Ville, laboratoire

1. 
   General information :
   

2. 
   Description of the 3 year doctoral research project
   

Our research team is involved for several years in the development by magnetron sputtering of thin films, whose potential applications concern the fields of electronics, energy (photovoltaics), optics, etc. Experimentally, a detailed understanding at the atomic scale of the different mechanisms involved during growth requires the development of in situ and real-time monitoring techniques (stress measurement by a multi-beam optical deflection method (MOSS), electrical resistivity). Studies carried out in the laboratory (Mo/Si thin films - Thesis Amelie Fillon 2010, Ta/Si and Pd/Si thesis Jonathan Colin 2015) have shown that a strong interplay between structural, chemical, elastic effects taking place at the surface or film bulk, and that very subtle changes at nano or even sub-nanometer scale can strongly influence the physical properties. That is why we are currently developing in the group a multiscale modeling activity based on a kinetic Monte Carlo algorithm (KMC) of thin fikm growth from atomic scale (DFT, molecular dynamics) up to the completion of the virtual material. In this context, the project of this PhD thesis is to implement a coupled modeling-experiment approach to study the influence of kinetic effects on growth mechanisms of metal films.

The key parameter for describing these kinetic effects is the intrinsic mobility of the deposited system, and this study will focus on low mobility metals (W), intermediate mobility (Pd) and high mobility (Ag, Au) taken as model systems. The in-situ monitoring of early growth stages by coupling MOSS measurement with electrical resistivity has allowed highlighting the percolation threshold and film continuity (see Fig. 1) or phase transition. These experimental results will be complemented by a detailed characterization of the microstructure and morphology of the films gained from ex situ investigations (X-ray reflectometry, X-ray Diffraction, AFM, HRTEM ...). The modeling of these phenomena, including KMC codes that are currently being developed, first requires the identification of diffusion processes at the atomic scale. Mo (see Fig.2) and Ag systems will be comparatively studied, using ab initio molecular dynamics to obtain stable atomic positions and associated energy.
Objectives

The aim of the PhD thesis is to build a comprehensive growth model, taking into account not only the kinetic effects described above but also the impact of energetic transfer specific to sputter-deposition process. The KMC simulation approach can be developed from the mechanisms described here at the atomic scale, and implementing the deposition conditions (energy and angular distribution of deposited species), based on quantitative experimental data that evidence the importance of point defects and grain boundaries. It will be interesting to extend these studies to more complex systems of binary or ternary compounds (FeAl, TiAlN) on single-crystal oxide substrates (MgO and / or Al₂O₃), for which the coupling epitaxy - ordering (stress) and surface segregation (kinetic effects) can play a crucial role.

E. Chason, Brown University, Etats-Unis