Electrical and structural characterization of ion implanted GaN

Ion implantation induced defects and their consequent electrical impact have been investigated. Unintentionally doped n-type gallium nitride was implanted with 100 keV Si⁺ and 300 keV Ar⁺ ions in a fluence range of 10¹⁴–10¹⁵ ions/cm². The samples were characterized with Rutherford backscattering/Channeling method for damage buildup. Time of flight elastic recoil detection analysis was implied on the Si implanted samples to see the ion depth distribution. Ar implanted GaN samples were studied electrically with scanning spreading resistance microscopy. Our results show that an Ar fluence of 5 × 10¹⁴ cm^?2 increases the resistance by five orders of magnitude to a maximum value. For the highest fluence, 6 × 10¹⁵ cm^?2, the resistivity decreases by two orders of magnitude.     

Thermal stress resistance of ion implanted sapphire crystals

Monocrystals of sapphire have been subjected to ion implantation with 86 keV Si⁻ and 80 keV Cr⁻ ions to doses in the range of 5 × 10¹⁴–5 × 10¹⁶ cm⁻² prior to thermal stress testing in a pulsed plasma. Above a certain critical dose ion implantation is shown to modify the near-surface structure of samples by introducing damage, which makes crack nucleation easier under the applied stress. The effect of ion dose on the stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance are determined. The critical dose for Si ions is shown to be much lower than that for Cr⁻ ions. However, for doses exceeding 2 × 10¹⁶ cm⁻² the stress resistance parameter decreases to approximately the same value for both implants. The size of the implantation-induced crack nucleating centers and the density of the implantation-induced defects are considered to be the major factors determining the stress resistance of sapphire crystals irradiated with Si⁻ and Cr⁻ ions.     

Annealing of ion implanted 4H–SiC in the temperature range of 100–800 °C analysed by ion beam techniques

Ion implantation induced damage formation and subsequent annealing in 4H–SiC in the temperature range of 100–800 °C has been investigated. Silicon Carbide was implanted at room temperature with 200 keV ⁴⁰Ar ions with two implantation fluences of 4 × 10¹⁴ and 2 × 10¹⁵ ions/cm². The samples were characterized by Rutherford backscattering and nuclear reaction analysis techniques in channeling mode using 2.00 and 4.30 MeV ⁴He ion beams for damage buildup and recovery in the Si and C sublattices, respectively. At low ion fluence, the restoration of the Si sublattice is evident already at 200 °C and a considerable annealing step occurs between 300 and 400 °C. Similar results have been obtained for the C sublattice using the nuclear resonance reaction for carbon, ¹²C(α,α)¹²C at 4.26 MeV. For samples implanted with the higher ion fluence, no significant recovery is observed at these temperatures.     

Study of modifications in Lexan polycarbonate induced by swift O6+ ion irradiation

Swift Heavy Ion (SHI) irradiation of the polymeric materials modifies their physico-chemical properties. Lexan polycarbonate films were irradiated with 95 MeV oxygen ions to the fluences of 10¹⁰, 10¹¹, 10¹², 10¹³ and 2 × 10¹³ ions/cm². Characterization of optical, chemical, electrical and structural modifications were carried out by UV–Vis spectroscopy, FTIR spectroscopy, Dielectric measurements and X-ray Diffraction. A shift in the optical absorption edge towards the red end of the spectrum was observed with the increase in ion fluence. The optical band gap (E_g), calculated from the absorption edge of the UV–Vis spectra of these films in 200–800 nm region varied from 4.12 eV to 2.34 eV for virgin and irradiated samples. The cluster size varied in a range of 69–215 carbon atoms per cluster. In FTIR spectra, appreciable modification in terms of breaking of the cleavaged C–O bond of carbonate and formation of phenolic O–H bond was observed on irradiation. A rapidly decreasing trend in dielectric constant is observed at lower frequencies. The dielectric constant increases with fluence. It is observed that the loss factor increases moderately with fluence and it may be due to scissoring of polymer chains, resulting in an increase in free radicals. A sharp increase in A.C. conductivity in pristine as well as in irradiated samples is observed with frequency and is attributed to scissoring of polymer chains. XRD analyses show significant change in crystallinity with fluence. A decrease of ～9.02% in crystallite size of irradiated sample at the fluence of 2 × 10¹³ ions/cm² is observed.     

