Contactless graphene conductivity mapping on a wide range of substrates with terahertz time-domain reflection spectroscopy

Abstract

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapour deposited graphene films on sapphire, silicon dioxide/silicon and germanium. We validate the technique against measurements performed with previously established conventional transmission based THz-TDS and are able to resolve conductivity changes in response to induced back-gate voltages. Compared to the transmission geometry, measurement in reflection mode requires careful alignment and complex analysis, but circumvents the need of a terahertz transparent substrate, potentially enabling fast, contactless, in-line characterisation of graphene films on non-insulating substrates such as germanium.

Graphene Conductivity Mapping using Terahertz Time-Domain Reflection Spectroscopy

“…For a fair comparison, spatial filtering was applied to the reflection conductivity map to emulate a spot size approximately 2.4 times greater in order to generate a spatially averaged conductivity map and histogram shown in figures 1f and h, respectively. This results show a much closer agreement against the transmission measurements [8].…”

“…In a manner similar to the literature [1,7] and as shown in Fig. 1, the graphene conductivity spectra on sapphire for our measurement does have a real part characterised by a flat spectral response near to its DC value well below the Drude roll-off frequency while the imaginary part is close to zero [8]. Figure 2a-c shows the Raman graphene D/G ratio map, frequency distribution, and 2D/G ratio map of the transferred graphene on sapphire.…”

Conductivity mapping of graphene on polymeric films by terahertz time-domain spectroscopy

“…Terahertz time-domain spectroscopy (THz-TDS) has proven useful as a fast non-contact method for extracting the conductivity, carrier scattering time, and mobility of graphene on highly resistive silicon (Si) substrates through transmission-mode measurements [2][3][4][5][6]. Recently, the conductivity of graphene on Si has also been extracted from reflection-mode THz-TDS, which avoids the requirement of the Si substrate to be highly resistive [7,8].…”

Review of Terahertz Pulsed Imaging for Pharmaceutical Film Coating Analysis

“…An example of a 3D coating thickness image is displayed in Figure 1c. The lateral resolution is limited by the wavelength of terahertz radiation, for example 150 and 400 µm at 1 and 2.7 THz, respectively [10,13], and the depth resolution is limited by the terahertz pulse duration of <200 fs. The achievable lateral resolution for pharmaceutical tablets is typically between 150-250 µm, at approximately 1.5 to 2.7 THz frequency range, and 30-40 µm for the depth resolution [6,7,10,14,15].…”