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., xc={[Li2O]/([Li2O]+[Ta2O5])}×100%=47.85–48.50%) grown by the single-crucible Czochralski method. A maximum value of the composition gradient ∇xc for a radial inhomogeneity of 0.163 and 0.036%/cm is measured for thin wafers diced from so-called congruent (vendors’ value of xc=48.50%) and quasi-congruent (xc=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 ∇xc≤0.006%/cm in near-stoichiometric VTE-treated crystals.