Michael J. Grundmann

Carrier distribution in (0001)InGaN∕GaN multiple quantum well light-emitting diodes

We study the carrier distribution in multi quantum well (multi-QW) InGaN light-emitting diodes. Conventional wisdom would assume that a large number of QWs lead to a smaller carrier density per QW, enabling efficient carrier recombination at high currents. We use angle-resolved far-field measurements to determine the location of spontaneous emission in a series of multi-QW samples. They reveal that, no matter how many QWs are grown, only the QW nearest the p layer emits light under electrical pumping, which can limit the performances of high-power devices.

Droop in InGaN light-emitting diodes: A differential carrier lifetime analysis

To investigate the variation in internal quantum efficiency in InGaN structures, we measure the differential carrier lifetime of an InGaN/GaN double-heterostructure light-emitting diode under varying electroluminescence injection conditions. By coupling this measurement to an internal quantum efficiency measurement, we determine the carrier density and the radiative and nonradiative contributions to the lifetime without making any assumptions on recombination processes. We find that droop is caused by a shortening of the nonradiative lifetime with current. The observed shortening of both radiative and nonradiative lifetimes with current is found to be in excellent agreement with an ABC model including phase-space filling.

Influence of polarization fields on carrier lifetime and recombination rates in InGaN-based light-emitting diodes

We study differential carrier lifetimes in InGaN light-emitting diodes (LEDs) of varying wavelengths. Increase in wavelength is correlated with an increase in lifetime, due to the impact of the polarization fields on carrier overlap. This effect explains the early onset of droop in longer-wavelength LEDs.

Bulk GaN based violet light-emitting diodes with high efficiency at very high current density

We present experimental results on III–nitride light-emitting diodes emitting at 410 nm, grown on low-defectivity bulk GaN substrates. The epitaxial layers are optimized for high peak efficiency and maintain efficiency at very high current densities. We use a volumetric device architecture with surface roughness to maximize light extraction efficiency. We report an external quantum efficiency of 68% at 180 A cm−2. No current crowding is observed at high current density. We also demonstrate flat-line reliable operation to over 1000 h.