Penetration Depth Of Electron Beam Research Articles

Decay kinetics in single crystals and ceramics based on yttrium aluminum garnet doped with rare earth ions

The paper is devoted to the study of cathodoluminescence decay kinetics in ceramics and single crystals of YAG:Nd and YAG:Eu. The decay kinetics of Nd and Eu cathodoluminescence bands were found to be double exponential. The decay kinetics dependence on the electron beam penetration depth was studied. The cathodoluminescence technique allowed varying the proportion of surface area in the excited region depending on the energy of the electron beam. The electron beam depth penetration calculation was performed using Monte Carlo method. In single crystals the second decay component appeared to be only on the surface of the samples, whereas in ceramics the second decay component was constant over electron beam penetration depth. Double exponential kinetics as assumed are connected with two types of RE ions – luminescent centers.

Characterization of a Radiofluorogenic Polymer for Low-Energy Electron Beam Penetration Depth Visualization.

Low-energy (80–300 keV) electron beam accelerators are gaining in importance in the radiation processing industry due to their ease of use and wide range of applications (e.g. product surface sterilizations or polymer curing and cross-linking). Due to their very low penetration depth (tens to hundreds of microns), currently used film dosimeters exhibit dose gradients over their thickness and do not resolve the dose response in the first microns of the irradiated material. Hence, the surface dose, defined as the dose in the first micron Dµ, cannot be measured directly. This study presents a polymer material as a dosimeter candidate for high-dose low-energy electron beam irradiations. The readout of the dose-dependent fluorescence intensity, originating from a pararosaniline dye reaction when irradiated, is measured using fluorescence microscopy. So far, no in-depth characterization of the material has been performed, leaving the stability and fluorescence properties of the material not fully optimized. We describe the improvements in polymer composition and the fabrication method, and characterize the material properties in terms of the thermal stability, glass transition temperature, refractive index, hardness, rheological behavior, and water affinity. All of these create a complex set of requirements a polymer needs to fulfill to become an effective dosimeter when measuring using confocal microscopy. The fluorescence readout procedure will be addressed in further studies.

Measurement of Nanometre-Scale Gate Oxide Thicknesses by Energy-Dispersive X-ray Spectroscopy in a Scanning Electron Microscope Combined with Monte Carlo Simulations.

A procedure based on energy-dispersive X-ray spectroscopy in a scanning electron microscope (SEM-EDXS) is proposed to measure ultra-thin oxide layer thicknesses to atomic scale precision in top-down instead of cross-sectional geometry. The approach is based on modelling the variation of the electron beam penetration depth and hence the depth of X-ray generation in the sample as a function of the acceleration voltage. This has been tested for the simple case of silica on silicon (SiO2/Si) which can serve as a model system to study gate oxides in metal-on-semiconductor field-effect transistors (MOS-FETs). Two possible implementations exist both of which rely on pairs of measurements to be made: in method A, the wafer piece of interest and a reference sample (here: ultra-clean fused quartz glass for calibration of the effective k-factors of X-ray lines from elements O and Si) are analysed at the same acceleration voltage. In method B, two measurements of the apparent O/Si ratio of the same wafer sample need to be made at different acceleration voltages and from their comparison to simulations the SiO2 layer thickness of the sample can be inferred. The precision attainable is ultimately shown to be limited by surface contamination during the experiments, as very thin carbonaceous surface layers can alter the results at very low acceleration voltages, while the sensitivity to ultra-thin surface oxides is much reduced at higher acceleration voltages. The optimal operation voltage is estimated to lie in the range of 3–15 kV. Method A has been experimentally verified to work well for test structures of thin oxides on Si-Ge/Si.

