keV Electron Irradiation Research Articles

In situ TEM observation of decomposition of high-purity sapphire

The decomposition of α-Al2O3 under 200 keV electron irradiation has been investigated by in situ high-resolution electron microscopy (HREM). It was confirmed that aluminium precipitated from α-Al2O3 under 200 keV electron irradiation for less than 1 min over the temperature range from 700 to 1273 K. The electron dose rate was of the order of 1023 e m−2 s−1 and the vacuum level of the microscope was better than 10−6 Pa. The mechanisms of α-Al2O3 decomposition were discussed based on two possible decomposition models: the thermally activated atom movement and the forced atom displacement.

Charge kinetics in spinel and yttria-stabilized zirconia

Spinel and zirconia were studied by measuring the total secondary electron emission (SEE) yield σ in a dedicated scanning electron microscope (SEM) especially equipped to study the fundamental aspects of the charge trapping in insulating materials during a 1.1 keV electron irradiation at room temperature. The variation of the total SEE yield with the injected dose for both spinel and zirconia is different. In spinel the coefficient σ starts from its intrinsic value σ 0 = 4 and reaches a plateau at σ = 1 at the end of irradiation, which corresponds to the self-regulated regime. The continuity of the curves, shot after shot, proves that the trapped charges are stable and does not spread out in the material as injection proceeds. In this case spinel is called “trapper insulator”. In contrast with the spinel, σ in zirconia, never reaches unity while the injected charge increases: it evolves from its intrinsic yield σ 0 = 2.3 to a steady value a few percent above 1. The curve shows the relaxation of the positive generated charge. In this case zirconia is called “conductive insulator”. The difference in the charging kinetics of the two materials is attributed to the difference in conductivities.

Mechanisms of unexpected reduction in hole concentration in Al-doped 4H-SiC by 200 keV electron irradiation

The hole concentration in Al-doped p-type 4H-SiC was found to be significantly reduced by electron irradiation when compared to the hole concentration in Al-doped p-type Si; this is an unexpected result. The temperature dependence of the hole concentration p(T) in Al-doped 4H-SiC irradiated with 200 keV electrons at various fluences was measured. Only substitutional C atoms in SiC can be displaced by irradiation with 200 keV electrons. The reduction in p(T) by the electron irradiation was found to be mainly due to a decrease in Al acceptors and not due to an increase in defects (e.g., C vacancies) located around the middle of the bandgap in SiC. Based on the analysis of p(T), two types of acceptor species were detected and the density and energy level of each acceptor species were determined. An equation describing the fluence dependence of each acceptor density is proposed. The results suggest that for the 200 keV electron irradiation, a substitutional C atom bonded to an Al acceptor was more easily displaced than a substitutional C atom bonded to four Si atoms.

Electron-stimulated desorption from the surface of alumina ceramics

The changes in the surface composition of UV-46, VK 94-1, and VK 100-1 alumina ceramics and leucosapphire under (1–2)-keV electron irradiation with doses D up to 4 × 1020 electrons cm−2 have been investigated by low-energy electron spectroscopy. The effect of the bulk composition of the ceramics and some other factors on the surface oxides destruction rate is shown.

Electron irradiation of solid nitrous oxide

We report the results of chemical processing induced by 1 keV electron irradiation of a solid N 2O ice sample formed at 25 K. Ozone and several oxides of nitrogen (NO 2, N 2O 2, N 2O 3, N 2O 4 and N 2O 5) were observed to form during the irradiation period. Such reactions have important implications for the ice chemistry of outer solar system planets/satellites and interstellar ices.

Electric Property Control of Carbon Nanotubes by Defects

Effects of local low-energy irradiation on the electric properties of metallic single-walled carbon nanotubes were studied. Defects formed by 20 keV-electron irradiation in an electron beam lithography system converted the room-temperature electric properties to p-type or ambipolar semiconducting. Coulomb oscillation was also observed at room temperature. The results also suggest that electric measurements are inconclusive for distinguishing whether a nanotube is metallic or semiconducting.

