On the formation of the porous structure in nanostructured a-Si coatings deposited by dc magnetron sputtering at oblique angles

The formation of the porous structure in dc magnetron sputtered amorphous silicon thin films at low temperatures is studied when using helium and/or argon as the processing gas. In each case, a-Si thin films were simultaneously grown at two different locations in the reactor which led to the assembly of different porous structures. The set of four fabricated samples has been analyzed at the microstructural level to elucidate the characteristics of the porous structure under the different deposition conditions. With the help of a growth model, we conclude that the chemical nature of the sputter gas not only affects the sputtering mechanism of Si atoms from the target and their subsequent transport in the gaseous/plasma phase towards the film, but also the pore formation mechanism and dynamics. When Ar is used, pores emerge as a direct result of the shadowing processes of Si atoms, in agreement with Thornton's structure zone model. The introduction of He produces, in addition to the shadowing effects, a new process where a degree of mobility results in the coarsening of small pores. Our results also highlight the influence of the composition of sputtering gas and tilt angles (for oblique angle deposition) on the formation of open and/or occluded porosity.

Nanocavities and germanium nanocrystals produced by Ge ion implantation in fused silica

High-resolution SEM images of germanium nanocrystals (Ge-nc) synthesized by ion implantation in fused silica samples annealed at temperatures below and above the melting point of Ge show a strong size-selective depth-distribution of nanostructures, as evidenced by the correlation between the dimension of the observed objects and the local concentration of implanted Ge measured by Rutherford backscattering spectroscopy (RBS). Whereas the Ge-nc nucleation seems to obey the Ostwald ripening process in samples annealed below 900 °C, Ge desorption effects, non-uniform in depth, in conjunction with the formation of large and spherical nanocavities, become dominant for annealing performed above the solid-liquid phase transition of Ge. Measurements for different annealing times at 1050 °C show two distinct processes in the Ge desorption dynamics: the first is related to direct Ge outgassing effects during the nucleation of Ge-nc, which occurs within the first minutes of the thermal annealing, while the second is due to the release of Ge from Ge-nc, associated with the formation of nanocavities. The formation rate of these nanocavities is more efficient at greater depth than in the vicinity of the sample surface. It appears to be strongly dependent on the local concentration of defects, responsible for the reduction of the Ge diffusion, and to be related to the breaking of Ge-O and Si-Ge bonds at the Ge-nc/SiO(2) interface.