Ferroelectric domain walls: new 2D functional materials for conventional and neuromorphic computing

Recently, it has been shown that a thin film LiNbO₃ (LNO) memristor device, with conductance levels spanning almost 12 orders of magnitude, could be realised by altering the domain wall microstructure with different applied electric fields. Different from ferroelectric tunnel junctions, the memristive response in these simple ferroelectric capacitor structures was controlled by altering the total number of conducting domain walls that straddle the interelectrode gap and by altering their inclination angle with respect to the polarisation axis.

Early signs of conduction plasticity have been seen in these devices, in which there is a gradual increase in conductivity, induced by applying multiple voltage pulses of equal magnitude. These findings led us to investigate neuromorphic aspects of domain wall transport in this ferroelectric system.<br><br>Building on the knowledge established in the earlier reports, this thesis explores the neuromorphic properties of domain wall conductivity in 500 nm thick LNO capacitors and investigates how neuromorphic functions and operating voltages change as ferroelectric film thickness decreases.

Two-probe current-voltage measurements supported by atomic force microscopy (AFM) characterisation have shown that the domain wall conductivity in LNO thin films is a complex history-dependent function of electric field and time. Field-time kinetics of microstructural change in this system can replicate important features of synaptic response such as potentiation & depression, spike-rate-dependent plasticity and spike-time-dependent plasticity.

Measurements on samples prepared by tomographic AFM (TAFM) and dimple grinding have demonstrated that operating voltages scale almost linearly with film thickness in the 50-500 nm regime. Similar neuromorphic behaviours have been observed in ≈100 nm thick capacitors at about five times lower voltages. Local variations in wall inclination angle resulted in diode-like behaviour in the dc conductance response, enabling half-wave rectification at low frequencies.

Boolean "AND" and "OR" logic gates have been experimentally demonstrated using domain wall diodes and resistors. Results show that ferroelectric domain walls in lithium niobate thin films might be useful not only for neuromorphic but also for conventional computing systems.<br><br><br>