P.E. Mijnarends

Helium and hydrogendashdecorated cavities in silicon

The formation of stable voids in Si by helium implantation and annealing is studied with the aid of the slow-positron depth profiling method. Decoration of the voids with helium or hydrogen reduces positronium formation in the voids and thus lowers the high value of the S-parameter found for large empty cavities in Si. Annealing at 800°C recovers the original empty cavities.

Two-detector Doppler broadening study of enhancement in Al

Enhancement due to positron-electron correlation has been studied in Al by calculating and measuring Doppler profiles and positron lifetimes. It is found that only the local density enhancement of Puska, Seitsonen and Nieminen can simultaneously reproduce the profile and the lifetime, but our analysis suggests that a GGA-type enhancement of reduced strength would likely give even better results.

Testing of a nuclear-reactor-based positron beam

Abstract This paper describes the testing of a positron beam which is primarily based on copper activation near the core of a nuclear reactor and extraction of the positrons through a beam guide tube. An out-of-core test with a 22 Na source and an in-core test with the reactor at reduced power have been performed. Both tests indicated a high reflectivity of moderated positrons at the tungsten surfaces of the moderation discs which enhanced the expected yield. Secondary electrons generated in the source materials during the in-core test caused electrical field distortions in the electrode system of the system by charging of the insulators. At 100 kW reactor power during one hour, positrons were observed with an intensity of 4.4 × 10 4 e + s −1 of which 90% was due to positrons created by pair formation and 10% by copper activation.

Positron beam analysis of semiconductor materials using a two-detector Doppler broadening coincidence system

Doppler-broadening measurements can be improved by using a second Ge-detector for the coincidence detection of the second annihilation photon. The coincidence condition in combination with an energy relation results in a reduction of the background by a factor of 100 compared to a single detector system. This background reduction opens up the possibility of performing accurate measurements of the high momentum part of Doppler-broadened annihilation spectra. We have used this technique for the analysis of a metal oxide semiconductor system and electron-irradiated, As- and Sb-doped silicon.

Buried oxide and defects in oxygen implanted Si monitored by positron annihilation

One- and two-detector Doppler broadening measurements performed on low (∼1014 to 1015 O+/cm2) and high dose (∼1017 to 1018 O+/cm2) oxygen-irradiated Si using variable-energy slow positrons are analyzed in terms of S and W parameters. After annealing the low-dose samples at 800 °C, large VxOy complexes are formed at depths around 400 nm. These complexes produce a clear-cut signature when the ratio of S to that of defect-free bulk Si is plotted. Similar behavior is found for samples irradiated with 2 and 4×1017 O+/cm2 and annealed at 1000 °C. After irradiation with 1.7×1018 O+/cm2 and anneal at 1350 °C a 170 nm thick almost-bulk-quality Si surface layer is formed on top of a 430 nm thick buried oxide layer. This method of preparation is called separation by implantation of oxygen. S−W measurements show that the surface layer contains electrically inactive VxOy complexes not seen by electron microscopy. A method is presented to decompose the Doppler broadening line shape into contributions of the bulk, surfa...

Magnetic focusing of an intense slow positron beam for enhanced depth-resolved analysis of thin films and interfaces

The intense reactor-based slow positron beam (POSH) at the Delft research reactor has been coupled to a Two-Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) setup. The design is discussed with a new target chamber for the 2D-ACAR setup based on Monte Carlo simulations of the positron trajectories, beam energy distribution and beam transmission in an increasing magnetic field gradient. Numerical simulations and experiment show that when the slow positron beam with a FWHM of 11.6 mm travels in an increasing axial magnetic field created by a strong NdFeB permanent magnet, the intensity loss is negligible above ∼6 keV and a focusing factor of 5 in diameter is achieved. Monte Carlo simulations and Doppler broadening experiments in the target region show that in this configuration the 2D-ACAR setup can be used to perform depth sensitive studies of defects in thin films with a high resolution. The positron implantation energy can be varied from 0 to 25 keV before entering the non-uniform magnetic field. 2D-ACAR depth-profiling results in He-irradiated Si obtained with the new setup are presented.

Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

The effect of the detector resolution on the Doppler broadening measurements of both valence and core electron–positron annihilation

Abstract The measured value of the Doppler profile of the electron–positron annihilation radiation depends on the resolution function of the detectors used in the experiment. This is illustrated by the results of calculations of the profile convoluted with resolution functions of different FWHM. These results are compared with Doppler broadening measurements for Al. The calculated results agree qualitatively with the experimental data. The positions of the peaks in the curves which give the ratio of a Doppler profile to a reference profile are found to shift towards higher momenta when the width of the resolution for the reference material was increased. Since the peaks in the ratio curves are used for identifying the chemical species, it is important that the peak positions be unambiguously characterized. Hence, the detector resolution should be carefully measured and quoted when reporting and comparing results of Doppler profiles.

Positron-annihilation 2d-acar measurements in Bi2Sr2CaCu2O8+x

Abstract We present a brief account of measurements of the two-dimensional angular correlation of annihilation radiation (2D-ACAR) in a single crystal of approximate composition Bi 2 Sr 2 CaCu 2 O 8+x , (T c = 85 K) at 6 and 92 K. The most noteworthy result is that the anisotropic part of the spectra displays a fine-structure which is sharpest at the highest of these two temperatures.

Direct observation of localization of the minority-spin-band electrons in magnetite below the Verwey temperature

Two-dimensional spin-uncompensated momentum density distributions, ?s2D(p)s, were reconstructed in magnetite at 12 and 300 K from several measured directional magnetic Compton profiles. Mechanical detwinning was used to overcome severe twinning in the single-crystal sample below the Verwey transition. The reconstructed ?s2D(p) in the first Brillouin zone changes from being negative at 300 K to positive at 12 K. This result provides the first clear evidence that electrons with low momenta in the minority-spin bands in magnetite are localized below the Verwey transition temperature.