Communication Light-Emitting Diodes
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Monolithic Flexible Vertical GaN Light-Emitting Diodes for a Transparent Wireless Brain Optical Stimulator Han Eol Lee, JeHyuk Choi, Seung Hyun Lee, Minju Jeong, Jung Ho Shin, Daniel J. Joe, DoHyun Kim, Chang Wan Kim, Jung Hwan Park, Jae Hee Lee, Daesoo Kim, Chan-Soo Shin, and Keon Jae Lee* clothes, automobiles, and buildings.[2–4] For example, flexible active-matrix organic light-emitting diodes (AMOLEDs) have been widely commercialized in mobile optoelectronics. However, organic lightemitting diodes have drawbacks including low power efficiency,[5] slow response time,[6] and short device lifespan in a humid condition.[7,8] Inorganic-based micro light-emitting diodes (µLEDs) have been considered as a key technology for the future full-color µLED TVs and flexible displays, due to their excellent optical properties (e.g., hue, brightness, saturation, and contrast values), low power consumption, short latency time, long lifespan, and high stability in harsh environmental conditions, as described in Table S1 (Supporting Information). Several research teams including ours have explored various III–V light-emitting diode (LED) materials, such as GaAs1−xPx, AlGaInP, and GaN for flexible and biomedical applications.[9–13] Kim et al. reported an injectable biostimulation tool by using flexible LEDs.[14] However, these flexible LEDs do not satisfy the industrial standards of power efficiency and thermal stability along with complex electrical interconnection on plastics. The power efficiency of flexible µLEDs is strongly dependent on the device structure and the length of current path between p- and n-electrodes. In general, there are two types of µLED structures; lateral-structured µLEDs and vertical-structured µLEDs (f-VLEDs),[11,15,16] as tabulated in Table S2 (Supporting Information). Our group reported the AlGaInP f-VLED with short current path (sub-10 µm), exhibiting long lifespan, low leakage current, and high light extraction efficiency (over 300% higher)[17] compared to lateral-structured LEDs.[14] In particular, the thermal stability issue of flexible µLEDs could be resolved by the cooling effect and high power efficiency of f-VLEDs.[18,19] Despite previous researches on flexible AlGaInP VLEDs, GaN f-VLEDs on plastic substrates have not been demonstrated yet due to the difficulty of perpendicular GaN interconnection (e.g., stress issue of inorganic-based laser lift-off (LLO),[20,21] the step coverage of VLED,[22] and the absence of conductive adhesive[23]). In addition, a flexible energy source for f-LEDs should be required for practical applications of fully flexible optoelectronic systems.
Flexible inorganic-based micro light-emitting diodes (µLEDs) are emerging as a significant technology for flexible displays, which is an important area for bilateral visual communication in the upcoming Internet of Things era. Conventional flexible lateral µLEDs have been investigated by several researchers, but still have significant issues of power consumption, thermal stability, lifetime, and light-extraction efficiency on plastics. Here, highperformance flexible vertical GaN light-emitting diodes (LEDs) are demonstrated by silver nanowire networks and monolithic fabrication. Transparent, ultrathin GaN LED arrays adhere to a human fingernail and stably glow without any mechanical deformation. Experimental studies provide outstanding characteristics of the flexible vertical μLEDs (f-VLEDs) with high optical power (30 mW mm−2), long lifetime (≈12 years), and good thermal/mechanical stability (100 000 bending/unbending cycles). The wireless light-emitting system on the human skin is successfully realized by transferring the electrical power f-VLED. Finally, the high-density GaN f-VLED arrays are inserted onto a living mouse cortex and operated without significant histological damage of brain.
With the emergence of the Internet of Things (IoT) era, visual IoT platforms have attracted significant interest, which can offer sensing, collecting, and processing of optical information in hyperconnected society.[1] Flexible displays are a potential candidate for bilateral visual communication, as they can be easily affixed anywhere, such as on the surfaces of human skin,
H. E. Lee, Dr. S. H. Lee, J. H. Shin, Dr. D. J. Joe, Dr. J. H. Park, J. H. Lee, Prof. K. J. Lee Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea E-mail:
[email protected] Dr. J. Choi, D. Kim, C. W. Kim, Dr. C.-S. Shin Photonic Device Lab Device Technology Development Division Korea Advanced Nano-Fab Center (KANC) 109 Gwanggyo-ro, Yeongtong-gu, Suwon Gyeonggi-do 16229, Republic of Korea Dr. M. Jeong, Prof. D. Kim Department of Biological Sciences Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
DOI: 10.1002/adma.201800649
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Figure 1. a) Schematic illustration of the fabrication procedure and biomedical application of the monolithic transparent GaN f-VLED. b) Irradiance of the various-sized LEDs as a function of increasing current density. The inset image shows a blue LED during the optical output measurement. c) A photograph of a GaN blue f-VLED array attached to a human fingernail. The inset shows a transparent f-VLED array, when the LEDs are in an offstate. d) A picture of high-density monolithic f-VLED arrays. The upper inset is a magnified optical microscopy image of the 30 × 30 PM array with 50 × 50 µm2-sized f-VLEDs. The lower inset exhibits a photograph of flexible GaN µLEDs in a bent state (bending radius: 5 mm); scale bar = 1 cm.
