Effect of thickness and morphology on terahertz and optical properties of ultrathin gold films

Abstract

Ultrathin noble-metal films combine electrical conductivity with optical transparency and are promising for transparent electrodes and broadband electromagnetic coatings. We investigate how thickness and morphology affect the optical and electrical response of gold films with nominal thicknesses of 2–8 nm. Two sample series were compared: continuous films fabricated by a graphene-inspired transfer process using a copper wetting layer, and island films formed by direct deposition in the Volmer–Weber growth regime. Morphology was characterized by scanning electron and atomic force microscopy. Optical transmittance was measured from 0.4 to 2.5 μm, complex sheet conductance was obtained by terahertz time-domain spectroscopy at 0.5–2.5 THz, and sheet resistance was measured by a four-point probe.

The transferred films were continuous and ultrasmooth, with an RMS roughness of about 0.4 nm. Their transmittance at 550 nm was 70–82%, while sheet resistance decreased from 450.1 to 7.6 Ω/sq as thickness increased from 2 to 8 nm. The real part of the terahertz sheet conductance was weakly frequency dependent and generally increased with thickness; contactless resistance estimates agreed with electrical measurements. Island films 2–6 nm thick remained highly transparent in the infrared and terahertz ranges and showed no measurable dc conduction. At 7–8 nm, their long-wavelength transmittance decreased and sheet resistance reached 110.8 and 60.6 Ω/sq, indicating the formation of conducting pathways. The results show that morphology controls the transition from dielectric-like to metallic response and provide guidelines for selecting ultrathin gold films for transparent electrodes and terahertz/infrared devices.

Speaker

Aleksandr Marakulin
Moscow Institute of Physics and Technology
Russia

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