Flexible Capacitive Piezoelectric Sensor with Vertically Aligned

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Flexible capacitive piezoelectric sensor with vertically aligned ultra-long GaN wires Amine El Kacimi, Emmanuelle Pauliac-Vaujour, and Joel Eymery ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15649 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Applied Materials & Interfaces

Flexible capacitive piezoelectric sensor with vertically aligned ultra-long GaN wires Amine El Kacimia, Emmanuelle Pauliac-Vaujoura and Joël Eymeryb,* 1

Univ. Grenoble Alpes, CEA, LETI, MINATEC Campus, 38000 Grenoble, France

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Univ. Grenoble Alpes, CEA, INAC-MEM, Nanostructures and Synchrotron Radiation Laboratory, 38000

Grenoble, France

ABSTRACT We report a simple and scalable fabrication process of flexible capacitive piezoelectric sensors using vertically aligned gallium nitride (GaN) wires as well as their physical principles of operation. As-grown N-polar GaN wires obtained by self-catalyst metal-organic vapor phase epitaxy are embedded into a PDMS matrix and directly peeled off from the sapphire substrate before metallic electrode contacting. This geometry provides an efficient control of the wire orientation and an additive contribution of the individual piezoelectric signals. The device output voltage and efficiency are studied by finite element calculations for compression mechanical loading as a function of the wire geometrical growth parameters (length, density). We demonstrate that the voltage output level and sensitivity increases as a function of the wire length and that a conical shape is not mandatory for potential generation as it was the case for horizontally assembled devices. The optimal design to improve the overall device response is also optimized in terms of wire positioning inside PDMS, wire density and total device thickness. Following the results of these calculations, we have fabricated experimental devices exhibiting outputs of several volts with a very good reliability under cyclic mechanical excitation.

KEYWORDS: sensor, piezoelectricity, flexible, gallium nitride, nanowire, metal-organic vapor phase epitaxy, self-powered

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INTRODUCTION Device flexibility has become of major interest for future electronics and its market has grown considerably in the past few years. It is being introduced in many conventional devices such as screens through OLED devices1,2 or light emitting diodes (LEDs),3,4 electronic printed circuit,5,6 MEMS7,8 and especially sensors.9 In this field, nanostructures such as nanofibers, nanowires and nanobelts have attracted a specific interest due to their outstanding physical properties in terms of mechanical robustness, flexibility and enhanced physical properties compared to bulk materials.10,11 These onedimensional structures have been used as the main building block of innovative devices for chemistry,12 biology,13-16 MEMS,17 optoelectronics18,19 and electronics applications.20 For example, zinc oxide finds applications in bio-inspired devices as molecule detectors21,22 as well as in mechanical energy harvesting at the nanoscale for self-powered devices thanks to its piezoelectric properties.23 Such devices deliver up to 1.5 V for 20 nA output current, which was proven to be sufficient to power a liquid crystal display.25 For nitrides, thin gallium nitride (GaN) nanowire-based nanogenerators grown by molecular beam epitaxy using Schottky barriers have also shown very good performances24 and core-shell GaN wires grown by metal organic vapor phase epitaxy (MOVPE) have been identified as an efficient solution for next generation flexible LEDs capable of emitting white light with an estimated quantum efficiency of 9.3 %.3,4 We have previously reported the feasibility of flexible piezoelectric sensors based on horizontally assembled MOVPE GaN wires delivering up to 700 mV for about 2 N applied force.29 In these devices, the wires are detached from their initial sapphire substrate and are chemically functionalized before being assembled horizontally on a flexible substrate using the Langmuir-Blodgett method.30 We demonstrated that a conical shape was mandatory for potential generation and that the control over wire relative orientation with respect to polarity was poor as the initial order resulting from the growth is lost once the wires are spread into a fluid. The output signal also strongly depends on the relative wire orientation because of the polar nature of the wires and of the dependence of the piezoelectric response on that polarity.29 Therefore, the control of wire orientation is necessary to limit potential degradation and device variability. To overcome this issue, this paper studies the transfer of vertical assemblies, which preserves the as-grown relative wire orientation resulting from the MOVPE growth method. We present first the fabrication process and the possible variants of device structures that can be achieved. Then using finite element calculations, we discuss the working mechanism relying on the vertical charge separation as well as the influence of the length and conical angle of the wires. The impact of growth density on the overall device efficiency is tackled and some basic fabrication guidelines are provided to enhance the device output signal. Finally, these calculations are validated by the realization of experimental devices with different wire lengths, which exhibit good piezoelectric performances. ACS Paragon Plus Environment

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EXPERIMENTAL SECTION Materials and methods. MOVPE wires are grown on a c-sapphire substrate with an in-situ injection of silane (SiH4) and small V/III molar ratio to promote vertical growth with very high growth rate (the wires used in this study are grown at about 100 µm/h). Further details about growth conditions and materials characterization can be found in references3,26,27 and important features can be summarized as followed. Wires are single crystals as demonstrated by high resolution transmission microscopy3,26,27 and are in a well-defined epitaxial relationship with sapphire (1100GaN //1210

