Microstructured Air Cavities as High-Index Contrast ... - ACS Publications

Apr 5, 2016 - Department of Physics, KAIST, Daejeon 34141, Republic of Korea ... CESs reveal that the high-index-contrast air/alumina core/shell patte...
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Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes Yoon-Jong Moon,† Daeyoung Moon,‡ Jeonghwan Jang,‡ Jin-Young Na,† Jung-Hwan Song,§ Min-Kyo Seo,§ Sunghee Kim,∥ Dukkyu Bae,∥,⊥ Eun Hyun Park,# Yongjo Park,‡,⊥ Sun-Kyung Kim,*,† and Euijoon Yoon*,‡,⊥,∇,○ †

Department of Applied Physics, Kyung Hee University, Gyeonggi-do 17104, Republic of Korea Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea § Department of Physics, KAIST, Daejeon 34141, Republic of Korea ∥ Hexa Solution Co., Ltd., Gyeonggi-do 16229, Republic of Korea ⊥ Energy Semiconductor Research Center, Advanced Institutes of Convergence Technology, Seoul National University, Gyeonggi-do 16229, Republic of Korea # Semicon Light Co., Ltd., Gyeonggi-do 17086, Republic of Korea ∇ Research Institute of Advanced Materials and ○Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-indexcontrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm2) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light. KEYWORDS: Light-emitting diodes, hollow cavities, high-index-contrast gratings, III-nitride compounds, FDTD simulation

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For example, submicron-thick vertical InGaN/GaN LED devices with 2D periodic GaN-etched patterns have produced an 80% extraction efficiency, which results from their enhanced diffraction strength combined with a reduction in the number of available guided modes.10 With the exception of vertical-geometry devices, most lateraland flip-chip-geometry GaN-based LED devices, which occupy a major portion of the current LED market, have adopted few-

wo-dimensional (2D) dielectric gratings, which are generally characterized by their refractive index contrast and pitch, have been integrated as reflecting or antireflecting elements in photonic applications including semiconductor lasers and solar cells.1−5 For light-emitting diodes (LEDs), rationally designed 2D dielectric gratings diffract optical modes trapped in high refractive index active media, thereby enhancing the outcoupling efficiency.6−14 In the case of GaN-based LEDs, 2D dielectric gratings are incorporated into the upper GaN surface8−10 or into the interface between the GaN medium and the growth substrate,6,7,11−14 depending on the diode configuration (i.e., vertical, lateral, or flip-chip geometries15). © XXXX American Chemical Society

Received: March 1, 2016 Revised: April 1, 2016

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Figure 1. Fabrication and optical characterization of high-index-contrast CES growth templates. (a) Camera image of a two-inch GaN/CES wafer. (b,c) Cross-section SEM image showing cylindrical (panel b) and hemispherical (panel c) CES templates. Scale bar, 1 μm. (Inset) SEM image magnifying an inner air cavity covered with a thin alumina shell; scale bar, 1 μm. (d) Schematic of a hemispherical CES template. Inset: schematic magnifying an inner air cavity covered with a thin alumina shell. (e) Confocal laser microscopy images of GaN/CES (top) and GaN/PSS (bottom) structures. Scale bar, 10 μm. (f) Transmission spectra for GaN/CES and GaN/PSS structures with the same pitch of 3 μm. (Inset) Schematics of measured structures.

micron-pitch patterned sapphire substrates (PSSs).16 PSSs serve as 2D dielectric gratings for improved light extraction as well as growth templates for pristine GaN crystallinity.6,7 However, breakthroughs in light extraction are limited because the refractive index contrast between GaN and sapphire is predetermined. In attempts to overcome this refractive index barrier, groups have proposed embedding patterned air gaps into GaN11−13 or using hollow silica nanosphere substrates.14 Although these high-index-contrast patterns may induce strong diffraction of guided modes and relatively high extraction efficiencies, additional fabrication steps (e.g., etching and regrowth processes) must be required to incorporate highindex-contrast patterns during GaN epitaxial growth. This could degrade the electrical characteristics (e.g., the leakage currents and operation voltages) and increase the fabrication cost. Furthermore, existing methods limit the structural control of patterns for tailored light extraction. Here, we report a new class of strong-diffraction, high-indexcontrast cavity engineered substrates (CESs) that act as monolithic growth templates to overcome the limitations typically encountered with patterned substrates. A standard photolithography process in conjunction with a simple atomic layer deposition (ALD) technique enables fabrication of well-

