Surface Chemistry Controls Anomalous Ferroelectric Behavior in Lithium Niobate

Abstract

Polarization switching in ferroelectric materials underpins a multitude of applications ranging from nonvolatile memories to data storage to ferroelectric lithography. While traditionally considered to be a functionality of the material only, basic theoretical considerations suggest that switching is expected to be intrinsically linked to changes in the electrochemical state of the surface. Hence, the properties and dynamics of the screening charges can affect or control the switching dynamics. Despite being recognized for over 50 years, analysis of these phenomena remained largely speculative. Here, we explore polarization switching on the prototypical LiNbO surface using the combination of contact mode Kelvin probe force microscopy and chemical imaging by time-of-flight mass-spectrometry and demonstrate pronounced chemical differences between the domains. These studies provide a consistent explanation to the anomalous polarization and surface charge behavior observed in LiNbO and point to new opportunities in chemical control of polarization dynamics in thin films and crystals via control of surface chemistry, complementing traditional routes via bulk doping, and substrate-induced strain and tilt systems.

Surface charged species and electrochemistry of ferroelectric thin films

“…[33][34][35][36] Near-ambient pressure X-ray photoelectron spectroscopy (AP-XPS), allowing the contributions of H 2 O and OH − groups to be considered separately, 15,17,37 has corroborated these observations, demonstrating an asymmetric adsorption of the charged species as a function of polarization orientation. 35,36 More recently, the combination of SPM with secondary ion mass spectrometry 38 has suggested that the evolution of chemical states during polarization switching, beside changes in surface chemistry, 39 can also include redistribution of bulk cations. 28,40 However, both the SPM and AP-XPS approaches present some inherent challenges, which can make a fully quantitative analysis of the interaction of water with ferroelectrics difficult.…”

To switch or not to switch – a machine learning approach for ferroelectricity

“…39 In a next step, we analyze spatially varying cKPFM response measured on macroscopically pre-poled lithium niobate (LN) that is subject to strong electrostatic interactions typical for that ferroelectric material. 10 To further corroborate the applicability of our analysis approach we process cKPFM response measured across the ferroelectricrelaxor phase transition on multiple grains of lanthanum zirconate titanate (PLZT), which even in the ferroelectric state exhibits peculiarities in hysteresis loop, as commonly observed for relaxors. 31 Fig.…”

“…The classication as ferroelectric switching or electrostatically driven artefacts, which is oen the main reason to apply this technique, has been only vaguely and qualitatively dened as the presence/ absence of formation of "two bands" and the cKPFM curve shapes resembling a hysteresis loop, as opposed to electrostatic artefacts which would appear as one band of straight lines. 10,39 Such an assignment is highly dependent on the probing versus writing voltage ratios, as well as the vagueness of the denition of a "good" hysteresis loop.…”

“…The LN sample had been macroscopically pre-poled and dynamic response on this material is typically subject to strong electrostatic interactions even in the presence of polarization switching. 10 In a rst step, PCA projects cKPFM maps into a lower dimensional feature space. 28,42 This way, component #0 contains the most information on variance in the data, followed by component #1 and so on.…”

“…[1][2][3][4][5][6][7][8][9] Interesting phenomena arise from local defect chemistry, chemical domains and interfaces such as domain walls, grain boundaries and the surface itself, necessitating local probing techniques to study functional material response. [10][11][12][13][14][15][16][17][18][19][20] Therefore, piezoresponse force microscopy (PFM) and related techniques that probe piezoelectric properties on the nm to mm scale have become increasingly popular. [21][22][23][24][25][26] In these techniques, AC and DC voltages are applied to a conductive tip in contact with the sample surface and the electromechanical response of the surface to the voltage stimuli is detected.…”