Visible Color Tunable Emission in Three-Dimensional Light Emitting

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Visible Color Tunable Emission in Three-dimensional Light Emitting Diodes by MgO Passivation of Pyramid Tip Ji-hyun Kim, Byeong Uk Ye, Joonmo Park, Chul Jong Yoo, Buem Joon Kim, Hu Young Jeong, Jin-Hoe Hur, Jong Kyu Kim, Jong-Lam Lee, and Jeong Min Baik ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08729 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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ACS Applied Materials & Interfaces

Visible Color Tunable Emission in Three-dimensional Light Emitting Diodes by MgO Passivation of Pyramid Tip

Ji-Hyun Kim†⊥, Byeong Uk Ye†⊥, Joonmo Park†, Chul Jong Yoo‡, Buem Joon Kim‡, Hu Young Jeong§, Jin-Hoe Hur||, Jong Kyu Kim¶, Jong-Lam Lee‡ and Jeong Min Baik†*

†

Department of Materials Science Engineering, KIST-UNIST-Ulsan Center for Convergent

Materials, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689798,Republic of Korea. ‡

Department of Materials Science and Engineering, Division of Advanced Materials Science,

Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea. §

UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and

Technology (UNIST), Ulsan, Korea ||

UNIST-OLYMPUS BIOMED IMAGING CENTER(UOBC), Ulsan National Institute of

Science and Technology (UNIST), Ulsan, Republic of Korea ¶

Department of Materials Science and Engineering, Pohang University of Science and

Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea.

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ABSTRACT: Visible color tunable three-dimensional pyramidal-shaped light emitting diodes are demonstrated, by depositing the MgO on and near the tip of the pyramid as an insulating layer. Here, we shows that the degradation of the materials (i.e. p-GaN) crystallinity and the built-in electric field due to the nanoscale geometry of the tip region is responsible for the large leakage current observed in LEDs. The confocal scanning electroluminescence microscopy images clearly showed that the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the tip surface, indicating that the MgO layer prohibited the carrier injection to the MQWs layer, suppressing the leakage occurring at or near the tip region of the pyramids. The color range of the LEDs can be also tuned by using the MgO layer, a blue-shift by 10.3 nm in the wavelength. This technique is so simple and scalable, providing a promising solution for developing 3D pyramidal-shaped LEDs with low leakage current and controllable light emission.

KEYWORDS: MgO, Current Path Control, Color Tunable, Pyramid, Light Emitting Diodes


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1. INTRODUCTION GaN-based light-emitting diodes (LEDs) have revolutionized solid-state lighting and large displays, due to their reliably long life, short response time, high radiance, and low energy consumption.1,2 However, the LED efficiency usually reaches a maximum value at low current density and then droops with the current density further increasing, which is referred to as “efficiency droop”. Many physical mechanisms have been proposed to explain this behavior, including Auger recombination and carrier delocalization.3,4 Conventional InGaN/GaN multiple-quantum-wells (MQWs) are often grown on the polar c-plane surface, however, a dramatic drop in efficiency at high current by the quantum-conﬁned Stark effect (QCSE) is one of the main problems limiting the growth of the solid-state lighting market. The QCSE may be avoided by growing the LED structures on semipolar and nonpolar substrates, however, these are not readily available because they are typically rather expensive to polar substrates and exhibit high density of defects such as threading dislocation and high density of stacking faults when grown on planar substrates.5,6 Three-dimensional (3D) pyramidal-shaped LEDs can be a good candidate due to their many advantages such as low dislocation density, high light extraction efficiency, and multiple color generation from different In compositions depending on location in QWs. Furthermore, the scalable patterning methods to fabricate micrometer-sized templates and the growth conditions similar to that on polar c-plane substrate will increase the commercialization success. Because of the potential benefits, several 3D pyramidal-shaped LEDs have been successfully demonstrated.7-9 The majority of these studies focused on the growth of the pyramidal-shaped LED structures and the pyramidal-shape dependent LED properties. Variations in surface polarity and growth rate led to variation in QW thickness and In composition, used to be a good approach to generating white light.10,11 However, the viability of this approach requires light emission to be controllable. Furthermore, most 3D pyramidalshaped LEDs have larger leakage current than the conventional LEDs. Identifying the origin ACS Paragon Plus Environment


