Sapphire Ribbons Research Articles

Imaging of micro-steps on as-grown surface of sapphire with X-ray phase contrast technique

Basal-faceted sapphire ribbons grown using the Stepanov–LaBelle technology have a low density of surface steps. We present our results on a study of steps on the surface of a ribbon, misoriented relative to the singular face (0001) by several arc minutes. The ribbon has been characterized by means of in-line phase contrast imaging technique at Pohang Light Source, South Korea. It was shown for the first time that a step height of about 1 μm can be determined directly from an image. The step height obtained using the phase contrast method was confirmed by atomic force microscopy measurements. We have found that the experimental contrast matches the theoretical simulations only if the calculated intensity profile has been convolved with a Gaussian function. The full width at half maximum of the Gaussian was independently got from previous measurements. We have obtained an analytical solution in the case of theoretical fully coherent phase contrast image. The inverse problem is easy to solve, since there is a direct proportionality between the contrast and the step height.

Study of Structural Defects in Sapphire Ribbons using X-Ray Topography and Coherent Imaging in Synchrotron Radiation

Study of Structural Defects in Sapphire Ribbons using X-Ray Topography and Coherent Imaging in Synchrotron Radiation

Large-Area Mapping of Voids and Dislocations in Basal-Faceted Sapphire Ribbons by Synchrotron Radiation Imaging.

The understanding of structural defects in basal-faceted sapphire ribbons was improved through X-ray imaging at a synchrotron source. The combination of phase contrast and X-ray diffraction makes it possible to visualize and characterize both gas voids and dislocations in the bulk of the ribbons grown by the Stepanov-LaBelle technology. Dislocations were directly related to gas voids. X-ray diffraction topography was employed to investigate the distribution, configurations, and character of the dislocations. The formation of voids of irregular shapes was detected by large-area mapping with spatial resolution in the μm range. Computer simulations of the experimental phase contrast images of microvoids were performed. The sizes of the spherical microvoids were determined. The results are discussed with reference to the available data on the emission of dislocations from the voids. The evolution of the shape, size, and arrangement of the voids during growth provides clues on the formation of block structure in basal-faceted sapphire ribbons.

Расчет остаточных напряжений при коноскопических измерениях в профилированных монокристаллах сапфира

The article deals with the measurement of residual stresses that are formed in shaped sapphire single crystals when they are grown from a melt. An algorithm is developed for calculating the normal components of the residual stress tensor from the data of optical measurements of residual stresses by the conoscopy method for optically uniaxial crystals in the form of plates. Examples of measuring residual stresses and calculating normal components for a square rod and a basal-faceted sapphire ribbon are given. It is shown that normal stresses are concentrated mainly along the periphery of the plates and reach up to 25 MPa in the rod and 70 MPa in the strip.

Numerical Analysis of Thermal Stress in Semi‐Transparent Oxide Crystals Grown by Czochralski and EFG Methods

AbstractCrystal growth of oxides is generally difficult since large curvatures of the growth interface in these systems generate high thermal stress, dislocations and crystal cracking. Three‐dimensional numerical modeling is applied to investigate thermal stress distribution in sapphire and langatate La3Ta0.5Ga5.5O14 (LGT) semi‐transparent crystals grown by Czochralski (Cz) and Edge‐defined Film‐fed Growth (EFG) techniques. The analysis of thermal stress distribution in a sapphire ingot grown in a Czochralski furnace shows high von Mises stresses distributed almost symmetrically on large areas in the crystal. Thermal stress computations for piezoelectric langatate crystals grown in a Czochralski configuration show non‐symmetrical von Mises distribution with higher stress on one side of the ingot. These numerical results are in agreement with experimental results showing non‐symmetrical cracking at the outer surface of the crystal. 3D modeling of multi‐die EFG growth of white sapphire ribbons shows that the von Mises stress is almost constant when the number of ribbons is increased from two to ten. Two models are applied to simulate the internal radiative heat transfer in the sapphire crystals: P1 approximation and the Rosseland radiation model. Numerical results show that applying Rosseland formula introduces significant errors in temperature field calculations especially in the case of the EFG configuration.

Tuning the sapphire EFG process to the growth of Al2O3/YAG/ZrO2:Y eutectic

In this work, a model is proposed, in order to analytically study the working point of the Edge defined Film-fed Growth (EFG) pulling of crystal plates. The model takes into account the heat equilibrium at the interface and the pressure equilibrium across the meniscus. It is validated on an industrial device dedicated to the pulling of sapphire ribbons. Then, the model is applied to pulling ceramic alloy plates, of the ternary eutectic Al2O3/YAG/ZrO2:Y. This allowed understanding the experimental difficulties of pulling this new material and suggested improvements of the control software. From these results, pulling net shaped ceramic alloy plates was successful in the same industrial equipment as used for sapphire.

Mechanical Characteristics of Sapphire Ribbons Grown Simultaneously by EFG Method

AbstractMechanical properties of ten sapphire ribbons are studied. These ribbons are grown simultaneously with prismatic orientation ( plane and direction) with the dimensions 2 × 85 × 400 mm. Strength and plastic characteristics of the crystals are determined on the as‐grown and mechanically treated surfaces. Peculiarities of the morphology and distribution of pores in thin subsurface layer are considered.

