Graphene-silicon device for visible and infrared photodetection.

The fabrication of graphene-silicon (Gr-Si) junction inolves the formation of a parallel metal-insulator-semiconductor (MIS) structure, which is often disregarded but plays an important role in the optoelectronic properties of the device. In this work, the transfer of graphene onto a patterned n-type Si substrate, covered by $Si_3N_4$, produces a Gr-Si device in which the parallel MIS consists of a $Gr-Si_3N_4-Si$ structure surrounding the Gr-Si junction. The Gr-Si device exhibits rectifying behavior with a rectification ratio up to $10^4$. The investigation of its temperature behavior is necessary to accurately estimate the Schottky barrier height at zero bias, ${\phi}_{b0}=0.24 eV$, the effective Richardson's constant, $A^*=7 \cdot 10^{-10} AK^{-2}cm^{-2}$, and the diode ideality factor n=2.66 of the Gr-Si junction. The device is operated as a photodetector in both photocurrent and photovoltage mode in the visible and infrared (IR) spectral regions. A responsivity up to 350 mA/W and external quantum efficiency (EQE) up to 75% is achieved in the 500-1200 nm wavelength range. A decrease of responsivity to 0.4 mA/W and EQE to 0.03% is observed above 1200 nm, that is in the IR region beyond the silicon optical bandgap, in which photoexcitation is driven by graphene. Finally, a model based on two back-to-back diodes, one for the Gr-Si junction. the other for the $Gr-Si_3N_4-Si$ MIS structure, is proposed to explain the electrical behavior of the Gr-Si device.