figure 6 outlines the fabrication process of our 
 devices . we start with a commercial silicon on insulator ( soi , from 
 soitec ) substrate with a 340 nm p - type ( 7  10 cm ) si layer on top of a 2 m buried oxide ( box ) . 

first , a 100 nm sin mask is deposited by plasma enhanced chemical 
 vapor deposition ( pecvd , oxford plasmalab100 ) onto the soi substrate 
 at 300 c ( figure 6b ) .
next , a si photonic waveguide and the pd area are defined by 
 electron beam lithography ( ebl , raith eline 150 ) using positive e - beam 
 resist ( zep 520a ) .
the ebl pattern is subsequently transferred to 
 sin by reactive ion etching ( rie ) ( oxford plasmalab 100 ) with a chf3/o2 gas mixture .
then the soi substrate is locally 
 oxidized ( wet , 1000 c ) to grow a sio2 layer only 
 in localized patterns defined by ebl where si is exposed to o2 , while at the same time a sin mask prevents o2 diffusion into the si in protected areas ( figure 6c ) .
after oxidation , the sacrificial sin 
 mask layer is etched in hot phosphoric acid ( h3po4 , 180 ) followed by sio2 removal in a buffered oxide 
 etch ( boe ) solution .
the ohmic contact to si is realized by al evaporation 
 followed by metal lift - off and thermal alloying at 460 c in 
 a forming gas ( h2/n2 , 5%/95% ) environment .
this 
 fabrication process is based on the technique of local - oxidation of 
 si ( locos ) in which a si waveguide is defined by oxide spacers rather than rie .
the locos process enables the 
 realization of low - loss ( 0.3 db / cm ) si photonic waveguides coupled to a schottky pd using the same fabrication 
 step . fabrication process of si  slg schottky pds integrated with 
 photonic waveguides .
( a ) planar soi substrate ; ( b ) pecvd deposition 
 and patterning of sin mask ; ( c ) local oxidation ; ( d ) etching of sin 
 and sio2 al ohmic contact to si ; ( e ) slg transfer ; ( f ) 
 formation of schottky contact and consequent slg etching .
slg is grown on a 35 m cu foil following 
 the process described in ref ( 98 ) .
the substrate is annealed in hydrogen atmosphere ( h2 , 20 sccm ) up to 1000 c for 30 min .
the substrate 
 is subsequently cooled in vacuum ( 1 mtorr ) to room temperature and 
 removed from the chamber .
after growth , the quality and uniformity 
 of slg are monitored by raman spectroscopy using a renishaw invia 
 equipped with a 100 objective ( numerical aperture na = 0.85 ) . 

