Study of structural defects and crystalline perfection of near stoichiometric LiNbO_3 crystals grown from flux and prepared by VTE technique

摘要

Near-stoichiometric LiNbO_3 (SLN) single crystals were grown/prepared by top seeded solution growth/vapor transport equilibration (VTE) technique, and investigated for stoichiometry, disorder and structural defects. The optical absorption and Raman line-width studies revealed higher stoichiometry (i.e., higher Li/Nb) for SLN prepared by vapor transport equilibration (SLN_V) technique in comparison to SLN grown from K_2O flux (SLN_K) and Li-rich melt (SLN_L). The nearly symmetric single diffraction curve (DC), though broad, as observed for SLN_L specimen in high resolution X-rays diffraction (HRXRD) analysis depicted lesser low angle grain boundaries. On the other hand, relatively sharp DC with lowest full-width at half-maximum (FWHM ～45 arc-sec) in HRXRD and lesser Urbach energy (~80 meV) in the absorption spectra for SLN_V crystal revealed less structural defects with respect to other SLN crystals. The higher FWHM of DCs in HRXRD for SLN_L and SLN_K is attributed to growth related imperfections usually observed in solution growth. Though, VTE process results in SLN crystals with better stoichiometry and lesser structural defects but the limitation being that samples up to ～1 mm thickness can be prepared with this technique. For bulk SLN, growth from K_2O flux resulted in better stoichiometry whereas Li-rich flux resulted in better structural quality. The absorption spectra of the grown SLN crystals depicted oxygen vacancy induced electronic defects (Nb~(4+), polarons), which was further authenticated by X-ray absorption near-edge structure (XANES) analysis at Nb K edge revealing lesser Nb~(4+) defects in SLN with respect to congruent lithium niobate (CLN) crystal.