Thin-GaN LED materials

In this chapter, thin film GaN-based light emitting diodes (LEDs) fabricated into n-side-up and p-side-up LEDs on mirror-substrate structures using a combination of wafer bonding, laser lift-off and surface texturing techniques were described. The effects of Pd, ITO/Al, NiO/Ag, NiO/Ag/Ni, and NiO/Au/Ag mirrors on the n-side-up GaN/mirror/Si LED properties were studied. It was found that the characteristics of the vertical-conducting n-side-up GaN/mirror/Si LEDs with a NiO/Ag/Ni mirror structure showed the best performance than the other mirror ones. After the thermal anneal process, the specific contact resistance of NiO/Ag/ Ni to p-GaN can be reduced. The output power of the n-side-up GaN/mirror/Si LED shows nearly three times in magnitude as compared with that of the original GaN/sapphire sample. On the other hand, the p-side-up GaN LEDs were fabri cated using a combination of omni-directional reflector (ODR) and double-sided textured surface (both p-GaN and undoped-GaN) techniques. An Essential Mac leod program was used to simulate the optimum thickness of the ODR structure. The reflectivity value of ODR structure used in work can reach 99%. On the top-side textured surface, the p-type GaN with hexagonal cavities was grown under low temperature (LT) conditions using metalorganic chemical vapor depo sition. The GaN LED with a suitable LT p-GaN cap layer thickness was also studied. Experimental results indicate that the GaN LED sample with the 200-nm hexagonal cavity GaN layer on the surface exhibits a 50% enhancement in luminance intensity. The luminance efficiency can be improved. This indicates that the thin-film structure can enhance the light extraction efficiency of GaN-based LEDs, especially for large chip sizes.