機能性単結晶育成の課題(II) : LiNbO_3の例(<特集>バルク成長(II))

抄録

Ferroelectric crystal LiNbO_3 has first utilized as nonlinear optical devices in 1960's. With development in surface accoustic wave (SAW) applications, the utility of this crystal has spread out to the region of high frequency niters. LiNbO_3 has a wide solid solution range between Li_2O and Nb_2O_5, and the SAW velocity significantly changes with the crystal composition. The determination of the congruent composition is thus essential for obtaining uniform SAW velocity in the crystals when they are grown from the melt. Since the starting reagents Li_2CO_3 and Nb2O_5 contain a considerable amount of impurities and water, the constituent in the melt often deviates from the desired, congruent composition, and consequently, a nonuniform SAW velocity is observed in the obtained crystals. The deviation is easily checked by measuring the SAW frequency on the obtained crystal wafers using newly developed equipment, and is eliminated by a suitable complement of deficiency in the composition. Finaly, the variety in the central SAW frequencies becomes less than 0.1%.