Electron Trapping Cross Section Research Articles

Transmission and Trapping of Low-Energy (1–10 eV) Electrons in Crystalline Ice Films

We studied the interactions between low-energy (1–10 eV) electrons and a crystalline ice film in a low fluence condition, where incident electrons interacted mostly with the pristine ice lattice. The electron beams were irradiated onto an ice film sample of large thickness (>100 monolayers) at 95 K. The kinetic energy and flux of incident electrons were maintained constant by applying an offset bias potential to the sample to compensate the charging voltage of an ice film developed by electron trapping. A Kelvin work-function probe was used to measure the charging voltage and estimate the electron trapping efficiency. The measurements for ice films with different thicknesses showed that low energy electrons transmitted through the ice films very efficiently. When electrons were trapped in the ice films, the trapping occurred preferentially at the surface of ice films. The electron trapping cross-section for the ice surface was estimated to be ≤(2.6 ± 0.3) × 10–23 m2 at the incident electron energy of 1–10 eV. The electron trapping cross-section for the ice interior was below ∼10–25 m2. In comparison, adsorbed SO2 and CFCl3 molecules on the ice surface were able to trap incident electrons much more efficiently than the surface water molecules, with corresponding cross sections of (1.1 ± 0.4) × 10–21 m2 and (2.8 ± 0.3) × 10–21 m2, respectively.

Evaluation of the effective cross-sections for recombination and trapping in the case of pure spinel.

In this paper, we have investigated the evolution of the secondary electron emission in the case of pure spinel during electron irradiation, achieved in a scanning electron microscope at room temperature, which is derived from the measurement of the induced and the secondary electron currents. It was observed from the experimental results, that there are two regimes during the charging process: a plateau followed by a linear variation, which are better identified by plotting the logarithm of the secondary electron emission yield lnσ as function of the total surface density of trapped charges in the material QT . For positive charging, E0 = 1.1 and 5 keV, the slope of the linear part, whose value is of about 10(-10) cm(2) charge(-1), is independent of the primary electron energy. It is interpreted as a microscopic cross section for electron-hole recombination. For negative charging of pure spinel, E0 = 15 and 30 keV, the slope is associated with an electron trapping cross section close to 10(-14) cm(2) charge(-1), which can be assigned to the microscopic cross section for electron trapping. This trapping cross section is four orders of magnitude lower than the recombination one.

Dependence of electron trapping on bubble geometry in laser-plasma wakefield acceleration

The effect of bubble shape in laser-plasma electron acceleration was investigated. We showed the general existence of an ellipsoid bubble. The electromagnetic field in this bubble and its dependence on bubble shape were determined through theory. The electron-trapping cross-section for different bubble aspect ratios was studied in detail. When the shape of the bubble was close to spherical, the trapping cross-section reached to the maximum. When the bubble deviated from a spherical shape, the cross-section decreased until electron injection no longer occurred. These results were confirmed by particle-in-cell simulation.

Dependence of secondary electron emission on surface charging in sapphire and polycrystalline alumina: Evaluation of the effective cross sections for recombination and trapping

The evolution of the secondary electron emission from sapphire and polycrystalline alumina during electron irradiation, achieved in a scanning electron microscope at room temperature, is derived from the measurement of the induced and the secondary electron currents. The semi-logarithmic plot of the secondary electron emission yield versus the surface density of trapped charges displays a plateau followed by a linear variation. For positive charging, the slope of the linear part, whose value is of about 10−9cm2, is independent of the primary electron energy, the microstructure and the impurities. It is interpreted as an effective microscopic cross section for electron–hole recombination. For negative charging of sapphire, the slope is associated with an effective electron trapping cross section close to 10−11cm2, which can be assigned to the dominant impurity trap. These effective values reflect the multiple interactions leading to the accumulation of charges. The yield corresponding to the plateau is controlled by the initial density of impurity traps. A charge transport and trapping >model, based on simplifying assumptions, confirms qualitatively these inferences.

Red-IR stimulated luminescence in K-feldspar: Single or multiple trap origin?

We investigate on the origins of the infra-red stimulated luminescence (IRSL) signals in 3 potassium feldspars based on IR-red spectroscopy (∼700–1050 nm) using a fiber-coupled tunable Ti:Sapphire laser, in combination with different thermal and optical (pre)treatments of the samples. We also measure dose-response curves with different wavelengths and at different stimulation temperatures so as to be able to distinguish between traps based on their electron trapping cross-sections. Our data suggest that the dosimetric signals, IRSL, and the post IR-IRSL in K-feldspars arise from a single electron trapping centre.

