Xenon-ion-induced and thermal mixing of Co/Si bilayers and their interplay

Studies on ion-irradiated transition-metal/silicon bilayers demonstrate that interface mixing and silicide phase formation depend sensitively on the ion and film parameters, including the structure of the metal/Si interface. Thin Co layers e-gun evaporated to a thickness of 50 nm on Si(1 0 0) wafers were bombarded at room temperature with 400-keV Xe⁺ ions at fluences of up to 3 × 10¹⁶ cm⁻². We used either crystalline or pre-amorphized Si wafers the latter ones prepared by 1.0-keV Ar-ion implantation. The as-deposited or Xe-ion-irradiated samples were then isochronally annealed at temperatures up to 700 °C. Changes of the bilayer structures induced by ion irradiation and/or annealing were investigated with RBS, XRD and HRTEM. The mixing rate for the Co/c-Si couples, Δσ²/Φ = 3.0(4) nm⁴, is higher than the value expected for ballistic mixing and about half the value typical for spike mixing. Mixing of pre-amorphized Si is much weaker relative to crystalline Si wafers, contrary to previous results obtained for Fe/Si bilayers. Annealing of irradiated samples produces very similar interdiffusion and phase formation patterns above 400 °C as in the non-irradiated Co/Si bilayers: the phase evolution follows the sequence Co₂Si → CoSi → CoSi₂.     

Mixing of Cr and Si atoms induced by noble gas ions irradiation of Cr/Si bilayers

Cr/Si bilayers were irradiated at room temperature with 120 keV Ar, 140 keV Kr and 350 keV Xe ions to fluences ranging from 10¹⁵ to 2 × 10¹⁶ ions/cm². The thickness of Cr layer evaporated on Si substrate was about 400 Å. Rutherford backscattering spectrometry (RBS) was used to investigate the atomic mixing induced at the Cr-Si interface as function of the incident ion mass and fluence. We observed that for the samples irradiated with Ar ions, RBS yields from both Cr layer and Si substrate are the same as before the irradiation. There is no mixing of Cr and Si atoms, even at the fluence of 2 × 10¹⁶ ions/cm². For the samples irradiated with Kr ions, a slight broadening of the Cr and Si interfacial edges was produced from the fluence of 5 × 10¹⁵ ions/cm². The broadening of the Cr and Si interfacial edges is more pronounced with Xe ions particularly to the fluence of 10¹⁶ ions/cm². The interface broadening was found to depend linearly on the ion fluence and suggests that the mixing is like a diffusion controlled process. The experimental mixing rates were determined and compared with values predicted by ballistic and thermal spike models. Our experimental data were well reproduced by the thermal spikes model.     

Ion-beam irradiation effects on reactively sputtered CrN thin films

The present study deals with CrN/Si bilayers irradiated at room temperature (RT) with 120 keV Ar ions. The CrN layers were deposited by d.c. reactive sputtering on Si(1 0 0) wafers, at different nitrogen partial pressures (2 × 10⁻⁴, 3.5 × 10⁻⁴ and 5 × 10⁻⁴ mbar), to a total thickness of 240-280 nm. The substrates were held at room temperature (RT) or 150 °C during deposition. After deposition the CrN/Si bilayers were irradiated up to fluences of 1 × 10¹⁵ and 1 × 10¹⁶ ions/cm². Structural characterization was performed with Rutherford backscattering spectroscopy (RBS), cross-sectional transmission electron microscopy (XTEM) and grazing angle X-ray diffraction (XRD). For the highest nitrogen pressure (5 × 10⁻⁴ mbar) a pure stoichiometric CrN phase was achieved. The results showed that Ar ion irradiation resulted in the variation of the lattice constants, micro-strain and mean grain size of the CrN layers. The observed microstructural changes are due to the formation of the high density damage region in the CrN thin film structure.     

