Size dependence of silica nanospheres embedded in 385 nm ultraviolet light-emitting diodes on a far-field emission pattern Young Jae Park,1 Nam Han,1 Beo Deul Ryu,1 Min Han,1 Kang Bok Ko,1 Tran Viet Cuong,1 Jaehee Cho,1 Eun-Kyung Suh,1 and Chang-Hee Hong1,* 1
School of Semiconductor and Chemial Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, South Korea * [email protected]
Abstract: We demonstrate that the use of silica nanospheres (SNs) with sizes close to the emission wavelength of light-emitting diodes (LEDs) can enhance the light output power and manipulate the far-field emission pattern. Near-ultraviolet (NUV)-LEDs grown on a patterned sapphire substrate embedded with 300 nm SNs show a three times higher light output power than that without SNs, when measured through the top side. For far-field emission measurements, the LEDs embedded with 300 nm SNs show the significant increase of front emission due to the improved crystal quality of epitaxial films as well as the increase of Mie scattering effect of SNs. These experimental results indicate the important role of the size of embedded SNs in enhancing the light output power for NUV-LEDs. © 2014 Optical Society of America OCIS codes: (250.0250) Optoelectronics; (290.0290) Scattering.
References and links 1.
D. Britalan and W. Nunley, Optoelectronics: Infrared-Visible-Ultraviolet Devices and Applications, 2nd ed. (CRC Press, 2009) 2. D. J. Chae, D. Y. Kim, T. G. Kim, Y. M. Sung, and M. D. Kim, “AlGaN-based ultraviolet light-emitting diodes using fluorine-doped indium tin oxide electrodes,” Appl. Phys. Lett. 100(8), 081110 (2012). 3. H. Lu, T. Yu, G. Yuan, X. Chen, Z. Chen, G. Chen, and G. Zhang, “Enhancement of surface emission in deep ultraviolet AlGaN-based light emitting diodes with staggered quantum wells,” Opt. Lett. 37(17), 3693–3695 (2012). 4. C.-Y. Cho, M. Choe, S.-J. Lee, S.-H. Hong, T. Lee, W. Lim, S.-T. Kim, and S.-J. Park, “Near-ultraviolet lightemitting diodes with transparent conducting layer of gold-doped multi-layer graphene,” J. Appl. Phys. 113(11), 113102 (2013). 5. T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). 6. R. Windisch, C. Rooman, S. Meinlschmidt, P. Kiesel, D. Zipperer, G. H. Döhler, B. Dutta, M. Kuijk, G. Borghs, and P. Heremans, “Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes,” Appl. Phys. Lett. 79(15), 2315 (2001). 7. J. R. Lee, S. L. Na, J. H. Jeong, S. N. Lee, J. S. Jang, S. H. Lee, J. J. Jung, J. O. Song, T. Y. Seong, and S. J. Park, “Low resistance and high reflectance Pt/Rh contacts to p-type GaN for GaN-based flip chip light-emitting diodes,” J. Electrochem. Soc. 152(1), G92 (2005). 8. J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Ludowise, N. F. Gardner, R. S. Kern, and S. A. Stockman, “High-power AlGaInN flip-chip light-emitting diodes,” Appl. Phys. Lett. 78(22), 3379 (2001). 9. R. H. Horng, W. K. Wang, S. C. Huang, S. Y. Huang, S. H. Lin, C. F. Lin, and D. S. Wuu, “Growth and characterization of 380-nm InGaN/AlGaN LEDs grown on patterned sapphire substrates,” J. Cryst. Growth 298, 219–222 (2007). 10. Y. J. Lee, T. C. Hsu, H. C. Kuo, S. C. Wang, Y. L. Yang, S. N. Yen, Y. T. Chu, Y. J. Shen, M. H. Hsieh, M. J. Jou, and B. J. Lee, “Improvement in light-output efficiency of near-ultraviolet InGaN-GaN LEDs fabricated on stripe patterned sapphire substrates,” Mater. Sci. Eng. B-Adv. 122, 184–187 (2005). 11. Y. J. Park, J. H. Kang, H. K. Kim, Y. S. Katharria, N. Han, M. Han, B. D. Ryu, E.-K. Suh, H. K. Cho, and C.-H. Hong, “Two-step lateral overgrowth of GaN for improved emission from blue light-emitting diodes,” J. Cryst. Growth 372, 157–162 (2013). 12. D. W. Hahn, “Light scattering theory,” University of Florida (2006).
