We report the growth of GaN micro-rods and coaxial quantum-well heterostructures on graphene films, together with structural and optical characterization, for applications in flexible optical devices. Graphene films were grown on Cu foil by means of chemical vapor deposition, and used as the substrates for the growth of the GaN micro-rods, which were subsequently transferred onto SiO 2 /Si substrates. Highly Si-doped, n -type GaN micro-rods were grown on the graphene films using metal–organic chemical vapor deposition. The growth and vertical alignment of the GaN micro-rods, which is a critical factor for the fabrication of high-performance light-emitting diodes (LEDs), were characterized using electron microscopy and X-ray diffraction. The GaN micro-rods exhibited promising photoluminescence characteristics for optoelectronic device applications, including room-temperature stimulated emission. To fabricate flexible LEDs, In x Ga 1–x N/GaN multiple quantum wells and a p -type GaN layer were deposited coaxially on the GaN micro-rods, and transferred onto Ag-coated polymer substrates using lift-off. Ti/Au and Ni/Au metal layers were formed to provide electrical contacts to the n -type and p -type GaN regions, respectively. The micro-rod LEDs exhibited intense emission of visible light, even after transfer onto the flexible polymer substrate, and reliable operation was achieved following numerous cycles of mechanical deformation.

polymer substrates represents a significant breakthrough in fabrication technology, which may enable next-generation optoelectronic devices that are flexible and can be manufactured using large-scale and low-cost processes. 1–3 325, 977 (2009). 1. S.-I. Park, Y. Xiong, R.-H. Kim, P. Elvikis, M. Meitl, D.-H. Kim, J. Wu, J. Yoon, C.-J. Yu, Z. Liu, Y. Huang, K.-C. Hwang, P. Ferreira, X. Li, K. Choquette, and J. A. Rogers, Science, 977 (2009). https://doi.org/10.1126/science.1175690 23, 4614 (2011). 2. C.-H. Lee, Y.-J. Kim, Y. J. Hong, S.-R. Jeon, S. Bae, B. H. Hong, and G.-C. Yi, Adv. Mater., 4614 (2011). https://doi.org/10.1002/adma.201102407 340, 211 (2013). 3. T.-I. Kim, J. G. McCall, Y. H. Jung, X. Huang, E. R. Siuda, Y. Li, J. Song, Y. M. Song, H. A. Pao, R.-H. Kim, C. Lu, S. D. Lee, I.-S. Song, G. Shin, R. Al-Hasani, S. Kim, M. P. Tan, Y. Huang, F. G. Omenetto, J. A. Rogers, and M. R. Bruchas, Science, 211 (2013). https://doi.org/10.1126/science.1232437 polymer substrates, however. Because of the limited thermal budget and lack of epitaxial growth on polymer substrates, growth of compound semiconductors is not straightforward. In addition, conventional inorganic semiconductor transfer techniques, such as laser-lift-off, require complicated processes—including numerous lithography and etching steps—to fabricate and assemble devices. 4 75, 1360 (1999). 4. W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romano, and N. M. Johnson, Appl. Phys. Lett., 1360 (1999). https://doi.org/10.1063/1.124693 1,5 325, 977 (2009). 1. S.-I. Park, Y. Xiong, R.-H. Kim, P. Elvikis, M. Meitl, D.-H. Kim, J. Wu, J. Yoon, C.-J. Yu, Z. Liu, Y. Huang, K.-C. Hwang, P. Ferreira, X. Li, K. Choquette, and J. A. Rogers, Science, 977 (2009). https://doi.org/10.1126/science.1175690 1, 1164 (2005). 5. K. J. Lee, J. Lee, H. Hwang, Z. J. Reitmeier, R. F. Davis, J. A. Rogers, and R. G. Nuzzo, Small, 1164 (2005). https://doi.org/10.1002/smll.200500166 growth of hybrid heterostructures composed of inorganic semiconductors grown on two-dimensional (2D) layered materials, such as graphene and hexagonal boron nitride. 2,6,7 23, 4614 (2011). 2. C.-H. Lee, Y.-J. Kim, Y. J. Hong, S.-R. Jeon, S. Bae, B. H. Hong, and G.-C. Yi, Adv. Mater., 4614 (2011). https://doi.org/10.1002/adma.201102407 330, 655 (2010). 6. K. Chung, C.-H. Lee, and G.-C. Yi, Science, 655 (2010). https://doi.org/10.1126/science.1195403 484, 223 (2012). 