Cs. Szeles

Role of implantation‐induced defects in surface‐oriented diffusion of fluorine in silicon

Open‐volume defects introduced in Si(100) crystals during fluorine implantation were investigated by variable‐energy positron beam depth profiling. The behavior of the implantation‐induced lattice defects upon high temperature annealing and their role in the surface‐oriented diffusion of F impurities were examined. The defects become mobile and undergo recovery at temperatures below 550 °C, i.e., well before the onset of fluorine diffusion as seen by secondary ion mass spectroscopy (SIMS) profiling. This behavior suggests that after irradiation and annealing the fluorine occupies substitutional sites to which positrons are insensitive. The anomalous F diffusion seen in SIMS has been explained through a two‐step diffusion mechanism, in which the diffusion kinetics is determined by dissociation of the substitutional F into an interstitial F and a vacancy, followed by a rapid diffusion of the interstitial F and the vacancy through the crystal to the surface.

Deep electronic levels in high-pressure Bridgman Cd1−xZnxTe

Abstract The behavior of deep electronic levels was studied as a function of Zn concentration in CdZnTe crystals grown by the high-pressure Bridgman technique using thermoelectric effect spectroscopy. A significant increase of the thermal ionization energies of hole traps was observed with the increasing Zn content of the ternary compound. The effect explains the stronger hole trapping and the resulting much shorter hole lifetime usually observed in CdZnTe as compared to CdTe. The behavior also suggests increased carrier recombination and explains the strong deterioration of electron collection in detectors fabricated from CdZnTe of high Zn concentration.

Defect distribution in low‐temperature molecular beam epitaxy grown Si/Si(100), improved depth profiling with monoenergetic positrons

The depth distribution of open‐volume defects has been studied in Si(100) crystals grown by molecular beam epitaxy at 300 °C by the variable‐energy monoenergetic positron beam technique combined with well‐controlled chemical etching. This procedure gave a 10 nm depth resolution which is a significant improvement over the inherent depth resolving power of the positron beam technique. The epitaxial layer was found to grow defect‐free up to 80 nm, from the interface, where small vacancy clusters, larger than divacancies, appear. The defect density then sharply increases toward the film surface. The result clearly shows that the nucleation of small open‐volume defects is a precursor state to the breakdown of epitaxy and to the evolution of an amorphous film.

Assay of weathering effects on protective polymer coatings using positron annihilation spectroscopy

Polymer coatings, both with and without pigments, have been subjected to solar radiation and water spray weathering. The degrees of penetration of the weathering effects have been measured by injecting positrons of varying energy, i.e. to variable depths, into the films and observing the Doppler broadening of the annihilation radiation. The method is capable of detecting changes due to weathering effects at very early stages, long before visual examination reveals degradation. As little as one week of exposure caused measurable changes in the polymer structure, which were reflected in the Doppler broadening. Given further development, positron spectroscopy could possibly become a useful complement to the other methods of determining weatherabilities of protective polymer coatings.

The Limits of Application of Variable-Energy Slow Positron Beams for Investigating TiN Hard Coatings Prepared by PVD

Samples of TiN hard coatings prepared by physical vapour deposition (PVD) were investigated by means of depth-sensitive positron annihilation spectroscopy. The results indicate that the samples are at the limits of the applicability of this method presumably due to the high defect concentration. Our findings also show that, though the samples are thoroughly characterized by other independent methods, they might not be sufficient to explain all aspects of positron-solid interactions in these cases.