A center from the family of "fourfold coordinated ͑FFC͒ defects", previously predicted theoretically, has been experimentally identified in crystalline silicon. It is shown that the trivacancy ͑V 3 ͒ in Si is a bistable center in the neutral charge state, with a FFC configuration lower in energy than the ͑110͒ planar one. V 3 in the planar configuration gives rise to two acceptor levels at 0.36 and 0.46 eV below the conduction band edge ͑E c ͒ in the gap, while in the FFC configuration it has trigonal symmetry and an acceptor level at E c − 0.075 eV. From annealing experiments in oxygen-rich samples, we also conclude that O atoms are efficient traps for mobile V 3 centers. Their interaction results in the formation of V 3 O complexes with the first and second acceptor levels at E c − 0.46 eV and E c − 0.34 eV. The overall picture, including structural details, relative stability, and electrical levels, is accompanied and supported by ab initio modeling studies.

4H-SiC epilayers were irradiated with either protons or electrons and electrically active defects were studied by means of deep level transient spectroscopy. Motion of defects has been found to occur at temperature as low as 350–400 K. Indeed, the application of an electric field has been found to enhance modifications in defect concentrations that can also occur during long time annealing at elevated temperature. Two levels have been revealed and labeled B and M. Two other levels, referred to as S1 and S2 and located at 0.40 and 0.71 eV below the conduction band edge have been studied in detail (capture cross sections, profiling, formation energy, activation energy during annealing). The S1 and S2 levels have been found to exhibit a one to one relation and are proposed to be two charge states of the same acceptor center, labeled the S center.

Monocrystalline n-type zinc oxide (ZnO) samples prepared by different techniques and containing various amounts of lithium (Li) have been studied by positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry. A distinct PAS signature of negatively charged Li atoms occupying a Zn-site (Li − Zn), socalled substitutional Li, is identified and thus enables a quantitative determination of the content of Li Zn. In hydrothermally grown samples with a total Li concentration of ∼2 × 10 17 cm −3 , Li Zn is found to prevail strongly, with only minor influence, by other possible configurations of Li. Also in melt grown samples doped with Li to a total concentration as high as 1.5 × 10 19 cm −3 , a considerable fraction of the Li atoms (at least 20%) is shown to reside on the Zn-site, but despite the corresponding absolute acceptor concentration of (2-3) × 10 18 cm −3 , the samples did not exhibit any detectable p-type conductivity. The presence of Li Zn is demonstrated to account for the systematic difference in positron lifetime of 10-15 ps between Li-rich and Li-lean ZnO materials as found in the literature, but further work is needed to fully elucidate the role of residual hydrogen impurities and intrinsic open volume defects.

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