Cathodoluminescence as an Effective Probe of Carrier Transport and Deep Level Defects in Droop-Mitigating InGaN/GaN Quantum Well Heterostructures

TL;DR

This study uses cathodoluminescence to investigate carrier transport and deep level defects in InGaN/GaN quantum wells, revealing insights that can guide improvements in LED efficiency and droop mitigation strategies.

Contribution

It introduces a novel correlation between deep level defects, electron beam induced cathodoluminescence, and quantum efficiency in InGaN/GaN heterostructures, advancing understanding of efficiency droop.

Findings

Deep level defects correlate with efficiency loss.

CL reveals carrier transport dynamics and defect states.

Quantum well width fluctuations are not the main cause of efficiency droop.

Abstract

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.