Congruent Lithium Tantalate Research Articles

Small electron polarons bound to interstitial tantalum defects in lithium tantalate

The absorption features of optically generated, short-lived small bound electron polarons are inspected in congruent lithium tantalate, LiTaO3 (LT), in order to address the question whether it is possible to localize electrons at interstitial Ta V :V Li defect pairs by strong, short-range electron–phonon coupling. Solid-state photoabsorption spectroscopy under light exposure and density functional theory are used for an experimental and theoretical access to the spectral features of small bound polaron states and to calculate the binding energies of the small bound Ta Li4+ (antisite) and Ta V4+ :V Li (interstitial site) electron polarons. As a result, two energetically well separated ( ΔE≈0.5 eV) absorption features with a distinct dependence on the probe light polarization and peaking at 1.6 eV and 2.1 eV are discovered. We contrast our results to the interpretation of a single small bound Ta Li4+ electron state with strong anisotropy of the lattice distortion and discuss the optical generation of interstitial Ta V4+ :V Li small polarons in the framework of optical gating of Ta V4+ :Ta Ta4+ bipolarons. We can conclude that the appearance of carrier localization at Ta V :V Li must be considered as additional intermediate state for the 3D hopping transport mechanisms at room temperature in addition to Ta Li , as well, and, thus, impacts a variety of optical, photoelectrical and electrical applications of LT in nonlinear photonics. Furthermore, it is envisaged that LT represents a promising model system for the further examination of the small-polaron based photogalvanic effect in polar oxides with the unique feature of two, energetically well separated small polaron states.

Study on the Fabrication and Acoustic Properties of Near-Stoichiometric Lithium Tantalate Crystal Surface Acoustic Wave Filters

Near-stoichiometric lithium tantalate (NSLT) wafers with different Li contents were prepared by vapour transfer equilibrium (VTE) method and fabricated into surface acoustic wave filters. The temperature coefficient of frequency, insertion loss, and bandwidth of the surface acoustic wave filters were tested using a special chip test bench and a network analyzer. The results show that the temperature coefficient of frequency shows a trend of first decreasing and then increasing with the increase in Li content, and the temperature stability of the surface acoustic wave filters is best when the Li content is 49.75%. It is also found that the surface acoustic wave filter fabricated from NSLT wafers has 21.18% lower temperature coefficient of frequency, 7.3% lower insertion loss, and 2.8% lower bandwidth than those fabricated from congruent lithium tantalate wafers. Therefore, NSLT crystals are more suitable for applications in acoustic devices, providing a new idea for performance enhancement of 5G communication devices.

Preparation, Properties, and Applications of Near Stoichiometric Lithium Tantalate Crystals

Lithium tantalate crystal is widely used in optical devices, infrared detectors and surface acoustic wave devices because of its excellent piezoelectric, acousto-optic and nonlinear optical properties. The Li content of near stoichiometric lithium tantalate (NSLT) crystal is higher than that of congruent lithium tantalate (CLT) crystal. Therefore, the performance of NSLT crystal is better than that of CLT crystal in some aspects. This article reviews the physical properties, preparation methods and current research status in acoustics and optics of NSLT crystals. It also looks forward to the improvement of NSLT crystal preparation methods and their applications in surface acoustic wave (SAW) filters and optics. With the increase of Li content, the acoustic performance of NSLT crystals is expected to be comprehensively improved, achieving the application of SAW filters in 5G communication.

Enhanced surface blistering efficiency of H+ implanted lithium tantalate by chemical reduction modification

The crystal ion slicing (CIS) fabricated heterogeneously integrated lithium tantalate (LiTaO3, LT) thin film is prospective in optical modulators, infrared sensors, and acoustic filters, therefore improving the transferring efficiency of the LT thin film will definitely promote its development and application. In the present work, the chemical reduction method was found effective to enhance the forming ability of H+ implantation-induced defects and surface blistering efficiency of lithium tantalate crystal during the CIS process. According to the investigations by XPS, XRD, RBS, Micro-Raman, and TEM on the reduction-induced defects, H+ implantation-induced defects, and the surface blistering behavior, the treated black lithium tantalate (BLT) present a higher blistering efficiency, confirming the contribution of reduction-induced defects in implantation and blister formation process than the referenced congruent lithium tantalate (CLT). In detail, the improvement is in ascribing to the existing intrinsic oxygen vacancy defects in BLT. These defects increase the forming efficiency of Ta defects in the implantation process and enhance the diffusion of H+ in the blistering formation process, thus decreasing the activation energy for the blistering process and increasing the film exfoliation area. These results highlight an avenue for enhancing ion slicing efficiency of piezoelectric and pyroelectric single crystal oxide.