Ion beam synthesis of SiC by C implantation into SIMOX(1 1 1)

We report the conversion of a 65 nm Si(1 1 1) overlayer of a SIMOX(1 1 1) into 30–45 nm SiC by 40 keV carbon implantation into it. High temperature implantation (600 °C) through a SiO₂ cap, 1250 °C post-implantation annealing under Ar ambient (with 1% of O₂), and etching are the base for the present synthesis. Sequential C implantations (fluence steps of about 5 × 10¹⁶ cm^?2), followed by 1250 °C annealing, has allowed to estimate the minimum C fluence to reach the stoichiometric composition as ～2.3 ×  10¹⁷ cm^?2. Rutherford Backscattering Spectrometry was employed to measure layer composition evolution. A two-sublayers structure is observed in the synthesized SiC, being the superficial one richer in Si. Transmission electron microscopy has shown that a single-step implantation up to the same minimum fluence results in better structural quality. For a much higher C fluence (4 × 10¹⁷ cm^?2), a whole stoichiometric layer is obtained, with reduction of structural quality.     

SHI induced re-crystallization of Ge implanted SiO2 films

Ge nanocrystals embedded in SiO₂ matrix have been synthesized by swift heavy ion irradiation of Ge implanted SiO₂ films. In the present study, 400 keV Ge⁺ ions were implanted into SiO₂ films at dose of 3 × 10¹⁶ ions/cm² at room temperature. The as-implanted samples were irradiated with 150 MeV Ag¹²⁺ ions with various fluences. Similarly 400 keV Ge⁺ ions implanted into Silicon substrate at higher fluence at 573 K have been irradiated with 100 MeV Au⁸⁺ ions at room temperature (RT). These samples were subsequently characterized by XRD and Raman to understand the re-crystallization behavior. The XRD results confirm the presence of Ge crystallites in the irradiated samples. Rutherford backscattering spectrometry (RBS) was used to quantify the concentration of Ge in the SiO₂ matrix. Variation in the nanocrystal size as a function of ion fluence is presented. The basic mechanism of ion beam induced re-crystallization has been discussed.     

Ion beam mixing in uranium nitride thin films studied by Rutherford Backscattering Spectroscopy

Thickness, composition, concentration depth profile and ion irradiation effects on uranium nitride thin films deposited on fused silica have been investigated by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He⁺ ions. The films were prepared by reactive DC sputtering at the temperatures of ?200 °C, +25 °C and +300 °C. A perfect 1U:1N stoichiometry with a layer thickness of 660 nm was found for the film deposited at ?200 °C. An increase of the deposition temperature led to an enhancement of surface oxidation and an increase of the thickness of the mixed U–N–Si–O layers at the interface. The sample irradiation by 1 MeV Ar⁺ ion beam with ion fluence of about 1.2–1.7 × 10¹⁶ ions/cm² caused a large change in the layer composition and a large increase of the total film thickness for the films deposited at ?200 °C and at +25 °C, but almost no change in the film thickness was detected for the film deposited at +300 °C. An enhanced mixing effect for this film was obtained after further irradiation with ion fluence of 2.3 × 10¹⁶ ions/cm².     

Point defects induced in ion irradiated 4H–SiC probed by exciton lines

The defects produced in 4H–SiC epitaxial layers by irradiation with a 200 keV H⁺ ion beam in the fluence range 6.5 × 10¹¹–1.8 × 10¹³ ions/cm² are investigated by Low Temperature Photoluminescence (LTPL–40 K).The defects produced by ion beam irradiation induce the formation of some sharp lines called “alphabet lines” in the photoluminescence spectra in the 425–443 nm range, due to the recombination of excitons at structural defects.From the LTPL lines intensity trend, as function of proton fluence, it is possible to single out two groups of peaks: the P₁ lines (e, f, g) and the P₂ lines (a, b, c, d) that exhibit different trends with the ion fluence. The P₁ group normalized yield increases with ion fluence, reaches a maximum at 2.5 × 10¹² ions/cm² and then decreases. The P₂ group normalized yield, instead, exhibits a formation threshold at low fluence, then increases until a maximum value at a fluence of 3.5 × 10¹² ions/cm² and decreases at higher fluence, reaching a value of 50% of the maximum yield.The behaviour of P₁ and P₂ lines, with ion fluence, indicates a production of point defects at low fluence, followed by a subsequent local rearrangement creating complex defects at high fluence.     

