Reference-free THz-TDS conductivity analysis of thin conducting films

Abstract

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Case studies of electrical characterisation of graphene by terahertz time-domain spectroscopy

“…Recently, it has been shown that it is also possible to perform reference-free extraction of dielectric and electrical properties of substrates and thin conducting films from THz-TDS measurements [28,29]. This is achieved by comparing individual transients from only the sample waveform, which means that the reference measurement can be omitted and furthermore decreases errors from signal fluctuations originating from the measurement setup such as timing jitter [26], periodic sampling errors [30], and uneven stage movement.…”

“…It is possible to determine d sub from the distance between peaks in frequency domain from the Fourier transform of the full THz waveform including all transients. In practice, the correction terms for the amplitude and Gouy phase shift can be determined from one single calibration-like measurement [29]. This is advantageous compared to the standard referenced analysis in equations ( 1)-( 3), where reference measurements must be acquired frequently to minimize effects from environment, mechanical stage movement and THz source fluctuations, all of which have no influence on the reference-free analysis.…”

“…This is advantageous compared to the standard referenced analysis in equations ( 1)-( 3), where reference measurements must be acquired frequently to minimize effects from environment, mechanical stage movement and THz source fluctuations, all of which have no influence on the reference-free analysis. Also, at lower frequencies, where commercial spectrometers are operating, it is possible to extract meaningful electrical parameters for graphene on non-absorbing substrates without having to correct for amplitude and Gouy phase shift [29].…”

“…Complementary measurements have been performed in many cases in order to validate the extracted electrical properties of graphene measured by THz-TDS [14]. The measurements conducted for comparison with THz-TDS varies across a range of characterization methods including optical microscopy [29,45], Hall measurements [6,9,12,46], Raman spectroscopy [5,8,41], Kelvin probe force microscopy [8,47], and micro four-point probe (M4PP) measurements [5,8,33,48]. M4PP is a unique metrology technology developed by DTU and Capres A/S where the conductance is measured across a conducting wafer or thin film, using a 12-point microfabricated probe, with a probe pitch down to 1 µm [5,14,33,[49][50][51][52].…”

“…For THz-TDS measurements of samples where the individual transients from the directly transmitted pulse and the internal reflections are wellseparated in time (see figure 1(d)) it is possible to extractσ s (ω) without performing the standard reference scan [29]. This is desirable for substrates that are fully covered by graphene where the reference measurement has to be performed on a different sample and in production scenarios where a THz-TDS system will be running for extended periods with possible fluctuations from THz source, environment, and mechanical movement being very likely.…”