Analysis on the efficiency droop and its solutions in InGaN-based light emitting diodes

Abstract

Group III-nitride based light-emitting diodes (LEDs) have several attractive peculiarities, including long lifetimes, low energy consumption, high durability, design flexibility, and ecological friendliness. As a result, they are promising candidates for use in solid-state lighting to replace traditional incandescent and fluorescent light sources, which have short lifetimes, slow response time, containing toxic mercury and relatively low energy efficiency. Outdoor and indoor lighting applications require high operating currents？typically greater than 350 mA and in some cases greater than 1 A. However, GaN-based high power LEDs generally exhibit unsatisfactory characteristics

that is, efficiency monotonically declines upon increasing the injection current density, which is known as “efficiency droop”. Many different contributions to efficiency droop have been identified and discussed in the literature: carrier overflow due to the asymmetry of the electron-hole concentration or polarization, Auger recombination, junction heating effect, carrier delocalization effect, and current crowding effect. However, the physical origin of the droop is still under intense debate. In this dissertation, we developed time-resolved electroluminescence (TREL) analysis technique to further understand the efficiency droop under injection current condition. We used two InGaN/GaN based light-emitting diode with different dislocation densities for analysis of carrier dynamics at the practical current injection condition. By using TREL technique, we measured decay times and internal quantum efficiency (IQE) of two samples without any theoretical assumption. From these results, we induced radiative and non-radiative recombination times from these results. We analyzed correlation between recombination processes and IQE to understand efficiency droop phenomenon, and found that efficiency droop was related to non-radiative recombination processes such as Auger recombination or carrier overflow in high injection current region. Based on the TREL results, we fabricated samples with reduced effective carrier density in quantum wells (QWs) by using conventional LEDs structure without any complex processes. In these structures, the total active region including the total well and barrier thicknesses were fixed, while the number of wells was increased with decreasing barrier thickness. Internal electric field was investigated by APSYS simulation tool, variation of effective carrier density in samples was observed through full width at half maximum of electroluminescence spectra. As the barrier thickness decreased, we found that the internal electric field in the wells was diminished, and effective well volume increased. We observed that the IQE showed increasing tendency with decreasing barrier thickness, while the efficiency droop decreased as the barrier thickness increased. We analyzed that these results were related to the decreased internal electric field and the increased effective well volume as the barrier thickness decreased. Finally, we fabricated samples for improving the efficiency by overcoming limitations such as surface damage and intrinsic dislocations showed in 3-dimensional (3-D) structures. In 3-D structures, the IQE and light extraction efficiency (LEE) were improved due to the blocking of the dislocations induced from interface between substrate by $SiO_2$ layer and air void and to decrease total internal reflection by structural shape. We can significantly reduce dislocation density in QWs of structure with $SiO_2$ layer and air void compare to without $SiO_2$ layer. These results were confirmed from transmission electron microscope (TEM) and cathodoluminescence. In addition, from Raman spectra, we confirmed that strain in sample with $SiO_2$ layer was much relaxed than that of sample without $SiO_2$ layer. In sample with $SiO_2$ layer, we observed that the leakage current was reduced, and light output power increased as the injection current increases. Furthermore, we found that the IQE (efficiency droop) in sample with $SiO_2$ layer increased (decreased). We analyzed that these results were attributed to the decreased dislocation density and internal electric field in sample with $SiO_2$ layer. Consequently, we developed a system to analysis the carrier dynamics in QWs under current injection condition. And, the effects of effective carrier density and internal electric filed in well were investigated for understanding efficiency droop. In addition, we fabricated the 3-D structure for improving the overall efficiency. From these experiment results, we believe that efficiency droop is attributed to the non-radiative recombination process such as Auger recombination and carrier overflow in high carrier injection region. Thus, to decrease efficiency droop phenomena, it is important to reduce the effective carrier density and internal electric field in QWs without degradation of IQE.