Impact of crystallinity and surface termination on bilayer diamond UV photodetectors

We report on the fabrication and optoelectronic characterization of UV photodetectors based on nano- and micro-crystalline diamond (NCD, MCD) layers synthesized by microwave plasma-assisted chemical vapor deposition under varying conditions. The structural, morphological and chemical properties of the diamond films were investigated using scanning electron microscopy and Raman spectroscopy, confirming the formation of crystalline sp 3 -bonded carbon with grain sizes ranging from ∼20 nm to several hundred nanometers. The optoelectrical performance was evaluated using AC photocurrent spectroscopy (200–1000 nm) and DC current-voltage (-2 to +2 V in the dark and under UV illumination) measurements, both before and after plasma oxidation. This surface treatment resulted in suppression of dark current by up to three orders of magnitude, consistent with the passivation of conductive grain boundaries and modification of surface charge states. After plasma oxidation, the NCD/MCD bilayer photodetector exhibited approximately 80% improvement in the photocurrent-to-dark current ratio. The study highlights the synergistic impact of bilayer architecture, crystallinity, deposition conditions, and surface termination on device performance, demonstrating the potential of layered diamond architectures for high-sensitivity, low-noise UV photodetectors.