Preparation, electrical, thermal and mechanical properties of black lithium tantalate crystal wafers

In this work, the effect of surface reduction treatment on the electrical, thermal and mechanical properties of Lithium tantalate (LiTaO3, LT) crystal wafers was investigated by comparing congruent Lithium tantalate (CLT) crystal wafers with deep-reduction black Lithium tantalate (BLT) crystal wafers using a mixture of high-purity aluminium powder and silicon powder as reducing agents. The structural changes of reduced wafers are discussed by XRD. The electrical conductivity of BLT wafers is 6.27 × 10–12 (Ω cm)−1, which is four orders of magnitude higher than the CLT crystal wafers. The Curie temperature and the thermal stability are basically the same with CLT crystal wafers, and the specific heat and hardness are reduced. The data show that high temperature annealing before reduction is conducive to the stress release of CLT crystal wafers, the blackening effect is more easily achieved, and the treated BLT wafer is more anti-static. The results show that the reduction treatment can obviously improve the conductivity of the wafer, so it is easier to improve and eliminate the discharge phenomenon caused by pyroelectric effects in the process of device preparation.

Green-pumped optical parametric oscillator based on fan-out grating periodically-poled MgO-doped congruent LiTaO3.

We report a green-pumped optical parametric oscillator (OPO) based on periodically poled MgO-doped congruent lithium tantalate (MgO:cPPLT). Pumped at 532nm by a frequency-doubled Q-switched Nd:YAG laser, and using a fan-out grating structure, the singly-resonant OPO provides continuous tuning across 689-1025nm in the signal and 1106-2336nm in the idler at room temperature by simple mechanical translation of the crystal. The tuning range can be further extended to 677nm and 2479nm in the signal and idler, respectively, by temperature tuning the crystal to 200°C. With a 29-mm-long crystal, the OPO generates 131mW of average idler power at 1476.5nm for an input pump power of 1.8W at 25kHz repetition rate, with a slope efficiency of 11.3%. Bulk damage in the MgO:cPPLT crystal has been observed for pump powers above ∼1.8 W, and at pump powers beyond ∼1.4 W under long-term operation. The passive power stability of the generated idler is 3.9% over 30min, in a Gaussian spatial profile.

Formation of the quasi-regular stripe nanodomain structures in lithium tantalate by scanning laser heating

The domain structure formation in the congruent lithium tantalate single crystals after IR laser irradiation by linear scanning of Z + polar surface has been studied. Two types of domain structures have been revealed experimentally: self-similar structure and quasi-regular stripe structure. The statistical processing has shown that the increase in the scanning velocity leads to increase in the area with stripe domain structure and to decrease in the average domain period. At the maximum scanning velocity 4.8 m/s, the quasi regular domain stripe structure with average period 0.8 μm has been obtained over the 70% of the switched area.

Effect of Nanoscale Defects on the Physical Properties of Lithium Niobate and Lithium Tantalate Crystals

The elastic, ferroelectric, and transport properties of congruent lithium niobate and lithium tantalate crystals are studied in the temperature range of 77–450 K, depending on the conditions for recovery annealing. Significant changes in the elastic moduli and electrical conductivity that correlate with an increase in the displacement of the off-center Nb5+ (Ta5+) ions along the trigonal $$\overline C $$ axis of the oxygen octahedra NbO6 (TaO6) are found in the interval 120 to 300 K as a result of more detailed studies. The attenuation of acoustic waves is suppressed as the temperature falls, which can be explained by an increase in the degree of ordering of NbO6 (TaO6) clusters. It may be assumed that the strong change in electrical conductivity correlates with the concentration of point nanoscopic defects (antisite defects Nb Li 5+ (Ta Li 5+ ), coupled polarons Nb Li 4+ (Ta Li 4+ ), and bipolarons).

Domain wall shape instability in congruent lithium tantalate during switching by ion beam

ABSTRACTThe formation of domain arrays and domain wall shape instability induced by ion beam irradiation have been studied in congruent lithium tantalate crystal covered by dielectric layer. The dependences of domain radii on exposure dose and array period were obtained. The domain wall shape instability effect (appearance of periodical oriented fingers) for doses above 20 pC was revealed. It was shown that dose increase led to increase of total length of the domain wall with fingers, while the finger number remained approximately constant. The observed results were explained by the kinetic approach based on analogy with first order phase transitions.

