Effect of N_2 and O_2 on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X)(0–0)band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source.The effect of the addition of molecular gases N_2 and O_2 to He plasma jets on OH generation was studied.Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species.Stark broadening of the hydrogen Balmer emission line(H_β)was used to estimate the electron density nein the jets.For both He/N_2 and He/O_2 jets, newas estimated to be on the order of 10~(15)cm~(-3).The effects of plasma power and gas flow rate were also studied.With increase in N_2 and O_2 flow rates, netended to decrease.Gas temperature in the He/O_2 plasma jets was elevated compared to the temperatures in the pure He and He/N_2 plasma jets.The highest OH densities in the He/N_2 and He/O_2 plasma jets were determined to be 1.0?×10~(16)molecules/cm~3 at x?=?4 mm(from the jet orifice)and 1.8?×?10~(16)molecules/cm~3 at x=3 mm, respectively.Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions.The presence of strong emissions of the N_2~+ bands in both He/N_2 and He/O_2 plasma jets, as against the absence of the N_2~+ emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of N_2~+ in these He plasma jets.     

Investigation of in-reactor creep and irradiation growth of zirconium-2.5 wt% niobium channel tubes

We summarize the diametral creep results obtained in the MR reactor of the Kurchatov Institute of Atomic Energy on zirconium-2.5 wt% niobium pressure tubes of the type used in RBMK-1000 power reactors. The experiments that lasted up to 30 000 h cover a temperature range of 270 to 350°C, neutron fluxes between 0.6 and 4.0 ×10¹³ n/cm² · s (E > 1 MeV) and stresses of up to 16 kgf/mm². Diametral strains of up to 4.8% have been measured. In-reactor creep results have been analyzed in terms of thermal and irradiation creep components assuming them to be additive. The thermal creep rate is given by a relationship of the type ε_th = A₁ exp [(A₂ + A _3σt) T] and the irradiation component by ε_rad = A_4σtø(T − A₅), where T = temperature, σ_t = hoop stress, ø = neutron flux and a₁ to A₅ are constants. Irradiation growth experiments carried out at 280° C on specimens machined from pressure tubes showed a non-linear dependence of growth strain on neutron fluence up to neutron fluences of 5 × 10²⁰ n/cm². The significance of these results to the elongation of RBMK reactor pressure tubes is discussed.     

Z variations in yields of multicharged ions scattered and recoiled from a silicon surface

The doubly charged and triply charged ion yields from keV ion-silicon surface scattering are found to have a strong dependence on the atomic number Z₁ of the incident ion. For Z₁ < Z₂ the yield of scattered multicharged ions increases with Z₁, so that these ions dominate the recoiling Si²⁺ and Si³⁺ ions by Z₁ = 9. In contrast, when Z₁ > Z₂, there are large yields of Si²⁺ and Si³⁺ ions, and the multicharged scattered ion yields are too small to detect. The interaction radius at which shell vacancies are produced is also found to change, suggesting that electron promotion occurs at a different level crossing on either side of Z₁ = Z₂.     

Synthesis of carbon-nitride films using a fast-switched dual-source low energy ion beam deposition system

Thin C_xN_y films were deposited in UHV using alternating low energy ion beams of C⁺ and N⁺ or N₂⁺ in the energy range of 5 to 100 eV. The ion beam deposition system is equipped with two Freeman ion sources, mass analysis and fast automated beam switching, allowing perpendicular bombardment of the target with a single ion beam at a time. The composition and density of the films were studied by ARS (in situ), XPS and RBS. The dependence of the film properties and growth mechanisms on ion energy, beam switching rate, and C-to-N arrival ratio have been investigated. The influence of the deposition parameters on the film stoichiometry is discussed. Exposure of the film to atmosphere leads to oxygen incorporation, resulting in a lowered surface concentration of nitrogen. The XPS N 1s and C Is binding energies vary in a relatively broad range indicating that several bond states may be present. The influence of the substrate material on film growth has also been studied. On Si{100}, film growth commences with the formation of an interfacial silicon nitride. No film growth was observed on gold, however deposition was possible on tantalum and molybdenum.     

Characterization of extended defects in SiGe alloys formed by high dose Ge implantation into Si

The synthesis of SiGe/Si heterostructures by Ge⁺ ion implantation is reported. 400 keV Ge⁺ ions were implanted at doses ranging from 3 × 10¹⁶ to 10 × 10¹⁶ ions/cm² into (001) Si wafers, followed by Si⁺ amorphisation and low temperature Solid Phase Epitaxial Regrowth (SPER). TEM investigations show that strained alloys can be fabricated if the elastic strain energy (E_el) of the SiGe layer does not exceed a critical value (E′_el) of about 300 mJ/m², which is independent of the implantation energy. Our analysis also suggests that “hairpin” dislocations are formed as strain relieving defects in relaxed structures. A “strain relaxation” model is proposed to explain their formation.     