designed CESs in which hexagonally arranged air cavities are covered with a thin alumina shell are placed on a sapphire substrate. To assess the impact of these high-index-contrast CESs, measurements of the diffraction strength with respect to the diffraction order were carried out. Additionally, InGaN/ GaN LED devices were fabricated on CES and PSS growth templates and the device performances were compared. The diffraction features of CESs were distinct from the behaviors in analogous PSSs. This suggests that CESs are promising diffracting elements that have the potential to be exploited in organic LEDs, solar cell devices, and the III-nitride semiconductor LEDs developed herein. CES templates were fabricated following literature procedures by defining the photoresist pattern with an i-line stepper, depositing alumina layer with an ALD system, oxidizing the photoresist, and then crystallizing alumina shell in a hot furnace (Supporting Information Figure S1a).17,18 The pitch and diameter of the CES templates were precisely controlled during the photolithography process, and the shape and volume of the inner air cavities were readily tuned by modifying the thermal reflow conditions (Supporting Information Figure S1b−d). A fabricated two-inch CES template (Figure 1a) includes a 2D hexagonal array of cylindrical (Figure 1b) or B

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Figure 2. Diffraction analysis of CES and PSS patterns. (a) Integrated (λ = 380−800 nm) and wavelength-resolved diffraction profiles for GaN/CES and GaN/PSS structures. Labeled profiles correspond to the transmission data labeled identically within Figure 1f. (b,c) Experimental (panel b) and calculated (panel c) wavelength-resolved diffraction strength with respect to the diffraction order (m = 1−5) for GaN/CES and GaN/PSS structures. For the first-order diffraction (m = 1), the higher intensity between the GaN/CES and GaN/PSS structures is defined as unity for the considered wavelengths. Inset (panel b): schematic illustrating the diffraction order (m). (d) Plots of the scattered fields of single CES and PSS hemispherical objects. These were acquired between λ = 400 and 800 nm with a step size of 50 nm. For the CES object, the inner air cavity was covered with an 80 nm thick alumina shell.

GaN/CES is highly structured; several high-contrast peaks assigned to successive Fabry−Perot modes are observed, despite the presence of embedded air cavities. Second, the GaN/CES exhibits higher transmittance than the GaN/PSS in the range of wavelengths that we considered (λ = 380−800 nm). The latter feature is quite remarkable when viewed by effective medium approximations in which the CES pattern is regarded as a low effective index homogeneous layer; this should lead to strong reflection. These results indicate that the optical features resulting from the few-micron-pitch dielectric gratings must be accounted for by wave optics. The wave optics aspects of the CES and PSS patterns are more apparent in the wavelength-resolved transmission profiles for normally incident light, which were obtained by using a home-built far-field scanner (Supporting Information Figure S2).21 The measured transmittance profiles highlight the different diffraction features between the CES and PSS patterns (Figure 2a). Both GaN/CES and GaN/PSS exhibit 6-fold symmetry diffraction spots that are further spread at longer wavelengths. Significantly, the centered spot (i.e., zeroth-order diffraction) and the nearest six spots (i.e., the first-order diffraction) are more visible for the CES for the wavelengths that we considered. The strong zeroth-order diffraction in the CES accounts for the pronounced Fabry−Perot modes in the transmittance spectrum (Figure 1f).

hemispherical (Figure 1c) air cavities with conformal alumina shells depending on the thermal treatment condition. A crosssection scanning electron microscopy (SEM) image of a single CES object clearly shows a well-defined air cavity that is sustained by an 80 nm thick alumina shell (inset, Figure 1c), which is also illustrated in Figure 1d. The alumina shell was crystallized into the α-phase by solid-phase epitaxy during the thermal annealing process, which permits monolithic GaN growth.17 A standard blue-emitting InGaN/GaN LED epitaxial growth recipe was adopted, using a metal−organic chemical vapor deposition (MOCVD) process for CES and PSS templates with the same pitch of 3 μm.19 To minimize unwanted run-to-run variations, LED structures on both templates were grown simultaneously in the same MOCVD chamber. To delineate the optical features of GaN-grown CES and PSS structures, we used high-resolution confocal laser microscopy to image the tops of samples. Comparison of the acquired brightfield images reveals that the GaN/CES exhibits a relatively low image contrast, which is indicative of an increase in the transmittance relative to the GaN/PSS (Figure 1e). To accurately evaluate the transmittance of both structures, we used an integrating sphere setup in which the wavelength of incident light was scanned through a monochromator.20,21 The measured transmittance spectra reveal key optical characteristics distinct to the CES (Figure 1f). First, the spectrum of the C

