Vapor Transport Equilibration Research Articles

Influence of Li‐Stoichiometry on Electrical and Acoustic Properties and Temperature Stability of Li(Nb,Ta)O3 Solid Solutions up to 900 °C

The current work is focused on the impact of the lithium stoichiometry on electrical conductivity, acoustic properties, and high‐temperature stability of single crystalline Li(Nb,Ta)O3 at high temperatures. The crystals grown from Li‐deficient melts are treated by the vapor transport equilibration (VTE) method, achieving near stoichiometric Li‐content. It is shown that the VTE‐treated specimens generally exhibit lower conductivity at temperatures below 800 °C, which is attributed to the reduced number of Li‐vacancies in near stoichiometric Li(Nb,Ta)O3, provided that the Li‐ion migration dominates the conductivity in this temperature range. Further, it is shown that above 600–650 °C different mechanism increasingly contributes to the conductivity, which is consequently attributed to the electronic conduction. Further, it is shown that losses in LNT strongly increase above about 500 °C, which is interpreted to originate from conductivity‐related relaxation mechanism. Finally, the thermal stability of Li(Nb,Ta)O3 is evaluated by the measurement of the conductivity and resonance frequency as a function of time. It is found that during annealing at 700 °C for 350 h, the resonance frequency of LiNbO3 remains in a ± 100 ppm range of the initial value of 3.5 MHz.

Electrical properties and temperature stability of Li-deficient and near stoichiometric Li(Nb,Ta)O3 solid solutions up to 900 °C

It is widely known that the intrinsic properties of LiNbO3, LiTaO3 and, consequently, of Li(Nb,Ta)O3 solid solutions are strongly dependent on lithium stoichiometry. In the current work the electrical properties of Li-deficient and near-stoichiometric single crystalline Li(Nb,Ta)O3 are studied as a function of temperature by means of impedance spectroscopy. The preparation of near-stoichiometric samples was performed by Vapor Transport Equilibration technique (VTE). It is shown, that VTE-treated samples generally exhibit lower conductivity, which implies better thermal stability. The latter was examined by the measurement of the electrical conductivity of the samples at 500 °C in air. After about 330 h of uninterrupted thermal treatment the conductivity of VTE-treated LiNb0.94Ta0.06O3 specimen was found to decrease for about 10%. To compare – the conductivity of congruent LiNbO3 specimen decreased for about 15% after 330 h of annealing at 500 °C.

Charged domain walls in lithium niobate and lithium tantalate crystals with composition gradients

The formation of charged domain walls (CDWs) in the bulk was studied in lithium tantalate LiTaO3 (LT) and lithium niobate LiNbO3 (LT) crystals with various composition gradients along the polar axis produced by partial vapor transport equilibration (VTE). Under the action of the pyroelectric field upon cooling after the phase transition, the shape of a CDW in LT changed significantly, in contrast to LN: the ledges appeared at CDW and grew toward the polar surfaces. The creation of tail-to-tail and head-to-head CDWs with controlled shape is important for various applications.

Near-stoichiometric Er3+ doped LiNbO3 thin films prepared by vapor transport equilibration treatment

A post-growth Li-rich vapor transport equilibration (VTE) technique was used to prepare near-stoichiometric Er3+ doped LiNbO3 thin films. Er3+ fluorescence property, defect feature and Li2O content of the films were characterized. The results show that the visible and near-infrared fluorescence intensity increases remarkably by ∼10 times the VTE time is increased from 0 to 5 h. The enhancement can be explained from viewpoint of Nb4+ defects concentration. X-ray photoelectron spectrometer study confirms presence of antisite NbLi4+ in the films and NbLi4+ is major defect. Its concentration decreases with increased VTE time. The Li2O content in the films is evaluated to be 48.8–49.7 mol% from the birefringence results. It increases with a rise in VTE time, matching with the characteristic of antisite defects. As defects concentration decreases, the non-radiative transition weakens , which is beneficial to improve fluorescence properties.

