Highly Efficient and Flexible Photosensors with GaN Nanowires

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Functional Inorganic Materials and Devices

Highly Efficient and Flexible Photosensors with GaN Nanowires Horizontally Embedded in Graphene Sandwich Channel Sangmoon Han, Seoung-Ki Lee, Ilgyu Choi, Jihoon Song, CheulRo Lee, Kangmin Kim, Mee-Yi Ryu, Kwang-Un Jeong, and Jin Soo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

Highly Efficient and Flexible Photosensors with GaN Nanowires Horizontally Embedded in Graphene Sandwich Channel Sangmoon Han,†,⊥ Seoung-Ki Lee,‡,⊥ Ilgyu Choi,† Jihoon Song,† Cheul-Ro Lee,† Kangmin Kim,‡ Mee-Yi Ryu,§ Kwang-Un Jeong,ǁ and Jin Soo Kim*,†

†

Department of Electronic and Information Materials Engineering, Division of Advanced

Materials Engineering, and Research Center of Advanced Materials Development, Chonbuk National University, Jeonju 54896, South Korea ‡

Applied Quantum Composites Research Center, Korea Institute of Science and Technology,

Wanju 55324, South Korea §

ǁ

Department of Physics, Kangwon National University, Chuncheon 24341, South Korea

Department of Polymer-Nano Science and Technology, and Polymer Materials Fusion

Research Centre, Chonbuk National University, Jeonju 54896, South Korea *corresponding author e-mail: [email protected] (J.S. Kim); Tel.: +82-63-270-2291; Fax: +82-63-270-2305

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ABSTRACT: In this study, we report highly efficient and flexible photosensors with GaN nanowires (NWs) horizontally embedded in a graphene sandwich structure fabricated on polyethylene terephthalate (PET). GaN NWs and the graphene sandwich structure are used as light-absorbing media and the channel for carrier movement, respectively. To form uniform high-quality crystalline GaN NWs on Si(111) substrates, the initial nucleation behavior of the NWs was manipulated by applying the new growth technique of Ga pre-deposition. Highresolution transmission-electron microscope images obtained along the vertical direction of the GaN NWs showed that stacking faults, typically observed in Si-based (In,Ga)As NWs, were rare. Consequently, narrow and strong optical emission was observed from the GaN NWs at the wavelengths of 365.12 nm at 300K. The photocurrent and photoresponsivity of the flexible photosensor with 802-nm-long GaN NWs horizontally embedded in the graphene sandwich channel were measured as 9.17 mA and 91.70 A/W, respectively, at the light intensity of 100 mW/cm2, which are much higher than those previously reported. The high optical-to-electrical conversion characteristics of our flexible photosensors are attributed to the increase in the effective interface between the light-absorbing media and the carrier channel by the horizontal distribution of the GaN NWs within the graphene sandwich structure. After 200 cyclic-bending test of the GaN NW photosensor at the strain of 3%, the photoresponsivity under strain was measured as 89.04 A/W at 100 mW/cm2, corresponding to 97.1% of the photoresponsivity obtained before bending. The photosensor proposed in this study is relatively simple in device design and fabrication, and it requires no sophisticated nano-structural design to minimize the resistance to metal contacts.

Keywords: GaN, nanowires, graphene channel, flexible photosensor, Si substrate, polyethylene terephthalate


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1. INTRODUCTION A flexible photosensor is one of critical components for wearable and portable device applications.1-5 In particular, photosensors operating in the ultraviolet (UV) to near-infrared (NIR) wavelengths are expected to be widely used for short-range optical wireless communications, missile-plume tracking, and flame monitoring.6-8 Theoretically, the III– nitride material system can cover the deep UV to NIR wavelengths by controlling the amounts of Ga, In, and Al in (Ga,In,Al)N. However, obtaining high-quality AlGaN with high Al content and InGaN with high In content, commonly used as active media for optoelectronic devices, is quite difficult.9-11 Recently, III–nitride-based nanostructures such as quantum dots and nanowires (NWs) have been extensively investigated to extend their emission wavelengths. GaN NWs formed on Si(111) were used as active media for optical devices because they showed high exciton-binding energies and relatively high carrier mobilities.12-15 The most well-known approach for the formation of GaN NWs is the vapor– liquid–solid (VLS) process, in which a metallic catalyst acts as seeds to promote the formation of anisotropic NWs.16,17 However, forming high-quality GaN NWs remains difficult because of the contamination originated from the metal catalysts.18-20 Several research groups have reported catalyst-free or self-catalyst GaN NWs using VLS or Volmer– Weber modes to reduce the chemical contamination caused by the metallic catalysts.21-25 However, because of the large difference in material parameters between GaN and Si, including the lattice constants and thermal expansion coefficients, methods for forming highcrystal-quality GaN NWs remain in development. That is, catalyst-free GaN NWs on Si substrates show various shapes and many structural defects, including stacking faults.21,25 From this consideration, it is first necessary to form GaN NWs with high crystal quality and uniformity.



