An approach to improve the defect density and internal quantum efficiency of near-ultraviolet emitters was proposed using a combination of epitaxial lateral overgrowth (ELOG) and patterned sapphire substrate (PSS) techniques. Especially, a complementary dot array pattern corresponding to the underlying PSS was used for the ELOG-SiO2 mask design. Based on the transmission-electron-microscopy and etch-pit-density results, the ELOG/SiO2/GaN/PSS structure can reduce the defect density to a level of 10(5) cm(-2). The internal quantum efficiency of the InGaN-based ELOG-PSS light-emitting diode (LED) sample showed three times in magnitude as compared with that of the conventional GaN/sapphire one. Under a 20 mA injection current, the output powers of ELOG-PSS, PSS, and conventional LED samples were measured to be 3.3, 2.9, and 2.5 mW, respectively. The enhanced output power could be due to a combination of the reduction in dislocation density (by ELOG) and improved light extraction efficiency (by PSS). Unlike the previous double ELOG approaches, the presented ELOG-PSS structure needs only one regrowth process and will have high potential in future high-quality ultraviolet emitters, even blue/green laser diode applications. (c) 2006 American Institute of Physics

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 3normalH2SnormalO4:1normalH3PnormalO4 mixture solution and the activation energy of this reaction is determined to be 28.2kcal∕mol . Three symmetric sidewall facets of the etched pyramidal hole were {112false¯kfalse¯} 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 20mA forward injection current, the output power of the conventional and pyramidal PSS LEDs (in epoxy lamp form, λnormalD=400nm ) were 7.45 and 9.35mW , 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.

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We present a micropillar surface structure based on the enhancement of the light extraction efficiency of the near-ultraviolet (409nm) vertical-conducting InGaN light-emitting diode (LED) with an electroplated Cu substrate. The micropillar InGaN∕Cu LED (chip size: 1×1mm2) was fabricated using a combination of patterned sapphire substrate (PSS), laser lift-off, and copper electroplating processes. The PSS and Cu substrate can offer the advantages of dislocation reduction and thermal heat sink, respectively. It was found that the light output power (at 350mA) of the micropillar InGaN∕Cu LED sample can be improved by 39% as compared with that of the conventional InGaN∕Cu LED one. This significant enhancement in output power could be attributed to the increase of the extraction efficiency which is a result of the increase in photon escaping probability caused by scattering the emission light at the micropillar surface. The light extraction efficiency can be further optimized by tuning the micropillar spacing, as evidenced by the ray-tracing simulation result.