Manufacture of sapphire ribbons with low dislocation density

Manufacture of sapphire ribbons with low dislocation density

Growth and defect structure of basal-plane-faceted sapphire ribbons

Polygonization processes in sapphire ribbons of basal orientation, 50 × 15 mm in cross-sectional dimensions have been studied using an optical polarization method, selective etching, and X-ray diffraction. Using a modified furnace, we have obtained block-free basal-plane-faceted crystals 450 mm in length.

Specific features of the morphology and distribution of gas inclusions in single-crystal sapphire ribbons grown by the Stepanov method

The distribution of gas inclusions in single-crystal sapphire ribbons with different crystallographic orientations, grown by the Stepanov method, is analyzed. In all ribbons gas inclusions are observed only in thin surface layers located at a depth of about 100 μm. One characteristic feature is the presence of several zones with different distributions and morphologies of inclusions. We observed inclusions of the two types: cylindrical pores and microbubbles. The differences in the morphology and zonality of the gas-inclusion distribution are consistent with the α-Al2O3 crystallography.

3D numerical investigation and improvement to the design of the thermal field before seeding in a multi-die edge-defined film-fed growth system for sapphire ribbon crystals

A multi-die edge-defined film-fed growth system was designed to simultaneously grow six sapphire ribbons in a crucible, to increase single-run throughput. The thermal field in the system must be precisely tailored to create conditions for stable growth of multiple crystals. A fully coupled 3D global model, including thermal conduction, thermal radiation, melt convection and gas flow, was established to investigate the thermal field in the system. On the basis of the simulation results, we proposed two methods to improve the thermal field and further simulations were carried out to validate their feasibility. The results indicate that both a reduction in heater height and the use of dies with differing heights are feasible for stable multiple crystal growth, although both have some disadvantages.

Analysis of the growth conditions of long single crystalline basal-plane-faceted sapphire ribbons by the Stepanov/EFG technique

The influence of the growth conditions and process parameters on the formation of blocks during growth of basal-plane-faceted single crystalline sapphire ribbons by the Stepanov/EFG technique was investigated. A number of modifications of the crystallization unit were used to vary systematically the crystal growth conditions. The results gave clearly the growth conditions which are favorable to get blocks-free single crystalline basal-plane-faceted sapphire ribbons.

Growth of sapphire ribbons with capillary channels for laser spectroscopy

Sapphire ribbons with extended capillary channels 450–600 μm in diameter were grown from melt by the Stepanov technique in order to develop a new scalpel capable of fluorescent diagnostics of the state of biological tissue. An automated system was built which allows one to control the shape and quality of sapphire ribbons at all growth stages.

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.

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).

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.

Analysis of the features of meniscus profile curves during growth of base-faceted sapphire ribbons

The dependence of the step parameters on the surface of base-faceted sapphire ribbons on the angle of deviation of crystallographic orientation has been measured. A model of step formation on the lateral ribbon surface is proposed. Hysteretic character of the dependence of the lateral ribbon surface inclination on the meniscus angle at the triple point is shown. The shapes of the lateral surfaces are calculated and the range of variation in the meniscus angle is determined.

Study of the effect of crystallization parameters on gaseous inclusions in sapphire ribbons grown by the Stepanov method

The effect of growth parameters on the two main types of gaseous inclusions in sapphire ribbons—bubbles in the crystal bulk and near-surface pores—have been investigated. The parameters, providing a minimum number of bubbles and a minimum thickness of the surface porous layer, are determined. The experimental data are obtained during the commercial production of sapphire single crystals by the Stepanov method using an expert software package that provides automatic collection and treatment of the process data and the results of a crystal-quality analysis. The effect of the following parameters is considered: the derivative of the mass sensor signal, which indirectly characterizes the temperature of the shaping surface and the melt meniscus height; the seeding power; the melt exposure time in a crucible before seeding; and the technique of shaper preparation.

Investigation of the heat exchange processes during growth of basal-plane-faceted sapphire ribbons by the Stepanov method

A three-dimensional model is proposed for the global heat exchange during growth of basal-plane-faceted sapphire ribbons. The model is based on the method for solving three-dimensional problems of radiative heat transfer in complex regions with diffusely and specularly reflecting boundaries. The temperature distributions in the ribbon and elements of the thermal zone are investigated. It is shown that the developed model correctly reflects the main features of the processes of heat exchange in the crystallization setup and makes it possible to purposefully modify the thermal zone.

Formation of the block structure during growth of basal-plane-faceted sapphire ribbons with different widths

The defect structure of basal-plane-faceted sapphire ribbons 10, 30, and 50 mm wide is investigated. It is shown that the high density of blocks in these crystals is associated with the specific features of the plastic deformation of the crystals with this orientation during crystal growth. In the basal-plane-faceted ribbons, the plastic deformation begins with the slip in the prismatic system. Dislocations of this system form characteristic low-angle boundaries, which then transform into block boundaries. It is revealed that the slip in the easiest basal system, which is characteristic of ribbons with all other orientations, is impossible because of the geometric constraints. Block-free basal-plane-faceted ribbons 50 mm wide are produced by decreasing thermal stresses in growing crystals.