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Transparent, Flexible Piezoelectric Nanogenerator Based on GaN Membrane Using Electrochemical Lift-Off Jin-Ho Kang, Dae Kyung Jeong, and Sang-Wan Ryu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15587 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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ACS Applied Materials & Interfaces

Transparent, Flexible Piezoelectric Nanogenerator Based on GaN Membrane Using Electrochemical Lift-Off Jin-Ho Kang, Dae Kyung Jeong, Sang-Wan Ryu*

Department of Physics, Chonnam National University, Gwangju 61186, Republic of Korea

Abstract

A transparent and flexible piezoelectric nanogenerator (TF PNG) is demonstrated based on a GaN membrane fabricated by electrochemical lift-off. Under shear stress on the TF PNG by finger force (~182 mN), the GaN membrane effectively undergoes normal stress and generates piezoelectric polarization along the c-axis, resulting in the generation of piezoelectric output from the TF PNG. Although the GaN layer is 315 times thinner than the flexible polyethylene terephthalate (PET) substrate, the low Young’s modulus of PET allows the GaN membranes to absorb ~41% of the applied strain energy, which leads to their large lattice deformation under extremely low applied stress. Maximum output voltage and current values of 4.2 V and 150 nA are obtained, and the time decay of the output voltage is discussed.

Keywords: electrochemical etching, energy-harvesting device, flexible electronics, GaN, piezoelectric nanogenerators

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1. Introduction With the extensive use of portable devices and various wireless sensors for the internet of things (IoT), harvesting energy from the surrounding environment to power small electronic devices has received a great deal of attention.1–3 Among various energy harvesters, piezoelectric nanogenerators (PNGs), which convert mechanical energy into its electrical counterpart, is considered a promising approach for self-powered devices due to the ubiquitous mechanical energy in the environment, such as movements and vibrations.4,5 In particular, transparent and flexible (TF) PNGs have attracted significant attention because they can be applied in wearable devices and convert infinitesimal movements into an electric signal.6-8 The combination of transparency and flexibility in a PNG can provide new types of energy harvesters, such as nanogenerator-embedded touch screens that convert touch action into electric energy and then operate the display.6 Recently, polyvinylidene fluoride (PVDF), lead zirconate titanate (PZT), zinc oxide (ZnO), and gallium nitride (GaN) are extensively studied as a piezoelectric medium for PNGs. Among them, PVDF has been considered as a good candidate for TF PNG due to flexibility, high piezoelectric coefficients, and biocompatibility. However, its high impedance and insulating properties lead to low output current and limit the applications in piezotronics and piezophototronics.9-12 Because of this, semiconductor-based PNGs have received great attention.13,14 GaN is a promising piezoelectric material due to its high mechanical robustness, chemical stability, environmental compatibility, and easy control of doping. However, the relatively high carrier concentration of unintentionally doped GaN (u-GaN) due to residual impurities and/or defects degrades the piezoelectric performance of PNGs because free carriers screen the piezoelectric polarization.14-16 Recently, forming a p-n junction barrier was proposed to suppress the detrimental screening behavior of free carriers, which showed significantly


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enhanced piezoelectric performance.17,18 When a p-n junction is formed in a piezoelectric device, screening current across the junction (called junction current screening) is the dominant degradation mechanism. The advantages of GaN, i.e., an epitaxially grown highquality p-n junction and the control of the built-in potential barrier by doping, offer promising piezoelectric performance due to suppressed junction current screening. Moreover, GaN is a non-toxic material with high chemical stability, so it is a strong candidate for an electrical power generator integrated with a device implanted into the human body. However, because growing GaN on a flexible substrate is challenging due to its high growth temperature (~1100 o

C), GaN TF PNGs have been rarely studied. Recently, various lift-off techniques to separate GaN films from sapphire substrates have

been studied for flexible devices;19-21 including laser, chemical, and electrochemical (EC) liftoff methods. Among them, EC lift-off, which selectively etches n-type GaN (n-GaN), is attractive because it causes little damage during the process, is low cost, and offers wide range of selectivity regardless of crystal orientations.22-24 Although there are several publications about fabricating free-standing GaN layers and devices through EC etching, producing and controlling large-area GaN membranes and applying them to PNGs remain challenging. In this work, we demonstrated large-area GaN membranes consisting of p-n homojunctions, which were produced based on the selective EC etching of n-GaN. The GaN membranes were transferred onto a flexible host substrate to complete TF PNG fabrication. The PNG was characterized by its optical transmittance and piezoelectric output.