The fabrication of flexible optical devices using GaN quantum structures is receiving significant attention. Peng et al. reported a flexible photoswitch using GaN film-based membranes on polyethylene terephthalate (PET).2 Photosensors using vertically aligned GaN NWs as light-absorbing media have been reported. Shi et al. and Zhang et al. demonstrated photosensors fabricated with vertically aligned GaN NWs and a polymer, where the polymer was used to protect the NWs in the device.26,27 However, the photosensors were fabricated by a complicated process, and optical losses might occur in devices via transmittance loss of light by the polymer. Moreover, the effective volume of vertically aligned GaN NWs absorbing light is lower than that of horizontally lying NWs. Lähnemann et al. and Spies et al. reported photodetection from a single GaN/AlN NW in various wavelength ranges, where the devices were fabricated on non-flexible Si substrates.5,28 Considering these previously reported results, it is necessary to change the device design in order to fabricate highly efficient and flexible photosensors. In this study, we report highly efficient and flexible photosensors with GaN NWs as absorption media, embedded horizontally and randomly in a graphene sandwich channel structure fabricated on PET. For comparison, GaN-NW photosensors with the graphene sandwich structure were also prepared on SiO2/Si substrates. For the formation of the GaN NWs used as light-absorbing media on Si(111) substrates, a new growth method of Ga predeposition was used with a plasma-assisted molecular-beam epitaxy (PA-MBE) system. The structural and optical characteristics of the GaN NWs were investigated by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The photocurrents of the GaN-NW photosensors were measured under various incident light conditions. In order to analyze the degree of flexibility, the GaN-NW photosensor was subjected to 200 cyclic-bending test under three different bending radii. The photoresponsivity of the photosensor under the bending radius of 3.1 mm,


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corresponding to the strain of 3%, was 89.04 A/W, which was 97.1% of that measured before bending. 2. EXPERIMENTAL SECTION Characterization of GaN NWs. GaN NWs were grown on Si(111) substrates by the PAMBE system. Figure 1 show a schematic of GaN-NW formation using the Ga pre-deposition technique. Before the growth of GaN NWs, the native oxide on the Si(111) substrates was removed by chemical wet etching and subsequent thermal annealing at a substrate temperature of 900 °C within the PA-MBE reactor. Only N-plasma flux was supplied to the Si(111) substrate for 10 min at the temperature of 800 °C (nitridation process), thereby forming a thin SiN layer with the thickness of 27 nm. After the nitridation process, a Ga flux was supplied to the SiN/Si(111) surface for 6 s without the N-plasma flux to form initial nucleation sites of Ga droplets for the GaN NWs. Figure 1a schematically shows the Ga droplets formed by supplying only Ga atoms without the N-plasma flux (process I). The size and density of the Ga droplets were determined by the supply time of Ga flux. As shown in Figure 1b, the Ga and N-plasma fluxes are simultaneously supplied to grow GaN NWs via the initial nucleation sites of the Ga droplets. The newly supplied Ga and N atoms move to Ga droplets and crystalize into GaN NWs (processes II and III). The Ga adatoms migrate along the lateral sides of the NW towards the top region (process IV), which is attributed to the driving force induced by the lower chemical potential of the Ga adatoms.29,30 To control the structural properties, the GaN NWs were grown under three different V/III ratios, defined as the flux ratio of N to Ga, of 130 (NW1 sample), 120 (NW2 sample), and 111 (NW3 sample). The V/III ratio was decreased by reducing N-flux at a fixed Ga flux of 4.5×10-8 Torr. To increase the height of the GaN NWs (NW4 sample), the growth time was increased by 1.67 times at the same V/III ratio used for the NW3 sample.