7. Y. Kobayashi, K. Kumakura, T. Akasaka, and T. Makimoto, Nature (London), 223 (2012). https://doi.org/10.1038/nature10970 polymer substrates. Moreover, a combination of different inorganic semiconductors and 2D layers on polymer substrates may lead to a range of flexible devices with diverse functionality, including solar cells, field-emission devices, optical communication devices, and light-emitting diodes (LEDs). Here we demonstrate flexible LEDs by growing high-quality GaN micro-rods and coaxial quantum structures on graphene films, and then transferring these structures onto polymer substrates. The preparation of high-quality inorganic compound semiconductors onsubstrates represents a significant breakthrough in fabrication technology, which may enable next-generation optoelectronic devices that are flexible and can be manufactured using large-scale and low-cost processes.A number of difficulties remain in the fabrication of high-quality compound semiconductors onsubstrates, however. Because of the limited thermal budget and lack of epitaxialonsubstrates,of compound semiconductors is not straightforward. In addition, conventional inorganic semiconductor transfer techniques, such as laser-lift-off, require complicated processes—including numerous lithography and etching steps—to fabricate and assemble devices.To overcome these problems, strategies using unconventional etching and release techniques have been used.One powerful solution has been exploiting theof hybridcomposed of inorganic semiconductorson two-dimensional (2D) layered materials, such asand hexagonal boron nitride.In particular, the 2D layers offer weak atomic bonding to the underlying substrate, as well as excellent mechanical flexibility, which can enable transfer of compound semiconductors to create flexible inorganic devices onsubstrates. Moreover, a combination of different inorganic semiconductors and 2D layers onsubstrates may lead to a range of flexible devices with diverse functionality, including solar cells, field-emission devices, optical communication devices, andHere we demonstrate flexiblebyhigh-qualitymicro-rods and coaxial quantum structures onand then transferring these structures ontosubstrates.

GaN microstructures and nanostructures have recently attracted much attention for light emitting devices because of their interesting characteristics, such as variable-color light emission, high density integration, and high material quality. 8–13 93, 021126 (2008). 8. M. Funato, K. Hayashi, M. Ueda, Y. Kawakami, Y. Narukawa, and T. Mukai, Appl. Phys. Lett., 021126 (2008). https://doi.org/10.1063/1.2956404 94, 213101 (2009). 9. C.-H. Lee, J. Yoo, Y. J. Hong, J. Cho, Y.-J. Kim, S.-R. Jeon, J. H. Baek, and G.-C. Yi, Appl. Phys. Lett., 213101 (2009). https://doi.org/10.1063/1.3139865 23, 3284 (2011). 10. Y. J. Hong, C.-H. Lee, A. Yoon, M. Kim, H.-K. Seong, H. J. Chung, C. Sone, Y. J. Park, and G.-C. Yi, Adv. Mater., 3284 (2011). https://doi.org/10.1002/adma.201100806 6, 012101 (2013). 11. K. Kishino, K. Nagashima, and K. Yamano, Appl. Phys. Express, 012101 (2013). https://doi.org/10.7567/APEX.6.012101 26, 3019 (2014). 12. Y. Tchoe, J. Jo, M. Kim, J. Heo, G. Yoo, C. Sone, and G.-C. Yi, Adv. Mater., 3019 (2014). https://doi.org/10.1002/adma.201305684 43, 451 (2013). 13. J. K. Hyun, S. Zhang, and L. J. Lauhon, Annu. Rev. Mater. Res., 451 (2013). https://doi.org/10.1146/annurev-matsci-071312-121659 graphene, excellent tolerance for mechanical deformation of these microstructures enable the fabrication of flexible and stretchable devices. Nevertheless, the key criteria for building reliable GaN microstructure LEDs on flexible substrates have rarely been studied, such as maintaining high crystallinity, control over doping, formation of heterostructures and quantum structures, and vertically aligned growth onto the underlying substrates. Here, the growth and fabrication of GaN micro-rod LEDs on graphene films are reported. Specifically, we investigated vertical growth of GaN micro-rods on graphene films in order to fabricate desirable vertical structure LEDs. Furthermore, visible emission was achieved by fabricating coaxial coatings of In x Ga 1–x N/GaN quantum-well heterostructures and the use of an Mg-doped