Al2O3

and 0001GaN //

0001Al2O3 within a few degrees of twist and tilt).3 They exhibit predominantly the N-polarity (i.e. the growth axis is along the GaN ̅-direction) with a hexagonal cross-section and m-plane facet sidewalls (i.e. 1010 crystallographic planes). Their shape is slightly conical (< 2°) and their length can be varied from 10 to 700 µm depending on growth time and gas mixture conditions.28 They can be considered as dislocation free along most of their length because lattice-mismatch (misfit) dislocations occurring at the Al2O3/GaN interface are bent to the sidewall surface very close to the bottom of the wire. This mechanical weakness probably facilitates the separation of the wire from the substrate at the interface. As reminded above in this growth method, silane flow is a necessary ingredient to favor the vertical growth of wires. This results in a quite heavy Si n-doping with carrier concentrations reaching about 1020/cm3. 26 The device structure itself consists of an array of vertical GaN wires embedded into a PDMS (polymethylsiloxane) matrix with contact electrodes on both sides of the membrane to achieve a capacitive structure. It is obtained through a straightforward fabrication process depicted in Figure 1.4 The PDMS prepared by mixing elastomer and curing agent with 10:1 ratio is spin-coated on the surface of a donor sapphire substrate with vertically as-grown MOVPE GaN wires. After annealing at 120 °C for 35 min, the flexible PDMS layer incorporating buried wires is peeled and contacted on both sides to make a capacitive structure.4 Several device designs can be realized by changing the wire position with respect to the electrodes. For example, in the geometry sketched up in Figure 1a, the wires are located at the bottom of the dielectric. In the second geometry (b), wires are positioned in the middle of the dielectric layer and surrounded by two equal thicknesses of PDMS. For a given load, the output voltages delivered by these two configurations will vary due to different mechanical deformations and electrostatics boundary conditions.


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Figure 1. Fabrication process flow for the realization of a flexible piezoelectric sensor with vertically assembled wires. (a) and (b) show two device variants easily achievable by this process.

Metallic electrodes can be deposited on both sides of the peeled flexible membrane. For example, a physical vapor deposition of Ti (10 nm) / Al (200 nm) can be carried out after PDMS O2plasma cleaning to promote adhesion. A metallic layer can be also obtained by spreading Ag nanowires on the dielectric surface3 or for the bottom part by using directly a polymer substrate with a conductive layer as it will be shown later. Note that this process will be demonstrated on 2-inch sapphire wafer, but it can be easily scaled to larger substrate wafer (6” or even 8” wafers) thanks to the scalability of the growth process and to the simplicity of technological steps. Within this geometry, the potential is generated by the deformation of the principal piezoelectric axis of the wires, i.e. the ̅-axis of N-polar wires. Charges are therefore separated vertically resulting in opposite potentials on top and bottom parts of the wires. To illustrate this point, Figure 2a represents the potential mapping in a structure consisting of a single wire embedded into a PDMS-based capacitive structure under a compression constraint of 2.5 /μ². This calculation is performed with a finite element method (FEM) using the COMSOL software with the piezoelectricity module,29 but without considering the free carriers introduced by the Si doping. A regular hexagonal cross-section is considered with 1010 crystallographic sidewall planes (i.e. m-plane facets). To be consistent with usual MOVPE growth, the conical angle α of the wire between the m-plane sidewall facets (defined on Figure 2a) is set to 1° and the top radius is set to 700 . Top and bottom metallic electrodes are added on both sides of the stack as terminals. In the calculation, the bottom electrode is grounded, while the top electrode is ACS Paragon Plus Environment

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defined as a floating potential where the value of the output resulting from the wire piezoelectric response is calculated. This type of device can operate both in compression mode, in which the deformation along the vertical axis is directly induced by the applied force and in the less efficient bending mode, where the strain along the principal axis of the wire is indirectly determined by Poisson’s effect. For the horizontal wire geometry,29 we demonstrated by finite element modelling that the wire conical shape played an essential role in potential generation. For the present vertical wire configuration, the same method will be used to study the impact of the shape and length of the wires on the generated voltage for a mechanical compression loading. Finite element calculations. The influence of the wire length on the device output level is studied by considering a single GaN wire ( 700 , 1°) embedded into a PDMS layer as depicted in Figure 2a. is varied between 60 and 300 μ. The ratio between the total PDMS dielectric layer thickness and the wire length is set to 1.5 to limit the number of parameters to be varied. The linear elasticity tensor of bulk GaN is considered with its usual elastic coefficients: 390 !"#, $ 145 !"#, & 106 !"#, && 398 !"#, (( 105 !"#. The GaN wire diameter obtained by MOVPE growth is usually larger than 100 nm and no giant piezoelectric effects predicted in nanostructures10,11,13 can be expected. Therefore, standard piezoelectric matrix coefficients are used with