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A FE-SEM (Hitachi Su-70) was used to analyze the structural properties of the NW samples and photosensors. The accelerating voltage for FE-SEM measurements was set to 10 kV. A TEM (JEOL JEM-2010) was used to investigate the structural properties of NWs depending on the positions. For the TEM measurements, the specimens were fabricated using focused ion-beam milling. The high voltage was set to 200 kV. The PL system was used to analyze the optical characteristics of the GaN NWs. A diode-pumped solid-state laser with the wavelength of 266 nm was used as an excitation source. A charge-coupled device with an electrically controlled cooling system was used to detect luminescence from the NW samples. A monochromator with a length of 0.5 m was used. The electrical characteristics of the photosensors

with

GaN

NWs

as

absorbing

media

were

measured

using

an

electroluminescence system. The voltage applied to the photosensors was varied from (−) 1.12 V to +1.12 V. Fabrication of Graphene and Flexible Photosensors with GaN NWs. Figure 2a shows a schematic of the fabrication of photosensors using the GaN NWs as light-absorbing media horizontally and randomly embedded in the graphene sandwich structure (single-layer graphene/NWs/single-layer graphene). For the fabrication of the flexible photosensors, Au electrodes with a thickness of 40 nm, a length of 0.50 mm, and a width of 1.02 mm were deposited on a PET sheet by using photolithography and a lift-off process. The distance between two electrodes (channel length) was 63.2 µm. For the growth of the graphene layer by chemical vapor deposition, 35-µm-thick Cu foil was loaded in the furnace. In the vacuum state, H2 was supplied at 100 sccm for 3 min and subsequently thermal annealing was performed at the substrate temperature of 1000 °C inside the furnace. Then, CH4 gas was supplied at 10 sccm for 30 min to form monolayer graphene on the Cu foil. A graphene layer was then transferred onto the PET sheet as the bottom channel for carrier movement (Step 1). To separate GaN NWs from Si(111) substrates, we used ultrasonic machining process31,


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where a small piece of the GaN-NW sample was dipped into isopropyl alcohol. After an ultrasonic process for 20 minutes, a dilute solution with GaN NWs was obtained and then dispersed on the bottom graphene layer transferred onto PET with Au electrodes (Step 2). A soft baking process was successively performed at 60 °C. And then, a second graphene layer was transferred to form a top channel (Step 3). A photosensor after transferring the top graphene layer is schematically shown in (Step 4). The Raman spectrum for the graphene sandwich channel for the photosensors is shown in Figure 2b. Three spectral features at 1,343.7, 1,588.6, and 2,682.2 cm−1 are observed, which correspond to the D-band, G-band, and 2D-band, respectively. The distinctive Raman peaks with narrow linewidths indicate the formation of a graphene sandwich structure with high crystal quality. To compare with the photosensors fabricated on the PET sheet, GaN-NW photosensors were also prepared on a SiO2/Si substrate. A SiO2 layer with a thickness of 300 nm was grown on a Si(001) substrate by thermal growth. The other fabrication processes for the photosensors with the GaN NWs and the graphene sandwich channel on the SiO2/Si substrate were the same as those used for the PET-based devices. The fabricated photosensors with GaN NWs embedded in the graphene sandwich channel are shown in Figure 2c, where the left and right images show the photosensors fabricated on SiO2/Si and PET sheets, respectively. A magnified image of a single photosensor is shown at the top right. To investigate the effect of effective volume of light-absorbing media, GaN NWs with different length and spatial density were used for the photosensors fabricated on SiO2/Si. For comparison, photosensors with only the graphene sandwich channels without GaN NWs were fabricated on SiO2/Si (Si-Ref). For the flexible photosensor on PET (PET-PS), GaN NWs with the length of 802 nm and the density of 1.67×108 cm-2 were used as light absorption media. Details on the length and density of GaN NWs for the photosensors are described in Table 1.



