  Commercial InGaN/GaN light emitting diode heterostructures continue to suffer
from efficiency droop at high current densities. Droop mitigation strategies
target Auger recombination and typically require structural and/or
compositional changes within the multi-quantum well active region. However,
these modifications are often accompanied by a corresponding degradation in
material quality that decreases the expected gains in high-current external
quantum efficiency. We study origins of these efficiency losses by correlating
chip-level quantum efficiency measurements with structural and optical
properties obtained using a combination of electron microscopy tools. The drop
in quantum efficiency is not found to be correlated with quantum well (QW)
width fluctuations. Rather, we show direct correlation between active region
design, deep level defects, and delayed electron beam induced
cathodoluminescence (CL) with characteristic rise time constants on the order
of tens of seconds. We propose a model in which the electron beam fills deep
level defect states and simultaneously drives reduction of the built-in field
within the multi-quantum well active region, resulting in a delay in
accumulation of carrier populations within the QWs. The CL measurements yield
fundamental insights into carrier transport phenomena, efficiency-reducing
defects, and quantum well band structure that are important in guiding future
heterostructure process development.
