Enhancement of crystalline, piezoelectric and optical quality of LiNbO3 single crystals by post-growth annealing and poling

LiNbO3 single crystals grown by Czochralski (CZ) method were characterized in their as-grown (in situ poled), post-growth annealed and post-growth poled states by high-resolution X-ray diffractometry (HRXRD), piezometry, UV-Vis and FTIR spectroscopy methods. From high-resolution diffraction curves (DCs), it was found that the as-grown specimens contain low and very low angle boundaries. Post-growth annealing at elevated temperatures results in removing the grain boundaries but leads to decrease in the piezoelectric constant d33 to 14 pC/N from its initial value of 17 pC/N, showing the misorientation of domains though the structural grain boundaries were removed. Such post-growth annealed samples were re-poled with different poling intensities. Poling leads to enhancement in piezoelectric constant d33 upto 23 pC/N. The decrease in half-width of the DC on poling of annealed specimens also indicates improvement in the crystalline perfection. Homogeneity of Li content was studied by UV-Vis spectra along the crystal boule. Annealing and subsequent poling treatments resulted in decrease of absorption coefficient and shifting of absorption edge. These results indicate the reduction of impurities and antisitic defects. FTIR measurements on post-growth annealed samples show significant reduction in OH− concentration and almost complete removal of CO32− ionic defects, which were otherwise observed in as-grown state.