3. RESULTS AND DISCUSSION Figures 3a, b, c, and d show the cross-sectional (left) and plan-view (right) FE-SEM images of the self-catalyst GaN NWs for the NW1, NW2, NW3, and NW4 samples, respectively. The average diameters (heights) of the NW1, NW2, NW3, and NW4 samples were measured as 151 (344), 130 (493), 61 (548), and 62 nm (802 nm), respectively. The plan-view FE-SEM images of the GaN NWs in Figures 3a and b clearly show the hexagonal shapes of the NW tops, which are evidence of the formation of the wurtzite (WZ) crystal structure. The inset of each plan-view image is a three-dimensional FE-SEM image of the GaN NWs with hexagonal facets, also indicating the WZ crystal structure. The white dotted circles in the plan-view images indicate coalescence between adjacent GaN NWs. This may generate defects between the facets of the GaN NWs.32,33 The GaN NWs of the NW1 sample show reverse-mesa shapes. As the V/III ratio is decreased, the uniformity in the diameter of the GaN NWs along the vertical direction (c-axis) is significantly improved. For the NW1 sample grown at a relatively high V/III ratio with more N atoms than the other NW samples, the migration length of Ga atoms is more likely to be limited by the N atoms.29-34 That is, the probability for bonding reactions between Ga and N adatoms is relatively increased; consequently, the probability of Ga atoms reaching the top surface of a GaN NW is decreased. Therefore, the horizontal growth along the diameter direction of the GaN NW is likely to be larger than the vertical growth, thus forming the reverse-mesa structure. For the NW3 sample grown at the V/III ratio of 111 realized by decreasing the N-plasma flux, however, the probability of the Ga atoms reaching the top surface of the NW along the lateral side of the NWs is increased. As a result, the uniformity in the diameter is improved along the vertical direction of the NWs. As the length of the GaN NW increases, the number of Ga atoms reaching the top surface of the NW along the lateral side should decrease for a constant V/III ratio of 111. Therefore, for the NW4 sample with a relatively high length compared to


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that of the NW3 sample, the diameter decreases toward the upper side of the GaN NW. With further decreasing V/III ratio (not shown here), the growth rate for NWs was significantly reduced due to the decrease in number of N atoms for the given Ga atoms, resulting in the decrease in an average length. The length, diameter, and density of the GaN NW samples are summarized in Table 2. Figures 4a, b, and c show the cross-sectional TEM images of GaN NWs for the NW1, NW2, and NW3 samples, respectively. The high-resolution TEM (HRTEM) images and diffraction patterns, obtained from three different positions of the NW samples along the vertical growth direction, are shown in the right-hand side of each cross-sectional TEM image. The HRTEM images (middle) show that some stacking faults appear in the bottom region of the GaN NWs. However, the stacking faults disappear as distance from the interface between the substrate and the NW increases, and no stacking faults are observed within most GaN NWs. The stacking faults observed in the bottom region of the GaN NWs can be attributed to the large lattice mismatch and difference in thermal expansion coefficients between the GaN and Si substrate. The diffraction patterns confirmed that the GaN NWs had a WZ structure and were grown in the [0001] direction, which is consistent with the FE-SEM images. For the bottom region of the NW1 sample shown in Figure 4a, the diffraction patterns are more complex, mainly because of irregularities in the crystalline structures of the WZ and zinc-blende (ZB) types, as well as the influence of stacking faults. More specifically, the diffraction patterns of [0002] and [011ത0] in the WZ structure appear, as well as those of [113], [1ത1ത3], and [2ത2ത4] in the ZB structure.35-39 Considering the lattice constants for GaN and Si(111), the difference in total system energy caused by the lattice mismatch between the WZ-GaN and ZB-GaN is small inside the GaN NW. Therefore, metastable multiphase structures can be formed at the bottom of the GaN NWs.35,40 Moving to the top region of the NW1 sample, only the diffraction patterns of [0002] and [011ത0] planes in the WZ structure



