Optical absorption and nonstoichiometry of YAG crystals grown by the HDC method in reducing atmosphere

Abstract

The features of the optical and spectral characteristics (190-1100 nm), and the mechanism of the formation of nonstoichiometry of yttrium-aluminum garnet (YAG) crystals during growth by the horizontal directional crystallization method in a reducing atmosphere containing CO, H₂ and annealing in environments with different redox potentials were investigated. It has been established that the two-tone coloration of the crystals with the formation of broad absorption bands (380-500 nm, 550-750 nm) is determined by the intrinsic centers arising as a result of nonstoichiometric evaporation of YAG, and include antistructural defects (Y_Al), cationic and anionic vacancies, as well as the associated F⁺–Y_Al, O⁻–Y_Al complexes, which are unevenly distributed in the crystal along the growth direction. The role of polyvalent impurities (Ti, Fe and С in the concentration range ≤0,1 wt%) is mainly to generate intrinsic defects, which ensure the electroneutrality of the lattice and lead to an increase in the intensity of the absorption without the formation of additional impurity bands. It is shown that the addition of excess Al₂O₃ (≤2 wt%) to the mixture leads to a noticeable decrease in the color and absorption intensity of the crystals in the entire spectral range studied.