M. D. Malinkovich

Effect of annealing on the structure and phase composition of thin electro-optical lithium niobate films

We have studied the formation of thin textured LiNbO3 films in originally amorphous samples produced by rf magnetron sputtering of a single-crystal target on silicon substrates containing a native oxide layer. The results demonstrate that postgrowth annealing leads to the formation of two phases, LiNbO3 and LiNb3O8, and that the percentage of the nonferroelectric phase LiNb3O8 is minimal after annealing at a temperature of 700°C. Annealing at 700°C is optimal because it ensures the lowest surface roughness of the film, the highest degree of structuring of the ferroelectric phase, and the maximum contrasts corresponding to the vertical and lateral components of the ferroelectric polarization in piezoresponse force microscopy.

Formation of bidomain structure in lithium niobate plates by the stationary external heating method

The method of development of the bidomain structure in single crystalline lithium niobate plates based on the creation of a given temperature gradient distribution through a sample thickness by stationary heating is considered. Heating the LiNbO3 plate, which is placed between two silicon plates, is implemented by light energy emitted by lamps of the photonic annealing setup, which is absorbed by silicon. The scheme of the technological cell provides the formation and control over heat fluxes penetrating a ferroelectric plate and forming temperature gradients required for the controlled formation of two domains with the opposite polarization vectors (a head-to-head domain structure). The efficiency of light absorption for the formation of heat sources, which can be used for symmetric and asymmetric heating, which determines the position of the conditional surface with a zero temperature gradient and, consequently, a domain boundary position, is confirmed experimentally. In the LiNbO3 plate with a thickness of 1.6 mm and length 60 mm, a symmetric bidomain structure with oppositely directed polarization vectors is formed. The dependence of the bending strain of a console-clipped sample on electric voltage is studied in the temperature range −300 to +300 V; the strain amplitude is more than 35 μm. The high linearity and repeatability of the electric voltage-bending strain characteristic is shown.

Interdomain region in single-crystal lithium niobate bimorph actuators produced by light annealing

The interdomain region of a bidomain strucrture formed in 127°-cut lithium niobate single crystals using light annealing has been studied by optical and scanning probe microscopies. A periodic subdomain structure on the 180° macrodomain wall is visualized by piezoresponse force microscopy. The piezoresponse signal (polarization) is shown to be a power-law function of the domain width with an exponent n = 0.53.

Propagation of polarization of ferroelectric grains in electrically isolated lithium niobate films

Piezoresponse force microscopy has been applied for the study of ferroelectric properties of lithium niobate thin films. The films have been obtained on a silicon surface {110}, coated with a thin layer of SiO2 using high-frequency magnetron spraying of lithium niobate. It has been demonstrated that after polarization of a sample surface segment with positive or negative potential using a conductive probe of an atomic force microscope the effect of propagation (spreading) of the region of grain polarized state with a decrease in the piezoresponse amplitude is observed. Moreover, the polarization occurs stepwise, that is, the Barkhausen effect takes place at the nanometer level.

Study of LiNbO3 single crystals with a regular domain structure by piezoresponse force microscopy

Lithium niobate crystals with a regular domain structure have been studied by piezoresponse force microscopy. The period of regular domains and the domain-wall width (w = 45 nm) have been calculated for Z- and Y-cut crystals based on an analysis of two-dimensional images of the domain-structure piezoresponse. It is shown that for the Y-cut crystal, both positive and negative domain boundaries can be identified when recording the lateral component of piezoresponse.

Bidomain structures formed in lithium niobate and lithium tantalate single crystals by light annealing

The bidomain structures produced by light external heating in z-cut lithium niobate and lithium tantalate single crystals are formed and studied. Interdomain regions about 200 and 40 μm wide in, respectively, LiNbO3 and LiTaO3 bidomain crystals are visualized and studied by optical microscopy and piezoresponse force microscopy. Extended chains and lines of domains in the form of thin layers with a width less than 10 μm in volume, which penetrate the interdomain region and spread over distances of up to 1 mm, are found.

Deformation Anisotropy of Y + 128°-Cut Single Crystalline Bidomain Wafers of Lithium Niobate

Bidomain single crystals of lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) are promising materials for use as actuators, mechanoelectrical transducers, and sensors capable of working in a wide temperature range. One need to take into account the anisotropy of the properties of the crystalline material when such devices are designed. In this study we investigated deformations of bidomain round shaped Y + 128°-cut wafers of lithium niobate in an external electric field. The dependences of the piezoelectric coefficients on the rotation angles were calculated for lithium niobate and lithium tantalate and plotted for the crystal cuts which are used for the formation of a bidomain ferroelectric structure. In the experiment, we utilized an external heating method and long-time annealing with the lithium out-diffusion method in order to create round bidomain lithium niobate wafers. Optical microscopy was used to obtain the dependences of the bidomain crystals’ movements on the rotation angle with central fastening and the application of an external electric field. We also modelled the shape of the deformed bidomain wafer with the suggestion that the edge movement depends on the radial distance to the fastening point quadratically. In conclusion, we revealed that the bidomain Y + 128°-cut lithium niobate wafer exhibits a saddle-like deformation when a DC electric field is applied.

Surface topography and crystal and domain structures of films of ferroelectric copolymer of vinylidene difluoride and trifluoroethylene

The crystallization of a copolymer from a solution at room temperature is found to lead to the formation of a metastable structure, characterized by the coexistence of ferroelectric and paraelectric phases. The fraction of the latter decreases after annealing above the Curie point. Atomic force microscopy (AFM) has revealed a difference in the surface topographies between the films contacting with air and the films contacting with a glass substrate. The microstructure of copolymer chains has been investigated by 19F NMR spectroscopy. The chain fragments with “defect” attached monomeric units are ejected to the surface. The character of the ferroelectric domains formed during crystallization and their size distribution are analyzed.

Formation of a bidomain structure in lithium niobate wafers for beta-voltaic alternators

The possibility of increasing the efficiency of a beta-voltaic generator due to using a single-crystal bimorph element made of lithium niobate as a piezoelectric converter. The known beta voltaic alternators consist of a piezoelectric cantilever and a source of β-electrons. The cantilever represents a resilient member made, for example, of silicon, on which a piezoelectric element made of PZT piezoceramics is mounted. It is proposed to replace the silicon cantilever structure with a piezoelectric element by a uniform cantilever that represents a thin wafer made of a bidomain single-crystal lithium niobate. Due to this, the efficiency of the mechanical oscillation conversion into electrical power, the system Q-factor, and the stability of the operating parameters simultaneously increase; and the operation temperature range also significantly increases (by several hundred degrees). The solution of the main problem—the formation of a bidomain structure in a thin wafer of lithium niobate—is considered in detail. A method for the high-temperature annealing of samples in a nonuniform electric field is proposed. It is demonstrated that one can predict the domain structure based on the developed model. Samples are obtained having the occurrence depth of the interdomain boundary ranging from 120 to 150 μm. At the same time, it is shown that the sharpness of the boundary depends on the potential difference between the striated electrodes of the technological cell and the external electrode. The method is efficient for manufacturing a bidomain structure in a wafer up to 300 μm thick.

Studying local conductivity in LiNbO3 films via electrostatic force microscopy

The distribution of conductivity is studied in nanocristalline lithium niobate thin films via electrostatic force microscopy. Conductivity images are obtained at different bias voltages. Grain boundaries are shown to play a predominant role in the conductivity in lithium niobate thin films.