are observed because of the drastic reduction in the portion of ZB crystal structure and stacking faults. In the diffraction patterns of the NW2 and NW3 samples, respectively shown in Figures 4b and c, the features corresponding to the stacking faults are almost invisible, except in the bottom region. After the generation of stacking faults at the interface, the influences of the lattice mismatch and the difference in thermal expansion coefficients on the formation of GaN NWs are drastically decreased moving to the upper regions of the NWs, thus yielding WZ-GaN NWs without stacking faults.17,34,41 Figure 5a shows the PL spectra of the GaN NW samples, measured at the light intensity of 3 mW/cm2. The free-exciton (FX) peaks were measured at the wavelengths of 369.43, 369.36, 365.12, and 365.12 nm for the NW1, NW2, NW3, and NW4 samples, respectively.42,43 The FX peaks of the NW1 and NW2 samples are slightly red-shifted compared to those of the NW3 and NW4 samples, because of the reduced quantum size effect in the lateral direction. Considering the exciton Bohr radius, the influence of the confinement of the carrier wave-function in the lateral direction is relatively large compared to that in the vertical direction. That is, since the GaN NWs in the vertical direction are very long in all NW samples, the carrier confinement does not differ significantly. The intensities of the FX peaks for the NW2, NW3, and NW4 samples are 7.37, 17.32, and 20.90 times stronger than that of the NW1 sample, respectively. This is attributed to the reduction in defect density including stacking faults, as confirmed from the TEM analysis. For the NW1 and NW2 samples, a broad yellow-band (YB) peak is weakly observed around 522 nm, which is related to defects.44-48 In addition, a broad peak around 462 nm originates from the polycrystalline (PC) GaN.35,44,49 Typically, it is very difficult to observe the FX peak from GaN NWs at 300K, largely because of the insufficient radiative recombination of carriers within the NWs over non-radiative recombination caused by defects and surface states.50-52 Once again, it should be noted that the FX peaks are clearly observed from our GaN NWs, confirming the


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formation of NWs with high crystal quality. Figure 5b shows the absorption spectra of the NW samples, where the main absorption wavelengths are 361.48, 361.35, 352.11, and 352.10 nm for the NW1, NW2, NW3, and NW4 samples, respectively. The absorption wavelengths are blue-shifted by 7.95 (NW1), 8.01 (NW2), 13.01 (NW3), and 13.02 nm (NW4), respectively, compared to the FX peaks of PL spectra. The variation in the wavelength is ascribed to the well-known Stokes shift. Figure 6a shows the plan-view FE-SEM image of the Si-Ref without GaN NWs, where continuous large-area graphene is transferred onto the target substrates without cracks or holes. Figures 6b, c, and d show the surface images of the photosensors with GaN NWs having the average length of 344 (Si-PS1), 548 (Si-PS2), and 802 nm (Si-PS3), respectively. The spatial densities of the GaN NWs for the Si-PS1, Si-PS2, and Si-PS3 were measured as 1.02×108, 1.01×108, and 1.06×108 cm−2, respectively. Figures 6e and f show the FE-SEM images for the photosensors with 802-nm long GaN NWs, corresponding to the NW4 sample, with the spatial density of 1.00×107 (Si-PS4), and 1.54×108 cm-2 (Si-PS5), respectively. In the FE-SEM images, there are some portions of GaN-NW agglomeration, occurred when dispersing and drying processes were performed for a diluted solution with GaN NWs. Figures 7a, b, c, d, e, and f show the current (I) versus voltage (V) characteristic curves of the Si-Ref, Si-PS1, Si-PS2, Si-PS3, Si-PS4, and Si-PS5, respectively, depending on the intensity of the white light (Xenon lamp) incident on the surface of photosensors. The photocurrent was calculated by subtracting the dark current from the light current.53 There is very small variation in the I–V characteristic curve of the Si-Ref with increasing light intensity. This is because of the nature of the electron–phonon scattering in the graphene structures caused by incident light.54-56 At the light intensity of 100 mW/cm2, the photocurrents of the photosensors were increased with increasing the length of GaN NWs. The photocurrent of SiPS3 was measured as 8.57 mA at the voltage of 1 V, which is higher than Si-PS1 (0.016 mA)



and Si-PS2 (1.86 mA). It can be explained by the increase in the effective volume of highquality crystalline GaN NWs. Figures 7e and f show the I-V characteristic curves of Si-PS4 and Si-PS5, respectively. The photocurrent of Si-PS5 was measured as 12.34 mA at the light intensity of 100 mW/cm2, which is higher than Si-PS3 and Si-PS4. This is also due to an increase in volume of the GaN NWs capable of absorbing light. In particular, the photocurrent of Si-PS5 is much higher than those reported in previous studies.2,26-28,57,58 Figures 7g and h show the summary on photoresponsivity, defined as the ratio of photocurrent to the “illuminated area × incident light density,” and resistance calculated from the I–V curves, respectively.57,58 The photoresponsivity (resistance) of the photosensors is increased (decreased) with increasing the length of GaN NWs, which is largely due to an increase in the effective volume of high-quality crystalline GaN NWs. In addition, the photoresponsivity (resistance) of the photosensors is increased (decreased) with increasing the spatial density of GaN NWs. The improved photocurrent and photoresponsivity can be attributed to the horizontally distributed highly crystalline GaN NWs, which effectively absorb and generate electron–hole pairs over an enlarged area. In addition, the atomically thin and pliable graphene realizes conformal contact with GaN NWs by wrapping the NW surface. The intimate contact facilitates transportation of the carriers between GaN NW and graphene by minimizing not only unintended artificial scattering centers but also recombination of excited carriers at the interface.59,60 Furthermore, the high carrier mobility of graphene allows prompt carrier transfer to the Au electrodes, thus increasing the photocurrent.8 Figure 8a and b show a schematic and an operating mechanism of a photosensor when illuminating white light. Since the conduction band of the GaN NWs is higher than the Fermi level of the graphene, the electrons on the conduction band, generated by absorbing white light, can moves to the graphene sandwich channel.8 And then, they moves to the Au electrodes contributing to the photocurrent.


