Multifrequency effects in Piezoresponse Force Microscopy

Resumen del póster presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016.Piezoresponse Force Microscopy (PFM) has become a standard for imaging ferroelectric domain patterns and for the study of piezoelectric and flexoelectric phenomena at the nanoscale. PFM, based on the converse piezoelectric effect of the material under test, uses a conductive AFM probe tip to apply an electrical voltage to the sample surface while simultaneously measure the mechanical response of local expansion and contraction according to the material piezoelectric coefficient. Usually an AC voltage (Vbias) is used to excite the sample, because it allows the use of a lock-in amplifier to read-out the tiny motion generated by piezoelectric effect. But this is also precisely the origin of other electromechanical phenomena such as electrodynamic effects which can veil the net piezoelectric effect and even induce pseudo-ferroelectric behavior. On the other hand, operation of PFM at resonance frequency using DART mode has also been widely used to enhance the tinny piezoelectric signal, especially in thin films. In this case, the contact resonance frequency is strongly influenced by topography, but it also includes information on the intrinsic electrical and mechanical properties of the tip-sample coupling. Most of the materials that exhibit piezoelectricity are insulators, making the measurement of this property easier because the applied voltage between AFM probe tip and sample substrate is fix and known. However, piezoelectricity can also be found in semiconductor crystals with non-central symmetry, a field of growing interest due to its wide range of new applications (e.g. energy harvesting, nanosensing, photonics, etc.). In this case, non-linear effects on the voltage drop on the piezoelectric sample due to Schottky barriers and sample conductivity leakages can be observed, leading to multiharmonic response of the PFM signals and nonlinear behavior of the piezoresponse amplitude signal. In this work, we have characterized the piezoelectric properties of ZnO thin films and nanowires (NWs) by PFM, a material with a wurtzite structure and a dual semiconductorpiezoelectric behavior. Moreover, ZnO nanostructures show the recently discovered nanopiezotronic effect. The performance of I-V curve before a piezoresponse measurement is crucial to understand the resulting data for piezoelectric coefficient determination, due to the rectifying barrier at the contact between tip and NW. To overcome this difficulty, conduction through the NW should be avoided. For instance, a thin layer of an insulator can be deposited by ALD over the sample or a different tip with appropriate working function or passivation coating can be used. Piezoelectric coefficients can be extracted from the forces curves generated when by sweeping the amplitude of Vbias at a certain ac frequency at the first and second harmonic of the ac deflection.Peer reviewe