Na0.5K0.5NbO3 Film Microwave Varactors

Na_0.5K_0.5NbO₃ (NKN) and Pb(Zr_0.53Ti_0.47)O₃ (PZT) films have been grown by rf-magnetron sputtering and pulsed laser deposition techniques, correspondingly, on sapphire (Al₂O₃-0112, r-cut), quartz (Y+36°-cut) and YAlO₃ + 1% Nd (Nd:YAlO₃-001) single crystal substrates with Interdigital Capacitor (IDC) of Coplanar Waveguide (CPW) structure. Photolithography and metal lift-off technique was used for processing of the tunable microwave capacitor. Microwave network analyzer with G-S-G Picoprobe and probe station performed microwave measurement with external DC bias. NKN film interdigital capacitors on Nd:YAlO₃ show superior performance in the microwave range from 1 to 40 GHz. Within this range, the voltage tunability (40 V, 200 kV/cm) was about 29%, loss tangent ～ 0.13, K-factor from 152% @10 GHz to 46% @40 GHz, voltage independent C _p was about 230 fF, tan δ _p changes from 0.14 @10 GHz to 0.36 @40 GHz, real and imaginary part of interconnect impedance increases with frequency from 0.13 Ω @10 GHz to 0.50 Ω @40 GHz and from 1.9 Ω @10 GHz to 5.9 Ω @40 GHz respectively.     

Ferroelectric Na0.5K0.5NbO3 thin films by pulsed laser deposition

Abstract

Highly c-axis oriented single phase Na_0.5K_0.5NbO₃ (NKN) thin films have been deposited onto polycrystalline Pt₈₀lr₂₀ substrates and SiO₂/Si(001) wafers using pulsed laser ablation of stoichiometric ceramic target. Strong self-assembling of NKN films along the [001] direction has been observed. Properties of NKN/Pt thin film structures have been successfully tailored by oxygen pressure control from the ferroelectric state, characterized by the remnant polarization of 12 uC/cm², dielectric constant ? ～ 520 and tan δ ～ 0.024 @ 100 kHz, to superparaelectric state with tan δ as low as 0.003 and ? = 210 with very small 1.7% dispersion in the frequency domain 0.4–100 kHz and less than 10% variation in the temperature range 77–415 K. NKN films grown onto SiO₂/Si(001) substrates show quadrupled super-lattice structure along c-axis, loss tan δ less than 0.01, and ? ～ 110 @ 1 MHz. C-V measurements for Au/NKN (270nm)/SiO₂/Si MFIS-diode structure yield memory window of 3.26 V at the programmable voltage of 8 V.     

Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract

Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002–0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 °C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO₂/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1–0.2 urn. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO₂ interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 μm. Devices with dielectric Q factors over 150 at 100 MHz (tan δ ～ 0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.     

Dielectric characteristic of nanocrystalline Na0.5K0.5NbO3 ceramic green body

Dielectric spectroscopy was applied to porous nanocrystalline Na_0.5K_0.5NbO₃ (NKN) ceramic green body, wherein influences of percolation effect and water adsorption at pore surface of the ceramic green body on dielectric response were examined over wide temperature (150 to 450?K) and frequency (100?Hz to 1?MHz) ranges. Dielectric permittivity of the ceramic green body is about 2–3 orders of magnitude higher than that of pure NKN powder or NKN ceramic. Furthermore, the high dielectric permittivity and high humidity sensitivity of the ceramic green body can appear again with aging a period of time in air. The data from this investigation make potential applications for NKN as a giant dielectric material or a humidity sensing material.     

Microwave Dielectric Properties of Ferroelectric Pb(Zr1 − x Ti x )O3 Thin Films Using Interdigital Capacitors

Ferroelectric Pb(Zr_{1 ? x} Ti _x )O₃ (PZT) films were deposited on (001) MgO single crystals using sol-gel method. Structural properties and surface morphologies of PZT films were investigated using an X-ray diffractometer and a scanning electron microscopy, respectively. The dielectric properties of PZT films were investigated with the dc bias field of 0–135 kV/cm using interdigitated capacitors (IDC) which were fabricated on PZT films using a thick metal layer by photolithography and etching process. The small signal dielectric properties of PZT films were calculated by a modified conformal mapping method with low and high frequency data, such as capacitance measured by an impedance gain/phase analyzer at 100 kHz and reflection coefficient (S-parameter) measured by a HP 8510C vector network analyzer at 1–20 GHz. The IDC on PZT films exhibited about 67% of capacitance change with an electric field of 135 kV/cm at 10 GHz. These PZT thin films can be applied to tunable microwave devices such as phase shifters, tunable resonators and tunable filters.     

