8 MeV electron irradiation effects in silicon photo-detectors

Results of investigations on the electrical properties of n⁺-p-p⁺ silicon (Si) photo-detectors irradiated with 8 MeV electrons are presented. The photo-detectors were irradiated with electrons of doses up to 100 kGy. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics under dark conditions were measured as a function of dose. A significant change in the diffusion component of the saturation current is observed after irradiation, while the generation-recombination component of the saturation current remains almost unchanged. The series resistance is found to increase with increasing dose while the shunt resistance and carrier concentration decrease with dose. Optoelectronic properties, namely short circuit current I_sc, open circuit voltage V_oc under air mass zero illumination and spectral response, were measured at various doses. From the spectral responses of the devices, the minority carrier diffusion length was estimated.     

Ag/Fe:TiO2 nano-catalysts prepared by Fe ion implantation and Ag nanoparticle deposition by electron beam irradiation

TiO₂ nano-catalysts made by the sol-gel method were modified by ion implantation and electron beam irradiation to obtain a more efficient photocatalytic function. The results of photodegradation of methyl orange in aqueous solution demonstrate firstly that the films have a photocatalytic activity which responds to visible light. Secondly, it demonstrates that under ultraviolet excitation the sample with a fluence of 6 × 10¹⁵ ions/cm² and electron beam irradiated with concentration of AgNO₃ aqueous solution at 1 × 10⁻³ M gives a more efficient photodegradation ability than pure TiO₂ film and other Fe-doped films display almost the same photodegradation ability as TiO₂ film. Thirdly it demonstrates that under sunlight, all modified films exhibit more photodegradation activity than TiO₂ film.     

Studies of InP nano dots formation after keV Ar irradiation

Temporal evolution of nano dots fabricated, in off-normal geometry but in the absence of rotation, on InP(1 1 1) surfaces by 3 keV Ar ion sputtering is reported here. After 10 min of sputtering, self-assembled nano dots with mean diameter of 24 ± 4 nm display square short range weak ordering. Fully developed square celled arrays of dots with mean diameter of 90 ± 26 nm, are seen beyond the non-linear coarsening regime at the critical time of 40 min. Inverse coarsening of dots in conjunction with surface smoothening, never seen in earlier studies of dot evolution, is observed beyond the critical time.     

Effect of swift heavy ion irradiation on spray deposited CdX (X = S, Te) thin films

Cadmium sulfide and cadmium telluride thin films are irradiated with high energy heavy ion beam to study the irradiation induced effects in these films. The polycrystalline thin film samples deposited by spray pyrolysis are irradiated with 60 MeV Oxygen ions using tandem Pelletron accelerator. The X-ray diffraction patterns exhibit a reduction in peak intensities in both CdS and CdTe films. The grain size decrease with fluence is observed for both CdS and CdTe films, with more decrease for CdTe films. The AFM results support this observation. The films show opposite trend in the variation of electrical resistivity with irradiation fluence. A decrease in resistivity is observed for CdS films due to an increase of carrier concentration arising by the creation of sulfur vacancies during the irradiation. The creation of sulfur vacancies is confirmed by XPS studies. The stoichiometric changes seen from XPS studies support this observation. An enhancement of grain boundary scattering due to the reduction of grain size leads to the increase of electrical resistivity for CdTe films.     

Gamma-rays irradiation effects on polysulfone at high temperature

The temperature dependence of the irradiation effects on polysulfone was studies by measuring the molecular weight, glass transition temperature, gel fraction and evolved gas. Polysulfone was irradiated with gamma-rays at room temperature, 100, 150, 180 and 210 °C. The change of molecular weight distribution and glass transition temperature showed occurrences of a main chain scission at room temperature and cross-linking at high temperature. The decrease of gel dose, the increases of gel fraction and total gas evolution with increasing temperature was observed. The evolution of CO, CO₂ and SO₂ gases increased at high temperature, while yield of evolved H₂ was independent of irradiation temperature. The probability of the cross-linking was clearly increased by irradiation at high temperature above 180 °C, though the chain scission was not changed very much.     

