LiNbO3 Single Crystals Research Articles

Non-steady-state crystal growth of LiNbO3 in the presence of an interface electric field

The redistribution of ionic species in LiNbO3 (LN) single crystals was analyzed under an abrupt change in the growth velocity in the presence of an interface electric field during growth using the micro-pulling-down technique. The unity value of the equilibrium partitioning coefficient, k0, for the true congruent LN, cs-MgO:LN, was modified by the interface electric field and converted to kE0 (≠1). This non-unity kE0 resulted in compositional variation in the crystal during non-steady-state growth. When a certain electric current was applied to the solid–liquid interface to counterbalance the Seebeck-effect-driven electric field to make kE0 = 1 for every ionic species, no change in solute concentration was found to have occurred during non-steady-state growth. These results demonstrated that the true congruent LN crystal maintained compositional uniformity irrespective of the growth conditions. We also investigated the effect of the velocity dependence of supercooling on the Seebeck-effect-induced potential at the interface.

Conductions through head-to-head and tail-to-tail domain walls in LiNbO3 nanodevices

Conducting domain walls in an insulating ferroelectric matrix are interesting for the development of next generation multifunctional nanodevices with large output powers. However, electrical conductions are diversified among head-to-head (H-H), neutral (N), and tail-to-tail (T-T) domain walls (DWs) with disputable conduction mechanisms. It is generally accepted that the charged DWs are more electrically conductive than the neutral DWs. However, the charged walls are unstable due to high depolarization energies. Here, we stabilized the H-H DWs, NDWs and T-T DWs within a LiNbO3 transistor by controlling charge injection in compensation of the domain boundary charge under applied drain–gate, drain–source and gate–source voltages. The walls were created through the local 180° domain reversals in different inclined angles, and the transistors in different sizes were fabricated at the surfaces of 5 mol.% MgO-doped LiNbO3 single crystals in monodomain patterns. The NDWs are positively charged due to small inclination angles (~1°) and local sideways meandering behavior of the charged dipoles with electrical conduction that is three orders of magnitude higher than that across the T-T DWs but is one order of magnitude lower than that across the H-H DWs. Voltage dependences of wall currents can be fitted according to the space-charge-limited current equation with an exponential coefficient varying between 2.1 and 3.7 in some specific voltage ranges, implying discontinuous domain retraction in different inclined wall angles against the reduced applied voltage to affect the wall current. This finding provides the fundamental physics to improve domain wall conduction via domain reconstruction and broadens the domain wall application in future nano-devices.

Hydrogen diffusion in proton-exchanged lithium niobate single crystals

Hydrogen diffusion in proton-exchanged congruent LiNbO3 single crystals is investigated in the temperature range between 140 and 200 °C. The proton-exchange process carried out in a mixture of benzoic acid and lithium benzoate (1 mol. %) results in an up to 2 μm thick surface layer where Li is substituted by H for about 60% (about 12 at. % H within LiNbO3) as determined by nuclear reaction analysis. For the diffusion experiments, deuterated benzoic acid is used as a tracer source and the hydrogen/deuterium isotope-exchange occurs at temperatures of 200 °C and below. Isotope sensitive depth profile analysis is done by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange and tracer diffusivities of deuterium within the exchanged layer are extracted. Both types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.2 eV, while the effective diffusivities are lower by three orders of magnitude. This result shows that the diffusion of hydrogen is not the rate-determining step for the proton-exchange process. Possible diffusion mechanisms of hydrogen tracer diffusion are discussed.

High-temperature poling treatment of congruent ferroelectric LiNb0.5Ta0.5O3 solid solution single crystals

Lithium niobate and tantalate are among the most important and widely used materials of acoustooptics and acoustoelectronics. They have high piezoelectric constants enabling their use as actuators. Their use is however restricted by the thermal instability of lithium niobate crystals and the low Curie temperature TC of lithium tantalate crystals. Overcoming these drawbacks typical of some compounds is possible by growing LiNb1-xTaxO3 single crystals. Good quality LiNb1-xTaxO3 single crystals have been grown using the Czochralsky technique. High-temperature poling process of LiNb1-xTaxO3 single crystals has been studied. The main differences between the process modes required for poling of congruent LiNb1-xTaxO3 single crystals and congruent LiNbO3 single crystals have been demonstrated. Parameters of high-temperature electric diffusion processing of LiNb1-xTaxO3 single crystals that provide for singledomain crystals for further study of physical properties have been reported.

