric, ceramic target on single crystal LaAlO₃(001) and Al₂O₃(0112) substrates. X-ray diffraction measurements revealed epitaxial quality of NKN/LaAlO₃ film structures, whereas NKN films on sapphire substrates were found to be preferentially c-axis oriented. A prism-coupling technique was used to characterize optical and waveguiding properties. A bright-line spectrum at λ = 632.8 nm, revealed sharp peaks, corresponding to transverse magnetic (TM) and electric (TE) waveguide propagation modes in NKN/LaAlO₃ and NKN/Al₂O₃ thin films. Using a least mean square fit the refractive index for the films and film thickness were calculated. The extraordinary and ordinary refractive indices were determined to n _e = 2.207 ± 0.002 and n _o = 2.261 ± 0.002, and n _e = 2.216 ± 0.002 and n _o = 2.247 ± 0.002 at λ = 632.8 nm for 2.0 μm thick NKN films on LaAlO₃ and Al₂O₃, respectively. This corresponds to a birefringence Δn = n _e ? n _o = ?0.054 ± 0.003 and Δn = ?0.031 ± 0.003 in the films, where the larger Δn for the NKN/LaAlO₃ structure can be explained by the superior crystalline quality compared to NKN/Al₂O₃. Atomic force microscopy images of the film surfaces revealed rms roughnesses of 2.5 nm and 8.0 nm for 1.0-μm thick NKN/LaAlO₃ and NKN/Al₂O₃ films, respectively. We believe surface scattering is one of the main sources of waveguide losses in the thin films.     

Microwave Characterization of BST Thin Films on LAO Interdigital Capacitor

The dielectric properties of (Ba_0.5Sr_0.5)TiO₃ (BST) thin films on LaAlO₃ (LAO) single crystals were studied over a wide frequency range. The samples with interdigital electrodes were prepared by microelectronic processing. The dielectric characterizations were carried out in the following steps: 1) the standard calibration of the instrument, 2) the removal of parasitic capacitance and the extraction of the capacitance of the interdigital capacitor (IDC) and 3) the extraction of the dielectric permittivity (?) of BST. It was found that ? of BST has a constant value (about 400) from 50 MHz to 2 GHz; at higher frequencies, ? gradually decreases. The BST interdigital capacitors exhibited good dielectric tunability.     

Structure and dielectric properties of Ca-doped (Ba0.7Sr0.3) TiO3 thin films fabricated by sol–gel method

A wide range of Ca-doped (Ba_0.7Sr_0.3)TiO₃ (BST) thin films (from 0 to 20 mol%) have been prepared on Pt/Ti/SiO₂/Si (100) substrates by sol–gel technique. The structural and dielectric properties of BST thin films were investigated as a function of Ca dopant concentration. The results showed that the microstructure and dielectric properties of the BST films were strongly dependent on the Ca contents. With the Ca dopant concentration increasing, the grain size, dielectric constant and dielectric loss of the BST thin films decreased. As the content of Ca dopant reaches 10 mol%, the dielectric constant, dielectric loss, tunability, the value of FOM and the leakage current density are 281, 0.0136, 16.7%, 12.3 and 5.5?×?10^?6 A/cm², respectively.     

CHARACTERIZATIONS OF W-DOPED (Ba0.5Sr0.5)TiO3 THIN FILMS USING LaNiO3 ELECTRODE

ABSTRACT

The (Ba_0.5Sr_0.5)TiO₃ thin films were deposited onto LaNiO₃ by RF-magnetron sputtering at 550°C. The influence of W content on microstructure and electrical properties of BST films were investigated. The surface and grain size become smoother and smaller as the W content increased. Besides, the dielectric constant, tunability, dissipation factor, and leakage current decreased when the W content increased. The 1% W is the optimal doping concentration. The resultant BST film, with proper dielectric constant and leakage current, has a tunability of 32%, a dissipation factor of 0.006, a FOM value of 53.3 under applied field of 450 kV/cm.     

