Analysis of Loss Processes in GaN-based LEDs

Abstract

GaN-based light-emitting diodes (LEDs) have attracted huge attention and developed rapidly in recent years due to advantages as light source regarding energy-saving and stability. However, several challenges still remain such as the green gap and droop. The main part of this thesis focusses on the following aspects: First a reliable value for the extraction efficiency for our single quantum well LED structure is obtained via calculation and calibration. According to the theory of light propagation in different media, the extraction efficiency is calculated based on the LED structure. A calibration is necessary because the real structure may be different from the ideal one. A series of samples was grown with nominal the same structure but different GaN contact layer thicknesses, which keeps the internal quantum efficiency constant while the extraction efficiencies are different. By scaling the calculated extraction efficiencies to fit the external quantum efficiencies determined by experiments, reliable extraction efficiencies are obtained. Secondly, a procedure to determine the carrier density at various injection currents is developed. The dependence of emission energy on carrier density is calculated taken into consideration the p-n junction built-in electric field, the piezoelectric field and screening effect of free carriers. According to the calculation, there is a minimum of the emission energy for increasing carrier density. A minimum of emission energy is indeed observed under electroluminescence condition when the current is varied from very low to high. Correct values for the carrier density are determined from comparing the minimum of the emission energy of simulation and measurement. For a specific sample the determination of the free carrier density was successful. Thirdly, the dependence of the radiative lifetime and the nonradiative lifetime on carrier density is studied utilizing the above procedure. For the radiative recombination, the dependence of the lifetime on carrier density shows that the electric field affects the radiative recombination by reducing the overlap of electron and hole wavefunction in the quantum well. The non-radiative recombination is unclear. A strange behavior of the dependence of non-radiative lifetime on carrier density (non-radiative lifetime is not constant at lower carrier density region) is observed for some samples, which we cannot explain.