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Figure 3. Device characteristics of InGaN/GaN LEDs fabricated on CES and PSS growth templates. (a) Representative optical power−current (L− I) characteristics of fabricated lateral-geometry CES and PSS LED devices with the same dominant wavelength of 468 nm. Top inset: plot of the current versus voltage (I−V) curves for the same devices. Bottom inset: optical microscopy image showing the near-field emission profiles of the lateral-geometry GaN/PSS LED device operated at 10 mA. (b) Integrated far-field profiles of flip-chip-geometry CES and PSS LED devices. (c) Average far-field intensity as a function of the polar angle (θ) for the flip-chip-geometry CES and PSS LED devices. Inset: plot of far-field intensity integrated within a span angle (Δ2θ) for the same devices. (d) Wavelength-resolved far-field profiles acquired at λ = 444 (435), 454 (445), and 463 (455) nm for the flip-chip-geometry CES (PSS) LED device.

stronger than the scattered light from the PSS object over the wavelengths that we considered (except at λ ∼ 600 nm). Provided that the near-field coupling between adjacent objects is very weak, the scattered light from each single object acts as an individual secondary wave source in the periodic CES and PSS patterns. Taken together, the diffraction measurements and analysis suggest that the index contrast in 2D dielectric gratings determines the diffraction strength with respect to the diffraction order. More importantly, the high-index-contrast CES preferentially excites the first-order diffraction, which enables an efficient outcoupling structure available for LED devices. The excitation of the first-order diffraction encourages efficient extraction of various high-order guided modes, which carry much of the light trapped within LED structures.24,25 The practical importance of strong-diffraction CES patterns was established by fabrication of large-area (1075 × 750 μm2) blue-emitting lateral-geometry InGaN/GaN LED devices.26 For fair comparison, these devices incorporated CES and PSS growth templates with the same pitch of 3 μm. The crosssection SEM images of the fabricated LED structures show that the CES and PSS patterns had no substantial differences in object volume; only their overall shape was different (Supporting Information Figure S5). For the CES (PSS) object, the diameter and height were 2.4 (2.7) and 1.5 (1.7) μm, respectively. Both LED devices were mounted on the same package and a standard integrating sphere measurement setup

The wavelength-resolved diffraction strengths of the GaN/ CES and GaN/PSS were quantitatively assessed and compared with respect to the diffraction order (m) (Figure 2b and Supporting Information Figure S3). For both structures, the diffraction strength drops off radically with increasing m. Notably, the CES excites the first-order diffraction (m = 1) much more strongly than the PSS, except at λ ∼ 600 nm. These aspects are clearly visualized by constructing the transmission profiles (Supporting Information Figure S4a) and plotting the diffraction strength (Supporting Information Figure S4b) on a logarithmic scale; for the CES, clean and sharp spots assigned to the first to second-order diffraction modes are observed in the transmission profiles. The measured wavelength-resolved diffraction strength was successfully reproduced by conducting Fourier transform simulations (Figure 2c and Supporting Information Figure S4c).22 For the first-order diffraction (m = 1), the CES significantly outperforms the PSS, which is consistent with the measurements. The small discrepancy between the simulation and measurement data is attributed to the finite angle step of the far-field scanner (1° for azimuthal and polar angles). The origin of the differences in the diffraction strength can be understood by comparing the interactions of the normal plane wave with single CES and PSS objects (Figure 2d).23 The plots of the electrical field intensity acquired at different wavelengths show that the scattered light from the CES object is much D