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1553
13. M. D. Lechner, “Influence of Mie scattering on nanoparticles with different particle sizes and shapes: photometry and analytical ultracentrifugation with absorption optics,” J. Serb. Chem. Soc. 70, 361–369 (2005). 14. T. S. Oh, H. Jeong, Y. S. Lee, A. H. Park, T. H. Seo, H. Kim, K. J. Lee, M. S. Jeong, and E.-K. Suh, “Light outcoupling effect in GaN light-emitting diodes via convex microstructures monolithically fabricated on sapphire substrate,” Opt. Express 19(10), 9385–9391 (2011).
1. Introduction AlGaN-based 380–400 nm near-ultraviolet light-emitting diodes (NUV-LEDs) have attracted much attention as a result of their promising applications in analytical instrumentation, medical treatments, biological agent identification, air/water purification, and white LEDs for solid-state lighting [1–3]. However, the efficiency of UV-LEDs needs to further improve in order to justify the replacement of current conventional UV lamps. Several methods have been developed to improve the light output power of UV-LEDs, such as the use of highly transparent conducting layers [4], surface texturing [5, 6], flip-chip technology [7, 8], and patterned sapphire substrates (PSS) [9, 10]. Among these methods, PSS has become widely used as a result of which the threading dislocation density in the GaN epilayer can be effectively reduced on PSS through epitaxial lateral overgrowth and also the sapphire patterns can act as a scattering center for the light trapped inside the LED structure. In the case of LEDs grown on PSS however, as more photons are readily extracted through the sapphire substrate, these photons are often refracted down and are absorbed by packaging materials in an LED package. It is, therefore, important to find novel technologies that reduce the amount of light that is guided and/or absorbed inside LEDs, and with respect to this issue, silica nanospheres (SNs) embedded in PSS are studied in this report in an attempt to enhance the light output power and to manipulate the direction of light propagation of an LED. Two different sizes of SNs are constructed to investigate those effects of SNs on LED performance. SNs with a size of 300 nm are close in size to the emission wavelength, and the other has a larger size (i.e. 500 nm) than the emission wavelength. Eventually, SNs that are similar size to the emission wavelength appear to be promising since they enhance light extraction efficiency to the top side by increasing light scattering and also improve the internal quantum efficiency by reducing the dislocation density. 2. Experiments Epitaxial layers were grown by using low-pressure metal-organic chemical vapor deposition (MOCVD). Trimethylgallium (TMGa) and trimethylaluminum (TMAl) were used as Ga and Al metal-organic precursors, respectively, and ammonia (NH3) was used as a nitrogen source. Bis (cyclopentadienyl)-magnesium (Cp2Mg) and silane (SiH4) were used as the p- and n-type dopant sources, respectively. Figure 1 shows the schematic diagram of the fabrication process of the SNs-embedded LED (SNE-LED) structure. First, a 30 nm thick GaN nucleation layer was deposited at 525 °C on PSS, followed by the growth of un-doped GaN at 1020 °C for 60 min. During this GaN growth step, well-aligned hexagonal depressions with {1-101} facets formed on the PSS template, as shown in Fig. 1(b). SNs of two different sizes (300 and 500 nm) and four different concentration of coating solution (5, 8, 10 and 12%) were then coated onto the hexagonal depressions via spin coating, as shown in Fig. 1(c). After coating, the samples were loaded back into the MOCVD chamber. In order to achieve a planar surface, 2 µm thick un-doped GaN was regrown at 1060 °C, followed by 2 µm thick Si-doped n-type GaN, as shown in Fig. 1(d). To obtain 385 nm emission, GaInN/AlGaN five period multiquantum wells/barriers were grown at 780/850 °C, respectively. A p-type Al0.2Ga0.8N and 150 nm thick Mg-doped p-type GaN were finally grown. After mesa etching, a Ni/Au bilayer was deposited and annealed for the transparent conductive layer at 550 °C for 60 sec under O2 ambient. Then, Cr/Au layers were deposited as the metal contacts for both the n- and ptype layers. The LED wafers were diced into 650 × 650 µm2 chips. The GaN epilayers were analyzed in a cross-section and a top-view by a scanning electron microscope (SEM). High-
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1554
resolution X-ray diffraction (HR-XRD) was used to evaluate the crystal quality of the GaN films. UV-VIS-NIR spectrophotometry was used to investigate the effects of the SNs on reflectance. Light output power vs. current (L-I) measurements were carried out using an optical detector connected in a parameter analyzer, and electroluminescence (EL) properties were characterized using a multichannel spectroradiometer (OL770).
Fig. 1. Schematic diagrams of key steps for fabricating SNs-embedded LED structures: (a) PSS substrate, (b) growth of inverted polygonal pyramid structures (IPPSs) grown on PSS, (c) self-assembled SNs on IPPSs, (d) the epilayer after re-growth.