Etching-induced damage in heavily Mg-doped p-type GaN and its suppression by low-bias-power inductively coupled plasma-reactive ion etching

Inductively coupled plasma–reactive ion etching (ICP–RIE)-induced damage in heavily Mg-doped p-type GaN ([Mg] = 2 × 1019 cm−3) was investigated by low-temperature photoluminescence (PL) and depth-resolved cathodoluminescence (CL) spectroscopy. From PL measurements, we found broad yellow luminescence (YL) with a maximum at around 2.2–2.3 eV, whose origin was considered to be isolated nitrogen vacancies (V N), only in etched samples. The depth-resolved CL spectroscopy revealed that the etching-induced YL was distributed up to the electron-beam penetration depth of around 200 nm at a high ICP–RIE bias power (P bias). Low-bias-power (low-P bias) ICP–RIE suppressed the YL and its depth distribution to levels similar to those of an unetched sample, and a current–voltage characteristic comparable to that of an unetched sample was obtained for a sample etched with P bias of 2.5 W.

Electron-beam recording of surface structures on As-S-Se chalcogenide thin films

The effect of electron beam irradiation on the amorphous chalcogenide film As38S36Se26 was studied. The formation of cones with a Gaussian profile on the surfaces of the films was found after local electron irradiation. Exposition dependent evolution of height surface nanostructures has been detected. The dependence of the height of surface nanostructures on the dose of irradiation is analyzed. Charge accumulation model into interaction region between the film and the electron beam was used to explain the electron-induced phenomena of the surface structure of amorphous As38S36Se26 films. Charges relaxation times, and electron beam penetration depth into film, and the initial and inverse doses are determined.

Electron linear accelerator system for natural rubber vulcanization

Development of an electron accelerator system, beam diagnostic instruments, an irradiation apparatus and electron beam processing methodology for natural rubber vulcanization is underway at the Plasma and Beam Physics Research Facility, Chiang Mai University, Thailand. The project is carried out with the aims to improve the qualities of natural rubber products. The system consists of a DC thermionic electron gun, 5-cell standing-wave radio-frequency (RF) linear accelerator (linac) with side-coupling cavities and an electron beam irradiation apparatus. This system is used to produce electron beams with an adjustable energy between 0.5 and 4MeV and a pulse current of 10–100mA at a pulse repetition rate of 20–400Hz. An average absorbed dose between 160 and 640Gy is expected to be archived for 4MeV electron beam when the accelerator is operated at 400Hz. The research activities focus firstly on assembling of the accelerator system, study on accelerator properties and electron beam dynamic simulations. The resonant frequency of the RF linac in π/2 operating mode is 2996.82MHz for the operating temperature of 35°C. The beam dynamic simulations were conducted by using the code ASTRA. Simulation results suggest that electron beams with an average energy of 4.002MeV can be obtained when the linac accelerating gradient is 41.7MV/m. The rms transverse beam size and normalized rms transverse emittance at the linac exit are 0.91mm and 10.48πmm·mrad, respectively. This information can then be used as the input data for Monte Carlo simulations to estimate the electron beam penetration depth and dose distribution in the natural rubber latex. The study results from this research will be used to define optimal conditions for natural rubber vulcanization with different electron beam energies and doses. This is very useful for development of future practical industrial accelerator units.

A semi-analytical model for calculating the penetration depth of a high energy electron beam in a water phantom with a magnetic field

PurposeAs an electron beam is incident on a uniform water phantom in the presence of a lateral magnetic field, the depth-dose distribution of the electron beam changes significantly and forms the well-known ‘Bragg peak’, with a depth-dose distribution similar to that of heavy ions. This phenomenon has pioneered a new field in the clinical application of electron beams. For such clinical applications, evaluating the penetration depth of electron beams quickly and accurately is the critical problem. MethodsThis paper describes a model for calculating the penetration depth of an electron beam rapidly and correctly in a water phantom under the influence of a magnetic field. The model was used to calculate the penetration depths under different conditions: the energies of electron beams of 6, 8, 12 and 15 MeV and the magnetic induction intensities of 0.75, 1.0, 1.5, 2.0 and 3.0 T. In addition, the calculation results were compared with the results of a Monte Carlo simulation. ResultsThe comparison results indicate that the difference between the two calculation methods was less than 0.5 cm. Moreover, the computing time of the calculation model was less than a second. ConclusionsThe semi-analytical model proposed in the present study enables the penetration depth of the electron beam in the presence of a magnetic field to be obtained with a computational efficiency higher than that of the Monte Carlo approach; thus, the proposed model has high potential for application.

