  Molecular dynamics simulations are often used to study sputtering and thin
film growth. Compressive stresses in these thin films are generally assumed to
be caused by a combination of forward sputtered (peened) built-in particles and
entrapped working gas atoms. While the former are assumed to hold a predominant
role, the effect of the latter on the interaction dynamics as well as thin film
properties are scarcely clarified (concurrent or causative). The inherent
overlay of the ion bombardment induced processes render an isolation of their
contribution impracticable. In this work, this issue is addressed by comparing
the results of two case studies on the sputter deposition of Al thin films in
Ar working gas. In the first run Ar atoms are fully retained. In the second run
they are artificially neglected, as implanted Ar atoms are assumed to outgas
anyhow and not alter the ongoing dynamics significantly. Both case studies have
in common that the consecutive impingement of 100 particles (i.e., Ar$^+$ ions,
Al atoms) onto Al(001) surfaces for ion energies in the range of 3 eV to 300 eV
as well as Al/Ar$^+$ flux ratios from 0 to 1 are considered. The surface
interactions are simulated by means of hybrid reactive molecular
dynamics/force-biased Monte Carlo simulations and characterized in terms of
mass density, Ar concentration, biaxial stress, shear stress, ring statistical
connectivity profile, Ar gas porosity, Al vacancy density, and
root-mean-squared roughness. Ultimately, implanted Ar atoms are found to form
subnanometer sized eventually outgassing clusters for ion energies exceeding
100 eV. They fundamentally govern a variety of surface processes (e.g., forward
sputtering/peening) and surface properties (e.g., compressive stresses) in the
considered operating regime.
