Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources such as incandescent and fluorescent lamps due to their high energy efficiency, long device lifetime, and integration with other digital devices. However, different material properties between GaN and sapphire substrates cause several problems such as high dislocation density in GaN epitaxial layers, serious wafer bowing, particularly in large-area wafers due to thermal expansion mismatch, and poor light extraction efficiency of GaN-based LEDs due to total internal reflection. In this presentation, we propose a new growth strategy for the fabrication of high efficiency LEDs by incorporating artificial hollow nanospheres. In particular, polystyrene (PS)/silica core-shell nanospheres were synthesized and coated as a monolayer on a sapphire substrate by a modified dip coating technique. Sapphire substrates containing a monolayer of silica hollow nanosphere (S-HNS) could be prepared by the calcination of PS core and subsequent fixation to the substrate in a furnace. The subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively with the maximum index contrast to increase the light extraction efficiency and reduced the wafer bowing due to partial alleviation of compressive stress in GaN. It was found that the GaN epilayer with embedded S-HNS had higher diffuse reflectance in all visible wavelength range, and reduced wafer bowing, and stronger integrated photoluminescence intensity. Detailed analyses of finite difference time domain (FDTD) simulation and finite element simulation were used to understand the effect of S-HNS on reflectance and structural properties, respectively. The blue LED with embedded S-HNS indeed had two-fold stronger light emission with respect to one without S-HNS, suggesting that the new GaN epitaxy scheme looks quite promising in achieving high external efficiency LEDs for solid-state lighting, particularly in the mass production of LEDs with large-area sapphire wafers.

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