Normally-off characteristics of LiNbO3/AlGaN/GaN ferroelectric field-effect transistor

With the help of MgO mask layer, LiNbO₃ (LN) ferroelectric films were etched effectively using wet etching method and LN/AlGaN/GaN ferroelectric field-effect transistors (FFETs) were fabricated. The electrical properties of the FFETs were studied. Due to the ferroelectric polarization nature of LN films, normally-off characteristics with a turn-on voltage of about + 1.0 V were exhibited in the device. The operation mechanisms of the LN/AlGaN/GaN FFET devices were proposed by the numerical calculations of the electronic band structure and charge distribution.     

Nanocrystallization of SrBi2Nb2O9 from glasses in the system Li2B4O7---SrO---Bi2O3---Nb2O5

Transparent glasses in the system (100−x)Li₂B₄O₇–x(SrO---Bi₂O₃---Nb₂O₅) (10≤x≤60) (in molar ratio) were fabricated by a conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via X-ray powder diffraction (XRD) and differential thermal analyses (DTA) respectively. Glass–ceramics embedded with strontium bismuth niobate, SrBi₂Nb₂O₉ (SBN) nanocrystals were produced by heat-treating the as-quenched glasses at temperatures higher than 500 °C. Perovskite SBN phase formation through an intermediate fluorite phase in the glass matrix was confirmed by XRD and transmission electron microscopy (TEM). Infrared and Raman spectroscopic studies corroborate the observation of fluorite phase formation. The dielectric constant (_r) and the loss factor (D) for the lithium borate, Li₂B₄O₇ (LBO) glass comprising randomly oriented SBN nanocrystals were determined and compared with those predicted based on the various dielectric mixture rule formalism. The dielectric constant was found to increase with increasing SBN content in LBO glass matrix.     

Nanocrystallization of ferroelectric strontium bismuth vanadium niobate in lithium tetraborate glasses

Transparent glass samples in (100-3x) (Li2O-2B2O3)-x(SrO-Bi2O3-0.7Nb2O5-0.3V2O5) (10 < or = x < or = 60, in molar ratio) system have been fabricated via conventional melt-quenching technique. The as-quenched samples, of all the compositions under study have been confirmed to be amorphous, by X-ray powder diffraction (XRD) studies. Differential thermal analysis (DTA) was employed to confirm the glassy nature of the as-quenched glasses. Glass composites comprising vanadium doped strontium bismuth niobate nanocrystallites were obtained by controlled heat-treatment of the as-quenched glasses at 783 K for 6 h. Perovskite SrBi2(Nb0.7VO3)2O9-delta phase formation was found to be preceded by an intermediate fluorite phase which was established via XRD and transmission electron microscopy (TEM). The dielectric constants (epsilonr) of the as-quenched glasses as well as the glass nanocrystal composites decreased with increase in frequency (100 Hz-10 kHz) at 300 K. Interestingly, the dielectric constant of the glass nanocrystal composite (heat-treated at 783 K/6 h) undergoes a maximum in the vicinity of the crystallization temperature of the host glass (Li2B4O7) reaching an anomalously high value (approximately 10(6)) at 800 K. Different dielectric mixture formulae were employed to rationalize the dielectric properties of the glass nanocrystal composite. The optical transmission properties of these glass nanocrystal composites were found to have strong compositional dependence.     

Cu ion in-diffusion in congruent LiNbO3 single crystals

Congruent LiNbO₃ (LN) crystals were grown by Czochralski technique. Thermal in-diffusion of transition metal ion Cu²⁺ into crystal wafers was performed at various temperatures. The absorption peaks in transmission spectra indicates the Cu²⁺ ion diffusion in LN crystal wafer. A broad absorption band centered at 948 nm was observed in the absorption spectra due to the vibronic transition associated with Cu²⁺ centers. The significant red shift in absorption edges indicates the decrease in Li/Nb ratio in Cu in-diffused crystal wafers. Powder XRD of pure and Cu in-diffused LN samples show no structural change during thermal diffusion.     

