The characteristics of Ni/indium tin oxide (ITO) ohmic contacts to p-type GaN (∼2×1017 cm−3) have been studied. The Ni/ITO (10 nm/250 nm) layers were prepared by thermal evaporation and rf magnetron sputtering, respectively. Although the as-deposited Ni/ITO contacts present rectified behavior, the linear current–voltage characteristics can be obtained. The contact resistance can be reduced significantly for the ITO/Ni/p-GaN samples after suitable rapid thermal process. The contact property of ITO/Ni/p-GaN shows lowest specific contact resistivity of 8.6×10−4 Ω cm2 and high transparency (above 80% for 450–550 nm) as the sample annealed at 600 °C in air. Possible mechanisms for the observed low contact resistance and high transparency will be discussed. The present process is compatible with the fabrication for the high-efficient GaN light-emitting devices.
Near-ultraviolet nitride-based light-emitting diodes (LEDs) with peak emission wavelengths around 410 nm were fabricated onto c-face patterned sapphire substrates (PSS). It was found that the electroluminescence intensity of the PSS LED shown 63% larger than that of the conventional LED. For a typical lamp-form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The improvement in the light intensity could be attributed to the decrease of threading dislocations and the increase of light extraction efficiency in the horizontal direction using a PSS
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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