# How Can InGaN/GaN Heterostructures Be Advantageous for the Piezoelectric Conversion of GaN Nanowires and their Energy Harvesting Applications?

Amaury Chevillard, Quang Chieu Bui, Pascal Chrétien, Nuño Amador-Méndez, Arup Kunti, Laurent Travers, Martina Morassi, Frédéric Houzé, Maria Tchernycheva, Noëlle Gogneau

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Amaury Chevillard, Quang Chieu Bui, Pascal Chrétien, Nuño Amador-Méndez, Arup Kunti, et al. How Can InGaN/GaN Heterostructures Be Advantageous for the Piezoelectric Conversion of GaN Nanowires and their Energy Harvesting Applications?. JNRSE 2023 - Journées Nationales sur la Récupération et le Stockage de l'Energie, Jun 2023, Paris, France. hal-05284589

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# How Can InGaN/GaN Heterostructures Be Advantageous for the Piezoelectric Conversion of GaN Nanowires and their Energy Harvesting Applications?

Amaury CHEVILLARD<fnref n="1" />,*, Quang-Chieu BUI<fnref n="1" />,<fnref n="2" />, Pascal CHRETIEN<fnref n="2" />, Nuño AMADOR<fnref n="1" />, Arup KUNTI<fnref n="1" />,

Laurent TRAVERS<fnref n="1" />, Martina MORASSI<fnref n="1" />, Frédéric HOUZE<fnref n="2" />, Maria TCHERNYCHEVA<fnref n="1" />, Noëlle GOGNEAU<fnref n="1" />,

Boulevard Thomas Gobert, 91120 Palaiseau - France

11 rue Joliot-Curie, 91190 Gif sur Yvette - France

Corresponding author e-mail: Amaury.chevillard@c2n.upsaclay.fr

**Abstract**— Due to their higher piezoelectric coefficients in comparison with GaN, InGaN appears as favorable for improving the piezoelectric response of active media and thus enhancing the efficiency of piezoelectric devices. We have developed the growth of axially heterostructured InGaN/GaN nanowires with In contents reaching up to 40%, presenting abrupt interfaces and a homogeneous In distribution. Based in this architecture, we demonstrate that the incorporation of the thick InGaN insertions in the volume of GaN nanowires (NWs) allows to enhance the piezoelectric response of the system in comparison with binary GaN NWs.

**Keywords**— InGaN/GaN heterostructures; Nanowires; Piezoelectric properties

## I. INTRODUCTION

The IoT demands for smart, integrated, miniaturized and low-energy wireless devices are constantly on a rise both in our daily life and in high-tech applications and the question of their energy suppling is now a key worldwide challenge. Nanowires (NWs)-based piezoelectric nanogenerators have appeared these last years as a new solution for replacing the batteries by converting the environmental vibrations into electrical energy. While these new energy sources have demonstrated power densities in the µW-mW/cm³ range, their use as micro-system power supply remains today rather limited. To consider them for supplying micro-devices, the improvement of the conversion efficiency per surface unit of the piezoelectric active layer is a pre-requisite. We demonstrated that by integrating In<sub>0.35</sub>Ga<sub>0.65</sub>N thick homogeneous insertion in the GaN NW volume, we improved the piezoelectric response of the GaN NWs due to the higher piezoelectric coefficients of InGaN [1]. Hence, an increase of the power density of about 20% in presence of InGaN is highlighted.

## II. METHODS

### A. Ga(In)N nanowire growth

Ga(In)N materials are very interesting for many applications, such as LEDs, LDs, PVs and piezoelectric devices. While growing high-quality 2D-InGaN layers with In contents exceeding 20% is a major challenge, the growth of axially heterostructured InGaN/GaN NWs with high In contents is

possible. To achieve this synthesis, the NWs are grown by plasma-assisted molecular beam epitaxy, a technique allowing to control finely the growth rate and the material composition and quality. Thanks to the mastering of the growth, the InGaN insertion thickness and composition (up to 40%) can be tuned and present abrupt interfaces as well as a homogeneous In distribution [2]. In the present study, all NW samples are grown on a n-doped (111) silicon substrate. Following the growth conditions, the NWs are vertically aligned and present a Wurtzite crystalline structure. Both GaN and 50-nm thick InGaN/GaN NWs are characterized by a height of 1.4 ± 0.1 µm, a diameter of 40 ± 10 nm and a density of 8x10<sup>9</sup> NWs/cm<sup>2</sup>.

