凯发

The gap between the rapidly upscaling of large-area graphene production compared to available electrical characterisation methods could become a major roadblock for emerging graphene applications. As an alternative to the often slow, cumbersome and destructive characterisation techniques based on electrical field effect or Hall measurements, THz time-domain spectroscopy (THz-TDS) [1] not only maps the conductance quickly and non-destructively, but also accurately. This is confirmed by direct comparison with micro four-point probe (M4PP) measurements, another low-invasive, well-established method already used by major semiconductor manufacturers for inline quality control. In addition to spatial maps of the sheet conductance, THz-TDS and M4PP offer unique information on otherwise hidden defects and inhomogeneities, as well as detailed scattering dynamics in the graphene film on nm to mm length scales [2-4]. We also show that THz-TDS can be used to map the carrier density and mobility, either by transferring graphene to a substrate equipped with a THz-transparent back gate [5], or by analyzing the frequency response in detail to extract the scattering time at a constant carrier density [6], which allows the mobility to be mapped even on insulating substrates. In contrast with conventional field effect and Hall measurements, THz-TDS measures the actual, intrinsic carrier mobility, i.e. not derived from a conductance (extrinsic) measurement. As field effect measurements are expected to remain useful for benchmarking, we have developed a fast (1 hour turn-around time) and clean (no solvents or water) method for converting a graphene wafer into 49 FET devices with electrical contacts, using a combination of a physical shadow mask and laser ablation [6,7]. Finally, the challenges of realizing in-line monitoring of the electrical properties in a graphene production scenario, and the prospects for establishing THz-TDS mapping as a measurement standard for large-area graphene films will be discussed.