2. Experimental Section 2.1. Lift-off of GaN Membranes and TF PNG fabrication. Figure 1 schematically shows the process flow for the fabrication of GaN membranes and the TF PNG. The epitaxial structures of c-axis GaN were grown by metalorganic chemical vapor deposition for the



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selective EC etching process. Since EC etching provides high selectivity based on the electrical conductivity of the GaN layer rather than its chemical constitution,25 a sacrificial layer with the highest conductivity was inserted into the structure. The epitaxial structure is shown in Figure 1a. The bottom n-GaN (n=2×1018 cm-3, 200 nm thick) provides enhanced lateral current flow, and it is protected by the upper u-GaN (300 nm thick), which is below the highly n-doped sacrificial GaN (n+-GaN, n=8×1018 cm-3, 400 nm thick). Before growing the p-doped GaN (p-GaN, 300 nm thick), u-GaN (300 nm thick) was grown again to form a p-n junction in the GaN membrane. Typical photolithography and chloride-based reactive ion etching (RIE) were carried out to open vias and expose sidewalls in the sacrificial layer. The diameter and distance between vias were 4 µm and 90 µm, respectively. The sacrificial layer was removed during EC etching (Figure 1 (b, c)), and the GaN membrane with the p-n junction

was

then

transferred

onto

a

new

flexible

host

substrate

of

sticky

polydimethylsiloxane (PDMS, 2 µm thick)-coated indium tin oxide (ITO, 300 nm) on polyethylene terephthalate (PET, 188 µm thick) (Figure 1d). By contacting with PDMS layer, GaN membrane loosely attached to sapphire substrate is transferred onto PET substrate. After the transferred GaN membranes were covered by PDMS (2 µm thick), a 300 nm layer of ITO was sputtered onto the PDMS as a transparent top electrode. The detailed information about PDMS coating and ITO deposition is narrated in Supporting Information. 2.2. Characterization. A scanning electron microscope (SEM) was employed to monitor the selective EC etching of n+-GaN. The transmittance of the TF PNG was measured by ultraviolet (UV)-visible spectroscopy. The piezoelectric performance of the TF PNG was characterized

by

a

high-speed

voltage-current

measurement

unit

(PARSTAT

4000/Potentiostat/Galvanostat/ELS Analyzer) while the device was periodically bent by finger force.


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3. Results and discussion 3.1. Electrochemical (EC) Etching and GaN Transfer onto Flexible Substrate. The EC etching of GaN has been actively studied to chemically lift off a GaN device from the substrate.24,26,27 The etching behavior strongly depends on the doping concentration and anodic bias; a high doping concentration and large anodic bias lead to completely etching a highly conductive sacrificial layer. In our design, due to the large difference between the conductivities of the u-GaN and n+-GaN layers, n+-GaN was selectively etched at the applied voltage of 30 V, as shown in Figure 2a. The EC etching started from the exposed n+-GaN by vias and proceeded laterally with etching time. As the lateral etching progressed, bright circles were observed in the micrograph after 5 min, indicating the formation of empty spaces between GaN layers as the sacrificial layer was etched away. As shown in Figure 2b, the circles coalesced after 15 min and eventually disappeared after the selective etching of n+GaN was complete. Initially, the lateral etch rate calculated from the micrographs was approximately 6 µm/min, but it decreased with the increasing etching length due to reduced electrolyte conductance.22 GaN membranes have been reported to spontaneously separate from the bottom GaN after selective etching due to the residual strain of the GaN layer grown on a sapphire substrate. However, in this structure, nanoscale etching residues formed at the boundaries between the bright circles (Figure S1) loosely hold the GaN membranes during EC etching. This loose support enables easy handling of the GaN membranes during the following steps. After bonding the membrane to the PET substrate, the membrane was intentionally detached by applying shear force. Using PDMS as a bonding material, the GaN membranes were transferred onto ITO-coated PET substrates. Figure 2c indicates that the transfer of a large-area (11 mm×26 mm) GaN membrane was successfully achieved. In contrast to thick sapphire, the thin, flexible substrate can provide high piezoelectric conversion efficiency because the low Young’s modulus of the flexible substrate leads to a 5 ACS Paragon Plus Environment