Optical index profile at an antiparallel ferroelectric domain wall in lithium niobate

Unexpected optical contrast at antiparallel domain walls is observed in non-stoichiometric lithium niobate. This is imaged using near-field scanning optical microscopy. A detailed modeling of the imaging process is performed, and a comparison of the experimental and simulation images is used to extract the index profile across a single antiparallel domain wall.     

Optical properties of the c-axis oriented LiNbO3 thin film

C-axis oriented Lithium Niobate (LiNbO₃) thin films have been deposited onto epitaxially matched (001) sapphire substrate using pulsed laser deposition technique. Structural and optical properties of the thin films have been studied using the X-ray diffraction (XRD) and UV-Visible spectroscopy respectively. Raman spectroscopy has been used to study the optical phonon modes and defects in the c-axis oriented LiNbO₃ thin films. XRD analysis indicates the presence of stress in the as-grown LiNbO₃ thin films and is attributed to the small lattice mismatch between LiNbO₃ and sapphire. Refractive index (n = 2.13 at 640 nm) of the (006) LiNbO₃ thin films was found to be slightly lower from the corresponding bulk value (n = 2.28). Various factors responsible for the deviation in the refractive index of (006) LiNbO₃ thin films from the corresponding bulk value are discussed and the deviation is mainly attributed to the lattice contraction due to the presence of stress in deposited film.     

Influence of substrate on the pulsed laser deposition growth and microwave behaviour of KTa0.6Nb0.4O3 potassium tantalate niobate ferroelectric thin films

Thin films of potassium tantalate niobate KTa_0.6Nb_0.4O₃ (KTN) were grown by pulsed laser deposition on five different substrates suitable for microwave devices: (100)MgO, (100)LaAlO₃, (1–102)sapphire (R-plane), (0001)sapphire (C-plane) and alumina. The high volatility of potassium at the film growth temperature required the addition of an excess of potassium to the ablation target. For optimized deposition conditions, Rutherford backscattering showed that the KTN films had a 1: 1 atomic ratio for K:(Nb + Ta). As grown KTN thin films were single-phase, without any particular orientation on sintered alumina, whereas an epitaxial growth with the (100) orientation was achieved on (100)MgO and (100)LaAlO₃ with a mosaicity Δω_(100)KTN close to 0.7°–1.5° and  0.4°–0.9°, respectively, attesting a high crystalline quality. In contrast, growth of KTN on R-plane sapphire results in a texture with the (100) orientation and the presence of the (110) orientation as a secondary one. The room temperature measurements carried out on Au interdigited capacitors patterned on KTN coated (100) LaAlO₃ and sapphire led at 1 GHz to an agility ΔC / C  4.6% and  7.2%, respectively, for a moderate applied field of 15 kV cm^− 1. Stubs patterned on the same systems led to an agility ΔF_r / F_r of  2.2% and 4.2%, respectively, for F_r = 7 GHz and the same applied field.     

New type of piezo-damping aluminum matrix composites with ZnO:Al-coated LiNbO3 particles

A new type of piezo-damping aluminum matrix composite containing ZnO:Al-coated LiNbO₃ particles was prepared. The dependence of the damping properties of composites on the resistivity of ZnO:Al coatings, altered by Al doping concentration, was investigated. Dynamic mechanical thermal analysis revealed that decreasing the resistivity of ZnO:Al coatings causes the loss factors of the composites to initially increase until the maximum value, before rapidly decreasing. Based on this piezo-damping material, the LiNbO₃ particles contribute to the transformation of mechanical vibration energy into electric energy, which is then converted into Joule's heat in the networks within the ZnO:Al coatings and metal matrix. An optimum formulation for piezo-damping metal matrix composites can be designed based on the results of this study.     

Surface and bulk properties of Cu nanocluster composites in LiNbO3

Implantation of 60 keV Cu⁻ ions into LiNbO₃ resulted in formation of complex nanocomposites consisting of metal Cu nanoparticles distributed among nanodomains of the host medium. The nanodomains of the host medium correspond to regions of a few hundred naometers in diameter with a constant refractive index. Distributions of the Cu nanoparticles and nanodomains in the implanted region do not correlate with each other. Variations of linear and non-linear optical absorption of the nanocomposites are mainly determined by the change of chemical composition and structure of implanted regions of LiNbO₃.     

