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Fully Flexible GaN Light-Emitting Diodes through Nanovoid-Mediated Transfer Jun Hee Choi,* Eun Hyoung Cho, Yun Sung Lee, Mun-Bo Shim, Ho Young Ahn, Chan-Wook Baik, Eun Hong Lee, Kihong Kim, Tae-Ho Kim, Sangwon Kim, Kyung-Sang Cho, Jongseung Yoon, Miyoung Kim,* and Sungwoo Hwang
Nonetheless, inorganic GaN and InGaN materials are being used commercially in green, blue, and white light-emitting diodes (LEDs) and have long lifetimes with high brightnesses and efficiencies.[5–7] Pioneering studies on flexible/ stretchable GaAs- or GaN-based LEDs have tried to exploit the advantages of inorganic LEDs.[8–10] Because of high growth temperature of Group III nitrides (for example, GaN: ∼1050 °C) and the requirement of single-crystalline growth, the key technology for flexible/stretchable inorganic LEDs is the transfer of GaN or GaAs grown on single-crystalline substrates onto flexible or stretchable substrates. In most cases, a sacrificial layer is employed that is selectively wet-etched, leaving the overlying LED structure that had been tentatively fixed by an anchoring layer during the selective wet etching. The LED structure is then transferred to other flexible or stretchable substrates such as polyimide, PET, or polydimethylsiloxane (PDMS) by PDMS stamping,[11,12] and the final devices are fabricated using insulating and metalizing processes. New mechanical transfer methods were recently demonstrated by employing quasi- 2D layered structures as substrates, e.g., graphene and hexagonal boron nitride/sapphire.[13–15] Weak binding or van der Waals forces between adjacent layers in multilayered graphene or boron nitride allowed easy mechanical release of the GaN LEDs and transfer to the target substrate. Reasonable GaN defect density (∼1010 cm−2) was reported despite the very large lattice mismatch values with GaN (29.7% to graphene, 27.1% to boron nitride), which was also attributable to weak bonding with the substrates.[15,16] These are promising results because the InGaN/GaN materials are well known to exhibit high radiative efficiencies even in the presence of high defect densities.[17,18] This hetero-epitaxial GaN growth shows great potential as an alternative transfer technique for flexible GaN/stretchable LEDs. Motivated by such recent progress, substantial efforts have been made to achieve more flexible, surface-emitting blue inorganic devices. However, all of the previous attempts were based on single-crystalline wafers or graphite flakes and limited in scalability unless multiple transfers were utilized. (See the Supporting Information, Table S1 for comparison of
Recently, achieving flexible and highly efficient light-emitting elements is the most noticeable demand for lighting or displays. Here, fully flexible gallium nitride (GaN) light-emitting diodes (LEDs) are demonstrated based on a unique transfer method. The LED structure consisting of GaN pyramid arrays are first fabricated on an amorphous glass-based template with a low-temperature gallium nitride/titanium (LT-GaN/Ti) hetero-interface, then released and embedded into a flexible or stretchable substrate using a specialized interface control. Nanovoids created during thermal annealing render the hetero-interface weaker than the other interfaces. This interface is further weakened by a post-mechanical treatment for gentle release of the GaN pyramid arrays from the interface during a transfer process. The LEDs typically have a total thickness of }70 lm and exhibit stable surface-emitting electroluminescence even at a bending radius of }2 mm with exceptionally high luminance values of 595 and 175 cd/m2 at peak wavelengths of 514 and 483 nm, respectively. The results suggest a route to high brightness, large, flexible/stretchable blue or green lighting or displays.