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large lattice deformation in the piezoelectric layer. Thus, mechanical energy can be efficiently utilized in the piezoelectric layer. 3.2. Optical Property of TF PNG. Raman spectroscopy was carried out to monitor the change in the strain after the transfer process. Due to the thermal expansion mismatch between GaN and sapphire, GaN layers grown on sapphire substrates typically have high residual compressive stress (corresponding to a Raman peak shift of approximately 570 cm-1 for the E2 (high) mode). This stress can be released by detaching the GaN layer from the sapphire substrate.24,28 As shown in Figure 3a, bottom GaN and as-grown GaN exhibited high compressive strain states similar to that of typical GaN grown on sapphire. On the other hand, transferred GaN showed a fully strain-relaxed value, and an approximate compressive stress of ~570 MPa was released by the transfer.28 This Raman peak shift indicates that GaN was successfully transferred to a flexible substrate, and this transfer process can be applied to the fabrication of various GaN-based flexible and transparent devices. Figure 3c shows how the transparency changed with each additional layer in the TF PNG (see inset). The transferred GaN layer (structure #2) presented approximately 78% transmittance at a wavelength of 560 nm, and this transmittance decreased with the increasing incident photon energy due to light absorption by the ITO layer. The upper PDMS and ITO layers decreased the transmittance of the TF PNG (structure #3) at all wavelengths, however, the photograph (Figure 3d) clarified that the TF PNG is sufficiently transparent to reveal the text behind it under fluorescent light. Note that the sudden decrease in the transmittance of the TF PNG at 363 nm is considered to be due to light absorption by the embedded GaN layer. The PL spectrum of the transferred GaN (Figure 3b) shows an emission peak of 367 nm,22 which is consistent with the transmittance spectra. 3.3. Measurement of Piezoelectric Outputs. The piezoelectric performance of the TF PNG was measured while the device was periodically bent by finger force (~ 182 mN, Figure S2), as shown in Figure 4. Compared to the PET substrate (188 µm thick), the ultra-thin GaN 6 ACS Paragon Plus Environment

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membrane (600 nm thick) effectively underwent tensile and compressive stress along the lateral and vertical directions, respectively, resulting in vertical piezoelectric polarization along the c-axis, as shown in the COMSOL simulation result (Figure 4b). The simulation structure and detailed conditions are narrated in Figure S4. The resulting piezoelectric output voltage and current were successfully measured, thus demonstrating a functional TF PNG based on the transferred GaN membrane, as shown in Figure 4c and 4d. The generation mechanism of the piezoelectric output is analyzed below. 3.4. Analysis of Measured Piezoelectric Outputs. Figure 5 illustrates the mechanism of voltage generation in the PNG when the GaN membrane is subjected to compressive stress along the c-axis by bending the PET substrate. Due to the non-centrosymmetry of its crystal structure, piezoelectric charges are generated at both ends of the GaN, and the conduction and valence bands are tilted immediately after applying the stress (Figure 5b).2,17 However, the high density of free charge carriers in GaN quickly screens the piezoelectric charges (called free-carrier screening). Without the p-n junction that forms a built-in potential barrier internally, the Fermi levels would be flattened after reaching a thermal equilibrium distribution of carriers. Due to the high electron mobility of u-GaN (160 cm2/Vs) and their short travel distance (600 nm), this process is expected to be complete within 100 ns.18 Since the p-n junction restricts the flow of free charge carriers within the n- or p-GaN region, Fermi levels at the n- and p-regions were separated after the completion of free carrier screening (Figure 5c). The separation between the Fermi levels represents the output voltage of piezoelectric generation. However, the junction current (Jscr) driven by the piezoelectric bias itself diminishes the piezoelectric charges, which is called junction current screening. This is the dominant mechanism to degrade the piezoelectric performance of a GaN p-n junction generator. Therefore, decreasing the junction current is necessary to obtain high piezoelectric output, and the depletion width and built-in potential should be optimized to suppress the junction current 7 ACS Paragon Plus Environment


