Optical anomalies and residual stresses in basal-plane-faceted ribbons of Stepanov-grown sapphire crystals

The effects of an anomalous biaxiality and a residual-stress field in basal-plane-faceted single-crystal sapphire ribbons were studied by an optical polarization method using a conoscopic light beam. Formulas that relate the difference between the principal stresses to the angle of optical-axis divergence are obtained. It is shown that the central regions of the ribbons undergo compressive stresses whereas their edges undergo tensile stresses. As compared to block-free ribbons, the stresses in block-containing ribbons increase more intensely with distance from the seed.