Light Extraction Efficiency Analysis of Flip-Chip Ultraviolet Light-Emitting Diodes With Patterned Sapphire Substrate

Abstract

This study investigates polarization-dependent light extraction efficiency (η extraction ) of AlGaN-based flip-chip ultraviolet (UV) light-emitting diodes (LEDs) emitting at 230 nm and 280 nm with microdome-shaped patterning on sapphire substrate based on 3-D finite-difference time-domain simulations. Three types of patterned sapphire substrates (PSS) have been analyzed: bottom-side PSS, top-side PSS, and double-sided PSS. Our results show that microdome-shaped patterning on sapphire substrate is predominantly beneficial in enhancing transverse-magnetic (TM)-polarized output. Specifically, TM-polarized η extraction enhancement of up to ∼4.5 times and ∼2.2 times can be obtained for 230 nm and 280 nm UV LEDs with bottom-side PSS, respectively, and ∼6.3 times and ∼1.8 times for 230 nm and 280 nm UV LEDs with top-side PSS, respectively. By employing double-sided PSS, up to ∼11.2 times and ∼2.6 times enhancement in TM-polarized η extraction can be achieved for 230 nm and 280 nm UV LEDs, respectively. In contrast, the microdome-shaped PSS act as a reflector for transverse-electric-polarized photons which leads to severe limitation in light extraction for both 230 nm and 280 nm flip-chip UV LEDs. Thus, it is expected that this study will serve as a guidance in designing PSS for high-efficiency mid-and deep-UV LEDs.

Temperature-Dependent Carrier Recombination and Efficiency Droop of AlGaN Deep Ultraviolet Light-Emitting Diodes

“…Deep ultraviolet light-emitting diodes (DUV LEDs), with emission wavelengths below 300 nm, are commonly manufactured with AlGaN-based multiple quantum wells (MQWs) [1]- [4]. Highphoton-energy emissions are propitious in various fields, such as water sterilization, biological agent detection, covert communications, and solid state lighting [1], [5]- [7]. LEDs generally suffer from current-droop (J-droop) and temperature-droop (T-droop) [8], [9].…”

AlGaN-Delta-GaN Quantum Well for DUV LEDs

“…Although a UV LED epi-stack would be grown on a sapphire substrate at least 200 µm thick, simulating such a device with an acceptable degree of accuracy using direct solution of Maxwell's equations would require significant simulation time, so a 200 nm-thick sapphire was used here. Use of a growth substrate of significantly reduced thickness is common practice when performing an FDTD simulation of LEDs as it allows for an order of magnitude reduction in the simulation run time while maintaining the accuracy of the simulation [33][34][35]. Table 2 lists the refractive index (n) and extinction coefficient (k) of the materials used in this study.…”

Enhancing Light Extraction Efficiency of Vertical Emission of AlGaN Nanowire Light Emitting Diodes With Photonic Crystal

“…The spectrum of TM dipole source has a Gaussian shape. The LEE of top surface is defined as the ratio of the emitted power going upward within a rectangular region above the AlN layer to the total power of the active region [38], [39]. In this work, we use a power monitor, which is a sufficiently large rectangular plane, to collect the light escaping from the top surface of the LED device.…”