3. Results and discussion Figures 2(a) and 2(b) respectively show the plan-view and cross-sectional SEM images of the inverted polygonal pyramid structures (IPPSs) grown on PSS. Most of the IPPSs have an opening of about 3 µm in diameter and a depth of around 2 µm. The pattern size and the spacing of PSS can be controlled to adjust the height and the diameter of the IPPSs [11]. It is necessary to optimize the size of the IPPSs because the size influences on obtaining a coalesced GaN template and confining a number of SNs in the IPPSs.
Fig. 2. SEM images of the (a) plan-view, (b) bird-view of IPPSs formed on PSS, and crosssectional view of regrown GaN on (c) 300 nm and (d) 500 nm-SNs.
The growth of {10-1-1} inclined facets and c-plane {0001} facets was controlled mainly through growth temperature, pressure, V/III ratio, fill factor and orientation of the PSS pattern. In particular, a change in the alignment of the PSS patterns from to led to a formation of c-plane (0001) on top of GaN pyramids or stripes with triangular cross-
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1555
sections. In this study, the PSS patterns were arranged along the direction, resulting in six triangular c-planes around a top of patterned sapphire that acted as a seed layer when the LED structures were regrown. After the growth of the IPPSs, SNs with 300 nm and 500 nm diameters were spin-coated under optimized conditions. Figures 2(c) and 2(d) clearly show that SNs are selectively self-stacked, forming multiple layers in the IPPS. The SNs stacked in the IPPSs can be a result of capillary force and geometric confinement. The capillary force originates from the liquid meniscus and seems to play the most important role for self-assembled confinement of SNs in the IPPS. We observe that SNs with 300 nm size (using 8 wt% solution) fill the depression region with higher density when compared to those with 500 nm size (using 10 wt% solution), as shown in Figs. 2(c) and 2(d). This means that a higher amount of 300 nm-SNs are confined in the IPPS than that of 500 nm-SNs. The higher density of embedded SNs provides the higher probability that photons are scattered to escape from the LED. The regrowth of GaN starts at the c-plane {0001} facets that are not covered by SNs. These {0001} facets serve as a seed layer for epitaxial regrowth of high-quality GaN, and consequently, a GaN layer is fully coalesced over the SNs, followed by growth of MQW and p-type GaN structures. To further investigate the effects of the density of the SNs on the structural properties of the regrown GaN, the full-width-at-half-maximum (FWHM) of asymmetric (10-12) and symmetric (0002) planes were measured via HR-XRD, as shown in Fig. 3(a). The density of embedded SNs in the IPPSs is controlled by the concentration of a colloidal solution that is composed of the silica particles diluted with an ethanol solvent. The FWHM values for the samples with SNs are significantly improved compared to the samples without SNs, irrespective of the size. For the symmetric (0002) plane, the FWHM value of the GaN regrown on 500 nm-SNs (10 wt% solution) decreases to 262 arcsec compared to that of GaN without SNs, 306 arcsec. The FWHM value of the asymmetric (10-12) rocking curves of the GaN regrown on SNs decreases from 286 to 234 arcsec as the concentration of the solution increased.
Fig. 3. (a) FWHM values according to the size and concentration of the SNs solution and (b) diffuse reflectance according to wavelength for 300 nm and 500 nm SNE-LEDs and for the LED without SNs.
The amount of SNs stacked in the IPPSs increases with an increase in the concentration of the colloidal solution. It should be noted, however, that it becomes increasingly difficult to obtain a planar GaN surface after regrowth as the concentration of the solution increases. The GaN is not coalesced in case of 500 nm-SNs (12 wt% solution) and 300 nm-SNs (10, 12 wt% solution). An excessive concentration of the solution leads to stacking the SNs even on the cplane surface, resulting in incomplete coalescence of GaN at the regrowth stage. It is wellknown that the ω-scan rocking curves of symmetric (0002) planes are related to screw-type dislocations, whereas those of asymmetric (10-12) planes are widened by edge and mixedtype dislocations. Based on the XRD results, all types of dislocations decreased considerably for the GaN films regrown on SNs. This implies that a reduction of dislocations is enabled by
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1556
bending and blocking the dislocations, thus prevention from propagating upward to the regrown GaN [11]. To confirm the effect of SNs on light extraction at the near-UV region, we investigated the diffuse reflectance of the GaN films regrown on SNs. The samples were illuminated from the front, and the reflected light was detected under diffuse reflection geometry in order to understand the light-scattering effect of the embedded SNs. Figure 3(b) shows the reflectances of the LEDs with 300 nm and 500 nm SNs as well as of those without SNs, and these are 44.4, 42.8, and 38.9%, respectively, at a wavelength of 385 nm. Interestingly, as the range of the wavelength increases, the reflectance of the 300 nm-SNE-LED gradually becomes the same as that of the 500 nm-SNE-LED at a wavelength in the 400 nm range. The reflectance goes into reverse at a wavelength in the range of 420 nm. The reflectance of 300 nm-SNE-LED is lower than that of 500 nm-SNE-LED above wavelengths of 420 nm. The reflectance goes into reverse at a wavelength in the range of 420 nm. The reflectance of 300 nm-SNE-LED is lower than that of 500 nm-SNE-LED in the wavelengths above 420 nm. The reflectances of LEDs with 300 nm and 500 nm-SNs are 47% and 48.6%, respectively, at a 500 nm wavelength. A significant finding in this study is that the reflectance has an inversion when the size of nanoparticles gets closer to the wavelength of light, as shown in the inset of Fig. 3(b). The reflectance of 500 nm-SNs is higher than that of 300 nm-SNs at the wavelength of 500 nm, whereas the reflectance of 300 nm-SNs is higher than that of 500 nmSNs at the wavelength of 380 nm. This could be explained by noting that Mie scattering plays an essential role in increasing the reflectance. Qualitatively it is clear that increasing the diameter will red-shift the Mie scattering peak [12, 13]. This means that the emitted light can be efficiently scattered using 300 nm-SNs that have a size close to that of the emission wavelength of the LEDs. Also, smaller particles must be better stacked inside the depression region, resulting in enhanced light output power.