### B. Fabrication of the transducers

The as-grown NWs are embedded in a PDMS matrix for mechanical consolidation, which is etched-back by reactive ion etching (RIE) to ensure that the NWs summits are free from the polymer. For macro scale characterization, a metal electrode is deposited on the top (Cr/Pt electrode) and bottom (Ti/Au electrode) of the transducers.

### C. Characterization of the transducers

We characterize the NW-based transducers with a multiscale analysis combining the characterization of the piezoelectric response of each NW composing the array, and the characterization of piezoelectric micro-devices integrating the NW in their active media. For single NW characterization, piezo-conversion measurements are performed by AFM equipped with Rescope module specifically adapted for piezoelectric conversion measurements [3]. For device characterization performed under environmental conditions (few Newton, 1-100 Hz, intermittent and permanent contact), the used mechanical input system is composed of a shaker controlled by an electrical generator, as well as an oscilloscope. An impedance adapter is needed to be able to modify the load resistance. The generated signal is numerically filtered by discrete FFT.

## III. EXPERIMENTAL RESULTS

The study on single NWs shows an increase of the output voltage up to 35% compared to GaN NWs, reaching

<footnote n="1">¹Centre for Nanoscience and Nanotechnology, CNRS/Paris-Saclay University/Paris-City University</footnote>

<footnote n="2">² Group of electrical engineering of Paris, Paris-Saclay University/CentraleSupélec/Sorbonne University/CNRS,</footnote>
respectively an average and a maximum value of 330 mV and 470 mV per NW [1]. This enhancement is due to the higher piezoelectric coefficients of InGaN versus GaN [4] and opens a way to the improvement of the micro-device performances. The study on micro-devices shows an increase in power generation at optimal load resistance of InGaN/GaN NWs compared to GaN NWs, reaching a maximum power density of 3.2 µW/cm² (23.1 mW/cm³) under intermittent contact solicitation, with a maximum peak-to-peak output voltage attaining 143 mV. According to the solicitation conditions, the raise in power density reaches at least 20%.

In concordance with the nanoscale analysis [1], these results obtained on a macroscopic device level demonstrate that InGaN incorporation improves the conversion efficiency of the active media.

## IV. CONCLUSIONS

We demonstrate that the insertion of the InGaN in the GaN NW volume increases the piezo-conversion capacity of the transducer, both at nano- and macro-scales studied under environmental-like solicitation conditions. This enhancement

allows us to approach the power requirements of microelectronical systems, such as medical implants (1µW-10µW).

## ACKNOWLEDGMENT

This work was supported by the French National Research Agency as part of the "Investissements d'Avenir" program (through the NanoVIBES project and by the ANR-PiezoSens project (ANR-19-CE42-0016)). This work was done within the C2N micro nanotechnologies platforms and partly supported by the RENATCH network and the General Council of Essonne.

[1] N. Jegenyes et al., High piezoelectric conversion properties of axial InGaN/GaN nanowires, Nanomaterials, 8, 367, 2018.

[2] M. Morassi et al., Morphology tailoring and growth mechanism of Indium-rich InGaN/GaN axial nanowire heterostructures by plasma-assisted molecular beam epitaxy, Cryst. Growth Des. 18, 2545, 2018.

[3] N. Gogneau et al, GaN nanowires for piezoelectric generators, Phys. status Solidi RRL, 8, No 5, 414-419, 2014.

[4] F. Bernardini et al., Spontaneous polarization and piezoelectric constants of III-V nitrides, Phys. Rev. B 56, R10024, 1997.

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