Tip-induced domain growth on the non-polar cut of lithium niobate with various stoichiometry deviations

Appearance of wedge-like domains during local switching by a biased tip of a scanning probe microscope (SPM) was studied in non-polar cut plates of lithium niobate (LN) with the spatial distribution of the deviation of Li concentration from stoichiometry composition (ΔcLi) created by vapor transport equilibration. The voltage dependences of the domain sizes were measured in the areas of LN plates with various values of ΔcLi. It was shown that the domain length increased linearly with the voltage, while the domain base width demonstrated a square root voltage dependence. The obtained dependence of the base width was attributed to the spatial distribution of the polar component of the external field produced by a biased SPM tip. The obtained results were considered in terms of the kinetic approach to domain growth. The growth of wedge-like domains was attributed to step generation and kink motion. The velocities of the base growth and kink motion were defined by the excess of the local value of superposition of the polar components of an external field produced by a biased tip, depolarization field, and screening fields over the threshold values. The average length of the elementary steps at the domain walls revealed from the domain length to width ratio demonstrated the square root voltage dependence. It was revealed that the domain length and the width of the base inversely depended on ΔcLi. The obtained dependences of the domain growth parameters on the composition allow for improving the periodical poling technique used for the fabrication of the nonlinear optical devices.

Shape of charged domain walls in bidomain lithium tantalate plates with composition gradients

Lithium tantalate LiTaO3 (LT) plates with various composition gradient along the polar axis were produced by partial vapor transport equilibration (VTE) with annealing temperature above phase transition. The proper annealing parameters allow creating a charged domain wall (CDW) in the center of the plate. The CDW morphology depends on the spatial distribution of stoichiometry deviation. The bidomain actuators manufactured from the LT plates after partial VTE procedure possess a linear voltage dependence of the displacement of the free end without any hysteresis. It is shown that increase in CDW roughness leads to a noticeable decrease in the effective piezoelectric coefficient.

AlN–Ce-doped yttrium aluminum garnet composite ceramic phosphor for high-power laser lighting

Ce-doped yttrium aluminum garnet (YAG:Ce) has been used as a phosphor emitting white light using a blue-light-emitting diode as a light source. Recently, studies on laser lighting as next-generation lighting have been carried out. However, because a high-power blue laser diode is used as the light source, the decrease in phosphor emission due to temperature increase (thermal quenching) has become a serious issue. Therefore, it is desirable to develop a method to suppress the temperature increase of the YAG:Ce phosphor. To increase the thermal conductivity of the YAG:Ce phosphor and suppress the temperature increase, we fabricated a composite ceramic of AlN (nonoxide), which has a high thermal conductivity, and YAG:Ce (oxide) using a modified vapor transport equilibration technique. The oxynitride was not completely removed, but we succeeded in adding up to 80 wt% AlN to YAG:Ce. The ceramic with 50% added AlN had a thermal conductivity up to 19.2 W m−1K−1 and emitted almost the same intensity as that of the nondoped YAG:Ce. This thermal conductivity exceeds the reported values of 12–13 W m−1K−1 for Al2O3–YAG:Ce composite ceramics. Moreover, the luminous efficacy exhibited by the ceramic with a 20 wt% AlN addition was 266.0 lmW−1, which is larger than the reported values obtained from composite ceramics.

Polarization reversal in lithium niobate with inhomogeneous stoichiometry deviation

Polarization reversal in lithium niobate (LN) with spatially inhomogeneous stoichiometry deviation (ΔcLi) obtained by reduction of annealing time of vapor transport equilibration procedure has been studied. The dependence of the threshold field for nucleation (Eth.n) on the ΔcLi has been measured in uniform LN plates. In the inhomogeneous samples, the threshold field for domain nucleation and immediate forward growth through the plate linearly depended on ΔcLi at the treated polar surface. The linear dependence on the stoichiometry deviation of the threshold field for formation of plane domain walls (Eth.s) and internal bias field for cyclic switching has been revealed.

Preparation and characterization of thick stoichiometric lithium tantalate crystals by vapor transport equilibration method

A stoichiometric lithium tantalate (SLT) crystal, 3.2 mm in the thickness and approximate 56 mm in the diameter, was prepared employing improved vapor transport equilibration (VTE) method. The chemical composition in SLT crystals is about 49.99 mol%. The compositional depth profile and uniformity along the radial direction were characterized. The composition deviation in the whole wafer is less than 0.02 mol%. The poling fields for ferroelectric domain reversal of SLT crystals were measured by applying ramped voltages using liquid electrodes. The forward and reverse poling fields are 159 V/mm and 145 V/mm, respectively, and their uniformity in the whole SLT crystal is promising.

