Size-controllable nanopyramids photonic crystal selectively grown on p-gan for enhanced lightextraction of light-emitting diodes

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1 Size-controllable nanopyramids photonic crystal selectively grown on p-gan for enhanced lightextraction of light-emitting diodes Chengxiao Du, 1 Tongbo Wei, 1,* Haiyang Zheng, 1 Liancheng Wang, 1 Chong Geng, 2 Qingfeng Yan, 2 Junxi Wang, 1 and Jinmin Li 1 1 Semiconductor Lighting Technology Research and Development Center, Institute of Semiconductors, Chinese Academy of Sciences, Beijing , China 2 Department of Chemistry, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing , China * tbwei@semi.ac.cn Abstract: Size-controllable p-gan hexagonal nanopyramids (HnPs)- photonic crystal (PhC) structures were selectively grown on flat p-gan layer for the elimination of total internal reflection of light-emitting diodes (LEDs). The LEDs with HnPs-PhC of 46.3% bottom fill factor (PhC lattice constant is 730 nm) showed an improved light output power by 99.9% at forward current of 350 ma compared to the reference LEDs with flat p- GaN layer. We confirmed the effect of HnPs-PhC with different bottom fill factors and the effect of nanopyramid-shaped and nanocolumn-shaped PhC on the light-extraction of LEDs was also investigated by using threedimensional finite-difference time-domain simulations Optical Society of America OCIS codes: ( ) Optoelectronics; ( ) Light-emitting diodes; ( ) Photonic crystals; ( ) Nanostructure fabrication. References and links 1. J. J. Wierer, A. David, and M. M. Megens, III-nitride photonic-crystal light-emitting diodes with high extraction efficiency, Nat. Photonics 3(3), (2009). 2. A. I. Zhmakin, Enhancement of light extraction from light emitting diodes, Phys. Rep. 498(4-5), (2011). 3. T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, Increase in the extraction ef ciency of GaN-based light-emitting diodes via surface roughening, Appl. Phys. Lett. 84(6), (2004). 4. T. B. Wei, Q. F. Kong, J. X. Wang, J. Li, Y. P. Zeng, G. H. Wang, J. M. Li, Y. X. Liao, and F. T. Yi, Improving light extraction of InGaN-based light emitting diodes with a roughened p-gan surface using CsCl nano-islands, Opt. Express 19(2), (2011). 5. K. Tadatomo, H. Okagawa, Y. Ohuchi, T. Tsunekawa, Y. Imada, M. Kato, and T. Taguchi, High output power InGaN ultraviolet light-emitting diodes fabricated on patterned substrates using metalorganic vapor phase epitaxy, Jpn. J. Appl. Phys. 40(Part 2, No. 6B), L583 L585 (2001). 6. H. Y. Gao, F. W. Yan, Y. Zhang, J. M. Li, Y. P. Zeng, and G. H. Wang, Enhancement of the light output power of InGaN/GaN light-emitting diodes grown on pyramidal patterned sapphire substrates in the micro- and nanoscale, J. Appl. Phys. 103(1), (2008). 7. C.-F. Lin, Z.-J. Yang, B.-H. Chin, J.-H. Zheng, J.-J. Dai, B.-C. Shieh, and C.-C. Chang, Enhanced light output power in InGaN light-emitting diodes by fabricating inclined undercut structure, J. Electrochem. Soc. 153(12), G1020 G1024 (2006). 8. J. K. Kim, S. Chhajed, M. F. Schubert, E. F. Schubert, A. J. Fischer, M. H. Crawford, J. Cho, H. Kim, and C. Sone, Light-extraction enhancement of GaInN light-emitting diodes by graded-refractive-index indium tin oxide anti-reflection contact, Adv. Mater. 20(4), (2008). 9. J. K. Kim, A. N. Noemaun, F. W. Mont, D. Meyaard, E. F. Schubert, D. J. Poxson, H. Kim, C. Sone, and Y. Park, Elimination of total internal re ection in GaInN light-emitting diodes by graded-refractive-index micropillars, Appl. Phys. Lett. 93(22), (2008). 10. A. David, H. Benisty, and C. Weisbuch, Photonic crystal light-emitting sources, Rep. Prog. Phys. 75(12), (2012). 11. T. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, III-nitride blue and ultraviolet photonic crystal light emitting diodes, Appl. Phys. Lett. 84(4), (2004). (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25373

