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Jeudi 3 octobre 2019 à 14h00,
Salle des séminaires, bâtiment A - CNRS
25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9

Orateur : Lucas Jaloustre
"Piezoelectricity in single III-Nitride nanowires for nanopiezotronics : a scanning force microscopy investigation"


Nanopiezotronics is a recent field which has increasingly contributed to a progress of smart electronics and energy harvesters that benefits from a conversion of small mechanical movements into electrical energy. Its foundation relies on the coexisting of piezoelectric and semiconducting properties in nano-objects based on non-centrosymmetric semiconductors such as III-N or II-V. Lately, III-N nanowires with their high aspect ratio have been proposed as a promising building block for this emerging research.
Nevertheless, the key mechanism in nanopiezotronics that governs a mechanical-toelectrical energy conversion must be clarified. The origin of the electrical signal from single nanowires detected by conductive scanning force microscopy was still under debate, whether it originates from piezoelectric effect or rather measurement artefacts. In addition, the reports of larger piezo-coefficients than bulk values in GaN nanowires need to be verified as it possibly results from the measurement misinterpretation.
This PhD work is focused on in-depth understanding of the piezoelectric effect in III-N nanowires which is a critical issue for nanopiezotronics. For this purpose, several scanning force microscopy modes such as Kelvin probe force microscopy, piezoresponse force microscopy, conducting scanning force microscopy were performed to locally probe direct and reverse piezoelectricity including other parameters that play an important role on the piezo-potential generation. Various device configurations and different electronic setups were applied.
The experimental results together with finite element simulations, reveal that the piezocoefficients of III-N nanowires are similar to those of bulk, and no giant piezoelectric effect was observed. The analyses of free and bound charges suggest that the effect of the piezocharges should be undetectable even with highest sensitive electronics. Besides, the electrical signal found in single nanowires by conductive-scanning force microscopy is rather related to the current passing through the nanowires which is often generated by an unappropriated grounded setup. The outcome of this work point out various parameters in piezoelectric semiconductor nanowires that must be taken into account before attributing their mechanical or electrical response to the piezoelectric effect. Those signals can be potentially caused by the detection setup itself, the measurement artefacts and the environments, resulting in the misinterpretations of the signal origins.

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