1. Introduction Advances in organic light-emitting diode (OLED) technology have enabled flexible displays and lighting with various form factors.[1–3] Recently, polymer-based OLEDs realized on 1.4-μmthick ultrathin polyethylene terephthalate (PET) substrates show promise for high flexibility and even some stretchability.[4]
Dr. J. H. Choi, Dr. E. H. Cho, Mr. Y. S. Lee, Dr. M.-B. Shim, Dr. H. Y. Ahn, Dr. C.-W. Baik, Dr. E. H. Lee, Dr. K. Kim, Dr. T.-H. Kim, Dr. S. Kim, Dr. K.-S. Cho, Dr. S. Hwang Samsung Advanced Institute of Technology Samsung Electronics Yongin, Kyunggi-do 446–712, South Korea E-mail:
[email protected] Prof. J. Yoon Department of Chemical Engineering and Materials Science University of Southern California Los Angeles, CA 90089, USA Prof. M. Kim Department of Materials Science and Engineering Seoul National University Seoul, 151–744, South Korea E-mail:
[email protected]
DOI: 10.1002/adom.201300435
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Figure 1. The device structure and process of fully flexible GaN LEDs. a) (left panel) Perspective view of the device formed on a flexible PET or stretchable PDMS substrate. (right panel) Cross-sectional view of the dotted line (a-a’) in (a). b) (left panel) Calculated axial strain distribution corresponding to right panel in (a), and (right panel) calculated principal strain distribution of the ITO upper electrode. c) Key fabrication steps: (left panel) nanovoid formation and (right panel) transfer of GaN pyramid stacks. LG, NV, UIP and LIP represent LT-GaN, nanovoid, the upper and lower insulating polymers, respectively.
various releasing methods for flexible GaN LEDs.) Recently, we reported the fabrication of GaN LEDs on glass that consisted of nearly single-crystalline pyramid stack [n-GaN (core)/ InGaN-GaN multiple-quantum well (MQW)/p-GaN (shell)] arrays using a hole-patterned SiO2/LT-GaN/Ti/glass substrate.[19,20] Here, we demonstrate a novel transfer method to realize fully flexible, surface-emitting GaN LEDs. This was enabled by accurately reducing the adhesion strength of the LT-GaN/Ti hetero-interface and embedding such stack arrays into ultrathin, flexible substrates. Furthermore, we show that the proposed LED structure is favourable not only for its high flexibility, but also for a high stretchability that is potentially applicable to large sizes.
2. Advantageous Structure for High Flexibility and Stretchability The flexible GaN LEDs consisted of light-emitting, isolated hard segments (GaN pyramid stack) embedded in a soft matrix (upper and lower insulating polymers UIP and LIP) on ultrathin, flexible substrates (Figure 1a,b). The fabrication flow of the flexible LEDs is schematically shown in Figure 1c (detailed in Supporting Information, Figure S1). We applied the following approach to obtain an extreme flexibility. First, the strain on the light-emitting pyramid stack was greatly reduced with the help of an embedded geometry (left panel
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of Figure 1b). The external strain was concentrated in the soft matrix because of large differences in the Young’s modulus values. This might be advantageous compared to the structure of prior arts that typically has a continuous light-emitting region. For the UIP and LIP layers (Figure 1a), we used a urethane acrylate-based elastomer, which was stable under the external strain. The UIP in particular could be stretched up to a strain of more than 50%, as determined from its stress-strain curves (see Supporting Information, Figure S2). This stretchability arises from the ether linkage in the chemical structure of the acrylate oligomer (see Supporting Information, Table S2 and Figure S2b). Second, we employed a hybrid structure composed of a continuous silver (Ag) layer and an isolated square or continuous indium tin oxide (ITO) layer as the upper electrode in Figure 1a (see Supporting Information, Figure S3).