Domain Walls In Lithium Niobate Research Articles

Electrophysical properties, memristive and resistive switching of charged domain walls in lithium niobate

Charged domain walls (CDWs) in ferroelectric materials raise both fundamental and practical interest due to their electrophysical properties differing from bulk ones. On a microstructure level, CDWs in ferroelectrics are 2D defects separating regions with different spontaneous polarization vector directions. Screening of electric field of the CDW's bound ionic charges by mobile carriers leads to the formation of elongated narrow channels with an elevated conductivity in initially dielectric materials. Controlling the position and inclination angle of CDW relative to the spontaneous polarization direction, one can change its conductivity over a wide range thus providing good opportunities for developing memory devices, including neuromorphic systems. This review describes the state of art in the formation and application of CDWs in single crystal uniaxial ferroelectric lithium niobate (LiNbO3, LN), as resistive and memristive switching devices. The main CDWs formation methods in single crystal and thin-film LN have been described, and modern data have been presented on the electrophysical properties and electrical conductivity control methods of CDWs. Prospects of CDWs application in resistive and memristive switching memory devices have been discussed.

Electrophysical properties, memristive and resistive switching in charged domain walls in lithium niobate

Charged domain walls (CDW) in ferroelectric materials are interesting from fundamental and applied points of view, since they have electrical properties different from bulk ones. At the microstructural level, CDW in ferroelectrics are two-dimensional defects that separate regions of the material with different directions of spontaneous polarization vectors. Compensation of the electric field of the bound ionic charge of the CDW by mobile carriers leads to the formation of extended narrow channels with increased conductivity in the original dielectric material. By controlling the position and angle of inclination of the CDW relative to the direction of spontaneous polarization, it is possible to change its conductivity in a wide range, which opens up broad prospects for creating memory devices, including for neuromorphic systems. The review presents the current state of research in the field of formation and application of CDW formed in single crystals of uniaxial ferroelectric lithium niobate (LiNbO3, LN) as resistive and memristive switching devices. The main methods for forming CDW in single crystals and thin films of LN are considered, and modern data on the electrophysical properties and methods for controlling the electrical conductivity of CDW are presented. The prospects for using CDW in memory devices with resistive and memristive switching are discussed.

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.

Tailoring of stable induced domains near a charged domain wall in lithium niobate by probe microscopy

Ferroelectric lithium niobate (LiNbO3) crystals with an engineered domain structure have a number of applications in optical systems for generation of multiple laser radiation harmonics, acoustooptics, precision actuators, vibration and magnetic field sensors, including those for high-temperature applications, and prospectively, in non-volatile computer memory. We have studied the effect of charged domain boundary on the formation of induced domain structures in congruent lithium niobate (LiNbO3) crystals at the non-polar x-cut. Bi- and polydomain ferroelectric structures containing charged “head-to-head” and “tail-to-tail” type domain boundaries have been formed in the specimens using diffusion annealing in air ambient close to the Curie temperature and infrared annealing in an oxygen free environment. The surface potential near the charged domain wall has been studied using an atomic force microscope (AFM) in Kelvin mode. We have studied surface wedge-shaped induced microscopic domains formed at the charged domain boundary and far from that boundary by applying electric potential to the AFM cantilever which was in contact with the crystal surface. We have demonstrated that the morphology of the induced domain structure depends on the electrical conductivity of the crystals. The charged “head-to-head” domain boundary has a screening effect on the shape and size of the domain induced at the domain wall. Single wedge-shaped domains forming during local repolarization of reduced lithium niobate crystals at the AFM cantilever split into families of microscopic domains in the form of codirectional beams emerging from a common formation site. The charged domain wall affects the topography of the specimens by inducing the formation of an elongated trench, coincident with the charged boundary, during reduction annealing.

Tilt control of the charged domain walls in lithium niobate

The utilization of charged domain walls (CDWs) as nanoelectronic devices requires three technological aspects to be developed: (1) controllable CDW creation, (2) tuning of CDW geometry, and (3) CDW removal. We have studied CDW formation in lithium niobate single crystals during polarization reversal using liquid and solid-state electrodes, as well as their combination. It was shown that CDW can be formed by two alternative procedures: (1) by forward switching for the liquid electrode at the Z+ polar surface and solid at the Z− one, (2) by backward switching for solid at the Z+ polar surface and liquid at the Z− one. The created CDW can be transformed to an almost neutral domain wall by the application of the field pulse of the reverse polarity for both procedures. As a result, the tilt of the formed CDW can be tuned reversibly in the range from 0.2° to 1.2° resulting in a change between isolated and highly conductive states. We have demonstrated that the created CDW can be used as a nanoelectronic channel for local electrolysis opening the additional possibilities for ferroelectric lithography.

Large and accessible conductivity of charged domain walls in lithium niobate

Ferroelectric domain walls are interfaces between areas of a material that exhibits different directions of spontaneous polarization. The properties of domain walls can be very different from those of the undisturbed material. Metallic-like conductivity of charged domain walls (CDWs) in nominally insulating ferroelectrics was predicted in 1973 and detected recently. This important effect is still in its infancy: The electric currents are still smaller than expected, the access to the conductivity at CDWs is hampered by contact barriers, and stability is low because of sophisticated domain structures or proximity of the Curie point. Here, we report on large, accessible, and stable conductivity at CDWs in lithium niobate (LN) crystals – a vital material for photonics. Our results mark a breakthrough: Increase of conductivity at CDWs by more than 13 orders of magnitude compared to that of the bulk, access to the effect via ohmic and diode-like contacts, and high stability for temperatures T ≤ 70 °C are demonstrated. A promising and now realistic prospect is to combine CDW functionalities with linear and nonlinear optical phenomena. Our findings allow new generations of adaptive-optical elements, of electrically controlled integrated-optical chips for quantum photonics, and of advanced LN-semiconductor hybrid optoelectronic devices.

