The use of surface charging in the SEM for lithium niobate domain structure investigation

The different lithium niobate crystals with opposite domain structure (ODLN) were studied in the SEM using e-beam negative surface charging. In the ODLN the polarization vector Ps alternates periodically “tail-to-tail” and “head-to-head” and arranges perpendicular to the domain boundaries. In the investigation we have used congruent LiNbO3 crystals (CLN) and Cr2O3- and In2O3-doped LN with the ODLN structure.Initial surface potential images in secondary electron (SE) mode and charge accumulation on the Y-cut surfaces of different LiNbO3 crystals were compared. The initial surface potential relief in ODLN was found to depend on the position of “tail-to-tail” and “head-to-head” domain walls in the structure. In the doped LN with the periodical domain structure formed during the growth process, the surface potential changes near the domain walls are much weaker than in the structures of CLN obtained by the aftergrowth thermoelectrical treatment technique. A well-defined SE image of “tail-to-tail” domain walls was observed upon special surface negative charging in all types of investigated LN crystals. The negative surface charging in the “tail-to-tail” domain wall areas proceeded slower and the SE exit from these areas decreased as compared to other crystal areas. The charging conditions for the domain wall observation in all samples were comparatively analyzed. The width and uniformity along the boundary images were different and correlated with doping in the crystals. The formation of different charge images could be explained by a variety of positive point defect distributions near the “tail-to-tail” domain walls in different LN crystals. The defects and doping ions screen polarization charges in the “tail-to-tail” wall area and can influence recombination rates of charges induced by an electron beam. Therefore, these domain wall areas are charged much weaker at negative surface charging than other domain areas. This SEM approach could afford a possibility to compare the distribution of the defects that screen the polarization charges and accumulate near the ferroelectric domain wall areas in LN crystals.