Structure-specific photoluminescence of ZnGa2O4:Eu3+ thin films deposited on sapphire C-plane substrates

Abstract

1 at. % Eu3+-doped ZnGa2O4 (ZnGa2O4:Eu) thin films were deposited on sapphire C-plane substrates by dual-target sputtering, and the correlation between their crystal structures and luminescent properties was investigated. When deposition was carried out with H2O vapor as the oxygen source gas and subsequently postannealing was done in vacuum, the ZnGa2O4:Eu films consisted of randomly oriented polycrystals. In contrast, ZnGa2O4:Eu films deposited with O2 consisted of (111)-oriented crystallites. The crystallites were epitaxial after postannealing at above 400 °C. At intermediate annealing temperatures between 250 and 400 °C, Ga2O3 and GaO(OH) precipitated as impurity phases with (111)-oriented ZnGa2O4. Upon excitation at 325 nm, the Eu3+ emission intensities from the randomly oriented ZnGa2O4:Eu films were comparable to those of the epitaxial films. The epitaxial films exhibited only the 5D0 → 7F2 transition signal, whereas the randomly oriented polycrystalline films showed considerable emissions from the 5D0 → 7F4 transition as well as 5D0 → 7F2. Furthermore, the emission signal width of the 5D0 → 7F2 transition was narrower for the epitaxial films compared with the polycrystalline films. Such different emission characteristics can be explained from the viewpoint that the single crystal structure allows Eu3+ ions to occupy only a limited number of sites, whereas randomly oriented polycrystalline crystallites formed by incorporating H+ and OH− species have abundant grain boundaries, and defect sites enable larger numbers of structurally and chemically different emission-active sites to hold Eu3+ ions.1 at. % Eu3+-doped ZnGa2O4 (ZnGa2O4:Eu) thin films were deposited on sapphire C-plane substrates by dual-target sputtering, and the correlation between their crystal structures and luminescent properties was investigated. When deposition was carried out with H2O vapor as the oxygen source gas and subsequently postannealing was done in vacuum, the ZnGa2O4:Eu films consisted of randomly oriented polycrystals. In contrast, ZnGa2O4:Eu films deposited with O2 consisted of (111)-oriented crystallites. The crystallites were epitaxial after postannealing at above 400 °C. At intermediate annealing temperatures between 250 and 400 °C, Ga2O3 and GaO(OH) precipitated as impurity phases with (111)-oriented ZnGa2O4. Upon excitation at 325 nm, the Eu3+ emission intensities from the randomly oriented ZnGa2O4:Eu films were comparable to those of the epitaxial films. The epitaxial films exhibited only the 5D0 → 7F2 transition signal, whereas the randomly oriented polycrystalline films showed considerable emissions from the...