Radiation-enhanced Diffusion Research Articles

Growth kinetics of CoSi formed by ion beam irradiation at room temperature

Growth kinetics of cobalt silicide layers formed by ion beam irradiation was investigated at a temperature between room temperature and 100 °C. The CoSi phase was identified by x-ray diffraction of Co/Si samples irradiated with 25 keV argon ions to a dose of 2.0×1015 cm−2. The number of intermixed silicon atoms in the CoSi layers was evaluated as a function of dose, dose rate, and nuclear energy deposition rate at the Co/Si interface for samples irradiated with 40 keV focused silicon ion beams. The growth is shown to be diffusion-limited and attributed to radiation-enhanced diffusion with an activation energy of 0.16 eV. The number of intermixed silicon atoms is approximately proportional to the nuclear energy deposition rate at the initial Co/Si interface, while it is independent of dose rate, which shows that the CoSi phase is formed without contribution of the sample heating caused by irradiation.

Role of defect diffusion in the InP damage profile

Channeling of incident ions and radiation-enhanced diffusion of the ion-created defects have been shown to be major components of the ion damage profile. Our earlier results showed a deeper damage profile in InP, compared to GaAs, when subjected to the same ion bombardment conditions. Computer simulations demonstrated that this can partially be attributed to the greater ion channeling range in InP. In this article the role of defect diffusion in InP, through experiments coupled with simulations, is delineated. The multiple quantum well (MQW) probe technique is used to determine the amount of damage by measuring the change in low temperature photoluminescence of quantum wells before and after argon ion bombardment. A blocking superlattice is added to the MQW heterostructure and is proven effective in preventing damage from propagating into the material below it. By correlating the experimental results with computer modeling, an estimate of the defect diffusion constant is obtained and it is found to be in the range of 4×10−15–1×10−14 cm2/s. These high values for diffusion are justified with experimental results that illustrate the presence of radiation-enhanced diffusion mechanisms during ion bombardment.

Characterization of the radiation-enhanced diffusion of dry-etch damage in n-GaAs

Radiation-enhanced diffusion of dry-etch damage observed from experiments has been further characterized with Schottky diodes and deep level transient spectroscopy (DLTS) measurements. The use of DLTS spectra to monitor the effects of changes in ion dose rate and the application of laser radiation shows that the ion-induced defects having high diffusivities during ion-assisted processes are basically associated with the components of primary point defects, such as interstitials and vacancies. The properties of ion-induced traps obtained from DLTS measurements may provide us with some information to refine our model on the low-energy ion-induced damage.

Atom transport under ion irradiation

Diffusion coefficients in nickel single crystals under heavy ion irradiation were measured near the irradiated surface by using multi-layer specimens. Analysis of the depth dependence of these diffusion coefficients yields information on the effective fraction η of the displacement rate, with which transporting, i.e., ‘freely migrating’ defects are produced, and on the radiation-induced sink strength. Upper limits of η = 7% are estimated for a self-ion irradiation at 950 K. By increasing the irradiation temperature from 800 to 950 K the effective sink strength was observed to decrease while η increases. The relation between swelling rates and the magnitude of the radiation-enhanced diffusion coefficient is briefly discussed.

Calculation of marker distortion at elevated temperatures under low energy ion bombardment

The contributions of radiation-enhanced diffusion and ion induced collisional mixing to the distortion of the sputter depth profile of a near surface pseudo-marker under 100 eV Ar ion bombardment of a Cu (100) surface at 500 K have been evaluated using a simple semi-infinite model of radiation — enhanced diffusion via vacancies and interstitials. The ion mixing coefficient and the production of ion-induced point defects have been obtained from Molecular Dynamics calculations which were discussed in a previous paper [G. Kornich, G. Betz, Nucl. Instr. and Meth. B 117 (1996) 81]. The distortion of such a pseudo-marker is mainly caused by radiation-enhanced diffusion and only weakly due to collisional mixing. Steady-state concentrations of point defects with depth have been obtained. It was found that they depend strongly on the efficiency of defect sinks at the surface. The influence of internal sinks and of mutual recombination of defects is much weaker.

