Improvement of the optical and electrical performance of GaN-based

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Improvement of the optical and electrical performance of GaNbased LEDs using transferrable ZTO microsphere monolayer Taek Gon Kim, Seong-Jin Park, Dohyun Kim, Dong Su Shin, and Jinsub Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01736 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Sustainable Chemistry & Engineering

Improvement of the optical and electrical performance of GaN-based LEDs using transferrable ZTO microsphere monolayer Taek Gon Kima, Seong-Jin Parkb, Dohyun Kima, Dong Su Shina, Jinsub Parka,b,c,* a

Department of Electronics and Computer Engineering, Hanyang University, 222 Wangsimni-ro

Seongdong-gu Seoul, 04763 South Korea b

Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro Seongdong-gu,

Seoul, 04763 South Korea b

Department of Electronic Engineering, Hanyang University, 222 Wangsimni-ro Seongdong-gu,

Seoul, 04763 South Korea Keyword Fast synthesis of ZTO, Microspheres, ZTO monolayer, Light extraction

AUTHOR INFORMATION

Corresponding Author *Jinsub Park. E-mail : [email protected]

ACS Paragon Plus Environment

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ABSTRACT

We report on the formation of transferable microspheres monolayers consisting of ZnSnO3 (ZTO) synthesized by fast ethanol precipitation and their application to GaN-based light-emitting diodes (LEDs) to improve device performance. ZTO microspheres with diameters of 820±20 nm (ZTO-1), 910±10 nm (ZTO-2), and 1200±10 nm (ZTO-3)were synthesized and arrayed to form a monolayer using polydimethylsiloxane (PDMS) and a unidirectional rubbing method. ZTO-1, ZTO-2, and ZTO-3 monolayers exhibited optical transmittance percentages of 94.4, 92.98, and 87.09% at 450 nm, respectively. Effects of ZTO monolayers on the light extraction efficiency of LEDs were evaluated using LED chips sized 800 x 800 µm2. LEDs with a ZTO-3 monolayer as a top-layer had a light extraction efficiency of about 144% relative to regular LEDs as well as improved electrical properties as evidenced by a decrease in Vturn on of ~0.6 V and a decrease in Ron by 0.036 kΩ. XPS analysis indicated that the improvement in the electrical properties of LEDs with ZTO monolayers is due to the presence of graphene-like carbon bonds present in carbon residue that remains on the surface of the LED after removal of polymer. In terms of light extraction, ZTO monolayers effectively guided light generation from active regions in LEDs towards air, which was confirmed by 3-dimensional finite-difference time-domain simulations.

INTRODUCTION Metal oxides (MOs) are the most abundant materials in the earth’s crust and have many advantages compared to conventional semiconductors such as silicon and III-V compounds with respect to materials design, electronic band structures, charge transport mechanisms, defect states, and optoelectronic properties1. Recently, MOs have been in the spotlight because of their


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versatile applications in transparent conducting oxides, thin film transistors (TFTs), photocatalysts, lithium-ion batteries, solar-cells, memory devices, and portable sensors2,3,4,5,6,7. Among multifunctional MOs, shape-controllable ZnSnO3 (zinc tin oxide, ZTO) nanomaterials such as nanowires, nanocubes, and hexagonal, polyhedral, hollow spherical, and hierarchical nanocages have been extensively investigated as new types of high gas-sensitive materials and photocatalysis materials8,9,10,11,12. ZTO has a higher carrier mobility than ZnO because of its Sn content, and has therefore been used as an active layer in high performance TFTs 13 . More interestingly, ZTO has optical properties that can be exploited to enhance light extraction from light emitting diodes (LEDs), namely a wide band gap (~3.6 eV), transmittance (~90%) in the visible region, and medium refractive index (~2.01) between that of GaN and air14,15,16. Among the various ZTO nanomaterial shapes, spheres have several advantages over the other shapes with regard to the formation of arrayed structures that can generally undergo surface modification to enhance the performance of optoelectronic devices17,18. Several research groups have performed surface treatment of substrate to form well arrayed monolayers with uniformity because of the high sensitivity of the formed nanospheres to hydrophilic and hydrophobic surface states. 19 , 20 To form a dense monolayer, complicated electrochemical deposition or additional high-pressure processes are needed after spin coating of nanospheres21,22. In this study, we describe the facile synthesis of ZTO microspheres and a monolayer formation method to transfer the formed ZTO monolayers onto GaN-based LED devices to improve their electrical and optical device performance. A unidirectional rubbing method using polydimethylsiloxane (PDMS) facilitated formation of a transferable ZTO monolayer 23 . Our ZTO microspheres monolayers are an excellent way to improve the performance of LEDs, in particular their light extraction and electrical properties.


