Oxidation inhibition in bismuth films bombarded with nitrogen ions

Vacuum-evaporated bismuth films were bombarded with (non-penetrating) N⁺₂ ions and the oxidation behaviour of these films during a subsequent thermal treatment was studied. The incorporation of oxygen in the films at a fixed temperature was indirectly monitored by measuring the variation in the electrical resistance with the oxidation time. The oxygen content in the partially oxidized films was directly evaluated by Auger depth profiling. The results of such measurements clearly indicate a considerable reduction in the oxidation rate of the bombarded films. A possible mechanism for this effect is discussed.     

Potential sputtering of hydrogen atoms on various surfaces with slow highly-charged ions

Several recent findings on a new sputtering mechanism of hydrogen with slow highly-charged ions are discussed. The sputtering yields of protons were proportional to q^∼5 for q≲10 independent of the surface condition for both untreated and well-defined surfaces, where q is the charge state of the ion. This q^∼5 dependence started to level off for q≲10. The yield for the Si(100)1×1–H surface was ten times larger than that for the Si(100)2×1–H surface although the stoichiometric hydrogen abundance of the former is only twice that of the latter. The key quantity to govern the yield is proposed to be surface roughness, which also influences the energy distribution of sputtered protons. These findings were consistently explained with a pair-wise potential sputtering model involving two successive electron transfers which follow the classical over barrier mechanism.     

Damage buildup in semiconductors bombarded by low-energy ions

The damage buildup in Ge and GaAs bombarded at room and elevated temperatures by keV Ne⁺ and Ar⁺ ions has been studied. Results show that the accumulation of disorder can be considered as a planar growth of an amorphous layer proceeding from the surface. A retardation of amorphous layer growth is experimentally observed in Ge for low ion doses. Results (for the case of room temperature irradiation of GaAs and for all the cases of irradiation of Ge except for the case of irradiation by Ar⁺ ions with high ion flux density) are explained based on the diffusion of mobile point defects to the surface with a subsequent interface segregation process. Calculations based on this model are in good agreement with experimental data.     

Structural damage in ZnO bombarded by heavy ions

The effect of implantation parameters on damage build-up in ZnO bombarded with Bi and Er ions is studied by Rutherford backscattering/channelling spectrometry. The results show that the damage accumulation behaviour in ZnO is different dramatically from that in other semiconductors. In particular, a variation of implantation parameters, such as collision cascade density, sample temperature and ion flux, has only a minor influence on the damage accumulation in the crystal bulk for the case of such heavy ions. Moreover, an intermediate damage peak, between the surface and bulk defect peaks, is observed for all the irradiation conditions studied. The cascade density affects the behaviour of this intermediate peak with increasing ion dose.     

Radiation-induced hydrogen yield from silicon bombarded with 710-MeV bismuth ions

The behavior of hydrogen in solids under the action of intense ionizing radiation is of importance in solving numerous problems of materials science for the nuclear and thermonuclear power industry, spacecraft engineering, hydrogen energetics, and microelectronics. Experimental results are presented that provide evidence of the radiation-induced yield of hydrogen from single crystal silicon bombarded with 710-MeV bismuth ions.     

Twinning of bismuth single crystals bombarded by boron ions

Twinning of ion-bombarded single crystals has been investigated for the first time. It has been established that bombardment of bismuth single crystals with boron ions stimulates mobility of twinning dislocations and quenches their sources. This result is explained using the dislocation model of a wedge-shaped twin. Pis’ma Zh. Tekh. Fiz. 24, 1–9 (April 26, 1998)     

Electron and photon beams interacting with plasma

A comparison is made between the interaction of electron bunches and intense laser pulses with plasma. The laser pulse is modelled with photon kinetic theory, i.e. a representation of the electromagnetic field in terms of classical quasi-particles with space and wave number coordinates, which enables a direct comparison with the phase space evolution of the electron bunch. Analytical results are presented of the plasma waves excited by a propagating electron bunch or laser pulse, the motion of electrons or photons in these plasma waves and collective effects, which result from the self-consistent coupling of the particle and plasma wave dynamics.     

