Shadow masked pulsed laser deposition of WSex films: Experiment and modeling

Pulsed laser deposition of WSex films was carried out in a buffer gas (Ar). The mask was placed between a WSe2 target and a substrate in order to prevent the deposition of micro- and nanoparticles. At Ar pressure of 2 Pa the laser plume was effectively scattered by the gas into a shadow area, and the ion bombardment of the film was the most intense. The radial distribution of the W and Se atoms on the substrate was measured, and a significant enrichment of the film by selenium at the center of the shadow area was found. The experimental results were compared with the results of Monte Carlo simulations. A model described the inertial motion of laser-initiated atomic flux bypassing the mask due to collisions of atoms with gas molecules. Model distribution of deposited atoms agreed qualitatively with the experimental data quite well. Some discrepancy could be due to sputtering, re-deposition, and segregation processes. Rutherford backscattering spectroscopy of helium ions, atomic force microscopy, and micro-Raman spectroscopy studies showed that selenium segregated to the surface and formed amorphous nanoparticles. The model predicted an effective bombardment by atoms with energies up to 150 eV which, together with the ion, could stimulate the transport processes in a subsurface layer and on the surface of the films.