Effects of Wavelength and Defect Density on the Efficiency of (In,Ga)N-Based Light-Emitting Diodes17 Aug 2017
We measure the electroluminescence of light-emitting diodes (LEDs) on substrates with low dislocation densities (LDD) at 106 cm -2 and low 108 cm -2 , and compare them to LEDs on substrates with high dislocation densities (HDD) closer to 1010 cm-2. The external quantum efficiencies (EQEs) are fitted using the ABC model with and without localization. The nonradiative-recombination (NR) coefficient A is constant for HDD LEDs, indicating that the NR is dominated by dislocations at all wavelengths. However, A strongly increases for LDD LEDs by a factor of 20 when increasing the emission wavelength from 440 to 540 nm. We attribute this to an increased density of point defects due to the lower growth temperatures used for longer wavelengths. The radiative recombination coefficient B follows the squared wave-function overlap for all samples. Using the observed coefficients, we calculate the peak efficiency as a function of the wavelength. For HDD LEDs the change of wave-function overlap (i.e., B) is sufficient to reduce the EQE as observed, while for LDD LEDs also the NR coefficient A must increase to explain the observed EQEs. Thus, reducing NR is important to improving the EQEs of green LEDs, but this cannot be achieved solely by reducing the dislocation density: point defects must also be addressed.