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of the leakage current path in this structure is still under investigation. Thus, it is crucial to identify and examine the main factors in improving the LED performance.12-14 Here, we demonstrate visible color tunable 3D pyramidal-shaped LEDs with enhanced electrical performance. Our method is to deposit the magnesium oxide (MgO) on and near the tip of the pyramid as an insulating layer before the deposition of indium tin oxide (ITO) on pGaN layer. The leakage current at revere bias of -5 V was found to be reduced by approximately two orders of magnitude by the MgO layer, which may ascribe to the significant decrease of the current injection to the QWs through the tip surface. This technique is also effective in controlling the wavelength of the emitted light, in which the light is blue-shifted by 10.3 nm in the wavelength by the removal of the MgO layer after the deposition of ITO. This technique is so simple and scalable, providing a promising solution for developing 3D pyramidal-shaped LEDs with low leakage current and controllable light emission. The origin of the visible color tunable light emission and low leakage current of the LEDs were also investigated using confocal scanning electroluminescence microscopy (CSEM) and high-resolution transmission electron microscopy (HR-TEM).

2. EXPERIMENTAL SECTION 2.1 Growth of 3D pyramidal-shaped LED Structures A 2 µm-thick undoped GaN buffer layer, a 2 µm-thick n-type GaN layer were grown in sequence on c-plane sapphire substrates using metal-organic chemical vapor deposition. Circular openings with a diameter of 3 µm and a center to center pitch of 7 µm are formed by a conventional photolithography technique. For lateral over-growth, a 100 nm-thick SiO2 mask layer with hole patterns of 3 µm-diameter was prepared on an n-type GaN template by RF-sputtering. The micrometer-sized hexagonal pyramid n-GaN was grown via MOCVD using trimethylgallium (TMGa) and ammonia (NH3) at 1050oC for 1h. InGaN/GaN MQW layers and Mg-doped p-GaN layers were heteroepitaxially grown on the entire surface of the ACS Paragon Plus Environment

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n-GaN hexagonal pyramid structures. The InGaN/GaN MQW LED structures consisted of an InGaN/GaN active layer, and a 120 nm-thick Mg-doped p-GaN layer. After growth, p-GaN was annealed at 750oC for 5 min in a nitrogen atmosphere. 2.2 Fabrication of conventional pyramidal-shaped LEDs A photoresist (SU8) layer was spin-coated to fill the space between 3D pyramidal-shaped LEDs, followed by O2 plasma ashing to expose the p-GaN surface. A 200 nm-thick indium tin oxide (ITO) layer was deposited on the LEDs by reactive e-beam evaporation in the presence of oxygen (At 250°C with 20 sccm O2 flow at a base pressure of 5.0 x 10-6 Torr) on the pGaN surface to serve as a transparent electrode and current spreading layer. For electrical contact, indium soldering was utilized on the pyramid surface and the n-GaN layer to serve as p- and n-metal contacts, respectively. 2.3 Fabrication of MgO-passivated and Tip-open structured pyramidal-shaped LEDs First, a SU8 resist layer was spin-coated to fully cover the 3D pyramidal-shaped LEDs, followed by O2 plasma treatment to etch the photoresist (AZ5214E) on and near the tip region of the pyramid. Then, a 100 nm-thick MgO layer was deposited at room temperature by an ebeam evaporator and the AZ5214E was removed with acetone. The remaining MgO layer on the pyramid tip surface serves as the insulating layer. As a top electrode, 200 nm-thick ITO was deposited on the LEDs by reactive e-beam evaporation in the presence of oxygen (At 250°C with 20 sccm O2 flow at a base pressure of 5.0 x 10-6 Torr), producing MgO-passivated LEDs. The MgO layer on and near the top region of the pyramid was then etched in the diluted H3PO4 solution (1 : 9) for 1min, by spin-coating AZ5214E on the ITO electrode. After removing the AZ5214E with acetone, the tip-open structured LEDs was fabricated. 2.4 Characterization The scanning electron microscopy (SEM) was done using a PHILIPS XL30S with an accelerating voltage of 5 kV. The high-resolution transmission electron microscopy (HRTEM) images were collected using a Cs-corrected JEM-2100 operated at 200 kV. For crossACS Paragon Plus Environment