As-grown domain structure in lithium tantalate with spatially nonuniform composition

ABSTRACTWe investigated the as-grown domain structure in LiTaO3 with different nonuniform spatial distributions of deviation from stoichiometry. The as-grown structure formed due to cooling below Curie temperature after vapor transport equilibration (VTE) process. The formation of wide charged domain boundary consisting of smooth head-to-head and zig-zag tail-to-tail charged domain walls and flat neutral domain walls was observed. The mechanisms of domain structure formation due to phase transition and subsequent cooling were explained. The different type of charged domain walls in ferroelectrics attracts interest due to possibilities of its applications.

Refractive index change in Ti-diffused near-stoichiometric LiTaO3 waveguide and its relation to Ti-concentration

Ti-diffused near-stoichiometric(NS) LiTaO3 multimode planar waveguide was fabricated by diffusion of Ti-film coated onto a congruent LiTaO3 substrate and post Li-rich vapor transport equilibration process. The crystalline phase in the Ti-diffused layer was analyzed by powder x-ray diffraction and the Li2O-content was evaluated from the measured birefringence. The Ti-concentration profile was analyzed by secondary ion mass spectrometry. The modes guided in the planar waveguide were characterized by prism-coupling technique. The refractive index profile was constructed from the measured mode indices, and correlated with the Ti-concentration profile. The results show that the waveguide retains the LiTaO3 phase and is in an NS composition environment. Both the refractive index and Ti-concentration follow Gaussian profile. The extraordinary and ordinary index change profiles are almost same, implying that the electric field profile of the mode guided in the NS Ti:LiTaO3 waveguide would be polarization-insensitive. The ordinary and extraordinary index changes and the Ti-concentration follow a similar exponential relationship with a power index ∼1.3. Comparison shows that the Ti:LiTaO3 waveguide shows considerable differences from the Ti:LiNbO3 in Ti diffusivity, index change profile and its relation to Ti-concentration, and polarization dependence of guided mode.

Electro-optic coefficients of a non-congruent lithium niobate fabricated by vapour transport equilibration: Composition effect

Abstract Composition effect on electro-optic (EO) properties of a LiNbO3 (LN) single-crystal has been investigated in a Li2O-content range of 47.0–49.95 mol%. Some non-congruent LN crystals with different Li2O-contents were prepared by performing Li-deficient or Li-rich vapour transport equilibration treatments on as-grown congruent LN crystals. Unclamped EO coefficients γ13 and γ33 of these samples were measured by a Mach–Zehnder interferometric method. The measurements show that in the Li-deficient regime both γ13 and γ33 increase by ∼8% as Li2O-content decreases from the congruent 48.6 mol% to the 47.0 mol% in the Li-deficient regime. The feature is desired for the EO application of the Li-deficient crystal. In the near-stoichiometric regime, both γ13 and γ33 reveal a non-monotonic dependence. As the Li2O-content increases from the 48.6 mol%, the EO coefficient decreases. Around Li2O-content 49.5 mol%, a minimum is reached. After that, the EO coefficient recovers slowly. At the stoichiometric composition, it recovers to a value close to that at the congruent point. Comparison shows that different crystal growth methods give rise to different defect structure features and hence different composition effects.

Substitution mechanisms and location of Co2+ ions in congruent and stoichiometric lithium niobate crystals derived from electron paramagnetic resonance data

