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Nanomaterials 2018, 8(6), 426;

Piezo-Potential Generation in Capacitive Flexible Sensors Based on GaN Horizontal Wires

University Grenoble Alpes, CEA, LETI, MINATEC Campus, F-38054 Grenoble, France
University Grenoble Alpes, CEA, LITEN, MINATEC Campus, F-38054 Grenoble, France
University Grenoble Alpes, CEA, INAC-MEM, Nanostructures and Synchrotron Radiation Laboratory, F-38000 Grenoble, France
Author to whom correspondence should be addressed.
Received: 28 April 2018 / Revised: 1 June 2018 / Accepted: 8 June 2018 / Published: 12 June 2018
(This article belongs to the Special Issue 1D Nanostructure-Based Piezo-Generators)
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We report an example of the realization of a flexible capacitive piezoelectric sensor based on the assembly of horizontal c¯-polar long Gallium nitride (GaN) wires grown by metal organic vapour phase epitaxy (MOVPE) with the Boostream® technique spreading wires on a moving liquid before their transfer on large areas. The measured signal (<0.6 V) obtained by a punctual compression/release of the device shows a large variability attributed to the dimensions of the wires and their in-plane orientations. The cause of this variability and the general operating mechanisms of this flexible capacitive device are explained by finite element modelling simulations. This method allows considering the full device composed of a metal/dielectric/wires/dielectric/metal stacking. We first clarify the mechanisms involved in the piezo-potential generation by mapping the charge and piezo-potential in a single wire and studying the time-dependent evolution of this phenomenon. GaN wires have equivalent dipoles that generate a tension between metallic electrodes only when they have a non-zero in-plane projection. This is obtained in practice by the conical shape occurring spontaneously during the MOVPE growth. The optimal aspect ratio in terms of length and conicity (for the usual MOVPE wire diameter) is determined for a bending mechanical loading. It is suggested to use 60–120 µm long wires (i.e., growth time less than 1 h). To study further the role of these dipoles, we consider model systems with in-plane 1D and 2D regular arrays of horizontal wires. It is shown that a strong electrostatic coupling and screening occur between neighbouring horizontal wires depending on polarity and shape. This effect, highlighted here only from calculations, should be taken into account to improve device performance. View Full-Text
Keywords: piezoelectricity; sensor; capacitive; wires; GaN; metal-organic vapour phase epitaxy; finite element modelling piezoelectricity; sensor; capacitive; wires; GaN; metal-organic vapour phase epitaxy; finite element modelling

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El Kacimi, A.; Pauliac-Vaujour, E.; Delléa, O.; Eymery, J. Piezo-Potential Generation in Capacitive Flexible Sensors Based on GaN Horizontal Wires. Nanomaterials 2018, 8, 426.

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