Papers by Author: Reinhard Krause-Rehberg

Abstract: Glasses with the compositions (100-x)(0.16Na₂O/0.10MnO/0.74SiO₂)/xFe₂O₃ (x = 0-15 mol%) were prepared and characterized using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening (CDB) spectroscopy. The PALS method applied seems to be sensitive to verify the Verwey phase transition, discovered for bulk magnetite (Fe₃O₄), from a high-temperature ‘bad metal’ conducting phase to a low-temperature insulating phase occurring at about 120 K in the glasses examined. It means that at relatively low concentration of Fe₂O₃ up to 15 mol%, the magnetite crystals could be synthesized in the amorphous matrix of Na₂O/MnO/SiO₂/Fe₂O₃ glass that is important for numerous practical applications. The CDB measurements showed that the majority of positrons in the glass samples studied are annihilated in the vicinity of oxygen anions and iron-oxide containing glasses have lower concentration of oxygen-vacancy defects compared to the non-iron containing base glass.

Abstract: The centers of bismuth (Bi) in silicon are being scrutinized as the defect qubits for mostly developed integrated electronics including its photonic component and we have applied the positron annihilation lifetime spectroscopy (PALS) to gain deeper insight into symmetry of the Bi impurity center whose configuration was modified by 15 MeV proton irradiation. It was revealed that hyperfine (hf) and super-hyperfine (shf) interactions of the nuclear and electron spin systems of the bismuth impurity center, ²⁰⁹Bi (J = 9/2), with the regular ²⁹Si (J = 1/2) atoms of silicon delay the essentially local event of emitting of a couple of annihilation gamma–quanta from within the crystal cell which comprises Bi impurity atom (J is the nuclear spin). This phenomenon is observed under increasing occupancy of Bi donor ground and excited states, in contrast to a profoundly enriched ²⁸Si (J = 0) material (so-called “semiconductor vacuum”) where content of ²⁹Si (J = 1/2) isotope was suppressed up to the value of ≈ 50 ppm. The many-body exciton-like states comprising a polyelectronic exciton {e⁻e⁺e⁻h} at Bi donor center are suggested for interpreting the data. The proton irradiation leads to acquiring by Bi impurity atom of an open volume ( V_op ) which is splitted in [V_op – Bi] complex. This defect possessing of D_3d symmetry dominates in the irradiated material. Being thermally stable up to ≈ 370 °C, [V_op – Bi] complex is annealed at ~ 470 – 500 °C. These data agree well with the results of ab intio cluster calculations performed on the basis of LDA-KKR formalism for exploring both the energy gain and symmetry of Bi–vacancy complex.

Abstract: We observed for the first time the thermally stable point positron-sensitive center of a vacancy type in n–FZ–Si (P) material irradiated at RT by ~ 0.9-MeV electrons. The center that emerges after isochronal annealing at T_anneal.≈ 260 – 280 oC is found to be similar to the vacancy-group-V-atom complex revealed in the same Si material irradiated by 15-MeV protons; the detecting of the centers by the positron trapping is finalized at T_anneal.≥ 520 oC. The annihilation gamma-quanta to be emitted from the positron trap gives rise to a characteristic positron lifetime τ₂ (I₂ ~ 38–19 %) ≤ 276 – 294 ps which is somewhat longer than the one predicted for unrelaxed single vacancy τ_V.≈ 254 – 261 ps. Our data suggested a configuration of the complex V_opPV_op, wherein the atom of phosphorus is tied to a split open vacancy volume 2V_op. It is argued that V_op volume detected by the positron trapping may be formed by extended semi-vacancy, V_s-ext , or by the relaxed inwards vacancy, V_inw , thus resulting in a distorted V_s-extPV_s-ext or V_inwPV_inw configurations.

Abstract: To determine the electrically inactive fraction of As or P in heavily doped as-grown Czochralski Si 4-point resistivity and SIMS measurements were carried out. No clear trend for the electrical inactive fraction was found with an increasing dopant concentration, though a mean electrical inactive fraction of 11.5% for As doping could be determined.Experimental results on a dopant-vacancy complex in as-grown Si are scarce, hence temperature-dependent positron annihilation lifetime spectroscopy (PALS) was carried out on several heavily As and P doped as-grown Si samples. The measured average positron annihilation lifetime τ_av is between 218 ps and 220 ps. No temperature dependent effect on τ_av could be observed. Therefore, it can be concluded that in the studied doping range the dopant-vacancy complexes do not exist. The reason for the inactivation of the dopant has to be found elsewhere. A possible explanation can be the formation of dopant precipitates.

Abstract: The free volume of the thermally stable vacancy center in n-FZ-Si:P has been probed by positrons. The defects were produced with 15 MeV protons, and then the irradiated material was subjected to the isochronal annealing. The positron lifetime has been determined over the temperature range ~ 30 K – 300 K; the samples-satellites have been characterized by Hall effect measurements. The microstructure of the center involves, at least, one atom of phosphorus and it manifests itself as a deep donor. The center is singly negatively charged and the cascade phonon-assisted trapping of positron proceeds over the length characteristic of the point defect, l₀ ~3.62 a. u. Obeying ~ T ^–3 law, the positron trapping cross section ranges 3∙10^–12 cm² (66K) to 2.5∙10^–14 cm² (266 K). The positron lifetimes ranging from ~240 ps to ~280 ps suggest that the atomic relaxation is directed inward towards the free volume of the deep donor involving, at least, two vacancies.

