Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

Abstract

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thinfilm flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrateremoval process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 µm/hr at a high RF bias (400 W), and ~7 µm/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.

Deep-ultraviolet integrated photonic and optoelectronic devices: A prospect of the hybridization of group III–nitrides, III–oxides, and two-dimensional materials

“…A consistent blueshift in the emission wavelengths was observed with decreasing nanowall widths because of the reduced distribution in tensile strain. The LEDs exhibited excellent current-voltage I-V characteristics, including a turn-on voltage of 7 V and current densities of greater than 170 A/cm 2 at 12 V [405] .…”

Nanophotonic crystals on unpolished sapphire substrates for deep-UV light-emitting diodes

“…For DUV-LEDs that emit from the back side, internal reflection from the unpolished SSP sapphire surface reduces the light-extraction efficiency (LEE) significantly, in addition to losses due to low reflectivity on the polished top side of the device. Recently, many techniques have been advanced to increase the LEE such as using a super-lattice hole-spreading layer with an Al reflector 20 ; flip chip DUV-LEDs 21 with a roughened AlN template surface 22 ; highly reflective photonic crystals on a p-AlGaN contact layer on top of the DUV-LED structure 23 ; and highly reflective layers and a patterned sapphire substrate 24 . However, improving the LEE by making a nanophotonic crystal (NPhC) on the unpolished side of the single-side-polished (SSP) sapphire substrate has not been done before (the polished side sapphire substrate is for semiconductor growth and the unpolished side is for creating NPhC, where the UV light transmits to the air).…”

AlGaN-Delta-GaN Quantum Well for DUV LEDs

“…It has been reported that the internal quantum efficiency (η IQE ) can be boosted from 1% to 60% if the dislocation density is decreased from 1 × 10 10 cm −2 to 5 × 10 8 cm −2 [1]. Recently, higher quality AlN and SiC substrates have been developed and used for the growth of DUV LEDs [12][13][14][15][16]. In contrast to substrate and growth optimization, few studies have focused on the engineering of the active region to reduce the both QCSE and tune the optical polarization of emitted light.…”