Nanoindentation and structural studies of MgO-doped congruent LiNbO3 single crystals

The mechanical properties of undoped and 2.0, 4.0 and 6.0 mol% Mg-doped LN single crystals, grown by the Czochralski technique, have been investigated using nanoindentation studies to understand the mechanical deformation behaviour as doping is increased. This has been correlated with structural investigations by powder XRD analysis and Raman spectroscopy. Powder X-ray diffraction measurements show a slight increase in the lattice parameters as the Mg content is increased in the crystal. The lattice strain developed due to the doping has been calculated by the Williamson-Hall relation. The influence of Mg incorporation on lattice vibrations was analysed using Raman spectroscopy, which indicated no shift in the peak positions with doping, and only slight variation in the intensity and width of the peaks. The grown crystals were subjected to nanoindentation and the Young's modulus and hardness values were obtained by using the Oliver-Pharr method. The results reveal the optimal doping levels of Mg which result in enhanced mechanical strength of lithium niobate single crystals.

Optical thermometry based on thermolabile intrinsic polarons in Tm3+ and Yb3+ co-doped congruent lithium niobate single crystal

By contrast with the conventional optical thermometry based on fluorescence intensity ratio (FIR) of thermally coupled levels (TCLs), we propose a novel thermometry strategy based on the diversity in thermal responses of non-TCLs of Tm3+ (3H4→3H6/1G4→3F4). Notably, the exacerbated response-contrast stems from the intrinsic structure variation induced by thermolabile polarons of NbLi3+ and NbNb4+ in congruent Tm3+,Yb3+:LiNbO3(LN) single crystal. The luminescence mechanism incorporated with energy transfer processes associated with polarons levels is revealed by investigating the abnormal temperature dependence of emission spectra under 980 nm excitation. The maximum absolute (SA) and relative (SR) sensitivities reach as high as 3.7% K−1 and 1.25% K−1 at 80 K, much higher than those of the TCLs strategy based on thermally coupled Stark sublevels of Tm3+ (3H4|1→3H6/3H4|3→3H6) in Tm3+,Yb3+:LN. This study demonstrates an effective pathway for developing new FIR strategies with improved sensitivities via taking advantage of host structure variation.

Domain modulation in LiNbO3 films using litho piezoresponse force microscopy

Domain engineering plays a pivotal role in the development of ferroelectric non-volatile memory devices. In this work, we mainly focus on the domain kinetic in ion-sliced single crystal LiNbO3 thin films under tip-induced electric fields using piezoresponse force microscope (PFM). Polarization reversal takes place when the electric fields are above threshold value (coercive voltage V c) of films. Besides, the dependence of domain dynamic on pulse duration and amplitude were investigated in detail, and specific local domain reversal (5 μm) was completed by the optimized poling condition. All the results reveal that tip-induced polarization reversal could be an effective way to domain engineering, which gives much more promising prospects regarding to the high density non-volatile ferroelectric memory devices.

Analytical Study of the Film Bulk Acoustic Resonators Based on Single Crystal LiNbO3 with Different Crystal Orientations

Film bulk acoustic resonator (FBAR) based on single crystal LiNbO3 are promising for next-generation high Q, wideband filters in telecommunications. The crystal orientation of LiNbO3 has significant impacts on the FBAR performance, and the bulk acoustic wave (BAW) velocity was calculated by solving the Christoffel’s equation in each crystal orientation. The electromechanical coupling coefficient of every cut was obtained. Moreover, the impedance information and the film displacement of various crystal orientations were analyzed by the finite element method (FEM). Analytical results demonstrate that the effective coupling coefficient and the number of BAW modes vary with the crystal orientation.

Erasable Ferroelectric Domain Wall Diodes* *Supported by the National Key Basic Research Program of China (Grant No. 2019YFA0308500), the Basic Research Project of Shanghai Science and Technology Innovation Action (Grant No. 17JC1400300), and the National Natural Science Foundation of China (Grant Nos. 61674044 and 61904034).

