Finite Element Analyses of Cracking in Lithium Niobate Single Crystal

Abstract

Trigonal single crystals such as LN (lithium niobate, LiNbO3) and LT (lithium tantalate, LiTaO3) are utilized not only as SAW (surface acoustic wave) devices but also as photonic devices. Their bulk single crystals are usually manufactured by the Czochralski (CZ) growth technique. Cracking of the crystals sometimes occurs during the CZ growth, especially during cooling down. Since cracking can be avoided by adjusting thermal conditions of a CZ furnace, the cracking is supposed to be caused by a thermal stress [1][2]. Lee et al. [3] performed experiments on LN growth to examine the conditions for the cracking by changing the growth conditions such as the growth rate and the vertical temperature gradient in a CZ furnace and keeping the cooling conditions constant. According to their experiments, the cracking sometimes occurs under a certain growth condition even though the cooling condition is unchanged. It is deduced from the experiments that the cracking may be initiated by the thermal stress during cooling down from the defects in a crystal which would be caused by the thermal stress during the growth. Although the qualitative natures of the cracking have been clarified through these works, there has been no research concerning quantitative estimation of the failure stress of trigonal single crystals.