Fig. 4. Light output power of three different LED chips versus forward injection current measured from (a) the front and (b) the back of the LED chips.
Light propagation is closely examined by measuring the light output power using two photo-diodes at normal incidence, that are located at the front and back of the LED chips. Figures 4(a) and 4(b) show the light output powers of the LEDs with 300 nm SNs, 500 nm SNs, and without SNs measured at the front and back of the LED chip, respectively. The LEDs with SNs show higher light output power than the LED without SNs, not only at the front, but also at the back, irrespective of the SN sizes. At an injection current of 20 mA, the light output power of the 300 nm-SNE-LED at the front is 1.3 times higher than that of the 500 nm-SNE-LED, as shown in Fig. 4(a). At the back, the light output power of the 500 nmSNE-LED is 1.7 times higher than that of the 300 nm-SNE-LED, as shown in Fig. 4(b). The major reason for the highest light output power of the 300 nm-SNE-LED at the front is that the larger amount of embedded SNs in IPPS induces an increase of the light extraction due to
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1557
the increased light scattering of the SNs. Once the incident light hits the top layer of SNs, parts of the scattered light would propagate towards the upper direction and the other parts would propagate in the horizontal direction to adjacent SNs, where they might undergo a chain of scattering events. The smaller particles could be stacked with higher density in the IPPSs, resulting in an increase in the probability that photons are scattered. This behavior is important when we want to increase the possibility for light extraction toward the front side, not to be captured in the packaging materials. To further validate these results, far-field radiation patterns from packaged LED chips were measured at an operating current of 20 mA, as shown in Fig. 5. In the case of LEDs that were grown on PSS, more photons are guided into the sapphire due to its hemispherical geometry, resulting in deflected it to back of sapphire. The light was observed to enter into the sapphire and to be guided toward the side even more by the hemispherical structure of the PSS, resulting in a large viewing angle in far-field radiation patterns [14]. In this experiment, viewing angles were measured from the front. A viewing angle of the LED is defined as the angle where the measured light intensity is half of its maximum value. The viewing angles for the LEDs with 300 nm, 500 nm-SNs and without SNs are 126.4°, 130.7°, and 137.1°, respectively. Smaller viewing angles and better intensity are obtained for the LED with 300 nm-SNs, and this indicates that more light is extracted towards the upward direction relative to other samples. Our results prove that smaller particles must be better stacked inside the depression region, and this provides higher probability for photons to be scattered to escape. Embedded SNs could also block photons to go toward the bottom through the substrate, resulting in the enhancement of light extraction at the front.
Fig. 5. Far-field radiation patterns of the LED chip packaging.
4. Summary In summary, we investigate the optical characteristics of SNs-embedded NUV-LEDs. The light output power improvement of the 385 nm AlGaN-based SNE-LEDs is a result of not only an improved crystal quality, but also of an increase in light scattering due to the large density of the SNs. Mie scattering is found to play an essential role in enhancing the light output power, and therefore, embedded SNs that are closer in size to the emission wavelength are more effective in extracting the light from a top side of LED since more photons are deflected upward. This study can be of benefit for a device operation by allowing for the manipulation of the effect of undesired emission patterns encountered with the use of PSS in NUV-LEDs. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (2010-0019694) and by the Ministry of Education (MOE), by the National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2013H1B8A2032197).