Effects of BF2+ implantation on the strain-relaxation of pseudomorphic metastable Ge0.06Si0.94 alloy layers

Metastable pseudomorphic Ge_0.06Si_0.94 alloy layers grown by molecular beam epitaxy (MBE) on Si (1 0 0) substrates were implanted at room temperature by 70 keV BF₂⁺ ions with three different doses of 3 × 10¹³, 1 × 10¹⁴, and 2.5 × 10¹⁴ cm⁻². The implanted samples were subsequently annealed at 800°C and 900°C for 30 min in a vacuum tube furnace. Observed by MeV ⁴He channeling spectrometry, the sample implanted at a dose of 2.5 × 10¹⁴ BF₂⁺ cm⁻² is amorphized from surface to a depth of about 90 nm among all as-implanted samples. Crystalline degradation and strain-relaxation of post-annealed Ge_0.06Si_0.94 samples become pronounced as the dose increases. Only the samples implanted at 3 × 10¹³ cm⁻² do not visibly degrade nor relax during anneal at 800°C . In the leakage current measurements, no serious leakage is found in most of the samples except for one which is annealed at 800°C for 30 min after implantation to a dose of 2.5 × 10¹⁴ cm⁻². It is concluded that such a low dose of 3 × 10¹³ BF₂⁺ cm⁻² can be doped by implantation to conserve intrinsic strain of the pseudomorphic GeSi, while for high dose regime to meet the strain-relaxation, annealing at high temperatures over 900°C is necessary to prevent serious leakages from occuring near relaxed GeSi/Si interfaces.     

Thermal,structural, and radiological properties of irradiated graphite from the ASTRA research reactor – Implications for disposal

The release of Wigner energy from the graphite of the inner thermal column of the ASTRA research reactor has been studied by differential scanning calorimetry and simultaneous differential scanning calorimetry/synchrotron powder X-ray diffraction between 25 °C and 725 °C at a heating rate of 10 °C min⁻¹. The graphite, having been subject to a fast-neutron fluence from ∼10¹⁷ to ∼10²⁰ n cm⁻² over the life time of the reactor at temperatures not exceeding 100 °C, exhibits Wigner energies ranging from 25 to 572 J g⁻¹ and a Wigner energy accumulation rate of ∼7 × 10⁻¹⁷ J g⁻¹/n cm⁻². The shape of the rate-of-heat-release curves, e.g., maximum at ca. 200 °C and a fine structure at higher temperatures, varies with sample position within the inner thermal column, i.e., the distance from the reactor core. Crystal structure of samples closest to the reactor core (fast-neutron fluence >1.5−5.0 × 10¹⁹ n cm⁻²) is destroyed while that of samples farther from the reactor core (fast-neutron fluence <1.5−5.0 × 10¹⁹ n cm⁻²) is intact, with marked swelling along the c-axis. The dependence of the c lattice parameter on temperature between 25 °C and 200 °C as determined by Rietveld refinement for the non-amorphous samples leads to the expected microscopic thermal expansion coefficient along the c-axis of ∼ 26 × 10⁻⁶ °C⁻¹. However, at 200 °C, coinciding with the maximum in the rate-of-heat-release curves, the rate of thermal expansion abruptly decreases indicating a crystal lattice relaxation. The ¹⁴C activity in the inner thermal column graphite ranges from 6 to 467 kBq g⁻¹. The graphite of the inner thermal column of the ASTRA research reactor has been treated by heating to 400 °C for 24 h in a hot-cell facility prior to interim storage.     

Swift heavy ion induced modifications of optical and microstructural properties of silver–fullerene C60 nanocomposite

In this paper, we study the optical and microstructural properties of silver–fullerene C₆₀ nanocomposite and their modifications induced by swift heavy ion irradiation. Silver nanoparticles embedded in fullerene C₆₀ matrix were synthesized by co-deposition of silver and fullerene C₆₀ by thermal evaporation. The nanocomposite thin films were irradiated by 120 MeV Ag ions at different fluences ranging from 1 × 10¹² to 3 × 10¹³ ions/cm². Optical absorption studies revealed that the surface plasmon resonance of Ag nanoparticles showed a blue shift of ～49 nm with increasing ion fluence up to 3 × 10¹³ ions/cm². Transmission electron microscopy and Rutherford backscattering spectroscopy were used to quantify particle size and metal atomic fraction in the nanocomposite film. Growth of Ag nanoparticles was observed with increasing ion fluence. Raman spectroscopy was used to understand the effect of heavy ion irradiation on fullerene matrix. The blue shift in plasmonic wavelength is explained by the transformation of fullerene C₆₀ matrix into amorphous carbon.     