Time-of-Flight Current Shapes in Molecularly Doped Polymers: Effects of Sample Thickness and Irradiation Side and Carrier Generation Width

The time-of-flight (TOF) current transients from solution-cast, free-standing films of p-diethylaminobenzaldehyde diphenyhydrazone in bisphenol A polycarbonate (DEH:PC) have been studied using electron gun induced charge generation. Changes in the shape of the current transient, cusp formation in particular, with film thickness, electron-beam penetration depth, and the side of the sample irradiated, have been analyzed with a two-layer multiple trapping model, and indicate that the time-of-flight transients of solution-cast films can be problematic in that plateau formation does not necessarily imply nondispersive charge transport. The results are consistent with the existence of thin surface layers which are depleted of the hole transport material. The depleted layer on the surface of the film that was exposed to air during coating/drying is always thicker than it is for the surface contacting the substrate. Depletion could occur through transport material sublimation, surface physical characteristics suc...

Study of the Distribution of Radiative Defects and Reabsorption of the UV in ZnO Nanorods-Organic Hybrid White Light Emitting Diodes (LEDs)

In this study, the low temperature aqueous chemical growth (ACG) method was employed to synthesized ZnO nanorods to process-organic hybrid white light emitting diodes (LEDs) on glass substrate. Electroluminescence spectra of the hybrid white LEDs demonstrate the combination of emission bands arising from radiative recombination of the organic and ZnO nanorods (NRs). Depth resolved luminescence was used for probing the nature and spatial distribution of radiative defects, especially to study the re-absorption of ultraviolet (UV) in this hybrid white LEDs structure. At room temperature the cathodoluminescence (CL) spectra intensity of the deep band emission (DBE) is increased with the increase of the electron beam penetration depth due to the increase of defect concentration at the ZnO NRs/Polyfluorene (PFO) interface and probably due to internal absorption of the UV. A strong dependency between the intensity ratio of the UV to the DBE bands and the spatial distribution of the radiative defects in ZnO NRs has been found. The comparison of the CL spectra from the PFO and the ZnO NRs demonstrate that PFO has a very weak violet-blue emission band, which confirms that most of the white emission components originate from the ZnO NRs.

Study of Au/ZnO nanorods Schottky light-emitting diodes grown by low-temperature aqueous chemical method

High quality vertically aligned ZnO nanorods (NRs) were grown by low-temperature aqueous chemical technique on 4H-n-SiC substrates. Schottky light-emitting diodes (LEDs) were fabricated. The current-voltage (I–V) characteristics of Schottky diodes reveal good rectifying behavior. Optical properties of the ZnO nanorods (NRs) were probed by cathodoluminescence (CL) measurements at room temperature complemented with electroluminescence (EL). The room-temperature CL spectra of the ZnO NRs exhibit near band edge (NBE) emission as well as strong deep level emission (DLE) centered at 690 nm. At room temperature the CL spectra intensity of the DLE was enhanced with the increase of the electron beam penetration depth due to the increase of defect concentration at the interface and due to the conversion of self-absorbed UV emission. We observed a variation in the DLE along the nanorod depth. This indicates a relatively lower structural quality near the interface between ZnO NRs and n-SiC substrate. The room-temperature CL spectra of SiC show very weak emission, which confirms that most of the DLE is originating from the ZnO NRs, and SiC has a minute contribution to the emission.

Depth-resolved cathodoluminescence study of zinc oxide nanorods catalytically grown on p-type 4H-SiC

Optical properties of ZnO nanorods (NRs) grown by vapour–liquid–solid (VLS) technique on 4H-p-SiC substrates were probed by cathodoluminescence (CL) measurements at room temperature and at 5 K complemented with electroluminescence. At room temperature the CL spectra for defect related emission intensity was enhanced with the electron beam penetration depth. We observed a variation in defect related green emission along the nanorod axis. This indicates a relatively poor structural quality near the interface between ZnO NRs and p-SiC substrate. We associate the green emission with oxygen vacancies. Analysis of the low-temperature (5 K) emission spectra in the UV region suggests that the synthesized nanorods contain shallow donors and acceptors.

