Near infrared broadband from Er-Tm codoped zinc oxide and temperature-dependent properties

Er-Tm codoped ZnO thin film is synthesized by co-sputtering from separated Er, Tm, and ZnO targets. A flat and broad emission band is achieved in a range of 1400-2100 nm by optimizing annealing temperature, and the observed 1460, 1540, 1640 and 1740 nm emission bands are attributed to the transitions of Tm3+: 3H4 →3F4, Er3+ 4I13/2 →4I15/2, Tm3+ 1G4 → 3F2 and Tm3+ 3F4 → 3H6 transitions, respectively, which cover S, C, L, U bands. The intensity ratios of 1640 to 1535 nm and 1740 to 1535 nm below 1000 ℃ are nearly constant, while the ratios increase sharply above 1000 ℃. The temperature dependence of photoluminescence (PL) spectrum is studied under 10-300 K. With increasing the operation temperature, the bandwidth of broadband is nearly invariable (340-360 nm), and the Tm3+ PL emission intensities of 1640 nm and 1740 nm from Er-Tm co-doped ZnO thin film decrease by a factor of 1.5 and 2, respectively. Moreover, the 1535 nm emission intensity is increased by a factor of 1.2. This phenomenon is attributed to the complicated energy transfer (ET) processes involving both Er3+ and Tm3+ and the increase of phonon-assisted ET rate with temperature as well. And the cross relaxation between Tm3+ ions does not occur.