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GaN wire-based Langmuir–Blodgett films for self-powered flexible strain sensors
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 375502 (http://iopscience.iop.org/0957-4484/25/37/375502) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 61.129.42.30 This content was downloaded on 25/04/2017 at 05:53 Please note that terms and conditions apply.
You may also be interested in: Effects of mechanical deformation on energy conversion efficiency of piezoelectric nanogenerators Jinho Yoo, Seunghyeon Cho, Wook Kim et al. A review of piezoelectric polymers as functional materials for electromechanical transducers Khaled S Ramadan, D Sameoto and S Evoy Investigating the energy harvesting capabilities of a hybrid ZnO nanowires/carbon fiber polymer composite beam N Masghouni, J Burton, M K Philen et al. From single III-nitride nanowires to piezoelectric generators: New route for powering nomad electronics N Gogneau, N Jamond, P Chrétien et al. The preparation of -crystalline non-electrically poled photoluminescant ZnO–PVDF nanocomposite film for wearable nanogenerators Santanu Jana, Samiran Garain, Sujoy Kumar Ghosh et al. Lateral bending of tapered piezo-semiconductive nanostructures for ultra-sensitive mechanical force to voltage conversion Rodolfo Araneo and Christian Falconi Optical properties of vertical, tilted and in-plane GaN nanowires on different crystallographic orientations of sapphire C Tessarek, S Figge, A Gust et al. Piezoelectric thin films: an integrated review on transducers and energy harvesting Asif Khan, Zafar Abas, Heung Soo Kim et al.
Nanotechnology Nanotechnology 25 (2014) 375502 (8pp)
doi:10.1088/0957-4484/25/37/375502
GaN wire-based Langmuir–Blodgett films for self-powered flexible strain sensors S Salomon1, J Eymery2 and E Pauliac-Vaujour1 1
University Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, MINATEC Campus, F-38054 Grenoble, France 2 University Grenoble Alpes, F-38000 Grenoble, France CEA, INAC-SP2M, F-38054 Grenoble, France E-mail: [email protected] Received 5 March 2014, revised 7 July 2014 Accepted for publication 21 July 2014 Published 27 August 2014 Abstract
We report a highly flexible strain sensor which exploits the piezoelectric properties of ultra-long gallium nitride (GaN) wires. Langmuir–Blodgett assembled wires are encapsulated in a dielectric material (parylene-C), which is sandwiched between two planar electrodes in a capacitor-like configuration. Through FEM simulations we show that encapsulating densely aligned conical wires in a properly designed dielectric layer can maximize the amplitude of the generated piezoelectric output potential. According to these considerations we designed and fabricated macroscopic flexible strain sensors (active area: 1.5 cm2). The sensor was actuated in three point configuration inducing curvature radii of less than 10 cm and has a typical force sensitivity of 30 mV N−1. Keywords: GaN wires, piezoelectricity, self-powered flexible strain sensors, Langmuir–Blodgett assembly (Some figures may appear in colour only in the online journal) 1. Introduction
coefficients are lower than the ones found in ceramics [6] and are degraded when submitted to temperatures higher than 80 °C [7]. During the last decade, piezoelectric nanowires made of wurtzite crystals such as zinc oxide (ZnO) [8] and gallium nitride (GaN) [9] have been successfully synthesized. These single crystal materials are not centrosymmetric along their c-axis and this direction can be selected under specific growth conditions to obtain elongated wires. These wires are intrinsically piezoelectric and do not require an external poling. Moreover, due to their high aspect ratio, they are highly flexible and their piezoelectric coefficients are estimated to be at least as large as the ones of the bulk material they are made of [10], which gives them mechanical advantage over brittle piezoelectric ceramics and makes them viable competitors for polymer technologies. Consequently, piezoelectric wires appear to be promising candidates for integration into flexible piezoelectric sensors that can be employed in harsh environments. Piezoelectric nanowires have already been embedded horizontally and vertically in polymers (PVDF) or in polymethyl methacrylate resists to be used as high-power-output flexible nanogenerators [11, 12]. Regarding vertically aligned nanowires, the substrate and the growth
Piezoelectric sensors, usually made of piezoelectric ceramics such as PZT, are widely used to monitor dynamic strains that may occur in a structure upon deformation induced by an external force or pressure. Such systems are of interest for monitoring a structure’s integrity for example (structural health monitoring (SHM)) and preventing failure in a view to optimizing maintenance and replacement of damaged parts [1]. They are commonly mounted as surface strain sensors [2] and are usually limited to planar surfaces due to piezoelectric ceramics brittleness. Indeed ceramics fracture quite easily when submitted to strains higher than 0.1% [3] and monitoring strains on curved and highly deformable surfaces could be of particular interest for foldable electronics, electronic skin applications [4] or for monitoring the structural integrity of highly deformable and curved composite materials. Commercially available flexible piezoelectric strain sensors are usually made of polymers such as polyvinylidene fluoride (PVDF) that can acquire piezoelectric properties upon poling when placed in an electric field [5]. Their compliance offers a good conformability, but their piezoelectric 0957-4484/14/375502+08$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 375502