Effective Green Semiconductor Lasers with Multiple CdSe/ZnSe QD Active Region for Electron Beam Pumping

Electron beam pumped (EBP) semiconductor lasers using ZnSe-based structures as active elements look very promising for the development green lasers because of no requirements of both p-n junction and low-resistivity contacts. Another advantage of EBP lasers is that the maximum conversion efficiency of electron beam energy to light is of ~30%. The possibility of using ZnSe-based quantum-size heterostructures for EBP green lasers demonstrated during last decade. Thus, room-temperature lasing at Ee<5keV and extremely low threshold current density jth=0.4-0.5 A/cm 2 (at Ee=8-10keV) have been demonstrated in laser heterostructures with short-period superlattice (SL) waveguide and single CdSe/ZnSe quantum dot (QD) active region [1]. The structure with three ZnSe/CdSe QD sheets arranged within 0.4um-thick extended SL waveguide has demonstrated laser generation with efficiency (per one facet) as high as ~4% [2]. Here we present the latest results on fabrication and study of ZnSe-based EBP laser heterostructure with multiple CdSe/ZnSe QD sheets. Laser heterostructure has been grown by molecular beam epitaxy pseudomorphically to GaAs (001) substrate and consists of 1.6μmthick ZnMgSSe cladding, active region with ten CdSe/ZnSe QD sheets equally spaced within a 0.6um-thick ZnSe/ZnSSe SL waveguide and cap 20-nm ZnMgSSe layer. The nominal thickness of CdSe QD layers was chosen to ~2.5 monolayers. All measurements were carried out at room temperature in transverse excitation geometry in pulse mode at electron beam energy Ee=10-30 keV. The lasing has been observed at the wavelength of ~542nm (T=300K) with FWHM of the emission line of ~2-3 nm. The maximum achieved values of pulse output power ~12W per facet has been registered at Ee=23keV. Maximum quantum efficiency of ~8.5% per facet has been obtained at the same electron beam energy and electron beam current Ie~5.7mA. The main reason of increase of laser efficiency seems to be more effective utilization of pumping energy by means of adjustment of both structure geometric parameters and electron beam penetration depth into the sample. Further increase of light output power is likely can be achieved by increasing Ie keeping constant electron beam energy Ee. The question on optimum number of active CdSe QD layers will be discussed. This work was partly supported by FASI Contract and ISTC Project No. 3754.

Cathodoluminescence of laser AIIBVI heterostructures

A Cathodoluminescence (CL) method has been used for studies of ZnSe-based heterostructures with a CdSe fractional-monolayer recombination region. Electron beams with energies of 1–25 keV provide the ability to analyze CL bands associated with different layers of the heterostructure and revealed on the CL spectrum at different electron-beam penetration depth. Comparative analysis of the CL bands both from the recombination region and the upper layers has been used to characterize the transport properties of the structure. The correlation between the density and size distribution of quantum dots and full width at half maximum and spectral position of the CL bands has been studied in detail.

Luminescence of bound excitons in epitaxial ZnO thin films grown by plasma-assisted molecular beam epitaxy

Luminescence properties of ZnO films, which have been epitaxially grown on c-sapphire (0001) substrates by plasma-assisted molecular beam epitaxy, are investigated by means of different excitation sources and their measurement conditions. With the increase of measurement temperature, photoluminescence spectra clearly present, the appearance of different bound-exciton peaks (I10 line) with an abrupt increase of emission intensity at the measurement temperature of 30-50K. Hypothetical explanations on the basis of thermalization effects, vibronic/rotational resonance states, and the involvement of the B-valence level in emission are given. In cathodoluminescence (CL), the deep level emission intensity was enlarged with the electron beam penetration depth due to the higher defect density near the interface between ZnO and the sapphire. From the CL image of the ZnO film, the dislocation density was estimated as 6×108-3×109∕cm2. The lasing phenomenon was observed at the threshold power density of 1.3MW∕cm2 at 300K.