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EXPERIMENTAL DETAILS For the fast synthesis of ZTO microspheres, we used an ethanol precipitation method. First, we synthesized ZnSn(OH)6 (zinc hydroxystannate, ZHS) microspheres by dissolving 0.658 g of Zn(CH3COO) 2H2O (zinc acetate) in 40 mL of solvent with magnetic stirring (Solution A). Then, 1.052 g of SnCl4 (tin chloride (IV)) was dissolved in 30 mL of solvent with magnetic stirring (Solution B) and 1.200 g of NaOH (sodium hydroxide) was dissolved in 30 mL of solvent (Solution C). The solvent consisted of deionized (DI) water and absolute grade ethanol at a 1:1 volume ratio. Solutions A and B were mixed together in a 250 mL round bottom flask at room temperature for 10 minutes with ultrasonic mixing and magnetic stirring. Solution C was then added dropwise to the flask, which was then allowed to sit for 30 minutes. Finally, ZHS microspheres were collected by centrifugation and washed with DI water and ethanol. ZHS were then dried in a vacuum at room temperature. The obtained ZHS microspheres were converted to ZTO microspheres through thermal treatment at 400ºC for 2 hours. Figure 1 shows a schematic of the steps used to transfer a ZTO monolayer onto GaN-based blue LED chips. LED chip manufacturing was performed prior to transferring the ZTO monolayer24,25 . Ga-polar surfaces of as-grown LED wafers were partially etched by multiplex inductively coupled plasma (ICP, STS) until the n-type GaN layer was exposed. Indium tin oxide (ITO, 20nm) was evaporated to surface of p-GaN for current spreading. Cr/Au electrodes (10/500 nm) were subsequently evaporated as both n- and p-type electrodes for contact. Finally, LED wafers were divided into chips with an area of 800 x 800 µm2. To form a transferrable ZTO spheres monolayer, we prepared the two PDMS (curing agent 10 wt%) pads. ZTO microspheres were loaded onto one PDMS pad. Then, the ZTO microspheres monolayers were formed by


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unidirectional rubbing using the other PDMS pad. Then, polyvinyl alcohol (PVA (87~89% hydrolyzed) solution at 10 wt% was coated on the surface of the LED chips using a spin coating method, and ZTO/PDMS pads were then placed on the LED chips. After aging for 12 hours under atmospheric conditions to cure the PVA solution, the PDMS pad was peeled from the LED chips. ZTO/PVA/LEDs structures were annealed at 500oC for 4 hours to evaporate the PVA. Structural properties of the synthesized ZHS and ZTO microspheres were investigated by scanning electron microscopy (SEM), and an X-ray diffraction (XRD) system (miniFlex600) equipped with Cu-Kα radiation and a Raman spectrometer (NRS-3100) with a 532 nm laser. Optical properties of the ZTO monolayers were analyzed by UV-Vis spectroscopy (Lambda 650S). To confirm the presence of carbon residue produced through the PVA evaporation process on the surfaces of chips, X-ray photoelectron spectroscopy (XPS, Theta probe) analysis was conducted. To evaluate the light extraction and electrical properties of the LEDs with ZTO monolayers, light-power-current-voltage (L-I-V) measurements were conducted.