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sectional TEM imaging, samples were milled with 30-kV-accelerated gallium ions using a focused ion beam machine (FIB; Quanta 3D FEG) in dual-beam mode. I–V characteristics of the 3D pyramidal-shaped LEDs were measured at an unpacked (on-wafer) by applying a DC voltage to the device using a source meter (Keithley 2400). The confocal scanning electroluminescence microscopy (CSEM) was done using a IX81 Motorized Microscope with a spatial resolution of 250 nm. Where it was finally detected using a cooled charge coupled device (CCD) detector (iXon3 888).

3. RESULTS AND DISCUSSION A schematic illustration of the 3D pyramidal-shaped LEDs is shown in Figure 1a. The LEDs fabrication process is also shown in Figure S1. The LED structures and the fabrication process were reported previously.9,14 Roughly, micrometer-sized (~ 4.5 µm in diameter and ~ 4 µm in height) hexagonal pyramid structures of GaN were formed by lateral over-growth with a 3 µm diameter hole-patterned mask on an n-type GaN template that was grown on a c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The 5-period InGaN/GaN MQW layers were grown on the pyramids, followed by growth of a p-type GaN. The angle between the lateral surface and the central axis of the pyramidal-shaped structures 1) facets. A schematic scanning electron microscope (SEM) image is about 36o, close to (101 of the pyramids confirms that the pyramid structures are uniformly grown on the substrate. The pyramidal-shaped LEDs were fabricated by depositing ITO on the surface of the p-GaN and the electrical measurement was carried out at room temperature. The representative current-voltage (I-V) curve shows the typical rectifying behavior of the LEDs, however, it seems that it exhibits quite large leakage current density of -208.49 mA/cm2 at -5 V and the current increases gradually with increasing the forward bias voltage. The LEDs also emit a partially greenish color at low forward bias due to high In composition at the tip region of the pyramid LED. As the injection current increases, the injected carrier overflow to the lower ACS Paragon Plus Environment

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portion of MQWs because the In composition decreases along the body of the LED from the tip to the sidewall, resulting in a broad-band emission, consistent with previous results.7,12 So far, in conventional flat GaN-based LEDs, there have been many reports on the origin of the leakage current such as, the carrier hopping through defect states, thermally assisted multi-step tunneling, field-emission tunneling, and trap-assisted tunneling associated with dislocations such as screw components.15-18 When the pyramidal-shaped LEDs are grown, the tip surface of the n-GaN template may be not covered by the thin p-GaN or many defects may be created near the tip surface of the LEDs, resulting in a large leakage current in such LED devices.19,20 Strategies and techniques from the growth aspect of the GaN-related materials, that is, by increasing the thickness of the p-GaN layer, by inserting additional p-AlGaN layer, were suggested to overcome the issues.21 However, the thickness of the p-GaN layer is typically limited to 0.1 ~ 0.2 µm due to the poor electrical properties which cause large leakage current and poor stability. Additional layer such as AlGaN, for which an accurate growth condition is necessary, may decrease the conductivity of the LEDs or induce stress in each epitaxial layer. Although there were some reports on the tip-free annular structured LEDs, highly sophisticated technologies for the growth and the fabrication process may be required.21-22 It has been also reported that there were so many defects at and near the lower part of GaN pyramid.23 Such defects were identified as dislocations parallel to the interface between the GaN and SiO2 layer. It was mainly due to the fast lateral growth, threading dislocations tend to bend towards the SiO2 layer. The GaN clusters formed on the SiO2 layer when the LED was grown also play as a path for the leakage current. However, these factors may be removed by passivating the defects with insulating polymer (SU8) before the deposition of the ITO, as shown in Figure S2. First, the crystal structures of the pyramid LEDs, focused on near the tip region of the LED,



