Figure 4-2 shows the surface morphologies and AFM images of LED surfaces. Clearly, the surface of CV-LED is much smoother than that of PR-LED. Their surfaces were measured using atomic force microscopy (AFM) to identify the degree of texturing. The root mean square (rms) roughness of p-GaN surface without any treatment is only 11.8 nm, while that of roughened p-type GaN surface is 71.6nm. As for the surface of the roughened undoped-GaN layers, as shown in Fig. 4-2(c), the surface was full with three-dimensional islands after the KOH solution treatment. The height and size of the GaN islands were 300~ 700 nm and 0.1~
0.4 µm, respectively. The rms roughness of this undoped-GaN surface was 91.9 nm.
The I-V characteristics of the PR-LED and DR-LED exhibited normal p-n diode behaviors with forward voltages about 3.3 V at 20 mA as shown in Fig. 4-3 and Fig. 4-4, which was similar to that of CV-LED (3.2 V at 20 mA). The Vf1 and Vf2 represent the forward voltage at 10 µA and 20 mA, Ir and Iv symbolize reverse current (µA) at –5V and luminance intensity (mcd) at 20 mA, WLP and WLD show the peak and dominant wavelength at 20 mA. This similarity indicated that the surface-roughening process,
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wafer-bonding process and laser lift-off process did not degrade the performance of DR-LED.
Furthermore, Fig. 4-4 display the double roughened InGaN-GaN LED devices were successfully fabricated on a 50mm sapphire substrate using double transfer method with high yield and better luminance intensity than that of PR-LED.
To further investigate the influence of roughened GaN surfaces on light-output performance of a LED chips. The luminance intensities of unpackaged LED were measured from both the frontside (top side through the transparent ITO layer) and backside (substrate side through the sapphire/transparent glue/glass) of the device. The light intensity as a function of injected forward current are shown in Fig. 4-5(a) and Fig. 4-5(b), respectively. It is obvious that roughened surfaces of LED did enhance the luminance intensities. Comparing with the CV-LED chip, the light intensity for the PR-LED chip with a roughened p-GaN surface was increased by 60 % for the frontside and by 56% for the backside at an injection current of 20 mA. These results are similar to the conclusions drawn by Hu, Lee, Kang and Park [4] during their microroughening of the p-GaN surface studies. They produced a microroughened p-GaN top surface using the metal clusters as a wet etching mask and measured the light-output powers of unpackaged LED chips from both frontside and the backside of the device. They also found the light-output powers were increased on both sides.
Comparing with the conventional LED chip, the light-output power for the LED chip with a microroughened top surface was increased by 52.4 % for the frontside and by 30 % for the backside, respectively. They believed the microroughened surface structure improve the escape probability of photons due to the angular randomization of photons inside the LED structure, resulting in an increase in the light extraction efficiency of LED.
As for the luminance intensity of our DR-LED, the light intensities from both sides were greatly enhanced. The frontside luminance intensity was 133 mcd, which was 2.77 times higher than that of the CV-LED, and 1.73 times higher than that of the PR-LED. As for the
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backside luminance intensity, it was 178 mcd, which was 2.37 times higher than that of the CV-LED, and 1.52 times higher than that of the PR-LED. Clearly, this tremendous enhancement was caused by the roughened p-GaN surface and the roughened undoped-GaN surface.
The light-extraction efficiency in the GaN-based LED is limited mainly due to the difficulty for light to escape from high refractive index semiconductors. The key to enhance the escape probability is to give the photons multiple opportunities to find the escape cone [3].
As shown in Fig. 4-6, roughened surfaces not only can provide photons multiple chances to escape from the LED surface, but also redirect photons, which were originally emitted out of the escape cone, back into the escape cone. Figure 4-6(a) shows the possible photon paths for CV-LED without any roughened surface. For a PR-LED, the angular randomization of photons can be achieved by surface scattering from the roughened p-GaN surface, as shown in Fig. 4-6(b). Thus, the roughened surface structure can provide photons multiple chances to escape from the LED, and redirect photons back into the escape cone. A DR-LED device has two roughened surfaces, as shown in Fig. 4-6(c). Compared to the PR-LED, the extra-roughened undoped-GaN layer can greatly increase the escape probability of photons, resulting in an increase in the luminance intensity of LED, as shown in Fig. 4-5.
Fig. 4-7 illustrates the external quantum efficiency and the output power of the CV-LED, PR-LED and DR-LED. The external quantum efficiency of the CV-LED, PR-LED and DR-LED was 13.9 %, 18.4 % and 21.4 % at 20mA. On the other hand, the output power observed from of the DR-LED was 16 % larger than that observed from of PR-LED, and 55%
higher than that of the PR-LED. With a DC 20mA current injection, it was found that the output powers were 11.38, 9.78 and 7.34 mW for the CV-LED, PR-LED and DR-LED, respectively.
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