Self-trapping of the d-d charge transfer exciton in rock-salt structured Zn1–xNixO evidenced by soft X-ray excited luminescence



Luminescence spectra of NiO and solid solutions Zn1–xNixO have been investigated under vacuum ultra-violet (VUV) and soft X-ray (XUV) excitation (DESY, Hamburg). Photoluminescence (PL) spectra show broad emission violet and green bands centered at about 3.2 and 2.6 eV, respectively. The PL excitation (PLE) spec-tral evolution and lifetime measurements reveal that the two mechanisms with short and long decay times, at-tributed to the d(eg)-d(eg) and p(π)-d charge transfer (CT) transitions in the range 4–6 eV, respectively, are responsible for the observed emissions. The XUV excitation makes it possible to avoid the predominant role of the surface effects in luminescence and reveal a bulk violet luminescence with puzzling well isolated doublet of very narrow lines. These lines with close energies near 3.3 eV are attributed to recombination transitions in the self-trapped d-d CT excitons formed by the coupled Jahn-Teller Ni+ and Ni3+ centers. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)