Open Access
Add to BibSonomyAbstract
Multi-quantum wells (MQWs) InGaN/GaN LEDs, 300μm × 300μm chip size, were fabricated with Ta2O5 / SiO2 dielectric multi-layer micro-mirror array (MMA) embedded in the epitaxial-lateral-overgrowth (ELOG) gallium nitride (GaN) on the c-plane sapphire substrate. MQWs InGaN/GaN LEDs with ELOG embedded patterned SiO2 array (P-SiO2) of the same dimension as the MMA were also fabricated for comparison. Dislocation density was reduced for the ELOG samples. 75.2% light extraction enhancement for P-SiO2-LED and 102.6% light extraction enhancement for MMA-LED were obtained over the standard LED. We showed that multiple-diffraction with high intensity from the MMA redirected the trap lights to escape from the LED causing the light extraction enhancement.
©2010 Optical Society of America
1. Introduction
Gallium-nitride (GaN) based light emitting diodes (LED) are drawing much attention nowadays for purpose of illumination and display. However, total internal reflection at the interface between GaN and air, as a result of the high refractive index contrast between the GaN and the air, traps the light from being totally extracted. In order to improve the light extraction efficiency, many methods, such as photonic crystal LED [1, 2], metal reflector, Bragg reflector [3], have been proposed. On the other hand, ELOG process for GaN has advantages of reducing dislocation density and hence increasing the internal quantum efficiency [4–6]. Micron size SiO2 patterned-mask for ELOG GaN [7, 8], ELOG patterned sapphire substrate (PSS) [9, 10], and pyramidal-shape patterned SiO2 [11] were reported to give higher light output power for laser diode and LED. Ray tracing with Monte Carlo method was used to demonstrate that the patterned structure has optical effect of enhancing the light extraction efficiency [12]. Finite difference time domain (FTDT) analysis on the patterned sub-micron size SiO2 array 2-D photonics crystals found the similar result [13]. 1st order diffraction together with high reflectance from the small period, 300nm, auto-cloned photonics crystal on the back side of sapphire were proven to cause the light extraction enhancement for the LED [14].
In this paper, we propose a light extraction enhancement structure by using the heat-resistive dielectric MMA embedding in the ELOG GaN. Taking advantages of reducing dislocation density by ELOG together with the capability of diffraction and high reflectance of the patterned structure from the MMA, higher light output power for the LED are expected. LED with patterned SiO2 and standard LED were fabricated and tested for comparison.
2. Structure and fabrication
Three types of LED were fabricated: standard LED (std-LED), LED with ELOG patterned SiO2 array (P-SiO2-LED), and LED with ELOG MMA (MMA-LED). Their structures are shown schematically in Fig. 1(a), 1(b), and 1(c), respectively. The MMA and the P-SiO2 array were formed on the GaN template and they have the same geometry and dimensions, Fig. 1(d) shows their top view; both the element and the lattice are in hexagon shape coinciding with the {0001} c-plane of the sapphire substrate and the GaN template. The elemental hexagon has diagonal length of 3μm, and the lattice hexagon has side length of 6μm. The MMA consisted of 5 pairs of Ta2O5 (52nm)/ SiO2 (68nm) thin films deposited by ion-beam sputtering. Photo-lithography followed by inductive-coupled-plasma etching was used to form the MMA array. The patterned SiO2 array was fabricated with the same lithographic and etching processes as the MMA. The MMA and the P-SiO2 array were then subjected to GaN ELOG process. The GaN ELOG processes for the P-SiO2-LED and the MMA-LED were identical. Details of the fabrication process for the ELOG and the MMA were given in Ref. [15].
Besides the ELOG process, the remaining processes for the epitaxial structures for all three types of LED were identical. The LED epitaxial structures were a MQW In-GaN/GaN, they composed of 600nm thick Si-doped n-type GaN, followed by 10 pairs of GaN(2nm)/InGaN(2nm) pre-strain layer, 5 pairs of GaN(13nm)/InGaN(3nm) multi-quantum wells, 20nm thick of AlGaN electron blocking layers (EBL), 200nm thick Mg-doped p-type GaN, and 10nm thick heavy Mg-doped p+-type GaN. The MOCVD unit for growing the LED epitaxial structures was a Taiyo Nippon Sanso SR-4000 reactor. Mesa structure, 300μm × 300μm square, was then formed and followed by deposition of Ti(100nm)/Au(300nm) electrode for p-contact, Ti(200nm)/Al(600nm)/Ti(200nm)/Au(200nm) for n-contact and Ni(0.5nm)/Au(4nm) for transparent p-contact.
