Piezoresponse force microscopy (PFM) is a powerful technique used to characterize nanoscale properties in electromechanically active materials, such as ferroelectrics and multiferroics.
Study: Quantification of the electromechanical measurements by piezoresponse force microscopy. Image Credit: remotevfx.com/Shutterstock.com
To date, most PFM-based quantitative estimates of physical parameters (e.g., domain wall velocity, domain wall roughness, local coercive fields, and polarization mechanism) have been derived using uncalibrated raw PFM signals.
Interpretation of uncalibrated data could lead to confusion while comparing similar materials. Interpreting calibrated PFM phase signals could be beneficial in analyzing the piezoelectric properties of new ferroelectrics.
A recent Advanced Materials study has reported two complementary methodologies to calibrate the PFM phase signal. These methods were used to analyze the piezoelectric behavior in ferroelectric hafnium oxide (HfO2)-based thin film capacitors, which exhibited intriguing variations in the longitudinal piezoelectric coefficient.
Principle and Application of PFM
The main principle behind PFM is that piezoelectrically active materials undergo deformation when subjected to an electric field. This phenomenon is known as the converse piezoelectric effect.
Conventional PFM has been associated with an oscillating electric field applied by a conductive nanoscopic atomic force microscopy (AFM) probe in contact with the sample, causing periodic sample deformation. This deformation is detected by a quadrant photodetector. Application of PFM to ferroelectrics provides great insights into the magnitude of the piezocoefficient.
Although the underlying principle of PFM is simple, the interpretation of PFM signals is challenging because it can be affected by various artifacts, including electrostatic signals, thermal effects due to Joule heating, electrochemical reactions due to ionic motion, and complicated cantilever dynamics.
PFM is a non-destructive technique that provides insightful information regarding the local polarization switching behavior of ferroelectric films (inorganic and organic), single crystals, polymers, and capacitors. It also helps explore the magnetoelectric couplings, topological vortex structures, and local disorder potential of electromechanically active materials.
PFM coupled with other scanning probe microscopy, has been utilized to analyze functional properties, such as tunneling electroresistance phenomena, domain wall conductivity, and optical modulation of polarization behavior of ferroic materials. Additionally, it has been recently used in analyzing biological samples and phase transitions in antiferroelectric batteries and fuel cells.
Quantification of PFM Phase Signal
The vertical PFM phase signals are related to the sign of the longitudinal piezoelectric coefficient. Several contradictory theoretical modeling and experimental analysis reports can be found on the piezoelectric properties of HfO2-based ferroelectrics. For instance, theoretical modeling predicted a negative longitudinal piezoelectric coefficient, while experimental studies reported a positive longitudinal piezoelectric coefficient for these materials.
The current study adopted two approaches to rectify the parasitic phase offset, commonly caused by cantilever displacement measurements by the optical beam detection (OBD) method. The first method utilizes a reference sample with a known longitudinal piezoelectric coefficient sign. The second method is associated with identifying the cantilever-sample electrostatic interactions to determine parasitic phase offset.
The methods proposed for the quantification of PFM phase signals were tested against standard reference ferroelectric samples with a known sign of longitudinal piezoelectric coefficient: Pb(Zr,Ti)O3 (PZT) capacitors and PbTiO3 (PTO) thin films. PZT and PTO have a positive longitudinal piezoelectric coefficient, while poly(vinylidene fluoride) (PVDF) thin films possess a negative longitudinal piezoelectric coefficient. Various samples’ signs of longitudinal piezoelectric coefficient were tested against HfO2-based capacitors.
Parasitic phase offset was identified based on PFM measurements of a reference sample with a known sign of the longitudinal piezoelectric coefficient. This phase offset could be balanced by modifying the initial offsets of the lock-in amplifiers. Additionally, when the polarization was pointing downwards in a material with a positive result, it could be manually adjusted by subtracting it from the raw PFM phase signal.
To determine no local influences, local quasi-static strain loops of the same directions as the PFM spectroscopic loops were used. The strain loop confirmed a positive sign. The use of correct initial phase offset ensured that the rotation of the phase signals was always in line with the longitudinal piezoelectric coefficient across all reference samples.
The calibrated phase responses at approximately 3 kHz were found to be in agreement with the phase values for PTO and PVDF films and PZT capacitors. Proper tracking of resonance is important for obtaining correct phase signals in the resonance-enhanced PFM measurements. The application of this method on 20 nanometer thick La:HfO2-based capacitors have shown similar results to PVDF, which indicated a negative sign.
In summary, it was observed that piezoelectric properties of the HfO2-based capacitors were dependent on the material thickness, electrodes as well as deposition method. These factors led to wide variations in longitudinal piezoelectric coefficient signs within a single device.
The main significance of calibrated PFM phase signal for the detection of the local piezoelectric coefficient was highlighted in this study. The newly developed methodologies helped investigate the uniqueness of the piezoelectric properties of HfO2-based ferroelectric capacitors. In the future, more structural research is required to elucidate the underlying physical mechanisms behind the observed variability.
Buragohain, P. et al. (2022) Quantification of the electromechanical measurements by piezoresponse force microscopy. Advanced Materials. https://doi.org/10.1002/adma.202206237
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