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Figure 9a shows the I-V characteristic curves of PET-PS fabricated on PET with 802 nmlong GaN NWs with spatial density of 1.67×108 cm-2. When the light intensity was 100 mW/cm2, the photocurrent of PET-PS was measured as 9.17 mA at the voltage of 1 V. Figure 9b shows the summary on photoresponsivities and resistances of Si-PS5 and PET-PS, calculated from the I–V curves at the light intensities from 0 to 100 mW/cm2. The photoresponsivity of the Si-PS5 was measured as 123.50 A/W, which is also much higher than those reported previously.8,32,58 The relatively high photoresponsivity indicates that the device design used in this study is very useful for the fabrication of a high-efficiency photosensor. The photoresponsivity of the PET-PS was evaluated as 91.70 A/W, lower than that of the Si-PS5. This is related to an electrical loss from damage of graphene by thermal expansion of the PET sheet because of heat caused by incident light.61 Without illumination, the resistances of the Si-PS5 and PET-PS were measured as 567 and 870 Ω, respectively. At the incident light intensity of 100 mW/cm2, the resistances of the Si-PS5 and PET-PS were drastically reduced to 80, and 107 Ω, respectively. This is attributed to the additional supply of the carriers generated in the GaN NWs by the incident light. From the photoresponsivity of a photosensor, we calculated the external quantum efficiency and detectivity.62-64 The photocurrent, photoresponsivity, external quantum efficiency, detectivity, and resistance of phosensors fabricated on SiO2/Si and PET are summarized in Table 3. Figure 10 shows the photoresponsivity with respect to the amount of strain controlled by the bending radius of the PET-PS, which was measured at the light intensity of 100 mW/cm2. 200 cyclic-bending test of a PET-PS was carried out at each bending radius. The speed of the bending machine that applied strain to the photosensors was 10.5 mm/s. A video of the bending test of the photosensors is attached in the Supporting Information. The adhesive energy between the GaN and graphene layer is estimated as 0.3–0.7 J/m2, which is relatively high compared to that between GaN and metal electrodes.32,65,66 Therefore, it is considered



that the durability of the photosensors is not significantly changed by external changes such as bending. When no strain is applied to the photosensors, the photoresponsivity is 91.70 A/W. The photoresponsivity of the photosensors measured after 200 cyclic-bending test at the bending radius of 9.3 mm, corresponding to the strain of 1%, is 89.90 A/W. With further increases in the curvature of the substrate, the photoresponsivity is slightly decreased. Under the bending radius of 3.1 mm, corresponding to the strain of 3%, the photoresponsivity is 89.04 A/W at the voltage of 1 V. This is equivalent to 97.1% of the photoresponsivity measured before the bending test. Considering these results, we can conclude that our proposed photosensors have not only high optical-to-electrical conversion efficiency, but also reliable flexibility and high mechanical strength.

4. CONCLUSIONS In conclusion, we fabricated PET-based flexible photosensor with GaN NWs horizontally embedded in a graphene sandwich structure. The FX peaks were clearly observed from the room-temperature PL spectra of the GaN NWs working as light-absorbing media. Typically, FX peak observation from NWs is difficult, largely because of insufficient radiative recombination of carriers inside the NWs relative to non-radiative recombination caused by defects and surface states. Considering the optical properties of the GaN NWs, the Ga predeposition method of controlling the initial nucleation kinetics, as suggested in this work, can be evaluated as an effective growth technique for NWs with high crystal quality. The photocurrents of the photosensors were increased with increasing GaN NW length at the same light intensity, largely because of the increase in the effective volume of the GaN NWs capable of absorbing light. The photocurrent and photoresponsivity of the photosensors with 802-nm-long GaN NWs fabricated on PET were 9.17 mA and 91.70 A/W at 100 mW/cm2,


