Graphene/Ge microcrystal photodetectors with enhanced infrared responsivity

We report on the electrical and optical properties of microcrystal arrays obtained by depositing Ge on a deeply patterned Si substrate. Finite difference time domain simulations indicate that the faceted morphology and high refractive index of Ge microcrystals lead to strong light trapping effects, enhancing infrared light absorption in the spectral window between the direct and indirect absorption edge of Ge (asymptotic to 1550-1800 nm). This is experimentally confirmed by fabricating microcrystal-based Ge-on-Si photodiodes employing graphene as a top transparent contact. In these devices, the ratio between the responsivities at 1550 and 1700 nm is more than ten times larger than that of photodiodes based on conventional Ge-on-Si epilayers.