The unipolar diode-like domain wall currents in LiNbO3 single-crystal nanodevices are not only attractive in terms of their applications in nonvolatile ferroelectric domain wall memory, but also useful in half-wave and full-wave rectifier systems, as well as detector, power protection, and steady voltage circuits. Unlike traditional diodes, where the rectification functionality arises from the contact between n-type and p-type conductors, which are unchanged after off-line production, ferroelectric domain wall diodes can be reversibly created, erased, positioned, and shaped, using electric fields. We demonstrate such functionality using ferroelectric mesa-like cells, formed at the surface of an insulating X-cut LiNbO3 single crystal. Under the application of an in-plane electric field above a coercive field along the polar Z axis, the domain within the cell is reversed to be antiparallel to the unswitched bottom domain via the formation of a conducting domain wall. The wall current was rectified using two interfacial volatile domains in contact with two side Pt electrodes. Unlike the nonvolatile inner domain wall, the interfacial domain walls disappear to turn off the wall current path after the removal of the applied electric field, or under a negative applied voltage, due to the built-in interfacial imprint fields. These novel devices have the potential to facilitate the random definition of diode-like elements in modern large-scale integrated circuits.

High temperature ferroelectric domain wall memory

For automotive and space applications, the nonvolatile ferroelectric domain wall random access memory (DWRAM) is required to work at a wide temperature range of −40 °C to +150 °C. However, the wall current generally decays with the retention time at high temperatures with the uncertain mechanism. Here we observed the peaked domain wall (DW) currents to decay with time at 450 K in LiNbO3 single crystals, which was correlated with the charge injection into domain wall regions in the mobility of 6.4 × 10−9 cm2/V·s. The extrinsic charge injection suppresses the intrinsic diode-like DW current due to the local Li/O vacancy redistribution to compensate the domain boundary charge. But the injected charge can be removed at an opposite poling voltage and can be frozen upon cooling down to 300 K. The traces of frozen wall regions become conducting upon the wall erasure. Once the memory write and read times are far shorter than the charge injection time, the DWRAM shows good retention of written information with a large on/off current ratio of ~104 at 450 K, suitable to the applications in some extreme conditions.

Определение концентрации ОН-групп и точечных дефектов в кристаллах ниобата лития.

The analysis of structural particularities of nominally pure and doped LiNbO3single crystals was performed by methods of Raman light scattering and infrared absorption. The concentration of OH groups, as well as the concentration of point defects is calculated from IR absorption spectra in the region of stretching vibrations of OH groups. A correlation has been determined in behavior of IR absorption spectra bands of strongly doped LiNbO3:Zn crystals with the behavior of the Raman spectra band that correspondsto stretching bridge vibrations of oxygen atoms in octahedra along the polar axis.

The effect of substrate bias on the piezoelectric properties of pulse DC magnetron sputtered AlN thin films

Substrate bias was applied for AlN deposition on rolled Ni sheet during pulse DC reactive sputtering to overcome the difficulty caused by thermal expansion mismatch between Ni substrate and AlN upon substrate heating. It was shown by Piezoresponse Force Microscopy (PFM) that the quality of the deposited AlN layer depends strongly on the negative substrate bias, i.e., the energy transferred via the bombardment of the accelerated positive ions on the sample. As the negative substrate bias becomes larger, the so formed layer shows higher piezoresponse, and better homogeneity. A Z-cut LiNbO3 single crystal was used as a reference to correct the PFM signals. The highest average d33 piezoelectric coefficient value, achieved at − 100 V substrate bias, is 3.4 pm/V indicating the feasibility of AlN deposition on rolled Ni substrate for vibration energy harvesting applications.