Analysis of the defect clusters in congruent lithium tantalate

A wide range of technological applications of lithium tantalate (LT) is closely related to the defect chemistry. In literature, several intrinsic defect models have been proposed. Here, using a combinational approach based on DFT and solid-state NMR, we demonstrate that distribution of electric field gradients (EFGs) can be employed as a fingerprint of a specific defect configuration. Analyzing the distribution of $^{7}\mathrm{Li}$ EFGs, the FT-IR and electron spin resonance (ESR) spectra, and the $^{7}\mathrm{Li}$ spin-lattice relaxation behavior, we have found that the congruent LT samples provided by two manufacturers show rather different defect concentrations and distributions although both were grown by the Czochralski method. After thermal treatment hydrogen out-diffusion and homogeneous distribution of other defects have been observed by ESR, NMR, and FT-IR. The defect structure in one of two congruent LT crystals after annealing has been identified and proved by defect formation energy considerations, whereas the more complex defect configuration, including the presence of extrinsic defects, has been suggested for the other LT sample. The approach of searching the EFG fingerprints from DFT calculations in NMR spectra can be applied for identifying the defect clusters in other complex oxides.

Self-organizing formation of dendrite domain structures in lithium niobate and lithium tantalate crystals

ABSTRACTIn the paper we review the self-organized formation of exotic dendrite-shape domain structures in ferroelectric single crystals. The main attention is paid to our recent detail experimental study of the dendrite domain growth during polarization reversal in stoichiometric lithium niobate LiNbO3 at elevated temperature and in congruent lithium tantalate LiTaO3 after pulse laser heating. Optical, confocal Raman, scanning electron, and piezoelectric force microscopy have been used for domain visualization at the surface and in the bulk. The key roles of isotropic domain growth at elevated temperature, correlated nucleation effect, and ineffective screening of the depolarization field have been revealed.

Vibrational properties ofLiNb1−xTaxO3mixed crystals

Congruent lithium niobate and lithium tantalate mixed crystals have been grown over the complete compositional range with the Czochralski method. The structural and vibrational properties of the mixed crystals are studied extensively by x-ray diffraction measurements, Raman spectroscopy, and density functional theory. The measured lattice parameters and vibrational frequencies are in good agreement with our theoretical predictions. The observed dependence of the Raman frequencies on the crystal composition is discussed on the basis of the calculated phonon displacement patterns. The phononic contribution to the static dielectric tensor is calculated by means of the generalized Lyddane-Sachs-Teller relation. Due to the pronounced dependence of the optical response on the Ta concentration, lithium niobate tantalate mixed crystals represent a perfect model system to study the properties of uniaxial mixed ferroelectric materials for application in integrated optics.

Formation of the nanodomain structures after pulse laser heating in lithium tantalate: experiment and computer simulation

ABSTRACTFormation of the micro- and nanodomain structures in congruent lithium tantalate single crystal under the action of pyroelectric field induced by pulse infrared laser irradiation at elevated temperatures was investigated. Three types of the self-organized domain structures were obtained: (1) the self-similar domain structure formed in crystal regions heated below Curie temperature, (2) the maze-type domain structure, and (3) irregular-shaped isolated domains formed in the regions heated above Curie temperature. The results of calculation of the time dependent spatial distribution of the temperature and pyroelectric field have been used for explanation of the peculiarities of domain structure formation.

Formation of snowflake domains during fast cooling of lithium tantalate crystals

Formation of the original dendrite snowflake-shape domains during fast cooling after heating above phase transition temperature by pulse laser irradiation was revealed in congruent lithium tantalate crystals. The effect was attributed to polarization reversal under the action of spatially nonuniform pyroelectric field. Two stages of the domain shape evolution at the surface were separated: (1) growth of circular domains by sideways motion of the domain walls and (2) backswitching leading to formation of the snowflake domains. The simulated spatial distribution of the pyroelectric field in regular two-dimensional structure was used for an explanation of the obtained results. The backswitching process in the surface layer has been attributed to change of the sign of the pyroelectric field at the domain wall. The snowflake domain shape is caused by the formation of isolated nanodomain fingers and hampering of their merging.