Characterization of low-temperature molecular-beam-epitaxy grown GaBiAs layers

GaBiAs layers with Bi content reaching 8.4% are grown by MBE technique at low temperatures. All layers were of p-type with carrier densities ranging from 3 × 1014 to 2 × 1015 cm−3 and resistivities exceeding 60 Ω cm. Energy bandgap of the gallium bismide alloys as determined from spectral measurements of the optical absorption, photoconductivity and photoluminescence decreases linearly with increasing Bi content. Optical pump–terahertz probe measurements made on these layers show that the carrier density dynamics is best described by a double-exponential decay. The shorter of the time constants corresponds to the electron trapping and the longer time constant corresponds to the trap emptying times. It has been found that the electron trapping cross-section is of the same order of magnitude as the corresponding parameter for As-antisite traps in LTG GaAs; therefore, it is reasonable to assume that As antisites play a significant role in carrier recombination processes in GaBiAs, too.

Absolute cross section for trapping low-energy electrons (–18eV) in molecular films of n-hexane

We propose an apparatus and method, based on the absorption of photon of energy ranging between 1.5 and 3.54eV, to measure the absolute cross section for trapping low-energy electrons in a molecular film deposited on a metal substrate. A simple model is described to analyze electron trapping in the film and charge release from the same film by photon excitation. Measurements of the trapping of electrons in multilayer films of n-hexane are presented for different film thicknesses, electron exposures and incident energies, and probing photon fluxes and energies. The electron trapping cross section in a five-layer film of n-hexane is characterized by a large maximum reaching a value of (4.7±0.4)×10−17cm2 at 10eV and having a 2.3eV full width at half maximum. This feature is ascribed to the formation of one or several core-excited resonances lying between 7 and 14eV and leading, via dissociative electron attachment, to the formation of stable H− fragments, which stabilize within the first outerlayer of the film.

Definition of curve fitting parameter to study tunneling and trapping of electrons in Si/ultra-thin SiO 2/metal structures

The tunneling of electrons through metal–oxide–silicon (MOS) structures with ultra-thin oxide is modeled using a linear model for the electron potential energy, an approach which simplifies the computation of both the interface potential and the field penetration distance in the substrate. The one-particle quantum problem is split into finding the metastable states induced by the internal field penetration in the substrate and the running states in the gate region. The two states are assumed to be connected by the condition for the continuity of the probability density at the substrate–dielectric interface. The electron probability current and the total gate current density are obtained for different gate voltages. As the model yields excellent fittings with experimental current–voltage ( I– V) data for MOS structures, it was further applied to constant current stressing analysis in order to obtain values for important electron trapping parameters in the oxide. The resultant estimates of the electron trapping cross-section fall in the range of other independent determinations in the literature.

Photoconductivity sampling of low-temperature-grown Be-doped GaAs layers

The dynamical electrical properties of annealed low-temperature-grown GaAs layers moderately doped with beryllium are studied using photoconductive sampling. Picosecond electrical pulses are sampled using an interdigitated metal-semiconductor-metal structure. These time-resolved experiments supported by numerical simulation allow us to determine electron and hole mobilities of, respectively, 540 and 90cm2∕V∕s. We also demonstrate that the free-electron trapping process is saturating under high-optical-density excitation. From these experiments, we get an electron trapping cross section of σn=1×10−13cm2.

Spin relaxation of charged centers in silicon in the presence of photocarriers (Si: Cr+, [Cr+−B−

Spin relaxation of deep charged centers Cr+ and of donor-acceptor pairs (Cr+−B−)0 in silicon is studied by nonstationary EPR spectroscopy at liquid-helium temperatures. We observed the effect of an increase in the spin-lattice relaxation rate under band-to-band sample illumination; the magnitude of the effect is proportional to the photoelectron concentration. The spin-dependent carrier trapping is shown to play a dominant role in spin relaxation under illumination for centers of both types. Coupled rate equations describing the interaction of various subsystems with one another and with the bath are solved. A comparison of experimental data with theory yielded the electron trapping cross sections σr(Cr+)≅4.9×10−12 cm2 and σr(Cr+−B−)≅1.6×10−12 cm2 at T=4.2 K. The results obtained are discussed in terms of the theory of trapping by attractive centers.

Deep states in silicon δ-doped GaAs

The density and electron trapping cross section of deep states in silicon δ-doped GaAs were investigated by means of measurements of the voltage and temperature dependences of the impedance of a Schottky contact to the structure. It was observed that density-of-states tails appear in the band gap when the silicon density in the d-layer exceeds 6×1012 cm−2. In our structures the energy characterizing the penetration depth of a tail was in the range 20–100 meV. The characteristic electron trapping cross section of deep states in δ-layers was of the order of 10−17 cm2. It was shown that saturation of the electron density in the δ-layer with increasing Si density is due to self-compensation of Si.

