GaN-based light-emitting diode (LED) was fabricated on the sapphire substrate with monolithic convex microstructures (CMs) array. Using confocal scanning electroluminescence (EL), we have directly observed the strong outcoupling phenomenon of the light confined in a LED via the CMs array. This outcoupled light could be efficiently converged on the convex center through consecutive reflections at the flat area and the curved slant area of the CMs array. Compared to the conventional LED, the ray tracing simulation and far field EL results of the LED with a CM array showed efficient light extraction toward the top surface, i.e., 0-5, 40-45 and 60-65 degree by the outcoupling effect. We conclude that the outcoupled optical path via CMs is the dominant factor of the enhanced light extraction in the LED with a CM array.
© 2011 OSA
GaN-based light-emitting diodes (LEDs) are in high demand because of their potential applications in terms of full-color displays, traffic signals and general lighting. However, large differences in the fundamental properties between the GaN layer and hetero-substrates can lead to deterioration of optical and structural properties. Furthermore, the occurrence of trapped or guided light in LEDs has resulted in an extremely low light extraction efficiency, ηLEE, and low reliability of GaN-based LEDs since they can be converted to heat in the LED. Quantitatively, wave-guided light in the GaN layer and sapphire substrate comprises around 66% and 22% of the total emission, respectively. Therefore, the external quantum efficiency, ηEQE, for conventional blue LEDs is still limited to a value of around 30% . In order to improve the quantum efficiency, a few methods have been proposed such as the use of 2-dimensional photonic crystals, substrate patterning, lateral epitaxial overgrowth (LEO), Bragg reflector, a graded-refractive index (GRIN) structure and surface plasmons coupling [2–6]. Among these methods, the monolithic patterning method of sapphire substrates has been explored extensively for the achievement of high quantum efficiency [7,8]. It is well known that this method promises effective outcoupling of the wave-guided light in both the GaN layer and sapphire substrate trapped by total internal reflection, whereas photonic crystal, GRIN and LEO methods only focus on the light guided in the GaN layer. In case of curved shape patterns, in particular, it can provide larger variation in the angle at the surfaces of the pattern. A simulation based on ray-tracing has suggested that micro-sized curvature patterning of the sapphire substrate, i.e microlens or hemispherical shapes, can produce high ηLEE compared to those of cone, cylindrical shapes, etc . Although, there have been various demonstrations of GaN-based LEDs fabricated using curved slant-patterning methods of a substrate [10–12], the majority of scientific efforts have been concentrated on improved crystallinity and output power efficiency. In other words, direct observation of outcoupling phenomena and their effect on local electroluminescence within LEDs has shown extremely limited achievement.
In this work, convex microstructure (CM) arrays have been fabricated monolithically on the sapphire substrate using the photoresist reflow . We show experimentally the effective light-outcoupling phenomenon in LED structures by the use of confocal scanning electroluminescence microscopy (CSEM) and demonstrate the enhanced light extraction via the light outcoupling effect.
The LEDs epistructures on (0001) sapphire with CM array and conventional flat sapphire substrate were grown by metalorganic chemical vapor deposition (MOCVD), consisting of 20-nm-thick GaN nucleation layer, 2-µm-thick undoped GaN, 2 µm n-type GaN, InGaN/GaN multiple-quantum-wells MQWs active layers, and 250-nm-thick p-type GaN layers. During the MOCVD growth, trimethylgallium, trimethylindium, and NH3 have been used as precursors for Ga, In, and N, respectively. Hydrogen was used as the carrier gas except when nitrogen was used for the InGaN MQWs and GaN barrier layers. To fabricate LEDs, the grown LED epi-wafer was partially etched until the n-type GaN layer is exposed. 5 nm/5 nm thick Ni/Au was deposited as a transparent conductive layer. The thickness of Cr/Au n- and p-electrodes was 50 nm/250 nm, respectively.
The local electroluminescence (EL) of the fabricated LED has been investigated by CSEM. Modified confocal microscopy (Witec Gmbh) with a high numerical aperture of 0.9 objective lens was utilized to collect the light emitted from the 300 x300 μm2 LED device. Static current by the KEITHLEY 2400s was applied to the LED chip during the scanning. Spatial EL distribution within a LED was characterized on various focal planes, i.e. from the interface between LED epistructures and sapphire substrate to the LED surface, varied by 500 nm steps, and focal planes were precisely controlled by the adjustment of a piezoelectric stage. Light collected from focal planes were delivered to a monochromator through a multimode optical fiber and detected by a cooled charge coupled device detector. Wherein, the optical fiber acts as the role of a pinhole. Also, the light output power measurement of the fabricated LEDs was performed as a function of injection current. The fabricated LED chips were tested on the wafer form, where the light output power was estimated using a calibrated Si photo-detector placed 5 mm from the front semitransparent p-metal contact.