Calculated molecular mean excitation energies for some small molecules

Using the random phase approximation, we have calculated the total and directional components of the mean excitation energy for stopping (I₀) and the stopping anisotropy ( ) as well as the first moment (I₁) of the dipole oscillator strength distribution for 20 small molecules and molecular ions containing 6 to 30 electrons. We find that for excitations polarized orthogonal (perpendicular) and parallel to the molecular high symmetry axis I₀^O > I₀^P for all linear molecules, while I₁^O < I₁^P for all molecules other than HF. We note that the Bloch rule holds only for small, compact molecules, and that the Bragg rule should not be used to determine the mean excitation energies of the molecules that we consider here. The stopping anisotropy is positive for all but the C_3v molecules, and is largest for the linear, rod-shaped molecules.     

Damage production in GaAs during MeV ion implantation

The influence of the nuclear and electronic energy loss on the damage production in GaAs has been studied by Se⁺ ion implantation at T_I = 293 K with energies ranging from 2 MeV up to 20 MeV. The ion dose was varied between 5 × 10¹² /cm² and 1 × 10¹⁵ /cm². The damage production was investigated using RBS in channeling regime. Temperature and energy dependent backscattering measurements and TEM investigations were performed to study the kind of defects in more detail. The resulting defect profiles are compared with the depth distribution of the nuclear and electronic energy loss which were simulated by TRIM 87. The results show that the remaining defect concentration strongly decreases with increasing implantation energy even if the same energy density is deposited into nuclear processes. We suppose, that the electronic energy loss increases the defect transformation and annealing during implantation at T_I = 293 K. The defects in the samples implanted with energies greater than 5 MeV are characterized as point defects, point defect clusters and small dislocation loops; the kind of defects are the same over the whole implantation depth and the existence of amorphous zones can be widely excluded.     

The kinetics and mechanism of the uranium-water vapour reaction — an evaluation of some published work

The published results of Grimes and Morris on the rate of the uranium-water vapour reaction which were obtained using interferometry have been recalculated using the best values derived from the literature for the complex refractive indices of uranium and uranium dioxide (3.1–3.91 for uranium and 2.2-0.51 for uranium dioxide). The kinetics have been described by Haycock's model and the linear rate constant is given by K₁ = 1.3 × 10⁴P^1/2_H2O exp( − 9.0 kcal/RT )mg U/cm² h, where P_H2O is the water vapour pressure in torr or K₁ = 3.48 × 10⁸r^1/2 exp( −14.1 kcal/RT)mg U/cm² h, where r is the fractional relative humidity, R is the gas constant and T is the absolute temperature.

A mechanism is described which accounts for the observed dependence of the rate of uranium-water vapour reaction on the square root of the water vapour pressure.     

Ion beam induced nucleation in amorphous GaAs layers during MeV implantation

To investigate the nonlinear dose dependence of the thickness of the recrystallized layer during ion beam induced epitaxial recrystallization at amorphous/crystalline interfaces GaAs samples were irradiated with 1.0 MeV Ar⁺, 1.6 MeV Ar⁺ or 2.5 MeV Kr⁺ ions using a dose rate of 1.4 × 10¹² cm⁻² s⁻¹ at temperatures between 50°C and 180°C. It has been found that the thickness of the recrystallized layer reaches a maximum value at T_max = 90°C and 135°C for the Ar⁺ and Kr⁺ implantations, respectively. This means that the crystallization rate deviates from an Arrhenius dependence due to ion beam induced nucleation and growth within the remaining amorphous layer. The size of the crystallites depends on the implantation dose. This nucleation and growth of the crystallites disturbes and at least blocks the interface movement because the remaining surface layer becomes polycrystalline. Choosing temperatures sufficiently below T_max the thickness of the recrystallized layer increases linearly with the implantation dose indicating that the irradiation temperature is too low for ion induced nucleation.     

RBS studies of the lattice damage caused by 1 Me V Si implantation into Al0.3Ga0.7As/GaAs superlattices at elevated temperature

The lattice damage accumulation in GaAs and Al_0.3Ga_0.7As/GaAs superlattices by 1 MeV Si⁺irradiation at room temperature and 350°C has been studied. For irradiations at 350°C, at lower doses the samples were almost defect-free after irradiation, while a large density of accumulated defects was induced at a higher dose. The critical dose above which the damage accumulation is more efficient is estimated to be 2 × 10¹⁵ + Si/cm² for GaAs, and is 5 × 10¹⁵ Si/cm² for Al_0.8Ga_0.7As/GaAs superlattice for implantation with 1.0 MeV Si ions at 350°C. The damage accumulation rate for 1 MeV Si ion implantation in Al_0.3Ga_0.7As/GaAs superlattice is less than that in GaAs.     