[21] That fulfilled the requirement of high electrical conductivity and transparency under varying strain conditions. The ITO layer functioned as a transparent, current-spreading electrode for the light-emitting array, i.e., a transparent and electrically ohmic contact to p-GaN while the Ag layer served as a current conducting electrode. The stability of an evaporated Ag film under various types of stress (stretching or bending) can be found in the literature.[22,23] We also confirmed that the Ag film was fairly stable up to 30% tensile strain (see Supporting Information, Figure S4a) and was still conducting under 80% tensile strain. Under severe strain, the Ag film had inter-granular cracks, but the cracks were still
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3. Fabrication of the Flexible GaN LED Structure The described architecture was achieved by developing a simple process that easily released GaN pyramid arrays from a glass template and embedded them into a flexible polymer formed on a flexible substrate. The releasing and embedding processes were performed when the device was nearly completed (right panel of Figure 1c). After embedding, only one additional simple step (i. e., metallization of the lower electrode; Figure S1 (h)), was needed to complete the full device structure in Figure 1a. This enabled transfer of the LED structure to ultrathin flexible substrates, of which a variety of form factors were available. The LED structure was based on a GaN pyramid stack array [n-GaN (core)/5-period InGaN–GaN MQW (inner shell)/p-GaN (outer shell)] that was selectively grown on a template with a hole-patterned SiO2 /LT-GaN/Ti/glass (left panel of Figure 1c). The core-shell structure eliminated the surface recombination issue, hence guaranteed a high radiative recombination efficiency even in a micron-scale. In the typical mesa structure, the surface recombination issue typically becomes serious as the emitter size decreases.[25] In order to release such an LED structure gently, the LT-GaN/Ti interface was intentionally weakened relative to the other interfaces in the assembly (Figure S1f). Here, we controlled the LT-GaN/Ti interfacial strength using the following approaches; (1) by controlling nanovoids at the LT-GaN/Ti interface, (2) by controlling intrinsic or thermal film stresses, and (3) by using a post- mechanical process. First, nanovoids that were created during the growth of the HT-GaN pyramids at elevated temperatures (left panel of Figure 1c) partly separated the LT-GaN from the Ti layer. Nanovoids were found neither in the LT-GaN/glass without
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connected and sustained percolating electric current paths (see Supporting Information, Figure S4b). To minimize cracking of the brittle ITO, we can design the structure to locate the neutral mechanical plane (NMP), which is the zero strain contour, near the flat region of the ITO electrode. This could be done either by decreasing the PET thickness to 15 μm or by placing the dummy PET substrate having moderate thickness on the other side (see Supporting Information, Figure S5). The material data and device structures for local strain simulations are shown in Table S3 and Figure S5. The ITO covering the side walls of the pyramids was rarely strained even though it was not on the NMP (right panel of Figure 1c). Poly(3,4-ethylenedi oxythiophene):poly(styrenesulfonate) (PEDOT:PSS)[24] or multilayered graphene[12] might be possible alternatives to ITO as the isolated transparent electrode. An ultrathin PET substrate was valuable for our process that used a transfer scheme of a “nearly completed device”, as will be discussed later. In addition to holding promise for a thinner device, the thin geometry also enforced a smaller local strain under the same amount of bending and a smaller stiffness than with a thicker material (See see Supporting Information, Figure S6). As a lower electrode having an isolated form as shown in Figure 1a (see Supporting Information, Figure S3), we used a single evaporated Ag layer, which had a high reflectance and stability to external strain.