The influence of processing parameters on luminescent oxides produced by combustion synthesis

Rare-earth activated oxide phosphors have application in high energy photoluminescent (plasma panels) and cathodoluminescent (field emission devices) flat panel displays. These phosphors are composed of a highly insulating host lattice with fluorescence arising from the 3d! 3d, 5d! 4f or 4f! 4f transitions in transition metal or rare earth ions. Fabrication of complex host compositions Y 2SiO5 ,Y 3Al5O12, Y2O3, and BaMgAl10O27 along with controlled amounts of the activators (Cr 31 ,M n 21 ,C e 31 ,E u 21 ,E u 31 ,T b 31 ,T m 31 ) represent a challenge to the materials synthesis community. High purity, compositionally uniform, single phase, small and uniform particle size powders are required for high resolution and high luminous efficiency in the new flat panel display developments. This paper will review the synthesis techniques and present physical and luminescent data on the resulting materials. q 1999 Published by Elsevier Science B.V. All rights reserved.

Photo- and cathodoluminescence of ZnSSe quantum well heterostructures grown by MOVPE

Lateral- and depth-resolved cathodoluminescence as well as photoluminescence of MOVPE-grown single- and multi quantum well ZnSSe-based heterostructures in the temperature range of 14–300 K were investigated. The intensity ratios between different emission peaks and their dependencies on the electron penetration depth together with the temperature dependence of photoluminescence spectra permit concluding that the transport of non-equilibrium carriers or excitons in the structure is hindered by potential fluctuations created by microvariations of the sulphur content. The deep centres are localized mostly in the near-substrate region which is indicated by an increase of the self-activated emission at 2.2 eV with increasing electron beam penetration depth. Further improvement of layer homogeneity as well as a sufficient distance between the quantum well and the substrate together with minimization of the interlayer strain is necessary to improve the radiative properties of ZnSe-based heterostructures.

Depth-Resolved Cathodoluminescence in GaAs Epilayers Grown on Si Substrates

Cathodoluminescence is used to study defects in undoped GaAs epilayers grown on Si. The GaAs epilayers include in situ thermal cycle (TC) processes and/or InGaAs-GaAs strained-layer superlattices (SLS's). In an in situ TC sample, the density of dark-spot defects is proved to increase with electron beam energy. Under the condition that the electron beam penetration depth reaches the position of the in situ TC, weak crosshatching contrasts appear, which are thought to be due to the movement and arrangement of 60°-type dislocations along the {111} planes during thermal cycle processes. In the sample with an in situ TC followed by SLS growth, distinct dark nonradiative recombination lines along <011 > directions appear in the luminescence image of the top GaAs layer. These <011 > aligned dark lines are thought to be caused by dislocations lying near the SLS/GaAs interface.

LEED Investigation on Temperature Dependence of Sublattice Magnetization of NiO (001) Surface Layers

Temperature dependence of the exchange diffraction spot intensity from the cleaved face (001) of nickel oxide is measured for the cases of very small and comparatively large penetration depth of electron beam concerning the surface barrier resonance in LEED. The exchange reflexion intensity is corrected for the Debye-Waller factors in order to reveal the temperature dependence of the sublattice magnetization. The effective critical exponent of the sublattice magnetization is determined to be 0.89±0.01 as the proper value for the top-most surface layer. The thickness of the surface layers, which is characterized by the same value of the critical exponent as the top-most surface layer, becomes thicker than the mean absorption distance when the temperature is raised up to 0.9 in T / T N .

Electron-beam penetration in GaAs

Measurements of the primary electron-beam penetration depth R in single-crystal GaAs have been made over the electron-beam energy range of 3–17 kV. The penetration depth is defined as the GaAs film thickness at which primary electrons of a given energy Ep just begin to emerge from the exit surface of the film with an energy of several hundred eV. The relationship between R and Ep is R=a(Ep)b μm, with a=0.027±0.003 and b=1.46±0.05. Ep is given in kV.