Yu. A. Serebryakov

Investigation of structural peculiarities of impure lithium niobate crystals by Raman spectroscopy

Abstract Polarized Raman spectra of lithium niobate single crystals doped with Mg 2+ , Gd 3+ , Y 3+ and Ta 5+ cations were studied. Threshold concentrations for Mg 2+ and Gd 3+ cations were shown to exist below which these dopants cause an ordering effect of the Li + and Nb 5+ cations and an octahedral alignment of the hollows along the crystal polar axis. This ordering can be seen in spectra as a narrowing of the 254 and 274 cm −1 modes corresponding to fundamental vibrations of the Li + and Nb 5+ ions located inside the oxygen octahedra and as a splitting into two components of the two-phonon line in the region of 100–120 cm −1 caused by the light scattering on two bound acoustic phonons of A 1 symmetry. Above the maximum concentrations Mg 2+ and Gd 3+ ions disorder the crystal structure, which leads to the loss of the fine structure for the two-phonon line whereas the 254 and 274 cm −1 modes considerably broaden and almost merge into one band. The Y 3+ and Ta 5+ ions worsen the ordering of the cations and the octahedral hollows along the polar axis and cause, in addition, deformation of the crystal oxygen carcass.

Effects of nonstoichiometry and doping on the curie temperature and defect structure of lithium niobate

LiNbO3 crystals doped with Gd, Zn, Cu, and Er to various levels were grown by the Czochralski technique. The structural parameters, Curie temperature, and density of the LiNbO3 crystals were determined as a function of composition. With increasing Gd content, thea parameter of the hexagonal cell increases, c remains constant within the experimental error, density increases, and Tc gradually decreases. The defect structure and properties of LiNbO3 doped with Gd, Zn, Cu, Er, Mg, and Ta are contrasted with those of nominally undoped LiNbO3 crystals differing in Li/Nb ratio. The results suggest that the variation of the Curie temperature with composition is determined primarily by the position that the dopant cations occupy in the structure of lithium niobate.

The structural ordering and photorefraction in lithium niobate admixed crystals

By Raman spectra photorefraction properties of lithium niobate congruent crystals doped with Mg2+, B3+, Gd3+, Y3+ ions have been studied. The photorefraction dependence of admixed ions concentration in the crystal has a clearly pronounced minimum in the area of minor impurity concentration. Lithium niobate cation sublattice there is characterized by the best ordering along the polar axis. The connection between lithium niobate crystalline structure ordering and its photorefraction properties is discussed.

Numerical and experimental investigations of convection and heat/mass transfer effect in melts on inhomogeneity formation during Ge crystal growth by the Bridgman method

Abstract The role of convection in heat and mass transfer processes under terrestrial and space conditions is investigated on the basis of the assessment of dimensionless parameters. It has been shown that convection near the crystallization front will be of nonstationary character due to reconstruction of vortex regions with the decrease of the melt height. Numerical calculations of Oberbeck–Boussinesq equations have shown that nongravitational motion sources (Marangoni convection) generate under space conditions a basic flow which is of higher intensity than under earth conditions. The total time of flow evolution increases essentially. This nonlinear effect of space conditions on hydrodynamics will bring about qualitative changes in heat and dopant transfer processes. The experiments were performed on crystallization of germanium doped with gallium at reduced natural convection. It has been shown that inhomogeneities appear in the crystal in the form of dopant striations at achieving a certain level of convection. Theoretical predictions are in agreement with experimental data obtained.

Structural ordering and ferroelectric properties of LiTaxNb1-xO3 solid solutions

Abstract Roman scattering and second harmonic generation in the LiTaxNb1−xO3 solid solutions have been studied. The structure disordering with increasing x was shown to occur first in the cation sublattice due to violation of cation ordering along the polar axis. Further increase of x leads to growth of this disordering and to deformation of the oxygen structure of the crystal. Behaviour of spectral parameters for some lines and of spontaneous polarization indicates the existence of specific concentrations points at x ∼ 0.4 and x ∼ 0.6 with maximal deformation of the crystalline unit cell.

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals grown under various conditions of heat and mass transfer are studied by methods of X-ray topography, high-resolution X-ray diffractometry, and digital image processing. It is established that the inhomogeneity of crystals is determined by specific features of impurity microsegregation during growth under conditions of nonstationary convection in a melt and by peculiarities of the dislocation structure of crystals. The processes related to the initial stage of the decay of the Si supersaturated solid solution in GaSb contribute considerably to the inhomogeneity of crystals on the micro- and macrolevels.

Fast ionic transport in Li x Na1-xTa y Nb1-yO3 (LNTN) ferroelectric solid solutions

Abstract The complex admittance and dielectric properties of LNTN solid solutions have been studied as functions of frequency in the range of 10 Hz to 10 MHz. The obtained energy and kinetic characteristics of ionic transport are discussed.

Some results of the growth of semiconductor crystals in microgravity conditions (to the 50th anniversary of Yuri Gagarin’s flight into space)

The history of the growth of semiconductor crystals aboard space vehicles and their subsequent investigation has been described shortly. It has been shown using Ge(Ga), GaSb(Si), and GaSb(Te) crystals as an example that the formation of segregation growth striations can be avoided during their recrystallization by the vertical Bridgman method in conditions of physical simulation of microgravity on the Earth, mainly due to the essential weakening of the thermal gravitation convection. By their structure and impurity distribution, they approach the crystals grown in space. The investigation of recrystallization of Te has made it possible to determine the role of the detachment effect characteristic of the microgravity conditions and the features of the microstructure of the samples that crystallize with a free surface. The analysis of the results obtained from experiments in space allows us to better understand the processes occurring during the crystallization of the melts and to improve the crystal growth in terrestrial conditions.

Growth of high-homogeneity GaSb:Te crystals for thermophotovoltaic energy converters

Problems concerning the technology of growing highly homogeneous semiconductor crystals are discussed. The dependence between the inhomogeneity of GaSb:Te substrates and the efficiency of thermophotovoltaic converters (TPVCs) fabricated on their base via the diffusion technique is examined. The macro- and microhomogeneities of grown crystals can be substantially increased on account of the theoretically substantiated and experimentally implemented approximation to diffusive mass transfer conditions in the melt. The characteristics of TPVCs on substrates made of crystals with the most homogeneous properties exceed the analogous characteristics of other samples.

Comparative studies of the features of the formation of impurity heterogeneity in GaSb:Te crystals in the case of directed crystallization under space and ground conditions

The results of comparative studies concerned with the formation of impurity heterogeneity in GaSb:Te crystals grown at the POLIZON facility by the Bridgman method under space and ground conditions are presented. Crystallization is carried out in the same temperature-time modes of the heaters: on board the Foton-M 3 spacecraft in space and by the vertical Bridgman method under conditions of weakened thermogravitational convection on Earth. In both cases, Marangoni convection is eliminated, and the effect of vibration microaccelerations on the ampoule that contains the melt is minimized. The microuniformity of the dopant distribution in the crystal areas that are recrystallized by the Bridgman method is higher than that in the seed grown by the Czochralski method. Microuniformity is higher under space conditions than under ground conditions with weakened thermogravitational convection. Spectral Fourier analysis of the spreading resistance distribution Rs reveals the characteristic periods of heat and mass transfer processes in the melts under ground and space conditions.