p-GaN layer. By meeting the above criteria and taking advantage of flexibility of graphene substrates, reliable and flexible micro-rod LEDs were demonstrated. microstructures and nanostructures have recently attracted much attention forbecause of their interesting characteristics, such as variable-color light emission, high density integration, and high material quality.Moreover, when combined by flexible substrates such asexcellent tolerance for mechanical deformation of these microstructures enable the fabrication of flexible and stretchable devices. Nevertheless, the key criteria for building reliablemicrostructureon flexible substrates have rarely been studied, such as maintaining high crystallinity, control over doping, formation ofand quantum structures, and vertically alignedonto the underlying substrates. Here, theand fabrication ofmicro-rodonare reported. Specifically, we investigated verticalofmicro-rods onin order to fabricate desirable vertical structureFurthermore, visible emission was achieved by fabricating coaxial coatings of InGaN/GaN quantum-welland the use of an Mg-doped-GaN layer. By meeting the above criteria and taking advantage of flexibility ofsubstrates, reliable and flexible micro-rodwere demonstrated.

GaN micro-rods were grown on graphene films by metal-organic chemical vapor deposition (MOCVD), as shown in Fig. 1(a) graphene substrates, graphene films were synthesized on copper foil using chemical vapor deposition (CVD), and transferred onto supporting substrates of amorphous SiO 2 -coated Si (SiO 2 /Si) substrates. Typically, CVD-grown graphene films are semi-transparent multi-layer-graphene with an electrical resistance in the range 600–800 Ω/□. GaN micro-rods were grown on graphene films using a two-step temperatures of 750−850 °C for 3 min and 950−1050 °C for 30 min, followed by substrate heating at 1100 °C for 10 min with hydrogen. To grow the GaN micro-rods, trimethyl-gallium (TMGa), ditertiarybutyl-silane (DTBSi), and ammonia (NH 3 ) were employed as reactants, and nitrogen was used as the carrier gas. The flow rates of TMGa, DTBSi, and NH 3 were in the range 15–30 sccm, 1–3 sccm, and 100–500 sccm, respectively. The pressure of the reactor chamber was maintained at 300 Torr during the growth of the micro-rods, with mixture of hydrogen and nitrogen gases. Prior to the growth of the GaN micro-rods, a 2-μm-thick GaN buffer layer was grown to improve the vertical alignment of the micro-rods. GaN micro-rods were grown over the entire graphene film, with a uniform areal density of 107 cm−2, and were hexagonal. The length and aspect ratio of GaN micro-rods depended on the growth time. GaN micro-rods grown for 30 min exhibited a diameter of 1.0 ± 0.3 μm and a length of 7.5 ± 1.0 μm. Themicro-rods wereonby metal-organic chemical vapor deposition (MOCVD), as shown in Fig.. To prepare thesubstrates,were synthesized on copper foil using chemical vapor deposition (CVD), and transferred onto supporting substrates of amorphous SiO-coated Si (SiO/Si) substrates. Typically, CVD-grownare semi-transparent multi-layer-graphene with an electrical resistance in the range 600–800 Ω/□.micro-rods wereonusing a two-step temperatures of 750−850 °C for 3 min and 950−1050 °C for 30 min, followed by substrate heating at 1100 °C for 10 min with hydrogen. Tothemicro-rods, trimethyl-gallium (TMGa), ditertiarybutyl-silane (DTBSi), and ammonia (NH) were employed as reactants, and nitrogen was used as the carrier gas. The flow rates of TMGa, DTBSi, and NHwere in the range 15–30 sccm, 1–3 sccm, and 100–500 sccm, respectively. The pressure of the reactor chamber was maintained at 300 Torr during theof the micro-rods, with mixture of hydrogen and nitrogen gases. Prior to theof themicro-rods, a 2-μm-thickwasto improve the vertical alignment of the micro-rods.micro-rods wereover the entirewith a uniform areal density of 10cm, and were hexagonal. The length and aspect ratio ofmicro-rods depended on thetime.micro-rodsfor 30 min exhibited a diameter of 1.0 ± 0.3 μm and a length of 7.5 ± 1.0 μm.

