K0.5Na0.5NbO3 Films Research Articles

Fundamental Issues in the Synthesis of Ferroelectric Na0.5K0.5NbO3 Thin Films by Sol−Gel Processing

In contrast to films fabricated by physical vapor deposition methods, thin films of Na0.5K0.5NbO3 (NKN) made by metal alkoxide sol−gel routes generally fail to exhibit polarization-electric field responses typical of a ferroelectric. This study sets out to investigate the reasons for the problems in producing sol−gel NKN films by examining the thermochemistry of the gel to ceramic conversion. The NKN precursor gels displayed multiple heating DTA crystallization exotherms in the temperature range 480−550 °C, which are attributed to compositional segregation of NKN components. At higher temperatures, 800−850 °C, a heating DTA endotherm and cooling DTA exotherm are indicative of melting and recrystallization of sodium/potassium carbonate secondary phases. Additionally, repeated thermal analyses, after storage of the gel decomposition product under ambient conditions, revealed a tendency for hydration and carbonation on exposure to air. Together, these are critically limiting reasons for the absence of strong...

Volatilization of alkali ions and effects of molecular weight of polyvinylpyrrolidone introduced in solution-derived ferroelectric K0.5Na0.5NbO3 films

Ferroelectric K0.5Na0.5NbO3 (KNN) thin films were prepared by a chemical solution deposition approach with polyvinylpyrrolidone (PVP) of different molecular weights introduced in the precursor solutions. The volatilization of the alkali ions and the effects of the molecular weight of PVP were examined with x-ray diffraction (XRD), thermal analysis, mass spectrometry, and x-ray photoelectron spectroscopy (XPS). The results clearly showed that the volatilization of the alkali ions mainly happened at moderate temperatures before the crystallization of the KNN perovskite phase. Loss of Na was more significant than K ions during the heating process of KNN. The introduction of PVP with the appropriate molecular weight could effectively promote the crystallization of the KNN perovskite phase at reduced temperature and substantially suppress the loss of the alkali ions before crystallization. Therefore, a high dielectric constant, piezoelectric coefficient, and well saturated ferroelectric hysteresis loops were obtained in the KNN films in which PVP of the right molecular weight were introduced.

Curie Temperature, Biaxial Elastic Modulus, and Thermal Expansion Coefficient of (K,Na)NbO3 Piezoelectric Thin Films

We investigated fundamental parameters such as the Curie temperature, biaxial elastic modulus, and thermal expansion coefficient of (K,Na)NbO3 (KNN) films on Pt/Ti/SiO2/Si substrates. From the temperature dependence of the dielectric constant of the KNN films, their Curie temperature was approximately 360 °C; this value was confirmed from critical changes in the lattice parameters of the films and the stress induced in the films at approximately this temperature. By using the optical lever method, the biaxial elastic modulus and thermal expansion coefficient of the KNN films were found to be 92 GPa and 8.0×10-6 (1/°C), respectively. The fundamental properties of the KNN films were similar to those of widely used Pb(Zr,Ti)O3 films, indicating that KNN films are potential candidates for lead-free piezoelectric thin films in micro-electro-mechanical system (MEMS) applications.

Crystalline Structure of Highly Piezoelectric (K,Na)NbO3 Films Deposited by RF Magnetron Sputtering

We have previously reported (K,Na)NbO3 (KNN) films, whose piezoelectric properties are the highest reported thus far. In this study, we investigate the detailed crystalline structures of these KNN films after deposition on Pt/MgO and Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. The KNN film on Pt/MgO was epitaxially grown on the Pt lower electrode with a perfect <001> orientation in the perovskite structure. The KNN film on Pt/Ti/SiO2/Si was polycrystalline with a preferential <001> orientation in the perovskite structure having dense columnar grains. X-ray diffraction measurements revealed that the KNN films grown on Pt/MgO and Pt/Ti/SiO2/Si were tetragonal; the lattice parameters c and a were related as c/a > 1. The KNN film on Pt/MgO had a higher c/a value than the KNN film on Pt/Ti/SiO2/Si, indicating that the former had more compressed strain. We conclude that this difference in compressed strain may contribute to the difference in piezoelectric properties of the KNN films on Pt/MgO and Pt/Ti/SiO2/Si.

Sol-Gel Processing and Characterization of (Na,K)NbO3 Lead-Free Ferroelectric Films

Lead-free piezoelectric Na0.5K0.5NbO3 (NKN) thin films were prepared via sol-gel processing on both Pt (111)/Ti/SiO2/Si(100) and Si(100) substrates. XRD analysis showed that perovskite structure was achieved after annealing at 800∼900°C. The SEM morphology showed the NKN films with a total thickness of about 0.5 micron had relatively dense and uniform microstructure. Ferroelectric properties of NKN thin films are characterized at room temperature and the P-E hysteresis loop proved the ferroelectricity of the NKN films. The compensation of volatile components can help control the stoichiometry of the films and improve the film quality.