Surface hardening of Al by high current Fe-ion implantation

Surface modification of Al was studied by high current Fe-ion implantation using a metal vapor vacuum arc ion source. In the implantation process, two parameters were adjusted, that is, current density and ion dose, corresponding to varying the temperature and time for growing surface Fe-aluminide compound. The major hardening phase was identified to be Al13Fe4 and its depth profile was found to depend on the current density and dose. At a fixed dose of 3×1017 Fe/cm2, implanting with a low current density from 25 to 51 μA/cm2, the Al13Fe4 compound penetrated into a depth between 2500 and 4000 Å. Whereas implanting with a high current density up to 101 μA/cm2, the Al13Fe4 compound not only be situated in a 1000 Å surface layer, but also extended into a deep region when the dose was increased to 1×1018 Fe/cm2. Another dual implantation was conducted with two different current densities and it resulted in a modified region of 4500 Å thick with a high concentrated Al13Fe4 compound in a 1000 Å surface layer. Accordingly, the microhardness of the implantation treated Al films was considerably increased. The formation of the hardening phase of Al13Fe4 compound was responsible for the improvement of the surface mechanical property and was discussed in terms of temperature rise, irradiation time, and radiation-enhanced diffusion in the process of implantation.

Radiation enhanced transport of hydrogen in SiO 2

2.2 MeV 4He + and 7 MeV 15N 2+ ion beams have been used to investigate, by in situ measurements, the hydrogen desorption processes in SiO 2 thin films deposited by plasma enhanced chemical vapor deposition (PECVD). An effective cross-section of 3 × 10 −16 cm 2 for He and 17 × 10 −16 cm 2 for N has been measured for the ion-thin-film interaction phenomena. The structure crystalline silicon/thermally grown SiO 2/PECVD film has also been investigated for the hydrogen radiation-enhanced diffusion in thermally grown SiO 2. The data are consistent with a diffusion process with a diffusion coefficient of (5.0 ± 0.5) × 10 −26 cm 2/ion from a constant source at a concentration C 0 = (2.5 ± 0.2) × 10 21 at/cm 3.

Electron-irradiation-induced nucleation and growth in amorphous LaPO4, ScPO4, and zircon

Synthetic LaPO4, ScPO4, and crystalline natural zircon (ZrSiO4) from Mud Tanks, Australia were irradiated by 1.5 MeV Kr+ ions until complete amorphization occurred as indicated by the absence of electron-diffraction maxima. The resulting amorphous materials were subsequently irradiated by an 80 to 300 keV electron beam in the transmission electron microscope at temperatures between 130 and 800 K, and the resulting microstructural changes were monitored in situ. Thermal anneals in the range of 500 to 600 K were also conducted to compare the thermally induced microstructural development with that produced by the electron irradiations. Amorphous LaPO4 and ScPO4 annealed to form a randomly oriented polycrystalline assemblage of the same composition as the original material, but zircon recrystallized to ZrO2 (zirconia) + amorphous SiO2 for all beam energies and temperatures investigated. The rate of crystallization increased in the order: zircon, ScPO4, LaPO4. Submicrometer tracks of crystallites having a width equal to that of the electron beam could be “drawn” on the amorphous substrate. In contrast, thermal annealing resulted in epitaxial recrystallization from the thick edges of the TEM samples. Electron-irradiation-induced nucleation and growth in these materials can be explained by a combination of radiation-enhanced diffusion as a result of ionization processes and a strong thermodynamic driving force for crystallization. The structure of the amorphous orthophosphates may be less rigid than that of their silicate analogues because of the lower coordination across the PO4 tetrahedron, and thus a lower energy is required for reorientation and recrystallization. The more highly constrained monazite structure-type recovers at a lower electron dose than the zircon structure-type, consistent with recent models used to predict the crystalline-to-amorphous transition as a result of ion irradiation.

