Direct Growth of Graphene on UVC-LEDs by a Plasma-Enhanced Low Temperature CVD Process

Among others, drinking water supply, medical care and air pollution control are major and global challenges of our time. For this, an efficient disinfection, e.g., by using ultraviolet (UV) light is required. It is expected that UV light-emitting diodes (UV-LEDs) based on aluminum-gallium nitride (Al_xGa_1-xN) heterostructures will replace Hg-containing light sources in the near future as they are considered to be an ecologically and economically attractive alternative. However, the efficiencies of UV-LEDs in the technologically relevant UVC-regime are still limited. One reason is the poor conductivity of the p-Al_xGa_1-xN layer that requires the integration of a transparent current spreading layer (TCSL) for LEDs in standard geometry. Due to the poor transparency in the UVC range, ITO as commonly used for visible LEDs is not suitable. The high electrical conductivity and the high transparency over the whole UV-VIS-spectrum make graphene a highly promising candidate for the TCSL [1]. However, up to now, graphene used as TCSL in UVC LEDs is not yet reported.<br/>In this work, we present the successful direct growth of a graphene layer on top of the p-Al_xGa_1-xN cladding layer in a UVC-LED. For the growth, we employ a direct current plasma-enhanced (PE)CVD process with a methane / nitrogen mixture at temperatures as low as 670°C, expanding our previous work on GaN [2]. Optimizing gas mixture ratio, growth temperature and growth time, we obtained graphene with Raman intensity ratios I_D/I_G ≈ 2.1 and I_2D/I_G ≈ 1.3, respectively. The optical transparency of the as-grown graphene layer exceeds 90% over the whole spectral range spanning from 270 nm to 800 nm and we achieved a sheet resistance of around 5 kΩ/sq. Our UVC-LED with graphene as a TCSL shows a diode-like I-V behavior with a current density of approximately 0.1 mA/cm² at 4 V and 0.23 mA/cm² at 5 V, respectively. The emission maximum of the LED is at a wavelength of 275 nm (5 V). A distinct current spreading effect is observed.<br/>[1] Zhang et al., Materials, 15(6), 2203 (2022)<br/>[2] Mischke et al., 2D Materials 7, 035019 (2020)