Luminescent nanoclusters array fabricated by pulsed laser beam irradiation

Ordered luminescent nanoclusters array in the form of grating structures are fabricated on silicon (1 0 0) surface by Q-switched Nd:YAG laser beam irradiation of second harmonic wavelength (532 nm) in vacuum. Blue-green photoluminescence (PL) spectrum from the ordered nanoclusters array exhibits two asymmetrical peaks at 2.58 eV and 2.88 eV in the blue-green region corresponding to the bimodal distribution of nano size clusters. The size of the nanoclusters is estimated from the three dimensional quantum-confined model incorporating Gaussian size distribution. When subjected to rapid thermal annealing at 710 °C for 10 min in N₂ atmosphere there is an enhancement of the PL intensity without any change in the peak emission energy and broadening suggesting that the origin of PL is related to quantum confinement effect in Si nanocrystallite. The surface morphology of the irradiated surface varies considerable with the number of laser shots, laser fluence and ambient conditions.     

Implantation effects of low energy H and D ions in germanium at −120 °C and room temperature

Germanium was implanted with 5 keV H and D ions at −120 °C or room temperature and thermally annealed in several steps. The samples were analysed at various stages by atomic force microscopy, ion channeling and Raman spectroscopy of Ge-H/D local vibration modes. The results are discussed in comparison with those in the well studied silicon. In general, the evolution of the different types of defects, in germanium at a given temperature, tends to be similar to that of the corresponding defects in silicon at 100-300 °C higher temperature. However, the behaviour of the defects detected by ion channeling (interstitials, lattice distortions) often appears unrelated to the chemical evolution measured by Raman scattering and to the temperature and isotope dependence of blistering.     

Barrier modification of Au/n-GaAs Schottky diode by swift heavy ion irradiation

The effect of swift heavy ion (72.5 MeV ⁵⁸Ni⁶⁺) irradiation on Au/n-GaAs Schottky barrier characteristics is studied using in situ current-voltage measurements. Diode parameters are found to vary as a function of ion irradiation fluence. The Schottky barrier height (SBH) is found to be 0.55(±0.01) eV for the as deposited diode, which decreases with ion irradiation fluence. The SBH decreases to a value of 0.49(±0.01) eV at the highest ion irradiation fluence of 5 × 10¹³ ions cm⁻². The ideality factor is found to be 2.48 for unirradiated diode, and it increases with irradiation to a value of 4.63 at the highest fluence. The modification in Schottky barrier characteristics is discussed considering the energy loss mechanism of swift heavy ion at the metal-semiconductor interface.     

Radiation effects on ohmic and Schottky contacts based on 4H and 6H-SiC

A systematic study of Ni based ohmic and Schottky contacts (SCs) onto the n-4H-SiC and n-6H-SiC under relatively low-dose (1 × 10¹² e⁻ cm⁻²) and high-energy (6, 12, 15 MeV) electron irradiation (HEEI) has been introduced. Lower specific contact resistivity has been reached for Ni based ohmic contact structures on both 4H and 6H-SiC after each electron irradiation. This finding has been explained by the displacement damage produced by the collision of electrons with atoms of Ni contact material. It has been observed that the HEEI caused to increase in the ideality factors of both SCs indicating deviation from thermionic emission theory in current transport mechanism. While the Schottky barrier height (SBH) for Ni/4H-SiC SC remains nearly constant, an increase has been observed for the Ni/6H-SiC SC. Donor concentrations for both diodes have decreased with increasing electron energy probably due to the trapping effect of the irradiation induced defect(s).     

Mechanisms of damage formation in semiconductors

The damage accumulation in ion-implanted semiconductors is analysed using Rutherford backscattering spectrometry (RBS). When energetic ions are implanted in a material, they transfer their energy mainly into atomic collision processes (nuclear energy loss) and in electronic excitations (electronic energy loss). For a given material this primary energy deposition is determined by the mass and energy of the implanted ions and the ion fluence (number of ions per unit area). However, the damage concentration which is measured after implantation does not only depend on the primary energy deposition, but is strongly influenced by secondary effects like defect annealing and defect transformation. For the latter processes the target temperature and the ion flux (number of ions per unit area and time) play an important role. In this presentation the influence of the various parameters mentioned above on the damage accumulation is demonstrated for various materials. Simple empirical models are applied to get information about the processes occurring and to systematize the results for the various semiconductors.     