AES analysis of nitridation of Si(100) by 2–10 keV N +2 ion beams

Ion beam nitridation of Si(100) as a function of N + 2 ion energy in the range of 2–10 keV has been investigated by in-situ Auger electron spectroscopy (AES) analysis and Ar + depth profiling. The AES measurements show that the nitride films formed by 4–10 keV N + 2 ion bombardment are relatively uniform and have a composition of near stoichiometric silicon nitride (Si 3N 4), but that formed by 2 keV N + 2 ion bombardment is N-rich on the film surface. Formation of the surface N-rich film by 2 keV N + 2 ion bombardment can be attributed to radiation-enhanced diffusion of interstitial N atoms and a lower self-sputtering yield. AES depth profile measurements indicate that the thicknesses of nitride films appear to increase with ion energy in the range from 2 to 10 keV and the rate of increase of film thickness is most rapid in the 4–10 keV range. The nitridation reaction process which differs from that of low-energy (< 1 keV) N + 2 ion bombardment is explained in terms of ion implantation, physical sputtering, chemical reaction and radiation-enhanced diffusion of interstitial N atoms.

Cohesive energy effects on the atomic transport induced by ion beam mixing

Atomic transport in the radiation enhanced diffusion (RED) region has been studied from the shifts of a marker layer in ion beam mixed Pd Co and Pd Au bilayers. 80 keV Ar + with a dose of 1.5 × 10 16 ions/cm 2 were irradiated into the bilayers at temperature region from 90 K to 700 K. In the Pd Co system, the atomic flux of Pd ( J Pd) transported across the interface is nearly same with J Co in the thermal spike region, while J Pd is always larger than J Co in the RED regio However, in the Pd Au system, J Pd is nearly same with J Au in both of the thermal spike and RED regions. We have developed a model to describe the atomic transport in the RED region, which predicts that the atom with small cohesive energy has more mobility than that with large cohesive energy.

Argon incorporation and surface compositional changes in InP(100) due to low-energy Ar+ ion bombardment

Angle-resolved x-ray photoelectron spectroscopy (ARXPS) has been used to study the Ar incorporation and surface compositional changes in InP(100) after 1–5 keV Ar+ bombardment at various ion fluences. The ARXPS measurements showed that the incorporated Ar concentration achieved saturation at ion bombardment fluences of &amp;gt;1016 cm−2. The surface Ar concentration decreased with increasing bombardment energy. No Ar bubbles were observed by atomic force microscopy, suggesting that Ar bubble formation was not the main Ar trapping mechanism. The altered layers were, on average, In rich up to the sampling depth of the ARXPS technique. However, the altered layers were inhomogeneous as a function of depth and appeared more In rich at the surface than in the subsurface region. The results are compared with those obtained by other authors and discussed in the context of preferential sputtering, radiation-enhanced diffusion and segregation, and Ar incorporation. Although the altered layers were In rich, a P-rich phase induced by Ar+ bombardment was identified in the altered layers.

Ion-beam induced compositional changes in alloys at elevated temperatures

Near-surface compositional changes in alloys during ion bombardment have been studied theoretically. The scheme employed ensures pressure relaxation of the target, and the effects of preferential sputtering, collisional mixing, radiation-enhanced diffusion, and Gibbsian and radiation-induced segregation are allowed for. High-fluence composition profiles were determined directly from a nonlinear integro-differential equation by means of an efficient iteration procedure developed recently. The dependence of the composition profile on input parameters such as target temperature and defect mobility was examined for NiCu and NiPd alloys.

