Improving Light Extraction From GaN Light-Emitting Diodes by Buried Nano-Gratings

Recent experimental work has demonstrated that the light extraction enhancement due to scattering by a metallic nano-grating in an InGaN/GaN quantum well (QW) structure can be improved significantly by burying the grating in a dielectric, such as polyvinyl alcohol (PVA). In this paper, we employ the fluctuational electrodynamics method to investigate the origin of this improvement and to provide guidelines on how to optimize emission efficiency in these structures. Our results show that metallic grating diffracts efficiently the high-intensity PVA resonances trapped in the structure, because of the large permittivity contrast between the metal and semiconductor, providing the reported exceptional enhancement for s-polarization. We also study the effect of two important physical factors in the enhancement: 1) the thickness of the GaN barrier separating the QW from the grating and 2) the thickness of the InGaN QW. Results reveal that the enhancement efficiency can be maintained even when the QW is not in the near field of the grating, for a QW-grating separation of up to 1 μm. This is in contrast to plasmonic structures, where enhancement strongly decreases as the separation is increased. However, the enhancement factor can also vary strongly at the local level in smaller spatial intervals. Results also show that the enhancement significantly decreases with the QW thickness due to the losses in the QW.