G. Moya

Stability of trapped charges in sapphires and alumina ceramics: Evaluation by secondary electron emission

The stability of trapped charges in sapphires and alumina ceramics is characterized via an experimental parameter expressing the variation of the secondary electron emission yield between two electron injections performed in a scanning electron microscope. Two types of sapphires and polycrystalline alumina, which differ mainly by their impurity content, are investigated in the temperature range 300–663K. The stable trapping behavior in sapphires is attributed to trapping in different defects, whose nature depends on the purity level. In alumina ceramics, the ability to trap charges in a stable way is stronger in samples of high impurity content. In the low impurity samples, stable trapping is promoted when the grain diameter decreases, whereas the reverse is observed in high impurity materials. These behaviors can stem from a gettering effect occurring during sintering. The strong dependence of the variation of the secondary electron emission yield on the grain diameter and impurities enables a scaling of...

Advanced measurement techniques of space-charge induced by an electron beam irradiation in thin dielectric layers

Abstract In dielectric materials, various electron beam techniques have been developed recently for space charge measurements. If charges injection is carried out with the primary electron beam of a scanning electron microscope, two methods can be developed, i.e. the mirror mode and the influence charge mode. In the first method, if the secondary electron emission yield σ is less than one, after charge implantation a net negative charge is trapped which acts as a mirror reflecting the electron trajectories corresponding to a smaller scanning energy beam. From this method, we can measure the electric charges trapping properties (amount of trapped charges, trap energy, and spreading of charges). In the second method, during the charge injection the measurement of the current, induced by the trapped charges in the metallic sample holder of the microscope, gives information in real time as function of the injected dose on the trapped charge quantities and on the dynamics of trapping–detrapping phenomena. This article describes how to use a scanning electron microscope to perform such measurements. It is shown that the characterisation of thin oxide layers requires primary energies which generally, because of the electron penetration depth, prohibit using the mirror method (due to net positive charge corresponding to σ>1). This topic is debated and some results carried out, using the influence charge method, particularly on SiO2 oxide layers of different nature are reported. This method provides a better understanding of aging and breakdown processes in thin charged dielectrics.

New technique to characterise thin oxide films under electronic irradiation

To characterise insulating materials, measurements of their ability to trap or release injected charges have been developed recently by induced charge measurement (ICM). This new technique allows to investigate the charging properties of bulk or thin oxide films deposited on non-conductive substrates during the electronic irradiation carried out in a scanning electron microscope (SEM). In this work, ICM is used to investigate in MgO single crystal and thin MgO layers deposited on glass or enamel, the effects of substrate, injected charge density and primary electron energy on the kinetics and the nature of the net trapped charge.

Study of quenched-in vacancies in the intermetallic compound NiSb by differential scanning calorimetry

Abstract Differential scanning calorimetry performed on specimens of polycrystalline NiSb, quenched from 750 and 800°C, has revealed only one annealing stage at about 280°C. The activation energy (Ea=1·15 eV) and the order of reaction for this stage (m = 1) have been determined by a kinetic analysis. These results are discussed and it is suggested that the stage is due to the annealing of Ni quenched-in monovacancies.

Analysis of isochronal calorimetric measurements of point defect elimination in a quenched Co48Ga52 intermetallic compound

Abstract In the intermetallic compound Co48Ga52, high vacancy supersaturations have been produced by quenching from 900°C. Differential scanning calorimetry performed on quenched samples has revealed one annealing stage at about 550°C. An accurate method for the determination of the activation energy from anisothermal calorimetric data is presented. The results are discussed and it is suggested that the stage observed is due to the annealing out of quenched-in monovacancies by the formation of clusters.

Positron annihilation in the quenched and electron-irradiated NiSb intermetallic compound

Samples of the near equiatomic NiSb compound were irradiated by 3 MeV electrons at 20 K or quenched from 1103 K and 1333 K and subsequently annealed isochronally. The behaviour of defects created by quench or irradiation were studied by the positron annihilation technique. Only one recovery stage was found around 425 K for quenched specimens, but two distinct stages (100 K and 425 K) were observed after irradiation. The 425 K stage is ascribed to the migration of Ni vacancies giving dislocation loops. The recombination of mobile interstitials with vacancies after irradiation is assumed to occur between 100 K and 250 K. Doppler broadening and lifetime variations of positrons as a function of the measuring temperature in these irradiation samples are discussed.

A positron lifetime study in quenched Pb(Ag) alloys

Positron lifetime measurements have been made in quenched or irradiated pure Pb and in quenched Pb(Ag) alloys. From positron investigation of annealing behaviour, the precipitation of silver atoms in dilute alloys should be understood in terms of (Ag-Pb) interstitially migrating pairs. The presence of di-interstitials (Ag-Ag) or complexes [Ag(S)-V] as mobile defects responsible for the Ag transport process in concentrated alloys is discussed.

Fast solute diffusion in solid and supersaturated solid solutions

Abstract In this paper, we review the experimental data concerning fast diffusion of solute in Pb matrix and Pb (solute) solid solutions. Using both calorimetric and radiotracer techniques the temperature dependence of solute diffusion coefficients in Pb (solute) supersaturated solid solutions is determined. On the basis of experimental results it is shown that these coefficients can only be understood by assuming different defect states for solutes in solid and supersaturated solid solutions.

Modeling of secondary electron emission and charge trapping in an insulator under an electronic beam

Charge accumulation in an insulator, as achieved by electron irradiation in a SEM, is governed by complex phenomena. The understanding of the relevant processes (the generation of free carriers, transport and trapping/de-trapping) is a prerequisite for the design of materials having improved breakdown strengths. A simple modeling, which meets the planar geometry conditions, is proposed. The predictions of the evolution of secondary electron emission yield (as function of the quantity of trapped charges) agree well with the experimental results at room temperature, in pure amorphous SiO2. The agreement is obtained with trapping cross-sections near 10-10 cm2, which are much higher than the typical values (about 10-16 cm2). Trapping appears as an “effective” process that reflects the Brownian motion of secondary electrons.

Method for characterizing charge spreading in sintered alumina by secondary electron emission: Effect of microstructure and temperature

The competition involving charge trapping, diffusion and mobility in insulators may give rise to some charge spreading. This phenomenon is investigated by measuring the recovery of the secondary electron emission. A parameter, which assesses the recovery of the secondary electron emission yield during two successive electron irradiations performed with a Scanning Electron Microscope, is defined. This parameter reflects the evolution of the charging state and the electric field of the material near surface. Alumina samples sintered to different grain diameters and containing only 150 ppm of various impurities (mainly silicon), are studied. The experiments are carried out at temperatures lying in the range 300-663 K. It is deduced that when the temperature increases charge spreading is enhanced. For a given temperature, spreading is promoted in larger grains. The competition between charge spreading and trapping is discussed by calling for the trapping centres induced by silicon dissolution in the bulk and at grain boundaries. It is found that trapping at grain boundaries is more stable than in the bulk. This method constitutes the first step towards setting up standard characterization procedures.