the Ti layer (Figure 2a), nor in the as-deposited state, where the LT-GaN was directly grown on an evaporated Ti layer at 560 °C without any thermal annealing (left panel of Figure 2b). In both cases, the LT-GaN interfaces were flat and continuous. This indicates that chemical interaction or alloying of LT-GaN with Ti was responsible for the formation of the nanovoids (central and right panels of Figure 2b), similar to the case of LT-GaN on a Si wafer under thermal annealing.[26,27] Indeed, chemical interaction at the interface was revealed by energydispersive X-ray spectroscopic (EDS) analyses of central and right panels of Figure 2b, showing significant nitridation of the Ti layer as shown in Figure 2c (see Supporting Information, Figure S7a). The Ti nitridation was further studied by normalmode X-ray diffraction (XRD) considering the 2θ values of Ti (002) or TiN (111) peaks in Figure 2d (see Supporting Information, Figure S8).[28] For as-deposited LT-GaN at 560 °C, the Ti was slightly nitrided in the form of a Ti-N solid solution due to NH3 source itself existing in the LT-GaN growth, showing with a slight decrease in the 2θ value of Ti (002). As thermal annealing was performed at higher temperatures, for example, at 850 and 950 °C, the Ti near the interface was first converted into TiN to form the TiN/Ti double-layer. This is indicated by the coexistence of Ti (002) and TiN (111) peaks (Figure 2d). As the annealing temperature was increased to 1040 °C, Ti is mostly converted into TiN. The LT-GaN dissociation and successive Ti nitridation is explained by the greater thermodynamic stability of TiN compared with GaN.[29] The remaining Ga atoms diffused into the SiO2 layer as nitridation proceeded. Notably, a higher Ga concentration was detected in the form of lines in the amorphous SiO2 layer (left panel of Figure 2b,c, and Figure S7b). These lines are derived from the sharp tips of the LT-GaN columns that entered the SiO2 layer, revealing that Ga diffusion occurred through the column boundaries in the LT-GaN. The process of nanovoid formation is schematically shown in Figure 2e. The nanovoid size evidently increases with increasing annealing temperature (Figure S7c), supporting the described process. Additional experimental finding that increasing Ti thickness (tTi) drastically weakened the LT-GaN/ Ti interface also supports this explanation. This was because of enlargement of the nanovoids with increasing tTi through more interaction of the LT-GaN with Ti. On the other hand, comparative study with and without the Ti layer using grazing incidence x-ray diffraction (GIXRD) revealed that the grain or crystallite growth of LT-GaN was enhanced in the presence of the Ti layer (see Supporting Information, Figure S9), meaning that material transport or rearrangement in the LT-GaN layer itself such as Ostwald ripening, partly contributed to the nanovoid formation.[30] The repeated appearance of nanovoids even for singlecrystalline GaN LEDs on sapphire wafers (see Supporting Information, Figure S10) proves that the proposed process can have versatile applications. Second, stress on the interface weakens the bonding, which was established by examining the LT-GaN/Ti interface stability after GaN pyramid formation on templates having different SiO2 thicknesses (tSiO2). It should be noted that, after the GaN pyramid stack formation, the bottom layer structure consisted of a layer stack with p-GaN/InxGa1-xN /SiO2/LT-GaN/Ti/glass (see Supporting Information, Figure S1 (a) and Figure S11). The overall film stress imposed by the layer stack affected the
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Figure 2. Bright-field TEM images of a) SiO2/LT-GaN/glass after thermal annealing at 1040 °C and b) SiO2/LT-GaN/Ti/glass; (left panel) the as-deposited state, (central panel), the thermal annealed state at 950 °C, and (right panel) magnified image of a dotted rectangle in (b). c) EDS composition analyses at different positions roughly marked in central and right panels of (b). d) Normal mode XRD analyses exhibiting Ti niridation with increasing annealing temperature. e) A schematic diagram illustrating nanovoid formation at the LT-GaN/Ti hetero-interface. Scale bars are 200 nm (a), and left and central panels of (b) and 50 nm (right panel of (b)).