Temperature‐dependent Formation of Ozone in Solid Oxygen by 5 keV Electron Irradiation and Implications for Solar System Ices

Wehaveinvestigatedtheformationofozonebyelectronimpactinsolidmolecularoxygenicesbetween11and30K. The amount of ozone formed is shown to be strongly dependent on the sample temperature. As the ice temperature increases,thecolumndensityoftheozonemonomerisfoundtodiminish.Thisisascribedtothelossofoxygenatomsby recombinationwith aneighboring oxygen atom to ‘‘recycle’’ molecular oxygen.In the ‘‘warm-up’’ phase after irradiation, two additional temperature-dependent reaction mechanisms were observed to synthesize ozone: (1) a reaction of oxygen atoms from a [O3 .. .O] complex with a neighboring oxygen molecule and (2) a reaction of trapped oxygen atoms with oxygenmoleculestoyieldtheozonemonomer.Theseexperimentshaveimportantimplicationstotheoxygenchemistryin icy satellites throughout our solar system. Subject headingg astrochemistry — ISM: molecules — methods: laboratory — molecular processes — planets and satellites: general

Mechanisms of Decrease in Hole Concentration in Al-Doped 4H-SiC by Irradiation of 200 keV Electrons

From the temperature dependence of the hole concentration p(T) in a lightly Al-doped 4H-SiC epilayer irradiated with several fluences of 200 keV electrons, the density of Al acceptors with 0.2 V E + eV decreases significantly with increasing fluence, whereas the density of unknown defects with 0.37 V E + eV increases with fluence and then decreases slightly. Although only C vacancies increase with fluence because 200 keV electrons can displace only C atoms, only the increase in the density of C monovacancies cannot explain the changes of p(T) by 200 keV electron irradiation. It may be necessary to consider the relationship between C vacancies and Al acceptors.

Capacitance-voltage characterization of electron beam induced surface cross-linked functional monomers

A metal-insulator-semiconductor (MIS) capacitor was fabricated using surface cross-linked functional monomers on silicon substrate. Functional monomer: multifunctional acrylates with 3:1 composition were spin coated and later cross-linked under the influence of keV electron irradiation on the surface of silicon to generate a surface-anchored cross-linked network bearing functional moieties. Capacitance-voltage (CV) measurements were carried out before and after electron irradiation. Doping concentration and bulk potential were calculated from the CV curve.

Low-Energy Irradiation Damage in Single-Walled Carbon Nanotubes

AbstractWe show that low-energy (20 eV–20 keV) electron or photon irradiation extinguishes the characteristic physical and chemical properties of single-walled carbon nanotubes, indicating that the irradiation damages the nanotubes. The irradiation-induced defects convert the electric properties of metallic SWNTs to semiconducting, and the nominal bandgap can be tuned simply by the irradiation dose. The defects also have the following interesting properties. The damage and recovery are reversible, indicating that the number of carbon atoms is preserved. The damage and recovery strongly depend on the diameter, suggesting that the damage is prominent in a rolled up graphene sheet, but not in a planar one. The activation energy of the defect healing is so small, depending on the diameter, that the defects can be healed even at room temperature or below.

Characteristics of Radiation-induced Defects in Fluorite Structure Oxides Formed by Electron Irradiation

AbstractOxide ceramics with fluorite structure attracts considerable attention as a host of inert matrix fuels and a transmutation target for minor actinides and long life fission products. We will report the characteristic features of defect clusters in oxide ceramics with fluorite structure, such as yttria stabilized zirconia (YSZ: 13 mol% Y2O3-ZrO2), ceria (CeO2), which were investigated in situ by transmission electron microscopy under electron irradiation. Results on calcium fluorite (CaF2) will be also shown for a comparison [1]. Electron irradiation with 100 to 1000 keV was found to induce large defect clusters in YSZ, which accompany strong black/black lobes contrast and multiply dislocation networks when they grow to a critical diameter of about 1.0 ìm. The defect clusters were considered to be charged oxygen platelets formed by the selective displacement damage of oxygen sublattice. A large difference in mass of constituent ions (O and Zr/Y ions) attributes the selective displacement damage under electron irradiation [2]. Similar defect clusters with strong diffraction contrast were formed in CaF2 under 200 keV electron irradiation, and they also multiplied dislocations at a critical diameter [1]. In the case of CeO2, a higher density of defect clusters was formed compared with YSZ and CaF2. Defect clusters formed under electron irradiation less than 1250 keV were found to be faulted-prismatic-interstitial loops lying on {111} planes with Burgers vector parallel to <111> directions [3], whereas electron irradiation with 1500, 2000 and 3000 keV induces perfect dislocation loops of 1/2<110>{110} in nature. Formation of radiation-induced defects is found to depend on electron energy for crystals with fluorite structure investigated in this study. Results will be discussed through selective displacement damage and charge accumulation. This work was financially supported by the Budget for Nuclear Research of MEXT, based on the screening and counseling by the Atomic Energy Commission, and by the Grant-in-Aid for Scientific Research (B) from JSPS. [1] M. Watanabe, T. Noma, K. Yasuda, K. Yasunaga, S. Matsumura and C. Kinoshita, Proc. of 16th Electron Mmicroscopy Congress, (2006) 1854. [2] K. Yasuda, C. Kinoshita, S. Matsumura, A.I. Ryazanov, J. Nucl. Mater., 319 (2003) 74. [3] K. Yasunaga, K. Yasuda, S. Matsumura and T. Sonoda, Nucl. Instr. and Meth. B, 250 (2006) 114.

