Air-ring microstructure arrays for enhanced light extraction from a face-up light-emitting diode.

In this Letter, a light-emitting diode (LED) with prism-shaped-air-ring microstructures (PSAMs) formed on flat sapphire substrate is demonstrated as an alternative design to face-up LEDs on patterned sapphire substrate (PSS) for enhanced light extraction efficiency. In this LED design, the emitted photons can be deflected to the top of the chip for its effective extraction, contrary to the PSS-LED wherein photons are guided to sapphire and get absorbed by packaging materials. The PSAM-LED showed an enhancement in the radiometric power as high as 10% with a low far-field angle of 129° over that of a PSS-LED under an injection current of 20 mA.

Impact of interlayer processing conditions on the performance of GaN light-emitting diode with specific NiOx/graphene electrode.

This paper reports on the evaluation of the impact of introducing interlayers and postmetallization annealing on the graphene/p-GaN ohmic contact formation and performance of associated devices. Current-voltage characteristics of the graphene/p-GaN contacts with ultrathin Au, Ni, and NiO(x) interlayers were studied using transmission line model with circular contact geometry. Direct graphene/p-GaN interface was identified to be highly rectifying and postmetallization annealing improved the contact characteristics as a result of improved adhesion between the graphene and the p-GaN. Ohmic contact formation was realized when interlayer is introduced between the graphene and p-GaN followed by postmetallization annealing. Temperature-dependent I-V measurements revealed that the current transport was modified from thermionic field emission for the direct graphene/p-GaN contact to tunneling for the graphene/metal/p-GaN contacts. The tunneling mechanism results from the interfacial reactions that occur between the metal and p-GaN during the postmetallization annealing. InGaN/GaN light-emitting diodes with NiO(x)/graphene current spreading electrode offered a forward voltage of 3.16 V comparable to that of its Ni/Au counterpart, but ended up with relatively low light output power. X-ray photoelectron spectroscopy provided evidence for the occurrence of phase transformation in the graphene-encased NiO(x) during the postmetallization annealing. The observed low light output is therefore correlated to the phase change induced transmittance loss in the NiO(x)/graphene electrode. These findings provide new insights into the behavior of different interlayers under processing conditions that will be useful for the future development of opto-electronic devices with graphene-based electrodes.