S Salomon et al
seed are often part of the final device influencing the nanowire density and the device flexibility [13]. Horizontally aligned nanowires are usually detached from their growth substrate by dry contact printing [12, 14] or more conventionally by sonication before being reassembled on a substrate of interest [15]. Upon dry contact printing, the alignment of the piezoelectric polarization axes along the nanowire lengths (c-axes in these wurtzite wires) can be preserved for relatively low surface density arrangements. By patterning interdigitated electrodes on nanowires assembled by dry contact printing, one can read out the overall generated piezoelectric potential. Considering wires dispersed by sonication, the c-axes are randomly oriented and further improvements have to be developed to take advantage of additive effects. Previous works have shown that in order to measure a strain with randomly oriented nanowires in a capacitor-like electrode structure it was mandatory to have a wire shape anisotropy such as conicity [15–17]. In such devices, the measured piezopotential is increased by stacking several layers of nanowires and dielectric material. In the present work, we bring up experimental and theoretical studies of the collective piezoelectric properties of conical GaN wires assembled in an ordered and compact monolayer coupled to an optimized capacitor-like electrode structure. The main goal was to maximize the piezoelectric charge generation through the sensor design to minimize signal conditioning and processing. We first exploited the high compacity of wire layers assembled using a Langmuir–Blodgett (LB) trough to obtain 1.5 cm2 active area
sensors with over 1.5 million wires participating to the global piezoelectric response of the device. Using predictive finite element (FEM) modeling, we then focused on the coupling between the wire dimensions, shape, ordering and position in the inter-electrode dielectric material in view of device optimization through the identification of the key design parameters that enhance the charge generation and sensor sensitivity. A model embedding two separated wires was simulated to take into account their electrostatic coupling. Finally, the piezoelectric potential generated by a densely packed GaN wire monolayer integrated in a flexible, selfpowered force/pressure sensor was measured and yielded sensitivities of 30 mV N−1 and a minimum detection limit of the order of 1 kPa (skin sensitivity) prior to signal conditioning and processing (no amplification). These results compare well with the vast state-of-the-art of flexible force and pressure sensors based on ZnO nanowires [18], PVDF [19, 20] or other less common materials [21–23]. Moreover, obtained sensitivities are very competitive with regards to standard strain gage technologies used in SHM [24, 25] (ability to detect μm-scale deformations).
2. Materials and methods 2.1. Synthesis of GaN wires
Ultra-long self-organized GaN wires with hexagonal sections were grown on sapphire substrates by metal–organic vapor
Figure 1. SEM image of MOVPE GaN wires (a) after the growth (view perpendicular to the sapphire substrate) and (b) after sonication in a 2propanol solution followed by dispersion on a Si substrate. The inset shows evidence of their conical shapes. Wire (c) length and (d) radius distributions on ∼100 wires deduced from SEM image analysis. 2
Nanotechnology 25 (2014) 375502
S Salomon et al
phase epitaxy (MOVPE) using low III/V ratio and silane addition to favor the one-dimensional growth and to have a very high growth rate (>100 μm h−1) [9, 26]. The wire growth direction is along the (−c)-axis (they are called N-polar wires), which corresponds to the spontaneous and main piezoelectric polarization axis of the wurtzite structure. These self-organized and non-catalyzed wires have an average conical semi-angle of 0.3° with {1100} sidewall facets (see figures 1(a) and (b)). After sonication in a 2-propanol solution (IPA) and dispersion on a silicon substrate, the measured characteristic length is 120 ± 50 μm for a 0.8 μm average radius (see figures 1(c) and (d)). The dispersion of the wire lengths is related to the growth process, but also enlarged by sonication which can break the high aspect ratio wires. Dispersion in radii is mainly linked to the distributed size of the selforganized nucleation sites. This point can be largely improved by using a selective growth method and a substrate patterning or sedimentation/centrifugation separation protocols at the cost of the method complexity. Ultra-long ZnO nanowires with comparable dimensions (130 μm in length, 250 nm in diameter) were recently obtained using catalysts to take advantage of larger piezoelectric coefficients [27]. In our case, the GaN MOVPE growth process is catalyst free and offers more flexibility regarding silicon n-doping (which can be tuned between 5 × 1019 and 5 × 1020 atoms cm−3) and the realization of heterostructures (core-shell quantum wells/dots and p–n junctions). The wire shape can be changed from cylindrical to conical by adjusting growth conditions such as doping concentration, temperature and carrier gas pressure.