adhesion strength of the LT-GaN/Ti interface. The LT-GaN/ Ti interface in the layer stack was also drastically weakened with decreasing tSiO2 because of higher compressive stresses, as determined by extrapolation of the stress curve to the growth temperatures of n-GaN, InGaN, and p-GaN (see Supporting Information, Figure S12). The adhesion strength of the LT-GaN/Ti interface could be controlled by varying the tSiO2 or tTi, but precise control of the interfacial strength was difficult. For example, the interface was highly stable for tTi = 50 nm and tSiO2 = 300 nm, while full peel-off occurred for the template with tTi = 200 nm and tSiO2 = 200 nm. The peel-off of the p-GaN/ InxGa1-xN/SiO2/LT-GaN layer stack at the LT-GaN/Ti interface occurs like a sheet, exhibiting a typical aspect of compressively stressed peel-off (see Supporting Information, Figure S13).[31,32] The typical tTi and tSiO2 values used for device fabrication were 120 and 300 nm, respectively. Third, we developed a post mechanical process that selectively weakened the LT-GaN/Ti hetero-interface. The process consisted of attaching, heating, and removing an adhesive tape. The complex three-dimensional geometry of the GaN stacks made it difficult for the adhesive tape to conform to the entire surface and hindered the application of a uniform mechanical force to the interface (see Supporting Information, Figure S1b and Figure S14a). Conformal contact of the adhesive was achieved using a silicon adhesive at 170 °C (Figure S1c and 4
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Supporting Figure S14b), which is above the glass transition temperature as confirmed by differential scanning calorimetry (DSC; Supporting Figure 14c). Then, uniform mechanical force was exerted to the interface during removal of the tape. The interface structures before and after the post-mechanical treatment (see Supporting Information, Figure S15a,b) clearly reveal a reduction in the contact area between the LT-GaN and the Ti. Note that the covalent bonding of a conventional epitaxially grown interface is so strong that the interface is hardly weakened by such a mechanical treatment. Comparison of areas of the single glass template with and without the postmechanical treatment clearly revealed its macroscopic effect (Figure S15c–e). In addition to the LT-GaN/Ti interface, many other interfaces are involved in the assembly of the glass template and the flexible substrates: Ti/glass, SiO2/LT-GaN, flexible polymer/pGaN or InGaN, ITO or Ag/flexible polymer, and PET/flexible polymer (Figure S1f). The post-mechanical treatment selectively weakened the LT-GaN/Ti interface when considering the relative interface strength and the process sequence (Figure S1b,c). The use of an adhesion promoter before applying the flexible polymer coating, selection of the appropriate polymer chemistry, and adjustment of the evaporated film conditions were optimized to ensure that these interfaces had a stronger adhesion force than that of the LT-GaN/Ti hetero-interface.
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indicating that the release occurred exactly at the LT-GaN/Ti interface. The released pyramid array was well embedded in UIP on the flexible substrate (Figure 3b–h). Cross-sectional scanning electron microscopy (SEM) images (Figure 3d) show that the full device structure, corresponding to Figure S1g, was well embedded. Furthermore, the embedded structure could have a total thickness of ∼30 μm (Figure 3e,f). The typical thicknesses of the UIP, LIP, and PET were 6–20, 5–8, and 50 μm, respectively (Figure 3g and Supporting Information (Figure S16 and Supporting Figure S17)). Therefore, the total device thickness could be controlled to be ∼70 μm. As discussed, such an ultrathin device was achievable by using a transfer process scheme for a “nearly completed device”. Based on these microscopic observations, we found that the lot-to-lot yield of the transfer process into flexible substrates is ∼50% with good uniformity except the edge of the glass template. Moreover, we achieved successful transfer results even using stretchable PDMS as a substrate (Figure 3h, Figure S16f, and Figure S17b). But, the substrate is often found to be torn because of its low mechanical toughness.
