A comparative EPR study of ion implantation induced damage in Si, Si1−xGex (x ≠ 0) and SiC

Electron Paramagnetic Resonance (EPR) measurements have been made to investigate the build up of damage in silicon in relaxed crystalline Si_1−xGe_x (x = 0.04, 0.13, 0.24, 0.36) and in 6H-SiC as a result of increasing the ion dose from low levels (10¹² cm⁻²) up to values (10¹⁵ cm⁻²) sufficient to produce an amorphous layer. Si, Si_1−xGe_x (x ≠ 0) and SiC were implanted at room temperature with 1.5 MeV Si, 2 MeV Si and 0.2 MeV Ge ions respectively. A comparison is made between the ways in which the type and population of paramagnetic defects depend on ion dose for each material.     

2 MeV Si ion implantation damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3 μm thick relaxed Si_1−xGe_x layers with 2 MeV Si⁺ ions has been measured as a function of Ge content (x = 0.04, 0.13, 0.24 or 0.36) and Si dose in the dose range 10¹⁰–10¹⁵ cm⁻². The accumulation of damage with increasing dose has been studied as a function of Ge content by Rutherford Backscattering Spectrometry, Optical Reflectivity Depth Profiling and Transmission Electron Microscopy and an increased damage efficiency in Si_1−xGe_x with increasing x is observed. The characteristics of implantation-induced defects have been investigated by Electron Paramagnetic Resonance. The results are discussed in the context of a model of the damage process in SiGe.     

Study of ion beam induced swelling in fluorite as an inert matrix model

As a model material of fluorite type structure, single crystals of CaF₂ were irradiated with various heavy ions (from ³²S to ²⁰⁹Bi) of specific energies between 1 and 12 MeV u⁻¹. Using a profilometer, out-of-plane swelling was measured by scanning over the border line between irradiated and virgin areas of the sample surface. The step height as a function of the ion fluence exhibits a linear increase in the initial phase and saturates at high fluences. Track radii contributing to the swelling effect are deduced. Above a critical energy loss of about 5 keV nm⁻¹, the initial swelling rate per incident ion shows two separated regimes. Finally, the response of CaF₂ submitted to high electronic energy loss is discussed in combination with earlier track observations by transmission electron microscopy.     

Photoluminescence of Au formed in 12CaO · 7Al2O3 single crystal by Au-implantation

Au⁺ ion implantation with fluences from 1 × 10¹⁴ to 3 × 10¹⁶ cm⁻² into 12CaO · 7Al₂O₃ (C12A7) single crystals was carried out at a sample temperature of 600 °C. The implanted sample with the fluence of 1 × 10¹⁵ cm⁻² exhibited photoluminescence (PL) bands peaking at 3.1 and 2.3 eV at 150 K when excited by He–Cd laser (325 nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au⁻ ion having the electronic configuration of 6s², judged from their similarities to those reported on Au⁻ ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (2.3 × 10²¹ cm⁻³), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au⁺ implantation if an appropriate fluence is chosen.     

Measurements of natural concentrations of I in uranium ores by accelerator mass spectrometry

An accelerator mass spectrometry system is described and utilized for measurements of ¹²⁹I concentrations in natural and environmental samples. We report here on measurements of ¹²⁹I isotopic abundances in iodine reagents and in iodine of mineral origin and of ¹²⁹I concentrations in uranium ores of different origins. The ¹²⁹I isotopic abundances for two measured contemporary iodine reagents and for iodine from a deep underground brine are 1.3 × 10⁻¹³ and about 4 × 10⁻¹⁴, respectively. ¹²⁹I/U ratios in the range 10⁻¹³–10⁻¹² are measured and compared to a simple model of ¹²⁹I production by spontaneous and induced fission of uranium. No clear correlation with the uranium concentrations or residence times is observed.     

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.     