the raman spectrum of slg on cu at 514 nm is shown in figure 7b ( green curve ) .
the 2d peak is a single sharp lorentzian with full width at half - maximum , 
 fwhm(2d )  29 cm , a signature of slg .
different ( 20 ) point measurements show 
 similar spectra , which indicate uniform quality .
the position of the 
 g peak , pos(g ) , is 1589 cm , with fwhm(g ) 
  13 cm .
the 2d peak position , pos(2d ) 
 is 2698 cm , while the 2d to g peak intensity 
 and area ratios , i(2d)/i(g ) and a(2d)/a(g ) , are 2.6 and 5.8 , respectively , 
 indicating a p - doping 300 mev , which corresponds to a carrier concentration 5  10 cm .
( a ) raman spectra of ( red curve ) si substrate 
 and ( black curve ) slg transferred on si .
( b ) raman spectra of ( green 
 curve ) slg on cu , and ( blue curve ) after normalized , point - to - point 
 subtraction of the si substrate spectrum ( shown in ( a ) , red curve ) 
 from the spectrum of slg transferred on si ( shown in ( a ) , black curve ) .
a 500 nm thick layer of poly(methyl 
 methacrylate ) ( pmma ) is spin coated on the slg / cu sample and then 
 placed in a solution of ammonium persulfate ( aps ) in di water until 
 cu is completely etched . after cu etching , 
 the pmma membrane with attached slg is transferred to di water for 
 cleaning aps residuals . to obtain a schottky interface between 
 the si waveguide and slg without the native oxide layer ,
we perform 
 the transfer in diluted hydrofluoric acid ( hf ) and di water ( 1:100 ) . 
 after cleaning from aps residuals ,
a slg / pmma membrane is placed on 
 a plastic beaker containing 5 ml/500 ml hf and di water .
next , the 
 target soi chips are first dipped in boe for 5 s to etch the si native 
 oxide and then used to lift the floating slg / pmma membrane from diluted 
 hf . as a result ,
during drying , the presence of hf at the slg / si interface 
 prevents si oxidation and allows formation of  oxide free  
 slg / si schottky contacts . after drying ,
pmma is removed in acetone , 
 which leaves slg to entirely cover the soi .
we also transfer slg from 
 the same cu foil with the same transfer procedure onto si .
the raman spectrum of slg transferred on si is shown in figure 7a ( black line ) .
this 
 is measured at 514.5 nm and with laser power below 300 w to 
 avoid possible heating effects or damage .
the d peak region overlaps 
 the bands at 12001500 cm , attributed 
 to third order raman scattering from to phonons in the si substrate .
the peaks at 1550 and 2330 
 cm in the raman spectrum of si substrate ( red 
 line ) arise from molecular vibrations of ambient oxygen ( o2 ) and nitrogen ( n2 ) .
the raman spectra of the transferred slg film 
 ( black line ) and reference si substrate ( red line ) are acquired using 
 identical exposure time and laser power .
after the intensity of the 
 third order si peak at 1450 cm in the 
 si reference spectrum ( red line ) is normalized to the same peak in 
 the spectrum of the transferred slg film ( black line ) , a point - to - point 
 subtraction is implemented ( figure 7b , blue line ) .
the 2d peak retains its single - lorentzian line - shape with fwhm(2d ) 
  33 cm , validating that slg has been successfully 
 transferred .
pos(g )  1584 cm , fwhm(g )  17 cm , and pos(2d )  2687 cm , while i(2d)/i(g ) and a(2d)/a(g ) are 3.2 and 5.9 , respectively , suggesting a p - doping 
 4  10 cm ( 200 
 mev ) .
after slg transfer , we use an additional 
 ebl step followed by o2 plasma etching to selectively remove 
 slg from the substrate area containing five waveguides and dedicated 
 to the reference m  si devices .
then a schottky contact is prepared 
 by evaporation and liftoff of an 3 nm/100 nm cr / au metal strip intersecting 
 the si waveguide with slg on top ( figure 6f ) ( or without slg for reference devices ) 
 and forming a schottky interface for photodetection .
finally , the 
 samples are placed in a reactive o2 plasma to remove superfluous 
 slg .
the tfe current is given 
 by1where a * * is the 
 effective richardson constant , k is the boltzmann 
 constant , t is the temperature , and q is the electron charge .
the contribution of tfe to charge injection 
 across the m  si interface can be evaluated by comparing the 
 thermal energy kt to e00 , defined as2where  is the reduced 
 planck constant , n is the si doping , m * is the effective mass of the charge carriers in si , and s is the dielectric permittivity of si . when kt  e00 , the tfe process mainly 
 contributes to charge carriers injection across the schottky interface . the parameters e0 and  are analytically defined as34 in our case , for si doping 7  10 cm using eq 2
, we get e00 45 mev , comparable to the thermal 
 energy at room temperature of 26 mev , reflecting a significant tfe 
 contribution to carriers injection at the schottky interface .
 we report an on - chip integrated metal 
 graphene  silicon plasmonic schottky photodetector with 85 ma / w 
 responsivity at 1.55 m and 7% internal quantum efficiency . 
 
 this is one order of magnitude higher than metal  
 silicon schottky 
 photodetectors operated in the same conditions . at a reverse bias 
 of 3 v , we achieve avalanche multiplication , with 0.37a / w responsivity 
 and avalanche photogain 2 . this paves the way to graphene 
 integrated silicon photonics . 
Methods