Fig. 1. (color online) Structures for the (a) standard LED (std-LED), (b) patterned SiO2 array LED (P-SiO2-LED), (c) micro mirror array LED (MMA-LED), and (d) orientation of the hexagonal array (top view).
3. Quality of the ELOG GaN
Figures 2(a) and 2(b) show the SEM pictures for the side-view of the P-SiO2-LED and the MMA-LED, respectively. It is clear that ELOG GaN with ~ 2.5μm thickness has come to a complete coalescence with flat top surface, and the LED epitaxial structures were laid on the flat surface. The P-SiO2 array and the MMA were intimately embedded in the ELOG GaN. The Ta2O5 / SiO2 multi-layers micro-mirrors were intact by the 1200ºC ELOG process, consistent with what was reported in Ref [15].
Figure 3 shows the TEM pictures, viewed through the (112̄0) GaN direction, for the MMA-LED. The empty patches (in white) were caused by over-polishing of the sample in the TEM preparation process. The dislocation defect lines are clearly shown in the TEM pictures. The dislocation defect density is high within the GaN template. In between the MMA, the dislocation defect become less dense as the ELOG GaN grows, few dislocation defects propagate to the LED epitaxial structures. The region above the MMA is dislocation-free. These observations are consistent with the general characteristics of the ELOG GaN [16]. Figure 4 shows the TEM pictures of the LED epitaxial structures, fine structures of the epitaxial layers can be clearly seen with high resolution. Figure 5 shows the room temperature photo-luminescence (PL) spectra for three types of LED measured by 325 nm He-Cd laser and integrating sphere. The yellow bands of the PL spectra for all three types of LED were inhibited. The MMA-LED shows the strongest PL effect and the std-LED shows the weakest PL effect. Both the higher internal quantum efficiency due to the reduced dislocation density from ELOG and the increased optical extraction by the patterned array are possible causes for the stronger PL spectra of the MMA-LED and the P-SiO2-LED.
4. Electrical and optical properties of the LEDs
Figure 6 shows the optical microscope pictures for three types of LED together with their blue electro-luminescence (EL) at 5mA forward current (If). The underneath P-SiO2 array and the MMA can be clearly seen. The MMA-LED has the brightest image and the std-LED has the dimmest. Since the external illumination level in the optical microscope was set to be the same for observation of three types of sample, therefore, the visual difference in the brightness of the samples under the optical microscope directly implies that there were differences in the light extraction efficiency between the samples. Figure 7 shows the I-V characteristics for three types of LED. Figure 8(a) shows the EL spectra for three types of LED driving at 20mA forward current and measured by using integrating sphere. Figure 8(b) shows the optical output power vs. forward current for three types of LED measured by using integrating sphere. The data points in Fig. 7 and 8 were average values taken from ~10 pieces of sample on a wafer for each type of LED, the error bars were not shown for purpose of figure clarity. Table 1 summarizes the relevant electrical and optical parameters extracted from Fig. 7 and 8, average values together with the standard deviations are shown.
Fig. 3. TEM pictures of the cross-sectional view for the MMA-LED, viewed through (112̄0) direction of GaN.
Fig. 4. TEM pictures of the cross-sectional view for (a) the epitaxial structure of the In-GaN/GaN MQWs and (b) close-up view.
Table 1 shows that, in terms of series resistance, forward voltage at 20mA and reversed current at -5V, the MMA-LED is slightly superior to the P-SiO2-LED, and they both are slightly superior to the std-LED in the electrical properties. However, the optical output power, measured at 20mA forward current, is 12.16mW, 10.52mW and 6.00mW for the MMA-LED, P-SiO2-LED, and std-LED, respectively. The light extraction enhancement, taking the std-LED as the comparison base, is 102.6% for MMA-LED and 75.2% for P-SiO2-LED.
Figures 9(a)–9(c) show the far field EL distributions for three types of LED measured by using an Imaging Sphere from Radiant Imaging. Figure 9(d) shows the polar distributions for three types of LED. The polar distributions in Fig. 9(d) were obtained by taking the average of the polar distribution from 18 meridional plans of the distributions in Figs. 9(a)–9(c). Part of the sideway guided intensity of the std-LED appeared in the far field distribution. Basically, all three types of LED follow a Lambertian far field distribution.
