Optimized design on High-power GaN-based Micro-LEDs

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1 Optimized design on High-power GaN-based Micro-LEDs Jingmei Fan, Liangchen Wang, Jinxia Guo, Xiaoyan Yi, Zhiqiang Liu, Guohong Wang, Jinmin Li R&D Center for Semiconductor Lighting, Institute of Semiconductors, CAS, 183, China ABSTRACT The structure of micro-leds was optimized designed. Optical, electrical and thermal characteristics of micro-leds were improved. The optimized design make micro-leds suitable for high-power device. The light extraction efficiency of micro-leds was analyzed by the means of ray tracing. The results shows that increasing the inclination angle of sidewall and height of mesa, and reducing the absorption of p and n electrode can enhance the light extraction efficiency of micro-leds. Furthermore, the total light output power can be boosted by increasing the density of micro-structures on the device. The high-power flip-chip micro-leds were fabricated, which has higher quantum efficiency than conventional BALED s. When the number of microstructure in micro-leds was increased by 57%, the light output power was enhanced 24%. Light output power is 82.88mW at the current of 35mA and saturation current is up to 8mA, all of these are better than BALED which was fabricated in the same epitaxial wafer. The I-V characteristics of micro-leds are almost identical to BALED. Key words: GaN-based LED, Micro-LEDs, Light extraction efficiency, Ray tracing, Flip-chip. 1. INTRODUCTION Recently, Ⅲ-nitride wide band gap semiconductors have attracted considerable interest due to their applications for optoelectronic devices, which are active in the blue and ultraviolet wavelength regions and electronic devices capable of operation at high power conditions. [1],[2] GaN-based LEDs are a potential candidate for light sources in solid-state lighting because of their advantages of low energy consumption, high reliability, no pollution, and long life,but there is a long way to 2lm/W. At present, the bottlenecks to limit LEDs efficiency are the total reflection and the interior of material which is absorbing light. There are many methods to improve light extraction efficiency, some of them are used widely, for example, flip-chip, truncated inverted pyramid(tip) [3], surface roughened [4][5], substrate lift-off [6] ; some of them are still in research, for example, resonant cavity [7], photonic crystal [8] and micro-arrays [9-13]. Research results indicated that low-power micro-leds external quantum efficiency can increase 6% compared to normal LEDs. Furthermore, micro-leds have simple produce process and low request to material. However, micro-leds still have some problems in research, including difficulty in electrode connection, reduction of active area and bad radiating. Therefore micro-leds research concentrates on low-power device. Solid State Lighting and Solar Energy Technologies, edited by Jinmin Li, Yubo Fan, Ling Wu, Yong-Hang Zhang, Michael E. Coltrin, Yuwen Zhao, Nuofu Chen, Vladimir M. Andreev, Jai Singh, Proc. of SPIE Vol. 6841, 68418, (27) X/7/$18 doi: / Proc. of SPIE Vol

2 This paper improves micro-leds structure and technology to enhance device s electric and light characteristics, changes included unit shape, mesa height and inclination angle of side wall. At the same time, we used electrode-connection to improve micro-leds electric characteristic and reduce electrode absorption. We also adopted flip-chip to solve radiating problem. 2. METHODOLOGY Light s transverse absorption is one of reasons which cause broad area LEDs extraction efficiency is low. Usually, broad area LEDs transverse size is hundreds microns, that is far more than blue light s absorption length (47nm) in GaN. Therefore, mostly transverse light will be absorbed in propagation. However, micro-leds transverse size is nearly or smaller than absorption length, so the light which generates from active region can escape before absorption, this is the main reason that make micro-leds extraction efficiency improved greatly; secondly, micro-leds side wall also play important role to improve extraction efficiency [14] : (1) micro-leds side wall became rough through ICP etching, this way can restrain light s total reflection from side wall and improve light exit probability; (2) micro-leds light emergent area is much larger than broad area LEDs, given surface area to active area is ξ, micro-leds unit is smaller and theξ is larger, light escaped probability from LED is increased whenξ is raised. Using light stimulation software, we can find that there are several factors to affect micro-leds extraction efficiency, including micro-leds incline angle of side wall, unit density, mesa height and electrode absorption. For micro-leds unit was quadrangular with inclination angle, extraction efficiency changed when incline angle increased from 3 to 45, the results were shown in figure Light extraction efficiency(%) Inclination angle Fig. 1. Extraction efficiency changed along with incline angle s increase In figure 1, micro-leds extraction efficiency was improved greatly when inclination angle was above 35, micro-leds extraction was the highest when incline angle was 45. Device total area (24 µ m 24 µ m) was fixed and micro-leds Proc. of SPIE Vol

