The Effect of Sapphire Substrates on Omni-Directional Reflector Design for Flip-Chip Near-Ultraviolet Light-Emitting Diodes

Abstract

In this paper, we investigate the effect of omni-directional reflectors (ODRs) on the light extraction efficiency (LEE) for flip-chip near-ultraviolet light-emitting diodes (LEDs) on patterned (PSS) and flat sapphire substrate (FSS) using three-dimensional finite-difference time-domain method. Different design principles of ODR for the flip-chip LEDs on PSS and FSS are proposed to attain optimum LEE. For the flip-chip LED on FSS, the LEE curve is oscillatory with changing the thickness of the SiO 2 layer as a result of the coherent interference in the ODR and the optical cavity tuning effect for light source. Therefore, the thickness of the p-GaN needs to satisfy even times of quarter wave and the thickness of SiO 2 needs to be quarter wave. However, for the flip-chip LED on PSS, both the total internal reflection and the surface plasmon polariton resonance absorption play a major role leading to LEE increasing as the thickness of SiO 2 layer increases. As a result, SiO 2 layer with a thickness of over one wavelength is better, because the energy of evanescent wave decays to sufficiently negligible when it reaches the Al metal.

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 .…”

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

“…The lateral dimension for the calculated structure is set to 8 × 8 μm 2 . The boundary conditions for the four lateral boundaries are assumed to have a reflectance of 100% such that the finite lateral dimensions can be speculated to be infinite [25]. The conditions for the top and the bottom boundaries are set to have perfectly matched layer (PML), which can entirely absorb the electromagnetic energy.…”

Formation Mechanism of Self-Formed Triangular Pyramidal Patterns on Sapphire Substrate

“…On the other hand, the inclined facets of the patterns on PSS can destroy the total internal reflection, which enhances the light extraction efficiency (LEE) of the LEDs. Also, people have reported that the light angular distribution of the LED die depends on the pattern structure and the morphology of the patterns on PSS …”

“…Also, people have reported that the light angular distribution of the LED die depends on the pattern structure and the morphology of the patterns on PSS. 7 Not only can the PSS technique improve IQE and LEE but also it provides the chance to adjust the light angular distribution of blue GaN-based LEDs, which can be developed for new applications and solve the issues of whitelight GaN-LEDs, such as low angular color correlated temperature deviation and human−system interface. 8 There are many ways of fabricating the patterned sapphire substrate, such as dry-and wet-etching processes.…”