Behavior of crystalline silicon under huge electronic excitations: A transient thermal spike description

Recent experimental works devoted to the phenomena of mixing observed at metallic multilayers Ni/Si irradiated by swift heavy ions irradiations make it necessary to revisit the insensibility of crystalline Si under huge electronic excitations. Knowing that Ni is an insensitive material, such observed mixing would exist only if Si is a sensitive material. In order to extend the study of swift heavy ion effects to semiconductor materials, the experimental results obtained in bulk silicon have been analyzed within the framework of the inelastic thermal spike model. Provided the quenching of a boiling (or vapor) phase is taken as the criterion of amorphization, the calculations with an electron-phonon coupling constant g(300 K) = 1.8 × 10¹² W/cm³/K and an electronic diffusivity D_e(300 K) = 80 cm²/s nicely reproduce the size of observed amorphous tracks as well as the electronic energy loss threshold value for their creation, assuming that they result from the quenching of the appearance of a boiling phase along the ion path. Using these parameters for Si in the case of a Ni/Si multilayer, the mixing observed experimentally can be well simulated by the inelastic thermal spike model extended to multilayers, assuming that this occurs in the molten phase created at the Ni interface by energy transfer from Si.     

Secondary electron yield of Au and Al2O3 surfaces from swift heavy ion impact in the 2.5-7.9 MeV/amu energy range

We report on the secondary electron yields of Au and oxidized aluminum (Al₂O₃) by impact of heavy ions with energies ranging from 7.92 MeV/amu (¹²C₆) to 2.54 MeV/amu (¹⁰⁷Ag₄₇). The obtained results, the first in this energy range using medium-heavy ions, extend the validity of proposed scaling laws obtained with lighter ions. Measurements have been performed using the SIRAD irradiation facility at the 15 MV Tandem of the INFN Laboratory of Legnaro (Italy), to evaluate the performance of ion electron emission microscopy at SIRAD.     

Beneficial effect of gamma irradiation on the N-deacetylation of chitin to form chitosan

The effect of gamma irradiation on the N-deacetylation of chitin to form chitosan was studied. Chitin from crab shells was irradiated up to 20 kGy and N-deacetylated in aqueous NaOH solution (40% and 60% w/w) at 60 and 100 °C for 60 min. The degree of N-deacetylation (DD) of non-irradiated and irradiated samples was determined by IR-band ratio method. It was found that higher extent of N-deacetylation was achieved for the chitin samples irradiated up to 20 kGy doses as compared to non-irradiated chitin. The DD values of chitin, prepared from non-irradiated and 20 kGy irradiated chitins by N-deacetylation at 60 °C with 40% NaOH for 60 min, were found to be 38% and 60%, respectively. The increase in DD by irradiation was interpreted as a result of reduction in molecular weight of chitin. Low dose irradiation of chitin has provided the possibility of its N-deacetylation into chitosan at much milder reaction conditions.     

380 keV proton irradiation effects on photoluminescence of Eu-doped GaN

The effect of 380 keV proton irradiation on the photoluminescence (PL) properties has been investigated for undoped and Eu-doped GaN. As the proton irradiation exceeds , a drastic decrease of PL intensity of the near band-edge emission of undoped GaN was observed. On the other hand, for Eu-doped GaN, the PL emission corresponding to the ⁵D₀→⁷F₂ transition in Eu³⁺ kept the initial PL intensity after the proton irradiation up to . Present results, together with our previous report on electron irradiation results, suggest that Eu-doped GaN is a strong candidate for light emitting devices in high irradiation environment.     

Effects of the density of collision cascades: Separating contributions from dynamic annealing and energy spikes

We present a quantitative model for the efficiency of the molecular effect in damage buildup in semiconductors. Our model takes into account only one mechanism of the dependence of damage buildup efficiency on the density of collision cascades: nonlinear energy spikes. In our three-dimensional analysis, the volume of each individual collision cascade is divided into small cubic cells, and the number of cells that have an average density of displacements above some threshold value is calculated. We assume that such cells experience a catastrophic crystalline-to-amorphous phase transition, while defects in the cells with lower displacement densities have perfect annihilation. For the two limiting cases of heavy (500 keV/atom ²⁰⁹Bi) and light (40 keV/atom ¹⁴N) ion bombardment of Si, theory predictions are in good agreement with experimental data for a threshold displacement density of 4.5 at.%. For intermediate density cascades produced by small 2.1 keV/amu PF_n clusters, we show that dynamic annealing processes entirely dominate cascade density effects for PF₂ ions, while energy spikes begin contributing in the case of PF₄ cluster bombardment.     

