Raman spectroscopy study of compositional inhomogeneity in lithium tantalate crystals

Abstract

Raman spectra of LiTaO₃ single crystals with various stoichiometries were measured to investigate the compositional uniformity of these crystals. Raman spectra mapping demonstrates a spatial variation of the widths of the phonon bands for stoichiometric, congruent, and quasi-congruent samples. A significant radial compositional inhomogeneity is found to be a common feature of commercially available wafers having a near-congruent crystal composition (i.e., x _c ={[Li₂O]/([Li₂O]+[Ta₂O₅])}×100%=47.85–48.50%) grown by the single-crucible Czochralski method. A maximum value of the composition gradient ∇ x _c for a radial inhomogeneity of 0.163 and 0.036%/cm is measured for thin wafers diced from so-called congruent (vendors’ value of x _c =48.50%) and quasi-congruent (x _c =47.88%) crystals, respectively. In crystals grown from highly Li-rich melts (starting composition 54.5 mol% Li₂O), a drastic spatial dependence of Raman bandwidths, indicating a significant gradual compositional inhomogeneity throughout the crystal, is found, which is due to a change of the melt composition during crystal growth. In contrast, the Raman bandwidths of near-stoichiometric crystals fabricated by a vapor transport equilibrium (VTE) technique are found to be constant, i.e. these crystals are practically compositionally uniform. This conclusion has been confirmed by mapping the photoluminescence intensity, evidencing ∇ x _c ≤0.006%/cm in near-stoichiometric VTE-treated crystals.