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Crystal Quality and Light Output Power of GaN-Based LEDs Grown on Concave Patterned Sapphire Substrate

The crystal quality and light output power of GaN-based light-emitting diodes (LEDs) grown on concave patterned sapphire substrate (CPSS) were investigated. It was found that the crystal quality of GaN-based LEDs grown on CPSS improved with the decrease of the pattern space (percentage of c-plane). However, when the pattern space decreased to 0.41 μm (S0.41-GaN), the GaN crystallinity dropped. On the other hand, the light output power of GaN-based LEDs was increased with the decrease of the pattern space due to the change of the light extraction efficiency.

In this study, micro-sized concave-PSS (CPSS) were fabricated with different pattern spaces. After the growth of the GaN layer, the crystallinity and performance of LEDs were compared and discussed.

Experimental
Two-inch c-plane sapphire with 200-nm SiO2 hard mask was immersed in a mixture etchant of H2SO4:H3PO4 (3:1) at 270 °C. The SiO2 mask was then removed by a buffer-oxide etching (BOE) solution. The pattern pitch was 3 μm with different pattern spaces. Figure 1 shows scanning electron microscope (SEM) images of top view and cross-sectional view of CPSS. The diameter of the concave pattern was 2.59 μm and the space was 0.41 μm, which was denoted as S0.41. The related detail and percentage of c-plane were shown in Table 1. S0.60 and S0.87 CPSS were also used in this study.  After the clean process, the LED structures were grown by metalorganic chemical vapor deposition (MOCVD). The structures consisted of a AlN as buffer layer on sapphire substrate, an undoped-GaN layer film, a n-GaN layer, a Si-doped AlGaN cladding layer, an InGaN-GaN multiple quantum wells (MQWs), a Mg-doped AlGaN cladding layer and a p-GaN layer. In this study the chip had an area of 10 × 23 mil 2 (254 × 584 μm 2 ). For the purpose of comparison, LED structures were also grown on flat sapphire without any pattern and denoted as FLAT-GaN/S3.00-GaN (~pattern space 3 μm, diameter 0 μm). Figure 2 shows the cross-sectional SEM image of S0.41-GaN. Voids were found at the GaN/sapphire interface. The GaN crystal quality was analyzed by (1) X-ray diffraction (XRD); (2) photo luminescence (PL) and (3) screw dislocation density, which can be characterized by etching pit density (EPD). The nature of GaN crystal qualities on sapphire substrates was first analyzed by XRD rocking curves. The full width at half maximum (FWHM) of (002) symmetric plane and the (102) asymmetric plane of GaN are tabulated in Table 2. A more or less continuous increase in GaN crystallinity (decrease in FWHM width) was expected with decreasing pattern space (percentage of c-plane). This is because most of the growth of GaN was initiated not from sidewall surfaces but c-planes [14][15][16][17]. As the growth time increased, GaN epilayers on the top c-plane covered these concave patterns by lateral growth causing the threading dislocation to bend. At the same time, voids were formed when growth front boundaries coalesced, as shown in Figure 2. The crystal quality was improved with the increase of lateral growth area of GaN. In other words, it improved with the decrease of percentage of c-plane (the pattern space). However, when the pattern space decreased to 0.41 μm, the crystallinity of S0.41-GaN dropped. We believe this drop is because further decrease in c-plane area makes epitaxy of GaN film on PSS very difficult [18]. PL was also used to investigate the quality of muti-quantum wells (MQWs). The excitation source was a 325 nm, 30 mW He-Cd continuous wave laser. As shown in Figure 3, the peak intensity increased with the decrease of pattern space. Then, the intensity drop when pattern space decreased to 0.41 μm.
The light output power characteristics of LEDs are listed in Table 2, the forward voltage of S0.41-GaN at 20 mA was 2.79 V, which was almost the same as that of S0.87-GaN and S0.60-GaN. However, surprisingly, the light output power (LOP) of S0.41-GaN at 20 mA was 140.7 mW, which was better than that of S0.60-GaN (115.5) and S0.87-GaN (99.9), even though the GaN crystal quality of S0.41-GaN was not as good as that of S0.60-GaN and S0.87-GaN. This observation suggested that the LEE of S0.41-GaN must be higher than that of S0.60-GaN.
Theoretical study indicates that EQE and LOP are a product of IQE and LEE. IQE is a function of crystal quality, while LEE is a function of interface/surface texture. In our study it was found that the crystallinity of S0.60-GaN was better than that of S0.87-GaN and S0.41-GaN. However, the LEE of S0.41-GaN was comparable and higher than other LEDs. As a result, the LOP (EQE) of S0.41-GaN was better than that of other LEDs.

Conclusions
In this study, CPSS was fabricated by wet-etching method. The pattern pitch was 3 μm with different pattern spaces. The crystal quality and light output power of GaN-based LEDs grown on CPSS were investigated. XRD rocking curves, PL and EPD analysis revealed that the GaN crystallinity was increased with the decrease of pattern space (percentage of c-plane). This is because most of the growth of GaN was initiated from c-planes. As the growth time increased, GaN epilayers on the c-plane covered these concave patterns by lateral growth causing the threading dislocation to bend. However, when the pattern space decreased to 0.41 μm, the crystallinity of S0.41-GaN dropped. This is because further decrease in c-plane area makes epitaxy of GaN film on PSS very difficult. The light output power of S0.41-GaN was better than others. This is because the LEE of S0.41-GaN was larger than other LEDs.