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Figure 4. Full-vectorial electromagnetic simulations of CES and PSS LEDs. (a) Simulated extraction efficiencies of lateral-geometry GaN/CES LEDs (r = 0.35a) with various filling fractions of the inner air cavity for three different pitches: a = 600, 1000, and 3000 nm. The dashed black line represents the extraction efficiency of a nonpatterned GaN LED. The red arrow indicates the simulated extraction efficiency for the fabricated CES LED device with a = 3000 nm and t = 80 nm. Inset: schematic of the simulated structures. (b) Simulated extraction efficiencies of lateral-geometry GaN/CES LEDs (a = 3000 nm, r = 0.35a, and t = 80 nm) with various refractive indices of the inner cavity. Inset: schematic of the simulated structures. (c) Simulated far-field profiles within the sapphire background for flip-chip-geometry GaN/CES and GaN/PSS LEDs (a = 3000 nm, r = 0.35a, and t = 80 nm). The centered and four-sided closed regions, drawn by white dashed lines, indicate the light extraction cones through the top and side sapphire/air interfaces. (d) Comparison of total extraction efficiencies into sapphire and air media and the individual extraction efficiencies via top and side escape routes for the flip-chip-geometry GaN/CES and GaN/PSS LEDs. Inset: schematic illustrating top and side extraction routes in a flip-chip-geometry GaN LED.

was used for recording their optical power.26 The representative optical power−current (L−I) characteristics show that the CES LED exhibits a constant enhancement in the optical power at low-to-high injection currents relative to the PSS LED (Figure 3a). The L−I characteristics were recorded from typical CES and PSS LED devices that possessed the same spectra with a dominant wavelength (λD) of 468 nm (Supporting Information Figure S6a). Also, measurements of the current−voltage (I−V) data reveal that the CES LED has no degradation in its electrical properties (e.g., the leakage currents, operation voltages, and uniform current spreading) (top and bottom insets, Figure 3a). This clearly demonstrates that the thermally treated alumina shells serve as GaN growth templates that are as good as the standard PSS. Finally, the CES LED exhibits an ∼39% increase in the wall-plug efficiency compared to the PSS LED at an injection current of 240 mA. On-wafer probe measurements confirm that the enhanced optical power of the CES LED is statistically reliable (Supporting Information Figure S6b). For different dominant wavelengths, the CES LED still outperforms the PSS LED; the wall-plug efficiency is improved by 8% and 9% for λD = 456 and 462 nm, respectively (Supporting Information Figure S6c,d). We highlight that the impact of designed outcoupling structures becomes more significant in absorptive LED devices (e.g., InGaN/GaN LEDs with a high fraction of the In composition).24 Previous reports have shown that for PSS LED devices cone patterns provide more efficient light extraction and improved GaN crystallinity

as compared to hemisphere patterns.27−29 Therefore, we postulate that even larger wall-plug efficiency could be obtained in CES LEDs that incorporate cone patterns. Additionally, the inner air cavities in the CES pattern help release the stress accumulated during the heteroepitaxial growth of GaN layers on the sapphire substrate, which leads to reduced wafer bow (Supporting Information Figure S7).30 This mechanical feature is beneficial and improves the manufacturability of LED devices.31 Measurements of far-field profiles revealed another important diffraction feature unique to the CES LEDs (Figure 3b). For these far-field measurements, CES and PSS LEDs with a flipchip geometry were used. Both CES and PSS LEDs exhibit farfield profiles with 6-fold symmetry due to their hexagonallattice patterns, although the lattice fringes are more pronounced for the CES LED. The high-contrast fringes of the CES LED result in a relatively narrow emission profile compared to the PSS LED (Figure 3c). For example, the CES and PSS LEDs possessed ∼60% and ∼50% of the total optical power within ±45°, respectively (inset, Figure 3c). The wavelength-resolved far-field data show that even small differences in the emission wavelength cause substantial changes in the far-field profile (Figure 3d).10 Taken together, the high-contrast-index dielectric grating in the CES leads to enhanced optical power and improved directionality in LED devices, as compared to an analogous PSS. E

