  Graphene / silicon (G/Si) heterostructures have been studied extensively in
the past years for applications such as photodiodes, photodetectors and solar
cells, with a growing focus on efficiency and performance. Here, a specific
contact pattern scheme with interdigitated Schottky and
graphene/insulator/silicon (GIS) structures is explored to experimentally
demonstrate highly sensitive G/Si photodiodes. With the proposed design, an
external quantum efficiency (EQE) of > 80 % is achieved for wavelengths ranging
from 380 to 930 nm. A maximum EQE of 98% is observed at 850 nm, where the
responsivity peaks to 635 mA/W, surpassing conventional Si p-n photodiodes.
This efficiency is attributed to the highly effective collection of charge
carriers photogenerated in Si under the GIS parts of the diodes. The
experimental data is supported by numerical simulations of the diodes. Based on
these results, a definition for the true active area in G/Si photodiodes is
proposed, which may serve towards standardization of G/Si based optoelectronic
devices.
