Nat. Commun. 4, 1811 (2013)

Credit: © 2013 NPG

Conventional field-effect-transistor-based graphene photodetectors suffer from poor responsitivity (<10 mA W−1) owing to low absorption in monolayer graphene and the short recombination lifetime of the photogenerated carriers. Now, by employing bandgap engineering in graphene, Yongzhe Zhang and colleagues from Singapore and China have developed a device with a much improved performance of 8.61 A W−1 — around three orders of magnitude higher than that of previous devices. The researchers report that their graphene photodetector, as well as being highly sensitive, also has a very broad photoresponse spanning from the visible (532 nm) to the mid-infrared (10 μm). The team introduced a bandgap and a defect mid-gap states band (MGB) by fabricating a structure that resembled an array of graphene quantum dots. Defect MGB electron-trapping centres then formed on the boundary and surface of the graphene quantum dots, thus creating a bandgap due to quantum confinement. The team attributed the large and broad photoresponse to electron trapping in the defect MGB, which was enhanced by carrier impact ionization. The proposed approach is compatible with CMOS technology. Although the operating speed of their device was low, the researchers say that this could be overcome by defect and nanostructure engineering to decrease the trapped electron lifetime in graphene.