Electron paramagnetic resonance (EPR) spectra and their angular dependencies were measured for Co2+ trace impurities in stoichiometric samples of lithium niobate doped with rhodium. It was found that Co2+ substitutes for Li+ in the dominant axial center (CoLi) and that the principal substitution mechanism in stoichiometric lithium niobate is 4Co2+ ↔ 3Li+ + Nb5+. The four Co2+ ions can occupy the nearest possible cation sites by occupying a Nb site and its three nearest-neighbor Li sites, creating a trigonal pyramid with C3 symmetry, as well as non-neighboring sites (e.g. a CoNb–CoLi pair at the nearest sites on the C3 axis with two nearby isolated single Co2+ ions substituted for Li+). In congruent crystals and samples with Li content enriched by vapor transport equilibrium treatment the excess charge of the Co2+ centers is compensated by lithium vacancies located rather far from the Co2+ ions for the dominant axial center or in the nearest neighborhood for low-symmetry satellite centers (the Co2+ ↔ 2Li+ substitution mechanism). The use of exact numerical diagonalization of the spin-Hamiltonian matrices explains all the details of the EPR spectra and gives a value for hyperfine interaction A|| that is several times smaller than that obtained using perturbation formulae. The refined values of A and g-tensor components can be used as reliable cornerstones for ab initio and cluster calculations.

Crystalline phase, profile characteristics and spectroscopic properties of Er3+/Tm3+-diffusion-codoped LiNbO3 crystal

Er3+/Tm3+-codoped LiNbO3 crystal was prepared by co-diffusion of stacked Er and Tm metal films coated onto surface of off-congruent, Li-deficient LiNbO3 substrate produced by Li-poor vapor transport equilibration technique. The crystalline phase on the diffused surface was analyzed by X-ray single-crystal diffraction. The Er3+ and Tm3+ profile characteristics were studied by secondary ion mass spectrometry. The emission spectra were measured under the 980 or 795nm wavelength excitation, and the emission and absorption cross section spectra were calculated based upon McCumber theory. The lifetimes of some emissions were measured. The results show that the Er3+ and Tm3+ ions presence is in the form of LiNbO3 phase. Both ions obey to Gaussian profile with a diffusion depth 21.5μm. In the codoping case, both ions keep their respective spectroscopic features of only doping case and do not affect each other. The codoping enables to combine the wavelength emissions of both ions and the resultant emission band in the telecommunication window around 1.5 μm is as wide as 150nm, providing the possibility of S+C+L broadband amplification by employing commercial 980 and 795nm laser diodes as the pump sources. The Er3+/Tm3+-codoped LN is a promising host material for integrated optics.

Note: Electro-optic coefficients of Li-deficient MgO-doped LiNbO3 crystal

A number of Li-deficient MgO-doped LiNbO3 (LN) crystals with different Li2O contents ranging from 43.4 mol. % to 44.5 mol. % were prepared by carrying out the Li-poor vapor transport equilibration treatment on 5 mol. % (in growth melt) MgO-doped LN crystals. Unclamped electro-optic (EO) coefficients γ13 and γ33 of these crystals were measured by Mach-Zehnder interferometry. The results show that γ13 (γ33) increases linearly by ∼14% (11%) as the Li2O content decreases from 44.5 mol. % of the as-grown state to 43.4 mol. % of the Li-deficient state. This feature is desired for the EO application of the Li-deficient MgO:LN crystal.

Near-stoichiometric Ti:Sc:LiNbO_3 strip waveguide for integrated optics

We demonstrate near-stoichiometric Ti:Sc:LiNbO3 strip waveguide fabrication starting from a congruent LiNbO3 substrate with a technological process in sequence of Sc3+-diffusion-doping, Ti diffusion, and post Li-rich vapor transport equilibration. We show that the waveguide is in a near-stoichiometric composition environment, well supports single-mode propagation at 1.5 μm wavelength under both TE and TM polarizations, shows considerable polarization dependence, and has a loss ≤ 0.4/0.7 dB/cm for TE/TM mode. The Ti4+ surface profile can be fitted by a sum of two error functions, the depth profile can be fitted by a Gaussian function, and the Sc3+-profile part which has a concentration above the threshold of photorefractive damage entirely covers the waveguide, showing that the waveguide is expected to be optical-damage-resistant.