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were carefully characterized by low-magnified bright-field TEM and high-resolution TEM (HR-TEM) images, as shown in Figure 2. Figure 2a shows the direct imaging of single LED pyramid covered by MQWs. It is clearly seen that the LED grew to about 4 µm diameter and 4 µm height. The smooth tip at the tip region of the pyramid may ascribe to the ions bombardment while milling for sample preparation. The HR-TEM image for the MQWs at 1) the sidewall in Figure 2b shows that high-quality MQWs were grown on the semipolar (101 GaN planes containing 5 periods of 15.1 nm thick GaN barriers and 2.3 nm thick InGaN wells. The QWs are getting thinner and thinner because the desorption effect dominate and thus the growth rate decreases. In composition of the Pyramid InGaN MQWs was measured by the energy dispersive X-ray spectroscopy (EDS). It is clearly seen that the composition increases from 4.39 % to 12.18 %, along the facet from the tip to bottom of the pyramid, as shown in Figure S3. It may be ascribed to almost defect-free and atomically precise single crystallinity of the GaN pyramid used as a substrate, as shown in Figure 2c. The lattice spacing between adjacent planes was ~ 0.52 nm, corresponding to the d-spacing of GaN (0001) planes. However, high magnification at the p-GaN layer, near the tip region of the pyramid shows several stacking faults (see the Fourier filtering image toward the [0002] direction in Figure 2d), while no such defects in the p-GaN layer at the sidewall were observed. Furthermore, the diffraction spots were split into several smaller spots or became larger, suggesting the material crystallinity is degraded. The degradation of the materials may ascribe to the strong internal strain by the high In composition at the tip region of the pyramid structure, which induce the large lattice mismatch between InGaN and GaN.12,24,25 Here, a new fabrication process of 3D pyramidal-shaped LEDs is suggested, to deposit the MgO layer on the pyramid tip surface as an insulating layer. Figure 3a illustrates the novel process. The MgO, evaporated by e-beam evaporator at room temperature and found to be a good insulator, was expected to prohibit the current injection to the MQWs through the tip region. A 200-nm-thick ITO layer was then deposited and the electrical transport ACS Paragon Plus Environment

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measurements were carried out at room temperature. Compared with the I-V curve in the conventional pyramid LEDs, it is clearly seen that the device shows lower reverse-biased leakage current, -2.97 mA/cm2 at -5 V. The current at forward bias also increases steeply with the applied voltage although the turn-on voltage is higher than that of the conventional device. This means that the MgO layer formed at the tip region is effective in improving the performance of the LEDs. It is also well-known that the lower turn-on voltage in the conventional LED is attributed to the decrease in leakage of carrier through defects at or near the surface. This means that the MgO layer can suppress the defect assisted leakage occurring near the tip region of the pyramids. The MgO layer was then etched in the diluted H3PO4 solution, producing the tip-open structured LEDs. This increases the leakage current from 2.97 to -31.46 mA/cm2 at -5 V although there is a little decrease in the turn-on voltage. Actually, we expected the lower leakage current because leakage current can flow across the MgO layer. It may be ascribed to the damage of ITO film by the etching solution, increasing the sheet resistance and current spreading resistance, thereby, deteriorating electrical properties. The improvement of the LEDs by the MgO may be also understood in terms of the nanoscale geometry and relevant physical properties. The electric field is usually concentrated at the pointed extremity (here, tip region), of the pyramid, resulting in an accumulation of the electrons at the point. It may be responsible for electric-field-assisted tunnelling current (Poole-Frenkel effect) which involves the presence of the electrostatic potential within the tunnelling barrier.24 Furthermore, the lattice mismatch between InGaN and GaN due to high In composition at the MQWs near the tip region will induce a strong internal electric field. Thus, the large leakage current may ascribe to the strong induced internal field and the nonradiative recombination at the tip region by the degradation of the materials of 3D pyramidal-shaped LEDs.24,26



