The effects of sapphire substrates processes to the LED efficiency

We investigate the relation between the thickness of sapphire substrates and the extraction efficiency of LED. The increasing about 5% was observed in the simulations and experiments when the sapphire thickness changed from 100μm to 200μm. But the output power increasing is inconspicuous when the thickness is more than 200μm. The structure on bottom face of sapphire substrates can enhance the extraction efficiency of GaN-based LED, too. The difference of output power between the flip-chip LED with smooth bottom surface and the LED with roughness bottom surface is about 50%, where only a common sapphire grinding process is used. But for those LEDs grown on patterned sapphire substrate the difference is only about 10%. Another kind of periodic pattern on the bottom of sapphire is fabricated by the dry etch method, and the output of the back-etched LEDs is improved about 50% than a common case.