RESULT AND DISSCUSSION SEM images of the ZHS microspheres synthesized using different solvents (50, 55, and 60 vol% ethanol) are shown in Fig. 2(a), (b), and (c), respectively. All ZHS microspheres had a rough surface and their sizes were well controlled with diameters of 890±20 nm, 1010±10 nm, 1310±10 nm, respectively. Interestingly, the size of the ZHS microspheres increased with an increase in ethanol concentration. Upon dropping Solution C into the mixture of Solutions A and B, the primary particles nucleated by aggregation with small subunits. This indicated that Ostwald ripening, where primary particles adsorb to each other to reduce surface energy and


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grow into larger particles, took place. However, when more Solution C was dropped into the mixture of Solutions A and B, a significant amount of OH- ions were incorporated in the ZHS spheres. Ultimately, the edges of the ZHS were etched by OH- ions, resulting in microspheres with a rough surface 26. The formation of ZHS microspheres can be explained by Eqs. (1)-(2): + + 6 → (1) + 4 →

+ (2)

During ethanol precipitation, the solvent can affect particle growth after nucleation, because particle interaction potentials differ in different solvents. As the ethanol ratio in the alcohol/water mixed solvent increased, the formation of stable ionic bonds increased, resulting in an increase in particle size. This phenomenon is related to Coulomb’s law (3): =

(3)

where q1 and q2 are the signed magnitudes of the charges, the scalar r is the distance between the charges, and εr is the dielectric constant. The dielectric constant of the solvent decreased as the ethanol ratio in the alcohol/water mixed solvent increased. The denominator part of equation (3) increased with a decrease in the dielectric constant of the solvent, indicating that the electrostatic forces between charged particles increased27. To obtain ZTO microspheres, as-synthesized ZHS microspheres were annealed. The phasechange of ZHS to ZTO was confirmed by XRD measurements and Raman spectra. Figure 3(a) shows the XRD spectra of the ZHS and ZTO microspheres. The XRD patterns of the assynthesized ZHS microspheres were in good agreement with that of ZHS crystals (JCPDS no. 74-1825), and are indexed by the black line in Fig. 3(a) 28 . This indicated that the ZHS microspheres synthesized by ethanol precipitation had no additional impurities due to


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stoichiometrically controlled co-precipitation of Zn2+ and Sn2+ in excess alkaline solvent. For the ZTO microspheres, there was no strong peak as in the XRD pattern of the crystal phase, as shown by the blue line in Fig. 3(a). Consistent with our results, a previous study reported ZTO with a broadened, weak XRD peak, which is referred to as amorphous ZTO29. Black and red lines in Figure 3(b) indicate the Raman spectra of ZHS and ZTO, respectively. The Raman spectra of ZHS included four Raman modes for ZHS at 298, 380, 435, and 603 cm -1 arising from the breathing vibration of long M-OH (M: Zn or Sn) bonds and M-OH-M bending modes. After annealing, Raman modes shifted to two new peaks at 540 and 671 cm-1. Raman analysis results confirmed phase-change of the amorphous ZTO structure through an annealing process as reported by other groups30,31. Based on the XRD and Raman results, we confirmed that pure ZHS microspheres were synthesized by ethanol precipitation and that these were converted to ZTO microspheres through annealing. To gain insight into the effects of annealing of ZHS on the morphology and size of the resulting ZTO structures, we performed SEM analysis. Figures 4(a), (b), and (c) show crosssectional SEM images of monolayers consisting of ZTO microspheres with particle diameters of 800 nm (ZTO-1), 900 nm (ZTO-2), and 1.2 µm (ZTO-3), respectively. After annealing, the ZTO microspheres shrunk by about ~100 nm, which is described in Eq. (4):

!!" # + 3 (4) The size of all microspheres was reduced by annealing due to the evaporation of H2O, but the surface of the microspheres was still rough and did not show drastic changes. Top views of ZTO monolayers with different diameter ZTO microspheres are shown in Fig. 4(d), (e), and (f). A well-arrayed ZTO microspheres monolayer formed on PDMS due to unidirectional rubbing.