)& −0.338 +. ,$ , )&& 0.667 +. ,$ , )- −0.167 +. ,$.32,33

The

Young modulus,

Poisson’s ratio and relative permittivity of PDMS are taken equal to . 750 /"#, 0 0.5 and 1 2.8 respectively.34 Compressive constraints, defined as a load per unit area Fs, are rigidly applied at the top face of the device while the bottom face is fixed. Figure 2b shows the potential evolution taken at the top electrode, as a function of the wire length L for the different values of the load (2.5, 7.5 and 12.5 / μ²). In this range, the potential varies quasi linearly as a function of L and its variation increases with the mechanical loading. As plotted in Figure 2c, we can define a sensitivity parameter η as the ratio between the output potential V and the applied force per unit surface Fs: η = V/Fs. This parameter varies linearly in the 60-300 µm range and can be approximated by V/Fs=-0.0078+1.47x10-4xL (R2=0.9978) confirming that longer wires are preferable to generate larger potential. As L depends on the MOVPE growth time and defines the overall device thickness, a compromise between efficiency, sensitivity and size needs to be made according to targeted applications.


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Figure 2. (a) Potential field V (in volts) across a 180 µm thick device embedding a single 120 μ long GaN wire for a 2.5 /μ² compression load applied to the top electrode (bottom electrode is fixed and grounded). The top radius of the wire is 700 and the conical angle is set to 1°. (b) Evolution of the potential V taken at the top floating electrode as a function of the wire length for Fs = 2.5, 7.5 # 2 12.5 /μ² applied loads. (c) Evolution of the sensitivity parameter η=V/Fs as a function of the wire length. (d) Evolution of the output voltage as a function of the conical angle for three different wires length 60, 150 # 2 300 μ for 2.5 /μ² compression.

It has been shown in a previous works29 that a conical shape was mandatory to generate a potential in horizontally aligned GaN wire-based sensors (this shape allows charge separation and accumulation on wire lateral facets that induce potential creation in between electrodes). But in the case of vertically aligned wires, the potential generation mechanism is different. It relies on the vertical charge separation along the ̅- axis through the applied strain. To study this point, we calculated the output voltage taken at the top electrode as a function of the conical angle for the structure described previously in Figure 2a. Wire lengths are fixed to 60, 150, 300 μ with a top radius 700 . The conicity angle is varied between 0.1 and 2° to be consistent with scanning electron microscopy measurements and a compressive load of 2.5 /μ² is applied to the top face electrode. Figure 2d shows that the output voltage V increases slightly with . Indeed, the relative variations of 3 ACS Paragon Plus Environment

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for 60, 150, 300 μ in the 0.1 - 2° range is about 12, 12 and 8% respectively. Note also that 3 is non-null for a cylindrical shape 5 0°) contrary to what occurs in horizontal device assemblies.29 Therefore, we can conclude that the conicity variation has no significant impact on the device response for vertical wire assemblies. The PDMS layer plays a key role in device performance for both mechanical and electrical point of views. On one hand, its thickness and rigidity contribute (with the substrate) to the mechanical properties of the device in terms of flexibility, conformity, and sensitivity to the applied force. On the other hand, thickness also affects the potential level readout at the top electrode because of its dielectric nature. Indeed, the maximum of the piezoelectric potential occurring at the top of the wire is attenuated by the upper PDMS thickness (see Figure 2a). Consequently, the measured potential on the top floating electrode depends on the distance separating the metal from the wires, which dependent on both the wire length and PDMS thickness 6. To clarify this point we investigate in the following the dependence of the device output voltage with respect to the parameter 7 6/ for the two configurations (a) and (b) schematized in Figure 1 and having the same wire length and PDMS overall thickness 6. The top electrode voltage yielded by a structure consisting of a single GaN wire embedded into a capacitive structure is calculated in Figure 3 by finite element modelling. The wire conical angle, length and top diameter 52 ) are set to 1°, 120 μ and 1.4 μ respectively to fit the usual dimensions of MOVPE wires measured by scanning electron microscopy. A compressive load of 5 /μ $ is considered and 7 6/ is varied from 1.5 to 4.

Figure 3. Comparison of the potential generated by a single GaN wire buried into PDMS for the configurations (a) and (b) of Figure 1 as a function of the ratio p between the PDMS thickness t to the wire length L (7 6/). The potential is calculated at the top floating electrode of the device for 120 μ, 700 and 1°. The applied load is 5 /μ² on the 200x200 µm2 surface cell area. The mappings show the potential (in 89:6;) and the module of the total displacement (in µm) across the structures.


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The output signal delivered by the device decreases as a function of 7 6/ regardless the configuration: a thicker dielectric layer increases the attenuation of the piezo-potential and gives a lower potential at the metal electrode. In practice, a PDMS thickness to length ratio lower than about 1.5 will be therefore preferred to enhance device efficiency. It is also shown that configuration (a) provides higher output voltage regardless of the PDMS thickness. It tends towards a minimum saturation value of about 10 3 for t in the 400 - 600

Flexible Capacitive Piezoelectric Sensor with Vertically Aligned

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