2 12. J. J. Wierer, M. R. Krames, J. E. Epler, N. F. Gardner, M. G. Craford, J. R. Wendt, J. A. Simmons, and M. M. Sigalas, InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures, Appl. Phys. Lett. 84(19), (2004). 13. D.-H. Kim, C.-O. Cho, Y.-G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q.-H. Park, Enhanced light extraction from GaN-based light-emitting diodes with holographically generated twodimensional photonic crystal patterns, Appl. Phys. Lett. 87(20), (2005). 14. A. David, T. Fujii, R. Sharma, K. McGroddy, S. Nakamura, S. P. DenBaars, E. L. Hu, C. Weisbuch, and H. Benisty, Photonic-crystal GaN light-emitting diodes with tailored guided modes distribution, Appl. Phys. Lett. 88(6), (2006). 15. E. Matioli, E. Rangel, M. Iza, B. Fleury, N. Pfaff, J. Speck, E. Hu, and C. Weisbuch, High extraction ef ciency light-emitting diodes based on embedded air-gap photonic-crystals, Appl. Phys. Lett. 96(3), (2010). 16. H. Kitagawa, M. Fujita, T. Suto, T. Asano, and S. Noda, Green GaInN photonic-crystal light-emitting diodes with small surface recombination effect, Appl. Phys. Lett. 98(18), (2011). 17. T. B. Wei, K. Wu, D. Lan, Q. F. Yan, Y. Chen, C. X. Du, J. X. Wang, Y. P. Zeng, and J. M. Li, Selectively grown photonic crystal structures for high efficiency InGaN emitting diodes using nanospherical-lens lithography, Appl. Phys. Lett. 101(21), (2012). 18. A. Lundskog, U. Forsberg, P. O. Holtz, and E. Janzén, Morphology control of hot-wall MOCVD selective area grown hexagonal GaN pyramids, Cryst. Growth Des. 12(11), (2012). 19. W. Wu, A. Katsnelson, O. G. Memis, and H. Mohseni, A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars, Nanotechnology 18(48), (2007). 1. Introduction GaN-based light-emitting diodes (LEDs) are becoming an increasingly attractive alternative to conventional light sources due to their compact structures, high efficiency and long lifetime. Ongoing research is dedicated to improving their performance through the use of more efficient light-generating and light-extracting structures [1]. As a consequence of the total internal reflection (TIR) caused by the large discrepancy of the refractive index between the GaN (n = 2.52) and the air (n = 1.0), a large amount of light emitted from the active region is trapped inside the LEDs, resulting in low light-extraction efficiency (LEE) for LED devices [2]. To solve this problem, many methods have been used to model the propagation of emitted light by the surface/interface morphology modification, including the use of surface texturing [3,4], sapphire substrates patterning [5,6], LED chips shaping [7], gradedrefractive-index (GRIN) layer coating [8,9], and two-dimensional (2D) photonic crystal (PhC) structures integrating [1,10 17]. Specially, using PhC as the diffraction gratings in LEDs is a possible next step for improved performance [1]. PhC is dielectric perturbations on the scale of the wavelength, where spontaneous emission into guided modes is allowed but these modes are subsequently out coupled by the diffractive properties of PhC. Compared to random surface roughening LEDs, integrating PhC in LEDs can provide more information about the propagation and distribution of emitted-light in the LED slab, leading for more efficient light-extraction design. More freedom of designing PhC is available while designing random surface roughening LEDs is usually not possible. The record of unencapsulated light-extraction is indeed maintained by the PhC-LEDs [1,10]. On the other hand, the PhC structures are usually fabricated by e-beam lithography [11,12], holographic lithography [13], and nanoimprint [1,15]. However, the drawback of the e-beam and holographic lithography techniques is low throughput, and nanoimprint technique is required to produce the expensive hard mold for imprinting. Therefore, developing simple and low-cost methods to fabricate large-scale PhC are critical for the widespread use of PhCs in commercial LEDs. In this letter, we developed a simple and effective method to grow size-controllable p- GaN hexagonal nanopyramids (HnPs) through selective area growth (SAG) [18] on the flat p- GaN layer for high LEE. The GaN HnPs were grown from patterned SiO 2 mask fabricated by nanosphere lithography (NSL) technology [17,19], which is simple and low-cost. We studied the growth characteristics of GaN HnPs in order to easily control their size. The LEE enhancement factor as a function of HnPs GaN bottom fill factor (f) of LEDs was first investigated in details. We further compared the effect of the HnPs-PhC and the hexagonal (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25374