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The transmittance of ZTO monolayers is very important if these monolayers are to be used as the light extraction layer of LEDs. Therefore, we investigated the optical properties of ZTO monolayers using UV-Vis measurements. To evaluate the transmittance of the ZTO monolayers, each type of ZTO monolayer was transferred to double-sided polished sapphire substrates. Figure 5(a) shows the transmittance spectra of ZTO-1 (black line), ZTO-2 (red line), and ZTO-3 (blue line) monolayers. As the diameter of the ZTO microspheres increased, the transmittance of the ZTO microsphere monolayer decreased. Nevertheless, the transmittances of ZTO-1, ZTO-2, and ZTO-3 monolayers were 94.43, 92.98, and 87.09% at 450 nm, respectively. In addition, we calculated the optical band gap of the ZTO monolayers from Tauc plots.32. Based on Eq. (5), αℎ'(/was plotted against hv to obtain the optical band gap (see Fig. 5(b)):

αℎ' = *ℎ' − ,- (5) Here . is an absorption coefficient and . = 2.303∗*/thickness. A is the absorbance and A = log T, where T is transmittance. The calculated optical band gaps for ZTO-1, ZTO-2, and ZTO-3 monolayers were 3.55, 3.54, and 3.57 eV, respectively. The high transparency in the visible region and the wide band gap indicate that ZTO monolayers can potentially be used as light extraction layers in GaN-based blue LEDs. To investigate the effects of ZTO-1, ZTO-2 and ZTO-3 monolayers transferred to LED chips on light extraction efficiency (LEE), L-I-V measurements were performed. The injection current range of LED chips for the evaluation of LEE was 0 to 80 mA, and results are shown in Fig. 6(a). Current–voltage (I-V) curves from Fig. 6(a) show that the ZTO monolayers improved the diode characteristic of LEDs. For LEDs with ZTO monolayers, VTurn on decreased by about 0.6 V, and the Ron of LEDs with ZTO-1, ZTO-2, and ZTO-3 monolayers decreased by 0.024, 0.03, and 0.036 kΩ, respectively. This indicates that a ZTO monolayer can simultaneously enhance the


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light extraction and electrical properties of LED chips. To quantify the performance of the LEDs, output power was divided by injection power (I x V). LEDs with ZTO-1, ZTO-2, and ZTO-3 monolayers showed a 118, 131, and 144% improvement in performance compared to a conventional LED operated at 80 mA as shown in Fig. 6(b). Although a slight decrease in transmittance occurred in ZTO-3/LEDs relative to the other ZTO/LEDs, LED chips with a ZTO3 monolayer showed the highest device performance. Optical microscopy images of emitted light obtained from LED chips with different sized ZTO monolayers are shown in Fig. 6(c). LED chips with a ZTO-3 monolayer had the brightest emission intensity. This indicates that ZTO microsphere monolayers can dramatically improve the electrical and optical properties of LED chips. We attributed the improvement in current spreading, which increased light extraction of LEDs, to residues of carbon on the ZTO microspheres causing a decrease in surface resistivity. To determine why the electrical properties of the devices improved when a ZTO monolayer was used, we focused on changes in surface properties. PVA solution was used as an adhesive material when the ZTO monolayer was transferred to the LED chip surface. After transfer of the ZTO monolayer onto the surface of the LED chip, PVA was removed because of its blocking effect on light. During the PVA removal process, carbon residues remained on the surface. The carbon residues not only made the ZTO monolayer stick strongly to the surface of the LED chip, but also connected ZTO microspheres16. To investigate the surface state after annealing to remove PVA, XPS measurements were performed. Figures 7(a), (b), and (c) show the XPS spectra recorded from ZTO-1, ZTO-2 and ZTO-3 monolayers after annealing, respectively. The C 1s spectrum consisted of three peaks related to C-C, C-O-C, and O-C=O bonds. Among the three carbon bonds, C-C bonding, such as graphene-like bonding, improves the conductivity of


