Defects In Near-surface Region Research Articles

Assessment of changes in desorption mechanism of MgH2 after ion bombardment induced destabilization

This paper deals with non-isothermal kinetics models of hydrogen desorption from MgH2 altered by ion bombardment and stresses the importance of the MgH2 surface during its decomposition. In the case of argon-irradiated samples, where defects are induced in the near-surface region, the Avrami Erofeev mechanism with parameter n = 2 can be adopted while in the case of boron-irradiated samples, where defects are created deeper in the bulk, the desorption mechanism is the same with n = 3. The difference is possibly related to the concentration and good dispersion of defects in near-surface region in the samples.

Protonic conduction processes of gamma-ray-irradiated perfluorosulfonic acid membranes

New protonic conduction processes of perfluorosulfonic acid membranes by gamma-ray irradiation at a dose of up to 240 kGy at room temperature in air have been found by direct current resistance measurement. The conductivities of the membranes at 373 and 300 K in vacuum are enhanced to approximately one and two orders of magnitude higher, respectively, than the conductivities of an unirradiated membrane. The new and original activation energies required for the conductivities in the temperature range below and above 343 K are determined to be 0.12 ± 0.05 and 0.84 ± 0.03 eV, respectively, which correspond to the potential energy of the hydrogen diffusion due to radiation-induced defects and the existing sulfonate group. By means of ultraviolet, visible, and infrared optical absorption and by hydrogen-ion-exchange capacity measurements, it is revealed that radiation-induced defects such as fluorocarbon and peroxy radicals, and C = O bonds included in carbonyl groups are related to the new proton conduction processes. The modification of hydrogen absorption characteristics due to the radiation-induced defects in near-surface regions induces the enhancement of the proton conductivity.

Improved depth profiling with slow positrons of ion implantation-induced damage in silicon

Variable-energy positron annihilation spectroscopy (VEPAS) has been extensively applied to study defects in near-surface regions and buried interfaces, but there is an inherent limit for depth resolution due to broadening of the positron implantation profile. In order to overcome this limit and obtain optimum depth resolution, iterative chemical etching of the sample surface and VEPAS measurement are employed. This etch-and-measure technique is described in detail and the capabilities are illustrated by investigating the depth profile of defects in Si after B and P implantations with 2×1014/cm2 at 100 keV followed by annealing. Defect tails can be accurately examined and the extracted defect profile is proven to extend beyond the implanted ion range predicted by the Monte Carlo code TRIM. This behavior is more remarkable for P ion implantation than B, and the mass difference of the implanted ions is strongly related to it. No significant difference is recognized in the annealing behavior between B and P implantations. After annealing at 300 °C, the defect profile is hardly changed, but the ratio of the characteristic Doppler broadening, S, a parameter for defects, to that for the bulk Si rises by 0.01, indicating that divacancies, V2, are transformed into V4. Annealing at more than 500 °C causes diffusion of the defects toward the surface and positron traps are annealed out at 800 °C. It is proved that this resolution-enhanced VEPAS can eliminate some discrepancies in defect profiles extracted by conventional means.

Improved defect profiling with slow positrons

Monoenergetic positrons are widely used to study defects in near-surface regions and buried interfaces of solids. Depth information is usually obtained by varying the positron implantation energy. However, at energies larger than 10 keV the stopping profile becomes much broader than the positron diffusion length. The study shows that optimum depth resolution can be obtained by stepwise removal of the surface and measurement with the smallest possible positron implantation depth. The removal from the surface can be done by ion sputtering or chemical etching. Furthermore, excellent defect depth profiles can be obtained when a sample is wedge-shaped polished (wedge angle about 1°). A line scan using a scanning positron microbeam along the wedge with a small positron implantation depth gives then the defect profile with optimum depth resolution.

Formation of Defects in Near-Surface Region over Li or Mn-Doped MgO Studied by Mg K-Edge XANES

Abstract The near-surface region of Li or Mn-doped Mg was characterized by Mg K-edge XANES spectroscopy. The oxygen defects were formed in near-surface region when the content of Li or Mn ion is relatively low. The amount of the defects in near-surface region does not depend on the crystallinity.

Variable-energy positron beam system and its application to depth-selective defect analysis

The gamma-radiation emitted when positrons are annihilated in a material has a peak whose line width depends upon the Doppler broadening. We have developed an isotope-based, variable-energy positron beam system at the National Institute of Materials and Chemical Research (NIMC) which allows us to use this Doppler broadening technique for depth-selective analysis of damage and defects in near-surface regions. In this paper an outline of our positron beam system which uses 22Na is given. Its use is described in investigating ion-irradiated amorphous poly(aryl-ether-ether-ketone) (PEEK) and amorphous silicon. The damaged region of Au + bombarded PEEK was found to extend to a greater depth than the average implantation depth for the same material. This is in contrast to the case of O + bombardment. Different widths of annihilation lines observed for as-received and heat-treated samples of amorphous silicon were discussed in relation to structural changes due to annealing.