New paper : Does N2 gas behave as a surfactant during Ag thin-film sputtering deposition? Insights from in vacuo and real-time measurements

02 February 2024
Does N2 gas behave as a surfactant during Ag thin-film sputtering deposition? Insights from in vacuo and real-time measurements

Ramiro Zapata a,b, Matteo Balestrieri a, Iryna Gozhyk a, Hervé Montigaud a, Rémi Lazzari b,∗
a Surface du Verre et Interfaces, UMR 125 CNRS/Saint-Gobain Recherche, 39 Quai Lucien Lefranc BP 135, F-93303 Aubervilliers, France
b CNRS, Sorbonne Université, Institut des NanoSciences de Paris, UMR 7588, 4 Place Jussieu, F-75005 Paris, France

ABSTRACT

Circumventing the thermodynamic trend of 3D growth of noble metals on dielectric substrates to form smooth,
continuous ultra-thin films remains a challenge for many applications involving transparent conductive layers
such as photovoltaic devices or smart windows. In this context, the use of gaseous species as ‘‘surfactants’’
during the sputtering deposition has attracted much attention in the past few years. The effect of N2 on the Ag
growth on amorphous SiO2 is revisited herein in light of film resistivity and UV-visible differential reflectivity
real-time measurements combined with in vacuo x-ray photoemission spectroscopy. Increasing the N2
content in the Ar gas flow (from 0 to 40%) gives rise to a drastic decrease of the percolation threshold thickness,
but at the expense of film conductivity. Photoemission suggests an apparent lack of chemistry between N and
Ag, with the absence of N in the bulk and at the surface of the Ag film despite a nitridation of the substrate
due to activated species in the plasma. The plasmonic response of Ag nanoparticles evidences the dynamic
presence of a N adsorbate that desorbs under vacuum once the growth is stopped. The latter decreases the metal
surface energy, thus inducing a flattening of Ag islands before percolation as proved by optics. The adsorption
is anisotropic between (111) and (100) facets. All of this explains (i) an earlier percolation resulting from
flattened objects and a delayed coalescence, (ii) the change of film texture as seen by ex situ diffraction and
(iii) the appearance of unfavourable grain boundaries for electrical transport in continuous films. Considering
its immiscibility in the bulk of Ag, its float-out behaviour, its impact on the surface energy and its tendency
to desorb at the end of growth, nitrogen has all the characteristics of a ‘‘surfactant’’ for Ag growth.

https://doi.org/10.1016/j.apsusc.2024.159546