5. Mechanism for light extraction enhancement
In order to explore the mechanism for light extraction enhancement, we measured the front side far field diffraction patterns for samples of bare sapphire/GaN-template, bare sapphire/GaN-template/P-SiO2-array and bare sapphire/GaN-template/MMA, there were no ELOG GaN and epitaxial LED layers on these samples. The incident light was a He-Cd laser beam at 441.6nm. The front side far field diffraction patterns were recorded for different angle of incidence θi. Figure 10 illustrates the measurement configurations and show the images for the front side far field diffraction patterns on the screen. The white dash circles in the images indicating the diffraction angles. The diffraction patterns showed hexagonal symmetry consistent with the symmetry of the array pattern. The diffraction orders shifted away from the incident beam as the angle of incidence increased.
Fig. 6. (color online) Optical microscope pictures and the blue EL images, driving at 5mA, for (a) std-LED, (b) P-SiO2-LED, and (c) MMA-LED.
Fig. 8. (color online) (a) The electro-luminescence spectra driving at 20mA, (b) optical output power vs. forward current for the LEDs.
Fig. 9. (color online) 3-D far field intensity distribution for (a) std-LED, (b) P-SiO2-LED, (c) MMA-LED, and (d) 2-D far field intensity distribution for three types LED.
Table 1. Electrical and optical properties of the LEDs.
The diffraction intensity distributions along the hemi-circle on the plane of incidence were measured and are shown in Fig. 11 for three types of LED at each angle of incidence. The gray area, ±23°, in Fig. 11 corresponds to the hypothetical escape cone defined by the critical angle of the total internal reflection at the GaN and the air interface. The critical angle is ~ 23° when the refractive index of GaN is taken as 2.54. The light within the gray area will escape from the LED into the air and the light outside the gray area will be reflected back to the LED due to total internal reflection at the GaN/Air interface. For std-LED, the light outside the gray area will be guided sideway in the LED and can not be escaped into the front air. However, for P-SiO2-LED and MMA-LED, the light outside the gray area will be totally reflected at the GaN/Air interface and back to the diffraction array, those lights will get second diffraction by the array, some of the second diffracted lights will have chance to be within the gray area to escape, and the remaining lights will then be totally reflected back and go through the diffraction process third times. Thus, through multiple diffractions, eventually all the lights will be escaped into the front air provided that LED is infinitely extended laterally and the transmittance is zero. This is the mechanism that the diffraction array serves for the light extraction enhancement.
The difference between the P-SiO2-LED and the MMA-LED is that the MMA is a high reflector, the intensity of the front side diffracted lights are higher for MMA than for P-SiO2. This fact is revealed in Fig. 11, as one can see in Fig. 11 that the intensity of the diffraction orders for MMA are higher than that of the corresponding diffraction orders for the P-SiO2. The intensity of the transmitted lights for the MMA should be therefore lower than that of the P-SiO2. Under this circumstance, more lights can be extracted into the front air through multiple diffractions within the finite lateral extent of the LED for the MMA-LED than for the P-SiO2-LED.
We would like to point out that although the reduced dislocation density by ELOG process can increase the internal quantum efficiency of the MQW InGaN/GaN, and the relation between the internal quantum efficiency and the dislocation density was investigated [17], however, it is not trivial to relate the electrical parameters of the devices in Table 1 to the dislocation density quantitatively. It is not clear at this point whether the extraction efficiency enhancement is purely an optical effect or if it is enforced to some extent by the increased internal quantum efficiency from the reduction in dislocation density, despite the fact that the differences in the electrical parameters between three types of LED are small according to Table 1.
Our MMA can sustain the 1200ºC MOCVD process as we have reported previously [15]; conventional high reflecting metal mirrors and its array that can not sustain 1200ºC high temperature without degradation are not suitable for this purpose.
Fig. 10. (color online) Diffraction measurement configuration and the diffraction distribution for (a) sapphire/GaN-template, (b) sapphire/GaN-template/P-SiO2-array, and (c) sapphire/GaN-template/MMA.
Fig. 11. (color online) (a) Experimental setup for measuring the intensity distributions of the diffraction orders and (b)~(f) intensity distribution of the diffraction orders for different angle of incidence (θi) for sapphire/GaN-template (black line), sapphire/GaN-template/P-SiO2-array (red line), and sapphire/GaN-template/MMA (blue line).
6. Conclusion
Fabrication of MMA-LED and P-SiO2-LED were demonstrated, their electrical and optical properties were measured and compared to the standard LED. 102.6% and 75.2% light extraction enhancement were obtained for the MMA-LED and the P-SiO2-LED, respectively. The MMA serves as a high reflective and high temperature-resistive diffraction grating. Through multiple diffraction with high intensity from the MMA array, and possibly the increased internal quantum efficiency due to the dislocation density reduction by the ELOG process, high level light extraction efficiency for the MMA-LED were therefore obtained.
