Direct Imaging of Hot Spots in Domain Wall Devices Using Scanning Thermal Microscopy

Domain walls (DWs) in ferroelectrics are an exciting category of reconfigurable functional interface, with properties that can differ significantly from bulk. Their electrical properties are of particular interest since domain walls can exhibit semiconducting, metallic-like, and even superconducting behaviour, while the bulk material remains comparatively insulating. A decade of research has focused on using conducting DWs for use in new types of voltage-reconfigurable electronic devices. Lab-level transistor [1] and memristor devices [2] have been reported, where device functionality is derived entirely from the number of conducting DWs connecting device electrodes. Functionally, the DWs perform the equivalent role of conductive nanofilaments seen in metal-oxide resistive switching materials. However, while self-heating and local temperature are known to be important factors for resistive switching [2,3], much less is known about the self-heating properties of domain walls and their influence on device behaviour.

We have been investigating the electrothermal properties of thin-film LiNbO3 domain wall devices using Scanning Thermal Microscopy (SThM). In LiNbO3, electrically conducting domain walls can be up to thirteen orders of magnitude more conducting than the highly insulating surrounding bulk. It is therefore reasonable to assume that any device current must pass through the walls only, which therefore act as extremely-thin buried sources of Joule heat. The resulting surface temperature profile of the in-operando device is mapped using SThM, in which the scanning probe is used as a mobile nanoscale temperature sensor. We directly image temperature ‘hot spots’ on the order of 10 K on the surface electrodes and piezoresponse force microscopy corroborates that they originate from sub-surface domain wall heating. Since heat spreading occurs from the domain walls into the surrounding ferroelectric film and top electrode, the measured surface temperatures likely represent a lower bound for the intrinsic temperature rise within the domain walls.

[1] Nat. Commun. 11, 2811 (2020).
[2] Adv. Funct. Mater. 30, 2000109 (2020).
[3] Sci. Adv. 8, eabk1514 (2022).
[4] ACS Appl. Mater. Interfaces 14, 29025 (2022).