growth of GaN micro-rods is desirable for light-emitting devices because it allows the fabrication of coaxial quantum-well structures, which exhibit useful optical properties, including color-tunable light emission and a high integration density. However, GaN micro-rods grown directly on pristine graphene without any surface treatment or additional buffer layer exhibited poor vertical alignment, with random growth directions, as shown in Fig. 1(b) growth of nanostructures on CVD-grown graphene depends on the surface roughness of the graphene, and ledges or kinks in the graphene film act not only as nucleation sites but also result in random growth directions. 14 12, 1431 (2012). 14. Y. J. Hong, W. H. Lee, Y. P. Wu, R. S. Ruoff, and T. Fukui, Nano Lett., 1431 (2012). https://doi.org/10.1021/nl204109t GaN micro-rods on the graphene, we grew a GaN buffer layer prior to the growth of the GaN micro-rods. Vertically alignedofmicro-rods is desirable forbecause it allows the fabrication of coaxial quantum-well structures, which exhibit useful optical properties, including color-tunable light emission and a high integration density. However,micro-rodsdirectly on pristinewithout any surface treatment or additionalexhibited poor vertical alignment, with randomdirections, as shown in Fig.. According to other previous research, verticalof nanostructures on CVD-growndepends on the surface roughness of theand ledges or kinks in theact not only as nucleation sites but also result in randomdirections.Thus, the use of high-quality single-layer-graphene is critical to obtain vertically aligned structures. Meanwhile, to improve the vertical alignment of themicro-rods on thewe grew aprior to theof themicro-rods.

1(c) GaN micro-rods grown on graphene with a GaN buffer layer. The FE-SEM image clearly reveals surface morphologies of GaN buffer layer and GaN micro-rods. The vertical alignment of the GaN micro-rods was significantly improved compared with that of GaN micro-rods grown on pristine graphene, exhibiting excellent vertical alignment on the graphene using a GaN buffer layer. This result strongly suggested that the GaN buffer layer played a critical role for vertically aligned growth of GaN micro-rods on graphene films. Figuresshow a 30° tilted-view field-emission scanning electron microscopy (FE-SEM) images of themicro-rodsonwith aThe FE-SEM image clearly reveals surface morphologies ofandmicro-rods. The vertical alignment of themicro-rods was significantly improved compared with that ofmicro-rodson pristineexhibiting excellent vertical alignment on theusing aThis result strongly suggested that theplayed a critical role for vertically alignedofmicro-rods on

growth of GaN micro-rods on graphene was further investigated by measuring X-ray diffraction (XRD). Figure 1(d) θ−2θ scan results of GaN micro-rods grown on graphene with (the solid line) and without (the solid line with white circles) a GaN buffer layer. The range of angles was 20°–80°. When the GaN micro-rods were grown on graphene with a buffer layer, only two wurtzite crystal peaks were observed, corresponding to GaN (002) and GaN (004) at 34.61° and 72.96°, respectively. (The additional peak at 69.18° is attributed to cubic Si(100) from the SiO 2 /Si substrate.) However, additional peaks corresponding to GaN (100), GaN (101), GaN (102), GaN (110), and GaN (103) peaks were observed in the XRD spectra at 32.44°, 36.89°, 48.15°, 57.82°, and 63.47° from GaN micro-rods grown on graphene without a GaN buffer layer, respectively. Rocking curves of the GaN micro-rods grown on the buffer layer were also measured, as shown in Fig. 1(e) GaN micro-rods was estimated to be 3°, which is consistent with the GaN buffer layer grown on graphene (2.5°). In addition, transmission electron microscopy (TEM) data for individual GaN micro-rods showed that they were single crystalline (data not shown