Erosion behavior of soft,amorphous deuterated carbon films by heat treatment in air and under vacuum

The erosion of soft a-C:D films by heat treatment in air and under vacuum is studied by ion-beam analysis. When the films are heated in air above 500 K, the film thickness and the areal densities of C and especially D decrease, and oxygen is incorporated in the films. The initial atomic loss rates of carbon and deuterium from the films are 2.6 × 10¹⁷ C atoms cm⁻² h⁻¹ and 4.8 × 10¹⁷ D atoms cm⁻² h⁻¹ at 550 K. However, after D depletion the films show a resistivity against further erosion due to annealing in air. When the films are heated under vacuum erosion starts at about 600 K and all components including D decrease proportionally to the film thickness. Thermal desorption spectroscopy of the films reveals the evolution of C_xD_y type hydrocarbons. Infrared analysis shows that the incorporated oxygen is chemically bonded to carbon. The thermally-activated decomposition of the soft a-C:D films is compared to that of hard a-C:D films and a reaction scheme is suggested.     

Investigation of hydrogen concentration and hardness of ion irradiated organically modified silicate thin films

A study of the effects of ion irradiation of organically modified silicate thin films on the loss of hydrogen and increase in hardness is presented. NaOH catalyzed SiNa_wO_xC_yH_z thin films were synthesized by sol–gel processing from tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES) precursors and spin-coated onto Si substrates. After drying at 300 °C, the films were irradiated with 125 keV H⁺ or 250 keV N²⁺ at fluences ranging from 1 × 10¹⁴ to 2.5 × 10¹⁶ ions/cm². Elastic Recoil Detection (ERD) was used to investigate resulting hydrogen concentration as a function of ion fluence and irradiating species. Nanoindentation was used to measure the hardness of the irradiated films. FT-IR spectroscopy was also used to examine resulting changes in chemical bonding. The resulting hydrogen loss and increase in hardness are compared to similarly processed acid catalyzed silicate thin films.     

Dose-dependent bonding environment of oxygen implanted in GaN

The bonding environment of oxygen implanted in GaN is studied using Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The implantation of 70 keV O ions in GaN results in the formation of a 200 nm – thick subsurface layer that is highly defective or amorphous depending on the implantation fluence which ranges from 1 × 10¹⁵ to 1 × 10¹⁷ cm^?2. The NEXAFS spectra are simulated using the FEFF8 code assuming models that account for the formation of point defects (various configurations of O interstitial and O substitutional in N and Ga sites) as well as chemical effects such as the formation of various polymorphs of Ga oxides and oxynitrides. The implantation-induced lattice disorder is modeled by displacing atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers that belong to normal distributions. The simulations reveal that when the fluence is 1 × 10¹⁵ cm^?2, the O implants occupy interstitial sites preferentially in the empty channels aligned parallel to the c-axis in the plane that contains the Ga atoms and/or in the columns that consist of Ga and N atoms along the c-axis. When the fluence is equal to 1 × 10¹⁶ cm^?2 the O ions substitute for N while at 1 × 10¹⁷ cm^?2 they participate in the formation of mixed GaO_xN_y phases.     

Characterisation of Ni+ implanted PEEK,PET and PI

Polyimide (PI), polyetheretherketone (PEEK) and polyethyleneterephthalate (PET) were implanted with 40 keV Ni⁺ ions at room temperature at fluences ranging from 1.0 × 10¹⁶ to 1.5 × 10¹⁷ ions cm^?2 and with ion current density varying between 4 and 10 μA cm^?2. The depth profiles of the implanted Ni atoms determined by the RBS technique were compared with those predicted by the SRIM and TRIDYN codes. Hydrogen depletion as a function of the ion fluence was determined by the ERDA technique, and the compositional and structural changes of the polymers were characterised by the UV–vis and XPS methods. The implanted profiles differed significantly from those predicted by the SRIM code while the lower fluences were satisfactorily described by the TRIDYN simulation. A significant hydrogen release from the polymer surface layer was observed along with significant changes in the surface layer composition. The UV–vis results indicated an increase in the concentration and conjugation of double bonds.     

