Graphene has interesting optoelectronic properties due to its zero bandgap, electron–hole symmetry, and high carrier mobility. For example, graphene can convert absorbed light into photocurrent. The origin of this photoresponse in graphene junctions has been attributed to either thermoelectric or photovoltaic (PV) effects. However, identifying which photocurrent mechanism is dominant is challenging because the polarities of PV and thermoelectric currents measured in metal–graphene or graphene p–n junctions are the same.
Recently, M. Freitag, T. Low, F. Xia, and P. Avouris at IBM’s T. J. Watson Research Center have investigated the mechanism of photocurrent response in biased, homogeneous graphene. By meas-uring the photoconductivity of the homogeneous graphene channel in a graphene field-effect transistor (FET) fabricated on Si/SiO2, wherein photocurrent polarities due to PV and thermoelectric effects are opposite, the researchers determined that the dominant photocurrent generation mechanism at low electrostatic doping is photovoltaic, and that the thermoelectric effect is an order of magnitude smaller.
The researchers report in an article published online on December 16, 2012 in Nature Photonics (DOI: 10.1038/NPHOTON.2012.314) that they biased their graphene FET at one contact with a moderate drain voltage of about –1 V. Doping was controlled electrostatically with a global silicon backgate, and the alternating photocurrent amplitude and phase were measured while a chopped, focused laser beam at a wavelength of 690 nm was scanned over the sample. The researchers observed that the photocurrent displays polarity reversal during a backgate voltage sweep, an effect which was attributed to alternation between two dominant mechanisms—photovoltaic and photoinduced bolometric effects.
In the photovoltaic effect, photoexcited electrons and holes are accelerated in opposite directions by an electric field, and the carriers produce a photocurrent either by reaching the contacts while still hot or by establishing a local photovoltage that drives the photocurrent through the device. In the bolometric effect, the incident electromagnetic radiation raises the local temperature of the graphene, which alters the resistance of the device, producing a change in direct current under bias. Modulation of the photocurrent polarity and magnitude by electrostatic doping allowed the researchers to probe the nonequilibrium characteristics of graphene’s hot carriers as well as phonons, which are involved in the dominant energy-loss pathway.
“Our work opens up the possibility of engineering the hot carrier photoresponse, which plays an essential role in applications such as bolometers, calorimeters and photodetectors,” said the researchers.