Process of the One-Dimensional Motion of Small Interstitial-Type Dislocation Loops in Iron

Extensive simulations based on classical molecular dynamics have shown that small interstitial-type perfect dislocation loops in various metals and alloys have the structure of bundles of crowdions and a loop can easily makes the one-dimensional glide motion due to almost independent motion of crowdions in the loop. However, the experimental knowledge on the motion of loops is not enough. The present study dynamically examined the motion process of loops in pure iron under 1000 keV electron irradiation and thermal annealing by using transmission electron microscopy under which loops could move. Two types of loops were formed by irradiation. Loops of one type possessed the Burgers vector of 1/2<111> and the habit plane of {011}, and loops of the other type were <001> {001}. Loops of the former type made back-andforth glide motion and expansion towards the direction along their Burgers vectors when they were smaller than about a few-ten nanometers in diameter. This strongly suggests that these small 1/2<111> loops have the structure of the bundle of crowdions. Loops of the latter type only rarely moved less frequently when they were smaller than about the same size. When loops of two types grew larger than about 50 nm, the characteristics of the motion of loops changed drastically. Dislocation segments of each large loop made long-distance glide independently of their opposite segments, and the habit plane deviated from the original ones. This kind of motion means that selfinterstitial atoms at the central region of such large loops are no longer the crowdions.

Deep Levels in As-Grown and Electron-Irradiated P-type 4H-SiC

AbstractDeep levels in as-grown and electron-irradiated p-type 4H-SiC have been investigated by deep level transient spectroscopy (DLTS). Three hole traps, namely HK2, HK3, and HK4, could be detected in the temperature range from 350K to 700K. Activation energies of the hole traps were estimated to be 0.84 eV for HK2, 1.27 eV for HK3, and 1.44 eV for HK4 from the Arrhenius plot of emission-time constants assuming temperature-independent capture cross section. By double-correlated DLTS (DDLTS), they were revealed to be donor-like (+/0) traps. The concentrations of HK3 and HK4 centers were clearly increased by low-energy (116 keV) electron irradiation. Based on thermal stability of the HK3 and HK4 centers up to 1350°C and the dependence of HK4 concentration on the electron fluence, they may originate from a complex including defect(s) caused by carbon displacement.

Electron beam induced surface cross-linking of functional monomers coated on silicon substrate

A 3:1 composition of functional monomer:multifunctional acrylate was spin coated and later cross-linked under the influence of keV electron irradiation on the surface of silicon to generate a surface-anchored cross-linked network bearing functional moieties. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) as well as wetting angle measurements were used for the analysis of functional monomer cross-linked surfaces. Results of the surface reconstruction of surfaces and electron irradiated on coated silicon wafers reveal that long-term hydrophilic surfaces can be achieved. Thus, the surface architecture can be favorably manipulated by using this remarkable technique with a suitable combination of functional monomers and cross-linkers.

Chain-oxygen ordering in twin-freeYBa2Cu3O7−δsingle crystals driven by20−keVelectron irradiation

We have examined the effects of 20 keV electron irradiation on [-Cu(1)-O(1)-]n chain oxygen arrangements in oxygen deficient but otherwise twin-free YBa2Cu3O6+x single crystals. Comparison of polarized Raman spectra of non-irradiated and irradiated areas provides evidence that electron bombardments instigate the collective hopping of oxygen atoms either from an interstitial at O(5) site to a vacant O(1) chain site or by reshuffling the chain segments to extend the average length of chains without changing the overall oxygen content. This oxygen ordering effect, while counter-intuitive, is analogous to that found in the photoexcitation induced ordering in which temporal charge imbalance from electron-hole pair creation by inelastic scattering of incident electrons causes a local lattice distortion which brings on the atomic rearrangements.