Comparative investigation of electrophysical characteristics of ceramic and single crystal LiNbO3

Electrophysical characteristics have been studied in ceramic and single crystal LiNbO3 of congruent and near-congruent composition. The comparative study has been carried out in a wide temperature range (290-800 K). Dispersion of the real part of the dielectric constant ε′ has been considered. Temperature dependences ε′(T) and σ(T) have been obtained and value of static conductivity has been determined in ceramic and single crystal LiNbO3 in the given temperature range. Temperature dependences of specific static conductivity σ dc have been obtained due to impedance spectroscopy data analysis. Ceramic LN conductivity has been shown to be much higher in the whole studied temperature range. Charge carriers activation enthalpy in high-temperature area (above 420 K) is H a = 0.88 eV for ceramic LN and H a = 1.24 eV for single crystal LN.

Nonlinear Bound States in the Continuum of Etchless Lithium Niobate Metasurfaces

Optical bound states in the continuum (BICs) have recently been studied in a wide range of material systems, where light is perfectly confined in the continuous spectrum of radiating modes. In this paper, we reported periodic nonlinear metasurfaces on the etchless LiNbO3 platform, realizing the enhancement of second-order generation (SHG). All-dielectric heterogeneous metasurfaces are constructed by patterning a low-refractive-index polymer on a high refractive-index LiNbO3 film without etching. Due to BICs, light is localized in the LiNbO3 film where the excellent optical second-order nonlinearity is exploited. We demonstrated that with normal incident waves, symmetry-protected BICs are formed at near-infrared wavelength showing a vanishing linewidth in the transmission spectrum. In addition, to manifest the invisible BICs to detectable supercavity resonances at normal incidence, asymmetry is introduced into the system, degrading the symmetry-protected BICs to sharp resonances with a high Q factor. Furthermore, the second harmonic generation (SHG) of the etchless lithium niobate metasurface is studied, predicting that the SHG efficiency can exceed 10−3 with 30 MW/cm2 of the pump intensity. The proposed strategy without facing the fabrication challenge of etching single-crystal LiNbO3 film opens a new avenue for the utilization of BIC in nonlinear optics of all-dielectric heterogeneous metasurfaces.

Energy-Efficient Ferroelectric Domain Wall Memory with Controlled Domain Switching Dynamics.

High readout domain-wall currents in LiNbO3 single-crystal nanodevices are attractive because of their application in a ferroelectric domain wall random access memory (DWRAM) to drive a fast memory circuit. However, the wall current at a small read voltage would increase nonlinearly at a much higher write voltage, which could cause high energy consumption. Here, we resolved this problem by controlling the two-step domain forward growth within a ferroelectric mesa-like cell that was formed at the surface of an X-cut LiNbO3 single crystal. The mesa-like cell contacts two side Pt/Ni electrodes that extend over the cell surface by 90 nm for the generation of an in-plane inhomogeneous electric field. The domain forward growth processes at first in the formation of an inclined charged 180° domain to span the in-plane electrode gap under a write voltage of 5 V in a large readout wall current, and then, the domain expands fully throughout the entire cell in the formation of a neutral 180° wall to reduce the wall current by 10 times at a higher write voltage of 6 V. Meantime, the domain below the mesa-like cell in an opposite orientation is unchanged to serve as the reference. A higher wall current at a lower read voltage and a lower wall current at a higher write voltage can satisfy both requirements of low energy consumption and fast operation speeds for the DWRAM.

Ferroelectric domain wall memory with embedded selector realized in LiNbO3 single crystals integrated on Si wafers.

Interfacial 'dead' layers between metals and ferroelectric thin films generally induce detrimental effects in nanocapacitors, yet their peculiar properties can prove advantageous in other electronic devices. Here, we show that dead layers with low Li concentration located at the surface of LiNbO3 ferroelectric materials can function as unipolar selectors. LiNbO3 mesa cells were etched from a single-crystal LiNbO3 substrate, and Pt metal contacts were deposited on their sides. Poling induced non-volatile switching of ferroelectric domains in the cell, and volatile switching in the domains in the interfacial (dead) layers, with the domain walls created within the substrate being electrically conductive. These features were also confirmed using single-crystal LiNbO3 thin films bonded to SiO2/Si wafers. The fabricated nanoscale mesa-structured memory cell with an embedded interfacial-layer selector shows a high on-to-off ratio (>106) and high switching endurance (~1010 cycles), showing potential for the fabrication of crossbar arrays of ferroelectric domain wall memories.

