Researchers at Zhejiang University×ª University of California, The Chinese University of Hong Kong, Peking University, Aalto University, University of Cambridge and Nanjing University have developed a new graphene-based photodetector that could detect light within a broader bandwidth. Their device could be used to develop new and more advanced imaging technologies.
The teams work is based on traditional charge-coupled device (CCD) and complementary metal-oxide-semiconductor (CMOS) imaging technologies. The researchers stated that their imaging devices, combining CCD's MOS photogate for high sensitivity and CMOS's independent pixel structure, can significantly benefit monolithic integration, performance, and readout.
Typically, CCD and CMOS imagers based on silicon (Si) can only detect light in the visible range, due to their intrinsic bandgap absorption limit. The team was able to broaden the bandwidth of the light that can be picked up by their photodetector by incorporating graphene into it.
"Our goal was to improve the responsivity and spectral performance of Si-based image sensors," Prof. Yang Xu from Zhejiang University explained. "First, we formed a Schottky junction between the Si and the multilayer graphene (MLG), where the IR-light induced hot holes in MLG are injected into Si (n-type) under the electrical field of Vg via photo-thermionic (PTI) emission. Second, we formed a deep-depletion well in the SiO2/Si interface under the voltage pulse, storing and integrating photo-charges."
Ultimately, the researchers placed single-layer graphene (SLG) on top of the oxide in their device. This layer ultimately allows the photodetector to directly and non-destructively read out the carriers stored in the deep depletion well through the photogate effect, which is caused by the graphene's strong field.
"The design of our proposed device realizes in-situ pixel-level readout of the charges in the profound depletion potential well, avoiding the sequential charge transfer in traditional CCD devices," Xu said. "In addition, we broadened the detection bandwidth to IR by integrating a MLG charge injection path at the bottom of the device."
In initial tests, Xu and his colleagues found that their photodetector's detection bandwidth is significantly broader than that of traditional Si-based devices, which typically only detect light in the visible wavelength. As it integrates a readout channel SLG, a Si-based deep depletion well and a charge injection layer (MLG), the device could be particularly valuable for imaging applications with high-density integration.
"Our paper solves two crucial problems, by replacing sequential charge transfer with direct readout, which simplifies the architecture and data processing, while also broadening the response range to IR by infrared charge injection," Xu said.