Epitaxial thin film heterostructures of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3

Abstract

We have grown stoichiometric pure perovskite phase Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) thin films and PMN-PT/SrRuO₃ heterostructures on miscut (100) SrTiO₃ substrates by pulsed laser deposition. X-ray diffraction θ–2θ scans show that the PMN-PT films are purely c-axis oriented. The off-axis Φ scans show that the heterostructures grow “cube-on-cube” with an in-plane epitaxial arrangement of PMN-PT[100], [010] // SrRuO₃[001] // SrTiO₃[100] and PMN-PT[010], [100] // SrRuO₃[110] // SrTiO₃[010]. The crystalline quality of the films found to be comparable to that of bulk single crystals. The AFM images show that the SrRuO₃ and PMN-PT layers have smooth surfaces with root mean square roughness of 9Å. These epitaxial heterostructures can be used for the fabrication of piezoelectric devices.     

SYNTHESIS AND CHARACTERIZATION OF SB-SUBSTITUTED (K0.5Na0.5)NbO3 PIEZOELECTRIC CERAMICS

ABSTRACT

Lead-free piezoelectric ceramics (K_0.5Na_0.5)(Nb_1-xSb_x)O₃+0.5 mol.%MnO₂, where x = 0 ÷ 0.10, with single phase structure and rhombohedral symmetry at room temperature were prepared by conventional ceramic technology. The optimal sintering temperatures of compositions were within 1100°–1140°C. MnO₂ functions as a sintering aid and effectively improves the densification. The samples reached density from 4.26 g/cm³ for undoped (K_0.5Na_0.5)NbO₃ to 4.40 g/cm³ for Mn/Sb⁵⁺ co-doped ceramics. The co-effects of MnO₂ doping and Sb⁵⁺ substitution lead to significant improvement in dielectric and piezoelectric properties: ε at the T_c increased from 6000 (KNN) to 12400 (x = 0.04), d₃₃ = 92 ÷ 192 pC/N, k_p = 0.32 ÷ 0.46, k_t = 0.34 ÷ 0.48.     

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.     

Temperature dependent electrical properties of 0.95[(K0.5Na0.5)(1-x)Ag x NbO3]-0.05LiSbO3 ceramics

Lead free 0.95[(K_0.5Na_0.5)_1-x Ag _x NbO₃]-0.05LiSbO₃ (KNAN-LS) ceramics with x?=?0.02, 0.04, 0.06 and 0.08 have been synthesized by conventional solid state reaction route (CSSR). XRD analysis confirmed the presence of a mixed structure for x?=?0.06. The orthorhombic?Ctetragonal polymorphic phase transition (PPT) temperature and the Curie temperature (T_c) decreased with the increase in Ag⁺ ion content in KNAN-LS ceramics. The relationship between the PPT of the ceramics and the temperature dependence of electrical properties of KNAN-LS ceramics were discussed in detail. The KNAN-LS ceramics with x?=?0.06 showed better piezoelectric and electromechanical properties (d₃₃?=?227pC/N and k_p?=?42.5?%).     

Origin of giant dielectric response in ferroelectric thin film multilayers

Abstract

A thermodynamic theory of dielectric response in ferroelectric thin film multilayers is developed. The solution of Lame equation for static dielectric susceptibility has shown that susceptibility diverges at the transition temperature of the thickness induced ferroelectric phase. This divergence is shown to be the origin of the giant dielectric response observed in some multilayers. The theory gives an excellent fit to the temperature dependence of the giant susceptibility observed recently in multilayers of PbTiO₃-Pb_1-xLa_xTiO₃ (x = 0.28).     

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.     

Preparation,characterisation, and electrical properties of (K0.5Na0.5)NbO3 lead-free piezoelectric ceramics

Na_0.5?K_0.5NbO₃ (KNN) ceramics were sintered at different temperatures (970 °C, 1000 °C, 1030 °C, 1060 °C, and 1090 °C) for 3 h by a pressureless sintering method. The powders had been synthesised by sol–gel method, using citric acid as a coordination agent and ethylene glycol as an esterifying agent. The effects of temperature on the phase, microstructure, dielectric, ferroelectric, and piezoelectric properties of the as-prepared ceramics were analysed. The results revealed that all of the ceramics had a pure perovskite phase with orthorhombic symmetry. The piezoelectric constant (d ₃₃), the relative dielectric constant (ε _r), the planar electromechanical coupling coefficient (K _p), and the remnant polarization (P _r) initially increased and then decreased with increasing of temperature in such KNN ceramics. The volatilization of sodium and potassium increased with increasing sintering temperature. Over the range of temperatures studied, those ceramics sintered at 1060 °C had the following optimal properties: (ρ?=?3.97 g/cm³, d ₃₃?=?119 pC/N, ε _r?=?362.46, tan δ?=?0.05, K _p?=?0.23, P _r?=?11.97 μC/cm², E _c?=?10.35 kV/cm, and T _c?=?408 °C).     

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.     

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.     