4. Full Device Operation Figure 4a show the stable, surface-emitting electroluminescence (EL) at multiple sites during bending. This figure confirmed that our fabrication method successfully released the GaN pyramid arrays from the glass substrates and embedded them into the PET substrates. The surface-emitting EL was observed with increasing forward bias voltage (Vf) (see Supporting Information, Figure S18a). The device only adopting the ITO upper electrode was even stable toward the certain amount Figure 3. The transfer results of GaN pyramid stack arrays. a,b) Optical microscope images of bending (Figure S18b). As described, the (back-side plane view) of the glass template and the flexible device after the transfer process, method is amenable to ultrathin devices that respectively, corresponding to Figure S1g. c–h) SEM images of the flexible device structure of could also have enhanced mechanical bend(b): c) back-side plane view; d) an FIB-cut, tilted cross-section of a dotted line marked in (c); ability. For example, the flexible EL devices and e,f) bird’s-eye views. g,h) An FIB-cut SEM image of the embedded GaN array onto 50-μmthick PET and 3-mm-thick PDMS substrates, corresponding to Figure S1h. Magnified images had stable performances for various form of (g) and (h) are shown in Figure S17. Scale bars are 20 (a–c), 5 (d), 10 (e,f), and 20 μm (g,h). factors; surface-emitting EL was observed at multiple sites while the devices were folded through a radius of curvature (R) of ∼2 mm With such optimized interface control, we achieved excellent or wrapped around an 8 mm-wide-wrench (Figure 4b and Suprelease of the GaN arrays from the glass template (Figure 3a) porting Information (Figure S19)). It emits surface-type, clear and their embedding into the flexible polymers formed on blue light in stable manner under various bending conditions the flexible substrate (Figure 3b–f) during detachment step (Figure 4c). (Figure S1f,g). Transfer results are also detailed in Supporting The maximum luminance values of the flexible GaN LEDs Information (Figure S16 and Figure S17). These findings demwere 595 and 175 cd/m2 at the peak wavelength of 514 and onstrated that the glass template had only nitrided Ti or TiN on 483 nm, respectively, indicating possibility of high-brightits surface with some pyramid traces after the release (Figure 3a), ness green and blue flexible LEDs (Figure 4d and Supporting
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Figure 4. EL performance of flexible GaN LEDs. a–c) Photograph showing EL for various form factors. Top and middle panels of (a) show stable surface-emitting EL at multiple sites, and bottom panel of (a) and panel (b) shows EL under the extreme bending condition (nearly folded in the left and central panels and wrapped around a wrench in the right panel). c) Bright blue EL loaded in a bending jig. d) (left panel) EL spectra of LEDs having the MQW temperatures of 775 (red) and 790 °C (blue) before (GaN LEDs on glass, dotted line) and after (flexible GaN LEDs, solid line) the transfer process. (right panel) V-I-L curves of the red spectra in left panel after bending (radius of curvature of 10 mm) and unbending (flat) with a number of bending/unbending N. The V-I-L curves for the blue spectra in left panel are shown in Figure S20. e) EL photograph of the device having the transferred GaN array on a PDMS substrate at three different viewpoints. The magnified image of each light-emitting area is shown in inset. Scale bars are 10 (a–c) and 20 mm (e).
Information (Figure S20)). From the I-V curves (right panel of Figure 4d), the series resistance (Rs) of the flexible LED is ∼ 500 Ω that was calculated by dV/dI at Vf of 10.2 V, which is much larger than that of the planar device (∼15 Ω). Although the large Rs is commonly observed in 3D structured LEDs, Rs should be greatly reduced to make the flexible EL devices more efficient. The wall-plug efficiency of the flexible LEDs is still in a proof-of-concept level, much lower than that of the state-ofthe-art, InGaN-based LEDs. We, however, strongly believe that the efficiency should be significantly increased by relevant control of polymer material/interface and of the key issues such as the internal quantum efficiency (IQE) and efficiency-droop. The IQE can be enhanced by minimizing charge separation in QWs,[33,34] and the droop minimized by enhancing current injection efficiency[35] or by reducing Auger recombination.[36,37] The luminance-current and voltage-current characteristics were nearly the same after a few tenth of bending (right panel of Figure 4d and Figure S20a), which meant that the hybrid Ag/ ITO upper electrode had bending stability to the some extent. For the bending of >100 times, however, our proof-of-concept EL devices suffer from luminance degradation although I–V curves are fairly stable (see Supporting Information, Figure S20b), which also needs to be improved. Comparison of the EL spectra for the GaN-on-glass (before transfer) and flexibleGaN (after transfer) devices showed similar full-width at halfmaximum values. This indicated that our transfer method did not cause any electrical damage to the light-emitting structure (left panel of Figure 4d). The slight shift in the peak wavelength might be related to lot-to-lot uniformity. However, the
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flexible LEDs exhibit slight degradation after several tenths of bending test (right panel of Figure 4d). Inspection of the figure reveals that leakage current and series resistance increases with increasing number of bending. The device efficiency is still at the proof-of-concept level. The maximum luminance of flexible GaN LEDs is at least an order of magnitude lower than that of GaN LEDs on glass. These might be attributable to imperfect interface between the GaN array and LIP, brittleness of the ITO electrode, and low thermal spreading. Our fabrication scheme was applied to stretchable PDMS-based EL devices having earlystage EL. In the as-transferred state without any stretching, the device often exhibited uniform and bright EL characteristics (Figure 4e and Supporting Information (Figure S21)). The substrate and all of the other constituent layers are sufficiently stretchable, however, stretchability of the device (see Supporting Information, Figure S22) is not proved yet, possibly because of brittleness of the ITO upper electrode and instability in wiring the electrode. Work is currently underway to identify the optimal interface and transparent upper electrode for application as well as optimized thermal design.
