Current spreading in UV-C LEDs emitting at 235 nm

We present UV-C LEDs emitting around 235 nm grown by MOVPE on ELO AlN/sapphire substrates. In order to account for the low conductivity of high Al content AlGaN layers and the associated high contact resistances, we designed an optimized compact LED geometry based on electro-thermal simulations of the current spreading. Experimental data (layer and contact resistances) are collected on test structures and used as input parameters for 3-D current spreading simulations. With resistances of the layers (n and p) approaching 0.1 Ωcm, the use of a segmented p-area with broad n-contact fingers (10 μm or more) that are close to the mesa edge (5 μm) help to maximize the emission power in the center of the structure. Based on this knowledge a series of compact LEDs of size 500 μm x 500 μm is designed and simulated. We get confirmation that the segmentation of the p-area is the most critical parameter to limit the non-uniformity introduced by the high n-sheet resistances. Up to 17% in emission power can be gained when the n-contacts are designed properly. LEDs with the optimum geometry were processed and measured. We get a good confirmation of our model concerning the distribution of the emission power. Both simulations and measurements show current crowding at the edge of the n-contact, however the power loss in the middle of the chip is higher than predicted.