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Nano Letters To elucidate the optical features that differentiate the CES LED from the PSS LED, we carried out finite-difference timedomain (FDTD) simulations on lateral- and flip-chip-geometry LEDs (Supporting Information Figure S8a).22 To simplify the analysis, hemispherical objects were introduced for both CES and PSS patterns. The extraction efficiencies were calculated with different inner air cavity volumes for three different pitches (a = 600, 1000, and 3000 nm) (Figure 4a and Supporting Information Figure S8b). The simulation results show that all of the extraction efficiencies increase steadily as the air cavity volume is expanded, which accounts for the enhanced optical power of the CES LEDs with a = 3000 nm (Figure 3a). The simulated extraction efficiencies are almost the same for CES LEDs with all three pitches, whereas for the PSS LEDs, a pitch that is significantly larger than the emission wavelength is appropriate for light extraction. This illustrates that light extraction is dictated by the index contrast of the pattern rather than the pitch, which enables one to use a broad range of optimal pitches for high-index-contrast patterns.9 We note that these design rules are generic to both lateral- and flip-chipgeometry LEDs. It is also worthwhile to mention that the absolute extraction efficiencies in the simulation are strongly influenced by the extinction coefficients (k) imposed on the artificial absorption layers.24,32 The effect of index contrast was also confirmed by investigating how the extraction efficiencies of CES LEDs with a = 3000 nm depend on the refractive index (n) of the inner cavity (Figure 4b). Starting at 1.78 (i.e., the refractive index of sapphire), the simulated extraction efficiency increases monotonically until n approaches ∼1.0. Finally, near-to-far-field transformation simulations were performed on the flip-chip-geometry CES and PSS LED structures with a = 3000 nm.22 The in-plane electric and magnetic fields were obtained at a certain plane in the sapphire substrate, and the same far-field simulations were iterated while varying the position and polarization of a single dipole source (Supporting Information Figure S9). We note that this method yields far-field profiles in the background of the sapphire. Integrated far-field profiles, which are the sum of individual profiles acquired at different dipole source conditions, show that the CES LED exhibits highly localized 6-fold symmetry fringes compared to the PSS LED (Figure 4c), which supports the measured data (Figure 3b). Wavevector analysis on the simulated far-field profiles allowed us to calculate the fraction of light that was extracted via top and side escape routes, as illustrated in the inset of Figure 4d. For lateral- and flip-chipgeometry GaN LED devices, the light extracted through the four side facets of the thick and transparent sapphire substrate is considerable;33 however, existing wave optics simulations could not separate the top and side emissions. By comparing the extraction efficiencies of the top and side escape routes, we can see that the improvement in the extraction efficiency for CES LEDs is attributable to an increase in the top emission (Figure 4d), which is directly correlated with the strong vertical directionality (Figure 3c). Notably, for the amount of light extracted into the sapphire substrate, the PSS LED slightly surpasses the CES LED. Additional polarization analysis on the simulated extraction efficiencies shows that the CES pattern more effectively extracts the transverse magnetic (TM) polarized light that is tightly confined in the GaN layer (Supporting Information Figure S10). This feature suggests that a CES pattern will be also an efficient outcoupling structure for ultraviolet-emitting AlGaN LED devices where TM polarized emissions are dominant.34

In conclusion, we have demonstrated that high-indexcontrast CES patterns can dramatically increase light extraction in InGaN/GaN LEDs by enhancing the strength of low-order diffraction modes. Rationally designed CES templates yielded blue-emitting InGaN/GaN LED devices, exhibiting up to 39% larger wall-plug efficiency than state-of-the-art, PSS LED devices. The effect can be leveraged in any semiconductor and organic LEDs to increase the wall-plug efficiency at minimal cost by combining standard semiconductor processes (e.g., photolithography and thermal annealing) that allows for creating air cavities enclosed by thin dielectric shells. In addition, CES LED devices exhibited pronounced far-field fringes, leading to a substantial beaming effect even with their afew-micron-pitch pattern. We believe that our results, which show the benefits of utilizing high-index-contrast CES templates, represent a substantial contribution to the development of GaN-based LED devices. Moreover, CES templates tailored for different structures and spectra can be utilized as strong-diffraction dielectric gratings for other photonic applications including organic LEDs and solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00892. Supplementary figures S1−S10. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-K.K.). *E-mail: [email protected] (E.Y.). Author Contributions

Y.-J.M. and D. M. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Yong-Sung Jin for helpful discussions. S.-K.K. was supported by the Basic Science Research Program through the National Research Fo undat io n of Ko rea (NRF2013R1A1A1059423) funded by the Ministry of Science, ICT, and Future Planning and Korea Evaluation Institute of Industrial Technology (Grant 10049601) funded by the Ministry of Trade, Industry, and Energy, Korea. E.Y. was supported by BK21 Plus SNU Materials Division for Educating Creative Global Leaders through the National Research Foundation of Korea (Grant 21A20131912052) funded by the Ministry of Science, ICT, and Future Planning. Y.P. was supported by the Technology Innovation Program (Grant 10037886) funded by the Ministry of Trade, Industry and Energy, Korea. M.-K.S. acknowledges support for this work from the National Research Foundation of Korea (NRF) (2014M3A6B3063709).