Channeling of low energy light and heavy ions in single-wall nanotubes

The Monte Carlo simulation program has been used to study low energy channeling in the single-wall nanotube and its rope, in comparisons between beam sizes and between light (He) and heavy (Ar) ions. The simulation mainly shows that the critical angle Ψ_C = 48 E^−1/2 (E is incident energy) for the He (light) ion channeling but Ψ_C = 18 E^−1/2 for the Ar (heavy) ion channeling, in the (17,0) zigzag single-wall nanotube. Thus, it might be found in the simulation that Ψ_C strongly depends on the ion mass.     

Determination des proprietes elastiques dynamiques du systeme UO2-SiO2 dans la gamme de temperatures 0–300°C

The complex elastic modulus E*(=E₁ + iE₂) of UO₂-SiO₂ with 64 mol% UO₂ in the range 0–300°C was measured by means of the resonant constrained bar technique. The modulus was found to be constant in the range investigated. Using the elastic moduli of UO₂ and SiO₂, the modulus of the sample were estimated theoretically and found to be close to the measured values; the theoretical model used can thus justifiably be adopted for other volume fractions.     

Heavy-ion-induced luminescence of amorphous SiO2 during nanoparticle formation

Silica glass was implanted with negative 60 keV Cu ions at an ion flux from 5 to 75 μA/cm² up to a fluence of 1 × 10¹⁷ ions/cm² at initial sample temperatures of 300, 573 and 773 K. Spectra of ion-induced photon emission (IIPE) were collected in situ in the range from 250 to 850 nm. Optical absorption spectra of implanted specimens were ex situ measured in the range from 190 to 2500 nm.

IIPE spectra showed a broad band centered around 560 nm (2.2 eV) that was assigned to Cu⁺ solutes. The band appeared at the onset of irradiation, increased in intensity up to a fluence of about 5 × 10¹⁵ ions/cm² and then gradually decreased indicating three stage of the ion beam synthesis of nanoclusters: accumulation of implants, nucleation and growth nanoclusters. The IIPE intensity normalized on the ion flux is independent on the ion flux below 20 μA/cm²at higher fluences. The intensity of the band increased with increasing samples temperature, when optical absorption spectra reveal the increase of Cu nanoparticles size.     

PIXE measurements of Kr-sputtered TiN coatings

Titanium nitride films of 30–300 nm thickness deposited via dc magnetron sputtering were irradiated with 150–700 keV Kr ions at fluences up to 2.1 × 10¹⁷ cm⁻². These films were then scanned with a well-collimated 400 keV proton beam and the X-ray yield of Ti was measured both in and outside the Kr beam spot. This procedure results in a precision determination of the average film thickness (± 1% in the case of tens of nm films). The PIXE results are found to be consistent with RBS data of the same specimens. Sputtering yields were determined from the variation of X-ray yields assuming unchanged Ti/N stoichiometry in the implanted area. For thick TiN films (d₀ > 100 nm) the sputtering yields are in good agreement with predictions of the collisional cascade model by Sigmund. In contrast, sputtering of thin layers (d₀ = 30 nm) depended sensitively on the ion energy, being a factor of 2 higher at 150 keV than at 500 keV.     

Ion beam mixing of titanium films on stainless steel

The ion mixing of Ti-steel bilayers with N⁺, Ar⁺, Ti⁺, Kr⁺ and Xe⁺ ions was investigated by means of Rutherford backscattering spectroscopy (RBS). The mixing rates exhibit a linear scaling with the deposited damage energy f_D. No correlation between the properties of the mixing ion and the mixing efficiency was found. The results are compared with the predictions of ballistic and thermal-spike models.     

Research of Si and GaAs diode structures by Ion Beam Induced Charge (IBIC) collection

A Si pn junction diode and a GaAs Schottky diode were prepared for studying the basic mechanism of charge collection followed by high energy charged particle incidence in order to improve the resistance against single event upset. A 2 μm wide and 20 μm long rectangular Al electrode attached to a circular Al electrode with a 50 μm diameter was made on a 2.5 μm thick epilayer (minority carrier density 2 × 10¹⁵ /cm³). Both a Schottky electrode of Al (5 μm × 110 μm) and two ohmic electrodes of AuGe/Ni (110 μm × 110 μm) were made on a 2 μm thick epilayer (7.3 × 10¹⁵ /cm³) grown on a semi insulator GaAs substrate (1 × 10⁷ Ω cm). The internal device structure was examined by the IBIC (Ion Beam Induced Charge) method using a 2 MeV He⁺ ion microbeam. IBIC images clearly show an Al electrode, the SiO₂, and an epilayer. These results were then used to improve the qualities of the test diodes.     