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1558
School of Semiconductor and Chemial Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, South Korea * [email protected]
Abstract: We demonstrate that the use of silica nanospheres (SNs) with sizes close to the emission wavelength of light-emitting diodes (LEDs) can enhance the light output power and manipulate the far-field emission pattern. Near-ultraviolet (NUV)-LEDs grown on a patterned sapphire substrate embedded with 300 nm SNs show a three times higher light output power than that without SNs, when measured through the top side. For far-field emission measurements, the LEDs embedded with 300 nm SNs show the significant increase of front emission due to the improved crystal quality of epitaxial films as well as the increase of Mie scattering effect of SNs. These experimental results indicate the important role of the size of embedded SNs in enhancing the light output power for NUV-LEDs. © 2014 Optical Society of America OCIS codes: (250.0250) Optoelectronics; (290.0290) Scattering.
References and links 1.
D. Britalan and W. Nunley, Optoelectronics: Infrared-Visible-Ultraviolet Devices and Applications, 2nd ed. (CRC Press, 2009) 2. D. J. Chae, D. Y. Kim, T. G. Kim, Y. M. Sung, and M. D. Kim, “AlGaN-based ultraviolet light-emitting diodes using fluorine-doped indium tin oxide electrodes,” Appl. Phys. Lett. 100(8), 081110 (2012). 3. H. Lu, T. Yu, G. Yuan, X. Chen, Z. Chen, G. Chen, and G. Zhang, “Enhancement of surface emission in deep ultraviolet AlGaN-based light emitting diodes with staggered quantum wells,” Opt. Lett. 37(17), 3693–3695 (2012). 4. C.-Y. Cho, M. Choe, S.-J. Lee, S.-H. Hong, T. Lee, W. Lim, S.-T. Kim, and S.-J. Park, “Near-ultraviolet lightemitting diodes with transparent conducting layer of gold-doped multi-layer graphene,” J. Appl. Phys. 113(11), 113102 (2013). 5. T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). 6. R. Windisch, C. Rooman, S. Meinlschmidt, P. Kiesel, D. Zipperer, G. H. Döhler, B. Dutta, M. Kuijk, G. Borghs, and P. Heremans, “Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes,” Appl. Phys. Lett. 79(15), 2315 (2001). 7. J. R. Lee, S. L. Na, J. H. Jeong, S. N. Lee, J. S. Jang, S. H. Lee, J. J. Jung, J. O. Song, T. Y. Seong, and S. J. Park, “Low resistance and high reflectance Pt/Rh contacts to p-type GaN for GaN-based flip chip light-emitting diodes,” J. Electrochem. Soc. 152(1), G92 (2005). 8. J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Ludowise, N. F. Gardner, R. S. Kern, and S. A. Stockman, “High-power AlGaInN flip-chip light-emitting diodes,” Appl. Phys. Lett. 78(22), 3379 (2001). 9. R. H. Horng, W. K. Wang, S. C. Huang, S. Y. Huang, S. H. Lin, C. F. Lin, and D. S. Wuu, “Growth and characterization of 380-nm InGaN/AlGaN LEDs grown on patterned sapphire substrates,” J. Cryst. Growth 298, 219–222 (2007). 10. Y. J. Lee, T. C. Hsu, H. C. Kuo, S. C. Wang, Y. L. Yang, S. N. Yen, Y. T. Chu, Y. J. Shen, M. H. Hsieh, M. J. Jou, and B. J. Lee, “Improvement in light-output efficiency of near-ultraviolet InGaN-GaN LEDs fabricated on stripe patterned sapphire substrates,” Mater. Sci. Eng. B-Adv. 122, 184–187 (2005). 11. Y. J. Park, J. H. Kang, H. K. Kim, Y. S. Katharria, N. Han, M. Han, B. D. Ryu, E.-K. Suh, H. K. Cho, and C.-H. Hong, “Two-step lateral overgrowth of GaN for improved emission from blue light-emitting diodes,” J. Cryst. Growth 372, 157–162 (2013). 12. D. W. Hahn, “Light scattering theory,” University of Florida (2006).
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1553
13. M. D. Lechner, “Influence of Mie scattering on nanoparticles with different particle sizes and shapes: photometry and analytical ultracentrifugation with absorption optics,” J. Serb. Chem. Soc. 70, 361–369 (2005). 14. T. S. Oh, H. Jeong, Y. S. Lee, A. H. Park, T. H. Seo, H. Kim, K. J. Lee, M. S. Jeong, and E.-K. Suh, “Light outcoupling effect in GaN light-emitting diodes via convex microstructures monolithically fabricated on sapphire substrate,” Opt. Express 19(10), 9385–9391 (2011).