Herein, we report a high-performance flexible 30 × 30 GaN VLED array via a simple monolithic fabrication process. The GaN-based LED chips were separated from a sapphire wafer by the LLO process. The freestanding µLEDs (chip size of 50 × 50 µm2) were isolated by biocompatible polymer layers and vertically interconnected with a silver nanowire (AgNW)based conductor by resolving the high step coverage of f-VLEDs. Ultrathin, transparent, and flexible GaN VLEDs were conformally attached to a human fingernail with high optical power of 30 mW mm−2. The lifetime of the f-VLEDs was experimentally investigated by the high accelerated stress test (HAST) and theoretically estimated by a finite-element method (FEM) simulation. In addition, our f-VLEDs showed outstanding mechanical durability during periodic bending/unbending cycles. The wireless power supply system on the human skin was successfully demonstrated by transferring the electrical energy to the f-VLEDs. Finally, monolithic blue f-VLEDs successfully emitted a brilliant blue light on a live mouse brain without severe histological damage. Figure 1a schematically illustrates the hybrid µLED fabrication process (the LLO and monolithic fabrication process) and biomedical application of flexible transparent GaN VLEDs. The following is the detailed procedure: i) High-purity GaN LED layers were grown by metalorganic chemical vapor deposition on a sapphire substrate. A thick Cu foil was then uniformly electroplated on the GaN thin film with p-Ohmic contact (Ni/ Au), as a metallic heat spreader during the inorganic-based LLO (ILLO) process. A KrF laser with a wavelength of 248 nm
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was irradiated to the GaN layers through the transparent sapphire substrate, causing exfoliation of the LED layers with the Cu heat spreader by volume expansion of nitrogen gas (2GaN → 2Ga + N2) between the GaN layer and the sapphire substrate. After ILLO process, µLED chips with n-Ohmic contacts (Cr/Au) were made on the Cu foil and isolated by UVcured epoxy resin (SU8, thickness = 5 µm). ii) AgNW solution was spin coated and patterned on an SU8 layer for a transparent n-electrode. The SU8 contact holes were covered and filled with the AgNW network. The polymer-based isolation layer had two types of contact holes, which were n-electrode contact holes (marked by reddish A) and vertical connection passages (marked by reddish B) of the n-electrode from the bottom to top surface. A biocompatible passivation layer was formed on the top surface to protect the transparent electrode and µLED chips from mechanical damage. The passivated GaN LED device was flipped over, and then the thick Cu foil was selectively eliminated by a wet-etching process. iii) A transparent AgNW electrode was formed on the top surface of the flipped LED device for the bottom (n-) and top electrode pads (p-electrode pads). To fulfill the biomedical experiment, the passivated monolithic GaN f-VLED array was smoothly inserted under a living mouse skull through a narrow cranial slit. The monolithic GaN f-VLED array could wrap the curved and corrugated mouse brain, enabling irradiation of the blue light to the cerebral cortex (see Figure S1 in the Supporting Information for the detailed fabrication procedure of the monolithic GaN LED).