Study on chemical structures of poly (tetrafluoroethylene-co-perfluoroalkylvinylether) by soft-EB irradiation in solid and molten state

Poly (tetrafluoroethylene-co-perfluoroalkylvinylether) (PFA) was irradiated by soft electron beam (soft-EB) under nitrogen gas atmosphere in solid-state and its molten state, respectively. The changes of thermal property and chemical structures of irradiated PFA in solid-state and molten state were studied by differential scanning calorimetric analysis (DSC) and solid-state ¹⁹F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. By DSC analysis, the melting temperature shifted to lower temperatures, and crystallinity decreased with increasing soft-EB dose. By solid-state ¹⁹F MAS NMR spectroscopy, the new signals was observed and the detected new signals in irradiated PFA at 315 °C and at 30 °C were due to the tertiary carbon group with branching site (Y-type crosslinking site), perfluoro-propylene site and chain end methylene groups, respectively.Moreover, the molar ratio of perfluoroalkylvinylether (FAVE) structure to -CF₂- units decreased with increasing dose.     

Ion irradiation effects on surface mechanical behavior and shrinkage of hybrid sol-gel derived silicate thin films

A study of the effects of ion irradiation on the surface mechanical behavior and shrinkage of organic/inorganic modified silicate thin films was performed. The films were synthesized by sol-gel processing from tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES) precursors and spin-coated onto Si substrates. The sol viscosity and the spin velocity were adjusted so that the films produced had a final thickness ranging from 580 to 710 nm after heat treatment. The ion species and incident energies used were selected such that the projected ion range was greater than the film thickness, resulting in fully irradiated films. After heat treatment at 300 °C for 10 min, the films were irradiated with 125 keV H⁺, 250 keV N²⁺ and 2 MeV Cu⁺ ions with fluences ranging from 1 × 10¹⁴ to 1 × 10¹⁶ ions/cm². Both hardness and reduced elastic modulus were seen to exhibit a monotonic increase with fluence for all three ion species. Also, H loss was found to increase monotonically with increase in fluence, while the film thickness was found to decrease with increase in fluence.     

Effects of electron beam irradiation on binary polyamide-6 blends with metallocene copolymers

The effect of electron beam irradiation on thermal and mechanical properties, and SEM morphology of polyamide-6 (PA-6) blends with grafted copolymers was investigated. High toughness materials were obtained with ethylene-polypropylene-diene grafted copolymers without significant variations in their thermal properties and Izod impact strength at room temperature and −30 °C with the irradiation doses used.     

Radiation effects on styrene-butadiene-styrene copolymer

Styrene-butadiene-styrene copolymer (SBS) of different configuration and block ratio were irradiated by ⁶⁰Co γ-rays at 20 °C and 120 °C. The gel fraction and thermal properties before and after irradiation were discussed. The gel fraction increased with both absorbed doses and temperature. This means higher crosslinking efficiency was obtained at higher temperature. The radiation chemical yields were calculated according to the Charlesby-Pinner equation. The DSC analysis indicated that the crosslinking had taken place in the polybutadiene (PB) block while the chain scission of polystyrene (PS) block was dominant when irradiated at elevated temperature.     

Thermo-mechanical characteristics of UV-irradiated polyurethane elastomers extended with α, ω-alkane diols

A series of polyurethane elastomers were prepared by the reaction of poly ε-caprolactone and 4,4′-diphenylmethane diisocyanate. The prepolymer was extended using α, ω-alkane diols as chain extenders having 2-10 methylene units in their structure. The synthesized samples were irradiated for 50, 100 and 200 h in an ultra violet (UV) exposure unit. Modifications in the chemical structure before and after irradiation were characterized using Fourier Transform Infrared spectroscopy. The thermal and mechanical properties were affected by the ultra violet irradiation time and the number of methylene units in the chain extenders. The experimental results indicated that the morphological structure changed during irradiation as a consequence of hard segment and soft segment degradation.