Lithium Diffusion in Lithium Niobate Crystals with Different Initial Li2O Content at High Temperature

Lithium diffusion in lithium niobate crystals with different initial Li2O content (Cinitial) was investigated under Li‐rich environment at 1100°C. Lithium niobate crystals with widely varying diffusion‐limited Li2O content profiles were prepared through the vapor transport equilibration (VTE) technique using congruent lithium niobate crystals with different Cinitial, and the profiles were measured through Curie temperature by a thermal analyzer. A Boltzmann‐Matano analysis was employed to those profiles to estimate the Li+ diffusivity as a function of Li2O content in lithium niobate crystals. A trigonometric function method was applied to those profiles to correlate diffusion time and Li2O content. The results show that at the same composition of lithium niobate crystals after diffusing treatment, the less the Cinitial, the larger the Li+ diffusivity. The relation between diffusion time and Li2O content of the samples which have different Cinitial and thickness was derived. Based upon the Boltzmann‐Matano result, diffusion time can be estimated easily from the relation. It is concluded that increasing Cinitial contributes to shorten the diffusion time for preparing near‐stoichiometric lithium niobate crystals through the VTE technique, especially for thick crystal wafers.

Polarization-insensitive, shallow Ti-diffused near-stoichiometric LiTaO3 strip waveguide for integrated optics.

We report on a Ti-diffused near-stoichiometric (NS) LiTaO3 strip waveguide fabricated by diffusion of an 8 μm wide, 160 nm thick Ti-strip followed by Li-rich vapor transport equilibration. It is found that the waveguide surface caves in ∼60 nm below the crystal surface. X-ray single-crystal diffraction shows that the indentation is due to Ti-induced lattice contraction. Optical studies show that the waveguide is in an NS composition environment, supports TE and TM single-mode propagation at 1.5 μm wavelength, is polarization insensitive, and has a shallow mode field profile and a loss of 0.2/0.3 dB/cm for the TE/TM mode. Secondary ion mass spectrometry analysis shows that the Ti profile follows a sum of two error functions in the width direction and a Gaussian function in the depth direction of the waveguide. With the optimized fabrication condition, the waveguide is promising for developing an optical-damage-resistant device that requires a shallow mode field profile.

High-temperature solution growth and vapour transport equilibration of (1−x)K1−yNaYNbO3–xLiNbO3 lead-free piezo-/ferroelectric single crystals

In order to develop lead-free piezo-/ferroelectric materials, single crystals of 0.98K0.8Na0.2NbO3–0.02LiNbO3 (KNN–LN) have been grown in the perovskite structure using the high-temperature solution growth method. Dielectric measurements reveal structural phase transitions at TC (cubic to tetragonal)=411°C and TO–T (tetragonal to orthorhombic)=189°C, respectively, and an additional phase transition at a lower temperature of −78°C which corresponds to the transition from the orthorhombic to a rhombohedral phase. Single crystal structural refinements based on X-ray diffraction data indicate that there are no oxygen octahedral tilts present at room temperature and at −103°C, which suggests that the crystal is K-rich. Composition analysis by energy dispersive spectroscopy and laser ablation – inductively coupled plasma – mass spectrometry confirms the K-rich composition. A vapour transport equilibration technique is successfully developed to optimise the composition of the as-grown crystals by enriching the Li-content and it is demonstrated to be a viable approach to increase the Li-concentration of KNN–LN and other piezo-/ferroelectric crystals.

Spectroscopic analysis of erbium doped LiNbO3: Effect of dopant concentration, vapor transport equilibration and poling

Spectroscopic analysis of as-grown and vapor transport equilibrated erbium doped lithium niobate (Er:LiNbO3; Er:LN) single crystals has been performed on the basis of Judd–Ofelt (JO) theory. Effect of erbium dopant concentration (0.5, 1.0, 2.0, 4.0 and 8.0 mol.% Er) on the absorption characteristics has been investigated. The results reveal that both the line strengths and JO parameters of Er:LN increase till 2.0 mol. % of Er doping concentration and thereafter decrease for 4.0 and 8.0 mol. % Er. This is attributed to increase in the occupancy of Er3+ ions in the clustering site that leads to concentration quenching. In contrast, for vapor transport equilibrated samples the line strengths and JO parameters increased for 0.5 and 1.0 mol. % Er:LN and same was observed to decrease for 2.0 and 4.0 mol. % Er doped samples. In comparison to earlier reports [14,15], improvement in the absorption, line strengths and JO spectroscopic parameters are observed for 0.5 and 1.0 mol. % Er:LN samples on VTE treatment, whereas detrimental effect on VTE is seen on 2.0 and 4.0 mol. % samples. It is observed that poling also enhances the absorption characteristics and JO parameters of the Er:LN sample.