Formation of dislocation loops in silicon by ion irradiation for silicon light emitting diodes

We have studied the influence of the ion species, ion energy, fluence, irradiation temperature and post-implantation annealing on the formation of shallow dislocation loops in silicon, for fabrication of silicon light emitting diodes. The substrates used were (1 0 0) Si, implanted with 20-80 keV boron at room temperature and 75-175 keV silicon at 100 and 200 °C. The implanted fluences were from 5 × 10¹⁴ to 1 × 10¹⁵ ions/cm². After irradiation the samples were processed for 15 s to 20 min at 950 °C by rapid thermal annealing. Structural analysis of the samples was done by transmission electron microscopy and Rutherford backscattering spectrometry. In all irradiations the silicon substrates were not amorphized, and that resulted in the formation of extrinsic perfect and faulted dislocation loops with Burgers vectors a/2〈1 1 0〉 and a/3〈1 1 1〉, respectively, sitting in {1 1 1} habit planes. It was demonstrated that by varying the ion implantation parameters and post-irradiation annealing, it is possible to form various shapes, concentration and distribution of dislocation loops in silicon.     

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.     

Effect of ion beam irradiation on magnetic and structural properties of Pt/Cr/Co multilayers

This paper discusses the effect of ion beam irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe Co-Cr-Pt ternary alloy phase formation in 1 MeV N⁺ ion irradiated [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]_×6/Si multilayers for a fluence of 1 × 10¹⁶ ions cm⁻² and beyond. The observed phase formation is accompanied by an enhancement in the average grain size, surface roughness and coercivity. Monte Carlo simulation has been performed to study ion-induced defect evolution and atomic displacements to correlate the above observed effects.     

Resonant Rutherford backscattering spectrometry for carbon diffusion in silicon

300 keV C⁺ ion implantation onto Si(1 0 0) wafers was carried out at temperatures of 400, 500, 550, 600, 650 and 700 °C. Depth profile of C was determined by resonant Rutherford backscattering spectrometry (RRBS) measurements using ¹²C(α,α)¹²C resonant reaction with the α-particle energy of 4.27 MeV. The concentration of the implanted carbon at the surface as a function of inverse of implantation temperature shows an Arrhenius behaviour. The activation energy for diffusion of carbon in Si was measured and found to be 0.434 eV, which is smaller than the activation energy (0.88 eV) for the C diffusion in Si in equilibrium condition. The possible mechanism of C diffusion in Si during irradiation conditions existing in our experiments where large concentration of vacancies and interstitials are produced is discussed and we find that the C diffusion during irradiation conditions could be due to the drag the carbon towards the surface by the vacancy flux.     

Incidence-angle dependent Cs incorporation in Si during low-energy bombardment: A dynamic computer simulation study

The implantation of Cs atoms in silicon was investigated by dynamic computer simulations using the Monte-Carlo code T-DYN that takes into account the gradual change of the target composition due to the Cs irradiation. The incorporation of Cs atoms was studied for incidence angles ranging from 0° to 85° and for four impact energies (0.2, 0.5, 1 and 3 keV). The total implantation fluences were (1-2) × 10¹⁷ Cs/cm², well above the values required to reach a stationary state. The steady-state Cs surface concentrations exhibit a pronounced dependence on impact angle and energy. At normal incidence, they vary between ∼0.57 (at 0.2 keV) and ∼0.18 (3 keV), but decrease with increasing incidence angle. Under equilibrium, the partial sputtering yield of Si exhibits the typical dependence on incidence angle, first increasing up to a maximum value (at ∼70°-75°) and declining sharply for larger angles. For all irradiation conditions a strongly preferential sputtering of Cs as compared to Si atoms is found, increasing with decreasing irradiation energy (from 4.6 at 3 keV to 7.2 at 0.2 keV) and for nearer-normal incidence.     

