Quantum Dot and Nanowire Light Emitting Diodes and Lasers

The development of nitride-based multiquantum well (MQW) light emitting diodes (LEDs) has been plagued by the phenomenon known as "efficiency droop", observed at high injection current densities. This detrimental characteristic has been attributed to carrier leakage, Auger recombination, excition dissociation, and other effects. The first two are related to hot carriers. In addition, the large In content in the wells, particularly for longer wavelengths, leads to a large polarization field with two adverse effects: reduced electron-hole overlap and a strong quantum confined Stark effect (QCSE) with increasing injection accompanied by a blue-shift of the emission wavelength. The use of self-organized InGaN/GaN quantum dots (QDs) in the active region promises several advantages compared to quantum wells: lower density of structural defects in the quantum dots, smaller piezoelectric polarization field, smaller carrier lifetimes, smaller QCSE-related blue-shift of the emission wavelength, and localization of carriers in the QDs which inhibits the transport of carriers to surrounding dislocations and defects and their eventual non-radiative recombination. Secondly, we have introduced tunnel injection of cold carriers directly into the quantum dots in the active region, in order to minimize hot-carrier related problems. Significant improvements in the performance of the green-emitting LEDs are experimentally observed: shorter carrier lifetimes, vastly reduced QCSE, larger internal quantum efficiency (IQE) and external quantum efficiency (EQE) and reduced droop. Efficiency rollover in QD green (λ=500nm) LEDs is observed at 220A/cm2.

Speaker: Dr. Pallab Bhattacharya, University of Michigan, Ann Arbor

Room 521