Ion Irradiation Technique Research Articles

Stress-field in Sputtered Mo Thin Films and Mo/Ni Superlattices: Origin and Evolution after Ion-Irradiation

ABSTRACTIn the present study the ion irradiation technique is used to investigate the origin of the stress-field in Mo layers grown by ion beam sputtering. Strain measurements were performed by X- ray Diffraction (XRD) using the sin2ψ method. The evolution of the sin2ψ plots with ion irradiation shows that the usual assumption of a biaxial stress state is not adequate to determine the true stress-free lattice parameter a0 of the film. A new stress model based on a triaxial state of stress, which includes a hydrostatic component linked to point defects induced volume distortions, is derived to interpret the XRD results. For pure Mo films, the obtained a0 parameter is close to the bulk value, while for Mo sublayers in Mo/Ni superlattices, the a0 value is lower due to intermixing between Ni and Mo. These results demonstrate that ion irradiation is a powerful tool for stress relaxation, which allows to obtain additional information on the respective contribution of chemical effects and growth defects to the a0 value.

Ultimate performances of an all-optical sampler based on ultrafast saturable absorbers created by heavy ion irradiation

We analyze the ultimate performances of an all-optical sampler based on ultrafast bulk GaAs saturable absorbers created by ion irradiation. The ion irradiation technique provides a unique way to adapt the sampler resolution to the optical pulse width since the absorber relaxation time can be adjusted over more than three decades with the irradiation dose. Subpicosecond time resolutions with pulse contrast of /spl sim/100 are achievable by using high irradiation doses. In standard conditions, the sampler noise performances are limited by the detector noise and sampling pulse amplitude fluctuations. Picosecond laser diode pulses with an average power of less than one microwatt are already measurable. The ultimate limit is imposed by the absorber luminescence shot noise.

Magnetic properties of irradiated highly anisotropic materials

We have developed an ion irradiation technique that allows us to tune the magnetic properties of several magnetic alloys which are considered as promising candidates for ultra-high density magnetic recording such as FePd, FePt and CoPt/sub 3/ films. Depending both on the initial microstructure of the films and temperature used during irradiation, the chemical order and the resulting magnetic anisotropy can be either increased or decreased in a very controlled way. We discuss the possible mechanisms involved in ordering processes under irradiation.

Colloidal Ellipsoids with Continuously Variable Shape

Oblate and prolate ellipsoidal inorganic colloids can be synthesized by combining colloidal chemistry and ion irradiation techniques, as reported here. It is found that dramatic anisotropic plastic deformation of monodisperse silica and zinc sulfide microspheres occurs under 4 MeV Xe ion irradiation (see Figure). Possible application in the assembly of liquid crystals is demonstrated.

Ion irradiation effects in nonmetals: formation of nanocrystals and novel microstructures

Ion implantation is a versatile and powerful technique for producing nanocrystal precipitates embedded in the near-surface region of materials. Radiation effects that occur during the implantation process can lead to complex microstructures and particle size distributions, and in the present work, we focus on the application of these effects to produce novel microstructural properties for insulating or semiconducting nanocrystals formed in optical host materials. Nanocrystal precipitates can be produced in two ways: by irradiation of pure (i.e., non-implanted) crystalline or amorphous materials, or by ion implantation followed by either thermal annealing or subsequent additional irradiation. Different methods for the formation of novel structural relationships between embedded nanocrystals and their hosts have been developed, and the results presented here demonstrate the general flexibility of ion implantation and irradiation techniques for producing unique near-surface nanocomposite microstructures in irradiated host materials.

Characterization of an enhanced thermal oxide layer on 6H–SiC using ion irradiation

The oxidation rate of the Si-face of 6H–SiC(0001) was significantly enhanced by applying an ion irradiation technique. Such significant enhancement of the oxidation rate is ascribed to the breakage of the strong covalent bond of the SiC lattice using low-energy ion irradiation and the resultant chemical reactions. Thermal oxide layers on the ion irradiated SiC surface were studied by ellipsometer, secondary ion mass spectrometry (SIMS), cross-sectional transmission electron microscope (XTEM), I-V meter and C-V meter. The refractive indices for the samples, which were irradiated with high doses and oxidized for a long time, are 1.46±0.005, which well matches that of SiO 2 thermally grown on non-irradiated SiC substrates. SIMS analysis of the enhanced thermal oxide layer shows that thermal oxidation forms the homogeneous SiO 2 layer. The cross sectional TEM image of the enhanced thermal oxide layer shows an abrupt interface and good crystallinity of the substrate near the interface. The electrical properties of the interface between the thermal oxide layer and the SiC substrate depend on the residual damage induced by ion irradiation. A longer oxidation time and/or post oxidation annealing improve the electrical properties.

