Exploring Charged Defects in Ferroelectrics by the Switching Spectroscopy Piezoresponse Force Microscopy

Abstract

information storage, [1,2] memristors based on tunneling, [3,4] energy storage devices, [5] etc. Tailoring the defect structure permits fine tuning of the functional properties of ferroelectrics. [6][7][8][9][10] The surfaces and interfaces are directly influenced by the nearby defects, and bulk properties are rendered by the defects via multiple mechanisms. [11,12] Up to date, the number of techniques allowing studies of defects with a high spatial resolution is limited, and they are mainly based on local monitoring of the crystalline structure or chemical composition. [13][14][15][16] Most of these local compositional methods like transmission electron microscopy, X-ray tomography, electron holography, etc. do not have a high enough sensitivity or need challenging sample preparation. [13] The optical method of second harmonic generation can be used to monitor the defect concentration and off-stoichiometry, [16] however it is limited in resolving the type of charged defects and yields only micrometer-scale spatial resolution. Thus, the novel experimental approaches allowing accurate measurement of the defect concentration and/or corresponded functional properties at the nanoscale are highly desirable.In ferroelectrics, charged defects affect polarization reversal as they directly participate in the screening of the depolarization electric field [8,[17][18][19][20] since they can become mobile Monitoring the charged defect concentration at the nanoscale is of critical importance for both the fundamental science and applications of ferroelectrics. However, up-to-date, high-resolution study methods for the investigation of structural defects, such as transmission electron microscopy, X-ray tomography, etc., are expensive and demand complicated sample preparation. With an example of the lanthanum-doped bismuth ferrite ceramics, a novel method is proposed based on the switching spectroscopy piezoresponse force microscopy (SSPFM) that allows probing the electric potential from buried subsurface charged defects in the ferroelectric materials with a nanometer-scale spatial resolution. When compared with the compositionsensitive methods, such as neutron diffraction, X-ray photoelectron spectroscopy, and local time-of-flight secondary ion mass spectrometry, the SSPFM sensitivity to the variation of the electric potential from the charged defects is shown to be equivalent to less than 0.3 at% of the defect concentration. Additionally, the possibility to locally evaluate dynamics of the polarization screening caused by the charged defects is demonstrated, which is of significant interest for further understanding defect-mediated processes in ferroelectrics.

Spatially-Resolved Study of the Electronic Transport and Resistive Switching in Polycrystalline Bismuth Ferrite

“…Importantly, the measurements were done out-of-contact, before I–V measurements using CAFM, and thereby they represent undisturbed inspection of the contact potential difference in a fully screened sample. In ferroelectrics, polarization is usually compensated by the charged defects and adsorbates at the surface [ 53 ], which realizes the screening of the depolarization field [ 54 ]. The polarization can be expected to be partially unscreened under the application of DC voltage to the probe [ 54 ].…”

“…In ferroelectrics, polarization is usually compensated by the charged defects and adsorbates at the surface [ 53 ], which realizes the screening of the depolarization field [ 54 ]. The polarization can be expected to be partially unscreened under the application of DC voltage to the probe [ 54 ].…”

“…The apparent difference of the bias field, i.e., the shift of the hysteresis along the axis of voltages, can be noticed ( Figure 7 b,e). The origin of the bias field can be a built-in charge localized under the probe [ 54 ]. As distinguished from the results in ref.…”

“…As distinguished from the results in ref. [ 54 ], DC voltage is applied to the substrate, which assumes a correlation between the sign of the bias field and the sign of the built-in charge (correlation instead of anti-correlation in ref. [ 54 ]).…”

“…[ 54 ], DC voltage is applied to the substrate, which assumes a correlation between the sign of the bias field and the sign of the built-in charge (correlation instead of anti-correlation in ref. [ 54 ]). Similar to the case of bulk ceramic BFO, built-in charge can be attributed to the segregation of the charged defects or free carriers taking a part in the polarization screening.…”