Improvement of light output power of InGaN / GaN light-emitting diode by lateral epitaxial overgrowth using pyramidal-shaped SiO 2

We report on the improvement of light output power of InGaN/GaN blue light-emitting diodes (LEDs) by lateral epitaxial overgrowth (LEO) of GaN using a pyramidal-shaped SiO2 mask. The light output power was increased by 80% at 20 mA of injection current compared with that of conventional LEDs without LEO structures. This improvement is attributed to an increased internal quantum efficiency by a significant reduction in threading dislocation and by an enhancement of light extraction efficiency by pyramidal-shaped SiO2 LEO mask. ©2009 Optical Society of America OCIS codes: (230.0230) Optical devices; (230.3670) Light-emitting diodes; (230.4000) Microstructure fabrication; (310.6860) Thin films, optical properties References and links 1. E. F. Schubert, Light-emitting diodes (Cambridge University Press, Cambridge, U.K., 2003). 2. J. S. Speck, and S. J. Rosner, “The role of threading dislocations in the physical properties of GaN and its alloys,” Physica B 273-274(1-3), 24–32 (1999). 3. O. 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Introduction
High brightness group III-nitride based light-emitting diodes (LEDs) for visible light emission have many applications in display backlight units, automotive lighting and solid-state lighting. Although InGaN-based LEDs are already commercially available, further improvement of the light output power and the external quantum efficiency (η ext ) are required. The limited external quantum efficiency of LEDs is mainly attributed to the low internal quantum efficiency (η int ) and light extraction efficiency (η extraction ) [1]. The low internal quantum efficiency (η int ) results mostly from the very high density (10 9 -10 10 cm −2 ) of threading dislocations (TDs) that form due to the large mismatch of lattice constants and thermal expansion coefficients between GaN films and sapphire substrates. These threading dislocations in GaN act as nonradiative recombination centers and as a leakage path in the LEDs [2]. To reduce the dislocation density, several methods such as lateral epitaxial overgrowth (LEO) [3,4], pendeoepitaxy [5], and in situ SiN x nano-masking [6,7] have been developed. Another major reason for the low external quantum efficiency (η ext ) is a low light extraction efficiency (η extraction ), which is mainly due to the total internal reflection of light by the difference between the refractive indexes of GaN (n = 2.5) and air (n = 1). The improvement in light extraction efficiency is crucial, and several methods such as texturing of surface [8][9][10], a patterned sapphire substrate (PSS) [11,12], triangular chip design [13], and photonic crystal [14][15][16][17][18] have been proposed to enhance the light extraction efficiency of LEDs.
Recently, a few groups have reported that both η int and η extraction are enhanced by using patterned sapphire substrates [19,20] or air voids inserted between GaN epitaxial layer and patterned sapphire by chemical wet etching method [21]. Another group also improved the η int and η extraction of LEDs using inverted hexagonal pyramid dielectric LEO masks [22]. However, the effect of LEO mask shape on the improvement of the η int and η extraction was not clearly understood, compared to those of conventional GaN LED using a flat LEO mask.
In this study, we report on the InGaN/GaN blue LEDs grown on a GaN template using a pyramidal-shaped SiO 2 LEO mask. The pyramidal-shaped SiO 2 LEO mask inserted into an n-GaN was observed to reduce the threading dislocations. Furthermore, in comparison with conventional GaN LEDs using a flat SiO 2 LEO mask, the pyramidal-shaped SiO 2 LEO mask increased η extraction by changing the direction of light path. Particularly, the measurement of η int of LEDs by using temperature-dependent photoluminescence (PL) clearly showed that the η extraction can be enhanced by changing the LEO mask shape while maintaining the same pattern areas of SiO 2 masks.

