Mickaël Gilliot

Dielectric function of sol-gel prepared nano-granular zinc oxide by spectroscopic ellipsometry

ZnO thin films have been prepared by sol gel and deposited by spin coating. The dielectric function has been determined by spectroscopic ellipsometry. Ellipsometric spectra are inverted by a direct numerical method without using the standard fitting procedures. The obtained dielectric function presents a broad excitonic effect. The dielectric function is studied using Elliot excitonic theory including exciton plus band-to-band Coulomb interactions with standard Lorentzian broadening. A modification of this model dielectric function with independent bound and unbound exciton contributions is empirically proposed to improve modelling of the band gap excitonic peak.

Dielectric function of very thin nano-granular ZnO layers with different states of growth

Zinc oxide (ZnO) layers consisting of grains closely packed together are grown using a solgel synthesis and spin-coating deposition process. The morphologies are characterized by atomic force microscopy and X-ray diffraction, and their optical properties are investigated by spectroscopic ellipsometry at the different stages of the growth process. The optical observations are correlated with evolution of morphology and orientation. Two remarkable evolutions are observed: gradual evolution of morphology, crystallinity, and excitonic contribution with the first deposition steps; and transformation from a poorly oriented to a c-axis oriented crystalline state featuring a large contribution of bound excitons after thermal annealing. A modified Elliott model is used to obtain the optical parameters of ZnO, including bandgap and exciton energies. A simple growth mechanism is proposed to explain the evolution of the layers in accordance with the different deposition steps.

Etching of a-Si:H thin films by hydrogen plasma: A view from in situ spectroscopic ellipsometry

When atomic hydrogen interacts with hydrogenated amorphous silicon (a-Si:H), the induced modifications are of crucial importance during a-Si:H based devices manufacturing or processing. In the case of hydrogen plasma, the depth of the modified zone depends not only on the plasma processing parameters but also on the material. In this work, we exposed a-Si:H thin films to H2 plasma just after their deposition. In situ UV-visible spectroscopic ellipsometry measurements were performed to track the H-induced changes in the material. The competition between hydrogen insertion and silicon etching leads to first order kinetics in the time-evolution of the thickness of the H-modified zone. We analyzed the correlation between the steady state structural parameters of the H-modified layer and the main levers that control the plasma-surface interaction. In comparison with a simple doped layer, exposure of a-Si:H based junctions to the same plasma treatment leads to a thinner H-rich subsurface layer, suggesting a possible charged state of hydrogen diffusing.

Charging process of polyurethane based composites under electronic irradiation: Effects of cellulose fiber content

The study deals with the charging effect of polyurethanes-based composites reinforced with cellulose fibers, under electronic beam irradiation in a scanning electron microscope. The results indicate that the leakage current and the trapped charge as well as the kinetics of charging process significantly change beyond a critical concentration of 10% cellulose fibers. These features are correlated with the cellulose concentration-dependence of the electrical properties, specifically resistivity and capacitance, of the composite.

Electric field assisted diffusion of hydrogen in a-Si:H thin films during hydrogen plasma etching

We exposed a hydrogenated amorphous silicon (a-Si:H) p–i junction to H2 plasma immediately after deposition. The H-induced modifications during hydrogen etching of the device were tracked by in situ UV–visible spectroscopic ellipsometry measurements. Against all odds and contrary to what occurs in a single thin a-Si:H layer, the plasma exposure leads to a thinner H-modified subsurface layer and a higher etch rate in the p-doped layer in comparison with the i-layer of the junction. Solution of the partial differential equation for electric field assisted diffusion of hydrogen, during the plasma etching process, provides the time-evolution of the mean diffusion distance of hydrogen and its relationship with the intensity of the electric field. These results, which emphasize the charged state of hydrogen diffusing into the p-layer and the role of the built-in electric field of the junction in counteracting it, can enable better control in manufacturing and processing of a-Si:H based devices.