CaF2:Tm (TLD-300) thermoluminescent response and glow curve induced by γ-rays and ions

The thermoluminescent response of CaF₂:Tm after exposure to ⁶⁰Co γ-rays at doses from 0.44 Gy to 8.75 kGy and to low fluences (10⁵ ∼ 10⁸ cm⁻²) of 25 and 40 MeV ¹H, 75 and 120 MeV ³He, 180, 300 and 480 MeV ¹²C, 400 MeV ¹⁶O and 800 MeV ²⁰Ne ion beams, spanning a LET interval up to about 500 keV/μm, has been investigated. A careful deconvolution analysis of the glow curve has been performed in order to obtain information for individual peaks. The region of linear response to gammas extends up to ≈1 Gy, while that for ions includes the complete dose interval covered in the study (up to ≈1.3 Gy). The ratio between the high- and low-temperature structures in the glow curve is correlated with radiation quality and dependence on ion identity, besides LET, is strongly suggested by the data. The thermoluminescent efficiency to ion exposure, with respect to irradiation with ⁶⁰Co γ-rays, shows a different dependence on LET for each of the peaks. In general terms, the efficiency reaches a maximum between 1.2 and 1.4 near 6 keV/μm and decreases for higher LET. Peak 3 displays a unique trend, its relative efficiency is always less than 1.0 and shows a strong monotonic inverse dependence with LET.     

Fabrication of hollow Ag nanoclusters in silica by irradiation with Ar ions

Ion irradiation is an effective method to control the morphology, size and distribution of metal nanoclusters in substrates. In this work, Ag nanoclusters embedded in silica by 200 keV Ag⁺ ion implantation were irradiated at room temperature with Ar⁺ ions at 200 keV and 500 keV to different fluences. After irradiation, a transmission electron microscopy (TEM) study revealed that nanovoids are formed in the larger Ag nanoclusters. With the increase of fluence and energy of the Ar⁺ ions, the number and average size of the nanovoids grow combining with increases in the average size of the larger Ag nanoclusters within a projected range. During the ion irradiation process, the electronic energy and nuclear energy loss of the Ar⁺ ions determine the size of the hollow Ag nanoclusters and the change of the size and distribution of Ag nanoclusters in silica, leading to changes in the optical absorption spectra.     

Fabrication and nuclear analysis of isotopic N and N ion-implanted silicon standards

The fabrication of reliable isotopic nitrogen standards is achieved in Si through ¹⁴N and ¹⁵N ion implantation. 60 keV and ions were implanted at 400 °C up to ∼60% peak atomic concentration, yielding nitrogen-saturated silicon layers as measured using resonant nuclear reaction analysis. No isotopic effect has been observed. The nitrogen standards are validated by measurements of stability under ion irradiation. No significant desorption of nitrogen is observed either under a ⁴He⁺ ion fluence of 3.36 × 10¹⁶ cm⁻² or under a ¹H⁺ ion fluence of 8.60 × 10¹⁷ cm⁻², giving strong evidence that isotopic nitrogen standards can be achieved.     

Sub-surface retention of Pb atoms in silicon after low-energy ion implantation and electron beam annealing

Low-energy Pb ion implantation into (1 0 0) Si and subsequent high-vacuum electron beam annealing was used to study the potential of sub-surface retention of Pb atoms after applying a high temperature annealing process. 7 keV Pb⁺ ions were implanted into p-type (1 0 0) Si at room temperature with a fluence of 4 × 10¹⁵ ions cm⁻². The implantation results in a Pb depth distribution that has a calculated Pb peak concentration of 23.9 at.% at a depth of 8.0 nm. The Pb implanted Si substrates were annealed with a high-current 20 keV electron beam at 200-700 °C for 15 s. The Pb loss by out-diffusion was measured with RBS. Key results are: (i) minimal Pb loss in samples annealed up to 400 °C, (ii) emerging out-diffusion above 400 °C, (iii) retention of Pb atoms in the near-surface region in samples annealed up to 700 °C. Comparison of the RBS data with the calculated evaporation rate of Pb under similar conditions reveals two distinctive temperature ranges in which the measured Pb loss of the implanted samples disagrees with the calculated Pb loss: (1) Pb atoms diffused out of the samples at a higher rate in the temperature range up to 400 °C and (2) the Pb atoms diffused out of the samples at a much slower rate above 450 °C. Both phenomena are attributed to the ion implantation process.     

