  The efficiency of ultra violet LEDs is critically limited by the absorption
losses in p-type and metal layers. In this work, surface roughening based light
extraction structures are combined with tunneling-based top-contacts to realize
highly efficient top-side light extraction efficiency in UV LEDs. Surface
roughening of the top n-type AlGaN contact layer is demonstrated using
self-assembled Ni nano-clusters as etch mask. The top surface roughened LEDs
were found to enhance external quantum efficiency by over 40% for UV LEDs with
a peak emission wavelength of 326 nm. The method described here can enable
highly efficient UV LEDs without the need for complex manufacturing methods
such as flip chip bonding.