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Figure 1b presents the optical power density as a function of input current density for the different-sized monolithic GaN VLEDs. As seen in these curves, the irradiance of the monolithic GaN LEDs increased as the device size decreased. The smallest LED (chip size of 50 × 50 µm2) had a maximum irradiance of 30 mW mm−2 (2.05 × 107 cd m−2) at 250 mA mm−2, which was ≈2 × 103 times higher than the output power of a conventional AMOLED and a smart phone display (500–10 000 cd m−2).[24] The junction temperature of the small optical device was remarkably lower than that of the large device at the same current density. The phenomena of low junction temperature at high injection current came from the low internal electrical resistance of small µLED, compared to the large device.[25] Therefore, the small LED chip could maintain high optical power and withstand high current densities without breakdown, because of its low thermal degradations.[17] Figure 1c presents the transparent blue f-VLED arrays conformally attached onto the human fingernail, showing high luminescence of the f-VLEDS without exfoliation, wrinkles, and cracks. As shown in the inset of Figure 1c, the turned-off LEDs were indistinguishable from the fingernail, owing to their high transmittance and ultrathin total thickness of 15 µm (see Figure S2 in the Supporting Information). Figure 1d displays the high-density monolithic f-VLED arrays that emit a highpower blue glow. The passive-matrix (PM) f-VLED arrays were composed of 900 GaN µLEDs and vertically interconnected by top and bottom transparent AgNW conductors. The upper inset shows a magnified optical microscopy image of the 30 × 30 GaN LED array fabricated by the monolithic LED process. The 50 × 50 µm2-sized LED chips were used to PM f-VLED array for the demonstration of a high-resolution display.[26] This highdensity GaN f-VLED array was successfully operated on a round metal stick with bending radius of 5 mm (the lower inset of Figure 1d). A critical issue of vertically interconnecting the f-VLED electrodes is high step coverage (5–10 µm step height) caused by the vertical LED structure. The conventional physical thin-film deposition methods for electrode fabrication (e.g., sputtering, thermal evaporation, electron beam evaporation, and pulsed laser deposition) hardly interconnect the VLED electrodes with high step height, due to poor sidewall deposition (see Figure S3 in the Supporting Information). Therefore, the AgNW networks were employed for simple electrical/physical interconnection between the two vertical electrodes, due to their long (length of ≈50 µm) shape and conformal contact characteristics.[27,28] Figure 2a presents the schematic illustrations of the monolithic VLED structure, composed of the LED chip, polymer matrix, and top/bottom electrodes. The 3D and vertical interconnection of the AgNW electrodes was analyzed by scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) elemental mapping. Figure 2b and its inset present top-view and enlarged tilted-view SEM images of the AgNWs, respectively. The spincoated AgNWs could physically connect the top and bottom electrodes of the f-VLEDs. Figure 2c is a cross-sectional SEM image and EDS mapping results of the GaN f-VLED. The EDS mapping image of Ag element indicated the sites of the distributed AgNWs, which showed that monolithic f-VLED electrodes were successfully interconnected by the conformally coated AgNWs. The electrical and optical properties of the flexible transparent
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conducting electrodes (FTCEs) were optimized by controlling the number of NW coating layers, as shown in Figure 2d. As the number of the NW coatings increased, the conductance of the NW network was proportionally improved, while the NW transmittance was decreased. To achieve high conductance and transmittance of the AgNWs simultaneously, four times-coated AgNWs (conductance of 0.11 S, transmittance of ≈70%) were employed to fabricate the transparent electrodes of a monolithic f-VLED. The inset of Figure 2d displays a top-view micro scopy image of the f-VLED after applying the four times-coated AgNWs. Furthermore, the electrical and optical properties of the FTCEs were analyzed as a function of the f-VLED step height (Figure 2e). It should be noted that the conductance of the FTCE was proportional to the polymer step height, whereas the transmittance of the four-times-coated AgNWs was inversely proportional to the polymer thickness. These results could be explained by easy accumulation of AgNWs on high step coverage, compared to that on low step coverage. The thermal stability of an µLED is closely related with the device structure (vertical and lateral), which determines the lifetime of a flexible and free-standing thin-film LED without a proper heat sink.[29–31] The thermal distribution of vertical and lateral GaN f-LEDs was theoretically calculated by an FEM simulation, and their life expectancy was experimentally investigated by a high accelerated temperature/humidity stress test (HAST). Figure 3a,b depicts the temperature distribution of flexible lateral- and vertical-structured LEDs, respectively. The temperature of the f-LED was solved by the following heat transfer equation from the FEM heat flux model (1) Q = ρC
