2, Colorado School of Mines, Physics Department, Golden, Colorado, United States
The purpose of this research work is to gain a better understanding of the nanostructural properties of Molybdenum (Mo) thin films’ porosity, nanovoid heterogeneity and its volume fraction as the film growth sputtering pressure changes systematically. This knowledge shall assists in optimizing Mo film nano- and microstructural properties as desired for solar cells applications. Therefore, in this research work two separate series of Mo thin films (0.7 μm thick) were deposited on high purity (99.999%) Al-foil (10 μm thick), and Si/SiO2 substrates using direct-current (DC) planar magnetron sputtering. The sputtering pressure was varied from 0.8 mT to 12 mT, with a sputtering power density of 1.2 W/cm2. Small Angle Scattering (SAXS) technique was applied to examine the heterogeneity and existence of nano- and microvoids in the Mo-coated Al foils. Moreover, the porosity of the Mo films as a function of sputtering pressure was studied by transmission electron microscopy (TEM) on Mo-coated Si/SiO2 substrates.
Preliminary results showed that all Mo films are characterized by columnar morphology where open grain-boundaries structure started to develop in the films at intermediate and high sputtering pressure. SAXS intensity I(q) as a function of the momentum transfer q for Mo films showed a systematic reduction in intensities as the sputter pressure is decreased. It revealed larger features in the films induced by the higher pressures, in addition to the existence of anisotropic scattering from non-spherical, oriented objects. The void volume fraction exhibited an increase from 0.4% (at 0.8 mT) to ∼ 7% (at 12 mT). The average void diameter was found to increase from 5.7 nm for the Mo films sputtered at 0.8 mT to 12.5 nm for those films sputtered at 12 mT. The oriented ellipsoid model was utilized and it was consistent with a columnar-like microstructure where the voids have an elongated shape and are located along the column boundaries. Also, the measured Porod parameter indicated significant scattering from larger features which can be due either to surface roughness or to even larger voids. It was observed that as the growth sputtering pressure increases, the calculated average microvoid number density ξ, at first increases up to 2.0 × 1016 cm-3 at 2 mT; and then decreases to an average of about ∼0.6 × 1016 cm-3 above 4 mT. Whereas the average microvoid volume V increases monotonically from 582 nm3 at 0.8 mT up to 10227 nm3 at 12 mT. Furthermore; in good agreement with SAXS results, TEM data revealed the existence of slit-like void that extended through the film thickness, which is not a single void, but rather a void network of about 10 nm in width. However, further data and interpretation of the results of this research work will be presented in the detailed paper, where a correlation between the Mo thin film sputtering pressure and the microvoids number density, its volume fraction, and its dimensions will be presented.