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

Patrick R Whelan,Da Luo,Qian Shen,Peter U Jepsen,Meihui Wang,Binbin Zhou,Peter Bøggild,Rodney S Ruoff

doi:10.1364/oe.402447

Abstract

We present a reference-free method to determine electrical parameters of thin conducting films by steady state transmission-mode terahertz time-domain spectroscopy (THz-TDS). We demonstrate that the frequency-dependent AC conductivity of graphene can be acquired by comparing the directly transmitted THz pulse with a transient internal reflection within the substrate which avoids the need for a standard reference scan. The DC sheet conductivity, scattering time, carrier density, mobility, and Fermi velocity of graphene are retrieved subsequently by fitting the AC conductivity with the Drude model. This reference-free method was investigated with two complementary THz setups: one commercial fibre-coupled THz spectrometer with fast scanning rate (0.2-1.5 THz) and one air-plasma based ultra-broadband THz spectrometer for greatly extended frequency range (2-10 THz). Certain propagation correction terms for more accurate retrieval of electrical parameters are discussed.

Highlights

Terahertz time-domain spectroscopy (THz-TDS) is maturing as a large-area, fast, accurate and non-invasive measurement technique for extracting the electrical properties of graphene [1,2,3,4,5]
We present a reference-free method to determine electrical parameters of thin conducting films by steady state transmission-mode terahertz time-domain spectroscopy (THzTDS)
We demonstrate that the frequency-dependent AC conductivity of graphene can be acquired by comparing the directly transmitted THz pulse with a transient internal reflection within the substrate which avoids the need for a standard reference scan

Summary

Introduction

Terahertz time-domain spectroscopy (THz-TDS) is maturing as a large-area, fast, accurate and non-invasive measurement technique for extracting the electrical properties of graphene [1,2,3,4,5]. A technique for measuring the conductivity which do not require the standard reference waveform scan is very desirable This has previously been achieved for reflection-mode THz-TDS on THz transparent substrates, where the THz-TDS waveform contains transients from a directly reflected pulse from the front surface and from reflection at the back surface of the substrate. In transmission-mode THz-TDS, the recorded waveform consists of a directly transmitted pulse followed by transients from internal reflections (echoes) inside the substrate These echoes have previously been utilized to improve the retrieval of the conductivity of graphene by comparing the same order of echo signal from reference and sample measurements [8,13,14]. This type of reference-free methodology has been applied to determine the refractive index and wave impedance of dielectrics [15]

Methods

Results

Conclusion