here). Furthermore, the SEM images and XRD data strongly suggested that the GaN micro-rods exhibited good vertical alignment, and were of sufficient quality for the fabrication of vertically aligned micro-rod optoelectronic devices. The vertically alignedofmicro-rods onwas further investigated by measuring X-ray diffraction (XRD). Figureshows−2scan results ofmicro-rodsonwith (the solid line) and without (the solid line with white circles) aThe range of angles was 20°–80°. When themicro-rods wereonwith aonly two wurtzite crystal peaks were observed, corresponding to(002) and(004) at 34.61° and 72.96°, respectively. (The additional peak at 69.18° is attributed to cubic Si(100) from the SiO/Si substrate.) However, additional peaks corresponding to(100),(101),(102),(110), and(103) peaks were observed in the XRD spectra at 32.44°, 36.89°, 48.15°, 57.82°, and 63.47° frommicro-rodsonwithout arespectively. Rocking curves of themicro-rodson thewere also measured, as shown in Fig.. The full-width at half-maximum (FWHM) of themicro-rods was estimated to be 3°, which is consistent with theon(2.5°). In addition, transmission electron microscopy (TEM) data for individualmicro-rods showed that they were single crystalline (data not shown here). Furthermore, the SEM images and XRD data strongly suggested that themicro-rods exhibited good vertical alignment, and were of sufficient quality for the fabrication of vertically aligned micro-rod optoelectronic devices.

GaN micro-rods grown on graphene films for optoelectronic devices, we investigated the optical characteristics of the GaN micro-rods using a pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (355 nm) as an optical excitation source. Figure 2(a) photoluminescence (PL) spectra of the GaN micro-rods at various pump powers. At low excitation densities, the dominant PL peak was observed at 3.4 eV. As the excitation energy density increased, additional PL peaks appeared around the near-band-edge (NBE) emission, eventually becoming the dominant feature in the PL spectra. The inset of Fig. 2(a) PL intensity versus the excitation power density. As the pump power increased, the PL intensity increased linearly for low excitation powers; however, above a threshold of approximately 350 kW/cm2, the slope of the PL intensity as a function of the pumping power drastically increased, becoming superlinear. This strong and sharp increase in the PL spectra is indicative of stimulated emission. The pump power threshold for stimulated emission of the GaN micro-rods grown on the graphene films was comparable to that of GaN nanorods grown on single-crystal sapphire substrates. 15 20, 17873 (2012). 15. Q. Li, J. B. Wright, W. W. Chow, T. S. Luk, I. Brener, L. F. Lester, and G. T. Wang, Opt. Express, 17873 (2012). https://doi.org/10.1364/OE.20.017873 To usemicro-rodsonfor optoelectronic devices, we investigated the optical characteristics of themicro-rods using a pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (355 nm) as an optical excitation source. Figureshows room temperaturespectra of themicro-rods at various pump powers. At low excitation densities, the dominantpeak was observed at 3.4 eV. As the excitation energy density increased, additionalpeaks appeared around the near-band-edge (NBE) emission, eventually becoming the dominant feature in thespectra. The inset of Fig.shows a plot of the integratedintensity versus the excitation power density. As the pump power increased, theintensity increased linearly for low excitation powers; however, above a threshold of approximately 350 kW/cm, the slope of theintensity as a function of the pumping power drastically increased, becoming superlinear. This strong and sharp increase in thespectra is indicative of stimulated emission. The pump power threshold for stimulated emission of themicro-rodson thewas comparable to that ofnanorodson single-crystal sapphire substrates.