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REFERENCES

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Nano Letters (4) Day, R. W.; Mankin, M. N.; Gao, R.; No, Y.-S.; Kim, S.-K.; Bell, D. C.; Park, H.-G.; Lieber, C. M. Nat. Nanotechnol. 2015, 10, 345− 352. (5) van Lare, C.; Lenzmann, F.; Verschuuren, M. A.; Polman, A. Nano Lett. 2015, 15, 4846−4852. (6) Yamada, M.; Mitani, T.; Narukawa, Y.; Shioji, S.; Niki, I.; Sonobe, S.; Deguchi, K.; Sano, M.; Mukai, T. Jpn. J. Appl. Phys. 2002, 41, L1431−L1433. (7) Li, Y.; You, S.; Zhu, M.; Zhao, L.; Hou, W.; Detchprohm, T.; Taniguchi, Y.; Tamura, N. S.; Tanaka, C. Appl. Phys. Lett. 2011, 98, 151102. (8) Fujii, T.; Gao, Y.; Sharma, R.; Hu, E. L.; DenBaars, S. P.; Nakamura, S. Appl. Phys. Lett. 2004, 84, 855−857. (9) Kim, S.-K.; Cho, H. K.; Bae, D. K.; Lee, J. S.; Park, H.-G.; Lee, Y.H. Appl. Phys. Lett. 2008, 92, 241118. (10) Wierer, J. J.; David, A.; Megens, M. M. Nat. Photonics 2009, 3, 163−169. (11) Matioli, E.; Rangel, E.; Iza, M.; Fleury, B.; Pfaff, N.; Speck, J.; Hu, E.; Weisbuch, C. Appl. Phys. Lett. 2010, 96, 031108. (12) Chiu, C.-H.; Lin, C.-C.; Han, H.-V.; Liu, C.-Y.; Chen, Y.-H.; Lan, Y.-P.; Yu, P.; Kuo, H.-C.; Lu, T.-C.; Wang, S.-C.; Chang, C.-Y. Nanotechnology 2012, 23, 045303. (13) Sheu, J.-K.; Yeh, Y.-H.; Tu, S.-J.; Lee, M.-L.; Chen, P. C.; Lai, W.-C. J. J. Lightwave Technol. 2013, 31, 1318−1322. (14) Kim, J.; Woo, H.; Joo, K.; Tae, S.; Park, J.; Moon, D.; Park, S. H.; Jang, J.; Cho, Y.; Park, J.; Yuh, H.; Lee, G.-D.; Choi, I.-S.; Nanishi, Y.; Han, H. N.; Char, K.; Yoon, E. Sci. Rep. 2013, 3, 3201. (15) Mottier, P. “Packaging” in LEDs for Lighting Applications; Wiley: New York, 2009; p 216. (16) LED Packaging Technology and Market Trends 2014; Yole Development Press, 2014. (17) Jang, J.; Moon, D.; Lee, H.-J.; Lee, D.; Choi, D.; Bae, D.; Yuh, H.; Moon, Y.; Park, Y.; Yoon, E. J. Cryst. Growth 2015, 430, 41−45. (18) A hexagonal array of photoresist (PR, AZ GXR-601) rod patterns was defined on a sapphire substrate by standard photolithography using an i-line stepper (Nikon i7). The pitch and diameter of the PR pattern were 3.0 and 2.2 μm, respectively. To make a hemispherical PR pattern, a thermal reflow process was applied at 150 °C for 40 min in a convection oven. Then, an amorphous alumina layer was conformally deposited on the PR pattern as well as the sapphire substrate by atomic layer deposition (NCD LUCIDA D100). H2O and trimethylaluminum (TMAl) were used as oxygen and aluminum sources, respectively. The ALD process was operated at 110 °C to prevent further thermal deformation of the PR pattern. The number of ALD cycles was 1000, which assures the deposition of an 80 nm thick alumina layer. Thermal treatment was then carried out in a furnace at 1100 °C for 2 h. During this thermal process, the PR material was removed via oxidization. This resulted in the formation of air cavities, conformally covered with crystallized Al2O3 shells, placed on the sapphire substrate. Note that the initial amorphous alumina layer was crystallized into single crystalline α-Al2O3 by solid phase epitaxy, which enables the CES to serve as a growth template for InGaN/GaN LEDs. (19) The LED epitaxial layer was grown on two-inch CES and PSS templates with the same pitch of 3 μm by a MOCVD process. A 3 μm thick undoped GaN layer was grown on each patterned substrate. This was followed by the deposition of a 3 μm thick n-type GaN layer, six pairs of InGaN/GaN multiple quantum wells (MQWs), a 180 nm thick p-type AlGaN electron blocking layer, and a 50 nm thick Mgdoped p-GaN layer. Trimethylindium (TMIn) was used as a precursor for In. For n- and p-type GaN layers, silane and bis-cyclopentadienyl magnesium were used as dopant sources, respectively. (20) Spinelli, P.; Verschuuren, M. A.; Polman, A. Nat. Commun. 2012, 3, 692. (21) Transmittance spectra were measured by a spectrophotometer (Varian, Cary 5000) where an integrating sphere was used to collect all of the diffuse light. To measure wavelength-dependent diffraction profiles, a home-built far-field scanner was used. In this setup, a charge coupled device array detector (USB4000, Ocean Optics) is rotated