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.     

PREPARATION AND PROPERTIES OF LITHIUM-DOPED K0.5Na0.5NbO3 THIN FILMS BY CHEMICAL SOLUTION DEPOSITION

ABSTRACT

Lithium-doped K_0.5Na_0.5NbO₃ (KLNN) films were fabricated by chemical solution deposition on Pt/TiO₂/SiO₂/Si substrates. Homogeneous and stable precursor solutions were prepared by controlling the reaction of starting metal alkoxides. Perovskite KLNN single-phase thin films were successfully synthesized on Pt/TiO _x /SiO₂/Si substrates. The 0.75-μ m-thick KLNN film annealed at 650°C exhibited ferroelectric polarization hysteresis loops at ?250°C. The loop at room temperature was round, indicating the film contained leakage components. The dielectric constant under zero bias was 490 at room temperature. A typical upside-down butterfly DC bias-capacitance curve was obtained in the KLNN film capacitors at room temperature, indicating that polarization reversal occurred in the obtained KLNN films.     

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.     

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.     

Ferroelectric Properties of (Bi, Sm)4Ti3O12 (BST) Thin Films Fabricated by a Metalorganic Solution Deposition Method

Ferroelectric properties of samarium substituted Bi₄Ti₃O₁₂ films, Bi_3.15Sm_0.85Ti₃O₁₂ (BST), were evaluated for use as lead-free thin film ferroelectrics for FeRAM applications. The BST films were fabricated on the Pt/Ti/SiO₂/Si(100) substrates by a metalorganic solution deposition method. The measured XRD patterns revealed that the BST films showed only a Bi₄Ti₃O₁₂-type phase with a random orientation. The BST film capacitors showed excellent ferroelectric properties. For the film capacitor annealed at 700C, 2P_r of 64.2 C/cm² and 2E_c of 101.7 kV/cm at applied electric field of 150 kV/cm were observed. The capacitor did not show any significant fatigue up to 1.5 × 10⁸ read/write switching cycles at a frequency of 1 MHz, which suggests that the samarium should be considered for a promising lanthanide elements to make a good thin ferroelectric film for memory applications.     

Magnetron sputter-deposited multilayer (BaxSr1x)Ti1+yO3+z thin films for passive and active devices

Abstract

High permittivity (Ba_xSr_1?x)Ti_1+yO_3+z(BST) thin films are being investigated for integration into charge storage dielectrics and electric-field tunable elements for high frequency devices. For the latter application, it is desirable to have BST capacitors with high tunability and low losses. Therefore, we investigated the use of multilayer BST thin films consisting of very low dielectric loss BST/electrode interfacial layers ((Ba+Sr)/Ti = 0.73) sandwiching a high tunability, high permittivity primary BST layer ((Ba+Sr)/Ti = 0.9). BST capacitors with multiple layers of controlled composition can be effectively produced insitu by magnetron sputter deposition, using a single stoichiometric target and controlling the layer composition by changing the total process gas (Ar+O<₂) pressure. The layered BST film capacitors exhibit simultaneous low loss (tan Δ = 0.005), high tunability (76%), high charge storage energy density (34 J/cm³), low leakage, and high dielectric breakdown (>2.8 MV/cm).     

Contribution of dielectric and metallic losses in RF/microwave tunable varactors using (Ba,Sr)TiO3 thin films

Abstract

Recently, there has been significant interest in use of (Ba,Sr)TiO₃ (BST) thin films for tunable high frequency (RF and microwave) components. In a previous work we have shown that BST thin films grown by metalorganic chemical vapor deposition (MOCVD) exhibit films very low losses (as low as 0.003–0.004) and tunabilities over 50% at low operation voltages.