GaN micro-rods grown on graphene films were further examined using micro-PL spectroscopy with a He-Cd laser (325 nm). As shown in Fig. 2(b) PL spectra exhibit a strong NBE emission at 3.4 eV, with weak deep level emission at around 2.2 eV. The strong NBE emission and low deep-level emission indicate that the GaN micro-rods were of high quality, comparable to that of GaN micro-rods and nano-rods grown on single-crystal sapphire and Si substrates. 16–18 6, 1808 (2006). 16. S. D. Hersee, X. Sun, and X. Wang, Nano Lett., 1808 (2006). https://doi.org/10.1021/nl060553t 21, 015602 (2010). 17. R. Koester, J. S. Hwang, C. Durand, D. L. S. Dang, and J. Eymery, Nanotechnology, 015602 (2010). https://doi.org/10.1088/0957-4484/21/1/015602 12, 3838 (2012). 18. S.-M. Ko, J.-H. Kim, Y.-H. Ko, Y. H. Chang, Y.-H. Kim, J. Yoon, J. Y. Lee, and Y.-H. Cho, Cryst. Growth Des., 3838 (2012). https://doi.org/10.1021/cg300764f PL spectra obtained at the top (region I), middle (region II), and bottom (region III) of the GaN micro-rods did not exhibit any significant variation in the peak position. This suggests that the strain in the GaN micro-rods was negligible, because strain results in a significant shift in the wavelength of the PL spectra. 19 54, 13460 (1996). 19. W. Shan, R. J. Hauenstein, A. J. Fischer, J. J. Song, W. G. Perry, M. D. Bremser, R. F. Davis, and B. Goldenberg, Phys. Rev. B, 13460 (1996). https://doi.org/10.1103/PhysRevB.54.13460 The optical characteristics of individualmicro-rodsonwere further examined using micro-PL spectroscopy with a He-Cd laser (325 nm). As shown in Fig., typical room temperaturespectra exhibit a strong NBE emission at 3.4 eV, with weak deep level emission at around 2.2 eV. The strong NBE emission and low deep-level emission indicate that themicro-rods were of high quality, comparable to that ofmicro-rods and nano-rodson single-crystal sapphire and Si substrates.Furthermore, thespectra obtained at the top (region I), middle (region II), and bottom (region III) of themicro-rods did not exhibit any significant variation in the peak position. This suggests that the strain in themicro-rods was negligible, because strain results in a significant shift in the wavelength of thespectra.

LED structure, a multiple quantum well (MQW) stack of 8 periods of In x Ga 1–x N/GaN heterostructures and a p-type GaN layer were coated coaxially on the n-type GaN micro-rods using MOCVD. As shown in Fig. 3(a) GaN micro-rod LEDs were vertically aligned on the graphene substrate following this coating process. The structural characteristics of these coaxial GaN micro-rod LEDs were further investigated using TEM. Scanning TEM images of the top and sidewalls of the GaN micro-rod LEDs are shown in Figs. 4(b) and 4(c) x Ga 1–x N/GaN MQWs were formed at the top and sidewalls with abrupt interfaces, and the thicknesses of the In x Ga 1–x N QWs at the top and sidewall were 8 and 4 nm, respectively. As determined by energy-dispersive X-ray spectroscopy, the x value for In x Ga 1–x N was approximately 0.07. Additionally, room temperature cathodoluminescence (CL) spectra showed emissions at 439 nm and 414 nm for the top most QWs and sidewall QWs, respectively, indicating nonuniformities in thicknesses or composition of the QW heterostructures (data not shown here). Such results were observed in a previous report on multi-faceted nanostructure and microstructure LEDs. 9,10,12 94, 213101 (2009). 9. C.-H. Lee, J. Yoo, Y. J. Hong, J. Cho, Y.-J. Kim, S.-R. Jeon, J. H. Baek, and G.-C. Yi, Appl. Phys. Lett., 213101 (2009). https://doi.org/10.1063/1.3139865 23, 3284 (2011). 10. Y. J. Hong, C.-H. Lee, A. Yoon, M. Kim, H.-K. Seong, H. J. Chung, C. Sone, Y. J. Park, and G.-C. Yi, Adv. Mater., 3284 (2011). https://doi.org/10.1002/adma.201100806 26, 3019 (2014). 12. Y. Tchoe, J. Jo, M. Kim, J. Heo, G. Yoo, C. Sone, and G.