Effect of electric charge accumulation on low energy ion implantation in insulators

Charge accumulation at the surface of insulators during low energy ion implantation is related to two processes: ion impinging on the sample and secondary electron emission. Samples composed of a piece of Si (having the size of the ion beam) fixed on the centre of polyethylene (PE) coupons have been implanted with 2.2 keV H₂ ions to a fluence of 2 × 10¹⁶ H/cm². ERD (Elastic Recoil Detection) depth profiles of the implanted ions are shallower with an increase of the PE coupon size. The relative critical Si/PE size to repel all the incident ions is around 1.1 × 1.1 cm²/2.5 × 2.5 cm². The potential of the secondary electron suppressor has been varied from −500 V to +500 V. It changes the secondary electron distribution around the implanted area and, consequently, affects the accumulation of charges at the sample surface. When the potential is 0 V, a uniform ion implantation with little effect of charge accumulation for all sizes of PE coupons is obtained. A two-dimension model has been performed and gives a good explanation for the mechanism of the electric charge neutralisation.     

Energy window effect in chemisorption of C36 on diamond (0 0 1)-(2×1) surface

In this paper, the collision of a C₃₆, with D_6h symmetry, on diamond (0 0 1)-(2×1) surface was investigated using molecular dynamics (MD) simulation based on the semi-empirical Brenner potential. The incident kinetic energy of the C₃₆ ranges from 20 to 150 eV per cluster. The collision dynamics was investigated as a function of impact energy E_in. The C₃₆ cluster was first impacted towards the center of two dimers with a fixed orientation. It was found that when E_in was lower than 30 eV, C₃₆ bounces off the surface without breaking up. Increasing E_in to 30–45 eV, bonds were formed between C₃₆ and surface dimer atoms, and the adsorbed C₃₆ retained its original free-cluster structure. Around 50–60 eV, the C₃₆ rebounded from the surface with cage defects. Above 70 eV, fragmentation both in the cluster and on the surface was observed. Our simulation supported the experimental findings that during low-energy cluster beam deposition small fullerenes could keep their original structure after adsorption (i.e. the memory effect), if E_in is within a certain range. Furthermore, we found that the energy threshold for chemisorption is sensitive to the orientation of the incident C₃₆ and its impact position on the asymmetric surface.     

Argon irradiation damage at a Cu/Al2O3 interface for different alumina structures

The evolution of damages at a Cu/Al₂O₃ device interface after Ar⁺ irradiation, depending on alumina structure, and the effect of surface roughness on sputtering have been studied. A polycrystalline Cu/Al₂O₃ bilayer and polycrystalline Cu on amorphous alumina were irradiated with 400 keV Ar⁺ ion beam at doses ranging from 5 × 10¹⁶ to 10¹⁷ Ar⁺/cm² at room temperature. The copper layer thicknesses were between 100 and 200 nm. RBS analysis was used to characterize the interface modification and to deduce the sputtering yield of copper. The SEM technique was used to control the surface topography. A RBS computer simulation program was used to reproduce experimental spectra and to follow the concentration profile evolutions of different elements before and after ion irradiation. A modified TRIM calculation program which takes into account the sputtering yield evolution as well as the concentration variation versus dose gives a satisfactory reproduction of the experimental argon distribution. The surface roughness effect on sputtering and the alumina structure influence at the interface on mixing mechanisms are discussed.     

Ion implantation doping and electrical conductivity enhancement of C60 films

Pristine C₆₀ films sublimed onto sheet mica were implanted with 20 keV K⁺ ions and I⁺ ions at doses of 1.0 × 10¹⁶/cm², 3.0 × 10¹⁶/cm² and 5.0 × 10¹⁶/cm², and with 20 keV Ar⁺ ions at a dose of 5.0 × 10¹⁶/cm². The distributions of dopants were studied using Rutherford backscattering spectrometry (RBS). The temperature dependence of sheet resistivity of the films was investigated applying a four-probe system. It was proposed that the conductivity enhancement of K⁺ implanted C₆₀ films was due to the implanted ions in the films, while for I⁺ implanted C₆₀ films, both implanted I⁺ ions and irradiation effects of the ions contributed to the enhancement of conductivity.