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.     

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.     

A simple way to analyze infrared reflectivity spectra of p-type Hg0.78Cd0.22Te implanted in various conditions

Different ion-implanted p-type Hg_0.78Cd_0.22Te samples were analyzed by infrared reflectivity in the 2–20 μm wavelength range. We show how to derive some characteristic values of the free carriers induced by ion implantation from simple models of the implanted samples. For low energy implantations (Al (320 keV)) an excess of electrons with concentration n⁺ ≈ 5 × 10¹⁷ cm⁻³ for doses 10¹² and 10¹⁴ ions cm⁻² is observed between the surface and the projected range R_p of the ions, in agreement with the well-known change of type of the free carriers induced by the ion implantation in this kind of samples. High energy α particle (0.8 and 2 MeV, 10¹⁴ ions cm⁻²) implantations lead to a pronounced inhomogeneous concentration of free electrons with n⁺ ≈ 9.2 × 10¹⁶ cm⁻³ between the surface and R_p where a negligible amount of defects due to the nuclear energy loss is formed, and n⁺ ≈ 1.6 × 10¹⁷ cm⁻³ between R_p and R_p + ΔR_p, ΔR_p being the longitudinal straggling, where the defect production rate through the nuclear energy loss mechanism is maximum.     

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.     

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.     

Soft X-ray fluorescence measurements of irradiated polymer films

Fluorescent soft X-ray carbon Kα emission spectra (XES) have been used to characterize the bonding of carbon atoms in polyimide (PI) and polycarbosilane (PCS) films. The PI films have been irradiated with 40 keV nitrogen or argon ions, at fluences ranging from 1 × 10¹⁴ to 1 × 10¹⁶ cm⁻². The PCS films have been irradiated with 5 × 10¹⁵ carbon ions cm⁻² of 500 keV and/or annealed at 1000°C. We find that the fine structure of the carbon XES of the PI films changes with implanted ion fluence above 1 × 10¹⁴ cm⁻² which we believe is due to the degradation of the PI into amorphous C:N:O. The width of the forbidden band as determined from the high-energy cut-off of the C Kα X-ray excitation decreases with the ion fluence. The bonding configuration of free carbon precipitates embedded in amorphous SiC which are formed in PCS after irradiation with C ions or combined treatments (irradiation and subsequent annealing) is close to either to that in diamond-like films or in silicidated graphite, respectively.     

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.     

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.     

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.     

MeV Si ions bombardment effects on the thermoelectric properties of Co0.1SbxGey thin films

We have grown three different monolayer Co_0.1Sb_xGe_y (x = 2, 4, 11 and y = 15, 7, 15) thin films on silica substrates with varying thickness between 100 and 200 nm using electron beam deposition. The high-energy (in the order of 5 MeV) Si ion bombardments have been performed on samples with varying fluencies of 1 × 10¹², 1 × 10¹³, 1 × 10¹⁴ and 1 × 10¹⁵ ions/cm². The thermopower, electrical and thermal conductivity measurements were carried out before and after the bombardment on samples to calculate the figure of merit, ZT. The Si ions bombardment caused changes on the thermoelectric properties of films. The fluence and temperature dependence of cross plane thermoelectric parameters were also reported. Rutherford backscattering spectrometry (RBS) was used to analyze the elemental composition of the deposited materials and to determine the layer thickness of each film.     

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.     

Lithium concentration dependence of implanted helium retention in lithium silicates

Helium ions of 500 keV were implanted with a fluence of 1.4 × 10¹⁷ ion/cm² into various lithium silicates to investigate whether a threshold level of helium retention exists in Li-containing silicate ceramics similar to that found in SiO_x in previous work. The composition and phases of the as prepared lithium silicates were determined by proton backscattering spectrometry (p-BS) and X-ray diffraction (XRD) methods with an average error of ±10%. Electrostatic charging of the samples was successfully eliminated by wrapping the samples in Al foil. The amounts of the retained helium within the samples were determined by subtracting the non-implanted spectra from the implanted ones. The experimental results show a threshold in helium retention depending on the Li concentration. Under 20 at.% all He is able to escape from the material; at around 30 at.% nearly half of the He, while over 65 at.% all implanted He is retained. With compositions expressed in SiO₂ volume percentages, a trend similar to those reported of SiO_x previously is found.     

Microstructure of ion-implanted region in TiNi alloy

TiNi alloy samples implanted with various fluences of 3 MeV Cu²⁺ ions were characterized by transmission electron microscope (TEM) and X-ray diffractometer. Cross-sectional TEM images of the samples showed that amorphous region was seen at the fluence of 10¹⁴ ions cm^?2 in case of ion implantation at 300 K of the substrate temperature, but in case of ion implantation at 100 K it did not appear even at 10¹⁵ ions cm^?2. These results were also confirmed by X-ray diffraction profiles of the same samples. Consequently, the extent of microstructure change of TiNi alloy by ion implantation was different depending on the substrate temperature.     

