Article

Graphene Conductance Uniformity Mapping

Department of Photonics Engineering and ‡Department of Micro- and Nanotechnology, Technical University of Denmark , DK-2800 Kongens Lyngby, Denmark.
Nano Letters (Impact Factor: 12.94). 09/2012; 12(10):5074-81. DOI: 10.1021/nl301551a
Source: PubMed

ABSTRACT We demonstrate a combination of micro four-point probe (M4PP) and non-contact terahertz time-domain spectroscopy (THz-TDS) measurements for centimeter scale quantitative mapping of the sheet conductance of large area chemical vapor deposited graphene films. Dual configuration M4PP measurements, demonstrated on graphene for the first time, provide valuable statistical insight into the influence of microscale defects on the conductance, while THz-TDS has potential as a fast, non-contact metrology method for mapping of the spatially averaged nanoscopic conductance on wafer-scale graphene with scan times of less than a minute for a 4-in. wafer. The combination of M4PP and THz-TDS conductance measurements, supported by micro Raman spectroscopy and optical imaging, reveals that the film is electrically continuous on the nanoscopic scale with microscopic defects likely originating from the transfer process, dominating the microscale conductance of the investigated graphene film.

1 Follower
 · 
112 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: The presence of defects in graphene have for a long time been recognized as a bottleneck for its utilization in electronic and mechanical devices. We recently showed that micro four-point probes may be used to evaluate if a graphene film is truly 2D or if defects in proximity of the probe will lead to a non-uniform current flow characteristic of lower dimensionality. In this work, simulations based on a finite element method together with a Monte Carlo approach are used to establish the transition from 2D to quasi-1D current transport, when applying a micro four-point probe to measure on 2D conductors with an increasing amount of line-shaped defects. Clear 2D and 1D signatures are observed at low and high defect densities, respectively, and current density plots reveal the presence of current channels or branches in defect configurations yielding 1D current transport. A strong correlation is found between the density filling factor and the simulation yield, the fraction of cases with 1D transport and the mean sheet conductance. The upper transition limit is shown to agree with the percolation threshold for sticks. Finally, the conductance of a square sample evaluated with macroscopic edge contacts is compared to the micro four-point probe conductance measurements and we find that the micro four-point probe tends to measure a slightly higher conductance in samples containing defects. (C) 2014 AIP Publishing LLC.
    Applied Physics Letters 08/2014; 105(5):053115. DOI:10.1063/1.4892652 · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We calculate the room-temperature complex conductivity sigma(omega) of suspended and supported graphene at terahertz frequencies (100 GHz-10 THz) by employing a self-consistent coupled simulation of carrier transport and electrodynamics. We consider a wide range of electron (n = 10(12)-10(13) cm(-2)) and impurity (N-i = 8 x 10(10)-2 x 10(12) cm(-2)) densities. For graphene supported on SiO2, there is excellent agreement between the calculation with clustered impurities and the experimentally measured sigma(omega). The choice of substrate (SiO2 or h-BN) is important at frequencies below 4 THz. We show that carrier scattering with substrate phonons governs transport in supported graphene for N-i/n < 0.1. Electron-impurity interactions dominate for N-i/n > 0.1, and transport enters the electron-hole puddle regime for N-i/n > 0.5. The simple Drude model, with an effective scattering rate Gamma and Drude weight D as parameters, fits the calculated sigma(omega) for supported graphene very well, owing to electron-impurity scattering. Gamma decreases with increasing n faster than n-1/2 and is insensitive to electron-electron interaction. Both electron-electron and electron-impurity interactions reduce the Drude weight D, and its dependence on n is sublinear.
    Physical Review B 07/2014; 90(4). DOI:10.1103/PhysRevB.90.045431 · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: ifficulty in deposition and integration of fragile graphene-like samples for optoelectronic devices may prevent a multiple contact measurement procedure. We employed noncontact and nondestructive transmission and reflection terahertz (THz)-pulsed spectroscopy to investigate not only the electrical conductivity, but also to study the optical properties of one-dimensional and two-dimensional graphene-like samples. The Drude and non-Drude models were applied to observe and compare the ultrafast carrier transport parameters and high mobility characteristic of such high conductance-nanostructured thin films without requirement for postprocess patterning. The diffusive coefficient and nanoscopic characteristic length from noncontact THz measurement enables us to predict the cut-off frequency of such devices in relevant optoelectronic applications in sub-THz and THz frequencies. The results show that the cut-off frequency of the devices increases with a reduction of the channel length.
    Journal of Nanophotonics 01/2015; 9(1):093598. DOI:10.1117/1.JNP.9.093598 · 1.45 Impact Factor