3 nanocolums (HnCs)-PhC on the LEE of LEDs in order to study if the details of the shape of the PhC modify the light-extraction. 2. Experiments The blue LEDs with conventional InGaN/GaN multiple quantum wells (MQWs) were grown on c-plane sapphire substrates via metalorganic chemical vapor deposition (MOCVD). In our recipe, the 80-nm-thick p-gan layer was grown under 950 C and 130 Torr for 10.5 min using 15 sccm (sccm denotes standard cubic centimeters per minute) trimethylgallium (TMG), 6 slm (slm denotes standard liters per minute) ammonia (NH 3 ) as precursors and biscyclopentadienyl magnesium (Cp 2 Mg) as the Mg-source. The p-gan HnPs were regrown to fabricate HnPs-PhC LEDs [Fig. 1]. Firstly, 30-nm-thick SiO 2 layer was deposited on p-gan layer by using plasma enhanced chemical vapor deposition system. Next, 500-nm-thick photoresist was spun on the SiO 2 layer. And then, monolayer of self-assembly polystyrene (PS) spheres was transferred onto the photoresist followed by developing-exposure process using the focusing nature of PS spheres developed by our group previously. Detailed exposure and development process can be seen in our early work [17]. The nano holes pattern was transferred onto the SiO 2 layer through an inductively coupled plasma etching process. Finally, the photoresist was removed, leaving circular window openings in SiO 2 mask and the wafer was cleaned for regrowth of p-gan HnPs. SAG performed on these circular window openings and gallium polar (0001)-oriented p-gan templates as substrates resulted in uniform HnPs with six smooth semipolar {1-101} facets, which is stable under 950 C and 500 Torr, ensuring low loss of photons during the modulation of light flow from the LEDs. Fig. 1. Schematic illustration of process for growth of GaN HnPs on p-gan layer. 3. Results and discussion Figure 2(a) is the scanning electron microscopy (SEM) images of nanoholes with a diameter of ~400 nm which were developed thoroughly using 900-nm-diameter (also the lattice constant: Λ) PS spheres in the photoresist and the cross-section view is illustrated in the inset of it. We also used 730-nm-diameter PS spheres to develop nanopatterns in photoresist. We found that the diameters of nanoholes generated by 900-nm-diameter PS spheres are almost (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25375

4 the same to the ones generated by 730-nm-diamter PS spheres under modified exposuredevelop condition. This is because the little difference of the beam waist of 900 and 730-nmdiamter PS spheres under 365-nm-wavelength laser exposure [19]. The size of the grown HnPs is related to the initial size of the open circles in SiO 2 mask and the growth time in MOCVD [18]. A number of samples with varying growth time were grown to investigate the growth characteristics of HnPs. Figure 2(b) and its inset show the HnPs and truncated HnPs grown for 15 min and 5 min on Λ = 730 nm patterns, respectively. Figure 2(c) and its inset show the HnPs and truncated HnPs grown for 7.5 and 3 min on Λ = 900 nm patterns, respectively. The HnPs GaN has fixed sidewall angles from ridgeline (31.6 ) to the center line of the sidewall (28 ) [Fig. 2(d)]. The height (H) of the truncated HnPs and HnPs versus growth time is shown in Fig. 3. Two HnPs growth regions were Fig. 2. Bird view SEM images of (a) nano holes array in photoresist. The inset of (a) is the cross-section view of nano holes. (b) and (c) are HnPs GaN. The insets of (b) and (c) are the truncated pyramids GaN.(d) Tilted high resolution images of the HnPs GaN. (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25376

5 Fig. 3. The height of the HnPs/truncated HnPs as a function of growth time. observed in this plot. The linear increasing of H with the growth time in the I region is much faster than in the II region. This is caused by the much faster growth rates of the (0001) facet than the {1-101} facets under our growth parameters. In the initial growth stage, truncated HnPs is obtained and there is large (0001) facet area. The H of the truncated HnPs is increasing linearly until the (0001) facet disappears in region I, truncated HnPs turned into HnPs. Then, relatively low growth rate of {1-101} facets results in low H increasing rate in region II. A transition time between I and II region is exist at ~6.5 min (the intersection of two extension line of I and II region). Above all, we can gain varying sizes of HnPs according to our needs by changing the initial patterns size or the growth time in MOCVD. After removing SiO 2 mask by hydrofluoric acid solution, the HnPs (H = 400 nm, f = 26.3%, Λ = 900 nm) LED wafer (PhC1-LED), the HnPs (H = 430 nm, f = 46.3%, Λ = 730 nm) LED wafer (PhC2-LED) and a reference LED (R-LED) wafer with regrown flat p-gan layer in the same run with PhC1-LED wafer were fabricated with a conventional mesa area of 1 1mm 2 using indium tin oxide (ITO) deposited on p-gan layer as transparent conductive layer and Cr/Pt/Au as n- and p-electrodes by e-beam evaporation. Compared to R-LEDs, the light output power (LOP) of PhC1- and PhC2-LEDs measured by an integrating sphere is improved by 55.0% and 99.9% at an injection current of 350 ma duo to the effective PhC Bragg scattering effect, respectively [Fig. 4(a)]. The forward voltage of PhC1-LEDs and R-LEDs is similar at an injection current of 350 ma [Fig. 4(a)], which implies that the ITO ohmic contacts with the semipolar {10-10} facets are similar to the polar (0001) facet. However, the forward voltage of PhC2-LEDs is a little larger than R- and PhC1- LEDs for the elevated bulk resistance of thicker p-gan. The angular far-field emission (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25377