3 unit was quadrangular(2 µ m 2 µ m), changing the edge spacing of micro-leds unit and increasing the quantities of unit, when spacing length was µ m, the number of unit was ,calculated the extraction efficiency, the results were shown in figure 2. Extraction efficiency (%) Side Space of Microstructures δ Number of Microstructures Fig. 2. The effect of micro-leds unit density on extraction efficiency From figure 2, we found when the unit s edge spacing δ was reduced, the device s extraction efficiency changed slightly, extraction efficiency was about 17.2% when δ 2 µ m and it was basically unchanged; the extraction efficiency was reduced 1% when δ =4 µ m. Didn t increase the number of unit and just changed the edge spacing of unit, the change trend of extraction efficiency was as same as just increasing the number of unit. The increasing of surface area didn t improve LEDs extraction efficiency. Mesa distance was too closed which made light escaped from one unit and then entered the other unit, numbers of absorption made efficiency reduced. However, the probability of multiple absorptions was very low. At the same time, we found that micro-leds extraction efficiency was increased when the height of mesa was augment, but etching depth was too large to increase device s electrical properties. 3. DATA AND RESULTS In the same epitaxial wafer, we produced two kinds of micro-leds with same chip size 1mm 1mm but different unit density. The unit size of µ -LED1 was 2 µ m 2 µ m, unit shape was quadrangular with inclination angle, edge spacing length between units was 4 µ m, there were 288 units and total active area was 1152 µ m 2, and the microscope picture was shown in figure3(b); the unit size of µ -LED2 was 2 µ m 2 µ m, unit shape was quadrangular with inclination angle, every four units used the same n electrode, edge spacing length between units was 2 µ m and 3 µ m, there were 452 units and total active area was 188 µ m 2, microscope picture was shown in figure 3(c); at the same Proc. of SPIE Vol

4 time, traditional broad area LED was also produced in the same epitaxial wafer for comparing. Produce process was as following: (1)cleaning wafer with organic solvent and aqua regia; (2)washing wafer with de-ionized water; (3)blowing wafer to dry with N 2 ; (4)putting wafer in PECVD cavity to deposit SiO 2 which was for mask in etching mesa; (5)etching mesa by ICP, where etching gas was Cl 2 /Ar and etching depth was 1 µ m; (6)depositing Ti/Al/Ti/Au n-electrode on n-gan; (7)depositing SiO 2 on n-electrode as insulting layer and passivation layer using PECVD; (8)photolithography and etching SiO 2 for Ni/Au transparent electrode (There are 1 1 µ m 2 p-contact area on every unit); (9)annealing in oxygen at 5 for 5 min; (1)depositing Al/Ti/Au for p-thickening electrode. Moreover, the flip-chip method was used to improve radiating and p n electrode connection. LL t:::: g : t;ifl - :II!9E!1 (a) (b) (c) H±L ::'::_J ::'::::::::k::: :::::::: : :: :::LJ ::: :: :: ::: ::i::: : ::.: :::: JL :: jjç Fig.3. The microscope photograph of BALED µ -LED1 and µ -LED2 XJ4822 semiconductor character graphic instrument was used to measure I-V characteristic of three kind chips after packaging. The results were shown in figure BALED µ-led2 (452) µ-led1 (288) 3 Current(mA) Forward Voltage(V) Proc. of SPIE Vol

