Role of Contacts in graphene transistors: A scanning Photocurrent study

T. Mueller,F. Xia, M. Freitag, J. Tsang, and Ph. Avouris

IBM Thomas J. Watson Reserch Center, Yorktown Heights, New York 10598, USA


PHYSICAL REVIEW B 79, 245430 (2009)

Graphene is a 2D material with extraordinary electrical and optical properties, high electron and hole mobility, high thermal conductivity, temperature stability and tensile strength. All of these properties make graphene a very attractive material for nanoelectronics.

In most experiments and simulations for evaluating the transport properties of graphene transistors, the graphene channels are treated as being homogeneous. However, recent experimental work provided evidence that the metal contacts induce strong charge inhomogeneity and should be taken in the account especially near the Dirac point. Most recent experiments that assessed the transport properties of graphene were done in the far-field.  Scanning probe techniques that can measure on the nanoscale are required for an improved understanding of the role of metal contacts in graphene nanodevices.

Scanning the photocurrent (PC) is an effective tool to study the potential profiles as a function of the photoresponse on the nanoscale. In 

this paper, high resolution photoconductivity was studied with Near-field Scanning Probe Microscopy (NSOM). A Nanonics Multiview 2000 was used for the measurements, which provided three key advantages for photoconductivity measurements:

1)  tuning fork based actuation so there is no laser for feedback that can cause artifacts during PC imaging

2) scanning head with both tip and sample scanning.  With this hardware setup, the probe can be moved to the specific location on the sample with high accuracy and then scanned there locally, without moving the sample which is electrically connected.

3) completely open optical axis from the top and from the bottom. This open optical axis and the cantilevered NSOM probe with extended tip enables locating the NSOM probe relative to the sample features from on the opaque sample, such as graphene transistor on Si substrate with high accuracy.

A modulated Ar-ion laser (514 nm) was coupled into cantilevered NSOM probe (aperture diameter of 100 nm) to excite the photoconductivity on the graphene device.  An NSOM probe scanned across the graphene transistor, while the photocurrent was recorded simultaneously with topography. The pho

1) The presence of a strong PC voltage near the metal contact electrodes, at the point on the local electrical field near the graphene/metal interfacetocurrent was measured versus gate voltage under short-circuit conditions where the gate voltage was sweeping between -60 and 100V.   The PC results showed:

  1. 2) PC polarity switching at gate bias 20 V. This behavior of the PC can be described with the theoretical model that treats bending of the graphene bands as the result of the charge transfer between the graphene sheet and the metal electrodes.
  2. 3) Same band bending occurs when the single graphene layer contacts with multilayer graphene.


Thus near field photocurrent microscopy is a powerful technique to study the electrical properties of graphene nanodevices with nanoscale resolution.  This method showed modification of the electronic structure of the graphene device near the metal contacts and inhomogenity of the charge transfer. The obtained results provide further understanding of the asymmetric behavior of the electrons and holes in the graphene transistors. Photocurrent imaging can be also used to probe single-layer/multilayer graphene interfaces.


Published:  PHYSICAL REVIEW B 79, 245430 (2009)