Atomic force microscopy studies of alkali halide surfaces nanostructured by DIET

We report on surface topography modification of single crystal alkali halides due to creation of the excitonic states by keV electron irradiation. The DIET—structured surfaces have been studied with nanometer scale resolution by means of a dynamic (non-contact) atomic force microscopy (DFM) in UHV. The force microscopy studies reveal that randomly spread rectangular pits of monolayer depth in the topmost layer of the crystal are formed during irradiation. Growth and coalescence of the pits lead to almost layer-by-layer desorption mode. It is demonstrated that varying surface topography affects the yield of both the halogen and the alkali atom desorption component, as well as velocity spectrum of desorbing halogen atoms (thermal versus non-thermal ratio). We propose a model in which periodic changes of the surface topography with the increasing electron fluence (from initially flat to rough at about half monolayer desorbed, back to flat after a complete monolayer removal) are modulating the surface recombination probability for the excited F-centers. By controlling the population of traps in the bulk these surface processes are causing modulation of the diffusion range of mobile defects migrating from the bulk of the material towards its surface and the bulk recombination probabilities of F- and H-centres.

Response of CR-39 to medium energy electron irradiation

The effect of 5–15 keV electron irradiation on the etch rate of CR-39 nuclear track detectors has been investigated for surface doses from 0.09 to 13.85 J / cm 2 . Even before etching, electrons produce a surface depression proportional to the surface dose raised to the power 0.78 ± 0.01 . Etch rate enhancement was observed in the surface region (about 4 μ m deep) and was well approximated by a Gaussian function of depth. The depth and thickness of the etch rate enhancement increase with increasing electron energy, but are independent of dose. The maximum etch rate is proportional to the peak volumetric dose raised to the power of 1.45 ± 0.06 , independent of electron energy. No signs of a saturated response were found in the etch rate. This enhanced etching rate may significantly affect nuclear track identification in situations with an electron background.

Electron-irradiation-induced phase separation in GaSb nanoparticles

Electron irradiation effects in GaSb compound nanoparticles have been studied by transmission electron microscopy, in order to examine structural stability under electron irradiation in a nanometer-sized system. It is revealed that when 75 keV electron irradiation was carried out in approximately (10--25)-nm-sized GaSb particles kept at 423 K, a phase separation of the GaSb compound was induced to form a two-phase structure consisting of a crystalline antimony core and a liquid gallium shell. The phase separation was induced neither by 75 keV electron irradiation in GaSb nanoparticles kept at more than 443 K or less than 363 K nor by 200 keV electron irradiation in particles kept from 363 to 443 K. It is suggested that the phase separation in GaSb nanoparticles may be induced by electronic excitations rather than knock-on displacements.

Calibration of thin-film dosimeters irradiated with 80–120 kev electrons

A method for calibration of thin-film dosimeters irradiated with 80–120 keV electrons has been developed. The method is based on measurement of dose with a totally absorbing graphite calorimeter, and conversion of dose in the graphite calorimeter to dose in the film dosimeter by Monte Carlo calculations. A thermal model was developed to estimate the temperature contributions from the air above the calorimeter that is heated by the electron beam. As an example, Risø B3 thin-film dosimeters were calibrated by 80–120 keV electron irradiation and compared with a calibration carried out at 10 MeV electron irradiation. The two calibrations were found to be equal within the estimated uncertainties of ±10% at 1 s.d.

Creation of metastable defects in microcrystalline silicon films by keV electron irradiation

The effect of electron irradiation with 40 keV electrons on the sub-gap optical absorption, dark conductivity and photoconductivity of μc-Si:H films has been investigated. The experiments are carried out on undoped μc-Si:H film, prepared on quartz substrate by electron cyclotron resonance chemical vapor deposition (ECRCVD). The absorption coefficient in the sub-gap region ( hν<1.2 eV) as well as the photoconductivity of μc-Si:H films change considerably after electron irradiation. Annealing of the irradiated films for 1 h at 180 °C temperature restored the initial state. The observed parameter changes are attributed to the creation of metastable defects (silicon dangling bonds) in the disordered phase of the material near the surfaces of the microcrystalline columns in μc-Si:H films induced by electron irradiation. The inversely proportional dependence of photoconductivity on the concentration of these defects suggests that the dangling bonds created by electron irradiation are the dominant recombination centers in electron irradiated μc-Si:H films.