Degradation of far-ultraviolet light emitting diodes on AlN substrate
3. Optical devices
Shashwat Rathkanthiwar1 , Maki Kushimoto2, Yudai Shimizu3, Kazutada Ikenaga3, Mayank Bulsara3, Keitaro Ikejiri3, Hiroshi Amano4, Leo J. Schowalter1
1 Lit Thinking, Orlando, Florida 32826, USA
2 Graduate School of Engineering, Nagoya University, Aichi 464-8603, Japan
3 Taiyo Nippon Sanso, Innovation Unit, Yokohama, Kanagawa 220-8561, Japan
4 Center for Integrated Research of Future Electronics, Institute of Materials Research and System for Sustainability, Nagoya University, Nagoya, 464-8601, Japan
Abstract:
The safety and efficacy of far-UVC radiation (<240 nm) for indoor pathogen inactivation has spurred research on far-UVC light sources. While nitride LED technology promises to replicate the advantages of visible and near-UV LEDs like compact size, low operating power, and tunable wavelength, a major challenge is the sharp decrease in efficiency and lifetime (at practical current densities) at shorter wavelengths (high Al content). Perhaps this degradation is linked to the increased point defect incorporation during high Al-content AlGaN growth or to a greater susceptibility of point defects (or point defect complexes) to be activated by high current at high Al composition. The precise mechanism is unknown, and, in this study, we are investigating the impact of multi-quantum well (MQW), electron blocking layer (EBL), and graded-AlGaN hole injection layer (HIL) variations on 240-nm LED lifetimes. The LEDs were grown on AlN substrates using a low-pressure, resistive-heated, horizontal-flow metalorganic chemical vapor deposition (MOCVD) which achieved step-flow morphology and pseudomorphic growth. Cross-sectional transmission electron microscopy corroborated the high-quality epitaxial growth. The Si-doped Al0.79Ga0.21N n-contact layer exhibited a sheet resistance of 405 Ω/□. TLM contacts on the p+ GaN contact layer showed a linear I-V behavior confirming an Ohmic contact formation. LEDs exhibited a sharp electroluminescence with FWHM of ~11 nm. Accelerated constant-current degradation tests [1] were conducted in the range of 0.7 to 4 kA/cm2 (corresponding to a forward voltage ranging from 7.9 to 11.3 V, respectively, at the start of the test). A 12-fold reduction (from 520 to 42 seconds) in L50 lifetime (50% drop from the initial output power) was observed for the 6-fold increase in current density. Notably, increasing the HIL start composition from 90% to 95% Al led to 2.5 and 5 times decrease in the L50 lifetime at 0.7 and 4 kA/cm2, respectively, which supports the hypotheses that point defects at higher Al content are susceptible to higher degradation rates for far-UVC LEDs.
[1] Zhang et al., physica status solidi (a), 221 (2024) 2300946.