References and links
1. 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), 3885–3887 (2004). [CrossRef]
2. A. David, T. Fujii, R. Sharma, K. McGroddy, S. Nakamura, S.nP. DenBaars, E. L. Hu, C. Weisbuch, and H. Benisty, “Photonic-crystal GaN light-emitting diodes with tailored guided modes distribution,” Appl. Phys. Lett. 88, 061124 (2006). [CrossRef]
3. Y. S. Zhao, D. L. Hibbard, H. P. Lee, K. Ma, W. So, and H. Liu, “Efficiency enhancement of InGaN/GaN light-emitting diodes with a back-surface distributed bragg reflector,” J. Electron. Mater. 32, 1523–1526 (2003). [CrossRef]
4. T. Nishinaga, T. Nakano, and S. Zhang, “Epitaxial lateral overgrowth of GaAs by LPE,” Jpn. J. Appl. Phys. 27, L964–L967 (1988). [CrossRef]
5. A. Usui, H. Sunakawa, A. Sakai, and A. A. Yamaguchi, “Thick GaN epitaxial with low dislocation density by hydride vapor phase epitaxy,” Jpn. J. Appl. Phys. 36, L899–L902 (1997). [CrossRef]
6. O. H. Nam, M. D. Bremser, T. S. Zheleva, and R. F. Davis, “Lateral epitaxy of low defect density GaN layers via organometallic vapor phase epitaxy,” Appl. Phys. Lett. 71, 2638–2640 (1997). [CrossRef]
7. S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, “InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate,” Appl. Phys. Lett. 72(2), 211–213 (1998). [CrossRef]
8. S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, “Present status of InGaN/GaN/AlGaN-based laser diodes,” J. Crystal Growth 189/190 , 820-825 (1998). [CrossRef]
9. K. Tadatomo, H. Okagawa, Y. Ohuchi, T. Tsunekawa, Y. Imada, M. Kato, and T. Taguchi, “High output power In-GaN ultraviolet light-emitting diodes fabricated on patterned substrates using metalorganic vapor phase epitaxy,” Jpn. J. Appl. Phys. 40, L583–L585 (2001). [CrossRef]
10. M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, and T. Mukai, “InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode,” Jpn. J. Appl. Phys. 41, L1431–L1433 (2002). [CrossRef]
11. C-Y Cho, J-B Lee, S-J Lee, S-H Han, T-Y Park, J W Kim, Y C Kim, and S-J Park, “Improvement of light output power of InGaN/GaN light-emitting diode by lateral epitaxial overgrowth using pyramidal-shaped SiO2,” Opt. Express 18, 1462–1468 (2010). [CrossRef] [PubMed]
12. E. H. Park, J. Jang, S. Gupta, I. Ferguson, C. H. Kim, S. K. Jeon, and J. S. Park, “Air-voids embedded high efficiency InGaN-light emitting diode,” Appl. Phys. Lett. 93, 191103 (2008). [CrossRef]
13. M. K. Kwon, J. Y. Kim, I. K. Park, K. S. Kim, G. Y. Jung, S. J. Park, J. W. Kim, and Y. C. Kim, “Enhanced emission efficiency of GaN/InGaN multiple quantum well light-emitting diode with an embedded photonic crystal,” Appl. Phys. Lett. 92, 251110 (2008). [CrossRef]
14. C. Y. Huang, H. M. Ku, and S. Chao, “Light extraction enhancement for InGaN/GaN LED by three dimensional auto-cloned photonics crystal,” Opt. Express 17, 23702–23711 (2009). [CrossRef]
15. C. Y. Huang, H. M. Ku, W. T. Liao, C. L. Chao, J. D. Tsay, and S. Chao, “Heat resistive dielectric multi-layer micro-mirror array in epitaxial lateral overgrowth gallium nitride,” Opt. Express 17, 5624–5629 (2009). [CrossRef] [PubMed]
16. K. Hiramatsu, “Epitaxial lateral overgrowth techniques used in group III nitride epitaxy,” J. Phys: Condens. Matter 13, 6961–6975 (2001). [CrossRef]
17. Q. Dai, M. F. Schubert, M. H. Kim, J. K. Kim, E. F. Schubert, D. D. Koleske, M. H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, “Internal quantum efficiency and nonradiative recombination coefficient of GaInN/GaN multiple quantum wells with different dislocation densities,” Appl. Phys. Lett. 94, 111109 (2009). [CrossRef]
