Graphene/black phosphorus-based infrared metasurface absorbers with van der Waals Schottky junctions

Black phosphorus (BP) is a promising candidate for fabricating infrared (IR) photodetectors because its bandgap in the IR region can be controlled by varying the number of layers. BP-based metasurfaces have attracted considerable attention for applications in wavelength-selective and/or polarization-selective IR absorbers. Graphene and BP (Gr/BP) van der Waals (vdW) heterostructures are expected to enhance the performance of BP-based IR photodetectors. However, the Gr/BP vdW heterostructure forms a Schottky junction; thus, the electron transfer between Gr and BP should be investigated to determine the precise optical properties of Gr/BP vdW heterostructure-based metasurfaces. In this study, the electron transfer in the Gr/BP vdW heterostructure is investigated theoretically. The metasurface absorber structure proposed based on the results comprises periodic Gr/BP vdW heterostructure strips on top, a middle dielectric layer, and a bottom reflector. Numerical calculations indicated that the Gr/BP vdW heterostructure has strong wavelength- and polarization-selective near-unity IR absorption. The absorbance is increased and absorption wavelength is shortened compared with those of the monolayer-BP-based metasurface. The absorption wavelength can be controlled by changing the width of the Gr/BP strips owing to the hybrid localized surface plasmons of Gr/BP. This is attributed to the electron transfer through the Schottky junction between Gr and BP with enhanced localized surface plasmon resonance. The results suggest that the Gr/BP vdW heterostructure is a promising platform for realizing wavelength-selective and/or polarization-selective IR photodetectors and IR absorbers/emitters. The resulting photodetectors exhibit high responsivity and low noise because the BP bandgap corresponds to the IR wavelength region.