Formation of self-organized domain structures with charged domain walls in lithium niobate with surface layer modified by proton exchange

We have studied the self-organized dendrite domain structures appeared as a result of polarization reversal in the uniform field in lithium niobate single crystals with the artificial surface layer created by proton exchange. We have revealed the self-organized sub-micron scale dendrite domain patterns consisting of domain stripes oriented along the X crystallographic directions separated by arrays of dashed residual domains at the surface by scanning probe microscopy. Raman confocal microscopy allowed visualizing the quasi-regular dendrite domain structures with similar geometry in the vicinity of both polar surfaces. The depth of the structure was about 20 μm for Z+ polar surface and 70 μm for Z− one. According to the proposed mechanism, the dendrite structure formation at the surface was related to the ineffective screening of the residual depolarization field. The computer simulation of the structure formation based on the cellular automata model with probabilistic switching rule proved the eligibility of the proposed scheme, the simulated dendrite domain patterns at various depths being similar to the experimental ones.

Charged Domain Walls in Lithium Niobate with Inhomogeneous Bulk Conductivity

The surface treatments by annealing in vacuum and by low energy ion irradiation have been used for inhomogeneous modification of the bulk conductivity of lithium niobate single crystals. The obtained inhomogeneous conductivity resulted in significant decreasing of external electric field in the treated volume. The inhomogeneous field distribution allowed us to obtain the polarization reversal in the bulk only, which led to formation of the domains with charged domain walls. The geometry of charged domain walls has been investigated by various methods. The proposed treatment techniques can be used for domain engineering in lithium niobate crystals.

OS1402-347 LiNbO_3のドメインウォールにおける電気伝導性の制御

OS1402-347 LiNbO_3のドメインウォールにおける電気伝導性の制御

Local probing of the interaction between intrinsic defects and ferroelectric domain walls in lithium niobate

We demonstrate the capability of confocal Raman spectroscopy to characterize nanoscale interactions of defects with ferroelectric domain walls by identifying defect-related frequency shifts in congruent lithium niobate. These shifts resemble those observed for an external field applied anti-parallel to the ferroelectric axis, suggesting a small reduction of the electric polarization. Density functional theory calculations suggest that this reduction results from a change in the intrinsic defect cluster structure and polarization at the domain wall.

Raman Study of Neutral and Charged Domain Walls in Lithium Niobate

In this work, we demonstrate the application of micro-Raman spectroscopy for the investigation of the neutral and charged domain walls in the congruent and MgO-doped lithium niobate single crystals. We present local changes of the intensity and frequency of three Raman lines in the vicinity of the domain walls and discuss the influence of the bulk screening of depolarization field on these parameters.

Field-induced light deflection in lithium niobate and lithium tantalate: the possible configurations

Field-induced light deflection by ferroelectric domain walls in lithium niobate (LN) and lithium tantalate (LT) is theoretically studied. The phenomenon can occur not only when both the incident wave vector k and the electric field E are parallel to the z-axis—as demonstrated by experiments made so far—but also when E and/or k are perpendicular to z. In particular, for E parallel to x and k parallel to y, the deflection phenomenon is predicted to have the same characteristics as in triclinic or monoclinic ferroelastics: the large deflection angle is related to the natural birefringence, whereas the deflection amplitude is proportional to the small tilt angle of the neutral lines, which is here induced by the electric field. In periodic domain structures, interference between deflected waves occurs, and the deflected intensity is expected to be largely enhanced when the Bragg condition is satisfied. This transverse configuration is thus specially attractive to characterize periodically-poled crystals.

Defect based real-time diagnostics of ferroelectric domain wall motion

We investigated the static and dynamic interaction of defect dipoles and ferroelectric domain walls in lithium niobate. Using a home-built confocal microscope with high spatial and temporal resolution we are able to image both the static domain wall region and the domain wall regions associated with a moving domain wall by monitoring the emission from rare earth ions. We find that the extent of perturbation induced by the presence of a domain wall is increasing with the speed of the domain wall motion.

Local electromechanical response at a single ferroelectric domain wall in lithium niobate

We report the presence of an unexpected asymmetry in the local electromechanical response across a single ferroelectric domain wall in congruent lithium niobate at room temperature. We provide evidence that this is related to the presence of nonstoichiometric defects in the crystal, which interact with the domain wall.

Long-range strains and the effects of applied field at 180° ferroelectric domain walls in lithium niobate

Ferroelectric domains with antiparallel polarization are readily induced in congruent ${\mathrm{LiNbO}}_{3}$ with electric fields above 240 kV/cm at room temperature. Even in the absence of external fields, these 180\ifmmode^\circ\else\textdegree\fi{} walls exhibit wide regions of shear strain, on the order of ${10}^{\ensuremath{-}5},$ within a 10-\ensuremath{\mu}m range of the domain walls. Using x-ray topography on samples while applying electric fields of 0--90 kV/cm, we have observed large-scale reversible domain changes. A detailed strain analysis of the piezoelectric behavior at the domain walls, as well as within the domains, indicates that substantial surface displacement is associated with the high contrast of ferroelectric domains in x-ray topographs. These observations show that long-range strain interactions due to applied fields are present around domain walls long before permanent changes are induced.