Tunnel junction integrated ultraviolet nanowire LEDs

Efficiency of ultraviolet (UV) and Deep UV LEDs are mainly hindered by poor dopant ionization in wide bandgap AlGaN. Polarization induced doping via composition grading has been used to increase the amount of ionized dopants. Composition gradient over the full composition range is limited by the small critical thickness of the epitaxial graded layer in planar thin films. However, nanowires can accommodate significantly large amount of strain due to large surface to volume ratio. Taking this advantage, previously, polarization induced doping in whole composition range is demonstrated to fabricate UV and deep UV LEDs using catalyst-free nanowires grown by plasma assisted molecular beam epitaxy on silicon wafers. Due to majority N-face (0001̅) crystallographic direction of the nanowires polarization induced nanowire light emitting diodes (PINLEDs) needs to be grown on p-type silicon substrate which suffers from large turn-on voltage and poor hole injection at the p-Si/p-GaN interface due to large valence band discontinuity. In this work, we integrate an InGaN tunnel junction (TJ) at the base of the PINLEDs to lower the turn-on voltage and increase hole injection into the p-graded region of the PINLEDs.