along the polar (ϕ) and azimuthal (θ) directions with a step size of 1°. A Xe lamp source (450 W, Newport) was used as the incident broadband light. To make a collimated beam the Xe lamp source passed through a metal cylinder with two identical 2.0 mm diameter circular apertures just before reaching the specimen. Total diffraction profiles were obtained by integrating each wavelength-resolved profile over λ = 380−800 nm. The wavelength-dependent diffraction strength with respect to the diffraction order (m) was evaluated from the measured wavelength-resolved diffraction profiles. (22) Home-built FDTD algorithms were used for all of the optical simulations. To calculate the wavelength-dependent diffraction strength with respect to the diffraction order, electric and magnetic scattered fields ( fsingle) were obtained from single CES and PSS objects with a normal plane wave. This was done by using the total-field scattered-field method. The incident plane wave was scanned over the wavelength between 350 and 800 nm with a step size of 5 nm. A spatial resolution of 5 nm was set along the x-, y-, and z-directions in the simulated structures. The near-fields ( f periodic) of periodic CES and PSS patterns were obtained by calculating the convolution of fsingle and the comb function ( fcomb) of a hexagonal lattice. The radiant flux profiles of the periodic CES and PSS patterns were then obtained by applying a near-field to far-field transformation on the f periodic. The diffraction strength for a specific diffraction order (m, n) was calculated by taking the radiant flux in the direction of the reciprocal vector: v = mv1 + nv2. Here, v1 and v2 are the primitive reciprocal vectors of the hexagonal lattice. The polar (θ) and azimuthal (φ) angles at the diffraction order are determined by v = 2πnsapp/λ (sin θ cos φ, sin θ sin φ), represented in Cartesian coordinates, where nsapp and λ are the refractive index of sapphire and the free-space wavelength, respectively. The diffraction strength with the diffraction order m was defined by averaging the two orthogonal cases of v = mv1 and mv2, which is identical to the experimental analysis. To calculate the extraction efficiencies of CES and PSS LEDs, a spatial resolution of 5 nm was used for the x-, y-, and z-directions and periodic boundary conditions were applied along the x- and y-directions. A single dipole source with TE (in-plane) or TM (vertical) polarization was excited at the position of the multiple quantum wells in a real InGaN/GaN LED device. A 100 nm thick absorption layer with an extinction coefficient (k) of 0.01 was introduced. Total extraction efficiencies were defined by averaging the extraction efficiencies for dipole sources with TE and TM polarizations (TE/TM = 2:1). To obtain the total far-field profiles of CES and PSS LED structures (Figure 4c), the near-field to far-field Fourier transformation method was used. In this method, near-field profiles were obtained at a certain plane in the sapphire substrate. For these FDTD simulations, a spatial resolution of 20 nm was used for the x-, y-, and z-directions. The near-field profiles were calculated while varying the position of the dipole source, and the far-field transformation was performed for the near-field profiles. The integrated far-field profiles were obtained by averaging the individual far-field profiles acquired at different dipole source conditions. Extraction efficiencies via each escape route were calculated from the integrated far-field profiles. This was done by evaluating the fraction of far-field energy within a specific range of solid angles, which is given by the refractive index contrast between sapphire and air. The extraction efficiency into air was defined as the sum of the top and side extraction efficiencies. (23) Kim, S.-K; Zhang, X.; Hill, D. J.; Song, K.-D.; Park, J.-S.; Park, H.-G.; Cahoon, J. F. Nano Lett. 2015, 15, 753−758. (24) Wiesmann, C.; Bergenek, K.; Linder, N.; Schwarz, U. T. Laser Photonics Rev. 2009, 3, 262−286. (25) David, A.; Fujii, T.; Sharma, R.; McGroddy, K.; Nakamura, S.; DenBaars, S. P.; Hu, E. L.; Weisbuch, C.; Benisty, H. Appl. Phys. Lett. 2006, 88, 061124. (26) For fabrication of lateral (1075 × 750 μm2) InGaN/GaN LEDs, a mesa depth of 1 μm was defined by an inductively coupled plasma process. A 150 nm thick ITO layer was deposited as a current spreading layer on the p-type GaN layer and distributed Bragg reflectors (DBRs), consisting of TiO2/SiO2, were used as bottom reflectors. Then, Ti/Al/Ni/Au multilayers were deposited as n-type G