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respectively, which were much better than those previously reported. This result is attributed to the enlarged effective interface between the GaN NW absorption media and graphene channel because of the horizontal distribution of the NWs inside the graphene sandwich structure. From these results, flexible photosensors with high optical-to-electrical conversion efficiency can be fabricated by using GaN NWs and a graphene sandwich structure. After 200 cyclic-bending test of the GaN NW photosensors, the photoresponsivity remained at 97.1% of that measured before bending. This indicates that the photosensors have high mechanical flexibility and durability. Most of all, the flexible photosensor proposed in this study is relatively simple in structure and fabrication, and has the merit of avoiding sophisticated nano-structural designs to minimize the resistance to metal contact.



ASSOCIATED CONTENT Supporting information. Video S1: Bending experiments of the GaN NW photosensors fabricated on PET (AVI). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J.S. Kim) ORCID Sangmoon Han: 0000-0001-7924-2050 Seoung-Ki Lee: 0000-0002-8786-0251 Ilgyu Choi: 0000-0002-5566-3055 Jihoon Song: 0000-0002-4318-4191 Cheul-Ro Lee: 0000-0003-2082-8828 Kangmin Kim: 0000-0002-9836-9329 Kwang-Un Jeong: 0000-0001-5455-7224 Jin Soo Kim: 0000-0002-5522-2255 Author contribution ⊥

Seoung-Ki Lee and Sangmoon Han contributed equally.

Note The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS This work was supported by a National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2015042417) and by the Ministry of Education (2015R1D1A1A01060681 and 2018R1D1A1B07043442).



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Figure 1. Schematic illustration for the (a) formation of Ga droplets using Ga pre-deposition technique and (b) growth process of GaN NWs via the Ga droplets by simultaneously supplying Ga and N-plasma sources on a Si(111) substrate.



Figure 2. (a) Schematic for the fabrication process of a GaN-NW photosensor, (b) Raman spectrum of graphene transferred on a Si substrate, and (c) an optical microscope image of the fabricated devices.


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Table 1. Photosensors with various lengths and spatial densities of GaN NWs.



Figure 3. Cross-sectional (left) and plan-view (right) FE-SEM images for the (a) NW1, (b) NW2, (c) NW3, and (d) NW4 samples, where the insets are the three-dimensional views. (Scale bars: 500 nm)


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Table 2. Summary on the length, diameter, and density of the GaN NWs.



Figure 4. Cross-sectional TEM (left), HRTEM (middle) images, and diffraction patterns of the GaN NW from the (a) NW1, (b) NW2, and (c) NW3 samples, measured at three different vertical positions. (Scale bars of HRTEM images: 10 nm) ACS Paragon Plus Environment

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Figure 5. (a) PL and (b) absorption spectra for the GaN-NW samples measured at 300K.



Figure 6. Plan-view FE-SEM images of the (a) Si-Ref, (b) Si-PS1, (c) Si-PS2, (d) Si-PS3, (e) Si-PS4, and (f) Si-PS5. The insets are magnified images of the GaN NWs horizontally embedded in the graphene sandwich channel. (Scale bars: 10 µm)


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Figure 7. I–V characteristic curves for the (a) Si-Ref, (b) Si-PS1, (c) Si-PS2, (d) Si-PS3 (e) Si-PS4, and (f) Si-PS5. Summary on the photoresponsivity and resistance depending on the (g) length and (h) density of the GaN NW measured at the light intensity of 100 mW/cm2 and the voltage of 1 V. ACS Paragon Plus Environment


Figure 8. (a) Schematic diagram of a photosensor and (b) transfer process of carriers from GaN NWs to graphene channels.


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Figure 9. (a) I–V characteristic curves of PET-PS and (b) summary on the photoresponsivity and resistance of Si-PS5 and PET-PS.



Table 3. Summary on the photocurrent, photoresponsivity, external quantum efficiency, detectivity, and resistance of the photosensors.


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Figure 10. Photoresponsivity with respect to strain as controlled by the bending radius of the flexible photosensor (PET-PS) at a voltage of 1 V.



TOC graphic


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Highly Efficient and Flexible Photosensors with GaN Nanowires

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