Analysis and Improvement of Internal Quantum Efficiency in GaN-based LEDs

Abstract

In order to extract the recombination coefficient and the internal quantum efficiency (IQE) of the GaN-based LEDs, a fast and reliable measurement method using transient characteristics is developed. For accurate extraction of the recombination coefficients and the IQE, an improved rate equation model for GaN-based LEDs considering the effective volume of the active region is also proposed. Through TCAD simulations, it is confirmed that the IQE, especially efficiency droop is related with small effective volume. Also, it is confirmed that the effective volume is controlled by polarization charge, the barriers between the quantum wells, and current density. The trap and its impact on the GaN-based LEDs are also analyzed by measurement and TCAD simulation. A reversible increase in the current of GaN-based blue LEDs is observed when constant forward voltage is applied. This characteristic is assumed to be the result of trapping process, and a trap activation energy of 0.30 eV is extracted. Through TCAD simulations, it is confirmed that the multi-quantum well (MQW) barrier height is reduced by the hole trapping process and that the current is increased by lowering this barrier. It is also confirmed that the effect of this trap on the optical characteristics of GaN-based blue LEDs by TCAD simulation and measurement. To improve the IQE of GaN-based LEDs, a novel structure for GaN-based LED featuring p-type trench in the MQW is proposed. Through TCAD simulation, it is confirmed that the proposed structure shows quite uniform hole distribution in the MQW than that of the conventional structure, because holes are injected efficiently into the MQW along the p-type trench. It is also confirmed that the proposed structure also has a significant effect on strain relaxation and reduction in quantum confined stark effect by cathodo-luminescence (CL) measurement. In addition, two simple fabrication methods using e-beam lithography and selective wet etching for manufacturing the proposed structure are also proposed. From the measurement results of the manufactured GaN-based LEDs, it is confirmed that the proposed structure using e-beam lithography or selective wet etching shows improved light output power compared to the conventional structure because of more uniform hole distribution and strain relaxation effect. From this study, methods for analyzing the IQE of the GaN-based LEDs and its limiting factors are proposed and verified. It is also demonstrated that the p-type trench structure in the MQW will be the promising candidate for solving the efficiency droop problem of the GaN-based LEDs.