5 Fig.4. The I-V character of µ -LED1 µ -LED2 and BALED From the linear part of I-V character curve, we can calculate the series resistance of BALED, µ -LED1 and µ -LED2 was 2.32Ω 2.6Ω and 3.46Ω, start voltage was 2.6V 2.7V and 2.7V, and the series resistance of µ -LED1 was largest. The p-electrode contact area of µ -LED1 was much smaller than µ -LED2 and BALED s, it was only 64% to µ -LED2 and 5% to BALED. Although p-electrode contact area of µ -LED2 was much smaller than BALED, the series resistance of µ -LED2 was only.28ω higher than BALED s, this is because of broad thickening electrode connection which made all units paralleled and device s total resistance reduced. Furthermore, though reducing ICP etching damage and improving the compactability of SiO 2 which was for insulating and passivation layer, the power micro-leds inverse leakage current was small to.1~5 µ A under -5V. µ -LED1 µ -LED2 and BALED s lighting photograph after flip-chip were shown in figure 5(a),(b),(c). F s.s5ss..5 I...S ss.S...S. S..s.SS.SS fr.11 SSSSSSS SSSSSSSSS...5.S5SSSS SSSS SS..S..S.S.S S5SS SSS555 5SS IS5SS SSSSS S S S.S.S S5 S5SSSSSS S..SSS5SSSSISSS55S SS...S5 S S S.S.S.SS..S.SS.SSSS S.S.S S Ss (a) (b) (c) Fig.5. The lighting photograph of µ -LED1 µ -LED2 and BALED At the same electric current, the relative luminescent intensity for three kinds of LED chips were shown in figure 6. When electric current was smaller than 45mA, BALED s luminescent intensity increased faster than µ -LED1 and µ -LED2 with incremental current. However, BALED s relative luminescent intensity was decreased rapidly when current continued increased and saturation current was 65mA; µ -LED1 s saturation current was above 45mA; µ -LED2 wasn t saturated until 8mA and its luminescent intensity was stronger than BALED when current was over 65mA. BALED s relative luminescent intensity was 9.9% higher than µ -LED2 with 35mA. The active area of µ -LED1 was 1/5.2 to BALED and µ -LED2 was 1/3.3 to BALED. At the same current, micro-leds current density was higher than BALED which made micro-leds relative luminescent intensity increased slowly with current. µ -LED1 s current density was highest. µ -LED1 s current density was 35A/cm 2 when current was 45mA, at the same condition, µ -LED2 s current density was 249A/cm 2 and BALED s was 74.5A/cm 2, so µ -LED2 reached Proc. of SPIE Vol

6 saturation quickly. The results were shown in figure 6. Relative luminous intensity(a.u.) BALED u-disk LED (288) u-disk LED (452) Current(mA) Fig.6. Relative luminescent intensity versus current of µ -LED1, µ -LED2 and BALED after flip-chip In figure 7, we showed luminous power density of three kind chips changed with current density before package, we found that micro-leds luminous power density was higher than BALED, it indicated micro-leds quantum efficiency was higher than BALED. Light output power density(mw/mm 2 ) µ-led1 µ-led2 BALED Current density J (A/cm 2 ) Fig.7. The output luminous power density versus current density of µ -LED1, µ -LED2 and BALED at same current 4. CONCLUSION The high-power flip-chip micro-leds were fabricated firstly and successfully, which had higher quantum efficiency than conventional BALED, When the amount of microstructures in micro-leds was increased by 57%, the light output power was enhanced 24%. The light output power of micro-leds was 82.88mW at the current of 35mA, which was higher than BALED. The I-V characteristics of micro-leds were almost identical to BALED. Proc. of SPIE Vol

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