Papers by Author: W. Anwand

Abstract: Electroluminescence in SiO2 layers can be created by Ge implantation and a subsequent heat treatment, leading to the formation of Ge nano-particles inside the SiO2. An additional implantation of Er, connected with a further annealing, can lead to an improvement of the luminescent properties. However, the intensity of electroluminescence was found to decrease drastically after exceeding an optimum concentration of the Er doping. Slow positron implantation spectroscopy (SPIS), both in single (DB) and coincidence (CDB) Doppler broadening mode, was applied to probe processes at a microscopic level which might have an impact on the optical response. It shows that the increasing intensity of the electro-luminescence is connected with a crystalline structure of the SiO2 covering the nano-particles and also with the improved reverse energy transfer process between Er and Ge.

Abstract: Hydrogen is an important impurity in zinc oxide and can be incorporated into the lattice in several ways. Hydrogen can be also bound by vacancies that can be studied using positron annihilation techniques. Here we examine theoretically oxygen and zinc vacancies in ZnO, the latter also with hydrogen atoms inside. Several computational approaches are employed to determine the defect geometries and related positron characteristics. Positron-induced forces are also taken into account. Calculated positron lifetimes are compared with those observed in experiment, which gives an indication of the presence of zinc vacancy-hydrogen complexes in ZnO materials.

Abstract: SIMOX (Separation-by-Implantation-of-Oxygen) is an established technique to fabricate silicon-on-insulator (SOI) structures by oxygen ion implantation into silicon. The main problem of SIMOX is the very high oxygen ion fluence and the related defects. It is demonstrated that vacancy defects promote and localize the oxide growth. The crucial point is to control the distribution of vacancies. Oxygen implantation generates excess vacancies around RP/2 which act as trapping sites for oxide growth outside the region at the maximum concentration of oxygen at RP. The introduction of a narrow cavity layer by He implantation and subsequent annealing is shown to be a promising technique of defect engineering. The additional He implant does not initiate oxide growth in the top-Si layer of SOI.

Abstract: In the present work, positron annihilation spectroscopy (PAS) is employed for microstructure investigations of various ultra fine grained (UFG) metals (Cu, Ni, Fe) prepared by severe plastic deformation (SPD), namely high-pressure torsion (HPT) and equal channel angular pressing (ECAP). Generally, UFG metals prepared using both the techniques exhibit two kinds of defects introduced by SPD: dislocations and small microvoids. The size of the microvoids is determined from the PAS data. Significantly larger microvoids are found in HPT deformed Fe and Ni compared to HPT deformed Cu. The microstructure of UFG Cu prepared by HPT and ECAP is compared and the spatial distribution of defects in UFG Cu samples is characterized. In addition, the microstructure of a pure UFG Cu prepared by HPT and HPT deformed Cu+Al2O3 nanocomposite (GlidCop) is compared.