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metal oxide layers 33 . The XPS spectra we obtained were dominated by graphene-like C-C bonding on the surface, which can improved the electrical properties of the LEDs. To design an adequate ZTO monolayer, various parameters such as refractive index, lattice constant, and the size of spheres should be considered. Among the parameters described here, we paid special attention to the effect of the size of the spheres on the LEE of LEDs. To evaluate LEE as a function of the size difference of spheres, we used 3-D FDTD simulations. Figure 8 compares cross sections of the power field for general planar LED and LEDs with ZTO sphere monolayers comprising differently sized spheres. It is apparent from Fig. 8(a) that emitted light was trapped in LED chips due to an abrupt change in the refractive index of p-GaN and air. However, when a ZTO monolayer was present on the LED chips, more light was extracted from the LEDs through the ZTO microspheres, as shown in Fig. 8(b), (c), and (d). In particular, the ZTO-3 monolayer allowed greater extraction of light from the LEDs compared with the other ZTO monolayers. For more detailed analysis, we investigated the escape angles of LEDs with ZTO monolayers using a far-field monitor and 3-D FDTD simulations. Light emission of general planar LEDs was confirmed to be between 10 and 20 degrees, as shown in Fig. 9(a). Figures 9(b), (c), and (d) show that as the size of ZTO spheres in the ZTO monolayers increased, so did the light-emission intensity, and the escape angle became more vertical. Furthermore, LEDs with a ZTO-1 monolayer exhibited a light-emission intensity 1.18-fold higher than that of LEDs without any structures, while LEDs with ZTO-2 and ZTO-3 monolayers showed a 1.30-fold and 1.36-fold enhancement over the reference LEDs, respectively.

CONCLUSIONS


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We described a method for rapid synthesis of various sized ZTO microspheres by ethanol precipitation and transfer of these ZTO microspheres to form a monolayer. After transfer of ZTO monolayers to LED chips, PVA removal resulted in carbon residue with graphene-like c-c bonds on the LED surface, as confirmed by XPS. ZTO monolayers with carbon residues enhanced light extraction and current spreading on the surface of GaN-based blue LEDs. In particular, the ZTO3 monolayer consisting of 1.2 µm diameter ZTO microspheres improved the LEE of the LEDs by about 144% compared to the reference sample. A possible mechanism for the improvement in LEE suggested by 3-D FDTD simulations was the more vertical direction of light extraction of light trapped inside the LEDs by the ZTO spheres. Transfer of ZTO microsphere monolayers onto the surfaces of LEDs using the method described here is a very effective way to improve light extraction from LEDs.


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FIGURES Figure 1. Schematic diagram of the process used to fabricate LED chips with ZTO monolayers. Figure 2. SEM images of ZHS microspheres synthesized with (a) 50, (b) 55, and (c) 60 vol% ethanol. Figure 3. (a) XRD and (b) Raman spectra of ZHS and ZTO microspheres. Figure 4. SEM images of the cross-sections of (a) ZTO-1, (b) ZTO-2, and (c) ZTO-3 monolayers. SEM images of the top view of (d) ZTO-1, (e) ZTO-2, and (f) ZTO-3 monolayers. Figure 5. (a) Transmittance spectra of ZTO-1, ZTO-2, and ZTO-3 monolayers. (b) Tauc plot to calculate the optical band gap of ZTO monolayers with different diameter spheres. Figure 6. (a) L-I-V curves of general LED chips and LED chips with ZTO-1, ZTO-2, or ZTO-3 monolayers. (b) Efficiency enhancement rate calculation graph of LED chips with different ZTO monolayers. (c) Comparison of the emission of LED chips with different ZTO monolayers. Figure 7. XPS spectra (C 1s) of (a) ZTO-1, (b) ZTO-2, and (c) ZTO-3 monolayers. Figure 8. Cross-section of the power field of (a) general LEDs and LEDs with (b) ZTO-1, (c) ZTO-2, or (d) ZTO-3 monolayers based on FDTD simulations. Figure 9. Far-field intensity of (a) general LED and LEDs with (b) ZTO-1, (c) ZTO-2,or (d) ZTO-3 monolayers. (e) Light extraction enhancement of LEDs with different ZTO monolayers based on FDTD simulations.