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In the devices suggested, we assumed that the MgO layer formed on and near the tip region would prohibit the carrier injection to the MQWs layer, however, we still should provide the evidence needed. The CSEM images in Figure 4 clearly demonstrate the electroluminescence (EL) properties such as light propagation and local light output. For the conventional LEDs, the light is extracted out of the hexagonal tip facet, as explained by the wave-guiding effect, due to the large refractive index difference between GaN (n = 2.2) and air (n = 1). Although some light is also emitted out of the side facet, the intensity is not as great as that from the tip facet. On the contrary, for the MgO-passivated LEDs, the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the hexagonal tip. This clearly shows that the carrier injection to the MQWs through the hexagonal tip is prohibited by the MgO layer. For the tip-open structured LEDs, although some light is emitted out of the tip region of the pyramid, the intensity is not also as great as that from the pyramidal facet. The enhanced emission out of the side facet of the pyramid can tune the visible color range of the 3D pyramidal-shaped LEDs. As mentioned above, the In composition increases along the body of the LED from the side wall to the tip region, resulting in a broad-band emission. It has been suggested as one of strategies to fabricate white LEDs by using the pyramid-shaped structure without phosphors, however, this method is not intentional. The light emission from the active regions of MgO-passivated and tip-open structured LEDs is mainly found at the side facet. The EL spectra and the CSEM images with 30 mA injection current for the three pyramid-shaped LED arrays are shown as a function of wavelength in Figures 5a and 5b. For the conventional LEDs, light at wavelengths of 470 - 550 nm is emitted with an additional light emission at 575 nm, a green emission. For the MgO-passivated LEDs, the intensity at the higher wavelength side of the main peak decreased and there was no light emission at 575 nm, meaning that the emission at 575 nm may be originated from the light extracted out of the tip region. There is also a significant blue-shift by 10.3 nm in the wavelength of the light emitted ACS Paragon Plus Environment

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in tip-open structured LEDs. Actually, the light emission at high wavelength is not observed in Figure 5b. From the EL spectra, the In composition can be estimated to be approximately >12 % and 4.39 ~ 12.18 % in QWs of the tip region and the side wall, respectively.19,20 Although the estimated In composition is larger than the values measured by EDS, this shows that the visible color range can be well-controlled by using the MgO layer on the tip region of the 3D pyramidal-shaped LEDs.

4. CONCLUSIONS In summary, we demonstrated visible color tunable three-dimensional pyramidal-shaped light emitting diodes with enhanced electrical performance, by depositing the MgO on and near the tip of the pyramid as an insulating layer. The degradation of the materials (i.e. p-GaN) crystallinity and the built-in electric field due to the nanoscale geometry of the tip region in the pyramid is responsible for the large leakage current observed in LEDs. The confocal scanning electroluminescence microscopy images clearly showed that the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the tip surface, indicating that the MgO layer prohibited the carrier injection to the MQWs layer, suppressing the leakage occurring at or near the tip region of the pyramids. The color range of the LEDs can be also tuned by using the MgO layer, by prohibiting the light emission out of the tip region of the pyramid. This technique is so simple and scalable, providing a promising solution for developing 3D pyramidal-shaped LEDs with low leakage current and controllable light emission.

