Near-ultraviolet-sensitive graphene/porous silicon photodetectors.

Porous silicon (PSi) is recognized as an attractive building block for photonic devices because of its novel properties including high ratio of surface to volume and high light absorption. We first report near-ultraviolet (UV)-sensitive graphene/PSi photodetectors (PDs) fabricated by utilizing graphene and PSi as a carrier collector and a photoexcitation layer, respectively. Thanks to high light absorption and enlarged energy-band gap of PSi, the responsivity (Ri) and quantum efficiency (QE) of the PDs are markedly enhanced in the near-UV range. The performances of PDs are systemically studied for various porosities of PSi, controlled by varying the electroless-deposition time (td) of Ag nanoparticles for the use of Si etching. Largest gain is obtained at td = 3 s, consistent with the maximal enhancement of Ri and QE in the near UV range, which originates from the well-defined interface at the graphene/PSi junction, as proved by atomic- and electrostatic-force microscopies. Optimized response speed is ∼10 times faster compared to graphene/single-crystalline Si PDs. These and other unique PD characteristics prove to be governed by typical Schottky diode-like transport of charge carriers at the graphene/PSi junctions, based on bias-dependent variations of the band profiles, resulting in novel dark- and photocurrent behaviors.

High photoresponsivity in an all-graphene p-n vertical junction photodetector.

Intensive studies have recently been performed on graphene-based photodetectors, but most of them are based on field effect transistor structures containing mechanically exfoliated graphene, not suitable for practical large-scale device applications. Here we report high-efficient photodetector behaviours of chemical vapor deposition grown all-graphene p-n vertical-type tunnelling diodes. The observed photodetector characteristics well follow what are expected from its band structure and the tunnelling of current through the interlayer between the metallic p- and n-graphene layers. High detectivity (~10(12) cm Hz(1/2) W(-1)) and responsivity (0.4~1.0 A W(-1)) are achieved in the broad spectral range from ultraviolet to near-infrared and the photoresponse is almost consistent under 6-month operations. The high photodetector performance of the graphene p-n vertical diodes can be understood by the high photocurrent gain and the carrier multiplication arising from impact ionization in graphene.

Graphene p-n vertical tunneling diodes.

Formation and characterization of graphene p-n junctions are of particular interest because the p-n junctions are used in a wide variety of electronic/photonic systems as building blocks. Graphene p-n junctions have been previously formed by using several techniques, but most of the studies are based on lateral-type p-n junctions, showing no rectification behaviors. Here, we report a new type of graphene p-n junction. We first fabricate and characterize vertical-type graphene p-n junctions with two terminals. One of the most important characteristics of the vertical junctions is the asymmetric rectifying behavior showing an on/off ratio of ~10(3) under bias voltages below ±10 V without gating at higher n doping concentrations, which may be useful for practical device applications. In contrast, at lower n doping concentrations, the p-n junctions are ohmic, consistent with the Klein-tunneling effect. The observed rectification results possibly from the formation of strongly corrugated insulating or semiconducting interlayers between the metallic p- and n-graphene sheets at higher n doping concentrations, which is actually a structure like a metal-insulator-metal or metal-semiconductor-metal tunneling diode. The properties of the diodes are almost invariant even 6 months after fabrication.