Experiments
In the present study, the LEDs were grown on a c-plane (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). After the growth of a 25 nm-thick GaN nucleation layer at 550 °C, a 3 μm-thick n-GaN epitaxial layer was grown at 1020 °C. Then, a 3 μm-thick SiO 2 layer was deposited as an LEO mask on n-GaN by plasma-enhanced chemical vapor deposition (PECVD). An array of pyramidal-shaped SiO 2 masks on the n-GaN were obtained by undercut etching of SiO 2 in buffered oxide etchant (BOE) for 5 min after the photolithography process as shown in Fig. 1(a). Figure 1(a) shows that the pyramidal-shaped SiO 2 LEO masks with a width of 7 μm and a height of 1.7 μm were formed on the n-GaN layer. The spacing between SiO 2 masks was 3 μm. To confirm the effect of mask shape, the conventional LEO mask was also fabricated. In case of conventional LEO mask, after the deposition of a 100 nm-thick SiO 2 layer on n-GaN, an array of square-shaped SiO 2 masks were obtained on the n-GaN by etching SiO 2 in BOE for 30 sec and the photolithography process. The conventional LEO mask had the same width and spacing between SiO 2 masks as those of PSLEO mask. After the overgrowth of 4 μm-thick undoped GaN, a 2 μm-thick n-GaN was grown on the GaN template covered with pyramidal-shaped SiO 2 masks. Figure 1(b) shows a cross-sectional SEM image of a coalesced pyramidal-shaped lateral epitaxial overgrowth (PSLEO) GaN template. Full coalescence of GaN was achieved and the pyramidal-shaped masks were fully covered by the GaN epilayer. Then five periods of InGaN/GaN multiple quantum wells (MQWs) were grown at 770 °C, followed by the growth of a 200 nm-thick p-GaN layer at 950 °C. To fabricate LEDs, a p-GaN layer was etched by an inductively coupled plasma (ICP) etching process using Cl 2 /CH 4 /H 2 /Ar source gases until the n-GaN layer was exposed for n-type ohmic contact. Then the LEDs with a size of a 300 × 300 μm 2 were fabricated using indium tin oxide (ITO) with a thickness of 200 nm as a transparent current spreading layer and Cr/Au as n-and p-pad electrodes by e-beam evaporation, respectively.

Atomic force microscope (AFM) analysis
The surface morphology of the PSLEO GaN template was characterized by atomic force microscope (AFM) as shown in Fig. 2(c). To evaluate the relative quality of the epilayer, the GaN templates grown without a SiO 2 mask (nonLEO template) and the conventional LEO GaN templates grown with square-shaped SiO 2 mask (LEO template) were also characterized by AFM as shown in Figs. 2(a) and 2(b), respectively. As shown in Figs. 2(b) and 2(c), there was a significant improvement in the surface morphology of LEO and PSLEO GaN templates grown with the inserted of SiO 2 masks. This result is attributed to a reduction of threading dislocations in the GaN by insertion of a SiO 2 LEO mask. Most threading dislocations in GaN propagate from the nucleation layer on the substrate to the top GaN surface. In the LEO GaN, however, the dislocations are terminated when they encounter the SiO 2 mask, resulting in a decrease in dislocation density of the LEO GaN layer [3,4]. Figures 2(b) and 2(c) also show that the PSLEO and LEO GaN templates have a similar surface pit density because both templates have the same area of SiO 2 masks.

Photoluminescence (PL) measurement
Figure 3(a) shows the PL spectra of InGaN/GaN MQWs grown on PSLEO, LEO, and nonLEO GaN templates. The PL spectra were obtained at room temperature using a He-Cd laser (λ = 325 nm) with an excitation laser power of 50 mW. As shown in Fig. 3(a), the PL intensity of InGaN/GaN MQWs grown on PSLEO GaN was much higher than those of other samples. The enhancement of PL intensity was attributed to the improvement in the η int due to the reduction of dislocation density. Moreover, the higher PL intensity of MQWs grown on PSLEO GaN compared to that of LEO GaN indicates that the η extraction of MQWs is also improved by the pyramidal-shaped SiO 2 mask. To more fully elucidate the improvement of η int , the temperature-dependent PL was measured in a temperature range from 10 to 300K. The η int of InGaN/GaN MQWs can be estimated by integrating the PL intensity by assuming that the η int is 100% at 10K [18]. Figure 3(b) shows an Arrhenius plot of the normalized PL intensity for the InGaN/GaN MQWs grown on different GaN templates. The η int of InGaN/GaN MQWs grown on a PSLEO GaN template was estimated to be 18.2%, which is about twice higher than that of a nonLEO GaN template (9.3%). This result indicates that a significant reduction of defects such as screw and edge-type threading dislocations in the n-GaN and MQWs contributes to the improvement of the η int value [18]. In the case of the InGaN/GaN MQWs grown on a LEO GaN template, the η int was 17.1%. This value was similar to that of PSLEO because both templates have the same area of SiO 2 masks and similar dislocation densities.