1. Introduction AlGaN-based 380–400 nm near-ultraviolet light-emitting diodes (NUV-LEDs) have attracted much attention as a result of their promising applications in analytical instrumentation, medical treatments, biological agent identification, air/water purification, and white LEDs for solid-state lighting [1–3]. However, the efficiency of UV-LEDs needs to further improve in order to justify the replacement of current conventional UV lamps. Several methods have been developed to improve the light output power of UV-LEDs, such as the use of highly transparent conducting layers [4], surface texturing [5, 6], flip-chip technology [7, 8], and patterned sapphire substrates (PSS) [9, 10]. Among these methods, PSS has become widely used as a result of which the threading dislocation density in the GaN epilayer can be effectively reduced on PSS through epitaxial lateral overgrowth and also the sapphire patterns can act as a scattering center for the light trapped inside the LED structure. In the case of LEDs grown on PSS however, as more photons are readily extracted through the sapphire substrate, these photons are often refracted down and are absorbed by packaging materials in an LED package. It is, therefore, important to find novel technologies that reduce the amount of light that is guided and/or absorbed inside LEDs, and with respect to this issue, silica nanospheres (SNs) embedded in PSS are studied in this report in an attempt to enhance the light output power and to manipulate the direction of light propagation of an LED. Two different sizes of SNs are constructed to investigate those effects of SNs on LED performance. SNs with a size of 300 nm are close in size to the emission wavelength, and the other has a larger size (i.e. 500 nm) than the emission wavelength. Eventually, SNs that are similar size to the emission wavelength appear to be promising since they enhance light extraction efficiency to the top side by increasing light scattering and also improve the internal quantum efficiency by reducing the dislocation density. 2. Experiments Epitaxial layers were grown by using low-pressure metal-organic chemical vapor deposition (MOCVD). Trimethylgallium (TMGa) and trimethylaluminum (TMAl) were used as Ga and Al metal-organic precursors, respectively, and ammonia (NH3) was used as a nitrogen source. Bis (cyclopentadienyl)-magnesium (Cp2Mg) and silane (SiH4) were used as the p- and n-type dopant sources, respectively. Figure 1 shows the schematic diagram of the fabrication process of the SNs-embedded LED (SNE-LED) structure. First, a 30 nm thick GaN nucleation layer was deposited at 525 °C on PSS, followed by the growth of un-doped GaN at 1020 °C for 60 min. During this GaN growth step, well-aligned hexagonal depressions with {1-101} facets formed on the PSS template, as shown in Fig. 1(b). SNs of two different sizes (300 and 500 nm) and four different concentration of coating solution (5, 8, 10 and 12%) were then coated onto the hexagonal depressions via spin coating, as shown in Fig. 1(c). After coating, the samples were loaded back into the MOCVD chamber. In order to achieve a planar surface, 2 µm thick un-doped GaN was regrown at 1060 °C, followed by 2 µm thick Si-doped n-type GaN, as shown in Fig. 1(d). To obtain 385 nm emission, GaInN/AlGaN five period multiquantum wells/barriers were grown at 780/850 °C, respectively. A p-type Al0.2Ga0.8N and 150 nm thick Mg-doped p-type GaN were finally grown. After mesa etching, a Ni/Au bilayer was deposited and annealed for the transparent conductive layer at 550 °C for 60 sec under O2 ambient. Then, Cr/Au layers were deposited as the metal contacts for both the n- and ptype layers. The LED wafers were diced into 650 × 650 µm2 chips. The GaN epilayers were analyzed in a cross-section and a top-view by a scanning electron microscope (SEM). High-
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1554
resolution X-ray diffraction (HR-XRD) was used to evaluate the crystal quality of the GaN films. UV-VIS-NIR spectrophotometry was used to investigate the effects of the SNs on reflectance. Light output power vs. current (L-I) measurements were carried out using an optical detector connected in a parameter analyzer, and electroluminescence (EL) properties were characterized using a multichannel spectroradiometer (OL770).
Fig. 1. Schematic diagrams of key steps for fabricating SNs-embedded LED structures: (a) PSS substrate, (b) growth of inverted polygonal pyramid structures (IPPSs) grown on PSS, (c) self-assembled SNs on IPPSs, (d) the epilayer after re-growth.
3. Results and discussion Figures 2(a) and 2(b) respectively show the plan-view and cross-sectional SEM images of the inverted polygonal pyramid structures (IPPSs) grown on PSS. Most of the IPPSs have an opening of about 3 µm in diameter and a depth of around 2 µm. The pattern size and the spacing of PSS can be controlled to adjust the height and the diameter of the IPPSs [11]. It is necessary to optimize the size of the IPPSs because the size influences on obtaining a coalesced GaN template and confining a number of SNs in the IPPSs.
