Aomar Hadjadj

Analysis of electrical charging and discharging kinetics of different glasses under electron irradiation in a scanning electron microscope

This paper presents a comparative study of electrical charging and discharging behavior of different glasses submitted to electron beam irradiation in scanning electron microscope. Charge storage and charge spreading in these glasses have been examined with help of a time resolved current method. Our interest concerns more particularly the dynamic behavior and the amount of the space charge build-up during and after electron irradiation under different experimental conditions of primary beam energy and current density. The precise contributions of different possible self regulation processes (leakage current and secondary electron emission) for charge accumulation are analyzed. Moreover, to characterize the ability of glasses to store charges in a stable way we introduce a relevant parameter that expresses quantitatively the variation in the released charge. The primary beam energy and the current density effects on the evolution of secondary electron emission yield during irradiation are also examined. As expected, the charge storage and spreading processes appear to be extremely dependent on the incident beam energy, current density and on the chemical composition of the studied glasses.

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.

Charging effects of PET under electron beam irradiation in a SEM

This paper deals with charge trapping and charge transport of polyethylene terephthalate (PET) polymer subjected to electron irradiation in a scanning electron microscope (SEM). Measurement of displacement current and leakage current using an arrangement adapted to the SEM allows the amount of trapped charge during and after electron irradiation to be determined and the charge mechanisms regulation to be studied. These mechanisms involve several parameters related to the electronic injection, the characteristics of insulator and the effects of the trapped charge itself. The dynamic trapping properties of PET samples are investigated and the time constants of charging are evaluated for various conditions of irradiation. The determination of the trapping cross section for electrons is possible by using the trapping rate at the onset of irradiation. Many physical processes are involved in the charging and discharging mechanisms; among them surface conduction is outlined. Through the control of irradiation conditions, various types of surface discharging (flashover phenomenon) behaviour are also observed. The strength of the electric field initiating surface discharge is estimated.

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.

Polypropylene-nanoclay composites under electron irradiation in SEM: Structure, charge trapping and electron emission properties

In this work, polypropylene (PP) and its polymer nanocomposites (PNCs) films containing very low contents (2–6 wt %) of Cloisite 20A natural montmorillonite clay platelets were investigated. In a first, they were characterized by means of several analytical techniques such as X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), Differential Scanning Calorimetry (DSC) and UV-visible spectroscopy. Secondly, the nanocomposites are submitted to an electron beam in a Scanning Electron Microscope (SEM) and the induced currents are measured using a time-resolved current method. These currents are used to determine the trapped charge and the electron emission yield. The aim is to contribute to the understanding of the effect of different concentrations of nanoclay platelets on the studied properties by correlating the results obtained by both techniques. The analytical characterization showed that upon the increase of nanoclays concentration an intercalated structure and an increase of crystallinity are induced. On the other hand, the optical band gap energy is modified because the increasing of the density of localized states in the forbidden energy band. As regards the measurement of the trapped charge, it was found, surprisingly, that not only the leakage current increases as a function of clay loading level but also the trapped charge (or the surface potential). This could be related to the increase of conductivity in one hand and to proliferation of interfaces between nanoparticles and neighboring materials on the other hand.

Study of temperature effects on the electrical behavior of polypropylene-clay nanocomposites submitted to electron beam irradiation in a SEM

Charge transport and electron emission properties in polypropylene and its nanocomposites filled with nanoclay particles submitted to an electron irradiation, in a Scanning Electron Microscope (SEM), are investigated using induced displacement and leakage currents. The measurements have been performed at various temperatures ranging from 20°C to 75°C at a primary beam energy of 20keV and a primary beam current of 1nA with the aim to highlight the effect of temperature and nanoclay content on these properties. The results show, at a given temperature, that the incorporation of clay in polypropylene (PP) matrix paradoxically leads to a concomitant increase in the electrical conductivity and the charge accumulated. In contrast, if the clay content is fixed, there is an increase in conductivity and a reduction of the charge accumulated when the temperature increases. The mobility of charge carriers and the corresponding activation energy are deduced from the measured leakage current during discharging step. The mobility was found to be an order of magnitude higher for the nanocomposites. The study of the influence of the temperature and nanoclay concentration on electron emission yield is also addressed.

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.