Preparation of Ferroelectric K&lt;sub&gt;0.5&lt;/sub&gt;Na&lt;sub&gt;0.5&lt;/sub&gt;NbO&lt;sub&gt;3&lt;/sub&gt; Films by Sol-Gel Processing

Orthorhombic perovskite-type K0.5Na0.5NbO3 (KNN) films on Pt/Ti/SiO2/Si substrates were prepared by sol-gel processing. The morphology of the films was characterized using SEM and AFM. The KNN films with a total thickness of about 0.5 micron had relatively dense and uniform microstructure. The ferroelectricity of the KNN films was confirmed by polarization-electric field (P-E) hysteresis loop.

Visible and IR Light Waveguiding in Ferroelectric Na0.5K0.5NbO3Thin Films

ABSTRACTHigh-quality ferroelectric thin films are attractive materials for integrated optics applications including electro-optic waveguide modulators and frequency doubling second-harmonic generators. Several ferroelectric thin film materials, such as BaTiO3, KNbO3, LiNbO3, and (Pb,La)(Zr,Ti)O3, have been investigated regarding their optical and waveguiding properties. Recently the first results on waveguiding in ferroelectric Na0.5K0.5NbO3 (NKN) thin films were presented. Perovskite NKN films have previously been investigated as electrically tunable material for low loss rf and microwave applications. Na0.5K0.5NbO3 thin films of thickness 0.5–1.0 μm have been deposited on Nd:YAlO3(001) and Al2O3(0112) substrates using rf-magnetron sputtering of a stoichiometric, high-density ceramic target. X-ray diffraction measurements confirmed films grown highly (00 l) oriented on the perovskite Nd:YAlO3 substrate and preferentially c-axis oriented on the single crystal r-cut sapphire substrate. Optical and waveguiding properties were characterized using a Metricon 2010 prism-coupling apparatus with a rutile prism. Dark-line spectra were obtained at visible light (λ = 632.8 nm) as well as at infrared optical communication wavelengths, λ = 1319 nm and λ = 1549 nm, in both transverse electric ( TE) and transverse magnetic ( TM) polarizations. Sharp dips corresponding to waveguide propagation modes in the thin film layers where observed for both substrates. The calculated refractive index values and corresponding birefringence (Δ n = n TM− n TE = n e − n o ) as a function of wavelength has been compared. Generally a larger birefringence is observed for the NKN film on Nd:YAlO3, which is in agreement with the larger degree of preferential c-axis orientation measured by XRD.

Electro-Optical Properties of Na0.5K0.5NbO3Films on Si by Free-Space Coupling Technique

AbstractWe report electro-optic performance of highly polar axis oriented Na0.5K0.5NbO3(NKN) films grown directly on Pt(100nm)/Ti(10nm)/SiO2/Si(001) substrates by rf-magnetron sputtering. Semitransparent gold electrodes (diameter Ø= 2 mm) were deposited ontop the NKN films by a thermal evaporation through the contact mask. Processing parameters have been specially optimized to obtain “electrosoft” NKN films with a non-linear fatigue-free P-E characteristics: low remnant Pr = 3.6 μC/cm2 and high induced polarization P = 26 μC/cm2 @ 522 kV/cm, and the coercive field Ec= 39 kV/cm. Electro-optical characterization of NKN/Pt/Si films has been performed using waveguide refractometry: a free-space coupling of a light beam into the thin-film waveguide modes. Intensity of TM- andTE-polarized light of 670 nm laser diode reflected from the free surface of NKN film and Au-cladding NKN/Pt/Si waveguide was recorded at zero and 30 V (100 kV/cm) bias electric field. Extraordinary and ordinary refractive indices as well as electro-optic coefficient have been determined by fitting these experimental data to the Fresnel formulas. Applying 160 V (530 kV/cm) across the parallel plate NKN capacitor (Ø= 2 mm, thickness 3 μm), modulation of the reflected light as high as 40% was achieved.

Na0.5K0.5NbO3 Film Microwave Varactors

Na0.5K0.5NbO3 (NKN) and Pb(Zr0.53Ti0.47)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.

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

Highly crystalline Na0.5K0.5NbO3 (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.

