N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs

A high efficiency, high brightness, and robust micro or sub-microscale red light emitting diode(LED) is an essential, yet missing, component of the emerging virtual reality and future ultrahigh resolution mobile displays.We report, for the first time, to our knowledge, the demonstration of an N-polar In GaN/GaN nanowire submicroscale LED emitting in the red spectrum that can overcome the efficiency cliff of conventional red-emitting micro-LEDs. We show that the emission wavelengths of N-polar In GaN/GaN nanowires can be progressively shifted from yellow to orange and red, which is difficult to achieve for conventional In GaN quantum wells or Ga-polar nanowires. Significantly, the optical emission intensity can be enhanced by more than one order of magnitude by employing an in situ annealing process of the In GaN active region, suggesting significantly reduced defect formation. LEDs with lateral dimensions as small as ~0.75 μm, consisting of approximately five nanowires, were fabricated and characterized, which are the smallest red-emitting LEDs ever reported, to our knowledge. A maximum external quantum efficiency ~1.2% was measured, which is comparable to previously reported conventional quantum well micro-LEDs operating in this wavelength range, while our device sizes are nearly three to five orders of magnitude smaller in surface area.

Strain-engineered N-polar InGaN nanowires:towards high-efficiency red LEDs on the micrometer scale

The absence of efficient red-emitting micrometer-scale light emitting diodes(LEDs),i.e.,LEDs with lateral dimensions of 1μm or less is a major barrier to the adoption of microLEDs in virtual/augmented reality.The underlying challenges include the presence of extensive defects and dislocations for indium-rich InGaN quantum wells,strain-induced quantum-confined Stark effect,and etch-induced surface damage during the fabrication of quantum well microLEDs.Here,we demonstrate a new approach to achieve strong red emission(>620 nm)from dislocation-free N-polar InGaN/GaN nanowires that included an InGaN/GaN short-period superlattice underneath the active region to relax strain and incorporate more indium within the InGaN dot active region.The resulting submicrometer-scale devices show red electroluminescence dominantly from an InGaN dot active region at low-to-moderate injection currents.A peak external quantum efficiency and a wall-plug efficiency of 2.2%and1.7%were measured,respectively,which,to the best of our knowledge,are the highest values reported for a submicrometer-scale red LED.This study offers a new path to overcome the efficiency bottleneck of red-emitting microLEDs for a broad range of applications including mobile displays,wearable electronics,biomedical sensing,ultrahigh speed optical interconnect,and virtual/augmented reality.