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 thin-film 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 SF6 etch parameters to develop a substrate-removal 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–1 at a high RF bias (400 W), and ∼7 μm hr–1 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 ∼3× after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency and power conversion efficiency UV-C LEDs.

Highlights

  • AlGaN ultraviolet light-emitting diodes (UV LEDs) and ultraviolet laser diodes (UV LDs) in the range of 265–280 nm are needed to develop novel disinfection and sterilizing technologies to improve access to clean water [1,2,3,4], improve public health [5,6], and enable other biotech applications [7,8,9,10]

  • Researchers have largely overlooked AlGaN LEDs grown on SiC substrates [66,67,68,69,70,71,72,73,74] because SiC absorbs strongly below its optical bandgap (3.2 eV and 3.0 eV for 4H-SiC and 6H-SiC, respectively), but this disadvantage can be overcome with a novel thin-film flip-chip (TFFC) LED architecture in which the SiC is removed with a highly selective SF6 chemical plasma etch between SiC and AlN

  • We demonstrate in this paper that AlGaN LEDs grown on SiC can be processed into thin-film LEDs with high light extraction efficiency (LEE) [82,83,84,85]; for example, finite-difference time-domain (FDTD) simulations by Ryu et al show that transverse magnetic (TM) emission’s LEE in textured thin-film LEDs is significantly higher (>6X) than in volumetric AlGaN FC-LEDs [48]

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Summary

Introduction

AlGaN ultraviolet light-emitting diodes (UV LEDs) and ultraviolet laser diodes (UV LDs) in the range of 265–280 nm are needed to develop novel disinfection and sterilizing technologies (water, air, and surfaces) to improve access to clean water [1,2,3,4], improve public health [5,6], and enable other biotech applications [7,8,9,10]. Researchers have largely overlooked AlGaN LEDs grown on SiC substrates [66,67,68,69,70,71,72,73,74] because SiC absorbs strongly below its optical bandgap (3.2 eV and 3.0 eV for 4H-SiC and 6H-SiC, respectively), but this disadvantage can be overcome with a novel thin-film flip-chip (TFFC) LED architecture in which the SiC is removed with a highly selective SF6 chemical plasma etch between SiC and AlN. InGaN thin-film blue LEDs were developed with very high LEE using laser lift-off for substrate removal, N-face GaN KOH photoelectrochemical (PEC) roughening, and a pside reflective mirror [86,87,88]. We demonstrate high LEE thin-film flip chip (TFFC) AlGaN LEDs grown on SiC. The AlGaN LED’s LEE improved by ~3X after roughening in KOH [0.25 M] for 70 sec at room temperature

SiC substrate thinning characteristics
SF6 etch chemistry characteristics
Findings
Conclusions
Full Text
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