6 Fig. 4. (a) LOP-I-V curves of the LEDs. (b) Far field radiation patterns of the LEDs. patterns of PhCs LEDs show omnidirectional enhancement in the overall integrated intensity due to the Bragg scattering of the PhCs [Fig. 4(b)]. The full-width-at-half maximum (FWHM) of emission divergence for the PhC1-LEDs and PhC2-LEDs are and 145.1, respectively, which is a little smaller compared to that of for R-LEDs. This implies that the PhC s directional characteristic on the light emission in our experiment isn t strong because the period of embedded-phc is large enough to have many diffraction orders in the blue light regime resulting in light leakage along many directions. These slightly smaller emission divergence may be caused by partial side emission of the conventional LEDs is redirected to the top and bottom (has a metal reflector) escape-cone by the PhCs effect [12]. We used the 3D FDTD solutions tools (Lumerical Solutions, Inc.) to quantitatively investigate the LEE enhancement factor as a function of HnPs GaN f at Λ = 900 nm of the HnPs-PhC LEDs. The simplified simulated LED structure consisted of 200-nm-thick ITO layer, 150-nm-thick p-gan layer, 120-nm-thick active region, 2-μm-thick n-gan layer, and 1-μm-thick sapphire. The simulation area was 7 7 μm 2. The mesh size and the time step size are 0.25 nm and femtosecond, respectively. The simulation area is much smaller than actual size of LEDs. In order to truncate the lateral dimension of actual LED structure, we used four perfect mirrors at the edges of the LED. A silver reflector under the sapphire substrate was used for the simulations. We also used symmetry to reduce the number of simulations to reduce the calculation time and two point dipoles polarized along the x and y directions was used as a radiating source and placed in the middle of the MQW layer [19]. The light source wavelength was set to 460 nm. The light extraction was calculated from the top surface only. Figure 5(a) shows the simplified HnPs-PhC LED structures for theoretical calculation. The simulation results clearly show that, when f < 70%, the enhancement of LEE is increasing with the f of HnPs GaN [Fig. 5(b)]. This is because large f leading to more opportunity for the strong interaction between the guided modes and the HnPs-PhC. The radiation profiles of the horizontal dipole sources along the x-axis (d x ) and the y-axis (d y ) for the R-LED and PhC1-LED with f = 50% was illustrated in Fig. 5(c). It was found that most of (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25378

7 Fig. 5. (a) Schematic of the 3D FDTD simulation domain for the HnPs-PhC LED. (b) Calculated LEE enhancement factor of the HnPs-PhC and the HnCs-PhC LED with different f. (c) Radiation profiles of the horizontal dipole source (d x ) and (d y ) in the R-LED and HnPs-PhC LED with f = 50%, respectively. the radiation from the d x and d y dipoles of the R LED is propagating inside the GaN epitaxial layer and the sapphire layer and cannot escape to the air, resulting in serious light absorption. However, emission from HnPs-PhC LED is significantly extracted into the air. Interestingly, one may wonder if the details of the shape of the scattering feature modify this result. So, we compared the effect of HnCs-PhC with the same f to HnPs-PhC on the LEE of LEDs [Fig. 5(b)]. It can be clearly seen that the LEE enhancement factor of both kind of PhC is similar at f < 50%. This is not surprising since it is governed by the average index of the scattering layer and not on the details of its geometry [10]. When the f is large than 70% for HnCs-PhC, significantly decreasing of LEE enhancement factor is shown. We believe that the average index of the HnCs-PhC with large f is almost the same as the index of flat p-gan layer, resulting in insufficient light-extraction. 4. Conclusions In summary, size-controllable p-gan HnPs-PhC for high light-extraction by suppression of the TIR has been fabricated via SAG method based on NSL technology. The size of GaN HnPs can be controlled by changing the size of the mask patterns and the growth time in MOCVD. The LOP of PhC2-LEDs (f = 50%) has been improved significantly by 99.9% due to the sufficient PhC Bragg scattering effect, which was confirmed by using 3D FDTD simulations. We also compared the effect of HnCs-PhC and HnPs-PhC on the LEE of LEDs. (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25379

8 We find that when f < 50%, the LEE enhancement is not sensitive to the details of the shape of PhC. However, the effect on the LEE of LEDs is strongly affected by the shape of the PhC when it has large f. Acknowledgments This work was supported by the National Natural Sciences Foundation of China under Grant Nos and , by the National Basic Research Program of China under Grant No. 2011CB301902, and by the National High Technology Program of China under Grant No. 2011AA03A103. The authors are grateful for the FDTD solutions tools from Lumerical Solutions, Inc. (C) 2013 OSA 21 October 2013 Vol. 21, No. 21 DOI: /OE OPTICS EXPRESS 25380