5. Conclusion We successfully demonstrated the fabrication of fully flexible, surface-emitting blue and green GaN LEDs by the heterointerface-controlled transfer. GaN stack arrays were reliably released and transferred from a glass substrate and, then embedded in an ultrathin flexible stretchable substrate. The
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6. Experimental Section Template Preparation and Formation of GaN Pyramid Arrays: The full device fabrication flow is described in Figure S1.Templates were formed on 2-inch fused-silica glass substrates. A template had a structure of hole-patterned SiO2/LT-GaN/Ti layers. The Ti pre-orienting layer with variable thickness (50–200 nm) was electron-beam evaporated at room temperature. The LT-GaN nucleation layer (130 nm) was grown at 560 °C by metal–organic chemical vapour deposition (MOCVD, Sysnex Inc.) using a mixture of trimethylgallium, NH3 and H2. The circular hole-patterned SiO2 layers with variable thicknesses (200–350 nm) and with 2.4-μm hole diameters were formed by plasma-enhanced CVD, conventional photolithography, and reactive ion etching with a CF4/O2 gas mixture. The GaN pyramid arrays were fabricated on the template using the same MOCVD process. The template was heated and annealed at 850, 950, and 1040 °C in a NH3/H2 atmosphere for 5 min, during which time nanovoids formed. Subsequently, selective growth of the GaN pyramid stack arrays with p–n junctions including MQWs was performed by MOCVD. All thicknesses in the following are the values for single-crystalline GaN grown on c-plane sapphire wafers. A 1.4-μm-thick GaN:Si (n-type GaN) layer was grown at 1040 °C, followed by five-period InGaN (2.0 nm)/GaN (19.4 nm) MQWs, and a GaN:Mg (300 nm, p-type GaN) layer. The growth temperatures for the MQWs (InGaN well/GaN barrier) and the p-type GaN were 755, 775 or 790/850 and 970 °C, respectively. During n-GaN pyramid formation, GaN was not nucleated on the SiO2 mask surface because of the high growth temperature (∼1040 °C). However, InxGa1-xN was nucleated and grown on the SiO2 mask surface because of its sufficiently low growth temperature (∼750 °C), and p-GaN was successively grown on InxGa1-xN poly-crystals (Figure S11). P-GaN was activated at 700 °C for 30 min in a nitrogen atmosphere. Release of GaN Pyramid Arrays and Transfer and Embedding into Flexible Substrates: The entire area of the glass template with the GaN pyramid arrays was covered with a 30-μm-thick silicone adhesive tape with silicone adhesive (Kepton, 3M Inc.) (Figure S1b), then heated to 170 °C, which is or above the glass transition temperature of the silicone adhesive. The tape was finally detached at a speed of ∼5 mm/s to weaken the LT-GaN/Ti hetero-interface (Figure S1c). The LT-GaN/ Ti hetero-interface was accurately weakened by this post-mechanical treatment without causing any removal or damage to other interfaces involved in the glass template. The spaces between the GaN pyramid stack arrays were filled with spin-coated LIP: we used a urethaneacrylate, UV-curable elastomer (Minuta Technology Inc.). The basic chemical structure of this polymer is shown in Table S2 and Figure S2b. The surface of the sample was partially removed by a CF4/O2 gas mixture plasma until the p-GaN surface was exposed, forming the upper electrode contact. As a hybrid upper electrode, ITO (200 nm) and Ag (200 nm) were deposited as current-spreading and current-delivering layers, respectively, through different shadow masks by electron-beam evaporation (Figure 1b and Figure S1e). Then the template was bonded to PET substrates of different thicknesses (50 and 125 μm) through a stretchable, UV-curable UIP with variable thickness Figure S1f). By detaching the PET substrate from the glass template at the template edge, GaN pyramid arrays were released from the glass substrate and