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screening. In this regard, a p-n junction with excellent controllability of junction properties by doping is a great virtue in high-performance GaN PNGs. To enhance the piezoelectric performance by decreasing the junction current, semi-intrinsic p-GaN was fabricated without post-annealing the as-grown p-GaN for Mg activation.29,30 As reported previously,17 highly resistive p-GaN (resistivity of approximately 0.5 MΩcm) successfully reduced the junction current and led to a high output voltage (3.4 V) with a slow decay (decay time constant of 170 ms) of the piezoelectric voltage due to the suppressed screening. Under constant piezoelectric polarization, the voltage decay is a measure of junction current screening, and a slow decay constant leads to high output voltage. By forming the p-n junction, the TF PNG showed maximum output voltage and current values of 4.2 V and 150 nA, respectively. As shown in the bottom plot of Figure 4c, the TF PNG exhibited a short decay time constant of 4 ms with a relatively high voltage output (4.2 V). This result deserves close attention in light of our previous results (170 ms decay time, 3.4 V output voltage) with a PNG formed on a sapphire substrate.17 The enhanced performance of the TF PNG could originate from the high piezoelectric polarization generated under a low applied force (~ 182 mN). Although the PET substrate (188 µm) was much thicker than the GaN film (0.6 µm), the extremely low Young’s modulus (2 GPa) of PET allowed the GaN layer (390 GPa) to absorb ~41% of the applied mechanical energy,31 resulting in extremely high lattice deformation compared to that on sapphire. It is supported by the calculated values of the stored mechanical energy and the work done by the bending force applied to the TF PNG (see S3, Supporting Information). However, unexpected Mg activation of p-GaN may occur during the RIE process and is suspected to increase the free hole concentration. A high free hole concentration could increase the junction current significantly, leading to the fast decay of the output voltage. This is consistent with the low energy conversion efficiency of TF PNG (see S2, Supporting Information) that was estimated as the ratio of the generated 8 ACS Paragon Plus Environment

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electric energy to the stored mechanical energy.32 Therefore, the piezoelectric performance and energy conversion efficiency of the TF PNG would be further enhanced by more carefully controlling the processing conditions.

4. Summary In conclusion, we demonstrated the transfer of a large-area GaN membrane onto a flexible PET substrate and the fabrication of a TF PNG. The morphological and structural analyses were conducted on transferred GaN membranes together with determining their optical characteristics. By bending the TF PNG, the piezoelectric polarization in the GaN membrane was generated along the c-axis, resulting in piezoelectric output voltage and current values of 4.2 V and 150 nA, respectively. Despite the short decay time of 4 ms from high junction current screening, the high output voltage of the TF PNG exhibited a large lattice deformation and enhanced piezoelectric charge generation in the GaN membrane due to bending with the aid of the small Young’s modulus of the flexible PET substrate. Therefore, the TF PNG based on the GaN membrane is promising as a high performance energy harvester for piezotronic and piezo-phototronic applications.

ASSOCIATED CONTENTS Supporting Information. Detailed EC etching, COMSOL simulation, and calculation of piezoelectric conversion efficiency (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2016R1A2B4008622).



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Figure 1. Process flow of separating the GaN membrane and fabricating the TF PNG: (a) Patterning GaN structures to expose the sacrificial layer (n+-GaN), (b) selective EC etching of the sacrificial layer in nitric acid, (c) the loosely bound GaN membrane due to nanoscale etching residues, (d) transferring the GaN membrane onto an ITO/PDMS-coated PET substrate, and (e) fabricating the GaN-membranebased TF PNG and applying shear stress to measure the piezoelectric output.

Figure 2. (a) Cross-sectional SEM image of the GaN membrane after EC etching and (b) top-view microscope images of the patterned GaN during EC etching. The etch depth was monitored using bright circles, and the sacrificial layer was fully etched after 15 min. (c) Photographs of the GaN membranes transferred onto the PET substrate and bottom GaN (residual GaN layers on sapphire). The left and right images show the surface morphologies of the transferred and bottom GaN, respectively. Blue arrows indicate the vias fabricated by RIE etching.



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Figure 3. (a) Raman spectroscopy for the transferred GaN, bottom GaN, and as-grown GaN, (b) room temperature photoluminescence of transferred GaN, (c) measured transmittance spectra of each layer of the TF PNG, as depicted in the inset, and (d) photographic images of structures #1 and #3 under fluorescent light.

Figure 4. (a) Photograph of the TF PNG bent by finger force for measuring the piezoelectric output, (b) COMSOL-simulated piezoelectric potential of a GaN membrane under shear stress on the TF PNG, and (c) piezoelectric output voltage and (d) current measured from the TF PNG while it was periodically bent. The lower plots show magnified views of the output voltage and current during a single bending cycle.


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Figure 5. Conceptual illustration of band diagrams that exhibited the generation and screening of piezoelectric output voltage in the TF PNG under normal stress. (a) Equilibrium before applying stress, (b) piezoelectric charge generation (black symbols) and band tilting immediately after the application of stress. (c) Free carriers in the n- and p-GaN layers (red symbols) screen piezoelectric charges, which lead to Fermi-level separation between the n- and p-regions. The difference in Fermi levels is measured as the output voltage (Vout). (d) The decrease in Fermi-level separation by the junction current (Jscr) across the p-n junction.

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Transparent, Flexible Piezoelectric Nanogenerator ... - ACS Publications

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