-C. Yi, Adv. Mater., 3019 (2014). https://doi.org/10.1002/adma.201305684 For thestructure, a multiple quantum well (MQW) stack of 8 periods of InGaN/GaNand a-typelayer were coated coaxially on the-typemicro-rods using MOCVD. As shown in Fig., themicro-rodwere vertically aligned on thesubstrate following this coating process. The structural characteristics of these coaxialmicro-rodwere further investigated using TEM. Scanning TEM images of the top and sidewalls of themicro-rodare shown in Figs., respectively. The InGaN/GaN MQWs were formed at the top and sidewalls with abrupt interfaces, and the thicknesses of the InGaN QWs at the top and sidewall were 8 and 4 nm, respectively. As determined by energy-dispersive X-ray spectroscopy, the x value for InGaN was approximately 0.07. Additionally, room temperature cathodoluminescence (CL) spectra showed emissions at 439 nm and 414 nm for the top most QWs and sidewall QWs, respectively, indicating nonuniformities in thicknesses or composition of the QW(data not shown here). Such results were observed in a previous report on multi-faceted nanostructure and microstructure

GaN micro-rod LEDs was completed by depositing metal contacts, and film transfer onto the polymer substrate, as shown schematically in Fig. 3(d) growth of the coaxial GaN micro-rod LEDs, the gaps in the micro-rod LEDs were filled using a polyimide insulating layer, and oxygen plasma etching was carried out to expose the top of the GaN micro-rod LEDs. A Ni/Au bi-layer was then deposited on p-type GaN and thermally annealed, resulting in Ohmic contacts. To form n-type GaN Ohmic contacts, the LEDs were removed from the SiO 2 /Si substrate by wet etching the sacrificial SiO 2 layer of the substrate. A Ti/Au metal layer was then deposited on the bottom of the LEDs and an additional thick Ag layer was coated onto the Ti/Au layer to provide reliable current injection. The coaxial GaN micro-rod LEDs were then transferred onto the polyimide substrate. The fabrication of themicro-rodwas completed by depositing metal contacts, and film transfer onto thesubstrate, as shown schematically in Fig.. Followingof the coaxialmicro-rodthe gaps in the micro-rodwere filled using a polyimide insulating layer, and oxygen plasma etching was carried out to expose the top of themicro-rodA Ni/Au bi-layer was then deposited on-typeand thermally annealed, resulting in Ohmic contacts. To form-typeOhmic contacts, thewere removed from the SiO/Si substrate by wet etching the sacrificial SiOlayer of the substrate. A Ti/Au metal layer was then deposited on the bottom of theand an additional thick Ag layer was coated onto the Ti/Au layer to provide reliable current injection. The coaxialmicro-rodwere then transferred onto the polyimide substrate.

3(e) LEDs on the polymer substrate with an applied current of 10 mA. The contact area of the LED device was 50 × 50 μm2. Individual light spots from the micro-rod LEDs can be observed clearly. Furthermore, the LED emitted strong blue light, which could be observed using the naked eye under normal interior lighting conditions. Additionally, we observed green and orange light emissions from some micro-rod LEDs at an applied current of 6 mA, which changed to blue as the current increased up to 10 mA. Because the n-type GaN micro-rods and the In x Ga 1–x N/GaN QW heterostructures exhibited extremely weak deep-level PL and CL emissions, respectively, the emission of green or orange light is presumably attributable to yellow emission from the p-type GaN layer. However, we believe the yellow emission can be reduced by optimizing the growth condition. 20 30, L1708 (1991). 20. S. Nakamura, M. Senoh, and T. Mukai, Jpn. J. Appl. Phys., L1708 (1991). https://doi.org/10.1143/JJAP.30.L1708 Figureshows a magnified optical image of the optical emission from the micro-rodon thesubstrate with an applied current of 10 mA. The contact area of thedevice was 50 × 50 μm. Individual light spots from the micro-rodcan be observed clearly. Furthermore, theemitted strong blue light, which could be observed using the naked eye under normal interior lighting conditions. Additionally, we observed green and orange light emissions from some micro-rodat an applied current of 6 mA, which changed to blue as the current increased up to 10 mA. Because the-typemicro-rods and the InGaN/GaN QWexhibited extremely weak deep-leveland CL emissions, respectively, the emission of green or orange light is presumably attributable to yellow emission from the-typelayer. However, we believe the yellow emission can be reduced by optimizing thecondition.