Infrared spectroscopy of a silicon surface bombarded by nitrogen ions

Transmission Fourier-transform infrared spectroscopy was used to study the formation of a ripple topology on a silicon surface bombarded by nitrogen ions, together with the formation of silicon nitride, the evolution of its composition and structure. For the first time, an attempt is made to study the evolution of the formation of a ripple topology on the surface of silicon by analyzing the main spectral characteristics (amplitude, position, and profile) of the infrared absorption bands. It is shown that the change in the profile of the characteristic absorption band and the position of its peak correlate with the characteristics of formation of the ripples on the silicon surface. It is demonstrated that infrared transmission spectroscopy can be used to study surface structuring processes for semiconductors bombarded by ions of chemically active elements. Pis’ma Zh. Tekh. Fiz. 24, 18–23 (March 26, 1998)     

Changes in silicon samples bombarded by 900–2300 eV hydrogen ions

We have performed a series of statistically designed experiments to evaluate the changes which occur in silicon samples as a result of hydrogen ion bombardment. Single-crystal silicon and polycrystalline silicon, and also edge-defined film-fed growth (EFG) solar cell devices, were exposed to a hydrogen ion beam produced by a Kaufman ion source. The experimental parameters which were systematically varied include the maximum hydrogen ion energy (900, 1600 and 2300 eV), the energy spread of the ions in the beam, the ion current density (0.8, 1.4 and 2.0 mA cm^?2), the total dose (1 × 10¹⁸, 2 × 10¹⁸ and 4 × 10¹⁸ ions cm^?2) and the bulk sample temperature during bombardment (200, 275 and 350 °C). We observed the changes in the short-circuit current, the open-circuit voltage, the photovoltaic conversion efficiency and the fill factor associated with the EFG devices, the change in the spectral reflectivity of the single-crystal silicon samples and the ratio of the numbers of SiH to SiH₂ groups present in the polycrystalline silicon samples. The results of our study indicate that the properties of hydrogen-ion-bombarded silicon change with both the maximum ion energy and the ion current density with a significant parametric interaction between the current density and the bulk sample temperature.The energy distribution of the ions present in the hydrogen ion beam affects the spectral reflectivity of single-crystal silicon. The changes associated with hydrogen ion bombardment are independent of dose for the conditions studied. The average absolute air mass 1 efficiency of EFG devices without intentionally applied antireflective coatings increased from 8.7% before exposure to 10.2% after exposure to the optimal conditions determined by this study.     

On mechanisms of defect formation in aluminum crystals bombarded by low-energy heavy ions

The mechanisms of formation of vacancies and radiation-induced adsorbed and interstitial atoms in the cascades of atomic collisions induced by 25-, 40-and 50-eV Ar and Xe ions normally incident onto the Al(100) surface at 300 K are discussed within the framework of the molecular dynamics approach. The numbers of bulk and surface vacancies formed in the course of a collision cascade exhibit two maxima in the case of Xe ions and a single maximum in the case of Ar ions.     

Molecular effect on surface topography of GaN bombarded with PF4 ions

We study surface topography and thickness of GaN layers implanted at room temperature with 1.3 keV/amu F, P, and PF₄ cluster ions. Results show that the density of collision cascades has a dramatic effect on the surface roughness and the thickness of implanted layers. Surface roughness increases with increasing cascade density. For very dense cascades produced by PF₄ ions, the evolution of layer thickness is dominated by ion-induced sputtering. In contrast, for the case of P ions producing less dense cascades, ion-induced swelling is observed.     

Stripping of fast helium-like ions in multiple collisions with diatomic molecules

Cross sections of single and double stripping of fast helium-like Fe²⁴⁺ ions upon collisions with N₂ molecules have been calculated by nonperturbative methods for the impinging ion energies of 10, 100, and 1000 MeV/nucleon. It is shown that an allowance for the multiple collisions leads to significant differences between the projectile stripping cross sections for the target molecules oriented parallel and perpendicular to the incident particle velocity, whereas for chaotic orientations, this difference is insignificant.     