Fig. 2. SEM images of the (a) plan-view, (b) bird-view of IPPSs formed on PSS, and crosssectional view of regrown GaN on (c) 300 nm and (d) 500 nm-SNs.
The growth of {10-1-1} inclined facets and c-plane {0001} facets was controlled mainly through growth temperature, pressure, V/III ratio, fill factor and orientation of the PSS pattern. In particular, a change in the alignment of the PSS patterns from to led to a formation of c-plane (0001) on top of GaN pyramids or stripes with triangular cross-
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1555
sections. In this study, the PSS patterns were arranged along the direction, resulting in six triangular c-planes around a top of patterned sapphire that acted as a seed layer when the LED structures were regrown. After the growth of the IPPSs, SNs with 300 nm and 500 nm diameters were spin-coated under optimized conditions. Figures 2(c) and 2(d) clearly show that SNs are selectively self-stacked, forming multiple layers in the IPPS. The SNs stacked in the IPPSs can be a result of capillary force and geometric confinement. The capillary force originates from the liquid meniscus and seems to play the most important role for self-assembled confinement of SNs in the IPPS. We observe that SNs with 300 nm size (using 8 wt% solution) fill the depression region with higher density when compared to those with 500 nm size (using 10 wt% solution), as shown in Figs. 2(c) and 2(d). This means that a higher amount of 300 nm-SNs are confined in the IPPS than that of 500 nm-SNs. The higher density of embedded SNs provides the higher probability that photons are scattered to escape from the LED. The regrowth of GaN starts at the c-plane {0001} facets that are not covered by SNs. These {0001} facets serve as a seed layer for epitaxial regrowth of high-quality GaN, and consequently, a GaN layer is fully coalesced over the SNs, followed by growth of MQW and p-type GaN structures. To further investigate the effects of the density of the SNs on the structural properties of the regrown GaN, the full-width-at-half-maximum (FWHM) of asymmetric (10-12) and symmetric (0002) planes were measured via HR-XRD, as shown in Fig. 3(a). The density of embedded SNs in the IPPSs is controlled by the concentration of a colloidal solution that is composed of the silica particles diluted with an ethanol solvent. The FWHM values for the samples with SNs are significantly improved compared to the samples without SNs, irrespective of the size. For the symmetric (0002) plane, the FWHM value of the GaN regrown on 500 nm-SNs (10 wt% solution) decreases to 262 arcsec compared to that of GaN without SNs, 306 arcsec. The FWHM value of the asymmetric (10-12) rocking curves of the GaN regrown on SNs decreases from 286 to 234 arcsec as the concentration of the solution increased.
Fig. 3. (a) FWHM values according to the size and concentration of the SNs solution and (b) diffuse reflectance according to wavelength for 300 nm and 500 nm SNE-LEDs and for the LED without SNs.
The amount of SNs stacked in the IPPSs increases with an increase in the concentration of the colloidal solution. It should be noted, however, that it becomes increasingly difficult to obtain a planar GaN surface after regrowth as the concentration of the solution increases. The GaN is not coalesced in case of 500 nm-SNs (12 wt% solution) and 300 nm-SNs (10, 12 wt% solution). An excessive concentration of the solution leads to stacking the SNs even on the cplane surface, resulting in incomplete coalescence of GaN at the regrowth stage. It is wellknown that the ω-scan rocking curves of symmetric (0002) planes are related to screw-type dislocations, whereas those of asymmetric (10-12) planes are widened by edge and mixedtype dislocations. Based on the XRD results, all types of dislocations decreased considerably for the GaN films regrown on SNs. This implies that a reduction of dislocations is enabled by
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1556
bending and blocking the dislocations, thus prevention from propagating upward to the regrown GaN [11]. To confirm the effect of SNs on light extraction at the near-UV region, we investigated the diffuse reflectance of the GaN films regrown on SNs. The samples were illuminated from the front, and the reflected light was detected under diffuse reflection geometry in order to understand the light-scattering effect of the embedded SNs. Figure 3(b) shows the reflectances of the LEDs with 300 nm and 500 nm SNs as well as of those without SNs, and these are 44.4, 42.8, and 38.9%, respectively, at a wavelength of 385 nm. Interestingly, as the range of the wavelength increases, the reflectance of the 300 nm-SNE-LED gradually becomes the same as that of the 500 nm-SNE-LED at a wavelength in the 400 nm range. The reflectance goes into reverse at a wavelength in the range of 420 nm. The reflectance of 300 nm-SNE-LED is lower than that of 500 nm-SNE-LED above wavelengths of 420 nm. The reflectance goes into reverse at a wavelength in the range of 420 nm. The reflectance of 300 nm-SNE-LED is lower than that of 500 nm-SNE-LED in the wavelengths above 420 nm. The reflectances of LEDs with 300 nm and 500 nm-SNs are 47% and 48.6%, respectively, at a 500 nm wavelength. A significant finding in this study is that the reflectance has an inversion when the size of nanoparticles gets closer to the wavelength of light, as shown in the inset of Fig. 3(b). The reflectance of 500 nm-SNs is higher than that of 300 nm-SNs at the wavelength of 500 nm, whereas the reflectance of 300 nm-SNs is higher than that of 500 nmSNs at the wavelength of 380 nm. This could be explained by noting that Mie scattering plays an essential role in increasing the reflectance. Qualitatively it is clear that increasing the diameter will red-shift the Mie scattering peak [12, 13]. This means that the emitted light can be efficiently scattered using 300 nm-SNs that have a size close to that of the emission wavelength of the LEDs. Also, smaller particles must be better stacked inside the depression region, resulting in enhanced light output power.
