Te-doped GaAs crystals were heat-treated under various arsenic vapor pressures, and the defects related to nonstoichiometry were studied. The acceptor density formed by heat treatment shows minimum at a certain arsenic vapor pressure (PAs, min), and the relation between the annealing temperature (TH) and this arsenic vapor pressure PAs, min is given as PAs, min\fallingdotseq2.6×106exp
{-1.05 eV/(kTH)} Torr. The density of acceptors related with defects, probably two sorts of acceptors generated at lower pressures and at higher pressures respectively, is at the same time in proportion to the concentration of the initial donor impurity (Te). Therefore complexes associated both with the donor impurity (Te) and with the defects due to the deviation from the stoichiometric composition are formed. These complexes are responsible for both the 0.18 eV acceptor level and the 1.30 eV emission band at 77 K. Furthermore defects with acceptor levels of 0.12 eV and 0.15 eV, are investigated. These defects are unstable and disappear by reannealing at comparatively low temperatures (300–400°C) leaving acceptor levels at 0.18 eV.
Polarization-dependent x-ray absorption measurements were performed on a crystalline ZnO three-dimensional array consisting of highly oriented microrods as well as on particulate thin film consisting of monodisperse spherical nanoparticles. Strong anisotropic effects have been observed for the highly oriented ZnO rods, unlike for the isotropic spherical ones. Full-potential calculations of orbital-resolved x-ray absorption of a ZnO wurtzite periodic crystal, including the Zn 3d as part of the valence states, shows a very good agreement with the experimental findings. Comprehensive fundamental knowledge of the electronic structure of ZnO is obtained by probing and demonstrating the orbital symmetry of oxygen and its contribution to the conduction band of this important II-VI semiconductor.
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