Thickness scaling of ferroelectricity in BiFeO ₃ by tomographic atomic force microscopy

Abstract

SignificanceIntrinsic and extrinsic properties of ferroelectric materials are known to have strong dependencies on electrical and mechanical boundary conditions, resulting in finite size effects at length scales below several hundred nanometers. In ferroelectric thin films, equilibrium domain size is proportional to the square root of film thickness, which precludes the use of present tomographic microscopies to accurately resolve complex domain morphologies in submicrometer films. We report a subtractive experimental technique with volumetric resolution below 315 nm3, that allows for three-dimensional, tomographic imaging of materials properties using only an atomic force microscope. Multiferroic BiFeO3 was chosen as a model system for illustrating the capabilities of tomographic atomic force microscopy due to its technological relevance in low-power, electrically switchable magnetic logic.

Depolarizing-Field Effects in Epitaxial Capacitor Heterostructures

“…affect the domain state. However, in earlier tomographic PFM characterization, this was not observed [32]. The absence of BTO surface degradation or mechanical switching induced by this process [67] was verified by AFM in the nanomachined area [26].…”

Investigation of AFM-based machining of ferroelectric thin films at the nanoscale

“…[10][11][12][13] Given the simplicity of this approach, one would expect that it could be adapted to ferroic materials, which find extensive applications in data-storage, and in particular to ferroelectric oxides where there is strong interest in both the fabrication of nanostructures 14 and volumetric investigations. [15][16][17] Despite this promising utility, few AFMbased machining studies have been applied to ferroelectrics [18][19][20] (i.e., materials possessing a spontaneous polarisation that is reversible under an applied electric field). One notable exception is the recent work by Steffes et al, 18 where volumetric imaging of ferroelectric domains in a BiFeO 3 thin film was achieved through sequential nanometric removal of layers of the film with a large (~ 11.4 μN) loading force while monitoring the domain structure via piezoresponse force microscopy (PFM).…”

“…[15][16][17] Despite this promising utility, few AFMbased machining studies have been applied to ferroelectrics [18][19][20] (i.e., materials possessing a spontaneous polarisation that is reversible under an applied electric field). One notable exception is the recent work by Steffes et al, 18 where volumetric imaging of ferroelectric domains in a BiFeO 3 thin film was achieved through sequential nanometric removal of layers of the film with a large (~ 11.4 μN) loading force while monitoring the domain structure via piezoresponse force microscopy (PFM).…”

“…37,38 In this context, AFM-based machining provides an alternative low-cost option to both fabricate nanostructures as well as undertake tomographic functional investigations, while avoiding problems related to ion-injection from FIB, 39 mask limitations (where the nanostructure is limited by the shape and dimensions of the mask) with mask-assisted bottom-up approaches 14 and low resolution (~ 1 µm) of confocal Raman microscopy. 18 The AFM setup also enables data to be collected in-situ during the machining process, thereby providing insight into the mechanical as well as functional properties, such as the conductivity, of the material. 18 In order to exploit these advantages, the technique must be able to reliably machine a broad range of depths into the ferroelectric sample and also be able to fabricate reproducible nanostructures with small (< 100 nm) lateral dimensions for high-density applications.…”

“…18 The AFM setup also enables data to be collected in-situ during the machining process, thereby providing insight into the mechanical as well as functional properties, such as the conductivity, of the material. 18 In order to exploit these advantages, the technique must be able to reliably machine a broad range of depths into the ferroelectric sample and also be able to fabricate reproducible nanostructures with small (< 100 nm) lateral dimensions for high-density applications. The sensitivity and precision of AFM enables a range of parameters such as loading force, tip velocity, scan angle, etc.…”