Researchers from Wuhan University in China and the LED company Changelight have reported developing high power, reliable GaN-based vertical LEDs (VLEDs) on 4-inch p-type silicon substrate. They achieved this using an optimised metallisation scheme with an SiO2 current blocking layer. In addition, by surface texturing the emitting surface of the VLED with KOH wet etching, they say they have boosted the light extraction efficiency.
“We may have found access to more cheap and reliable next-generation solid-state lighting devices," said Shengjun Zhou who led the Wuhan team.
Deposited metallisation schemes can buckle and delaminate in humid environments, which is a major concern for the reliability of VLEDs. In the study, published in Optics Express, the researchers demonstrated that the metallisation reliability can be improved by depositing Pt/Ti protective layers surrounding the Ag/TiW films to protect the interface from environmental humidity.
Figure 1 above shows optical microscope images of different metallisation scheme designs suffering from moisture in the air: (a) Conventional multilayer metallisation stacks without lateral protection; (b) Multilayer metallisation stacks with Pt/Ti capping layers surrounding the Ag/TiW films.
Light absorption by the metal contacts is one source of optical loss sources in LEDs. However, most of current is crowded in proximity of the electrodes due to the non-ideal conductivity of the semiconductor layers, leading to a localisation of emission around the electrode pads.
The researchers show that 'current crowding' can be significantly relieved by the insertion of a SiO2 current blocking layer under the p-electrode. A more uniform current distribution with the application of SiO2 current blocking layer was demonstrated using simulation results.