In order to integrate BST thin films in tunable devices, the objectives of this work are : (i) study the effect of bottom and top electrode on the performance of thin film based capacitors, (ii) correlate low frequency (10 kHz) and high frequency (45 MHz - 1 GHz) measurements, (iii) separate the contribution of dielectric losses and metallic losses at both low and high frequency and finally (iv) show the potential usefulness of series capacitors structures.     

Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films from DC to Ka Band

Much of the interest in Ba_0.5Sr_0.5TiO₃ (BST) thin films has focused on DRAM applications. For this application, the most studied characteristics have been the dielectric constant and the leakage current, both of which are usually measured either at DC or at low frequencies. A few studies have made use of impedance measurements in the 100 Hz to 100 MHz range. Unfortunately, all these measurements fall far short of current DRAM speeds, which are quickly approaching 1 GHz. Other technologies, such as digital computing and wireless communications are already well into the microwave range. The importance of microwave characterization of BST films is therefore clear. Such a characterization technique is described and results from DC to Ka band are presented.     

The resistance degradation of (Ba0.5Sr0.5)TiO3 thin films

Abstract

The leakage current and dielectric properties of (Ba_0.5Sr_0.5)TiO₃(BST) thin films prepared by pulsed laser deposition (PLD) were investigated. It was found that leakage currents for positive bias voltage were higher than that for negative bias voltage, which was attributed to the lattice mismatch between bottom Pt electrode and BST thin film. The time-dependent breakdown process under positive voltage was observed, which was interpreted as the increase of the internal electric field in the film near the bottom electrode. However, the internal electric field can be decreased and eventually recovered by applying negative bias voltage. It was found that internal electric field near the interface can influence the capacitance of the BST thin film capacitor. An explanation for the thickness effect of BST thin films was given.     

Planar and Parallel Dielectric Properties of Compositionally Graded (Ba,Sr)TiO3 Thin Films for Tunable Microwave Applications

Compositionally graded (Ba_x,Sr_{1 ? x})TiO₃ [BST] ferroelectric thin films have been received much attention in graded ferroelectric devices due to their unique properties, such as large pyroelectric coefficients, large polarization offset and small temperature coefficient of dielectric constant for microwave tunable devices. Compositionally graded BST thin films were deposited epitaxially on LaAlO₃ [LAO] and Nb-doped SrTiO₃ [STO:Nb] substrates by pulsed laser deposition. The planar and parallel dielectric properties of compositionally graded BST epitaxial thin films ware investigated in the frequency ranges of 100 Hz ～ 1 MHz as a function of the direction of the composition gradient with respect to the substrate at room temperature. The dielectric properties of the graded BST films depended strongly on the direction of the composition gradient with respect to the substrate. The graded ST → BT films grown on LAO and STO:Nb substrates exhibited a excellent dielectric properties than the graded BT → ST films.     

Annealing Effects on Structural and Dielectric Properties of Tunable BZT Thin Films

Ba(Zr, Ti)O₃ thin films have attracted great attention in recent years for their potential use in DRAMs and MCMs due to their high dielectric constant and relatively low leakage current. However, their tunable dielectric properties were rarely investigated and the corresponding potential for tunable microwave applications was seldom reported. In this paper, we present the tunable dielectric behavior of BZT thin films deposited by RF magnetron sputtering from a Ba(Zr_0.3Ti_0.7)O₃ ceramic target on MgO single crystal substrates. The composition, thickness and crystallinity of the thin films were analyzed by Rutherford backscattering (RBS), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The dielectric constant and loss tangent were measured as a function of electric field (0–7 kV/mm) and temperature (–140 to +160°C) at frequencies up to 1 MHz, using interdigital capacitors (IDC) with Au electrodes on thin films. By optimizing the preparation process, a tunability {defined as = [ (0) – (E_max)]/ (0)} of 76% at E_max = 7 kV/mm and a low loss tangent of 0.0078 can be achieved. In addition, the influence of annealing temperature on the dielectric properties of the thin films is also discussed.