Effects of exciton localization on internal quantum efficiency of InGaN nanowires

The optical properties of InGaN nanowires with different emission wavelengths of 485, 515, 555, and 580 nm have been studied by means of photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The PL peak energy of the nanowires exhibited an anomalous shift to higher energy and then to lower energy with increasing temperature. Analysis of the temperature-dependent variations in the PL peak energy let us evaluate the localization energies of excitons, which increased with increasing indium composition. TRPL measurements also revealed that the PL decay time of the nanowires increased and then became constant with decreasing emission energy, which was typical of localized excitons and enabled us to evaluate the characteristic energies of localized states. The characteristic energy increased with increasing indium composition, indicating that the density of localized states broadened with increasing indium composition. In addition, a correlation was clearly observed between the internal quantum efficiency (IQE) and localization energy of the nanowire: the IQE increased with increasing localization energy. The increase in the IQE was attributed to the increase in the degree of exciton localization as the indium composition of the nanowire increased. Moreover, it was found that with increasing excitation power density, a reduction in the IQE occurred simultaneously with a PL blue shift. This indicated that the reduction in the IQE was associated with saturation of localized states.