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Schematic of the fabrication process flowfor the graphene/p-type Si MSM photodetectors: (a) Graphene is grown on a copper foil, (b) PMMA is deposited on top of graphene as a support layer followed by etching of the copper foil, (c) p-type Si substrates with thermally grown SiO2 layers are cleaned, (d) windows are opened in the oxide layer, and (e) graphene is transferred onto the fabricated Si/SiO2 substrate, the PMMA layer is removed, and graphene is patterned into interdigitated fingers. Ti/Au (5 nm/50 nm) metal contacts are patterned on the graphene areas lying on SiO2 for electrical probing and wire bonding. (f) The Raman spectrum of graphene transferred onto SiO2 measured at a laser wavelength of 632 nm depicting the locations and relative intensities of the D, G, and 2D peaks. (g) Schematic of a graphene/p-type Si MS junction that is fabricated using the same process flow as the MSM devices. (h) SEM image of a fabricated graphene/p-type Si MSM device with finger width W = 5 μm, finger spacing S = 5 μm, active area feature length FL = 300 μm, and active area feature width FW = 300 μm. (i) AFM image of patterned graphene fingers on Si, where the cross-sectional height profiles over the fingers labeled “A” and “B” are also depicted on the right. “x” denotes distance and “z” denotes height over the fingers.
(a) Current-voltage characteristics for a graphene/p-Si Schottky junction with 2.5 × 105 μm2 area at various temperatures ranging from 260 K to 380 K. The upper inset depicts a magnified view of the low forward-bias region of the same I-V characteristics as in the main panel. The lower inset is an Arrhenius plot of the reverse saturation current at 2 V bias in the temperature range 95 K to 380 K for the same device as in the main panel, which shows the transition from thermionic emission to tunneling transport. (b) The experimental Richardson plot (log I/T 2 vs. 1/T) for the device in part (a) at a reverse bias of 2 V in the thermionic emission dominated temperature region and the linear best-fit, which yields the Schottky barrier height. (c) The experimentally extracted Schottky barrier height Φ B (left y-axis) and the calculated change in the Schottky barrier height due to the Fermi level shift in graphene (right y-axis) as a function of reverse bias VR for the same device as in part (a). Note that the two curves exhibit relatively similar slopes.
Energy band diagram of the graphene/p-Si Schottky junction (a) at thermal equilibrium and (b) under reverse bias VR . Evac is the vacuum level, χ, EC , ESi , EF , Si , and EV are the electron affinity, conduction band, bandgap, Fermi level, and valence band of Si, respectively. Furthermore, Vbi is the built-in voltage ,Φ g is the workfunction of intrinsic graphene, is the graphene Fermi-level shift, and is the Schottky barrier height. The superscripts “0” in part (a) denote thermal equilibrium (i.e., zero-bias) values. Note that the graphene Fermi level shifts further down relative to the Dirac point under reverse bias, decreasing Φ B .
Dark current and photocurrent as a function of bias voltage measured at room temperature for a graphene/p-Si MSM photodetector with finger width W = 10 μm, finger spacing S = 10 μm, active area feature length FL = 400 μm, and active area feature width FW = 400 μm. The photocurrent is measured under 633 nm He-Ne laser illumination with 5.1 mW power and ∼830 μm spot size. The inset shows the dark I-V characteristics for the same device as in the main panel.
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