Ferroelectric Domain Observations with Helium Ion Microscopy

We present a systematic demonstration of the capability of helium ion microscopy (HIM) to visualize the polar and nonpolar domains in ferroelectric LiNbO₃ and ErMnO₃ single crystals. Through a wide range of acceleration voltages and beam currents of He⁺ ions, HIM reproduces domain and domain wall maps with significant spatial resolution, consistent with the observations by piezoresponse force microscopy and conductive atomic force microscopy at the same locations. In the HIM domain images, the contrast of the polar domains with out-of-plane polarization can be attributed to the differences in the surface potential or the work function between oppositely poled domains. Intriguingly, HIM also exhibits contrast for nonpolar domains with in-plane polarization. During initial HIM scanning on the nonpolar surface of an ErMnO₃ single crystal, only dmain walls are visible, but consecutive scanning leads to the growth of domain contrast along its polar axis, implying a complex interaction between He+ ion implantation and the bulk photovoltaic effect of the ferroelectric materials. Our observations make HIM included in the list of the characterization tools for ferroelectric domains.