Abstract: A new type of a positron annihilation lifetime spectroscopy (PALS) system has been set up at the superconducting electron accelerator ELBE [ at Helmholtz-Zentrum Dresden-Rossendorf. In contrast to existing source-based PALS systems, the approach described here makes use of an intense photon beam from electron bremsstrahlung which converts through pair production into positrons inside the sample under study. The article focusses on the production of intense bremsstrahlung using a superconducting electron linear accelerator, the production of positrons inside the sample under study, the efficient detector setup which allows for annihilation lifetime and Doppler-broadening spectroscopy simultaneously. Selected examples of positron annihilation spectroscopy are presented.

Abstract: Positron lifetime annihilation spectroscopy (PALS) was used to study the temperature- dependence of sub-nanometer size local free volumes in two atactic poly(n-octadecyl methacrylate)s (PODMA) with different molecular weight. These materials exhibit short range layered structure with a self-assembly and crystallization of side chains. From the ortho-positronium (o-Ps) lifetime the size of free volume holes and its distribution are calculated. At a temperature Tm=311 ± 5 K, the mean hole volume shows an abrupt and strong increase from 0.15 nm3 to 0.2 nm3, which comes from the melting of side-chains. The reverse effect, attributed to side chain crystallization, was observed during cooling the samples at slightly lower temperatures. The lifetime result of PODMA is compared with semifluorinated polyesters in which the side chain has an oxydecylperfluorodecyl structure (-O-(CH2)10-(CF2)9-CF3). Long chain polymers without side-chains such as polyethylene and short side-chain poly(n-alkyl methacrylate)s: poly(methyl methacrylate) (PMMA) and poly(n-hexyl methacrylate) (PHMA) are also compared with these polymers.

Abstract: The electron momentum distribution and microstructure of centers incorporating a vacancy (vacancies) and a group-V-impurity atom (P, As, Sb, or Bi) in oxygen-lean n-Ge crystals have been investigated by means of the angular correlation of the annihilation radiation (ACAR). The vacancy-group-V-impurity atom complexes have been induced by irradiation with 60Co γ – rays at Tirr. ≈ 280K. A split between the intensities of the high-momentum emission of the annihilation radiation measured before and after n-p-conversion has been revealed for the complexes containing smaller ion cores (P, As) and the larger ones (Sb, Bi), respectively. After n-p-conversion the electron density decreases slightly (but markedly) around the positron localized at the vacancy complexes incorporating P, Sb, and Bi impurity atom. This decrease is accompanied by a lessening of intensity of the high-momentum emission of the annihilation radiation thus bringing in a direct evidence of a multi-vacancy structure of the vacancy-group-V-impurity atom complexes after n-p-conversion; the electron density was found to be affected by the localized deep acceptor states related to these centers. The relaxation inward open volume is a common feature which is pronounced for As-containing complexes. Subvalent band states are suggested to contribute the high-momentum annihilation most markedly. The electron momentum density around the positron is due to rather by the elemental specificity of the surrounding atoms than by changes of the electron-positron many-body interaction in the vacancy-group-V-impurity atom complexes.

Abstract: Positron annihilation lifetime spectroscopy (PALS) is used to study a series of controlled pore glasses (CPG). The lifetime spectra were decomposed into four components using the routine LifeTime, version 9.0 (LT9). The largest lifetime τ4, which is attributed to the annihilation of ortho-positronium (o-Ps) localized at mesopores, varied at 300 K between 21 and 131 ns. The size of mesopores (mean free path), D, was determined by N2 adsorption and Hg intrusion techniques to vary between 1.8 and 56 nm. It is shown that the Tao-Eldrup model extended to cylinders of infinite length and diameter d = D describes well the experiment for an overlap parameter δ fitted to be δ = 0.193 nm. The spectra were also analyzed allowing a distribution of o-Ps lifetimes. A method is developed to calculate the pore size distribution n(d) from the τ4 distribution. This method is of particular interest since PALS is very sensitive to pores being too small to be exactly analyzed by conventional porosimetry.

Abstract: Angular correlation of annihilation radiation technique (ACAR) has been used for studying a microstructure of the vacancy-group-V-impurity complexes (DV) formed by irradiation with 60Co γ – rays at Tirr. ≈ 280K in oxygen-lean n-Ge doped with group-V-impurity atoms As, Sb, and Bi. The probability of annihilation of positrons with the core electrons of DV complexes to be reconstructed from ACAR spectra has been analyzed on the basis of Chapman-Kolmogorov formalism; the Coulomb repulsion is proved to regulate the penetration of a positron into Ge4+ and D5+ ion cores. In passing from AsV to SbV and BiV complexes the ion cores D5+ are found to contribute more effectively to the probability of the positron annihilation in the core region. These data correlate well with the augmentation of the entropy of ionization (4S ~ 2,9 ÷ 4,2K) observed by means of capacitance transient techniques with the use of Au-Ge Schottky barriers in the same row of a similar vacancy-impurity complexes. The results obtained by ACAR spectroscopy suggest the full-vacancy configuration of DV pair with relaxation of atoms inward towards the vacancy.