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Metal-semiconductor-metal photodetectors based on graphene/p-type silicon Schottky junctions
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Image of FIG. 1.
FIG. 1.

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.

Image of FIG. 2.
FIG. 2.

(a) Current-voltage characteristics for a graphene/p-Si Schottky junction with 2.5 × 105μm2 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.

Image of FIG. 3.
FIG. 3.

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 .

Image of FIG. 4.
FIG. 4.

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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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Metal-semiconductor-metal photodetectors based on graphene/p-type silicon Schottky junctions