This report summarizes results from a three-year Laboratory Directed Research and Development (LDRD) project. The collaborative effort of researchers from Sandia National Laboratories and Rensselaer Polytechnic Institute was supported by Sandia’s National Institute for Nano-Engineering and focused on the study and application of nanoscience and nanoengineering concepts to improve the efficiency of semiconductor light-emitting diodes (LEDs) for solid-state lighting applications.
The project explored LED efficiency advances with two primary thrusts: (1) the study of nanoscale indium gallium nitrade (InGaN) materials properties, particularly nanoscale crystalline defects, and their impact on internal quantum efficiency (IQE), and (2) nanoscale engineering of dielectric and metal materials and integration with LED heterostructures for enhanced light extraction efficiency.
In the course of this LDRD project, a wide range of topics were investigated with the overarching goal of understanding and ameliorating the present limitations to InGaN LED efficiency. On the broad-ranging topic of InGaN materials, the researchers focused on the impact of nanoscale crystalline defects on LED internal quantum efficiency. Within this framework, their studies addressed two of the most severe chip-level roadblocks to realizing ultra-efficient solid-state lighting: the strong drop of LED efficiency at high current levels (efficiency droop) and the drop of efficiency of green and longer wavelength LEDs.
In the work on efficiency droop, the researchers examined whether the high density of threading dislocations found in typical InGaN LEDs contributed to the droop phenomenon. Through both electroluminescence characterization and modeling of a series of InGaN LEDs with different dislocation densities, it was determined that nonradiative recombination at threading dislocations is not the primary high current mechanism contributing to efficiency droop, while carrier leakage out of the InGaN active region is consistent with findings. The team further applied power-dependent photoluminescence (PL) to these LED samples to quantify both the IQE vs. carrier density relationship and the non-radiative coefficient A as a function of threading dislocation density.