Consequently, graphene is anticipated for use in optical sensors 2 with an ultra-wide wavelength region extending from ultraviolet to terahertz with high-speed photoresponse 3, 4. Monolayer graphene has been predicted theoretically to have uniform light absorption at ultra-wideband wavelengths 1. The charge accumulation induced by incident photons to the device is a crucially important aspect of imaging devices with sufficiently high sensitivity for single-photon detection. Image-sensing devices such as charge-coupled devices (CCDs) have become indispensable not only for daily life but also for industrial applications. These findings are expected to open avenues to realization of graphene photodetectors with high sensitivity toward single photon detection with optical memory function. Additionally, this light-induced relaxation imparts an optical memory function with retention time of ~5 s. Herein, we demonstrate a highly photosensitive graphene field-effect transistor with noise-equivalent power of ~3 × 10 −15 W/Hz 1/2 and with response time within milliseconds at room temperature, where the Au oxide on Au electrodes modulates the contact resistance because of the light-assisted relaxation of the trapped charge at the contact. In contrast, a charge accumulation at the contact would induce the efficient light-induced modification of the contact resistance, which would enhance its photosensitivity. However, the charge accumulation process slows the response to around a few tens of seconds to minutes.
To enhance the photosensitivity, hybridization of photosensitive material and graphene has been widely studied, where the accumulated photo-excited charge adjacent to the graphene channel modifies the Fermi level of graphene. Nevertheless, only the 2.3% light absorption of monolayer graphene and fast recombination time of photo-excited charge restrict its sensitivity. Graphene is a promising material for use in photodetectors for the ultrawide wavelength region: from ultraviolet to terahertz.
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