Active Region Part A. Internal Quantum Efficiency in LEDs

Abstract

In this chapter, we present different techniques used to assess the internal quantum efficiency (IQE) in light emitting diodes (LEDs). The commonly used technique based on temperature-dependent photoluminescence relies in strong assumptions which are discussed in this chapter. We introduce an alternative method to determine IQE based on electroluminescence, in which the external quantum efficiency (EQE) is measured from a single facet of the LED, where the light emission can be calculated with good accuracy. The IQE is ultimately obtained from the ratio of the EQE and the calculated light extraction efficiency. We develop an optical model of the light emission in a multilayered LED structure, from which we derive and validate an approximate model to easily calculate the extraction efficiency through the top facet of any LED structure. We address the various assumptions made to calculate the direct emission model through a single facet and evaluate the effect of photon recycling in the quantum wells. We also calculate the sensitivity of the model to the LED parameters and surface roughness. Finally, we apply this technique to calculate the IQE of both a state-of-the-art and a low performance GaN-based LEDs, highlighting the particular features in each structure.