Figure 2. (a) Langmuir–Blodgett method to assemble the wires. The inset shows an SEM image of a compact monolayer of GaN wires obtained with this method. (b) Schematic of the wire-based sensor stacking and (c) photograph of a processed flexible device.
Both metals and parylene-C were deposited by evaporation. Following the wire monolayer LB deposition, another conformal and pinhole free parylene-C layer was deposited by evaporation at ambient temperature, in order to insulate the wires from the Ti (20 nm)/Al (90 nm) top electrode. The latter was finally deposited to obtain the material stacking depicted in figure 2(b). The resulting sensor is equivalent to a parallel plate capacitor with a dielectric medium made of aligned wires and parylene-C and an example of a processed device is given in figure 2(c). Both the thickness of the parylene-C and the compacity/ordering of the wire layer are critical parameters that have been optimized through FEM simulations as described below.
2.2. Fabrication of the strain sensor
The vertical deposition of the GaN wire layer by the LB method is illustrated in figure 2(a). This method has been preferred to its horizontal counterpart (Langmuir–Schaefer) where the sample is approached parallel to the liquid surface mainly in order to maximize the device surface area. Prior to the LB assembly process, the GaN wires were dispersed in a solution of isooctane and 2-propanol (3 : 1 v/v ratio) containing 50 μL of 5 mM 1-octadecylamine in hexane per 1 mL of suspension. The resulting wire concentration was about 106 wires mL−1. Upon 12 h incubation wires were rinsed three times with an IPA/isooctane solution (3 : 1 v/v ratio) to remove the surfactant excess. These hydrophobic wires were then drop-spread on the water surface of the LB trough (Kibron Microtrough XS). After allowing the solvent to evaporate for 45 min, the Teflon barriers of the trough were compressed at 10 mm min−1. Once a surface pressure of 35 mN m−1 was reached, the substrate was lifted at 1.5 mm min−1 to complete the LB assembly process. A densely-packed monolayer of well-aligned wires was obtained as shown in the inset of figure 2(a). The substrate was a 125 μm flexible polyethylene naphthalate (PEN) film, previously coated with a Ti (20 nm)/Al (90 nm) electrode and with an insulating 1 μm thick paryleneC layer to get a controlled and chemically stable dielectric layer (εr ∼ 3.15) with a low leakage.
3. FEM modeling 3.1. Theoretical considerations
The relative piezoelectric potential profiles in cylindrical nanowires induced by bending [28] or uniaxial compression have been extensively studied [29] using analytical or FEM models [30]. In our system, supposing no shear between the wire and parylene, we assume that uniaxial tensile/compressive deformations are dominant with respect to bending. The actual measurement of mechanical shear losses at the wire/ dielectric interface is not straightforward and necessitates the development of local probe measurements correlated to the evaluation of bending curvature. This will be the object of 3
Nanotechnology 25 (2014) 375502
S Salomon et al
The potential distribution in the nanowire is usually derived by solving the Poisson equation assuming that there are no free carriers and that there are no surface residual charges. This can be modeled by the following equations:
⋅ (k E + P) = 0 ,
(1)
(k E + P) ⋅ n = 0 ,
(2)
where E is the electric field vector, P the piezoelectric polarization vector and k the permittivity matrix. Considering a tilted cone exhibiting a very small semiangle α (figure 3(c)) with one edge parallel to the z axis, the longitudinal piezoelectric polarization vector can be projected on the y and z axes [15] and rewritten as: P = Pz sin (α ) y + Pz cos (α ) z ,
(3)
where z′ is collinear to the (−c)-axis. Equation (3) shows that a polarization along the y axis (i.e. normal to the electrodes) results from tilting the wire by its semi-angle α. Making a first order approximation due to the small value of the semi-angle, this polarization is proportional to α. In this study, we focus on modeling by FEM simulations the coupling between adjacent wires with opposite c-axis directions, i.e. in the unfavorable configuration where the contributions along the z axis exactly cancel each other while the smaller contributions along the y axis are additive.
