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We report on the effect of embedded silica nanospheres on improving the performance of InGaN/GaN light-emitting diodes (LEDs). The silica nanospehres were coated on the selectively etched GaN using a spin-coating method. With the embedded silica nanospheres structures, we achieved a smaller reverse leakage current due to the selective defect blocking-induced crystal quality improvement. Moreover, the reflectance spectra show strong reflectance modulations due to the different refractive indices between the GaN and silica nanospheres. By using confocal scanning electroluminescence microscopy, a strong light emission from silica nanospheres demonstrates that the silica nanospheres acted as a reflector. We found that the optimized embedded silica nanospheres structure, whose the average size of the etched pits was about 3.5 μm and EPD was 3 x 107 cm−2, could enhance light output power by a factor of 2.23 due to enhanced the probability of light scattering at silica nanospheres.
©2011 Optical Society of America
1. Introduction
Nitride-based light-emitting diodes (LEDs) have attracted significant attention for a variety of applications such as traffic signals, back light units for liquid crystal displays, and next-generation solid state lighting. Although InGaN/GaN LEDs are commercially available, the output efficiency of these LEDs requires further improvement for feasible solid-state lighting. The primary limitations to optical power are low internal quantum efficiency (IQE), and the light extraction efficiency (LEE). The degradation of IQE mainly results from the high threading dislocation density (TDD) that forms due to a large mismatch of lattice constants and thermal expansion coefficients between GaN films and sapphire substrates. The threading dislocations (TDs) in GaN films acts as a nonradiative recombination center [1,2] and a leakage path in the LEDs [3]. Several methods have been reported to reduce TDs in GaN epitaxial growth such as epitaxial lateral overgrowth [4], and the inter layers such as in situ SiNx [5], ex situ TiNx [6], ScN [7] and silica microspheres [8]. Recently, we reported on the use of self-assembled silica nanospheres as a selective defect blocking mask for high quality GaN growth with low TDD [9]. Another major reason for the low external quantum efficiency is low light extraction efficiency. Extraction efficiencies remain low because of the total internal reflection occurring at the surfaces due to different refractive indices between GaN and air. To improve the light extraction efficiency, several methods have been reported such as a patterned sapphire substrate (PSS) [10], surface texturing [11], and photonic crystal structure formation [12]. Recently, some groups have reported that both IQE and LEE were enhanced by using a SiO2 inverted hexagonal pyramid dielectric mask-embedded structure [13], embedded SiO2 pillars and air gap array structures [14], and pyramidal-shaped SiO2 embedded structures [15]. However, the fabrication procedures used in these studies are complex and require expensive equipment.
In this study, we demonstrate the use of silica nanospheres as a mask to improve crystal quality by blocking the TDs and to increase the LEE by changing the direction of the light path. The process of coating the silica nanospheres in selectively etched pits was performed through a simple spin-coating method, and without photolithography patterning processes or expensive equipments. The proposed method has good potential to enhance the total optical output power by increasing both the IQE and LEE.
2. Experiments
All samples used in this study were grown on the c-plane (0001) with a 2 in. diameter sapphire substrate using metal organic chemical vapor deposition (MOCVD). Four different LEDs were fabricated to investigate the influence of silica nanospheres and the geometric shape of etched pits on crystal quality and light output power. A conventional LED, an LED with GaN etched for 7 min, an LED with silica nanospheres on flat GaN, an LED with silica nanospheres coated on GaN etched for 5 min, and an LED with silica nanospheres coated on GaN etched for 7 min are denoted as sample A, B, C, D, and E, respectively. First, a 2 μm-thick un-doped GaN layer was deposited by MOCVD. Then sample B, D, and E were immersed in phosphoric acid at 220 °C for 7 min, 5 min, and 7 min, respectively. Inverted hexagonal etched pits were generated using a selective chemical wet etching technique on the latent dislocations of the GaN template. Then, 500 nm diameter silica nanospheres were coated onto sample C with a non-etched flat GaN surface, and onto sample D and E with etched GaN surfaces using the spin coating method under the same conditions. Then, InGaN-based LED structures were regrown on all samples using MOCVD. The LED structures consisted of a 2 μm-thick undoped GaN layer, a 2 μm-thick n-type GaN layer, five pairs of InGaN/GaN multiple quantum well (MQW) active layers, and a 110 nm-thick magnesium-doped p-type GaN layer. After the growth of the LED epitaxial layers, mesa-structure LEDs with a 315x315 μm2 area were fabricated. Ni-Au was deposited as the transparent conductive layers, and annealed at 550 °C for 60 s in air. Then, the Cr/Au layers were deposited as metal contacts to both the n- and p-type layers. The LED wafers were diced into chips with a chip size of 350x350 μm2. The re-grown GaN was deposited at the same time to investigate the effect of silica nanospheres and the formation of selective etched pits on crystal quality. Also, by comparing sample C, D, and E, the increase in LEE can be explained by changing the density of the silica nanospheres. The surface morphologies were examined by scanning electron microscopy (SEM). The crystal quality of these samples was analyzed by high resolution x-ray diffraction. The current-voltage (I-V) and light output-current (L-I) measurements were carried out using a probe station system. To investigate the reflectance of the silica nanospheres, we recorded optical reflectance spectra using a UV-VIS-NIR spectrophotometer. The beam profile of the LED was characterized by an OL 770 multichannel spectroradiometer. The spatial distribution of light emission was analyzed using confocal scanning electroluminescence microscopy (CSEM).