Optical waveguiding in magnetron-sputtered Na0.5K0.5NbO3 thin films on sapphire substrates

Preferentially oriented perovskite-structured Na0.5K0.5NbO3 (NKN) thin films have been deposited on hexagonal Al2O3(011_2) substrates using rf magnetron sputtering of a stoichiometric, high-density, ceramic target. Structural and film surface properties were measured using x-ray diffraction and atomic force microscopy, respectively. Optical and waveguiding properties were characterized using a prism-coupling technique. We observed sharp and distinguishable TM and TE propagation modes and measured the refractive index of NKN thin films of different thicknesses. The ordinary and extraordinary refractive indices were calculated to be no=2.247±0.002 and ne=2.216±0.002 for a 2.0-μm-thick film at 632.8 nm. This implies a birefringence Δn=ne−no=−0.031±0.002 in the film. These first results show the potential use of rf-sputtered NKN films as an electro-optical active material.

High-performance epitaxial Na0.5K0.5NbO3 thin films by magnetron sputtering

Epitaxial Na0.5K0.5NbO3 (NKN) thin films have been grown on LaAlO3 substrates by rf magnetron sputtering of a stoichiometric, high-density, ceramic target. X-ray diffraction analysis showed c-axis oriented cube-on-cube growth. Micrometer size interdigital capacitor (IDC) structures were defined on the surface of the NKN film using photolithography. The electrical characterization at 1 MHz showed dissipation factor tan δ of 0.010, tunability 16.5% at 200 kV/cm and dielectric permittivity εr=470. The frequency dispersion of εr between 1 kHz and 1 MHz was 8.5% and the IDCs showed very good insulating properties with leakage current density on the order of 30 nA/cm2 at 400 kV/cm. The polarization loop exhibits weak ferroelectric hysteresis with maximum polarization 23.5 μC/cm2 at 600 kV/cm. These results are promising for tunable microwave devices based on rf sputtered NKN thin films.

Na 0.5 K 0.5 NbO 3 thin films for voltage controlled acoustoelectric device applications

Perovskite Na0.5K0.5NbO3 (NKN) thin films have been prepared on Y+36° cut single crystal quartz substrates using the pulsed laser ablation technique. X-ray diffraction θ–2θ and ω-scan data demonstrate almost perfectly c-axis oriented film textures with narrow mosaic broadening. Radio frequency dielectric spectroscopy showed that the films possess relatively high dielectric permittivities, low dielectric losses, and low frequency dispersions. Capacitance–voltage (C–V) measurements for a 2 μm slot NKN/quartz interdigital capacitor yield 23.1% tunability by applying 40 V bias at 1 MHz, while C–V hysteresis indicates polarization reversal. The considerable voltage tunability with superior crystallinity in piezoelectric NKN films on quartz substrates suggests their potential use for novel voltage tunable acoustoelectric devices.

Low-frequency and microwave performances of laser-ablated epitaxial Na0.5K0.5NbO3 films on high-resistivity SiO2/Si substrates

The dielectric properties of laser-ablated 0.5-μm-thick c-axis epitaxial Na0.5K0.5NbO3 (NKN) films on high-resistivity (&amp;gt;7.7 kΩ cm) silicon SiO2/Si substrates are studied experimentally in the temperature interval of 30–320 K and at frequencies of 1.0 MHz–40 GHz. The films are grown by laser ablation from a stoichiometric target. For the measurements, planar 0.5-μm-thick gold electrodes (interdigital and straight slot) are photolithography defined on the top surface of NKN films. The slot width between the electrodes is 2.0 or 4.0 μm. At low frequencies (f&amp;lt;1.0 GHz), the structure performance is that of a typical metal–dielectric–semiconductor type, where two of this type of capacitor are connected back to back. At these frequencies, the large change in the capacitance (more than 10 times at 1.0 MHz), due to the applied dc field, is mainly due to the changes in depletion layer thickness at the surface of silicon. The associated losses are also large, tan δ&amp;gt;1. At microwave frequencies (f&amp;gt;10 GHz), the voltage dependence of the capacitance is given by the NKN film. More than a 13% capacitance change at 40 V dc bias and a Q factor of more than 15 are observed at 40 GHz, which make the structure useful for applications in electrically tunable millimeter-wave devices.

Niobate Films for Microwave Applications

ABSTRACTSubmicron thick niobate films, Na0.5K0.5NbO3 (NKN) and Ag0.9Ta0.42Nb0.58O3-δ (ATN), have been pulsed laser deposited on MgO, Pt80Ir20, and Si substrates for microwave device applications. Strong bi-axial (001)-(011) texture observed in both films on MgO substrates indicates that there are major similarities in the growth mechanisms in these films. The dielectric permittivity ε′ of NKN film increases monotonously with temperature, while that of ATN shows a weak temperature dependence (about 21% of variation) in a wide temperature range from 77 K to 400 K. Measured tunability Δε′/ε′ and dielectric loss tanδ for niobate/MgO interdigital capacitors have been found to be (Δε′/ε′)NKN = 40%, tanδNKN = 1.4-2.3% and (Δε′/ε′)ATN = 4.3%, tanδATN = 0.23-0.25% at 1 MHz under maximum electric field of 100 kV/cm. Microwave spectroscopy studies for NKN/Si varactors show (Δε′/ε′)NKN/Si of 13% and tanδNKN/Si = 1.2-6.6% at 40 GHz @ 200 kV/cm.