Effect of electron beam writing parameters for ferroelectric domain structuring LiNbO3:Nd

We have successfully applied the versatile technique of direct electron beam writing to produce column shaped 180° micro-domains on a solid state laser crystal: Nd³⁺ doped LiNbO₃. The effect of the different parameters used during the electron lithography process has been analyzed on the morphology and on the optical properties of the inverted domains. Namely, we have studied the effect of the value of the accelerating voltage of the electrons and the effect of the electronic charge deposited onto the sample. By means of optical micrography and computer simulation we show that the accelerating voltage has a major effect on the diameter of inverted domains. On the other hand, the value of the electronic charge not only affects the domain size but it has also an additional and interesting effect on the optical spectroscopy of Nd³⁺ ions. Using low temperature site selective optical spectroscopy we demonstrate the possibility of acting selectively on the crystal field of the optically active Nd³⁺ centers in LiNbO₃ by means of the different electronic doses used to reverse the polarization of the matrix. The work constitutes an approach towards the control of the properties of reversal ferroelectric domains structures prepared by direct electron beam lithography and towards understanding the role of optically active defects on the switching behaviour in ferroelectric crystals.     

The possibility of tailoring the ne vs cLi relationship in lithium niobate optical waveguides

We present results of our study of concentration profiles of lithium (c_Li) in annealed proton exchanged (APE) waveguiding layers as measured by the neutron depth profiling (NDP) method. This non-destructive method, based on the ⁶Li(n,)³H reaction induced by thermal neutrons, allowed easy monitoring of c_Li profiles in a large number of samples fabricated under various fabrication conditions. Our systematic study revealed that, though every particular waveguide could be characterised by very similar mirror-shaped extraordinary refractive index (n_e) as well as c_Li depth profiles, in contrast with up to now generally accepted opinion, there was no linear relationship which unambiguously attributed Δn_e to Δc_Li. The most important fabrication step appeared to be the post-exchange annealing, during which the lithium atoms were transported towards the sample surfaces. The annealing regime pre-destined not only the depth distribution of the lithium atoms but, as a consequence of it, also other properties of the waveguiding region. That knowledge allows us to fabricate the APE waveguides with a priori given properties for a wide range of special applications. We have also formulated the n_e vs c_Li semi-empirical relationship, which was proved to fit all our fabricated APE waveguides.     

Properties of LiNbO3 based heterostructures grown by pulsed-laser deposition for optical waveguiding application

Epitaxial LiNbO₃ (LN) thin films have been grown onto (00.1) Al₂O₃ substrates and onto sapphire covered with a conductive ZnO buffer layer. For the two systems, the LN thin films are well crystallised and highly (00.1) oriented. Epitaxial relationships between the different layers are evidenced both on the LN/sapphire film and the LN/ZnO/sapphire heterostructure. The optical waveguiding propagation losses of the LN/sapphire films are very low (1 ± 0.5 dB/cm) while the LN/ZnO/sapphire heterostructure does not exhibit satisfying waveguiding properties mainly due to the high conductivity (600 S m^− 1) of the ZnO buffer layer.     

Luminescence and second harmonic generation in Eu/Eu embedded B2O3: LiNbO3 non-linear glass-ceramics

Multifunctional europium doped Li₂O-Nb₂O₃-B₂O₂ glass has been prepared by melt-quench method. Through subsequent heat treatments glass has then been transformed into glass ceramics containing ferroelectric LiNbO₃ phase. The glass ceramics have shown enhanced Eu³⁺ emission compared to parent glass when excited by 266 nm radiation. The emission measurements of glass ceramics have also shown the presence of Eu²⁺ state along with Eu³⁺ and Eu²⁺ state was found to increase when glass was heated in inert atmosphere. Lifetime of the ⁵D₀ level of the Eu³⁺ has been measured and a significant increase is found in case of glass ceramic prepared around glass transition temperature. Glass ceramics have also shown good second harmonic generation (SHG) with pulsed 1064 nm laser excitation.     

