Role of contacts in graphene transistors: A scanning photocurrent study

A near-field scanning optical microscope is used to locally induce photocurrent in a graphene transistor with high spatial resolution. By analyzing the spatially resolved photoresponse, we find that in the $n$-type conduction regime a $p\text{−}n\text{−}p$ structure forms along the graphene device due to the doping of the graphene by the metal contacts. The modification of the electronic structure is not limited only underneath the metal electrodes but extends $0.2–0.3\mu \text{m}$ into the graphene channel. The asymmetric conduction behavior of electrons and holes that is commonly observed in graphene transistors is discussed in light of the potential profiles obtained from this photocurrent-imaging approach. Furthermore, we show that photocurrent imaging can be used to probe single-layer/multilayer graphene interfaces.

Figure 1

(Color online) Schematic illustration of the experimental setup and sample structure.

Figure 2

(Color) The left picture shows the SEM image of a graphene transistor and the electrical setup for PC measurements. On the right we show seven PC images taken at gate biases between $-60$ and $+100\text{V}$. The dashed lines indicate the edges of the source and drain electrodes. The two scale bars on the bottom of the very right image are both $1\mu \text{m}$ long.

Figure 3

(Color online) PC profiles at (a) $V_{\text{G}}=-60\text{V}$ and (b) $V_{\text{G}}=100\text{V}$ along the arrows in Fig. 2. The dashed lines indicate the edges of the source and drain electrodes.

Figure 4

(Color online) (a) Band diagrams at $V_{\text{G}}=0\text{V}$ (dashed line) and $V_{\text{G}}=-60\text{V}$ (solid line) obtained by numerical integration of the PC profiles in Fig. 3. $\Delta \varphi$ describes the pinning of the Fermi level. Arrows indicate the flow of electrons and holes. (b) Band diagram at $V_{\text{G}}=100\text{V}$ obtained by numerical integration of the PC profile in Fig. 3. It shows the formation of a $p\text{−}n\text{−}p$ structure. The distance $d$ between the PC peaks is smaller than the device length $L$. There is no PC contribution from the contacts because the electric field at the electrode/graphene interface is nearly zero.

Figure 5

(Color) (a) SEM image of the device. Region 2 consists of single-layer graphene and regions 1 and 3 are multilayer. (b) PC image recorded in the $p$-type conduction regime. The black dashed lines indicate the edges of the source and drain electrodes. The white dotted lines mark the interfaces between SLG and MLG. (c) PC profile along the channel of the device. The red line indicates the PC that is generated at the graphene interfaces.