3. Results and Discussion
Figure 1(a) shows a typical SEM image of sample B with a selectively etched GaN surface. The average size of the etched pits was about 3.5 μm. The etched pit density (EPD) was about 3 x 107 cm−2. Figures 1(b)–1(d) show silica nanospheres on various surfaces. Sample C was sparsely dotted with silica nanospheres as shown in Fig. 1(b). The density of silica nanospheres of sample C was much lower than samples D and E because there was no geometric shape, such as inverted hexagonal etched pits, that could hold the silica nanospheres. It is known that the self assembly and spatial ordering of silica nanospheres in etched pits are mainly driven by capillary force and geometric confinement [16].
The average sizes of the etched pits of sample D and E were about 1.8 μm and 3.5 μm, respectively, as shown in Figs. 1(c) and 1(d). The EPD values of sample D and E were about 1 x 107 cm−2 and 3 x 107 cm−2, respectively. The latent TDs were more revealed as the etching time increased. The density of silica nanospheres in the etched pits of sample E was higher than that of sample D because the etched pit size and depth of sample E was larger than that of sample D. We found that more silica nanospheres were trapped as the size and depth of the etched pits increased.
The crystalline quality could be comprehended using the full-width at half-maximum (FWHM) of the ω-scan rocking curve for the (102) plane inferred the density of edge-type and mix-type dislocations. The FWHM of samples A-E were 463, 406, 400, 310, and 288 arc sec, respectively. The narrower FWHM of the XRD rocking curve implies that crystalline quality could be improved by using self-assembled silica nanospheres as a mask to block the propagation of threading dislocations on a defect selective etched GaN template. The FWHM value of sample B was narrower than sample A because deep and large etched pits were partially overgrown at the top parts of the pits only, whereas the bottom of the pits remained empty [17]. However, there was a slight increase in crystal quality because most etched pits were fully overgrown. In contrast to sample B, samples D and E, whose etched pits were filled with silica nanospheres, showed enhanced crystal quality by selectively blocking the TDs. In the case of sample C, the value of the FWHM was higher than that of a conventional LED because TDs could be blocked by randomly distributed silica nanospheres on the flat GaN surface [8]. We note that samples D and E showed a similar ability to improve the crystalline quality. We found that the FWHM of sample E was better than that of LED D because the density of the revealed etched pits of LED E was higher than that of LED D. In the case of LED E, more TDs were blocked by silica nanospheres. The detailed parameters used in the experiment are listed in Table 1 .
Table 1. Detailed parameters used in the experiment and values of etched pit size and density
Figure 2 shows a typical I-V characteristic of samples A-E. At an injection current of 20 mA, the forward bias voltages of samples A-E were 3.4, 3.42, 3.43, 3.45, and 3.45 V, respectively. In the forward bias regime, samples A and B had a leakage current that was due to the recombination of minority carriers in the depletion region [18]. There are many trap levels, such as TDs in the depletion region, that make such recombination events. In contrast, the leakage currents of samples C, D, and E, which had embedded silica nanospheres, were much smaller than those of samples A and B. Moreover, it has been reported that leakage current in the reverse voltage region increased with TD density [19,20]. The reverse current of sample E is about 2 μA at 10 V, which is one order of magnitude lower than that of sample A (17 μA). The smaller leakage current again indicates that sample E has better crystalline quality. By comparing the leakage current of each sample at the forward and reverse regions, we note that introducing the silica nanospheres was very important in decreasing both the TDD of the GaN template and the leakage current.
