Effects of Meshed p-type Contact Structure on the Light Extraction Effect for Deep Ultraviolet Flip-Chip Light-Emitting Diodes

Abstract

In this work, flip-chip AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with various meshed contact structures are systematically investigated via three-dimensional finite-difference time-domain (3D FDTD) method. It is observed that both transverse electric (TE)- and transverse magnetic (TM)-polarized light extraction efficiencies (LEEs) are sensitive to the spacing and inclined angle for the meshed structure. We also find that the LEE will not be increased when a large filling factor is adopted for the meshed structures, which is because of the competition among the p-GaN layer absorption, the Al metal plasmon resonant absorption, and the scattering effect by meshed structures. The very strong scattering effect occurring in the hybrid p-GaN nanorod/p-AlGaN truncated nanocone contacts can enormously enhance the LEE for both TE- and TM-polarized light, e.g., when the inclined angle is 30°, the LEE for the TE- and TM-polarized light can be increased by ~ 5 times and ~ 24 times at the emission wavelength of 280 nm, respectively.

Tripling the light extraction efficiency of a deep ultraviolet LED using a nanostructured p-contact

“…Due to the freedom of locally refined and unstructured meshes, and locally refined polynomial degrees, FEM is able to efficiently model the interaction of light with both dielectrics and metals 41 . Previous works optically modeling light emission in DUV-LEDs have mainly employed the finite difference time domain (FDTD) method for solving Maxwell's equations 27,29,37,42 . Due to the regular grid used in finite-difference time domain (FDTD), a higher computation effort is required to approximate the curved surfaces of the cylindrical nanostructure compared to FEM.…”

“…Unfortunately, by applying periodic boundary conditions, a dipole source inside the computational domain will also have its phase replicated, resulting in a periodic array of identical dipole sources emitting light coherently. To avoid this, previous work has tended to rely on computing large super-cells in the lateral direction 27,29,37 . Instead, in this work we employ a method of Brillouin zone integration to obtain a solution approximating an isolated dipole in a periodic device, without the need to calculate large supercells 38,39 .…”

“…Therefore, at least half of the emitted light will be lost due to absorption in the p-contact. Attempts to replace the highly absorbing contact with lower absorption materials constitutes an active research topic [26][27][28] . However, it is currently unclear if the voltage and lifetimes of these alternative contacts can stay within acceptable limits 5 .Absorption in the p-contact may also be reduced via nanostructuring.…”

“…However, nanostructuring the contact can undermine the electrical quality of the p-contact itself. This can be further exacerbated by using highly reflective materials for the electrode such as Al 27 which further increases the contact resistance. To avoid this, we seek nanostructures that can form a large area ohmic contact to the p-type part of the DUV-LED heterostructure for efficient carrier injection.…”

High-Efficiency and High-Reliability Deep-UV Light-Emitting Diodes Using Transparent Ni-Implanted AlN Ohmic Electrodes

“…Recently, the COVID-19 pandemic has accelerated the growth of DUV LED-related industries and markets, which in turn, has provided a boost to the research on DUV LEDs. However, the external quantum efficiency (EQE) and wall-plug efficiency of DUV LEDs are significantly lower than those of visible or near-UV LEDs [4], [5], mainly because most of the DUV light generated in the AlxGa1-xN (x > 0.6)-based active layer is absorbed in the conventional p-GaN or p-AlxGa1-xN (x < 0.4) contact layers [6], [7]. Typically, the Al content of the p-type AlGaN contact layer is lower than that of the AlGaN/AlGaN-based active layer by design to facilitate hole injection from the ptype electrode to the active region [8].…”