Fig. 4. Light output power of three different LED chips versus forward injection current measured from (a) the front and (b) the back of the LED chips.
Light propagation is closely examined by measuring the light output power using two photo-diodes at normal incidence, that are located at the front and back of the LED chips. Figures 4(a) and 4(b) show the light output powers of the LEDs with 300 nm SNs, 500 nm SNs, and without SNs measured at the front and back of the LED chip, respectively. The LEDs with SNs show higher light output power than the LED without SNs, not only at the front, but also at the back, irrespective of the SN sizes. At an injection current of 20 mA, the light output power of the 300 nm-SNE-LED at the front is 1.3 times higher than that of the 500 nm-SNE-LED, as shown in Fig. 4(a). At the back, the light output power of the 500 nmSNE-LED is 1.7 times higher than that of the 300 nm-SNE-LED, as shown in Fig. 4(b). The major reason for the highest light output power of the 300 nm-SNE-LED at the front is that the larger amount of embedded SNs in IPPS induces an increase of the light extraction due to
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1557
the increased light scattering of the SNs. Once the incident light hits the top layer of SNs, parts of the scattered light would propagate towards the upper direction and the other parts would propagate in the horizontal direction to adjacent SNs, where they might undergo a chain of scattering events. The smaller particles could be stacked with higher density in the IPPSs, resulting in an increase in the probability that photons are scattered. This behavior is important when we want to increase the possibility for light extraction toward the front side, not to be captured in the packaging materials. To further validate these results, far-field radiation patterns from packaged LED chips were measured at an operating current of 20 mA, as shown in Fig. 5. In the case of LEDs that were grown on PSS, more photons are guided into the sapphire due to its hemispherical geometry, resulting in deflected it to back of sapphire. The light was observed to enter into the sapphire and to be guided toward the side even more by the hemispherical structure of the PSS, resulting in a large viewing angle in far-field radiation patterns [14]. In this experiment, viewing angles were measured from the front. A viewing angle of the LED is defined as the angle where the measured light intensity is half of its maximum value. The viewing angles for the LEDs with 300 nm, 500 nm-SNs and without SNs are 126.4°, 130.7°, and 137.1°, respectively. Smaller viewing angles and better intensity are obtained for the LED with 300 nm-SNs, and this indicates that more light is extracted towards the upward direction relative to other samples. Our results prove that smaller particles must be better stacked inside the depression region, and this provides higher probability for photons to be scattered to escape. Embedded SNs could also block photons to go toward the bottom through the substrate, resulting in the enhancement of light extraction at the front.
Fig. 5. Far-field radiation patterns of the LED chip packaging.
4. Summary In summary, we investigate the optical characteristics of SNs-embedded NUV-LEDs. The light output power improvement of the 385 nm AlGaN-based SNE-LEDs is a result of not only an improved crystal quality, but also of an increase in light scattering due to the large density of the SNs. Mie scattering is found to play an essential role in enhancing the light output power, and therefore, embedded SNs that are closer in size to the emission wavelength are more effective in extracting the light from a top side of LED since more photons are deflected upward. This study can be of benefit for a device operation by allowing for the manipulation of the effect of undesired emission patterns encountered with the use of PSS in NUV-LEDs. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (2010-0019694) and by the Ministry of Education (MOE), by the National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2013H1B8A2032197).
#214118 - $15.00 USD Received 20 Jun 2014; revised 1 Aug 2014; accepted 18 Aug 2014; published 29 Sep 2014 (C) 2014 OSA 20 October 2014 | Vol. 22, No. S6 | DOI:10.1364/OE.22.0A1553 | OPTICS EXPRESS A1558