To investigate the optical properties of the silica nanospheres in the LED structure, diffuse reflectance spectra were used to characterize the reflection characteristics of each sample. The diffuse reflectance spectra of samples A-E are shown in Fig. 3 . The reflectance spectra show interference fringes due to the interfaces, and have an abrupt cutoff at the wavelength around the absorption edge of the MQWs in the GaN at 460 nm. For sample B, diffuse reflectance increased slightly more than for sample A due to a small amount of air in the GaN template. The diffuse reflectance of sample C with low density silica nanospheres was higher than that of sample B. In spectra, sample E had the highest diffuse reflectance. Figure 3 shows that the reflectance enhancement of sample E was 1.26 times higher than that of sample A, and 1.07 times higher than that of sample D. These results indicate that the diffuse reflectance increased significantly with an increase in the number of silica nanospheres. These results could be attributed to the different refractive indices between the GaN and silica nanospheres, or could suggest that the geometrical shape of the etched GaN might improve light output power by scattering the emitted photon to find the escape cone.
We measured the light output power to investigate the optical properties of the LEDs. Figure 4(a) shows the light output power of samples A-E as a function of injection current. As shown in Fig. 4(a), the light output power of sample E was much higher than that of the other samples; thus, the light intensity and reflectance were greatly enhanced. At an injection current of 20 mA, the light output power of sample E was 2.23 times higher than that of sample A, and 1.41 times higher than that of sample D. As the number of silica nanospheres increased, the optical output power was increased. As the size of the etched pits increased, a greater number of silica nanospheres were trapped in etched pits. However, there is a limit to the increase in size of the etched pits. It need adequate the flat GaN surface which provide the seed layer for regrowth. The flat GaN surface can disappear when the size of the etched pits is too large, indicating that a well-controlled etching process is required for regrowth. Also, the reflectance spectra and EL spectra, which showed a similar tendency, demonstrate that the multi-layer of silica nanospheres in etched pits resulted in the enhancement of LEE by changing the direction of the light path.
Fig. 4 (a) Light output power-current characteristic for all fabricated LEDs. (b) Beam profile for all samples.
The beam profiles of all the LED samples were measured at a 20 mA operating current as shown in Fig. 4(b). A divergent angle of an LED was identified as the angle of half-maximum emission intensity. In this experiment, divergent angles were measured from the front side. The divergent angles for samples A-E were 145, 143, 135, 133, and 128°, respectively. The decrease in the divergent angle for samples C, D, and E means that rays were extracted toward to the front side to a greater extent than for samples A and B due to light reflection and scattering at the embedded silica nanospheres. It is important to note that the density of the silica nanospheres dominated the divergent angle and light intensity.
To investigate the microscopic EL properties of the silica nanospheres, we employed confocal scanning electroluminescence microscopy (CSEM) with a spatial resolution of 200 nm, which is close to the spatial resolution (100 nm) of a near-field scanning optical microscope. CSEM is known as an effective experimental tool for measuring optical characteristics such as light propagation and local light output. One of the most significant findings of our study is the distribution of EL light emission from silica nanospheres. Figure 5(a) shows the CSEM image of sample E. Strong light emission was observed around the silica nanospheres compared to the horizontal {0001} facets. We found noticeable light emission with the contrast of luminescence intensity observed on silica nanospheres. A square part of Fig. 5(a) (white dash line) was enlarged to show a spatial 3-D image of EL light emission from silica nanosphere-embedded LEDs at an injection current of 1.5 mA as shown in Fig. 5(b). The spatial distribution of the light emission from the silica nanospheres demonstrates that the silica nanospheres acted as a reflector.
Fig. 5 CSEM image of sample E at an injection current of 1.5mA. (a) Top view CSEM image, and (b) spatial three-dimensional EL light emission from silica nanospheres.
4. Summary
In summary, we demonstrated the use of self-assembled silica nanospheres as a selective defect blocking mask for high quality GaN growth with low TDD, and the enhancement of light extraction efficiency. With the embedded silica nanospheres structures, we achieved a smaller reverse leakage current due to the selective defect blocking-induced crystal quality improvement. The reflectance spectra showed that the diffuse reflectance increased significantly with an increase in the number of silica nanospheres. These results could be attributed to the different refractive indices between the GaN and silica nanospheres. We found that the optimized embedded silica nanospheres structure, whose the average size of the etched pits was about 3.5 μm and EPD was 3 x 107 cm−2, could enhance light output power by a factor of 2.23 compared to conventional LEDs due to enhanced the probability of scattering at silica nanospheres to find the escape cone. By using confocal scanning electroluminescence microscopy, a strong light emission from silica nanospheres demonstrates that the silica nanospheres acted as a reflector.
Acknowledgements
This work was supported by the Strategic Technology Development Project of the Ministry of Knowledge Economy and the Ministry of Knowledge Economy (MKE) and Korea Institute for Advancement in Technology (KIAT) through the Workforce Development Program in Strategic Technology.
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