Radiation Effects in Nonmetals: Amorphization, Phase Decomposition, and Nanoparticles

AbstractRadiation effects in nonmetals have been studied for well over a century by geologists, mineralogists, physicists, and materials scientists. The present work focuses on recent results of investigations of the ion-beam-induced amorphization of the ABO4 compounds – including the orthophosphates (LnPO4; Ln = lanthanides) and the orthosilicates: zircon (ZrSiO4), hafnon (HfSiO4), and thorite (ThSiO4). In the case of the orthosilicates, heavy-ion irradiation at elevated temperatures causes the precipitation of a nanocrystalline metal oxide. Electron irradiation effects in these amorphized insulating ceramics can produce localized recrystallization on a nanometer scale. Similar electron irradiation techniques were used to nucleate monodispersed compound semiconductor nanocrystals formed by ion implantation of the elemental components into fused silica. Methods for the formation of novel structural relationships between embedded nanocrystals and their hosts have been developed and the results presented here demonstrate the general flexibility of ion implantation and irradiation techniques for producing unique near-surface microstructures in ion-implanted host materials.

Formation of the rim structure in high burnup fuel

The process of grain subdivision or polygonization in UO 2 was studied by using ion irradiation techniques and by investigating the rim structure in high burnup UO 2 fuel. Ion irradiation with fission product ions caused first the formation of subgrain boundaries, observed in high resolution transmission electron microscopy, HRTEM. At higher doses, polygonization was completed at a well defined threshold. High burnup UO 2 fuel showing well developed rim zones was investigated in optical, scanning and transmission electron microscopy. The oxygen potential Δ ¯G(O 2), the hardness H and the fracture toughness K Ic of the restructured material and the radial dependence of porosity, grain subdivision, H and K Ic were measured. K Ic was found to change at smaller radial positions than the other properties, indicating a nucleation process not affecting all properties to the same extent. The emerging picture for the formation of the rim structure in UO 2 is discussed.

Ballistic transport phenomena in crossed wire junction

A submicron, GaAsAlGaAs, crossed-wire junction with four side-gates is fabricated using electron beam lithography and 1keV Ar ion irradiation technique and the dependence of the bend resistance and the Hall resistance on gate voltage are examined. The Hall resistance at the low magnetic field is quenched and becomes negative with decreasing the gate voltage or decreasing the wire width. In addition, fluctuations are observed at the low magnetic field and on the last plateau. The amplitude increases with decreasing gate voltages.

In situ TEM study of ion-beam-induced amorphization of complex silicate structures

The in situ transmission electron microscopy (TEM) with ion-irradiation technique has been used for the first time to study the radiation-induced amorphization (metamictization) process of naturally occurring silicates: neptunite [Na 2KLi(Fe,Mn) 2Ti 2(SiO 3) 8], titanite (CaTiSiO 5), gadolinite (REE 2FeBe 2Si 2O 10), zircon (ZrSiO 4) and olivine [(Mg,Fe) 2SiO 4]. These phases were irradi 1.5 MeV Kr + ions in the Argonne HVEM-Tandem facility at room temperature with the electron diffraction pattern monitored in situ. The critical doses required for amorphization of the electron transparent thickness of neptunite, titanite, gadolinite, zircon and olivine have been determined to be 1.7, 2.0, 2.3, 4.8 and 6.0 × 10 14 ions/cm 2, respectively. The results show a correlation between amorphization dose and the chemical and structural complexity of these five phases. The most complex structure (e.g. neptunite) becomes amorphous at the lowest critical dose. The critical amorphization dose also increases with the increasing melting temperature of the minerals.

On the tailoring of polymers by radiation

Abstract Polymers are increasingly used in bioscientific device applications, such as dialysis units, vascular grafts, ocular contact lenses, comprehensive heart-and-lung machines, and sophisticated prosthetic devices such as hip-joints. Yet commercial polymers are in general unsatisfactory biomaterials. This is due to variability in composition, stability, surface smoothness, and the presence of impurities during production. On the other hand we have already successfully demonstrated that many of the mechanical and physical properties of polymers can be substantially improved by modifications of the surface by simple ion irradiation. Ion irradiation and track etch techniques have been used in a study of the surface properties of polymeric biomaterials, including polyetheylene, and teflon. The experimental results on surface improvement - especially wear, and stability under irradiation are discussed against the theoretical background of ion ranges and energy deposition in the polymer at the atomic/molecular level. Some new and exciting potential commercial applications are briefly referred to. There are implications for all polymeric biomaterials in this most recent practical approach to modification of basic physico-chemical properties. A high added value accompanies the much improved reliability of polymeric medical applications, and a greater compatibility with human tissue.