Twin defects in thick stoichiometric lithium tantalate crystals prepared by a vapor transport equilibration method

The twins were observed and investigated in vapor transport equilibration (VTE) treated lithium tantalate crystals by burying congruent lithium tantalate crystals (CLT) in a Li-rich polycrystalline powder. Twins and their etched patterns were observed under an optical polarizing microscope, and the geometry of the twins was discussed. Twin composition planes were the 011¯2 planes. The cause of twinning was analyzed and verified by experiment. The results indicate that the emergence of twins is due to sintering stress, which arises from sintered Li-rich polycrystalline powders at high temperature. 3.2mm thick stoichiometric lithium tantalate (SLT) crystals without twins were obtained by setting corundum crucibles over the top of the crystals to make crystals free from the sintering stress. In addition, cracks were observed at the intersection of twin bands, and the stress caused by the dislocation pile-up was considered to be the reason for the formation of cracks.

Self-Organized Nanodomain Structures Arising in Lithium Tantalate and Lithium Niobate after Pulse Heating by Infrared Laser

Formation of nanodomain structures during cooling after pulse heating by infrared laser has been studied in congruent lithium tantalate (CLT) and lithium niobate (CLN) single crystals. In situ study of the domain structure evolution allowed to reveal that in CLN the isolated domains appeared at the edges of the irradiated zone and grew to the center. In contrast, the quasi-regular stripe domain structure appeared in CLT near the edge of irradiated zone. The difference has been attributed to lower Curie temperature and larger pyroelectric coefficient of CLT. The self-organized domain structures can be used for periodical poling with submicron periods.

Polarization Reversal Process in MgO Doped Congruent Lithium Tantalate Single Crystals

The polarization reversal process has been studied in 8 mol.% MgO doped congruent lithium tantalate single crystal using in situ domain visualization and analysis of the switching current data based on original modification of the Kolmogorov-Avrami approach. Two revealed stages of domain structure evolution during cyclic switching representing slow growth of isolated domains and fast motion of macroscopic domain walls were taken into account during analysis of the switching current data. The coercive field for quasi-static switching about 1.2 kV/mm was revealed. The main parameters of the switching process have been compared with other representatives of lithium tantalate family.

Nano-domains and related phenomena in congruent lithium tantalate single crystals studied by scanning nonlinear dielectric microscopy.

Nanodomains and their related phenomena in congruent lithium tantalate (CLT) single crystals are studied using scanning nonlinear dielectric microscopy (SNDM). We carried out two specific investigations: super higher order nonlinear dielectric spectroscopy studies on thick multi-domain congruent CLT single crystals and electrical conduction in nanodomains in thin CLT single crystals. First, without using a special sharp tip, we achieve improved lateral resolution in SNDM through the measurement of super higher order nonlinearity up to the fourth order. We also found a marked enhancement of nonlinear dielectric constants when the applied tip-sample voltage exceeded a particular threshold value. This is due to domain nucleation activated by a huge electric field under the tip. Low frequencies (less than a few hundred hertz) do not enhance nonlinearity. An effectively lower electric field caused by ion conduction in the sample under the tip is a possible reason for the frequency-dependent characteristics of the enhanced nonlinearity for the applied voltage. Finally, electrical current flow behavior was investigated for nanodomains formed in a thin CLT single-crystal plate. When the nanodomains are relatively large, with diameters of about 100 nm, current flow is detected along the domain wall. However, when the nanodomains were about 40 nm or smaller in size, current flowed through the entire nanodomain. Schottky-like rectifying behavior was observed. A clear temperature dependence of the current is found, indicating that the conduction mechanism for nanodomains in CLT may involve thermally activated carrier hopping.

Electrical conduction in nanodomains in congruent lithium tantalate single crystal

The electrical current flow behavior was investigated for nanodomains formed in a thin congruent lithium tantalate (LiTaO3) single-crystal plate. When the nanodomains were relatively large, with diameters of about 100 nm, current flow was detected along the domain wall. However, when they were about 40 nm or smaller, the current flowed through the entire nanodomain. Schottky-like rectifying behavior was observed. Unlike the case of LiNbO3, optical illumination was not required for current conduction in LiTaO3. A clear temperature dependence of the current was found indicating that the conduction mechanism for nanodomains in LiTaO3 may involve thermally activated carrier hopping.

Shape of isolated domains in lithium tantalate single crystals at elevated temperatures

The shape of isolated domains has been investigated in congruent lithium tantalate (CLT) single crystals at elevated temperatures and analyzed in terms of kinetic approach. The obtained temperature dependence of the growing domain shape in CLT including circular shape at temperatures above 190 °C has been attributed to increase of relative input of isotropic ionic conductivity. The observed nonstop wall motion and independent domain growth after merging in CLT as opposed to stoichiometric lithium tantalate have been attributed to difference in wall orientation. The computer simulation has confirmed applicability of the kinetic approach to the domain shape explanation.