ABSTRACT
For evaluating the effect of light absorption in vertically structured thin film light-emitting diodes (VLEDs), we investigate the dependence of the efficiencies on the several specific parameters including thickness and doping concentration (N(D)) of the n-GaN layer, a design of hetero-structures of the n-GaN layer, and a number of pairs of multi-quantum wells (MQWs). Generally, there is a complementary relation between internal quantum efficiency (IQE) and light extraction efficiency (LEE). However, we confirmed that LEE determined by light absorption is more dominant than IQE in VLED structures with a textured surface, from numerical simulation and experimental results. Effect of light absorption is more prominent in the vertical chip with a textured surface than in that with a flat surface, because a travel length of light extracted from the textured surface is longer. Minimizing light absorption in VLEDs is a key technology for improving light output, and light absorption speaks for the index of enhancement by the general technologies for improving LEE.
ABSTRACT
We investigate the dependence of various efficiencies in GaN-based vertical blue light-emitting diode (LED) structures on the thickness and doping concentration of the n-GaN layer by using numerical simulations. The electrical efficiency (EE) and the internal quantum efficiency (IQE) are found to increase as the thickness or doping concentration increases due to the improvement of current spreading. On the contrary, the light extraction efficiency (LEE) decreases with increasing doping concentration or n-GaN thickness by the free-carrier absorption. By combining the results of EE, IQE, and LEE, wall-plug efficiency (WPE) of the vertical LED is calculated, and the optimum thickness and doping concentration of the n-GaN layer is found for obtaining the maximum WPE.