Monte-Carlo ray-tracing simulation analysis
To investigate the effect of SiO 2 mask shape on the η extraction , the η extraction of LEDs with PSLEO and LEO GaN templates was calculated using the Monte-Carlo ray-tracing method. The Monte-Carlo ray-tracing is regarded as the most suitable method to simulate light propagation due to the randomness of the spontaneous photon emission from the MQW active layer in LEDs [23]. The PSLEO-LED used for simulation consisted of an 80 μm-thick sapphire (n = 1.7) and pyramidal-shaped SiO 2 masks (n = 1.45) with a width of 8 μm and a height of 3 μm surrounded by 7 μm-thick GaN (n = 2.5). The LEO-LED contained the squareshaped SiO 2 mask which had the same width and spacing between SiO 2 masks as those of PSLEO mask. The active region with a η int of 100% was inserted into the GaN. The total amount of light emitted from the LED was detected by receivers in all directions. Figure 4 shows the results of ray-tracing simulation for PSLEO-LED and LEO-LED. As shown in Figs. 4(a) and 4(b), the light of PSLEO-LED is more effectively escaped from the LED than that of LEO-LED. Particularly, Figs. 4(c) and 4(d) clearly indicate that the path length of light escaping from the PSLEO-LED is much shorter than that from the LEO-LED. This demonstrates that the PSLEO-LED effectively reduces the internal reflection loss and absorption in PSLEO-LED because the light path is changed by refraction at the interface between GaN epilayer and pyramidal-shaped SiO 2 mask. Furthermore, Fig. 4(e) shows the η extraction of each face of PSLEO-LED and LEO-LED. The total η extraction of PSLEO-LED is much larger than that of LEO-LED. This result also indicates that the relative enhancement of η extraction is dominated mostly by the increase in light extraction through the top side of PSLEO-LED.

Light output power measurement
In order to investigate the optical properties of LED, the light output power was measured. The light output power of each LED was measured from the top side of LEDs using a 2 cmdiameter Si photodiode connected to an optical power meter. The distance between photodiode and LEDs was 10 cm. Figure 5 shows the light output power of PSLEO-LED, LEO-LED, and nonLEO-LED as a function of injection current. As shown in Fig. 5, the output power of PSLEO-LED is much higher than those of the other two LEDs. The light output power of PSLEO-LED is increased by 30% compared to that of the LEO-LED. This result is attributed to the improvement in the η extraction by the pyramidal-shaped SiO 2 mask. However, the enhancement of η extraction is relatively smaller than the simulation result because the light escaped from the epilayers is partially absorbed and reflected by ITO top contact layer which is not included in the simulation. Moreover, the difference is also attributed to some loss of light due to the long distance between photodiode and LEDs. The improvement of output power of the PSLEO-LED was 80% at an injection current of 20 mA compared with that of a conventional LED (nonLEO-LED). The large enhancement of output power was attributed to the reduction of dislocation density and to an improvement in the light extraction efficiency of PSLEO-LED by the pyramidal-shaped SiO 2 masks embedded in n-GaN.

Summary
In summary, we fabricated the InGaN/GaN blue LED using a PSLEO GaN template with pyramidal-shaped SiO 2 masks. The light output power of the PSLEO-LED showed an enhancement of 80% and 30% compared with nonLEO-LED and LEO-LED, respectively. The enhancement of light output power was attributed to a reduction in the threading dislocation and to an improvement in the light extraction efficiency of PSLEO-LED by pyramidal-shaped SiO 2 .