Dielectric and structural characterization of KNbO3 ferroelectric thin films epitaxially grown by pulsed laser deposition on Nb doped SrTiO3

KNbO₃ thin films were deposited on SrTiO₃ substrates by pulsed laser deposition. The X-ray diffraction patterns highlight an epitaxial growth according to the (011) orientation. This epitaxial growth was then confirmed by Electron Channeling Pattern. In agreement with the structural characteristics the dense microstructure consists in regular and ordered grains. Dielectric measurements were performed in the 20 Hz to 1 MHz frequency range on a KNbO₃ thin film grown on 2 at.% Nb doped (100)SrTiO₃ substrate in a large range of temperature in order to investigate the paraelectric-ferroelectric transition. Measurements at room temperature revealed a dielectric constant of 450 at 10 kHz and a minimum value of the loss tangent of 0.075 at 100 kHz. Dielectric study in the 20-600 °C temperature range showed a maximum of permittivity at the Curie temperature T_c = 410 °C and evidenced a “progressive” first-order phase transition, different from the classical “diffuse” transition.     

Influence of the dopant concentration on the OH absorption band in Fe-doped LiNbO3 single-crystal fibers

The growth of iron-doped single-crystal fibers of lithium niobate was performed by the laser heated pedestal growth technique for different Fe₂O₃ contents in the feed rods. The infra-red optical absorption spectra (OH⁻ band) were investigated by IR transmission microscopy. To explain the evolution of the OH⁻ ions concentration in the crystals, a model is proposed involving the partial dissociation of LiNbO₃ in the melt into Li₂O and Nb₂O₅. The constitution of the OH⁻ absorption band is also discussed on the basis of the Nb-vacancy structural model of lithium niobate.     

Fabrication and lithium intercalation properties of epitaxial Li2RuO3 thin films

Lithium intercalation in a lithium excess layered material Li₂RuO₃ was investigated using two-dimensional model electrodes with a restricted reaction plane of (002). Li₂RuO₃ films were synthesized on Al₂O₃(0001) substrate by a pulsed laser deposition, and X-ray diffraction measurements confirmed epitaxial growth of Li₂RuO₃(002). Electrochemical characterization using cyclic voltammetry and charge/discharge measurements indicated electrochemical reactions with a discharge capacity of 200 mAh g^− 1 for the film deposited at 400 °C followed by post-annealing at 550 °C. The electrochemical activity on the (002) plane indicated three-dimensional lithium diffusion in the two-dimensional layered rocksalt structure through the lithium sites in the transition metal layer.     

Blue shift of optical band-gap in LiNbO3 thin films deposited by sol-gel technique

Lithium niobate (LN) thin films were deposited on quartz substrates by sol-gel technique. The measured absorption and photoluminescence spectra show that the band structure of LN thin films is direct unlike indirect band-gap in bulk LN and the optical band-gap of these LN thin films was measured to be 4.7 eV which is ~ 1 eV greater than that for stoichiometric bulk LN. The dependence of the blue shift of band-gap on several parameters like quantum confinement, composition (Li:Nb ratios of LN thin films) and strain was also investigated. The results obtained show that the large blue shift in band-gap of LN thin films is primarily due to strain in the film.     

Electro-optic properties of c-axis oriented LiNbO3 films grown on Si(1 0 0) substrate

We developed a spectroscopic–ellipsometric approach to evaluate the electro-optic coefficient of highly c-axis oriented LiNbO₃ films on an Si(1 0 0) substrate grown by electron cyclotron resonance plasma sputtering. Applying an electric field between the TiN transparent top electrode and Si substrate, the interference fringe appearing in the tan Ψ spectrum was slightly modulated by phase retardation in the wavelength domain. The change in effective wavelength was converted to refractive index change, yielding dispersion in the Pockels coefficient (r₃₃) between 0.3 and 0.8 μm. At 633 nm, we obtained an r₃₃ that was 57% of the bulk LN crystal value.