Figure 3. (a) Cylindrical or (b) conical wires bent by a lateral force FLat or compressed by a vertical force FVert. (c) Schematic of a conical wire lying on a flat surface submitted to a tensile force FTensile.
further publication. For the time being, the no-shear approximation is considered sufficient.
Figure 4. (a) Simulated unit cell consisting of two aligned wires with opposite crystallographic c-axes. Wires are submitted to a tensile strain when the force FFEM is applied at the edge of the unit cell. (b) Geometry of the wire positioning: a facet is parallel to the bottom electrode and lies in the y = 1 μm plane. Variation of the piezoelectric potential between the ground and floating electrode as a function of (c) the conical semi-angle of the wires and (d) the applied force and the parylene-C thickness. 4
Nanotechnology 25 (2014) 375502
S Salomon et al
3.2. Simulations
In a first approximation, FEM calculations will neglect the spontaneous polarization of the wires [31]. Indeed, GaN wires exhibit polar charges on their bottom and top ±(0001) planes. Due to the wire high aspect ratio these charges can be considered as punctual and easily screened by foreign adsorbed molecules. Moreover, we can consider that this static electric field does not contribute significantly to the charge generation mechanism relying on dynamic carrier displacements at the capacitor electrodes. Parametric studies using FEM simulations will allow us to optimize the wire positioning in the dielectric layer of the functional device and to understand how the physical parameters (α, wire orientation) are linked to the average piezopotential. In order to optimize the piezoelectric potential generated by our device we studied the influence of several parameters such as the parylene-C thickness, wire density or alignment by FEM modeling using the COMSOL® Multiphysics software. In a first approximation, the piezoelectric coefficients of the GaN wires were considered to be equal to those of the bulk material although giant piezoelectricity has been predicted in GaN nanowires with smaller radii (
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My IOPscience
GaN wire-based Langmuir–Blodgett films for self-powered flexible strain sensors
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 375502 (http://iopscience.iop.org/0957-4484/25/37/375502) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 61.129.42.30 This content was downloaded on 25/04/2017 at 05:53 Please note that terms and conditions apply.
You may also be interested in: Effects of mechanical deformation on energy conversion efficiency of piezoelectric nanogenerators Jinho Yoo, Seunghyeon Cho, Wook Kim et al. A review of piezoelectric polymers as functional materials for electromechanical transducers Khaled S Ramadan, D Sameoto and S Evoy Investigating the energy harvesting capabilities of a hybrid ZnO nanowires/carbon fiber polymer composite beam N Masghouni, J Burton, M K Philen et al. From single III-nitride nanowires to piezoelectric generators: New route for powering nomad electronics N Gogneau, N Jamond, P Chrétien et al. The preparation of -crystalline non-electrically poled photoluminescant ZnO–PVDF nanocomposite film for wearable nanogenerators Santanu Jana, Samiran Garain, Sujoy Kumar Ghosh et al. Lateral bending of tapered piezo-semiconductive nanostructures for ultra-sensitive mechanical force to voltage conversion Rodolfo Araneo and Christian Falconi Optical properties of vertical, tilted and in-plane GaN nanowires on different crystallographic orientations of sapphire C Tessarek, S Figge, A Gust et al. Piezoelectric thin films: an integrated review on transducers and energy harvesting Asif Khan, Zafar Abas, Heung Soo Kim et al.
