  We report vertically-illuminated, resonant cavity enhanced, graphene-Si
Schottky photodetectors (PDs) operating at 1550nm. These exploit internal
photoemission at the graphene-Si interface. To obtain spectral selectivity and
enhance responsivity, the PDs are integrated with an optical cavity, resulting
in multiple reflections at resonance, and enhanced absorption in graphene. Our
devices have wavelength-dependent photoresponse with external (internal)
responsivity~20mA/W (0.25A/W). The spectral-selectivity may be further tuned by
varying the cavity resonant wavelength. Our devices pave the way for developing
high responsivity hybrid graphene-Si free-space illuminated PDs for free-space
optical communications, coherence optical tomography and light-radars