DOI: 10.1021/acs.nanolett.6b00892 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters and p-type electrodes. For fabrication of flip-chip (875 × 875 μm2) InGaN/GaN LED, DBRs, consisting of TiO2/SiO2, were used as the p-side reflectors and 60 nm thick ITO layer was used as a current spreading layer on the p-type GaN layer. The fabricated lateral CES and PSS LED devices were mounted on the same 5.0 × 5.0 mm2 lead frame package (Seoho, 5050 white EMC) using a die attach adhesive (Dow Corning, OE-8020) without any phosphor. The fabricated flipchip CES and PSS LED devices were mounted on 3.5 × 3.5 mm2 lead frame package (Semicon Light, MICI3535 PKG) with Ag−Sn−Cu alloy paste (Heesung, PFM-86 HS-HF). The optical power of the LED devices was measured by a standard 12 in. integrating sphere system equipped with a calibrated spectrometer (Instrument Systems, CAS 140CT). The electrical I−V characteristics of the LED devices were measured by a current−voltage meter (Keithley, Model 2400). (27) Lin, H.-C.; Liu, H.-H.; Lee, G.-Y.; Chyi, J.-I.; Lu, C.-M.; Chao, C.-W.; Wang, T.-C.; Chang, C.-J.; Chi, S. W. S. J. Electrochem. Soc. 2010, 157, H304−H307. (28) Huang, X.-H.; Liu, J.-P.; Kong, J.-J.; Yang, H.; Wang, H.-B. Opt. Express 2011, 19, A949−A955. (29) Kang, H. J.; Cho, S. U.; Kim, E. S.; Kim, C.-S.; Jeong, M. Y. Opt. Eng. 2013, 52, 023002. (30) The bow of GaN/CES and GaN/PSS LED wafers with a sapphire substrate thickness of 430 μm was assessed by a laser surface scanning technique at room temperature (Frontier Semiconductor, FSM 500TC). The thicknesses of the GaN LED structures were the same as 5.9 μm. The total scan length was 50 mm and the wavelength of the laser was 780 nm. A total of 2100 data points were acquired to fit the vertical displacement profile of the wafers. (31) Shin, I.-S.; Lee, D.; Lee, K.-H.; You, H.; Moon, D. Y.; Park, J.; Nanishi, Y.; Yoon, E. Thin Solid Films 2013, 546, 118−123. (32) Kim, S.-K; Song, H. D.; Ee, H.-S.; Choi, H. M.; Cho, H. K.; Lee, Y.-H.; Park, H.-G. Appl. Phys. Lett. 2009, 94, 101102. (33) Lee, H.-C.; Na, J.-Y.; Moon, Y.-J.; Kim, S.-K. J. Korean Phys. Soc. 2015, 66, 924−928. (34) Kolbe, T.; Knauer, A.; Chua, C.; Yang, Z.; Einfeldt, S.; Vogt, P.; Johnson, N. M.; Weyers, M.; Kneissl, M. Appl. Phys. Lett. 2010, 97, 171105.

H

DOI: 10.1021/acs.nanolett.6b00892 Nano Lett. XXXX, XXX, XXX−XXX