Low frequency characterisation of laser ablation deposited thin Na0.5K0.5NbO3 (NKN) films for microwave application

Experimental results on three types of Na0.5K0.5NbO3 (NKN) film capacitor structures are presented. The epitaxial NKN thin films have been deposited on (100) Si (high resistivity), SiO2/Si (low resistivity) and Pt/Si (low resistivity) substrates using laser ablation deposition. Both straight slot and interdigital electrode have been deposited on top of the NKN films. The leakage current and low frequency dielectric properties (I-V, C-V, tanδ-V) of the structures have been measured at 1 MHz as a function of electric field at room temperature. In all three types of capacitor structures the leakage currents in a-b plane are very small, while along c-axis there are extremely large leakage currents. On low resistivity silicon substrates the tunability of the dielectric permittivity is about 12% and loss tangent is low also. On high resistivity (ρ>10 kOhm cm) silicon substrate the tunability at 1 MHz is extremely high, about 10 times, and the losses are also high. On the other hand at microwave frequencies the losses are small (tanδ<0.1 at 50 GHz), which make this structure useful for application in microwave devices.

Thickness dependent performance of Na0.5K0.5NbO3/sapphire thin film varactors

Perfect c-axis oriented Na0.5K0.5NbO3 (NKN) films have been pulsed laser deposited on Al2O3(0112) single crystals (r-cut sapphire) for voltage tunable microwave device applications. Thickness dependence of dielectric performance of the NKN/sapphire interdigital capacitors (IDCs) has been studied. 40 V bias tunability and dielectric loss tan δ of 4 μm slot IDCs have been found to be 24.6 % and 2.86 % for 1.2 μm thick NKN film, and 6.1 % and 0.83 % for 0.14 μm thick NKN film, respectively. Low leakage currents and high breakdown voltages are observed in these structures.

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

Highly c-axis oriented single phase Na0.5K0.5NbO3 (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 ε ∼ 520 and tan δ ∼ 0.024 @ 100 kHz, to superparaelectric state with tan δ as low as 0.003 and ε = 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 ε ∼ 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.

Background oxygen effects on pulsed laser deposited Na0.5K0.5NbO3 films: From superparaelectric state to ferroelectricity

Ambient oxygen pressure in a pulsed laser deposition process has been observed to have a critical influence on the compositional, crystalline, and electrical properties of Na0.5K0.5NbO3 (NKN) thin films grown onto polycrystalline Pt80Ir20 and SiO2 (native oxide)/Si(111) substrates. Films prepared at high oxygen pressure (∼400 mTorr) were found to be single phase and highly c-axis oriented. X-ray diffraction θ–2θ scans and rocking curve data show a strong effect of NKN film self-assembling along the [001] direction regardless of the substrate texture. The high dielectric permittivity of 550, low dissipation factor of less than 3%, and high remanent polarization of 12 μC/cm2 indicate the high ferroelectric quality of the fabricated film. The role of the high-energy component of the erosion products has been proven to be crucial to film performance. On the other hand, films grown at low oxygen pressure (∼10 mTorr) have been found to be mixed phases of ferroelectric NKN and paraelectric potassium niobates. These films have shown superparaelectric behavior: 5% tunability at an electric field of 100 kV/cm, losses as low as 0.3%, and excellent stability to temperature and frequency changes.

Na 0.5 K 0.5 NbO 3 /SiO 2 /Si thin film varactor

Perfectly c-axis oriented micrometer thick Na0.5K0.5NbO3(NKN) films have been prepared on a thermally grown ultrathin SiO2 template layer onto a Si(001) wafer by the pulsed laser deposition technique. A x-ray diffraction θ–2θ scan reveals multiple-cell structuring of single phase NKN film along the polar axis, while films grown onto amorphous ceramic (Corning) glass show a mixture of slightly c-axis oriented NKN and pyrochlore phases. This implies a small amount of SiO2 crystallites distributed in an amorphous matrix inherit Si(001) orientation and promotes highly oriented NKN film growth. NKN film dielectric permittivity ε′ was found to vary from 114.0 to 107.2 in the frequency range 1 kHz–1 MHz, while the resistivity was on the order of 2.6×1010 Ω cm @ 20 kV/cm. The planar interdigital variable reactance device (varactor) based on the NKN/SiO2/Si thin film structure possesses a dissipation factor of 0.8% at 1 MHz and zero bias, electrical tunability of 3.1%, and nA order leakage current at 20 V bias at room temperature.