Study of 160 MeV Ni ion irradiation effects on electrodeposited polypyrrole films

Conducting polymer polypyrrole thin films doped with LiCF₃SO₃, [CH₃(CH₂)₃]₄NBF₄ and [CH₃(CH₂)₃]₄NPF₆ have been electrodeposited potentiodynamically on ITO coated glass substrate. The polymer films are irradiated with 160 MeV Ni¹²⁺ ions at three different fluences of 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². An increase in dc conductivity of polypyrrole films from 100 S/cm to 170 S/cm after irradiation with highest fluence is observed in four-probe measurement. X-ray diffractogram shows increase in the crystallinity of the polypyrrole films upon SHI irradiation, which goes on increasing with the increase in fluence. Absorption intensity increase in the higher wavelength region is observed in the UV–Vis spectra. The SEM studies show that the cauliflower like flaky microstructure of the surface of polypyrrole films turns globular upon SHI irradiation at fluence 5 × 10¹¹ ions cm⁻² and becomes smooth and dense at the highest fluence used. The cyclic voltammetry studies exhibit that the redox properties of the polypyrrole films do not change much on SHI irradiation.     

Optical and structural behaviour of Mn implanted sapphire

Sapphire single crystals were implanted at room temperature with 180 keV manganese ions to fluences up to 1.8 × 10¹⁷ cm⁻². The samples were annealed at 1000 °C in oxidizing or reducing atmosphere. Surface damage was observed after implantation of low fluences, the amorphous phase being observed after implantation of 5 × 10¹⁶ cm⁻², as seen by Rutherford backscattering spectroscopy under channelling conditions. Thermal treatments in air annealed most of the implantation related defects and promoted the redistribution of the manganese ions, in a mixed oxide phase. X-ray diffraction studies revealed the presence of MnAl₂O₄. On the contrary, similar heat treatments in vacuum led to enhanced out diffusion of Mn while the matrix remained highly damaged. The analysis of laser induced luminescence performed after implantation showed the presence of an intense red emission.     

Investigation of the radiation hardness on semiconductor devices using the ion micro-beam

Variation of the ion beam induced charge (IBIC) pulse heights due to ion irradiation was investigated on a Si pn diode and a 6H-SiC Schottky diode using a 2 Mev He⁺ micro-beam. Each diode was irradiated with a focused 2 MeV He⁺ micro-beam to a fluence in the range of 1×10⁹–1×10¹³ ions/cm². Charge pulse heights were analyzed as a function of the irradiation fluence. After a 2 MeV ion irradiation to the Si pn junction diode, the IBIC pulse height decreased by 15% at 9.2×10¹² ions/cm². For the SiC Schottky diode, with a fluence of 6.5×10¹² ions/cm², the IBIC pulse height decreased by 49%. Our results show that the IBIC method is applicable to evaluate irradiation damage of Si and SiC devices and has revealed differences in the radiation hardness of devices dependent on both structural and material.     

Large conductivity enhancement in polycrystalline 12CaO · 7Al2O3 thin films induced by extrusion of clathrated O ions by hot Au implantation and ultraviolet light illumination

Large enhancement in electrical conductivity from <10⁻¹⁰ S cm⁻¹ to 4 × 10⁻² S cm⁻¹ was achieved in polycrystalline 12CaO · 7Al₂O₃ (p-C12A7) thin films by hot Au⁺ implantation at 600 °C and subsequent ultraviolet (UV) light illumination. Although the as-implanted films were transparent and insulating, the subsequent UV-light illumination induced persistent electronic conduction and coloration. A good correlation was found between the concentration of photo-induced F⁺-like centers (a cage trapping an electron) and calculated displacements per atom, indicating that the hot Au⁺ implantation extruded free O²⁻ ions from the cages in the p-C12A7 films by kick-out effects and left electrons in the cages. These results suggest that H⁻ ions are formed by the Au⁺ implantation through the decomposition of preexisting OH⁻ ions. Subsequent UV-light illumination produced F⁺-like centers via photoionization of the H⁻ ions, which leads to the electronic conduction and coloration.     

Vacancy defects induced in the track region of 132 MeV C irradiated SiC

Positron annihilation lifetime spectroscopy (PALS) and electron paramagnetic resonance (EPR) have been used in this work to investigate vacancy defects induced in the track region of 132 MeV ¹²C irradiated silicon carbide. Irradiations have been performed at room temperature at a fluence of 2.5 × 10¹⁴ cm⁻² in N-low doped 6H–SiC and 3C–SiC monocrystals. Silicon monovacancies have been detected in both polytypes using EPR. Their charge state and concentration have been determined in the track and cascade region of the C⁺ ions. PALS measurements performed as a function of temperature have shown the presence of V_Si–C divacancies in the track region for both polytypes.     