Effects of ion irradiation on the structural transformation of sol-gel derived TEOS/MTES thin films

Ion beam processing of organic/inorganic thin films has been shown to be an effective means in converting polymeric films into their final ceramic-like state. In this study, hybrid sol-gel derived thin films based on TEOS (tetraethylorthosilicate) Si(OC₂H₅)₄ and MTES (methyltriethoxysilane) CH₃Si(OC₂H₅)₃ were prepared and deposited on Si substrates by spin coating. After the films were allowed to air dry, they were heat treated at 300 °C for 10 min. Ion irradiation was performed at room temperature using 125 keV H⁺ and 250 keV N²⁺ ions with fluences ranging from 1 × 10¹⁴ to 5 × 10¹⁶ ions/cm². FT-IR and Raman spectroscopies were used to quantify the chemical structural transformations which occurred including the evolution of the organic components, the cross-linking of silica clusters, and the clustering of carbon.     

Raman investigation of incident N-, Xe-ions induced effects in ZnO thin films

Highly c-axis orientation ZnO thin films with hundreds nanometers in thickness have been deposited on (1 0 0) Si substrate by RF magnetron sputtering. These films are implanted at room temperature by 80 keV N-ions with fluences from 5.0 × 10¹⁴ to 1.0 × 10¹⁷ ions/cm², implanted by 400 keV Xe-ions with 2.0 × 10¹⁴ to 2.0 × 10¹⁶ ions/cm², irradiated by 3.64 MeV Xe-ions with 1.0 × 10¹² to 1.0 × 10¹⁵ ions/cm², or irradiated by 308 MeV Xe-ions with 1.0 × 10¹² to 5.0 × 10¹⁴ ions/cm², respectively. Then the ZnO films are investigated using a Raman spectroscopy. The obtained Raman spectra show that a new Raman peak located at about 578 cm⁻¹ relating to simple defects or disorder phase appears in all ZnO films after ion implantation/irradiation, a new Raman peak at about 275 cm^-1 owing to N-activated zinc-like vibrations is observed in the N-implanted samples. Moreover, a new Raman peak at about 475 cm⁻¹ is only seen in the samples after 400 keV and 3.64 MeV Xe-ions bombardment. The area intensity of these peaks increases with increasing ion fluence. The effects of ion fluence, element chemical activity, atom displacements induced by nuclear collisions as well as energy deposition on the damage process of ZnO films under ion implantation/irradiation are discussed briefly.     

Change in magnetic properties of MgB2 thin films after irradiation by 200 MeV Au ion beam

The SHI irradiation induced effects on magnetic properties of MgB₂ thin films are reported. The films having thickness 300-400 nm, prepared by hybrid physical chemical vapor deposition (HPCVD) were irradiated by 200 MeV Au ion beam (S_e ∼ 23 keV/nm) at the fluence 1 × 10¹² ion/cm². Interestingly, increase in the transition temperature T_c from 35.1 K to 36 K resulted after irradiation. Substantial enhancement of critical current density after irradiation was also observed because of the pinning provided by the defects created due to irradiation. The change in surface morphology due to irradiation is also studied.     

Temperature-dependent surface modification of InSb(0 0 1) crystal by low-energy ion bombardment

Structural and compositional modification of InSb(0 0 1) single crystal surfaces induced by oblique incidence 2-5 keV Ar and Xe ion irradiation have been investigated by means of scanning tunneling and atomic force microscopies, and time-of-flight mass spectroscopy of secondary ion emission. In general, ion-induced patterns (networks of nanowires, or ripples) are angle of incidence- and fluence-dependent. Temperature dependences (from 300 to 600 K) of the RMS roughness and of the ripple wavelength have been determined for the samples bombarded with various fluences. Secondary ion emission from an InSb(0 0 1) surface exposed to 4.5 keV Ar⁺ ions has been investigated with a linear TOF spectrometer working in a static mode. Mass spectra of the sputtered In⁺, Sb⁺ and In₂⁺ secondary ions have been measured both for the non-bombarded (0 0 1) surface and for the surface previously exposed to a fluence of 10¹⁶ ions/cm². In⁺ and In₂⁺ intensities for the irradiated sample are much higher in comparison to the non-bombarded one, whereas Sb⁺ ions show a reversed tendency. This behavior suggests a significant In-enrichment at the InSb(0 0 1) surface caused by the ion bombardment.     

