Microstructure And Optical Properties Of Eu-doped Mg_xZn_1-xO Hexagonal Nanocrystals

The Mg_xZn_1-xO alloy system may provide an optically tunable family of wide band-gap materials that can be used in various UV luminescence and absorption applications in the range of 3.4-7.5 eV depending on the Mg composition percentage. The objective of this work was to understand the locations of Eu ion doped in Mg_xZn_1-xO nanocrystals and the role of Eu on the microstructure and optical properties of doped Mg_xZn_1-xO hexagonal nanocrystals. We also preliminarily explained the influence of Eu on the formation mechanism of Mg_xZn_1-xO hexagonal nanocrystals.Eu-doped Mg_xZn_1-xO hexagonal nanocrystals with wurtzite-type structure were fabricated on quartz substrates by electron beam evaporation (EBE) using Mg0.15Zn0.85O:Euy (0≤y≤0.08) target combined with thermal annealing followed with rapid cooling. The influence of Eu on the microstructure and optical properties of Mg_xZn_1-xO hexagonal nanocrystals has been investigated using x-ray diffraction spectra, x-ray photoelectron energy spectra, scanning electron microscopy, absorption spectra, and photoluminescence spectra.It was found that Eu-doped Mg_xZn_1-xO hexagonal nanocrystals annealed at 700 oC exhibit (002) preferred orientation, whereas undoped Mg_xZn_1-xO hexagonal nanocrystals annealed at 700 oC did not exhibit preferential growth. XPS images indicated Mg_xZn_1-xO hexagonal nanocrystals had been obtained and Eu2+ and Eu3+ existed in the Mg_xZn_1-xO:Eu hexagonal nanocrystals, although their concentration was quite low. The presence of Eu2+ might be because there was a lack of O2 when the nanocrystals grown. And the reason that the concentration of Eu ions was low in the nanocrystals was the seriously losing of Eu in the growth and post annealing stages.The Mg concentrations of Eu-doped and undoped Mg_xZn_1-xO hexagonal nanocrystals annealed at 700 oC were both 0.08 by the calculation of the absorption spectra data. However, the PL spectra of Eu-doped Mg_xZn_1-xO hexagonal nanocrystals with different Eu concentrations excited by the 325 nm line of a He-Cd laser at room temperature exhibited an evident phase separation for the Eu-doped Mg_xZn_1-xO hexagonal nanocrystals in UV region, but not for undoped sample. We obtained two UV emission peaks located at 3.46 eV and 3.31eV by fitting the photoluminescence spectra of Mg_xZn_1-xO:Eu hexagonal nanocrystals with different concentration of Eu. One peak derived from the area of Mg0.08Zn0.92O, and the other derived from that of Mg0.03Zn0.97O. But for the undoped sample, there was only one UV emission peak located at 3.46 eV. We had also tested the PL emission spectra of Mg_xZn_1-xO:Eu hexagonal nanocrystals excited by the 488nm Ar+ line. But there were not any luminescence related with Eu ions. Therefore, Eu might not be incorporated in the Mg_xZn_1-xO lattice. Furthermore, Zn-related species have a higher vapor pressure and can be easily desorbed at higher annealing temperature yielding Mg enriched alloy. But Eu-related species have a higher vapor pressure than that of Zn, so Eu will get together on the shell surface of nanocrystals preventing the evaporation of Zn. Consequently, the microstructure of Eu-doped Mg_xZn_1-xO hexagonal nanocrystals can be seen as the Mg0.08Zn0.92O/Mg0.03Zn0.97O/air single quantum well (SQW) core-shell structures. The results showed that the UV emissions mainly derived from the localized exciton recombination by fitting the photoluminescence spectra of Mg_xZn_1-xO:8% Eu hexagonal nanocrystals annealed at 700 oC at different temperatures. The binding energy of exciton in Mg0.03Zn0.97O (66 meV) was larger than that in Mg0.08Zn0.92O (50 meV) further indicated the Mg0.08Zn0.92O/Mg0.03Zn0.97O/air SQW core-shell structures had been formed.