Graphene-Enhanced UVC-LEDs

In anticipation of their promising applications in the deep-ultraviolet (UVC) spectral range, e.g., for disinfection, AlGaN-based UVC-Light Emitting Diodes (UVC-LEDs) have experienced a lot of attention over the last decade. However, these LEDs still have lower performance regarding Wall Plug Efficiency (WPE) and External Quantum Efficiency (EQE) than blue emitting LEDs [1,2]. One reason is the low electrical conductivity of the p-AlGaN cladding layer, which limits both, current injection in flip-chip geometry, and lateral current spreading for devices in standard geometry.<br/><br/>Graphene combines high electrical conductivity and high optical transparency in the UVC spectral range and has thus the potential to act as low absorbent contact layer in UVC-LEDs. Here we report on a transfer-free low-temperature approach for integrating graphene directly into AlGaN-based UVC-LEDs at temperatures far below 1000°C. We demonstrate functional UVC-LEDs in standard as well as in flip-chip geometry.<br/><br/>Based on our previous work [3,4] we used a plasma-enhanced CVD graphene process to directly grow graphene on the <i>p</i>-AlGaN layer of a UVC-LED at 670°C. The graphene has an optical transparency above 90% in the UVC region and an electrical sheet resistance of about 5 kΩ/sq. Using the directly grown graphene as a transparent contact layer, we were able to improve the EQE of UVC-LEDs emitting at 275 nm in standard as well as in flip-chip geometry to over 5%. With these proof-of-principle experiments, we have paved the way to a large-scale integration-friendly process of graphene-enhanced UVC-LEDs.<br/><br/>[1] M. Kneissl et al., <i>Nature Photonics</i> <b>13</b>, 233 (2019)<br/>[2] S. Liang and W. Sun, Adv. Mater. Technol. 7 (2022) 2101502<br/>[3] H. Zhang et al., <i>Materials</i>, <b>15</b>(6), 2203 (2022)<br/>[4] J. Mischke et al., <i>2D Materials</i> <b>7</b>, 035019 (2020)