Fabrication of pyramidal patterned sapphire substrates for high-efficiency InGaN-based light emitting diodes

Abstract

In this study, a wet-etched pyramidal patterned sapphire substrate (PSS) was used to fabricate the near-ultraviolet InGaN-based light-emitting diodes (LEDs). The pyramidal PSS was etched using a 3H(2)SO(4):1H(3)PO(4) mixture solution and the activation energy of this reaction is determined to be 28.2 kcal/mol. Three symmetric sidewall facets of the etched pyramidal hole were {11 (2) over bar(k) over bar} k on the (0001) sapphire. It was found that the GaN epi layer grew laterally from the top of the pyramid pit and overhung the cavity. An evident reduction in dislocation density of the GaN-on-PSS sample can be confirmed by the etch-pit-density, double-crystal X-ray, and micro photoluminescence measurement results. Under a 20 mA forward injection current, the output power of the conventional and pyramidal PSS LEDs (in epoxy lamp form,lambda(D) = 400 nm) were 7.45 and 9.35 mW, respectively. A 25% enhancement in output power was achieved in the pyramidal PSS LED as compared with that of the conventional LED sample. The enhanced output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using a pyramidal PSS. From light-tracing calculation, the pyramidal reflector arrays can offer more probability of escaping photons from the GaN/sapphire interface, resulting in an increase in light extracting efficiency. (c) 2006 The Electrochemical Society.