Enhancement of near-infrared photoluminescence in Mg:Er:LiNbO3 containing Au nanoparticles synthesized by direct ion implantation

Embedded gold (Au) nanoparticles (NPs) are formed in Mg:Er:LiNbO3 single crystals by Au ion implantation and subsequent thermal annealing. Absorption of the Mg:Er:LiNbO3 crystals with Au NPs is found to be enhanced significantly in the visible light wavelength band owing to the localized surface plasmon resonance (LSPR) effect. The calculated LSPR effect by Mie theory shows good agreement with the absorption spectra. A significantly enhanced Er related photoluminescence (PL) at 1.54 μm for crystals with Au NPs is also observed compared with samples without Au NPs. Energy transfer between Au NPs and Er is found to be responsible for the PL enhancement in the as-implanted samples while local field enhancement induced by LSPR is considered the dominant factor in the annealed samples. The dependence of PL enhancement on NP size makes it possible to tailor intensity by varying the annealing temperature. An avenue to enhance and modulate the PL of dielectrics with embedding Au NPs synthesized by ion implantation is presented in this study.

Control of Optical Homogeneity of Lithium Niobate Crystals with Admixtures of Rare-Earth Elements Gd, Er by Laser Conoscopy

Interference patterns of single crystals of LiNbO3:Gd, LiNbO3:Er, obtained by laser conoscopy, allow us to estimate the optical homogeneity of the samples under study. The observation of conoscopic patterns of the LiNbO3:Gd single crystal shows that with an increase in the percentage impurity concentration, the anomalous biaxiality in these samples manifests itself to a much greater degree. Anomalous biaxiality for the sample LiNbO3:Er [3.60 wt.%] is present at almost every point of the entrance face, which is manifested in conoscopic patterns in the form of enlightenment and rupture of the “Maltese cross”, which corresponds to the angle 2V~1° between the anomalous axes.

Integrated lithium niobate photonics

Abstract Lithium niobate (LiNbO3) on insulator (LNOI) is a promising material platform for integrated photonics due to single crystal LiNbO3 film’s wide transparent window, high refractive index, and high second-order nonlinearity. Based on LNOI, the fast-developing ridge-waveguide fabrication techniques enabled various structures, devices, systems, and applications. We review the basic structures including waveguides, cavities, periodically poled LiNbO3, and couplers, along with their fabrication methods and optical properties. Treating those basic structures as building blocks, we review several integrated devices including electro-optic modulators, nonlinear optical devices, and optical frequency combs with each device’s operating mechanism, design principle and methodology, and performance metrics. Starting from these integrated devices, we review how integrated LNOI devices boost the performance of LiNbO3’s traditional applications in optical communications and data center, integrated microwave photonics, and quantum optics. Beyond those traditional applications, we also review integrated LNOI devices’ novel applications in metrology including ranging system and frequency comb spectroscopy. Finally, we envision integrated LNOI photonics’ potential in revolutionizing nonlinear and quantum optics, optical computing and signal processing, and devices in ultraviolet, visible, and mid-infrared regimes. Beyond this outlook, we discuss the challenges in integrated LNOI photonics and the potential solutions.

Cleavage and surface energies of LiNbO3

“Cleavage” is one of the most well-known modes of mechanical failure of crystals. It describes their ability to break easily along specifically oriented atomically sharp planes. Although understanding cleavage properties of any crystalline functional material is mandatory, the current knowledge about cleavage is limited to simplest crystals like cubic silicon. With the goal of expanding the knowledge to the broader class of crystalline materials, the cleavage of trigonal LiNbO3 single crystal is investigated in this work. The experimental method to measure the cleavage energy is introduced. The method is based on adhering a pre-cracked specimen in a rectangular hole in aluminum loading frame; forces are applied to the cracked specimen when heating up the assembly and due to the thermal expansion coefficient mismatch between the specimen and the loading frame. The cleavage energies of two known cleavage planes are evaluated by this method followed by extraction of the surface energies, and third previously unknown cleavage plane is discovered. The findings are explained qualitatively by counting the number of Nb–O bonds in LiNbO3 structure, broken by these planes. The evaluated fracture properties can be used to prevent catastrophic failure or for optimal design of the LiNbO3 crystal at service.