A team from the IBM Nanoscale Science and Technology group has revealed some of the fundamental mechanisms of photoconductivity in graphene. In particular, the researchers demonstrated that the photoconductivity of graphene can be either positive or negative depending on its gate bias.
The research team includes Marcus Freitag, Tony Low, Fengnian Xia and Phaedon Avouris (who we interviewed on this blog for his work in creating a band gap in graphene). The group works out of IBM's T. J. Watson Research Lab, in Yorktown Heights, N.Y. and published their work (“Photoconductivity of graphene”) in the online version of Nature Photonics.
The impetus for their “classic photoconductivity experiment” was the appealing optoelectronic properties of graphene. It at once possesses high carrier mobility, zero bandgap, and electron-hole symmetry.
Graphene has proven itself capable of absorbing light and converting it into a photocurrent ranging from the ultraviolet to the visible and infrared spectra. Until now the photoresponse of graphene was attributed to either thermoelectric or photovoltaic effects. The IBM research team discovered that in this biased but otherwise homogenous graphene the thermoelectric effects of the photo response were insignificant. Instead both the photovoltaic and photo-induced bolometric effect dominate the photo response.
In the photovoltaic effect one expects the photocurrent to flow in the same direction as the dark current, which is a small current that runs through a device in the absence of light. But what the researchers discovered that under certain conditions the photocurrent would flow in the opposite direction to the dark current.
The photo-generated carriers, while propagating across graphene, emit quanta of lattice vibrations called phonons and thereby transfer their energy into the lattice. Heating up the lattice implies enhancing the electron-phonon scattering process and reducing the carrier’s mobility. This is the bolometric effect that the researchers discovered was a dominant effect in the photo response and is what leads to the photocurrent flowing in the opposite direction of the photocurrent.
In the experiment the researchers found that the efficiency of the graphene photodetector depended on keeping the electronics and phononic temperatures hot. The research team is currently exploring different engineering approaches to raise the temperatures beyond the 10 Kelvin and 1 Kelvin reached in the experiment for the electronic and phononic temperatures, respectively.
With the insights gained in this research, the IBM team believes that graphene may find uses as high-speed and broadband photodetectors driven by hot electrons or phonons.
Image: IBM Thomas J. Watson Research Center