DYNAMIC PROCESSES OF ALTERED LAYER FORMATION IN Cu-Pt ALLOYS UNDER ION BOMBARDMENT

Three different experimental approaches have been developed to study the dynamic process of subsurface altered layer formation in a Cu-Pt alloy under Ar + ion bombardment: (1) sputter neutral mass spectrometry by multiphoton ionization (MPI-SNMS) for the study of preferential sputtering caused by the collision cascade process in the very initial stage of sputtering; (2) ion scattering spectroscopy (ISS)-Auger electron spectroscopy (AES) sequential measurements for investigating radiation-enhanced Gibbsian segregation in the transient stage of sputtering; (3) an approach based on ISS monitoring by prompt switching of the ion bombardment with ( He ++ Ar +) ions to that with He + ions, for revealing the cooling effect in radiation-enhanced diffusion in the final steady state of sputtering. For this we have developed a specific coevaporating device for depositing Cu and Pt simultaneously on a substrate at constant deposition rate. The coevaporating device was attached to both of the specimen chambers of the Auger microprobe, JAMP-3, and of the MPI-SNMS apparatus. The results have clearly revealed: (i) ion bombardment causes a preferential sputtering of Cu atoms in the very initial stage of sputtering, (ii) followed by gradual formation of an altered layer as ion sputtering proceeds in the transient stage, and (iii) finally the alloy system approaches a steady state where the composition profile is controlled by cascade mixing, radiation-enhanced Gibbsian segregation and radiation-enhanced diffusion to satisfy the mass balance law. In the steady state the approach (3) has, first, revealed that the cooling effect does exist in radiation-enhanced diffusion.

Radiation-enhanced diffusion due to interstitials and dynamic crowdions

It is shown from measurements of the radiation damage rate in copper and in gold that dynamic crowdions cause spontaneous annihilation of radiation-produced vacancies and interstitials during irradiation so that the initial production rate of interstitials and vacancies is decreased by orders of magnitude with respect to the initial damage rate. The number of replacement collision sequences is N = 40 000 in pure materials decreasing to N = 15 if under- or over-sized atoms are added. It is shown that dynamic crowdions cause tracer-diffusion and inter-diffusion, e.g. of silver in nickel and of nickel in silver, and that they are able to destroy order very effectively. Three-dimensionally migrating radiation-produced point defects cause micro-structural changes above the temperature range above which they are sufficiently mobile, e.g. above 125°C in nickel and above − 16°C in copper. There is no three-dimensionally migrating defect below these respective temperatures in nickel and copper contributing to diffusion.

Radiation enhanced diffusion of low energy ion-induced damage

We have investigated the influence of concurrent above-band-gap laser illumination on the damage profile of GaAs/AlGaAs heterostructures subject to low energy (sub-keV) Ar+ ion bombardment. A dramatic change in damage profile was observed for these samples, compared with those that were not laser illuminated, and the degradation increases with the illuminated power intensity. Below-band-gap illumination results in a minimal increase in damage profile. Such results indicate the possibility of radiation-enhanced diffusion of defects, and may explain the observed high defect diffusivity at room temperature.

Lattice disorder and radiation enhanced annealing in La-implanted TiO 2 single crystals

Rutherford backscattering spectrometry and channeling were used to characterize the disorder produced by implantation of La ions into TiO 2 (rutile) single crystals at substrate temperatures T i between 77 and 1100 K. Complete amorphization was observed at T i of 77 and 300 K for La fluences of 1 × 10 15 cm −2 and 2 × 10 15cm −2, respectively. At T i equal to 300 K and fluences of 1 × l0 15cm −2, radiation-enhanced annealing starts to affect the disorder profile. The recovery occurs preferentially in the near-surface region. With increasing T i, two recovery stages of the lattice disorder, between 300 and 500 K and above 700 K, have been observed. These stages are about 200 K lower than those observed for subsequent isochronal annealing of an amorphous TiO 2 layer. At T i equal to 1100 K recovery of the lattice disorder was nearly complete. A minimum yield of 2.5% was reached, similar to that of the virgin crystal. Radiation-enhanced diffusion of La into TiO 2 leads to an estimated substitutional component of 35%.

