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

Abstract

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.

Electromechanical conversion efficiency of GaN NWs: critical influence of the NW stiffness, the Schottky nano-contact and the surface charge effects

“…Thanks to their large surface-to-volume ratio, high crystalline quality and nanometer scale dimensions, semiconductor nanowires (NWs) offer unique advantages for a wide range of applications. [1][2][3][4][5] In particular, NWs presenting piezoelectric properties appear as promising active nanostructures for a new generation of piezoelectric sensors [6][7][8][9][10] and harvesters. [11][12][13][14][15] Especially, the sub-100 nm-wide NWs present the particularity to exhibit novel properties, non-existing or non-significant at micrometric scales that can lead to a strong modulation/modification of their characteristics.…”

Detaching (In,Ga)N Nanowire Films for Devices Requiring High Flexibility and Transmittance

“…Flexible optoelectronic devices provide many novel functionalities and have the potential to open up a new branch of industry. − As another novel application, transparent devices have attracted considerable interest as next-generation display technology in various fields, including wearable intelligent electronics, automobile windshield navigation, and IoT (Internet of Things) applications. − Because of the extraordinary characteristics of low power consumption, being nontoxic, long lifetime, and high efficiency, GaN-based optoelectronic devices, e.g., light-emitting devices (LEDs), are promising for the above fields. , In addition, this material is also attractive for piezoelectric devices, such as piezoelectric sensors and piezoelectric transistors. − Thus, achieving flexible and transparent GaN-based films is a decisive step for future markets and applications. Over the past few years, the fabrication of flexible devices based on thin-film and micropyramid GaN-based materials has been intensively studied by developing procedures to transfer the epitaxial structures from the original rigid substrates used for growth to soft substrates. , These additional procedures usually require laser lift-off or selective wet etching of a thick sacrificial layer or the entire source substrate. ,− However, besides expensive laser equipment, the laser lift-off technology requires the bandgap of substrate should be larger than that of GaN, which limits its applications. , Furthermore, the sacrificial layers are normally thick, e.g., 2.5 μm in ref , or may need to be highly doped, which can increase the epitaxial cost and deteriorate the epitaxial quality.…”

1D Nanostructure-Based Piezo-Generators

“…The improvement of the electromechanical coupling of the 1D-nanostructures is benefits for the both applications. The paper entitled, “Piezo-Potential Generation in Capacitive Flexible Sensors Based on GaN Horizontal Wires” by Kacimi et al [13] highlights with simulations and experimental measurements that GaN NWs, with high piezoelectric properties, can also be very useful for developing high-sensitive piezoelectric sensors.…”