Systematic analysis of neutron yields from thick targets bombarded by heavy ions and protons with moving source model

A simple phenomenological analysis using the moving source model has been performed on the neutron energy spectra produced by bombarding thick targets with high energy heavy ions which have been systematically measured at the Heavy-Ion Medical Accelerator (HIMAC) facility (located in Chiba, Japan) of the National Institute of Radiological Sciences (NIRS). For the bombardment of both heavy ions and protons in the energy region of 100–500 MeV per nucleon, the moving source model incorporating the knock-on process could be generally successful in reproducing the measured neutron spectra within a factor of two margin of accuracy. This phenomenological analytical equation is expressed having several parameters as functions of atomic number Z_p, mass number A_p, energy per nucleon E_p for projectile, and atomic number Z_T, mass number A_T for target. By inputting these basic data for projectile and target into this equation we can easily estimate the secondary neutron energy spectra at an emission angle of 0–90° for bombardment with heavy ions and protons in the aforementioned energy region. This method will be quite useful to estimate the neutron source term in the neutron shielding design of high energy proton and heavy ion accelerators.     

Atomic relocations in a two-layer Al/Ni system bombarded with ions having energies close to the sputtering threshold

Collision cascades developed at 300 K in a Ni single crystal and in a system comprising a monolayer of Al atoms on the Ni(100) single crystal face bombarded at normal incidence by 25-and 50-eV Ar and Xe atoms were simulated by method of molecular dynamics. It was found that the number of atoms relocated through the Al-Ni interface markedly exceeds the number of analogous relocations in the nickel single crystal.     

Defect formation in a two-layer Al/Ni crystal bombarded with ions having energies close to the sputtering threshold

Atomic collision cascades developed in an Al/Ni system, comprising a monolayer of Al atoms on the Ni(100) single crystal face bombarded at normal incidence by 25-or 50-eV Ar and Xe atoms, were simulated by means of molecular dynamics. It is shown that the number of Al atoms relocated within the first layer of the Al/Ni crystal markedly exceeds the number of analogous relocations in a nickel single crystal, which is explained by differences in the number of vacancies in the first layer of two systems, and in the lattice constants of Al and Ni.     

Electron beams formed in a diode filled with air or nitrogen at atmospheric pressure

We have studied the electron beam formation in a diode filled with a molecular gas at atmospheric pressure. A beam current amplitude of up to ∼20 A at an electron energy of ∼70 keV was obtained in an air-filled diode. It is suggested that the main fraction of runaway electrons at low initial values of the parameter E/p (∼0.1 kV/(cm Torr)) is formed in the space between cathode plasma and anode. As the plasma spreads from cathode to anode, the electric field strength between the plasma front and anode increases and the E/p value reaches a critical level.     

Electron dynamics during laser neutralization of negative ions

A hydrodynamic approach is used to study the electron dynamics when laser radiation acts on a negative ion beam. An analytic solution of the problem is obtained. Pis’ma Zh. Tekh. Fiz. 24, 38–42 (May 12, 1998)     

Nitrogen oxides removed from flue gases with sulfur dioxide under the action of pulsed electron beams

The process of a model flue gas mixture purification from nitrogen oxides by microsecond-pulsed electron beam ionization in the presence of sulfur dioxide (SO₂) was experimentally studied. In mixtures with a small SO₂ content, interaction of this component with nitrogen oxides leads to a considerable increase in the specific energy consumption required for the gas purification (∼80 eV per NO molecule). In a gas mixture with approximately equal concentrations of SO₂ and NO, the energy consumption for NO removal decreases to a level close to that in the mixture free of sulfur dioxide.     

Air entrainment in fresh concrete with PFA

The results of a study into the influence of PFA on air entrainment in fresh concrete are discussed It is shown that the required dosage of AEA to produce an air content of 5.5 ± 0.5% in a PFA modified concrete mix is two-six times that required in the corresponding neat OPC concrete mix. The dosage of a vinsol based air entraining agent (AEA) required appears to be directly related to the PFA content of the mix. Similar direct relationships were obtained with a range of different PFAs. The dosage of an AEA based on the salt of a fatty acid appears to be sensitive to both PFA and OPC contents. For the type of PFA used, the variability of measured air content or the amount of air retained after continued agitation both indicated that vinsol based AEAs show the highest variability whilst fatty acid based AEAs show low variability. The between batch variability of air content was significantly improved by the addition of PFA regardless of the AEA used.