ASSOCIATED CONTENT Supporting Information The fabrication schematic process of Conventional LEDs, MgO-passivated LEDs and Tip-



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open structured LEDs; The fabrication schematic process and SEM images of Conventional LEDs; Cross-sectional low-magnification image of single LED covered by MQWs; In composition plot from the pyramid InGaN MQWs at various positions; Current-voltage curves measurements from different samples. The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author *Tel: +82(0)52-217-2324, FAX: +82(0)52-217- 2309, E-mail: [email protected] Author Contributions ⊥These

authors contributed equally to this work

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by the IT R&D program of MKE/KEIT [10035598, 180 lm/W High-efficiency nano-based LEDs], by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3063602), and by the Future Strategic Fund (1.130061.01) of UNIST(Ulsan National Institute of Science and Technology).

REFERENCES (1) Pimputkar, S.; Speck, J. S.; DenBaars, S. P.; Nakamura, S. Prospects for LED lighting. Nat. Photonics 2009, 3, 180-182.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2) Ye, B. U.; Kim, B. J.; Song, Y. H.; Son, J. H.; Yu, H. k.; Kim, M. H.; Lee, J.-L.; Baik, J. M. Enhancing Light Emission of Nanostructured Vertical Light-Emitting Diodes by Minimizing Total Internal Reflection. Adv. Funct. Mater 2012, 22, 632–639. (3) Piprek, J. Efficiency droop in nitride-based light-emitting diodes. Phys. Status Solidi A 2010, 207, 2217-2225. (4) Kim, A. Y.; Götz, W.; Steigerwald, D. A.; Wierer, J. J.; Gardner, N. F.; Sun, J.; Stockman, S. A.; Martin, P. S.; Krames, M. R.; Kern, R. S.; Steranka, F.M. Performance of HighPower AlInGaN Light Emitting Diodes. phys. stat. sol. (a) 2001, 188, 15-21. (5) Vampola, K. J.; Iza, M.; Keller, S.; DenBaars, S.P.; Nakamura, S. Measurement of electron overflow in 450 nm InGaN light-emitting diode structures. Appl. Phys. Lett. 2009, 94, 061116. (6) Chichibu, S. F.; Uedono, A.; Onuma, T.; Haskell, B. A.; Chakraborty, A.; Koyama, T.; Fini,

P. T.; Keller, S.; DenBaars, S. P.; Speck, J. S.; Mishra, U. K.; Nakamura, S.;

Yamaguchi, S.; Kamiyama, S.; Amano, H.; Akasaki, I.; Han,

J.; Sota, T.; Origin of

defect-insensitive emission probability in In-containing (Al,In,Ga)N alloy semiconductors. Nature Mater 2006, 5, 810. (7) Ko, Y.-H.; Kim, J.-H.; Jin, L.-H.; Ko, S.-M.; Kwon, B.-J.; Kim, J.; Kim, T.; Cho, Y.-H. Electrically driven quantum dot/wire/well hybrid light-emitting diodes. Adv. Mater 2011, 23, 5364-5369. (8) Hsu, C.-W.; Lundskog, A.; Karlsson, K. F.; Forsberg, U.; Janzen, E.; Holtz, P. O. Single excitons in InGaN quantum dots on GaN pyramid arrays. Nano Lett. 2011, 11, 2415-2418.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9) Choi, J. H.; Zoulkarneev, A.; Kim, S. I.; Baik, C. W.; Yang, M. H.; Park, S. S.; Suh, H.; Kim, U. J.; Son, H. B.; Lee, J. S.; Kim, M.; Kim, J. M.; Kim, K. Nearly singe-crystalline GaN light-emitting diodes on amorphous glass substrates. Nat. Photonics 2011, 5, 763. (10)

Riley, J. R.; Padalkar, S.; Li, Q.; Lu, P.; Koleske, D. D.; Wierer, J. J.; Wang, G. T.;

Lauhon, L. J. Three-Dimensional Mapping of Quantum Wells in a GaN / InGaN. Nano Lett 2013, 13, 4317-4325. (11)