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2018R1D1A1B07048382).


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(30) Chen, Y.; Yu, L.; Li, Q.; Wu, Y.; Li, Q.; Wang, T. An evolution from 3D face-centeredcubic ZnSnO3 nanocubes to 2D orthorhombic ZnSnO3 nanosheets with excellent gas sensing performance. Nanotechnology 2012, 23, 415501-415510. (31) Wu, J. M.; Chen, Y. N. The surface plasmon resonance effect on the enhancement of photodegradation activity by Au/ZnSn(OH)6 nanocubes. Dalton Trans. 2015, 44, 16294-16303. (32) Eperon, G. E.; Leijtens, T.; Bush, K. A.; Prasanna, R.; Green, T.; Wang, J. T.; Mcmeekin, D. P.; Volonakis G.; Milot R. L.; May R.; Palmstrom A.; Slorcavage D. J.; Belisle, R. A.; Patel, J. B.; Parrott, E. S.; Sutton, R. J.; Ma, W.; Moghadam, F.; Conings, B.; Babayigit, A.; Boyen, H.; Bent, S.; Giustino, F.; Herz, L. M.; Johnston, M. B.; McGehee, M. D.; Snaith, H. J. Perovskite-perovskite tandem photovoltaics with optimized band gaps. Science 2016, 354(6314), 861-865. (33) Johra, F. T.; Lee, J. W.; Jung, W. G. Facile and safe graphene preparation on solution based platform. J. Ind. Eng. Chem. 2014, 20(5), 2883–2887.

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We describe the facile synthesis of ZTO microspheres and a monolayer formation method to transfer the formed ZTO monolayers onto GaN-based LED devices to improve their electrical and optical performance.


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Figure 1. Schematic diagram of the process used to fabricate LED chips with ZTO monolayers. 326x169mm (150 x 150 DPI)


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Figure 2. SEM images of ZHS microspheres synthesized with (a) 50, (b) 55, and (c) 60 vol% ethanol. 307x76mm (150 x 150 DPI)



Figure 3. (a) XRD and (b) Raman spectra of ZHS and ZTO microspheres. 364x135mm (150 x 150 DPI)


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Figure 4. SEM images of the cross-sections of (a) ZTO-1, (b) ZTO-2, and (c) ZTO-3 monolayers. SEM images of the top view of (d) ZTO-1, (e) ZTO-2, and (f) ZTO-3 monolayers 305x153mm (150 x 150 DPI)



Figure 5. (a) Transmittance spectra of ZTO-1, ZTO-2, and ZTO-3 monolayers. (b) Tauc plot to calculate the optical band gap of ZTO monolayers with different diameter spheres. 353x137mm (150 x 150 DPI)


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Figure 6. (a) L-I-V curves of general LED chips and LED chips with ZTO-1, ZTO-2, or ZTO-3 monolayers. (b) Efficiency enhancement rate calculation graph of LED chips with different ZTO monolayers. (c) Comparison of the emission of LED chips with different ZTO monolayers. 236x148mm (150 x 150 DPI)



Figure 7. XPS spectra (C 1s) of (a) ZTO-1, (b) ZTO-2, and (c) ZTO-3 monolayers. 353x92mm (150 x 150 DPI)


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Figure 8. Cross-section of the power field of (a) general LEDs and LEDs with (b) ZTO-1, (c) ZTO-2, or (d) ZTO-3 monolayers based on FDTD simulations. 330x148mm (150 x 150 DPI)



Figure 9. Far-field intensity of (a) general LED and LEDs with (b) ZTO-1, (c) ZTO-2,or (d) ZTO-3 monolayers. (e) Light extraction enhancement of LEDs with different ZTO monolayers based on FDTD simulations. 173x280mm (150 x 150 DPI)


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Graphical Abstract 434x172mm (150 x 150 DPI)


Improvement of the optical and electrical performance of GaN-based

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