Lattice location and electrical conductivity in ion implanted TiO2 single crystals

Single crystals of TiO₂ (rutile) were implanted at room temperature with Ar, Sn and W ions applying fluences of 10¹⁵/cm² to 10¹⁶/cm² at 300 keV. The lattice location, together with ion range and damage distribution was measured with Rutherford Backscattering and Channeling (RBS-C). The conductivity, σ, was measured as a function of temperature. The implanted Sn and W atoms were entirely substitutional on Ti sites in the applied fluence region, where the radiation damage did not yet reach the random level. A large σ increase was observed for all implants at displacement per atom values (dpa) below 1. Above dpa = 1, σ reveals a saturation value of 0.3 Ω⁻¹ cm⁻¹ for Ar implants, while for W and Sn implants a further increase of σ up to 30 Ω⁻¹ cm⁻¹ was measured. Between 70 K and 293 K ln σ was proportional to T^−1/2, (Ar,W) and T^−1/4 (Sn), indicating that the transport mechanism is due to variable range hopping.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Dislocation imaging of an InAlGaAs opto-electronic modulator using IBICC

This paper presents Ion Beam Induced Charge Collection (IBICC) contrast images showing regions of differing charge collection efficiency within optoelectronic modulator devices. The experiments were carried out at the Sandia Nuclear Microprobe using 18 MeV carbon and 2 MeV helium ions. Lines of varying densities are observed to run along the different {110} directions which correlate with misfit dislocations within the 392 nm thick strained-layer superlattice quantum well of the modulator structure. Independent cross-sectional TEM studies and the electrical properties of the devices under investigation suggest the presence of threading dislocations in the active device region at a density of 10⁶ cm⁻². However, no clear evidence of threading dislocations was observed in the IBICC images as they are possibly masked by the strong contrast of the misfit dislocations. Charge carrier transport within the modulator is used to explain the observed contrast. The different signal to noise levels and rates of damage of the incident ions are assessed.     

Contribution of uranium diffusion on creep behavior of uranium dicarbide

Compressive creep tests of uranium dicarbide (UC₂) have been conducted. The general equation best describing the creep rate over the temperature range 1200–1400°C and over the stress range 2000–15000 psi is represented by the sum of two exponential terms ge =A(σ/E)^0.9 exp(−39.6 ± 1.0/RT) + B(σ/E)^4.5 exp(−120.6 ± 1.7/RT), where pre-exponential factors are A(σ/E)^0.9 = 12.3/h at low stress region (3000 psi) and B(σ/E)^4.5 = 3.17 × 10¹³/h at high stress region (9000 psi), and the activation energy is given in kcal/mol. Each term of this experimental equation indicates that important processes occurring during the steady state creep are grain-boundary diffusion of the Coble model at low stress region and the Weertman dislocation climb model at high stress region. Both mechanisms are related to migration of uranium vacancies.     

Release of nitrogen from SiOxNy films during RBS measurement

We have found that nitrogen atoms are released very rapidly from ultrathin SiO_xN_y films (2.6 nm) during RBS measurement with 500 keV He⁺ ions. The release behavior strongly depends on the preparation technique of the SiO_xN_y films. There is no release from the film prepared by thermal nitridation of SiO₂, while 80% of the nitrogen atoms are released from the film prepared by plasma nitridation at a fluence of 1×10¹⁶ cm⁻². The release cross-section for plasma SiO_xN_y films is of the order of 10⁻¹⁶ cm². This large cross-section cannot be explained by a simple recoil mechanism. The nitrogen release is also observed under irradiation with 5–10 keV electrons though the cross-section is of the order of 10⁻¹⁹ cm². These findings suggest that the observed nitrogen release is an electronic excitation induced process.     