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typical total thickness was ∼70 μm. Moreover, the transfer process even onto the stretchable substrate was successful. The release method was based on the formation of nanovoids at the LT-GaN/Ti hetero-interface and post-mechanical treatment that accurately weakened the hetero-interface. This method provides a new tool to realize flexible/stretchable blue or green EL devices having high performance, low cost, and scalability.[38–40] The suggested transfer method might also be used for transferring global epitaxy-based LED or vertical-cavity surface-emitting laser structures onto arbitrary substrates.
embedded in the PET (Figure S1g). For stretchable devices, the same transfer process was applied to a ∼3 mm-thick, thermally curable PDMS (OE-6340, Dow Corning Inc.) substrate. This material had greatly improved adhesion characteristics compared with conventional one (Sylgard 184, Dow Corning Inc.). Finally, as a lower electrode, Ti (5 nm)/ Ag (300 nm) was deposited through a shadow mask by electron-beam evaporation (Figure S1h). Structural Analyses: Nanovoid formation at the LT-GaN/Ti heterointerface in the SiO2/LT-GaN/Ti/glass structures was analyzed by SEM, normal-mode XRD, and transmission electron microscopy (TEM) with EDS. The TEM specimens were prepared using ion milling or focused ion beam (FIB) etching. A 200 kV field emission TEM (Tecnai F20, FEI Inc.) was used for electron diffraction, bright-field imaging and highresolution TEM. GIXRD using Cu-Kα radiation was employed to analyze LT-GaN crystallite size by analyzing the (100) planes in the horizontal direction during thermal annealing. Tensile strain-stress tests of UIP and LIP were performed on dog bones with a universal testing machine (LLOYD instruments, AMETEK Inc.) The effective film stress of the multilayers (SiO2/LT-GaN/Ti/Si wafer) was analyzed by a stress tester (FLX-2320, Tencor Inc.). Electrical Measurements: The sheet resistances of the Ag electrodes were measured using a tester (Fluke 187). Flexible EL devices were often mounted in a homemade bending jig that could control the radius of curvature of the device. EL spectra, and voltage-current-luminance (V-L-I) characteristics were recorded using a spectroradiometer (CS2000, Konica Minolta Inc.) coupled with a voltage and current source (M6100, McScience Inc.). Strain Calculations: The strain calculations were carried out using ABAQUS, a commercial finite-element package. All of the thin film layers in the flexible GaN LEDs were modeled as linear-elastic materials. The Young’s moduli, Poisson’s ratios, and film thicknesses are shown in Table S3 and Figure S5a.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements We thank T. W. Kim at Minuta Technology Inc. for fruitful discussions. M. K. acknowledges support by the Korea Science and Engineering Foundations (KOSEF) and the National Research Foundation of Korea grant funded by the Ministry of Education, Science and Technology (20120005637, 20120006644). Received: October 22, 2013 Revised: November 4, 2013 Published online:
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Adv. Optical Mater. 2013, DOI: 10.1002/adom.201300435
