A. S. Bykov

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.

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.

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.

The effect of silicon-substrate orientation on the local piezoelectric characteristics of LiNbO3 films

The domain structure of lithium-niobate thin films grown on Si(111) and Si(100) substrates coated with a native oxide layer with a thickness of no less than 2 nm is investigated by X-ray diffraction, scanning electron microscopy and piezoresponse force microscopy. The films are synthesized by the rf magnetron sputtering of a single-crystal lithium-niobate target. A high degree of grain orientation in the polycrystalline films is demonstrated. The piezoelectric coefficients dzz of the lithium-niobate films on Si(111) and Si(100) substrates are calculated from the measured dependences of the amplitude of the piezoresponse signal on the ac voltage applied between the cantilever tip and the substrate. Piezoelectric hysteresis loops are obtained in the remanent piezoelectric response regime

Application of Radioactive Isotopes for Beta-Voltaic Generators

The features of using radioactive isotopes when creating off-line power supplies are considered. The analysis of the substances used in radioisotope thermoelectric generators (RTGs) is carried out. The prospects for manufacturing beta-voltaic generators are justified and they are compared with other electric power sources. The mechanism of β-decay and its place among other types of nuclear transformations is considered. The basic requirements for radiation safety and the used materials of the frame and converter are formulated. Some designs of radioisotope beta-voltaic sources proposed earlier are considered. A list of isotopes that can be used as a power source in a beta-voltaic generator is presented. The methods for obtaining the radioactive materials demonstrating β-decay and their basic properties and natural isotopes are considered. It is concluded that the choice of nickel-63 isotope is preferable for use in beta-voltaic generators due to the optimal combination of its half lifetime, average particle energy, and radiation intensity.

ФОРМИРОВАНИЕ БИДОМЕННОЙ СТРУКТУРЫ В ПЛАСТИНАХ НИОБАТА ЛИТИЯ, ПРЕДНАЗНАЧЕННЫХ ДЛЯ БЕТА–ВОЛЬТАИЧЕСКИХ ГЕНЕРАТОРОВ ПЕРЕМЕННОГО ТОКА

This article discusses the possibility of increasing the efficiency of betavoltaic generators by using lithium niobate single−crystal bimorph as the piezoelectric transducer element. Existing betavoltaic alternating voltage generators consist of a piezoelectric cantilever and a  electron source, wherein the cantilever is a resilient member, for example silicon, to which a PZT ceramics piezoelectric element is connected. In this study we suggest changing the structure of the silicon cantilever with a piezoelectric element for a uniform cantilever which is a thin plate of bidomain lithium niobate single crystal. This increases the efficiency of converting mechanical vibrations to electrical power, Q of the system, and the stability of the working parameters, and furthermore significantly increases — up to several hundred degrees — the operation temperature range. We have considered in details the solution of the main task —formation of a bidomain structure in a thin lithium niobate plate. A method of the sample high−temperature annealing in a nonuniform electric field is proposed. The possibility of domain structure prediction on the basis of the developed model is shown. Samples with a domain boundary depth of 120—150 microns have been obtained, and we have shown that the clarity of the boundary depends on the voltage between the working cell strip electrodes and the external electrode. The method is effective for bidomain structure formation in plates of about 300 microns in thickness.

Initiation of a polarized state in thin lithium niobate films synthesized on isolated silicon substrates

The distributions of induced state and surface potential in thin lithium niobate films obtained via the deposition onto a (100)-oriented Si-oxidized substrate by the method of high-frequency magnetron sputtering were mapped using piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). Using an electrical field applied with a conducting cantilever, the possibility of the formation of an induced polarization state and of its subsequent visualization was shown. The induced state in KPFM measurements was shown to be retained for a much longer time than for PFM.