Quardratic Electromechanical Strain in Silicon Investigated by Scanning Probe Microscopy

TL;DR

This study reveals that silicon exhibits a quadratic electromechanical response due to ionic electrochemical effects, which can be distinguished from true piezoelectricity using specific measurement techniques.

Contribution

The paper demonstrates that silicon's apparent piezoresponse is dominated by ionic electrochemical effects, not true piezoelectricity, and introduces methods to differentiate these responses.

Findings

Silicon shows a quadratic electromechanical response related to ionic effects.

Second harmonic response dominates in vertical mode measurements.

Polarity of response can be switched by DC voltage, unlike ferroelectric materials.

Abstract

Piezoresponse force microscopy (PFM) is a powerful tool widely used to characterize piezoelectricity and ferroelectricity at the nanoscale. However, it is necessary to distinguish microscopic mechanisms between piezoelectricity and non-piezoelectric contributions measured by PFM. In this work, we systematically investigate the first and second harmonic apparent piezoresponses of silicon wafer in both vertical and lateral modes, and we show that it exhibits apparent electromechanical response that is quadratic to the applied electric field, possibly arising from ionic electrochemical dipoles induced by the charged probe. As a result, the electromechanical response measured is dominated by the second harmonic response in vertical mode, and its polarity can be switched by the DC voltage with evolving coercive field and maximum amplitude, in sharp contrast with typical ferroelectric materials we used as control. The ionic activity in silicon is also confirmed by scanning thermo-ionic microscopy (STIM) measurement, and this work points toward a set of methods to distinguish true piezoelectricity from the apparent ones.