3. Results and Discussion
Figure 1 presents the scanning electron microscopy (SEM) image of the CM array monolithically fabricated on sapphire substrate. The diameter and spacing of the fabricated convex microstructure were 3 and 2 μm, respectively. The LED epilayer was structured on the CM array patterned sapphire by MOCVD. Figure 1(b) shows the atomic force microscopy (AFM) image of the structured LED surface. Full coalescence of the epitaxial layer occurred over the whole area, but substantially rough surfaces were revealed with randomly positioned hexagonal pyramid hillocks, as frequently observed in the p-type GaN. In particular, many white islands of 20 ~40 nm diameter were exhibited. These small islands seem to appear during the thermal activation of the p-GaN, presumably caused by the presence of Ga-O complexes . Transmission electron microscopy (TEM) image of the as-grown LED epistructure on sapphire with CM arrays are as shown in Fig. 1(c). It appeared that a number of threading dislocations (TDs) persist on the CM regions as indicated by arrows, while flat regions between the CM array elements have few dislocations. Further, it is observed that some of TDs generated on CM regions have propagated into active layers of MQWs, which may cause the luminescence depression on CM regions. Details of the defect structures of the GaN epilayer grown on CM patterned sapphire substrate are described elsewhere .
Figure 2(a) shows near field optical microscopy photoluminescence (NSOM-PL) image mapped from the surface of the LED with a CM array. While the spatial resolutions of conventional micro photoluminescence (PL) systems are limited by the light diffraction, the NSOM-PL technique can provide higher spatial resolution down to 50~100 nm. In the NSOM-PL image, we can see a definite distinction in the contrast between the bright and dark regions. As indicated by dashed circle lines, these dark spots, which might be caused by relatively high density of non-radiative recombination channels, coincide with the CM array patterning positions. This luminescence distribution is in good agreement with that of cathodoluminescence in our previous work , and these phenomena have been supported by TEM results shown in Fig. 1(c). However, in Fig. 2(b), a micro-EL mapping image recorded under a 5 mA forward current inversely displays an emission distribution with respect to the NSOM-PL image. Amongst the positions marked by A, B and C, intense EL (λ ~448 nm) has been detected from regions above CM as can be seen in spectra shown in Fig. 2. (c); those regions have high defect density regions. Discrepancy in the luminescence intensity between PL and EL has been frequently observed in the blue InGaN MQWs luminescence. Onushkin et al.  suggested that it originates from the reduced local acceptor concentration at extended hillock regions of the p-GaN surface. In our case, however, it does not seem to have a direct interrelation between the surface roughness and luminescence properties as shown in the AFM image. On the other hand, it is clear that CM patterning dominantly affects the spatial EL distribution as well as defect structure of LEDs.
In order to investigate the physical origin of this discrepancy in EL and PL, 3-dimensional CSEM mapping was carried out at various focal plane depths as shown in the cross sectional SEM image of Fig. 3(a) . Figure 3(b)-(i) exhibit CSEM mapping images corresponding to positions marked by dashed line by (b)-(i) of Fig. 3(a), respectively, where focal plane positions are varied from (b), the interface between LED epi and sapphire, to (h), 1.5 μm below the surface with a 500 nm step distance. Finally, CSEM image from LED surface was mapped as shown in Fig. 3(i). When the focal plane was located near an interface between a sapphire and LED epilayer [Fig. 3(b)], relatively intense emission was observed near the circumference of CMs. As the focal plane approaches the LED surface, however, it was clearly observed that the EL distribution was enhanced at the center of the CM [Fig. 3(i)], despite of the significant structural deterioration caused by TDs above the center of the CM. In terms of the geometrical optics, the light from active layers seemed to converge on convex centre through strong outcoupling near the circumference of the CM arrays after they were emitted from MQWs layers or wave-guided in the LED structure.
