H. Schut

Analysis of positron profiling data by means of ‘‘VEPFIT’’

The program VEPFIT is presented which extrats the relevant parameters from positron measurements on implanted materials and layered structures. Measurements of the Doppler Broadening parameter S and the positronium fraction F vs the energy of the incident positrons are analyzed by means of a semi‐linear fitting procedure. The principles of the analysis method are given and the performance of the program on artificially and experimentally obtained data is demonstrated. Fitting strategies are outlined and the accuracy of the fitting results is discussed.

VEPFIT applied to depth profiling problems

Abstract The modelling and fitting program VEPFIT has been employed in recent years for resolving defect depth profiles and depth structures of deposited layers. Recent activities concerning program development include the testing of a new model of MOS systems for implementation into VEPFIT and a study into decomposition of Doppler-broadened photo-peaks. Further methods are proposed using VEPFIT for analysis of lifetime measurements and for modelling of positron transport with multi-energy groups.

Physical properties of nitrogenated amorphous carbon films produced by ion-beam-assisted deposition

Abstract Carbon films with up to 32 at.% N (a-C:N) have been prepared using an ion-beam-assisted magnetron, with an N2+ beam at energies between 50 and 300 eV. The composition and density of the films vary strongly with the deposition parameters. Electron energy loss spectroscopy shows that these a-C:N films are mostly graphitic with up to 20% C sp3 bonding. Rutherford backscattering spectroscopy and neutron depth profiling show that the density goes through a maximum as the average deposited energy per unit depth increases. X-ray photoelectron spectroscopy shows that nitrogen is mostly combined with carbon in triple (C≡N) and double (C=N) bonds. Positron annihilation spectroscopy shows that the void concentration in the films goes through a minimum with deposited energy. These results are consistent with a densification induced by the collisions at low deposited energy, and damage-induced graphitization at high deposited energy values.

Analysis of positron beam data by the combined use of the shape‐ and wing‐parameters

An improved approach is presented for the analysis of positron beam Doppler broadening data. Instead of analyzing the energy‐dependent shape parameter, the so‐called S(E) data, we combined the shape S(E) and wing W(E) data by plotting them as a trajectory in the S–W plane, using the implantation energy as a running parameter. It is shown that this plot is of particular interest for the qualitative interpretation of the data. Furthermore, it allows the independent determination of the characteristic shape and wing parameters of the different positron trapping layers without the use of a numerical simulation and fitting program. The method and its advantages and limitations are illustrated for three cases: a silicon sample implanted with helium, a metal–oxide–silicon system subjected to a bias voltage and a bare oxide layer on silicon.

Effect of ion beam assistance on the microstructure of nonhydrogenated amorphous carbon

Nonhydrogenated diamondlike carbon films have been prepared by dual ion beam sputtering and ion‐beam‐assisted magnetron. The assistance parameters—ion energy, ion mass, ion flux/atom flux—have been systematically varied, and the films have been characterized by Rutherford backscattering spectroscopy, high‐resolution transmission electron microscopy, electron energy loss spectroscopy, positron annihilation spectroscopy, Raman spectroscopy, and nanoindentation. It was found that the density and the degree of disorder of the films go through a maximum with ion energy, and the void concentration goes through a minimum. Microstructure analysis shows that the films are mostly sp2 bonded, with a maximum of about 16% concentration of sp3 bonding from the largest values of density. The evolution of density with ion flux and energy is consistent with a combined effect of atomic displacements in the film leading to densification, and damage buildup leading to progressive graphitization as the energy is increased. Th...

Rectangular nanovoids in helium-implanted and thermally annealed MgO(100)

Cleaved MgO(100) single crystals were implanted with 30 keV 3He ions with doses varying from 1×1019 to 1×1020 m−2 and subsequently thermally annealed from 100 to 1100 °C. Transmission electron microscopy observations revealed the existence of sharply rectangular nanosize voids at a depth slightly shallower than the helium-implantation range. Monitoring of the defect depth profile and the retained amount of helium was performed by positron-beam analysis and neutron depth profiling, respectively.

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.

Transport of positrons in the electrically biased metal-oxide-silicon system

This paper describes a study of the effect of an external electric field on the behavior of positrons in metal-oxide-silicon (MOS) systems. Doppler broadening measurements of the annihilation radiation were performed on capacitors with identical thermally grown SiO2 layers and with Al, W and Au layers as a gate. The data were analyzed by the combined use of the shape- and wing-parameters of the photo peak. The observed effects of the electric field are due to the field-driven transport of positrons through the SiO2, silicon and the interfaces. By applying a field of the order of 1 MV/cm the positrons can be efficiently transported through the approximately 100 nm thick SiO2 layer. From the transport behavior of the positrons it is concluded that the positron affinity is higher for SiO2 than for silicon and for the gate metal. By properly choosing the direction of the field, the positrons implanted into the SiO2 layer are collected either at the Si/SiO2 interface or at the SiO2/gate interface. For negative...

Positron beam defect profiling of silicon epitaxial layers

Epitaxial layers of silicon grown on a Si(100) substrate by molecular‐beam epitaxy (MBE) and solid‐phase epitaxy (SPE) have been investigated by slow positron beam analysis methods. Results of Doppler broadening measurements revealed that the S parameter of the SPE material is considerably higher than the value measured for the MBE layer, indicative of a higher concentration of open‐volume defects in the former material. This was confirmed by measurements of the positronium fraction at elevated temperatures.