Preparation and Ferroelectric Properties of K0.5Na0.5NbO3 Thin Films Derived from Non-alcohol Niobium Salt Sol-gel Process

Lead-free K_0.5Na_0.5NbO₃ (KNN) thin films were prepared on Pt (111) /Ti/SiO₂/Si (100) substrates by a polyvinylpyrrolidone (PVP)-modified sol-gel method without any metal alkoxides. The effects of PVP on the crystallization, surface morphology and electrical properties were investigated. It was found that the introduction of PVP into the sol could reduce the annealing temperature, enhance the surface quality and improve the KNN film of (100) oriented growth. Compared with the pure non-PVP-modified KNN thin films, the dielectric and ferroelectric properties of the KNN films were significantly enhanced by adding PVP into the sol. In particular, the PVP-modified KNN films which were annealed at 550°C exhibited relatively saturated polarization-electric field (P-E) hysteresis loops with high remnant polarization P_r of 22 μC/cm², dielectric constant of 280 and dielectric loss of 0.09, respectively, indicating promising lead-free piezoelectric film candidate for future applications.     

Electro-Optic Properties of Heteroepitaxial Pb(Zr,Ti)O3/La0.5Sr0.5CoO3 Film Structures

Vertical ferroelectric Pb(Zr,Ti)O₃ (PZT) 1 μm thick film capacitor was fabricated by pulsed laser deposition technique (PLD) onto conducting La_0.5Sr_0.5CoO₃(LSCO) 100 nm thick bottom electrode on both side polished YAlO₃ + 1% Nd₂O₃ (Nd:YAlO₃) single crystal substrate to operate as a Pockels cell optical modulator. On top of the PZT film, semitransparent 30 nm thick Au electrode was deposited by thermal evaporation. Intensity of the chopped 670 nm polarized laser radiation transmitted through the Au/PZT/LSCO/Nd:YAlO₃ cell was measured at various temperatures and bias voltage applied. Applying 20 V (200 kV/cm) across the capacitive cell, modulation of the transmitted light as high as 3% was achieved while the voltage tunability measured at 1 kHz from C-V characteristics was about 70%. Thermo-optical measurements performed for PZT/Nd:YAlO₃ sample in the range up to 400°C showed the phenomenon of critical opalescence in the vicinity of Curie temperature at 208°C. Optical transmission through the PZT film biased with electric field was studied in the range 400 to 1000 nm. Film thickness, refraction index and absorption coefficient have been determined from the interference pattern observed in the PZT transmission spectrum. A simple model yields the dispersion relation for the electro-optic coefficient.     

基于薄膜工艺的C频段交指滤波器小型化设计

为满足卫星通信某滤波器小型化需求,基于薄膜工艺设计了一款陶瓷基板的C频段交指滤波器。该滤波器由5阶平行耦合的谐振单元构成,输入输出形式采用抽头结构,相比于平行耦合输入输出更有利于实现滤波器的小型化。为了满足高抑制度的指标,该滤波器采用交叉耦合与并联1/4波长开路线的方式引入传输零点,有效的提高了滤波器的带外抑制度。同时采用了阻抗补偿的方式对滤波器驻波进行调节。测试结果显示,滤波器中心频率为3.7GHz,相对带宽为27%,通带插损为-1dB,在2.7GHz与4.8GHz处阻带抑制30dB,在2.5GHz与5GHz处阻带抑制40dB。滤波器尺寸为9.25m6.72mm。     

Deposition and characterization of low-loss epitaxial non-linear dielectric thin films for microwave devices

Abstract

Oxide ferroelectric thin films for frequency-tunable microwave devices, in which the dielectric constant of the non-linear dielectric is varied by application of electric fields, have been deposited using PLD. We have fabricated single phase epitaxial Ba_0.6Sr_0.4TiO₃ and KTaO₃ thin film capacitors for applications at 300K and 77K, respectively. Single phase KTaO₃ films were obtained only with excess potassium source in the target along with KTaO₃ perovskite phase. The films have been characterized for structure and morphology by X-ray diffraction and AFM. The dielectric properties were measured in the low frequency range from 100 kHz to 10 MHz, using interdigitated capacitors. Low loss tangents (0.002 at 300K) were observed for highly oriented Ba_0.6Sr_0.4TiO₃ films. The importance of low losses for various devices is discussed and the dielectric constants, loss tangents and tunability of these films are reported in this paper.     

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.     

Pulse width dependent polarizations of ferroelectric (Pb,La)(Zr,Ti)O3 thin film capacitors

Abstract

We report the measurements of pulse polarizations of (Pb, La)(Zr, Ti)O₃ ferroelectric capacitors as a function of pulse width. Pulse polarizations were measured from the switching current responses in the pulse width range from 1 μs to 1 s. The relation between pulse polarizations(switched(P?) and non-switched polarization(P?)) and hysteresis loop parameters was studied. In the case of all write/read pulse widths are same, P? increased but P? decreased with increasing pulse width. These results are explained by poling effects and confirmed by measuring pulse width dependencies with different patterns of pulse trains.