Effect of <200 keV proton radiation on electric properties of silicon solar cells at 77 K

The change in electric properties of back-field silicon solar cells was investigated under the irradiation of protons with the energies less than 200 keV at 77 K. Experimental results showed that the short circuit current, maximum output power and open circuit voltage decrease to different extent with increasing the fluence and energy of protons. Under the 120 keV proton irradiation for the fluence of 1 × 10¹⁶ cm⁻², a large amount of radiation-induced defects with the energy level H1 +0.47 eV were formed. In terms of analyzing the time dependence of electric properties, the performance lifetime of the silicon cells under the exposure of <200 keV protons was predicted.     

Light particle irradiation effects in Si nanocrystals

Si nanocrystals, formed by Si ion implantation into SiO₂ layers and subsequent annealing at 1150°C, were irradiated at room temperature either with He⁺ions at energies of 30 or 130 keV, or with 400 keV electrons. Transmission electron microscopy (TEM) and photoluminescence (PL) studies were performed. TEM experiments revealed that the Si nanocrystals were ultimately amorphized (for example at ion doses ∼10¹⁶ He cm⁻²) and could not be recrystallized by annealing up to 775°C. This contrasts with previous results on bulk Si, in which electron- and very light ion-irradiation never led to amorphization. Visible photoluminescence, usually ascribed to quantum-size effects in the Si nanocrystals, was found to decrease and vanish after He⁺ ion doses as low as 3 × 10¹²–3 × 10¹³ He cm⁻² (which produce about 1 displacement per nanocrystal). This PL decrease is due to defect-induced non-radiative recombination centers, possibly situated at the Si nanocrystal/SiO₂ interface, and the pre-irradiation PL is restored by a 600°C anneal.     

Stability of defects created by high fluence helium implantation in silicon

Stability of extended defects created by high fluence helium implantation (50 keV, 5 × 10¹⁶ cm⁻²) from room temperature to 800 °C has been studied using transmission electron microscopy. Our results clearly show that the cavities behave as good sinks for interstitial type defects generated during ion implantation, leading in some cases to the cavity dissolution. A three-dimensional “phase diagram” related to the formation and evolution of interstitial-type defects is also proposed. It is plotted in terms of quantity of damage, annealing time and implantation temperature.     

ERD analysis and modification of TiO2 thin films with heavy ions

Titanium dioxide (TiO₂) films have been deposited on Si substrates using reactive magnetron sputtering. The resulting films, having a polycrystalline anatase phase with a dense columnar structure, were analysed by time-of-flight elastic recoil detection analysis (ToF-ERDA) using 40 MeV I⁹⁺ ions. A clear decrease in the areal atomic density (atoms/cm²) of Ti and O was observed during measurement, but the stoichiometry remained essentially constant up to a fluence of 4 × 10¹³ ions/cm².To investigate this effect in more detail, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were applied in order to characterize the films prior to and after ion irradiation with fluences in the range of 10¹⁰–10¹³ ions/cm². Distinct morphological and structural changes of the polycrystalline film were observed. XRD revealed that the crystallinity of the film was gradually destroyed, and the film became amorphous at a fluence above 5 × 10¹² ions/cm². SEM and AFM measurements revealed topographical changes in the form of surface recession and smoothing compared to the pristine polycrystalline surface. The observed change in areal atomic density during ERD measurement is believed to be due to the combined effects of electronic sputtering, amorphization and ion hammering.     

Behaviour of implanted oxygen and nitrogen in halogen lamp annealed silicon

The behaviour of oxygen and nitrogen sequentially implanted into silicon at an energy of 100 and 175 keV with total doses of 5 × 10¹⁶ and 2 × 10¹⁷ cm⁻² is reported. The implantation sequence and energies were varied. In all cases the wafer temperature during implantation was maintained at 300°C. The samples were pulse annealed in using a halogen lamp to achieve temperatures of 1100°C and 1200°C for 2–60 s. The samples were analysed using FTIR, XRD and SIMS. For a total dose of 5 × 10¹⁶ cm⁻² no mutual redistribution of oxygen and nitrogen occurs during annealing. The impurity atoms (O and N) are found to bind to radiation defects. Annealing does not lead to any detectable new phase formation but does cause dissociation of oxygen–vacancy complexes and loss of oxygen from the bulk. For a dose of 2 × 10¹⁷ cm⁻² phase formation occurs during implantation. The mutual redistribution of the impurity atoms during annealing occurs but only if oxygen is implanted deeper than nitrogen. This redistribution is greatest if nitrogen is implanted prior to oxygen.