(Invited) Carrier Dynamics and Photon Management for Improvement in Quantum Efficiencies of GaN-Based Visible Light-Emitting Diodes

Data and analysis are presented employing several new methods to address carrier dynamics and photon management issues in order to improve internal quantum efficiency (IQE) and light-extraction efficiency (LEE) of GaN-based light-emitting diodes (LEDs).  First, the roles of a new InAlN electron blocking layer (EBL) and hole transport among quantum wells (QWs) are discussed with a focus on the mitigation of efficiency droop at high current densities.  While the origins of the efficiency droop remain to be controversial, currently suggested origins are all associated with carrier dynamics: the effect of the electron leakage being related to carrier confinement; hole transport to active region being pertinent to carrier injection and concentration in each QW, and Auger recombination becoming more dominant with increasing carrier density in each QW.  The strategy for the improvement, therefore, includes (1) confining electrons in the active region as much as possible, (2) injecting holes into the active region as much as possible, (3) distributing the both carriers among QWs as uniformly as possible.  In order to implement the strategies, we especially studied EBLs and hole injection and transport in the multiple QW (MQW) active region.  The effects of the various EBLs on the efficiency droop was compared (Figure 1), showing the lowest efficiency droop ratio of ~18% for the LEDs with InAlN EBL due to better electron confining/blocking effect than the standard EBLs.  Hole injection (into active region) and transport (across MQWs) are as important as electron confinement.  Also, they are not inter-related.  Uniform distributions of electrons and holes with the same concentration in each QW are a goal to effectively mitigate the efficiency droop.  Whereas the uniform distribution of electrons among QWs is achieved without difficulties, that of hole is quite challenging.  We employed triple-wavelength-emitting MQWs with different bandgap energies for the experimental evaluation of the hole distribution among MQWs (Figure 2), showing hole transport and resulting distribution of holes can be engineered by p-type layers.  Secondly, new way of surface patterning is demonstrated.  The periodic surface patterns having 2-dimensional hexagonal array were directly achieved by three-beam interference laser ablation technique without photolithography processes (Figure 3), resulting in improvement of LEE by ~20%.  This new direct laser patterning also maintained ohmic behavior of contacts on p-type surface, unlike the case of the patterning using plasma dry etching.