Wunderer, T.; Feneberg, M.; Lipski, F. Wang, J.; Leute, R. A. R.; Schwaiger, S.;

Thonke, K.; Chuvilin, A.; Kaiser, U.; Metzner, S.; Bertram, F.; Christen, J.; Beirne, G. J.; Jetter, M.; Michler, P.; Schade, L.; Vierheilig, C.; Schwarz, U. T.; Dra¨ger, A. D.; Hangleiter, A.; Scholz, F. Three-dimensional GaN for semipolar light emitters. Phys. Status Solidi B 2011, 248, 549-560. (12)

Hong, Y. J.; Lee, C.-H.; Yoon, A.; Kim, M.; Seong, H.-K.; Chung, H. J.; Sone, C.;

Park, Y. J.; Yi, G.-C. Visible‐Color‐Tunable Light‐Emitting Diodes. Adv. Mater 2011, 23, 3284–3288. (13)

Bae, S.-Y.; Kim, D.-H.; Lee, D.-S.; Lee, S.-J.; Baek, J. H. Highly integrated

InGaN/GaN semipolar micro-pyramid light-emitting diode arrays by confined selective area growth. Electrochem. Solid-State Lett. 2012, 15, H47. (14)

Choi, J. H.; Ahn, H. Y.; Lee, Y. S.; Park, K.; Kim, T.-H.; Cho, K. S.; Baik, C. W.;

Kim, S. I.; Yoo, H.; Lee, E. H.; Choi, B. L.; Kim, S.-D.; Kim, Y.-W.; Kim, M.; Hwang, S. GaN light-emitting diodes on glass substrates with enhanced electroluminescence. J. Mater. Chem.

2012, 22, 22942–22948.


Page 14 of 22

Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15)

Li, D. S.; Chen, H.; Yu, H. B.; Jia, H. Q.; Huang, Q.; Zhou, J. M. Dependence of

leakage current on dislocations in GaN-based light-emitting diodes. Appl. Phys. Lett. 2004, 96, 1111-1114. (16)

Lee, S. W.; Oh, D. C.; Goto, H.; Ha, J. S.; Lee, H. J.; Hanada, T.; Cho, M. W.; Yaoc,

T. Origin of forward leakage current in GaN-based light-emitting devices. Appl. Phys. Lett. 2006, 89, 132117-132117-3. (17)

Kim, J.; Tak, Y.; Kim, J.; Chae, S.; Kim, J.-Y.; Park, Y. Analysis of forward

tunneling current in InGaN/GaN multiple quantum well light emitting diodes grown on Si (111) substrate. Appl. Phys. Lett. 2013, 114, 013101 -013101-3. (18)

Shan, Q.; Meyaard, D. S.; Dai, Q.; Cho, J.; Schubert, E. F.; Son, J. K.; Sone, C.

Transport-mechanism analysis of the reverse leakage current in GaInN light-emitting diodes. Appl. Phys. Lett. 2011, 99, 253506-253506-3. (19)

Ra, Y.-H.; Navamathavan, R.; Kang, S.; Lee, C.-R. Different Characteristics of

InGaN/GaN Multiple Quantum Well Heterostructures Grown on m- and r-planes of a Single n-GaN Nanowire using Metalorganic Chemical Vapor Deposition. J. Mater. Chem. C 2014, 2, 2692-2701. (20)

Li, D. S.; Chen, H.; Yu, H. B.; Jia, H. Q.; Huang, Q.; Zhou, J. M. Dependence of

Leakage Current on Dislocations in GaN-Based Light-Emitting Diodes. J. Appl. Phys. 2004, 96, 1111-1114. (21)

Tchoe, Y.; Jo, J.; Kim, M.; Heo, J.; Yoo, G.; Sone, C.; Yi, G.-C. Variable-Color

Light-Emitting Diodes Using GaN Microdonut Arrays. Adv. Mater. 2014, 26, 3019-3023. (22)

