Effect of the Al0.3Ga0.7As interlayer thickness upon the quality of GaAs on a Ge substrate grown by metalorganic chemical vapor deposition
This study reports the development of GaN-based power-chip light-emitting diodes ͑LEDs͒ with sidewall roughness using natural lithography with polystyrene spheres as the etching mask. At an injection current of 350 mA, the LED with sidewall roughness increased the light output intensity of the InGaN/GaN multiple quantum well LEDs by a factor of 1.26, indicating that the LED with sidewall roughness had larger light extraction efficiency. The wall-plug efficiency of GaN-based LED was increased by 26.5% with sidewall roughness. After 1000 h life test, it was found that normalized output power of power-chip LED with sidewall roughness did not show any significant degradation. III-nitride wide bandgap light-emitting diodes ͑LEDs͒ have recently attracted considerable interest due to their various applications, such as traffic signals, back-side lighting in liquid crystal display, and illumination lighting by white light LEDs. 1 However, the external quantum efficiency of GaN-based LEDs is low because the refractive index of the nitride epitaxial layer differs greatly from that of the air. The refractive indexes of GaN and air are 2.5 and 1.0, respectively. Thus, the critical angle at which light generated in the InGaN-GaN active region can escape is approximately ͓ c = sin −1 ͑n air /n GaN ͔͒ ϳ 23°, which limits the external quantum efficiency of conventional GaN-based LEDs to only a few percent. 2,3 The light from LEDs can be enhanced either through the sample surface or through the sidewalls of the power chip. Research into improving the light extraction efficiency ͑external quantum efficiency͒ and brightness in the LEDs Refs. 3-15 has been intense. These processes all allow the photons generated within the LEDs to find the escape cone, by multiply scattering from a rough surface. A similar concept should also be applied to power chip sidewall. In other words, more photons should be able to escape from powerchip LEDs with sidewall roughness as compared to LEDs with conventional power-chip sidewall.By using plasma-enhanced chemical vapor deposition SiO 2 layer as the etching mask, Chang et al. successfully demonstrated a 10% output power enhancement of nitride-based LEDs with m-scale wavelike textured sidewalls. 8 Due to the lithography limits of their instruments, they used a mask with a large period to fabricate m-scale wavelike textured sidewalls. Further enhancement of the light output can be achieved if sidewall roughness can be reduced to the sub-m or nanoscale range. Recently, Horng et al. demonstrated the power enhancement of surface-textured ITO/GaN LEDs using a combination of natural lithography and dry etching techniques. 9 In this paper, GaN-based power-chip LEDs with chip sidewall roughness were fabricated by natural lithography and dry etching techniques to increase the light output efficiency of power chip LED. In addition, the life test result of power chips with sidewall roughness did not show significant degradation in the electrical properties of GaN-based power-chip LEDs.The GaN-based LED samples w...
The light extraction enhancement of GaN-based flip-chip indium-tin oxide light-emitting diodes ͑FC ITO LEDs͒ with an inclined sidewall coated with TiO 2 /SiO 2 omnidirectional reflectors ͑ODRs͒ is presented. At a driving current of 350 mA and a chip size of 1 ϫ 1 mm, the light output power and the light extraction enhancement of the FC ITO LEDs coated TiO 2 /SiO 2 ODRs with inclined sidewall reached 183 mW and 15% when compared with the results from the same device, FC ITO LEDs coated TiO 2 /SiO 2 ODRs with vertical sidewall. Furthermore, by examining the radiation patterns of the FC ITO LEDs, the increased optical power within 150°cone contributed to the stronger enhancement around the vertical direction of an inclined sidewall ODR within blue regime. Our work offers promising potential for enhancing output powers of commercial light-emitting devices.GaN-based material is a direct wide bandgap semiconductor that has attracted considerable interest in applications for blue, green and ultraviolet light-emitting diodes ͑LEDs͒. The availability of higher brightness, high power and large area of GaN-based LEDs has enabled their applications in traffic signals, back-side lighting in liquid crystal display and backlight for various handheld devices. 1 Due to the large refractive index difference between GaN of 2.5 and air of 1.0, the total internal reflection is mainly responsible for the photon trapping. Emitting photons that strike the GaN-air interface at angles exceeding the critical angle c are reflected back, the majority of which are guided laterally through the air-GaN-sapphire waveguide structure. Thus, the critical angle at which light generated in the InGaN-GaN active region can escape is approximately ͓ c = sin −1 ͑n air /n GaN ͔͒ ϳ 23°, which limits the external quantum efficiency of conventional GaN-based LEDs to only a few percent. However, there is a great need to improve the external quantum efficiency of GaN-based LEDs in order to further increase their light output power.Several approaches have been proposed to improve the external quantum efficiency and the output power of packaged LEDs; light emitted downward toward the substrate must be reflected upward in order to contribute to usable light output 2-4 and the output light could be enhanced through the sample surface or the sidewall profile. 5-8 Chang et al. reported 10% output power enhancement from the InGaN-GaN multiple quantum well ͑MQW͒ LEDs by the introduction of the wavelike textured sidewalls. 5 Kao et al. reported the experiment results for enhancing light extraction efficiency from the GaN-based LEDs with ϳ23°undercut sidewalls. 6 Lin et al. reported the enhancement of light extraction efficiency using photoelectrochemical wet oxidation on the GaN-based LEDs with inclined undercut sidewalls. 8 All these methods have one thing in common, which is that photons generated within the LEDs experience multiple opportunities to find the escape cone. Bhat, Ludowise, and Steigerwald 9 proposed a dielectric distributed Bragg reflector ͑DBR͒ coated ...
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