V. M. Krymov

Faceting of sapphire crystals grown from a melt by the Stepanov method

The appearance of faces on cylindrical sapphire single crystals grown from a melt by the Stepanov method is studied by photogoniography and optical microscopy. The crystallographic indices of the detected faces are established, and the microstructure of the growth layers is investigated. A relationship between the faceting and the growth conditions is found. The experimental results are compared with data on the faceting of the sapphire crystals grown from a solution-melt and with the calculated specific free surface energies of the faces.

Temperature distribution near the interface in sapphire crystals grown by EFG and GES methods

This paper presents results of experiments on in situ temperature measurements during sapphire shaped crystal growth. The temperature distribution difference between the crystals grown by EFG (edge-defined, film-fed growth) and GES (growth from an element of shape) methods is considered.

The formation of a dislocation structure in ribbon-shaped germanium single crystals under thermal stress

Abstract The formation of the dislocation structure in germanium single crystal ribbons grown from melt by Stepanovs method has been studied. It is shown that the relatively high dislocation density in the ribbons is due to thermal stresses present during growth and that the dislocation density can be controlled by properly varying the temperature of a radiation shield. Data are presented on the temperature distribution along the ribbon pulling axis, the tangential thermoelastic stress pattern, and the distribution of dislocations for three different shield geometries. The distribution of dislocations over the ribbon width is shown to correlate well with that of the rms tangential stresses. Samples of low dislocation density germanium ribbon were produced.

The influence of thermal screens on the temperature distribution, thermal stress, and defect structure during growth of shaped sapphire crystals

The ways in which a block structure is formed in shaped sapphire single crystals grown from melt by the Stepanov method are considered. The measured temperature distributions and results of a mathematical modeling of the heat exchange in the growth zones, as well as the calculated thermoelastic fields and measured residual stresses, are reported. The possibility of effectively controlling the thermal fields and growth of block-free crystals by choosing optimal screening is shown for single crystals in the form of tubes and basal-plane-faceted ribbons.

Residual stresses in sapphire rods grown by the Stepanov method

The residual stresses in cylindrical [0001] sapphire crystals have been studied using the polarization-optical method. The angle between the optical axes 2V and the difference in the normal components of the tensor of elastic residual stresses (σφ − σr) have been determined from the isogyre divergence. It has been found that a tangential tensile stress of no more than 20 MPa acts on the ingot surface. The residual stresses have been compared with the calculated thermoelastic stresses generated during the crystal growth in a given heating zone. It has been shown that the determined pattern of residual stresses can be caused by thermoelastic stresses developing in the immediate vicinity of the crystallization front.

Study of the real structure of basal-plane-faceted sapphire ribbons

The structure of basal-plane-faceted sapphire ribbons with cross sections of 30 × 1.5 and 50 × 1.5 mm, grown by the Stepanov method, has been investigated using X-ray diffraction topography and selective etching. The model of block formation in such ribbons through interaction of dislocations of prismatic slip system is confirmed. The role of gas bubbles as stress concentrators and dislocation sources is established. It is shown that the dislocations of basal-slip system are involved in the development the low-angle boundaries and block formation.

Blocks and residual stresses in sapphire rods of different crystallographic orientations grown by the Stepanov method

The formation of blocks in shaped sapphire rods of two crystallographic orientations has been investigated. It is shown that, when growth occurs in the direction of the optical c axis, blocks are formed with a higher probability than in the case of growth in the a direction. A model of formation of blocks in rods of different orientations is proposed. The distribution of residual stresses over sapphire rod cross sections is measured by conoscopy. It is found that stresses increase from the middle of a rod to its periphery and reach 20 MPa.

Control of temperature field and residual stresses in growing basal-plane-faceted sapphire ribbons

The influence of different heat shielding constructions on the distribution of temperature and thermoelastic and residual stresses in growing basal-plane-faceted sapphire ribbons (Al2O3) is studied. It is shown that inclined shields decrease thermoelastic stresses owing to the redistribution of heat fluxes from a heater to a ribbon, which allows growing block-free basal-plane-faceted sapphire ribbons.

Block structure and residual stresses in tubular sapphire single crystals grown by the Stepanov method

The formation of the block structure and residual stresses in sapphire single-crystal tubes grown from the melt by the Stepanov method has been studied. The distribution of the difference in the residual stress tensor components (σφ–σr) in a thick-walled [0001] sapphire tube has been measured by the conoscopy method. It has been shown that circumferential tensile σφ and compressive stresses act on the outer and inner tube surfaces, respectively. The maximum stress is ∼20 MPa. It has been demonstrated that the development of the block structure begins from the outer tube surface; as the crystal length increases during growth, blocks propagate over the entire cross section.

Thermal field simulation and heat zone optimization for the growth of 50-mm basal-plane-faceted sapphire ribbons

The global heat transfer during growth of 50-mm basal-plane-faced sapphire ribbons in a cylindrical heat zone has been numerically simulated for different heat shield configurations. The ribbon thermoelastic strains were computated to estimate the heat zone quality. It is shown that shield adjustment and redistribution of radiative heat fluxes inside the heat zone make it possible to essentially decrease the thermal field curvature in the ribbon around the crystallization front and, therefore, decrease the thermoelastic strains by a factor of 2 to 3 (to 20 MPa).