Nanotechnology Nanotechnology 25 (2014) 375502 (8pp)
doi:10.1088/0957-4484/25/37/375502
GaN wire-based Langmuir–Blodgett films for self-powered flexible strain sensors S Salomon1, J Eymery2 and E Pauliac-Vaujour1 1
University Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, MINATEC Campus, F-38054 Grenoble, France 2 University Grenoble Alpes, F-38000 Grenoble, France CEA, INAC-SP2M, F-38054 Grenoble, France E-mail: [email protected] Received 5 March 2014, revised 7 July 2014 Accepted for publication 21 July 2014 Published 27 August 2014 Abstract
We report a highly flexible strain sensor which exploits the piezoelectric properties of ultra-long gallium nitride (GaN) wires. Langmuir–Blodgett assembled wires are encapsulated in a dielectric material (parylene-C), which is sandwiched between two planar electrodes in a capacitor-like configuration. Through FEM simulations we show that encapsulating densely aligned conical wires in a properly designed dielectric layer can maximize the amplitude of the generated piezoelectric output potential. According to these considerations we designed and fabricated macroscopic flexible strain sensors (active area: 1.5 cm2). The sensor was actuated in three point configuration inducing curvature radii of less than 10 cm and has a typical force sensitivity of 30 mV N−1. Keywords: GaN wires, piezoelectricity, self-powered flexible strain sensors, Langmuir–Blodgett assembly (Some figures may appear in colour only in the online journal) 1. Introduction
coefficients are lower than the ones found in ceramics [6] and are degraded when submitted to temperatures higher than 80 °C [7]. During the last decade, piezoelectric nanowires made of wurtzite crystals such as zinc oxide (ZnO) [8] and gallium nitride (GaN) [9] have been successfully synthesized. These single crystal materials are not centrosymmetric along their c-axis and this direction can be selected under specific growth conditions to obtain elongated wires. These wires are intrinsically piezoelectric and do not require an external poling. Moreover, due to their high aspect ratio, they are highly flexible and their piezoelectric coefficients are estimated to be at least as large as the ones of the bulk material they are made of [10], which gives them mechanical advantage over brittle piezoelectric ceramics and makes them viable competitors for polymer technologies. Consequently, piezoelectric wires appear to be promising candidates for integration into flexible piezoelectric sensors that can be employed in harsh environments. Piezoelectric nanowires have already been embedded horizontally and vertically in polymers (PVDF) or in polymethyl methacrylate resists to be used as high-power-output flexible nanogenerators [11, 12]. Regarding vertically aligned nanowires, the substrate and the growth
Piezoelectric sensors, usually made of piezoelectric ceramics such as PZT, are widely used to monitor dynamic strains that may occur in a structure upon deformation induced by an external force or pressure. Such systems are of interest for monitoring a structure’s integrity for example (structural health monitoring (SHM)) and preventing failure in a view to optimizing maintenance and replacement of damaged parts [1]. They are commonly mounted as surface strain sensors [2] and are usually limited to planar surfaces due to piezoelectric ceramics brittleness. Indeed ceramics fracture quite easily when submitted to strains higher than 0.1% [3] and monitoring strains on curved and highly deformable surfaces could be of particular interest for foldable electronics, electronic skin applications [4] or for monitoring the structural integrity of highly deformable and curved composite materials. Commercially available flexible piezoelectric strain sensors are usually made of polymers such as polyvinylidene fluoride (PVDF) that can acquire piezoelectric properties upon poling when placed in an electric field [5]. Their compliance offers a good conformability, but their piezoelectric 0957-4484/14/375502+08$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 375502
S Salomon et al
seed are often part of the final device influencing the nanowire density and the device flexibility [13]. Horizontally aligned nanowires are usually detached from their growth substrate by dry contact printing [12, 14] or more conventionally by sonication before being reassembled on a substrate of interest [15]. Upon dry contact printing, the alignment of the piezoelectric polarization axes along the nanowire lengths (c-axes in these wurtzite wires) can be preserved for relatively low surface density arrangements. By patterning interdigitated electrodes on nanowires assembled by dry contact printing, one can read out the overall generated piezoelectric potential. Considering wires dispersed by sonication, the c-axes are randomly oriented and further improvements have to be developed to take advantage of additive effects. Previous works have shown that in order to measure a strain with randomly oriented nanowires in a capacitor-like electrode structure it was mandatory to have a wire shape anisotropy such as conicity [15–17]. In such devices, the measured piezopotential is increased by stacking several layers of nanowires and dielectric material. In the present work, we bring up experimental and theoretical studies of the collective piezoelectric properties of conical GaN wires assembled in an ordered and compact monolayer coupled to an optimized capacitor-like electrode structure. The main goal was to maximize the piezoelectric charge generation through the sensor design to minimize signal conditioning and processing. We first exploited the high compacity of wire layers assembled using a Langmuir–Blodgett (LB) trough to obtain 1.5 cm2 active area
sensors with over 1.5 million wires participating to the global piezoelectric response of the device. Using predictive finite element (FEM) modeling, we then focused on the coupling between the wire dimensions, shape, ordering and position in the inter-electrode dielectric material in view of device optimization through the identification of the key design parameters that enhance the charge generation and sensor sensitivity. A model embedding two separated wires was simulated to take into account their electrostatic coupling. Finally, the piezoelectric potential generated by a densely packed GaN wire monolayer integrated in a flexible, selfpowered force/pressure sensor was measured and yielded sensitivities of 30 mV N−1 and a minimum detection limit of the order of 1 kPa (skin sensitivity) prior to signal conditioning and processing (no amplification). These results compare well with the vast state-of-the-art of flexible force and pressure sensors based on ZnO nanowires [18], PVDF [19, 20] or other less common materials [21–23]. Moreover, obtained sensitivities are very competitive with regards to standard strain gage technologies used in SHM [24, 25] (ability to detect μm-scale deformations).
