  Sufficiently large depletion region for photocarrier generation and
separation is a key factor for two-dimensional material optoelectronic devices,
but few device configurations has been explored for a deterministic control of
a space charge region area in graphene with convincing scalability. Here we
investigate a graphene-silicon p-i-n photodiode defined in a foundry processed
planar photonic crystal waveguide structure, achieving visible - near-infrared,
zero-bias and ultrafast photodetection. Graphene is electrically contacting to
the wide intrinsic region of silicon and extended to the p an n doped region,
functioning as the primary photocarrier conducting channel for electronic gain.
Graphene significantly improves the device speed through ultrafast out-of-plane
interfacial carrier transfer and the following in-plane built-in electric field
assisted carrier collection. More than 50 dB converted signal-to-noise ratio at
40 GHz has been demonstrated under zero bias voltage, with quantum efficiency
could be further amplified by hot carrier gain on graphene-i Si interface and
avalanche process on graphene-doped Si interface. With the device architecture
fully defined by nanomanufactured substrate, this study is the first
demonstration of post-fabrication-free two-dimensional material active silicon
photonic devices.
