New Advances and Developments in the Stepanov Method for the ...

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NEW ADVANCES AND DEVELOPMENTS IN THE STEPANOV METHODS49V z(‡) (b)ZSeedηV xCurrent pointof the growing hemispherehrR sX1 cmShaperFig. 7. The growth of hemispheres by the GES technique: (a) the growth scheme using the shaper with an inclined end face;(b) photos of sapphire hemispheres up to 50 mm in diameter with walls up to 4 mm thick.should produce a certain effect on the level of thermallyinduced stresses. The EFG technique gives rise only toa nonlinear temperature distribution along the crystalgrowth axis, whereas in the GES technique, it is additionallysupplemented by distortion of the temperaturefield above the shaper, which moves in the course of thecrystal rotation.Despite all this, the performed estimates of thermoelasticstresses calculated using the method of finiteelements in the isotropic approximation for the case ofplanar stresses demonstrate that the stress level for theGES technique is not higher than that characteristic ofthe EFG technique [29]. To analyze the possible mechanismsof the stress formation in the crystals grown bythe GES technique, the results of the numerical simulationfor the time-dependent stressed and strained stateof the growing crystal were used [27]. The modelinvolved calculations of the temperature field, as wellas elastic and viscoplastic stresses. The nucleation ofthe initial crack parallel to the crystal growth directioncan stem from a high positive value (tensile force) ofthe hoopential component σ θθ of the stress tensor. Furtherevolution of the crack takes place occurs graduallywith the crystal growth and is related to the relaxationof the thermoplastic stresses continuously increasingwith the crystal size. Based on this model, the optimummodes for the growth of sapphire hemispheres werechosen to eliminate the cracking in the course of theirgrowth. To attain this optimum in the temperature rangeunder study, it is necessary to have a plastic deformationrate that is not lower than the rate characterizingthe growth of elastic stresses with the crystal size. Theapproximate estimates of these rates for the hemispheresallowed us to find the relationships between thegrowth parameters corresponding to the case when thecondition of the plastic relaxation is met for thermoelasticstresses. In Fig. 7, we present a schematicillustration for the growth of hemispheres by the GEStechnique using a shaper with an inclined end face, aswell as photos of sapphire hemispheres up to 50 mm indiameter with walls up to 4 mm thick [27].SHAPED CRYSTALS WITH DOPED SPATIALSTRUCTURESA topical task in the field of optoelectronics is thedevelopment of novel materials combining severalfunctions. One way to fulfill this task to grow shapedcrystals with modulated spatial structures. Differentversions of the Stepanov technique have paved the waynot only to the manufacture of crystals of any complicatedshape but also to the creation of a new class ofmaterials, namely, crystals with doped spatial structures.Manufacturing of periodic structures. To produceshaped crystals with periodic spatial structures, a versionof the GES technique was implemented whereintwo or more kinds of melt with different impurity contentwere used (Fig. 8a).The first layered crystals were produced based onthe LiF–LiF:Mg 2+ system (Fig. 8b) [25]; these crystalswere used as a model material for the study of mechanicalcharacteristics. These studies demonstrated that thenucleation of plastic shears occurs near the interfacebetween layers. This leads to a more uniform distributionof strains in the samples and hence to enhancementof their plasticity. The plastic shears always nucleate inthe impurity-free layers near the interfaces both at theCRYSTALLOGRAPHY REPORTS Vol. 47 Suppl. 1 2002
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New Advances and Developments in the Stepanov Method for the ...

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