2. Materials and methods 2.1. Synthesis of GaN wires
Ultra-long self-organized GaN wires with hexagonal sections were grown on sapphire substrates by metal–organic vapor
Figure 1. SEM image of MOVPE GaN wires (a) after the growth (view perpendicular to the sapphire substrate) and (b) after sonication in a 2propanol solution followed by dispersion on a Si substrate. The inset shows evidence of their conical shapes. Wire (c) length and (d) radius distributions on ∼100 wires deduced from SEM image analysis. 2
Nanotechnology 25 (2014) 375502
S Salomon et al
phase epitaxy (MOVPE) using low III/V ratio and silane addition to favor the one-dimensional growth and to have a very high growth rate (>100 μm h−1) [9, 26]. The wire growth direction is along the (−c)-axis (they are called N-polar wires), which corresponds to the spontaneous and main piezoelectric polarization axis of the wurtzite structure. These self-organized and non-catalyzed wires have an average conical semi-angle of 0.3° with {1100} sidewall facets (see figures 1(a) and (b)). After sonication in a 2-propanol solution (IPA) and dispersion on a silicon substrate, the measured characteristic length is 120 ± 50 μm for a 0.8 μm average radius (see figures 1(c) and (d)). The dispersion of the wire lengths is related to the growth process, but also enlarged by sonication which can break the high aspect ratio wires. Dispersion in radii is mainly linked to the distributed size of the selforganized nucleation sites. This point can be largely improved by using a selective growth method and a substrate patterning or sedimentation/centrifugation separation protocols at the cost of the method complexity. Ultra-long ZnO nanowires with comparable dimensions (130 μm in length, 250 nm in diameter) were recently obtained using catalysts to take advantage of larger piezoelectric coefficients [27]. In our case, the GaN MOVPE growth process is catalyst free and offers more flexibility regarding silicon n-doping (which can be tuned between 5 × 1019 and 5 × 1020 atoms cm−3) and the realization of heterostructures (core-shell quantum wells/dots and p–n junctions). The wire shape can be changed from cylindrical to conical by adjusting growth conditions such as doping concentration, temperature and carrier gas pressure.
Figure 2. (a) Langmuir–Blodgett method to assemble the wires. The inset shows an SEM image of a compact monolayer of GaN wires obtained with this method. (b) Schematic of the wire-based sensor stacking and (c) photograph of a processed flexible device.
Both metals and parylene-C were deposited by evaporation. Following the wire monolayer LB deposition, another conformal and pinhole free parylene-C layer was deposited by evaporation at ambient temperature, in order to insulate the wires from the Ti (20 nm)/Al (90 nm) top electrode. The latter was finally deposited to obtain the material stacking depicted in figure 2(b). The resulting sensor is equivalent to a parallel plate capacitor with a dielectric medium made of aligned wires and parylene-C and an example of a processed device is given in figure 2(c). Both the thickness of the parylene-C and the compacity/ordering of the wire layer are critical parameters that have been optimized through FEM simulations as described below.