GaN micro-rod LEDs were further investigated by measuring power-dependent EL spectra. Figure 3(f) x Ga 1–x N/GaN QWs and deep-levels of the p-type GaN layer, respectively. As the current was increased to 10 mA, the EL intensity gradually increased, and the dominant EL peak shifted from 437 nm to 407 nm. The inhomogeneities in indium composition or thickness of the QWs along the micro-rod LEDs may contribute to the large blue shift. 10,12 23, 3284 (2011). 10. Y. J. Hong, C.-H. Lee, A. Yoon, M. Kim, H.-K. Seong, H. J. Chung, C. Sone, Y. J. Park, and G.-C. Yi, Adv. Mater., 3284 (2011). https://doi.org/10.1002/adma.201100806 26, 3019 (2014). 12. Y. Tchoe, J. Jo, M. Kim, J. Heo, G. Yoo, C. Sone, and G.-C. Yi, Adv. Mater., 3019 (2014). https://doi.org/10.1002/adma.201305684 The electroluminescence (EL) characteristics of themicro-rodwere further investigated by measuring power-dependent EL spectra. Figureshows room-temperature EL spectra at applied currents in the range 2.6–10 mA. With a current of 2.6 mA, the dominant EL emission was observed at 437 nm, with a broad low-energy shoulder at 590 nm. The EL peaks at 437 and 590 nm presumably originate from the InGaN/GaN QWs and deep-levels of the-typelayer, respectively. As the current was increased to 10 mA, the EL intensity gradually increased, and the dominant EL peak shifted from 437 nm to 407 nm. The inhomogeneities in indium composition or thickness of the QWs along the micro-rodmay contribute to the large blue shift.

GaN micro-rod LEDs fabricated on large-area graphene films. Figure 4(a) GaN micro-rod LEDs became damaged by the mechanical deformation. The reliability of the LEDs was investigated by measuring the EL characteristics following repeated bending cycles. Figure 4(b) We investigated the flexibility and reliability of themicro-rodfabricated on large-areaFigureshows the EL spectra with bending radii of ∞, 6, and 4 mm. When the 20-mm-wide substrate was bent to a radius of curvature of 6 mm, the EL intensity was not observed to degrade, and the dominant EL peak wavelength did not change. For a radius of curvature of 4 mm, however, the EL intensity markedly decreased, indicating that themicro-rodbecame damaged by the mechanical deformation. The reliability of thewas investigated by measuring the EL characteristics following repeated bending cycles. Figureshows a plot of integrated EL intensity and wavelength of the dominant EL peak as a function of the number of cycles. Following 10 bending cycles, a slight degradation of the EL intensity was observed. Nevertheless, neither the wavelength of the dominant EL peak nor the integrated EL intensity exhibited significant changes following 1000 bending cycles.

In conclusion, we have described the growth of vertically aligned GaN micro-rods on large-area graphene films using MOCVD. The micro-rods exhibited optical characteristics comparable to those of GaN nano- and micro-rods grown on single crystal substrates. The GaN micro-rods exhibited excellent vertical alignment on the graphene substrate, which is desirable for fabricating 3D integrated devices. We grew coaxial In x Ga 1–x N/GaN MQW heterostructures to form LEDs, and transferred the structures to a polymer substrate. The resulting flexible LEDs exhibited intense EL, and were reliable, with no significant degradation in the optical performance following 1000 bending cycles. This transferable fabrication route to achieve flexible inorganic semiconductor/graphene heterostructures on a polymer substrate has numerous potential applications in flexible optoelectronic devices, and provides a method of integrating organic substrates and inorganic semiconductor materials without the thermal budget restrictions that are typical of polymer substrates.