Photoluminescence induced from hot Ge implantation into SiO2

Up to the present, by using the ion implantation technique, photoluminescence (PL) from Ge nanocrystals (Ge NCs) was obtained by room temperature (RT) Ge implantation into a SiO₂ matrix followed by a high temperature anneal. In this way two PL bands were observed, one at 310 nm and the second, with much higher yield at 390 nm. In the present work we have used another experimental approach. We have performed the Si implantation at high temperature (T_i) and then, we have done a higher temperature anneal (T_a) in order to nucleate the Ge NCs. With this aim we have changed T_i between RT and 600 °C. By performing the implantation at T_i = 350 °C we found a PL yield four times higher than the one obtained from the usual RT implantation at the same fluence. Moreover, by changing the implantation fluence between Φ = 0.25 × 10¹⁶ and 2.2 × 10¹⁶ Ge/cm² we observed that Φ = 0.5 × 10¹⁶ Ge/cm² induces a PL yield three times higher as compared to the usual RT implantation fluence. In conclusion, using a hot Ge implantation plus an optimal Ge atomic concentration, we were able to gain more than one order of magnitude in the 390 nm PL yield as compared with previous ion implantation results.     

Micro-Raman studies of swift heavy ion irradiation induced structural and conformational changes in polyaniline nanofibers

Polyaniline (PAni) nanofibers doped with camphor sulfonic acid have been irradiated with 90 MeV O⁷⁺ ions at different fluences (3 × 10¹⁰?1 × 10¹² ions/cm²) using a 15UD Pelletron accelerator under ultra-high vacuum. XRD studies reveal a decrease in the domain length and an increase in the strain upon SHI irradiation. The increase in d-spacing corresponding to the (1 0 0) reflection of PAni nanofibers with increasing irradiation fluence has been attributed to the increase in the tilt angle of the chains with respect to the (a, b) basal plane of PAni. Decrease in the integral intensity upon SHI irradiation indicates amorphization of the material. Micro-Raman (μR) studies confirm amorphization of the PAni nanofibers and also show that the PAni nanofibers get de-doped upon SHI irradiation. μR spectroscopy also reveals a benzenoid to quinoid transition in the PAni chain upon SHI irradiation. TEM results show that the size of PAni nanofibers decreases with the increase in irradiation fluence, which has been attributed to the fragmentation of PAni nanofibers in the core of amorphized tracks caused by SHI irradiation.     

Ion implantation effects in single crystal Si investigated by Raman spectroscopy

A study of the effects of Ar ion implantation on the structural transformation of single crystal Si investigated by confocal Raman spectroscopy is presented. Implantation was performed at 77 K using 150 keV Ar⁺⁺ with fluences ranging from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². The Raman spectra showed a progression from crystalline to highly disordered structure with increasing fluence. The 520 cm^?1 c-Si peak was seen to decrease in intensity, broaden and exhibit spectral shifts indicating an increase in lattice disorder and changes in the residual stress state. In addition, an amorphous Si band first appeared as a shoulder on the 520 cm^?1 peak and then shifted to lower wavenumbers as a single broadband peak with a spectral center of 465 cm^?1. Additionally, the emergence of the a-Si TA phonon band and the decrease of the c-Si 2TA and 2TO phonon bands also indicated the same structural transition from crystalline to highly disordered. The Raman results were compared to those obtained by channeling RBS.     

Doping of 6H–SiC pn structures by proton irradiation

The influence of proton irradiation on current–voltage characteristics, N_d − N_a values and parameters of deep centres in 6H–SiC pn structures grown by sublimation epitaxy has been studied. The irradiation was carried out with 8 MeV protons in the range of doses from 10¹⁴ to 10¹⁶ cm⁻². Irradiation with a dose of 3.6 × 10¹⁴ cm⁻² leaves the voltage drop at high forward currents (10 A/cm²) practically unchanged. For higher irradiation dose of 1.8 × 10¹⁵ cm⁻², the forward voltage drop and the degree of compensation in the samples increased ; partial annealing of the radiation defects and partial recovery of the electrical parameters occurred after annealing at T∼400–800 K. Irradiation with a dose of 5.4 × 10¹⁵ cm⁻² resulted in very high resistance in forward biased pn structures which remained high even after heating to 500°C. It is suggested that proton irradiation causes decreasing of the lifetime and formation of an i- or an additional p-layer.     

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.     

Early stage of the crystallization in amorphous Fe–Si layers: Formation and growth of metastable α-FeSi2

Crystallization processes of amorphous Fe–Si layers have been investigated using transmission electron microscopy (TEM). Si(1 1 1) substrates were irradiated with 120 keV Fe ions at ?150 °C to a fluence of 1.0 × 10¹⁷ cm². An Fe-rich amorphous layer embedded in an amorphous Si was formed in the as-irradiated sample. Plan-view TEM observations revealed that a part of the amorphous Fe–Si layer crystallized to the metastable α-FeSi₂ after thermal annealing at 350 °C for 8 h. The lattice parameter of c-axis decreased with thermal annealing. It was considered that the change in the lattice parameter originates from the decrease of the Fe occupancy at (0, 0, 1/2) and its equivalent positions in the unit cell of the metastable α-FeSi₂.     

Controlling the microstructure of ZnO nanoparticles embedded in Sapphire by Zn ion implantation and subsequent annealing

(0 0 0 1) α-Al₂O₃ single crystals (sapphire) were implanted with Zn ions of 60 keV at a fluence of 1 × 10¹⁷ ions/cm². Transmission electron microscopy and optical absorption spectroscopy studies show the formation of ZnO nanoparticles in the sapphire substrate after the implanted sample was annealed at 700 °C in oxygen ambient. The photoluminescence spectrum of the annealed sample indicates the formation of ZnO nanoparticles with perfect lattice structure. The selected-area electron diffraction pattern proves that the ZnO nanoparticles have the (0 0 0 2) orientation which follows the orientation of Al₂O₃ substrate. The result shows that the crystallographic orientation of nanoparticles obtained through ion implantation is defined by the substrate.