Enhanced terahertz conductivity in ultra-thin gold film deposited onto (3-mercaptopropyl) trimethoxysilane (MPTMS)-coated Si substrates

Youjin Lee,Volodymyr Shmid,Dasom Kim,Jugyoung Kim,Dai-Sik Kim,Parinda Vasa,Young-Mi Bahk,Jeeyoon Jeong,Oleg Korotchenkov

doi:10.1038/s41598-019-51085-0

Abstract

Various material properties change considerably when material is thinned down to nanometer thicknesses. Accordingly, researchers have been trying to obtain homogeneous thin films with nanometer thickness but depositing homogeneous few nanometers thick gold film is challenging as it tends to form islands rather than homogenous film. Recently, studies have revealed that treating the substrate with an organic buffer, (3-mercaptopropyl) trimethoxysilane (MPTMS) enables deposition of ultra-thin gold film having thickness as low as 5 nm. Different aspects of MPTMS treatment for ultra-thin gold films like its effect on the structure and optical properties at visible wavelengths have been investigated. However, the effect of the MPTMS treatment on electrical conductivity of ultra-thin gold film at terahertz frequency remains unexplored. Here, we measure the complex conductivity of nanometer-thick gold films deposited onto an MPTMS-coated silicon substrate using terahertz time-domain spectroscopy. Following the MPTMS treatment of the substrate, the conductivity of the films was found to increase compared to those deposited onto uncoated substrate for gold films having the thickness less than 11 nm. We observed 5-fold enhancement in the conductivity for a 7 nm-thick gold film. We also demonstrate the fabrication of nanoslot-antenna arrays in 8.2-nm-thick gold films. The nanoslot-antenna with MPTMS coating has resonance at around 0.5 THz with an electric field enhancement of 44, whereas the nanoslot-antenna without MPTMS coating does not show resonant properties. Our results demonstrate that gold films deposited onto MPTMS-coated silicon substrates are promising advanced materials for fabricating ultra-thin terahertz plasmonic devices.

Highlights

Though a considerable volume of literature has been published about material properties of metallic films[1,2], properties of ultra-thin metal films having thickness of few nanometers continue to attract attention
As the real and imaginary parts of electrical conductivity can be effectively extracted by performing time-domain spectroscopy, studying optical properties helps in understanding carrier dynamics in these thin films
To compare complex conductivity of gold films deposited onto silicon substrate with and without MPTMS treatment, we perform terahertz time-domain spectroscopy from 0.4 to 2.0 THz

Summary

Introduction

Though a considerable volume of literature has been published about material properties of metallic films[1,2], properties of ultra-thin metal films having thickness of few nanometers continue to attract attention. We investigate the enhanced terahertz conductivity of ultra-thin gold films deposited onto MPTMS-coated silicon substrates. To compare complex conductivity of gold films deposited onto silicon substrate with and without MPTMS treatment, we perform terahertz time-domain spectroscopy from 0.4 to 2.0 THz. Subsequent DC conductivity measurement is performed to confirm our results. Our results reveal that 5.8-nm-thick gold film having metallic character can be deposited using evaporation onto MPTMS-coated substrates, whereas those deposited onto bare substrates exhibit dielectric character.

Results

Conclusion