Stability of nano-scaled Ta/Ti multilayers upon argon ion irradiation

The effects of argon ion irradiation on structural changes in Ta/Ti multilayers deposited on Si wafers were investigated. The starting structures consisted of sputter deposited 10 alternate Ta (∼23 nm) and Ti (∼17 nm) layers of a total thickness ∼200 nm. They were irradiated at room temperature with 200 keV Ar⁺, to the fluences from 5 × 10¹⁵ to 2 × 10¹⁶ ions/cm². The projected ion range was around mid-depth of the multilayered structure, and maximum displacements per atom ∼130. It was found that, despite of the relatively heavy ion irradiation, individual nanocrystalline Ta and Ti layers remain unmixed, keeping the same level of interface planarity. The changes observed in the mostly affected region are increase in lateral dimensions of crystal grains in individual layers, and incorporation of bubbles and defects that cause some stretching of the crystal lattice. Absence of interlayer mixing is assigned to Ta-Ti immiscibility (reaction enthalpy ΔH_f = +2 kJ/mol). It is estimated that up to ∼5 at.% interface mixing induced directly by collision cascades could be compensated by dynamic demixing due to chemical driving forces in the temperature relaxation regime. The results can be interesting towards developing radiation tolerant materials based on multilayered structures.     

Damage creation in ion irradiated Si1−xGex/Si structures

Strained SiGe/Si structures have been proposed as substrates for fabrication of high speed metal oxide semiconductor transistors. However, influence of strain and/or presence of Ge atoms on damage creation during ion irradiation have not been explored to a significant extent. In this study, Rutherford backscattering spectrometry (RBS) was used to characterize Si_1−xGe_x/Si structures irradiated by 140 keV He⁺ ions at room temperature. When compared with pure Si, strained samples show enhanced damage accumulation as a function of He fluence. Channeling angular scans did not reveal any specific configuration of displacements. Possible mechanisms for enhanced damage in strained Si are discussed.     

Ion-induced electron emission monitoring of the structure and morphology evolution in HOPG

The temperature dependences of the ion-induced electron emission yield γ of highly-oriented pyrolytic graphite (HOPG) under high-fluence (10¹⁸-10¹⁹ ions/cm²) 30 keV Ar⁺ ion irradiation at ion incidence angles from θ = 0^o (normal incidence) to 80^o have been measured to trace both the structure and morphology changes in the basal oriented samples. The target temperature has been varied during continuous irradiation from T = −180 to 400 ^oC. The surface analysis has been performed by the RHEED and SEM techniques. The surface microgeometry was studied using laser goniophotometry (LGF). The dependences of γ(T) were found to be strongly non-monotonic and essentially different from the ones for Ar⁺ and N₂⁺ ion irradiation of the polygranular graphites. A sharp peak at irradiation temperature T_p ≈ 150 ^oC was found. A strong influence of electron transport anisotropy has been observed, and ion-induced microgeometry is discussed.     

Proton beam induced luminescence of silicon dioxide implanted with silicon

Light emission from a silicon dioxide layer enriched with silicon has been studied. Samples used had structures made on thermally oxidized silicon substrate wafers. Excess silicon atoms were introduced into a 250-nm-thick silicon dioxide layer via implantation of 60 keV Si⁺ ions up to a fluence of 2 × 10¹⁷ cm⁻². A 15-nm-thick Au layer was used as a top semitransparent electrode. Continuous blue light emission was observed under DC polarization of the structure at 8-12 MV/cm. The blue light emission from the structures was also observed in an ionoluminescence experiment, in which the light emission was caused by irradiation with a H₂⁺ ion beam of energy between 22 and 100 keV. In the case of H₂⁺, on entering the material the ions dissociated into two protons, each carrying on average half of the incident ion energy. The spectra of the emitted light and the dependence of ionoluminescence on proton energy were analyzed and the results were correlated with the concentration profile of implanted silicon atoms.