In Fig. 4 , the X-Z scanned CSEM mapping has revealed clearly the convergent outcoupling phenomenon on the CM regions, which assumes triangular ridge-like shapes. The dominant outcoupling path can be caused by consecutive reflections at the flat substrate and a curved slant position of the CMs. Here, it should be noted that this phenomenon was not observed in the PL image shown in Fig. 2(a). We believe that the reason is that PL distribution may be dominantly affected from defects, Indium inhomogeneities, strain effect or etc rather than outcoupling effect because only PL emission within active regions is detected. Consequently, the critical angle of the light escape cone does not change, but we can expect the stronger EL extraction at the specific angle ranges via light outcoupling effect.
If we measure internal quantum efficiency (ηIQE) experimentally, based on the equation given by ηEQE = ηIQE × ηLEE we can estimate the ηLEE of LEDs. In order to investigate the enhancement of the ηLEE in the LEDs with CMs, we measured PL spectra at 10 and 300 K and the EL output power as a function of the injection current, respectively. Assuming that the ηIQE is 100% at 10 K, the ηIQE could be estimated by comparison of PL intensities. However, ηIQE in the InGaN MQWs system is well known to be strongly dependent on injection carrier density. To exactly estimate the ηLEE enhancement by the outcoupling effect, ηIQE is calculated through the excitation power-dependent PL measurement. As shown in the inset of Fig. 5 , the ηIQE of LEDs with and without CMs were averagely estimated to be 35 and 24%, respectively, indicating that the ηIQE of the LED with CMs was increased 38% compared to that of the conventional LED. Such an increment in ηIQE may be attributed to the reduction of dislocation density through CMs-induced lateral growth behavior as shown in previous works . In the EL output power measurement, further the EL output power of the LED with CMs was 2.4 times higher than that of the conventional LED at normal operation current, 20 mA. Based on our results, the increase in ηLEE was estimated to be 73.5%, presumably due to dominantly resulted from the enhanced light extraction by CM array.
In order to understand the outcoupling effect on the enhanced light extraction of the LEDs with CM array, both an optical simulation, i.e., Light Tool based on Monte Carlo method, and far field EL measurement have been carried out as shown in Fig. 6 . In the simulation, the CM shape was assumed to be a hemisphere and the light loss by metals such as pad and transparent conducting layer was ignored. As shown in Fig. 6(a) and (b), compared to that of the conventional LED grown on flat sapphire, the light extraction efficiencies for top and bottom directions were increased up to 227% and 335%, respectively, while that for side wall directions was slightly increased by 26%. It clearly indicates that the light extraction from the LED with CMs is higher than that from the conventional LED because of the enhanced light coupling effect on the CM arrays of a substrate toward top and bottom directions rather than four sidewall directions. From this result, we expect that enhanced light extraction via an outcoupling phenomenon can affect the far field EL distribution from the LEDs with CM array. In order to clearly demonstrate its effect, the chip level EL gain was simulated and measured as a function of far field angles, respectively. Here, the gain is defined as the ratio of the EL intensity of LED with CM array to conventional LED. As shown in Fig. 6(c), relatively high gain peaks in the simulation result have revealed near angles of 0-5, 40-45 and 60-65 degree, respectively, which can be dominantly contributed to the light outcoupling effect. Such tendency reasonably agreed with experimental result. We carefully suggest that the discrepancy in gain values between simulation and measurement may be primarily due to light absorption into metallic layers or/and the significant difference in the IQE of LED epistructures. Consequently, our results have explained that the light confined in the LED can be efficiently outcoupled through CMs, and such a converged light path observed in CSEM mapping images maximizes the extraction efficiency at specific angle ranges compared to the conventional LED.
The light outcoupling phenomenon has been demonstrated experimentally in the GaN-based LED fabricated on the sapphire substrate with CM array. Based on the results of CSEM measurements, we directly observed that the light confined in LEDs can be effectively coupled out through CMs. The simulated and measured far field EL results of the LED with a CM array compared to that of the conventional LED have revealed an analogous tendency to lead the strong light outcoupling toward the top surface, around 0-5, 40-45 and 60-65 degree. We believe that this is the dominant factor of the enhanced light extraction efficiency in the LED with a CM array.
This research was supported by the Ministry of Knowledge Economy (MKE) and Korea Institute for Advancement in Technology (KIAT) through the Workforce Development program in Strategic Technology, and by Priority Research Centers Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (2009-0094032).
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