Ion beam mixing at copper/alumina interfaces using marker geometry

In the present paper results on ion beam effects such as ion beam mixing, radiation enhanced diffusion (RED) and phase stability in Cu Al 2O 3 marker interfaces are presented. Cu/Al 2O 3/Cu samples with marker geometry were prepared by vapor deposition of copper and alumina on polished glassy carbon and single crystal silicon substrates. The top Cu layer had a thickness of 70 nm and the marker thickness was 2.5 nm. In case of the Al 2O 3/Cu/Al 2O 3 samples a 2.5 nm Cu marker followed by an amorphous alumina layer with a thickness of 110 nm and a density of 2.9 g/cm 3 were deposited on single crystal alumina substrates. The marker samples were irradiated with 150 keV Ar + ions. Ion doses were in the range of 1.1 × 10 16 to 5.4 × 10 16 Ar +/cm 2. The sample temperature was varied between room temperature and 673 K. The mixing of the marker layers with the matrix was analyzed by depth profiling using Rutherford backscattering spectroscopy (RBS). Additionally, in case of the alumina marker system, to measure the aluminium depth distribution, nuclear reaction analysis (NRA) was applied using the 27Al(p, γ) 28Si reaction with a resonance energy of 992 keV. Film stability was checked by scanning electron microscopy (SEM)

Ion beam mixing, diffusion, and phase stability in Cu/Al2O3 interfaces

Ion beam mixing, diffusion properties, and phase stability have been investigated in Cu/Al2O3 bilayer samples. Specimens were prepared by vapor deposition and irradiated with 150 keV Ar+ ions up to a fluence of 1.5 · 1017 Ar+/cm2. Sample temperature under irradiation was varied between 77 K and 673 K. The mixing behavior was studied by analyzing the concentration depth profiles, determined by Rutherford Backscattering Spectroscopy. It was found that mixing efficiencies of Cu, Al, and O scale with Ar+ fluence. Radiation enhanced diffusion (RED), observed above room temperature, is separated from ballistic mixing and high temperature diffusion. The migration enthalpy for interdiffusion in the RED region (between RT and 300 °C) was estimated to be approximately 0.3 eV. Sputtering yields depending on temperature gradient near to sample and phase stability versus ion dose and temperature are also discussed.

Dynamic processes of altered layer formation on alloy subsurface: a Monte Carlo simulation for Cu-Pt alloys

To understand the basic processes of altered layer formation in subsurface regions of an alloy, we studied the dynamic change of the composition profile in the altered layer by Monte Carlo simulation. The simulation was performed for Cu 52Pt alloy under 3 keV Ar + ion beam irradiation at an incident angle of 45°. Model of the simulation is based on the descriptions of three processes; cascade mixing, radiation-enhanced Gibbsian segregation (REGS) and radiation-enhanced diffusion (RED). In the simulation we used the experimental results for assessing the composition of the 1st atomic layer and of different values of the RED coefficient. The results have revealed that the coefficient D ≈ 3 × 10 −16cm 2/s is most appropriate, satisfying the mass balance law at steady state attained after an experimentally determined ion dose. Preferential sputtering of Cu atoms is confirmed in the initial stage of the sputtering, and the composition ratio of sputtered Cu and Pt atoms approaches that of the bulk as sputtering proceeds.

Change of electrical properties of alloys and excitation of low-temperature atom mobility by ion bombardment

In situ measurements have been carried out of electrical resistance of submillimetre (100 μm thick) specimens of alloys of the (Pd 1− x Au x ) 3Fe system which have been submitted to ion bombardment by 20 keV Ar + ions at different temperatures (20–450°C). It has been established that ion irradiation brings about a process of atom ordering over the entire depth of initially disordered (quenched from 1000°C) specimens at temperatures substantially lower than thermal disorder-to-order critical temperature. A number of special features has been revealed in the observed action which are difficult to explain in terms of any of the known effects and, in particular, by radiation-enhanced diffusion. The hypothesis is suggested that the observed effect could be connected with atom regrouping at the solitary shock wave front induced by ion bombardment as a result of evolution of atomic collision cascades.