2.2. Fabrication of the strain sensor
The vertical deposition of the GaN wire layer by the LB method is illustrated in figure 2(a). This method has been preferred to its horizontal counterpart (Langmuir–Schaefer) where the sample is approached parallel to the liquid surface mainly in order to maximize the device surface area. Prior to the LB assembly process, the GaN wires were dispersed in a solution of isooctane and 2-propanol (3 : 1 v/v ratio) containing 50 μL of 5 mM 1-octadecylamine in hexane per 1 mL of suspension. The resulting wire concentration was about 106 wires mL−1. Upon 12 h incubation wires were rinsed three times with an IPA/isooctane solution (3 : 1 v/v ratio) to remove the surfactant excess. These hydrophobic wires were then drop-spread on the water surface of the LB trough (Kibron Microtrough XS). After allowing the solvent to evaporate for 45 min, the Teflon barriers of the trough were compressed at 10 mm min−1. Once a surface pressure of 35 mN m−1 was reached, the substrate was lifted at 1.5 mm min−1 to complete the LB assembly process. A densely-packed monolayer of well-aligned wires was obtained as shown in the inset of figure 2(a). The substrate was a 125 μm flexible polyethylene naphthalate (PEN) film, previously coated with a Ti (20 nm)/Al (90 nm) electrode and with an insulating 1 μm thick paryleneC layer to get a controlled and chemically stable dielectric layer (εr ∼ 3.15) with a low leakage.
3. FEM modeling 3.1. Theoretical considerations
The relative piezoelectric potential profiles in cylindrical nanowires induced by bending [28] or uniaxial compression have been extensively studied [29] using analytical or FEM models [30]. In our system, supposing no shear between the wire and parylene, we assume that uniaxial tensile/compressive deformations are dominant with respect to bending. The actual measurement of mechanical shear losses at the wire/ dielectric interface is not straightforward and necessitates the development of local probe measurements correlated to the evaluation of bending curvature. This will be the object of 3
Nanotechnology 25 (2014) 375502
S Salomon et al
The potential distribution in the nanowire is usually derived by solving the Poisson equation assuming that there are no free carriers and that there are no surface residual charges. This can be modeled by the following equations:
⋅ (k E + P) = 0 ,
(1)
(k E + P) ⋅ n = 0 ,
(2)
where E is the electric field vector, P the piezoelectric polarization vector and k the permittivity matrix. Considering a tilted cone exhibiting a very small semiangle α (figure 3(c)) with one edge parallel to the z axis, the longitudinal piezoelectric polarization vector can be projected on the y and z axes [15] and rewritten as: P = Pz sin (α ) y + Pz cos (α ) z ,
(3)
where z′ is collinear to the (−c)-axis. Equation (3) shows that a polarization along the y axis (i.e. normal to the electrodes) results from tilting the wire by its semi-angle α. Making a first order approximation due to the small value of the semi-angle, this polarization is proportional to α. In this study, we focus on modeling by FEM simulations the coupling between adjacent wires with opposite c-axis directions, i.e. in the unfavorable configuration where the contributions along the z axis exactly cancel each other while the smaller contributions along the y axis are additive.
Figure 3. (a) Cylindrical or (b) conical wires bent by a lateral force FLat or compressed by a vertical force FVert. (c) Schematic of a conical wire lying on a flat surface submitted to a tensile force FTensile.
further publication. For the time being, the no-shear approximation is considered sufficient.
Figure 4. (a) Simulated unit cell consisting of two aligned wires with opposite crystallographic c-axes. Wires are submitted to a tensile strain when the force FFEM is applied at the edge of the unit cell. (b) Geometry of the wire positioning: a facet is parallel to the bottom electrode and lies in the y = 1 μm plane. Variation of the piezoelectric potential between the ground and floating electrode as a function of (c) the conical semi-angle of the wires and (d) the applied force and the parylene-C thickness. 4
Nanotechnology 25 (2014) 375502
S Salomon et al
3.2. Simulations
In a first approximation, FEM calculations will neglect the spontaneous polarization of the wires [31]. Indeed, GaN wires exhibit polar charges on their bottom and top ±(0001) planes. Due to the wire high aspect ratio these charges can be considered as punctual and easily screened by foreign adsorbed molecules. Moreover, we can consider that this static electric field does not contribute significantly to the charge generation mechanism relying on dynamic carrier displacements at the capacitor electrodes. Parametric studies using FEM simulations will allow us to optimize the wire positioning in the dielectric layer of the functional device and to understand how the physical parameters (α, wire orientation) are linked to the average piezopotential. In order to optimize the piezoelectric potential generated by our device we studied the influence of several parameters such as the parylene-C thickness, wire density or alignment by FEM modeling using the COMSOL® Multiphysics software. In a first approximation, the piezoelectric coefficients of the GaN wires were considered to be equal to those of the bulk material although giant piezoelectricity has been predicted in GaN nanowires with smaller radii (
