In-situ SHG microscopy investigation of the domain-wall-conductivity enhancement procedure in lithium niobate

TL;DR

This study uses in-situ SHG microscopy to observe how domain walls in lithium niobate evolve during a conductivity enhancement process, revealing complex factors influencing their electrical properties beyond simple geometric considerations.

Contribution

The paper provides the first in-situ, time-resolved 3D imaging of domain wall evolution during conductivity enhancement in lithium niobate, challenging previous assumptions about the role of wall inclination angle.

Findings

Domain walls exhibit dynamic shape changes during voltage ramping.

The inclination angle alone does not determine conductivity.

Point defects significantly influence domain wall conductivity.

Abstract

Conductive domain walls (CDWs) in the uniaxial ferroelectric lithium niobate (LiNbO$_3$, LN) have attracted a lot of interest as potential elements in 2D nanoelectronics, due to their orders-of-magnitude larger electronic AC and DC conductivities as compared to the host material. On the way towards generating standardized CDWs into z-cut bulk LN crystals with controllable geometry and electrical properties, we have encountered setbacks recently: Although the first preparation step, i.e., the established UV-light-assisted liquid-electrode poling, reliably creates fully penetrating hexagonal domains with the DWs being aligned almost parallel to the polarization axis, the second step in the DW 'conductivity-enhancement' process through post-growth voltage ramping, resulted in randomly-shaped DWs as reflected in their different current-voltage (I-V) characteristics even after having applied the same process parameters. To clarify this phenomenon, we present here an \textit{in-situ} and time-resolved second-harmonic-generation (SHG) microscopy investigation of DW samples of different sizes, monitoring the DW evolution during that critical voltage ramping, which allowed us to reconstruct the 3D DW shapes both prior to and after the enhancement process. As a result, we are able to map the temporal changes of the local DW inclination angle $\alpha$, and to quantify the DW velocity. As a consequence, we need to re-assess and re-think the origin of the DW conductivity (DWC) in LN: The hitherto assumed simple connection between $\alpha$ and the DWC can not be generalized, since point defects accumulating along DWs act as extra sources for charge carrier trapping/release, significantly contributing to the DW current.