∂T − ∇ ( λ∇T ) ∂t
(1)
where Q is the total heat caused by the Joule heating, T is the device temperature, ρ is the density, C is the specific heat, and λ is the thermal conductivity of each material (n-GaN, multiquantum well (MQW), p-GaN, SU8, parylene-C, and AgNW). When the thin-film LED without a mother wafer emitted light of 20 mW mm−2, the temperature of the lateral and vertical f-LEDs rose to 428 K (Figure 3a) and 326 K (Figure 3b), respectively. These phenomena could be interpreted by isothermal mapping images, as shown in the inset of Figure 3a,b. The heat generated from the lateral f-LED was continuously concentrated and accumulated at the interface between the MQW and n-GaN layer, while the vertical f-LED lowered the device temperature by using the bottom electrode as a heat sink.[15,32,33] Figure 3c presents a graph of the simulated temperature at different light irradiance of the f-LEDs (1, 5, 10, 15, and 20 mW mm−2). Despite the intense light irradiation of 20 mW mm−2, the vertical f-LED showed high thermal stability. In contrast, the temperature of the lateral LED drastically elevated as the optical power of the f-LED rose, which exhibited junction temperature that is 102° Kelvin higher than that of the vertical f-LED at light irradiance of 20 mW mm−2. To estimate the life expectancy of monolithic f-VLEDs experimentally, the HAST evaluation of the f-VLEDs was utilized with the Arrhenius approach,[34–36] since the accurate lifetime measurement of f-µLEDs is realistically impossible.[37,38] The fully passivated optical devices were operated in a humidity chamber (85% humidity) with constant temperature (three different temperatures of 85, 95,
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Figure 2. a) Schematic illustrations of a monolithic LED with AgNW network as a transparent electrode. Transparent AgNW electrodes overcome the step height of the polymer matrix. b) SEM image of the monolithic LED electrode after AgNW-coating process. The inset shows a tilted SEM image of the electrode contact hole, covering the step of the polymer matrix by the AgNW network. c) Cross-sectional SEM image of monolithic GaN VLED with AgNW network and EDS elemental mapping results of Ga, Ag, C, and Pt. d) Electrical and optical properties of the FTCE as a function of the number of AgNW coatings. The inset is a magnified microscopy image of the FTCE based on the AgNW network. e) FTCE characteristics of f-VLEDs by the step height of the polymer matrix at four AgNW coatings.
and 105 °C) to accelerate the corrosion/deformation rate of the f-VLEDs by heat and moisture (Figure S8, Supporting Information). Figure 3d shows the HAST results of monolithic f-VLEDs based on the Arrhenius equation and acceleration factor (AF) at each temperature. The linear Arrhenius graph was defined by the following Equations (2) and (3) to predict the breakdown time of monolithic LEDs at room temperature (RT)[39] ln
Ea 1 = lnA − tf kT
t AF = tf 2 f1
(2) (3)
where tf is the device failure time, A is the preexponential factor, Ea is the activation energy, k is the Boltzmann’s
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constant, and T is the temperature condition of the HAST. The value tf was measured until LEDs-off, as presented in the inset of Figure 3d. In the reliability measurement chamber of 85% humidity, monolithic f-VLEDs emitted light until 130, 51, and 18 h at 85, 95, and 105 °C, respectively. According to Equations (2) and (3), Ea is derived as 1.15 eV by calculating the slope of the 1000/T versus ln(1/tf) graph. The tf of monolithic optical devices at RT (27 °C) could be predicted by extending the linear Arrhenius curve, as shown in Figure 3d. From the HAST and mathematical calculations, the lifespan of our flexible monolithic LEDs was estimated as ≈11.9 years (≈102 000 h). On the basis of these results, it is noteworthy that the monolithic f-VLEDs with long lifetime are suitable for various optoelectronic applications, including wearable phototherapeutic devices, smart devices, and full-color µLED displays.
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Figure 3. Temperature distribution by an FEM calculation when a) f-LLED and b) f-VLED emit light with an irradiance of 20 mW mm−2. The inset images are isothermal images when the generated heat is conducted from the junction, and the arrows indicate the direction of heat flux. c) The calculated LED temperature as a function of the irradiance. The graphs exhibit that the f-LLED has higher temperature than the f-VLED at various irradiance condition. d) Arrhenius curve of monolithic f-VLEDs to estimate the long-term stability of the devices during high accelerated stress test. The plot is linearly fitted to ln(1/tf ) as a function of 1000/T. The inset shows the long-term irradiance measurements of monolithic LEDs to evaluate device lifetime. The f-VLEDs in 85% humidity operated until 130 h at 85 °C, 51 h at 95 °C, and 18 h at 105 °C.