Surface modification of MgAl2O4 and oxides with heavy ions of fission fragments energy

In this paper we report on results of surface modification in several candidate materials for inert matrix fuel hosts (MgAl₂O₄, MgO and Al₂O₃) induced by (0.5–5) MeV/amu Kr, Xe and Bi ion bombardment in the fluence range of 2 × 10¹⁰–10¹² ions/cm². The size and shape of nanoscale hillock-like defects, each of which was created by the impact of a single ion, have been studied by using atomic force microscopy (AFM) technique. It was found that mean hillock height on sapphire and spinel surfaces depends linearly on the incident electronic stopping power. The hillocks are highest in MgAl₂O₄, having a lower threshold for the lattice disorder in the bulk material via relaxation of electronic excitations. As a possible reason for the hillocks formation, the plastic deformation due to the defects created by the Coulomb explosion mechanism in the target subsurface layer is suggested.     

The Stuttgart positron beam, its performance and recent experiments

A monoenergetic MeV positron (e⁺) beam, with a flux at present of 6 × 10⁴ e⁺/s in the energy range of 0.5 to 6.5 MeV, has been installed at the Stuttgart Pelletron accelerator. The stabilization and the absolute calibration of the energy E is monitored by a Ge detector with real-time feedback; a relative energy stability of ΔE/E 10⁻⁴ is obtained. So far, e⁺e⁻ scattering and annihilation-in-flight experiments for investigating the low-energy e⁺e⁻ interaction as well as β⁺ γ positron lifetime measurements in condensed matter have been performed. The advantages of the β⁺ γ method compared to the conventional γγ coincidence technique have been demonstrated. Recently, triple-coincidence positron “age-momentum correlation” measurements have been carried out on fused quartz. A brief account is given on the development of a “positron clock” aiming at a substantial improvement of the time resolution of the β⁺ γ positron lifetime measurements.     

Irradiation effects of swift heavy ions in actinide oxides and actinide nitrides: Structure and optical properties

Actinide oxides have been used as nuclear fuels in the majority of power reactors working in the world and actinide nitrides are under investigation for the fuels of the future fast neutron fission reactors developed in Forum Generation IV. Radiation damage in actinide oxides UO₂, (U_0.92Ce_0.08)O₂, and actinide nitride UN has been characterized after irradiation with swift heavy ions. Fluences up to 3 × 10¹³ ions/cm² of heavy ions (Kr 740 Mev, Cd 1 GeV) available at the CIRIL/GANIL facility were used to simulate irradiation in reactors by fission products and by neutrons. The macroscopic effects of irradiation remains very weak compared with those seen in other ceramic oxides irradiated in the same conditions: practically no swelling can be measured and no change in colour can be observed on the irradiated part of a polished face of sintered disks. The point defects in irradiated actinide compounds have been characterized by optical absorption spectroscopy in the UV–Vis–NIR wavelength range. The absorption spectra before and after irradiation are compared, and unexpected stability of optical properties during irradiation is shown. This result confirms the low rate of formation of point defects in actinide oxides and actinide nitrides under irradiation. Actinide oxides and nitrides studied are >40% ionic, and oxidation state of the actinides seems to be stable during irradiation. The small amount of point defects produced by radiation (<10¹⁶ cm⁻²) has been identified from differences between the absorption spectrum before irradiation and the one after irradiation: point defects in oxygen or nitrogen lattices can be observed respectively in oxides and nitrides (F centres), and small amounts of U⁵⁺ would be present in all compounds.     

Ion beam mixing of Au marker in Pt and Pt marker in Au by 7 MeV Ag ions

Mixing of a thin Au layer in Pt and in reversed conditions mixing of a thin Pt layer in Au due to bombardment with 7 MeV Ag ions has been measured. The Pt-Au multilayers deposited on a Si substrate were irradiated to doses of 1–6 × 10¹⁵ ions cm⁻² at room temperature. The mixed profiles were measured using a SIMS apparatus with O₂⁺ sputter ions at energy 2.5 keV. The width of the Pt marker increased from 90 to 260 Å with increasing dose. The width of the Au marker increased from 80 to 90 Å, respectively. The corresponding mixing efficiencies are 5 ± 3 (Au marker) and 90 ± 30 Å⁵/eV (Pt marker). The experimental results are compared with simulations based on a model which describes the atomic transport from the initial collisional phase to the late thermalized stage. The calculated values for mixing efficiencies agree reasonably well with experimental values.