An improved electron transmission method for measuring electron trapping cross sections at the surface of dielectric films

We investigate several problems inherent in the low energy electron transmission (LEET) technique for measuring cross sections σCT for charge trapping, by submonolayer (ML) quantities of a target molecule deposited onto the surface of a dielectric film. In particular we see that the energy of the incident electron beam while charging the film was poorly defined in the original method. Furthermore, we demonstrate that interactions between trapped charges and the metallic substrate set a limit of about 100 mV for the maximum surface potential that should be allowed in absolute measurements of CT cross sections. To surmount these problems, we show how the surface potential ΔV generated by electrons of specific incident energy can be obtained rapidly, by monitoring the relative transmitted current Ir at a single reference energy EREF, rather than by recording multiple LEET spectra (or injection curves). The approach allows several measurements at different incident electron energies to be made on a single film and increases markedly the rate of data acquisition. The overall uncertainty in these measurements has an upper limit of ∼50%.

A picosecond bleaching study of quantum-confined cadmium sulfide microcrystallites in a polymer film

We report a picosecond pump–probe study of 55 Å cadmium sulfide microcrystallites embedded in polymer films. Large negative absorbance changes at wavelengths corresponding to energies near the band gap are observed. This absorption bleaching and the associated changes in refractive index are mainly responsible for the large nonlinearity observed in degenerate four-wave mixing experiments. Based on photoluminescence data, the known electron-trapping cross section of defects, and these pump–probe experiments, we show that the conventional carrier density-dependent band-filling mechanism cannot account for the data, and the absorption bleaching is due to the saturation of the excitonic transition. We further show that the phase-space filling and exchange effects from exciton–exciton and exciton-free carrier interactions fail to account for the observed data. Instead, we propose that the exciton-trapped carrier interaction is mainly responsible for the observed bleaching of the excitonic absorption. This interaction is unique for small semiconductor clusters since the presence of a high density of defects (most likely on the surfaces) causes the extremely rapid trapping of free carriers. According to this model, the recovery time of the absorption bleaching is determined by the trapped-carrier relaxation time, which is sensitive to the fabrication methods and can be controlled by surface chemistry. Our study also demonstrates that one needs to understand the effects of surfaces and control the surface chemistry before the important question of size effects on the nonlinear optical properties can be addressed.

Electron trapping in electron-beam irradiated SiO2

In addition to the well-known positive space charge, electron irradiation of MOS capacitors with 25-keV electrons is shown to introduce additional uncharged electron traps into the oxide layer. These traps persist after most of the positively charged defects have been removed by the usual low-temperature (? °C) anneals. Their presence after this anneal is determined by injecting hot electrons into the oxide where they are captured by existing defects. The effective trap densities increase with increasing electron fluence and are reduced by forming-gas anneals at temperatures in excess of 500 °C. Observed electron-capture cross sections are between 10−15 and 10−18 cm2. The residual radiation damage in oxides exposed to 10−4 C cm−2 of 25-keV electrons and subsequently annealed at 400 °C results in an additional neutral density of 5×1011 traps cm−2 with cross sections distributed over the above range. Electron-trapping cross sections and effective trap densities associated with this damage are found to be identical at 77 and 295 K. The traps are possibly associated with dipolar defects formed when valence electrons localize around an ion after the bonds are broken.

Electron trapping and untrapping during the dark decay of flash-produced KC1 F′ and F′ > A(Na) bands between 290–350°K

Both the thermal lifetimes of the F′( F′ A ) centers and the ratios of the electron trapping cross-sections of ( F A ) to α( α A ) centers were derived from the decay kinetics of flash-produced F′( F′ A ) bands. In particular, the ratios dropped steeply with the temperature possibly due to a thermally activated de-excitation following excited state pre-trapping of the electron by the anion vacancy.

Competitive trapping in sodium disilicate glasses doped with Eu +3

Optical processes in X-irradiated sodium disilicate glasses with additions of Eu +3 up to 1% were studied. The added europium was assumed to have an electron trapping cross section proportional to its concentration and to compete with native defects for electrons generated by the X-rays; thus, the “free” electron concentration is suppressed by Eu +3. By considering these effects for small X-rays doses, it was possible to demonstrate that (i) an absorption band growing at 343 mμ was indicative of the photoreduction of europium; (ii) a composite absorption band ( E 1.2 − band) peaked in the red or orange and another band peaked at 305 mμ ( E 3 − band) were the absorptions of trapped electrons; and (iii) that two other bands peaked at 460 mμ ( H 2 +) and 620 mμ ( H 3 +) were absorption bands of trapped holes. Finally, the results show that luminescent processes involving a characteristic violet emission of silicate glasses should be interpreted as resulting from the recombination of a “free electron” at the site of a trapped hole ( H 1 + center). These results give additional support to the evidence for the nature of optical centers observed in alkali silicate glasses in the preceding paper.

Temporary Bleaching of Color Centers in KCl Crystal

Complex color centers of M , R and N in KCl crystal are found to be temporarily bleached under an intense auxiliary F -light illumination. The generations of M ', R ' and N ' centers consisting of complex centers and one more electron are suggested. The thermal stabilities of these F '-type centers are also investigated. In KCl crystals containing only F centers, F ' centers decay thermally by the bimolecular process and the electron trapping cross section of a negative ion vacancy is found to be twice that of F center at near -70°C.