No abstract

No abstract

We report the observation of slow emission bands overlapped with the near-band-edge steady-state luminescence of nominally undoped ZnO crystals. At low temperatures the time-resolved spectra are dominated by the emission of several high-energy bound exciton lines and the two-electron satellite spectral region. Furthermore, two donor-acceptor pair transitions at 3.22 and 3.238 eV are clearly identified in temperature-dependent time-resolved spectroscopy. These donor-acceptor pairs involve a common shallow donor at 67 meV and deep acceptor levels at 250 and 232 meV.

The wide-band-gap zinc oxide-based diluted magnetic semiconductors currently attract considerable attention due to their possible use in spintronic devices. In this work, we studied ZnO nanowire samples synthesized on 10×10 mm2 a-plane sapphire substrates by high-pressure pulsed laser deposition. The samples were characterized by scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) in the X-band (≃9.3 GHz) from T=4 to 300 K. According to the SEM pictures, the nanowires exhibit a length of about 1 μm and are aligned perpendicular to the substrate surface. The structures have a hexagonal cross section and their diameter ranges from 60 nm up to 150 nm. For the lowest nominal concentrations of xMn=3 at. % and xCo=5 at. %, we detect the anisotropic EPR spectra of isolated Mn2+ (3d5, S6) and Co2+ (3d7, F4), respectively, on Zn sites. The detection of the well-resolved anisotropic spectra proves a coherent crystallographic orientation of the nanowires. The linewidth was larger than the best values reported in the literature. Nevertheless, it was possible to identify two different components, A and B, of the reported spectra. From the temperature dependence of the EPR intensity, we found that both components exhibit paramagnetic behavior and are present in a concentration ratio of NB∕NA=1.4. In the case of the Mn-doped ZnO wires, the linewidth increases with increasing Mn concentration due to the dipole-dipole interaction of the paramagnetic ions. At the highest used nominal concentration, xMn=10 at. %, an additional broad single line is observed.

Mechanisms of non-radiative recombination of electron-hole complexes in Cd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations of Mn(2+) ions are analyzed within the framework of the single-electron model of deep 3d levels in semiconductors. In addition to the mechanisms caused by Coulomb and exchange interactions, which are related because of the Pauli principle, another mechanism due to sp-d mixing is considered. It is shown that the Coulomb mechanism reduces to long-range dipole-dipole energy transfer from photoexcited quantum dots to Mn(2+) ions. The recombination due to the Coulomb mechanism is allowed for any states of Mn(2+) ions and e-h complexes. In contrast, short-range exchange and sp-d recombinations are subject to spin selection rules, which are the result of strong lh-hh splitting of hole states in quantum dots. Estimates show that efficiency of the sp-d mechanism can considerably exceed that of the Coulomb mechanism. The phonon-assisted recombination and processes involving the upper excited states of Mn(2+) ions are studied. The increase in PL intensity of an ensemble of quantum dots in a magnetic field perpendicular to the sample growth plane observed earlier is analyzed as a possible manifestation of the spin-dependent recombination.