Black lithium niobate single crystal: influence of point defects on its microwave properties

Abstract

Lithium niobate, as an important piezoelectric substrate material, is widely used in various acoustic and microwave devices. When lithium niobate crystals are grown from a melt at high temperatures, they typically contain a high concentration of intrinsic point defects. These point defects (either intrinsic or related to impurities or external fields) often alter the material's physical constants and increase acoustic losses. In this study, in order to study the effect of defects on the performance of SAW devices, we prepared black lithium niobate crystals with different concentrations of bipolarons, lithium vacancies, niobium anti-sites and other defects in an argon–hydrogen atmosphere. The types and concentrations of defects in the blackened lithium niobate wafers at different reduction temperatures were analyzed using UV-vis and IR spectroscopy. The chemical state of oxygen on the surface of the sample was analyzed by XPS. Dual-port electrodes were fabricated on the samples using micro-nano processing technology, and the relationship between the substrate dielectric loss and defects in lithium niobate was established through S-parameter testing. The results indicate that the chemical reduction treatment increases the defects in lithium niobate, leading to lattice distortion and the formation of defect dipoles, thereby increasing the dielectric loss of the devices. These findings provide profound insights into defect engineering for the application of lithium niobate in acoustic devices.