Abstract
We demonstrate wafer-scale, non-contact mapping of essential carrier transport parameters, carrier mobility (µdrift), carrier density (Ns), DC sheet conductance (σdc), and carrier scattering time (τsc) in CVD graphene, using spatially resolved terahertz time-domain conductance spectroscopy. σdc and τsc are directly extracted from Drude model fits to terahertz conductance spectra obtained in each pixel of 10 × 10 cm2 maps with a 400 µm step size. σdc- and τsc-maps are translated into µdrift and Ns maps through Boltzmann transport theory for graphene charge carriers and these parameters are directly compared to van der Pauw device measurements on the same wafer. The technique is compatible with all substrate materials that exhibit a reasonably low absorption coefficient for terahertz radiation. This includes many materials used for transferring CVD graphene in production facilities as well as in envisioned products, such as polymer films, glass substrates, cloth, or paper substrates.
Highlights
Graphene synthesis technology is rapidly approaching a commercially viable level, where continuous graphene films of high structural integrity are routinely produced on wafer-scale [1,2,3,4,5,6] or even m2 scale [7,8], with projected throughput of order 105 m2/year for chemical vapor deposition (CVD) production tools [9]
We employ a THz time-domain spectrometer based on photoconductive antennas [13], to map the electromagnetic response from 0.25 to 1.2 THz of charge carriers in an approximately 95 mm wide CVD graphene film residing on a 4-inch wafer substrate
As described elsewhere [13,17], terahertz time-domain conductance spectroscopy (THz-TDS) is sensitive to free carrier absorption, and the fast Fourier transforms (FFT) of THz time-domain transients, E ref (ν ) = fft[Eref (t)] and Egraphene (ν ) = fft[Egraphene (t)], transmitted through silicon substrate and silicon substrate covered with graphene, respectively, can be related directly to the complex sheet conductance, σ s (ν ), of the graphene film at THz frequencies as: T
Summary
Graphene synthesis technology is rapidly approaching a commercially viable level, where continuous graphene films of high structural integrity are routinely produced on wafer-scale [1,2,3,4,5,6] or even m2 scale [7,8], with projected throughput of order 105 m2/year for CVD production tools [9]. While potentially very useful in the development and optimization phase of 2D material synthesis and transfer, the approach requires deposition of the graphene film on a gate-stack substrate, and is not directly applicable to the many applications and production scenarios involving graphene films on insulating and flexible substrates. These include some of the most lucrative technologies such as graphene photovoltaics, displays, and flexible electronics, where in-line characterization and quality-control require the THz-TDS analysis to be carried out on the native substrate. In addition to providing a means of non-destructive and rapid measurement of graphene drift mobility and carrier density in scenarios without a gate or external magnetic field, the method is significantly faster than previously demonstrated methods [16], and provides additional information on transport dynamics such as carrier scattering rate and mean free path
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