Comparison of swelling for structural materials on neutron and ion irradiation

The swelling of V-base alloys, Type 316 stainless steel, Fe-25Ni-15Cr alloy, ferritic steels, Cu, Ni, Nb-1% Zr, and Mo on neutron irradiation is compared with the swelling for these materials on ion irradiation. The results of this comparison show that utilization of the ion-irradiation technique provides for a discriminative assessment of the potential for swelling of candidate materials for fusion reactors.

Radiation induced phosphorus segregation in austenitic and ferritic alloys

The radiation induced surface segregation (RIS) of phosphorus in stainless steel attained a maximum at a dose of 0.8 dpa then decreased continually with dose. This decrease in the surface segregation of phosphorus at high dose levels has been attributed to removal of the phosphorus layer by ion sputtering. Phosphorus is not replenished since essentially all of the phosphorus within the irradiation zone has been segregated to the surface. Sputter removal can explain the previously reported absence of phosphorus segregation in ferritic alloys irradiated at high doses 1,2 (>1 dpa) since irradiation of ferritic alloys to low doses has shown measurable RIS. This sputtering phenomenon places an inherent limitation to the heavy ion irradiation technique for the study of surface segregation of impurity elements. The magnitude of the segregation in ferritics is still much less than in stainless steel which can be related to the low damage accumulation in these alloys.

Surface morphology and electronic properties of ErSi2

The surface of ErSi2, formed by the reaction of thin erbium layers with a single-crystal silicon substrate, is typically dominated by deeply penetrating regularly shaped pits. These pits are shown to have detrimental effects on the electronic performance of Schottky barrier diodes. Surface pits may be reduced in density or eliminated entirely (i) by the use of silicon substrate surfaces prepared under ultrahigh vacuum conditions prior to metal deposition, (ii) by means of ion irradiation techniques or (iii) by reacting erbium with an amorphous silicon (a-Si) layer. In this investigation, planar ErSi2 layers (pit free) are made using the third approach with a sample structure of a-Si/Er/c-Si where c-Si denotes crystalline silicon. The fast reaction between a-Si and erbium leads to a planar sample structure of ErSi2/c-Si with little or no reaction between erbium and the c-Si substrate. The electronic performance of pit-free ErSi2 diodes made in this manner is shown to be much superior to that of diodes made by reacting erbium with silicon substrates.

Surface morphology of erbium silicide

The surface of rare-earth silicides (Er, Tb, etc.), formed by the reaction of thin-film metal layers with a silicon substrate, is typically dominated by deep penetrating, regularly shaped pits. These pits may have a detrimental effect on the electronic performance of low Schottky barrier height diodes utilizing such silicides on n-type Si. This study suggests that contamination at the metal-Si or silicide-Si interface is the primary cause of surface pitting. Surface pits may be reduced in density or eliminated entirely through either the use of Si substrate surfaces prepared under ultrahigh vacuum conditions prior to metal deposition and silicide formation or by means of ion irradiation techniques. Silicide layers formed by these techniques possess an almost planar morphology.

Surface Morphology and Electronic Properties of ErSi2

The surface of ErSi2, formed by the reaction of thin erbium layers with a singlecrystal silicon substrate, is typically dominated by deeply penetrating regularly shaped pits. These pits are shown to have detrimental effects on the electronic performance of Schottky barrier diodes. Surface pits may be reduced in density or eliminated entirely (i) by the use of silicon substrate surfaces prepared under ultrahigh vacuum conditions prior to metal deposition, (ii) by means of ion irradiation techniques or (iii) by reacting erbium with an amorphous silicon (a-Si) layer. In this investigation, planar ErSi2 layers (pit free) are made using the third approach with a sample structure of a-Si/Er/c-Si where c-Si denotes crystalline silicon. The fast reaction between a-Si and erbium leads to a planar sample structure of ErSi2/c-Si with little or no reaction between erbium and the c–Si substrate. The electronic performance of pit–free ErSi2 diodes made in this manner is shown to be much superior to that of diodes made by reacting erbium with silicon substrates.