Ko, Y.-H.; Song, J.; Leung, B.; Han, J.; Cho, Y.-H. Multi-Color Broadband Visible

Light Source via GaN Hexagonal Annular Structure. Sci. Rep. 2014, 4, 5514-5518. ACS Paragon Plus Environment


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(23)

Tanaka1, S.; Kawaguchi, Y.; Sawaki, N.; Hibino, M.; Hiramatsu, K. Defect structure

in selective area growth GaN pyramid on (111)Si substrate. Appl. Phys. Lett. 2000, 76, 2701. (24)

Wildeson, I. H.; Colby, R.; Ewoldt, D. A.; Liang, Z.; Zakharov, D. N.; Zaluzec, N. J.;

García, R. E.; Stach, E. A.; Sands. T. D. III-Nitride Nanopyramid Light Emitting Diodes Grown by Organometallic Vapor Phase Epitaxy. J. Appl. Phys. 2010, 108, 044303-8. (25)

Chang, S.-P.; Chang, J.-R.; Sou, K.-P.; Liu, M.-C.; Cheng, Y.-J.; Kuo, H.-C.; Chang,

C.-Y. Electrically Driven Green, Olivine, and Amber Color Nanopyramid Light Emitting Diodes. Opt. Express 2013, 21, 23030-23035. (26)

Tseng, P.-H.; Hwu, J.-G. Corner Induced Non-Uniform Electric Field Effect on the

Electrical Reliability of Metal–Oxide–Semiconductor Devices with Non-Planar Substrates. Solid-State Electron. 2014, 91, 100-105.


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Figure 1. Schematic illustration and electrical property of 3D pyramidal-shaped LED arrays. (a) (upper) Schematic illustration of 3D pyramidal-shaped LED structure with InGaN/GaN MQWs. (middle) A schematic and SEM images of the LEDs. (bottom) A schematic and SEM images of the LEDs with ITO electrodes. (b) I–V curve of the LED arrays as a function of applied bias voltage (Inset) show the photographs of electroluminescence under 20 mA and at various bias voltage levels from 5 to 10 V



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Figure 2. The cross-sectional TEM images of a LED. (a) Cross-sectional low-magnification image of single LED covered by MQWs. High-magnification TEM images of MQWs at the sidewall (b), n-GaN template (c), p-GaN near the tip region (d), and p-GaN at the sidewall (e). The insets in (c), (d), and (e) show the corresponding SAED patterns. Fourier filtered images were also obtained and they show that p-GaN near the tip region has a few stacking faults. The Fourier filtering image toward the [0002] direction.


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Figure 3. Fabrication process of 3D pyramidal-shaped LEDs and the electrical properties. (a) New fabrication process of 3D pyramidal-shaped LEDs. The MgO layer is formed on the tip region, producing MgO-passivated LEDs. The MgO layer is removed to fabricate the tip-open structured LEDs. (b) SEM images of the two 3D pyramidal-shaped LEDs. (Upper) MgOpassivated LEDs and (bottom) Tip-open structured LEDs. The scale bars are 2 µm in the left images and 10 µm in the right images. (c) I–V curves of the LEDs as a function of the applied bias voltage plotted in both linear plot (upper panel) and logarithmic plot (bottom panel).



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Figure 4. (a) CSEM images and the normalized EL intensity of (a) Conventional LEDs, (b) MgO-passivated LEDs, and (c) Tip-open structured LEDs. The schematic and SEM images of the LED are also shown.


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Figure 5. (a) Normalized EL intensity as a function of wavelength under 30mA injection currents of the conventional LEDs, MgO-passivated LEDs, and Tip-open structured LEDs. The inset shows the corresponding light emission. (b) Electroluminescence images of conventional, MgO-passivated, and tip-open structured LEDs at various wavelengths 490 to 600 nm.



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Table of contents (TOC)


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Visible Color Tunable Emission in Three-Dimensional Light Emitting

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