Giant Dirac point shift of graphene phototransistors by doped silicon substrate current

Abstract

Graphene is a promising new material for photodetectors due to its excellent optical properties and high-speed response. However, graphene-based phototransistors have low responsivity due to the weak light absorption of graphene. We have observed a giant Dirac point shift upon white light illumination in graphene-based phototransistors with n-doped Si substrates, but not those with p-doped substrates. The source-drain current and substrate current were investigated with and without illumination for both p-type and n-type Si substrates. The decay time of the drain-source current indicates that the Si substrate, SiO2 layer, and metal electrode comprise a metal-oxide-semiconductor (MOS) capacitor due to the presence of defects at the interface between the Si substrate and SiO2 layer. The difference in the diffusion time of the intrinsic major carriers (electrons) and the photogenerated electron-hole pairs to the depletion layer delays the application of the gate voltage to the graphene channel. Therefore, the giant Dirac point shift is attributed to the n-type Si substrate current. This phenomenon can be exploited to realize high-performance graphene-based phototransistors.

Broadband photoresponse of graphene photodetector from visible to long-wavelength infrared wavelengths

“…Most research has focused on enhancement of the responsivity using various methods, such as p-n junctions, 13,14 plasmonic resonance, 15,16 asymmetric electrodes, 17,18 optical cavities, 19 waveguides, 20,21 nanoribbons, 8,22 and optical gating. [23][24][25][26][27] However, although graphene is highly sensitive to the surrounding materials and its influence changes according to the wavelength, the photoresponse mechanism in the broadband wavelength region from visible to long-wavelength IR (LWIR) has been less studied. 28 In particular, the materials surrounding graphene have a strong effect on its optical properties.…”

“…28 In particular, the materials surrounding graphene have a strong effect on its optical properties. 24,25 It is thus important to investigate the effects of these materials experimentally to reveal the photoresponse mechanism of graphene photodetectors. We have previously investigated the influence of the surrounding materials for broadband photoresponse; 29 however, no direct evidence of the photogating induced by the substrate from the visible to LWIR wavelengths was demonstrated.…”

Photogating for small high-responsivity graphene middle-wavelength infrared photodetectors

“…24 We have previously investigated various techniques for this purpose, including the use of p-n junctions, 25 plasmonic metamaterial absorbers, [26][27][28] and the photogating effect. [29][30][31][32][33][34] Here, we report a detailed mechanism for the photogating effect that significantly improves the responsivity of graphene photodetectors operating in the MWIR spectral band.…”

“…These photoelectrons/holes are subsequently separated via the application of V bg . Second, the photoelectrons accumulate in the depletion layer at the TEOS/InSb boundary as a consequence of V bg , 29 and V bg is modulated by the internal electric field generated by the accumulated photoelectrons. Third, the field effect in the graphene is changed by the modulation of V bg .…”

Carrier density modulation and photocarrier transportation of graphene/InSb heterojunction middle-wavelength infrared photodetectors

“…17,18 Although various studies have revealed Schottky-barrier formation and carrier transport phenomenon in graphene and semiconductors including Si, [19][20][21][22][23][24][25][26][27] Ge, 28,29 GaAs, [30][31][32] CdSe, 33,34 SiC, 35,36 and GaN, 37,38 it is not clear how the extraordinary high responsivity is obtained in the heterojunction structure. We have proved the graphene/insulator layer region underwent photogating, 3,17,[39][40][41][42][43][44] which is one of the most effective responsivity enhancement candidates among possible techniques, such as pn junctions; 45 turbostacking of graphene; 40 plasmonic metamaterial absorbers; [46][47][48] the addition of photosensitizers including MoS 2 , 49,50 ZnO, 51,52 organic semiconductor, 53 and quantum dots; [54][55][56][57] and optical waveguides. 58 Photogating modulates the surface carrier density of graphene by locating a photosensitizer in the vicinity of the graphene.…”