Transmission electron microscope studies of Ti-doped, congruent lithium niobate (LiNbO3) have shown that extended structural faults are only present within the Ti diffused layer (i.e. the wave guiding region). Structural faults have not been observed in undoped control crystals of congruent and stoichiometric LiNbO3. Therefore, it appears that the introduction of Ti is responsible for the formation of these defects. The chemical driving forces which may be controlling the formation of structural faults are discussed.
Diffraction contrast experiments, which have been interpreted in terms of two-beam dynamical theory for a centrosymmetric crystal, indicate that the faults are tensile in nature (i.e. formed by removing a plane of atoms – so-called intrinsic faults) and have a displacement vector of the type c/12/001] when indexed in the hexagonal coordinate system. That is, the displacement vector is along the c-axis. The detailed crystallographic character of the fault planes is not clear; both (118) and (1 1 12) planes have been confirmed from trace analyses and therefore the fault has a shear component. Additional contrast experiments will be required in order to clarify this feature of structural faults in Ti-doped LiNbO3.
Since the extent of these structural faults is tens of microns, they are clearly potential scattering sites for photons. In this regard, a systematic understanding of their origin and thermal stability is crucial to integrated optical device technologies based on LiNbO3 and on the Ti-doped waveguide fabrication technique.