Figure 4a shows the electroluminescence (EL) properties of our monolithic GaN VLEDs under flat and bent states. The f-VLEDs showed a sharp light emission peak (wavelength of 446 nm), regardless of bending and flat condition. The CIE 1931 color coordinates of the f-VLED blue light were (0.1508, 0.0414), which exhibited a high hue and chroma of the light. Figure 4b plots the cross-sectional light angular distribution of monolithic GaN f-VLEDs (sky blue region) and conventional LEDs (pink region).[40] The light distribution curves indicated that the monolithic VLEDs could emit light along the vertical direction with a small light divergence angle of 60°, which was resulted from total reflection at the interface between the polymer matrix and the passivation layer. In the light angular distribution curve, the monolithic VLED showed an isotropic radiation, due to the light dispersion in the AgNW electrode. The total reflection in the monolithic f-VLED could be expressed by the rearranged Snell’s law[41,42] n θ c = θ i = arcsin 2 n1
(4)
where θc is the critical angle for total reflection, θi is the angle of incident light, n1 is the refractive index of the passivation layer (n1 = 1.67), and n2 is the refractive index of the polymer matrix (n2 = 1.565). According to Equation (4), light with a larger θi than θc = 69.6° could be concentrated and radiated by high rectilinear propagation through the microscale polymer hole
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(see Figure S12 in the Supporting Information). This narrow light emission characteristic of the monolithic f-VLEDs can be effective for biomedical devices, due to its capability of localized optical biostimulation.[43–45] A mechanical stability test of the monolithic f-VLEDs was performed by monitoring the change of forward voltage (ΔVf) and irradiance (ΔE) at different bending curvature, as shown in Figure 4c. The GaN f-VLEDs presented a high stability of ΔVf and ΔE at a small bending radius. Despite the severe bending of f-VLEDs (bending curvature of 2.5 mm) on a curved poly(dimethylsiloxane) (PDMS) mold, the irradiance only decreased by 6.1% (1.82 mW mm−2), and the forward voltage increased by 6.3% (0.14 V). Figure 4d displays the excellent mechanical durability of the f-VLEDs under the periodical bending fatigue test at a bending radius of 2.5 mm. The transparent GaN f-VLEDs showed negligible changes of the forward voltage and optical power density during 100 000 bending and unbending cycles. The outstanding mechanical reliability could be attributed to the unique structure of the monolithic VLED, which could easily modify the mechanical neutral plane, regardless of the device thickness.[24,46–48] The flexible wireless power supply system for the ultrathin and lightweight f-VLEDs is an important tool for the practical application, such as mobile, smart, and biomedical modules.[12,49,50] To wirelessly transfer electrical energy to f-VLEDs, a total power supply system was designed, using the integration of electronic components including flexible antennas, a peak rectifier, and
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Figure 4. a) Electroluminescence spectra of a monolithic GaN LED at flat and bent states. The inset displays the CIE color coordinates of blue light by GaN f-LED. b) Light angular distribution of monolithic blue f-VLED. The sky blue region and the pink region are the light emission angle of monolithic GaN f-VLED and conventional LED, respectively. c) Electrical and optical characteristics of f-VLEDs with various bending radii. The inset image exhibits the blue f-VLED on a half-cylindrical PDMS mold. d) Mechanical durability test results of the forward voltage and optical output density during 100 000 bending/unbending cycles (bending radius of 2.5 mm). The inset images show the GaN blue LED in flat and bending states. e) Conceptual scheme of wireless GaN f-VLED system by resonant inductive coupling. f) The Q factor of the wireless receiver system. The inset shows a plot of time versus the AC input power and the transferred power. g) A photograph of a wireless GaN monolithic f-VLED on the human skin. The f-VLED is stably operated by near-field power transfer of the wireless system.
a voltage regulator (see more details in Figure S13 in the Supporting Information). Power transfer was controlled by a resonant inductive coupling method at the resonant frequency of Adv. Mater. 2018, 1800649
13.56 MHz (industrial, scientific, and medical radio bands).[51] Figure 4e presents a conceptual scheme of a wireless GaN f-VLED system. Alternative current (AC) power (13.56 MHz)
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Figure 5. a) A photograph of white f-VLED array, composed of a high-density GaN f-VLED array and yellow YAG phosphor. The left inset shows the CIE coordinates of the white f-VLED. The right inset is a distribution histogram with a forward voltage of 64 of LED devices. b) The measured voltage of f-VLED by current pulses (frequency of 10 Hz and pulse width of 10 ms). The inset image shows the pulsed blue light of the f-VLEDs in 1× PBS solution. Insertion surgery images of f-VLEDs under a living mouse skull: c) the small cranial slit was made on the mouse skull and d) the GaN f-VLED device smoothly slid into the subcranial gap. e) A picture of a head-fixed, living mouse with the high-density GaN f-VLED array. The 30 × 30 LED array displays the blue letters of “KAIST-KANC”, as shown in its inset; scale bar = 7 mm. f) Brain section images, after device insertion and light irradiation onto the surface of mouse cortex. Sections were sampled at anterior–posterior from the bregma.
was generated from a function generator (Agilent 33250A) and simultaneously transferred via the flexible transmitter antenna. After the flexible receiver antenna collected the wireless power in the near-field range, the peak rectifier converted the AC to direct current (DC) voltage. As presented in Figure 4f, the quality factor (Q factor) of the receiver system was specifically designed to a Q factor ≈10.88. The optimized Q factor (10 < Q factor
