3D Domain Arrangement in van der Waals Ferroelectric α-In2Se3

One of the exceptional features of the van der Waals (vdW) ferroelectrics is the existence of stable polarization at a level of atomically thin monolayers. This ability to withstand a detrimental effect of the depolarization fields gives rise to complex domain configurations characterized, among others, by the presence of layered "antipolar" head-to-head (H-H) or tail-to-tail (T-T) dipole arrangements. In this study, tomographic piezoresponse force microscopy (TPFM) is employed to study the 3D polarization arrangement in vdW ferroelectric alpha-In2Se3. Sequential removal of thin layers from the polar surface using the PFM tip reveals a complex 3D profile of the domain walls in the alpha-In2Se3 crystals. Antiparallel domain layers stacked along the polar direction are also observed by PFM imaging of the non-polar surfaces showing that H-H and T-T domain boundaries are commonly present in alpha-In2Se3. Application of TPFM to the electrically written domains allows evaluation of their geometrical lateral-to-vertical size aspect ratio, which shows a strong prevalence for the sidewise expansion in comparison to the forward growth. Local I-V measurements reveal a strong polarization direction dependence of conductivity due to the modulation of the energy barrier height as corroborated by theoretical modeling.