Luminescence properties of Eu doped ZnO PLD thin films: The effect of oxygen partial pressure

Eu (3 mol%) doped ZnO thin films were deposited by pulsed laser deposition at different oxygen partial pressures. The structure, morphology and luminescence properties of the films were studied. The scanning electron microscopy images revealed that the films deposited in vacuum have smoother surfaces compared to the oxygen pressure films. When excited by a He–Cd laser at 325 nm, the vacuum film exhibited an intense UV emission at ~379 nm, broad-weak deep level emission in the visible region (450 nm–700 nm), as well as a small peak associated with the characteristic emission of the 4f – 4f transitions of Eu³⁺ at 616 nm standing out from the deep levels emission for the oxygen atmosphere films. When the Eu³⁺ ions were selectively excited at 464 nm, only the characteristic emission of the 4f – 4f transitions of Eu³⁺ were observed at 536 nm, 578 nm, 595 nm, 616 nm, 656 nm and 707 nm corresponding to the ⁵D₁ – ⁷F₀, ⁵D₀ – ⁷F_J (J = 0, 1, 2, 3 and 4) transitions. When excited at 288 nm, the film deposited in vacuum only exhibited a broad peak centred at ~585 nm which was due to the ZnO deep defect levels. The samples prepared with oxygen exhibited characteristic emission of Eu³⁺ with an increase in intensity for increasing oxygen partial pressure. No cathodoluminescence was observed for the vacuum sample, whereas only the characteristic emission of Eu³⁺ was detected for the films obtained in oxygen atmosphere. Current-voltage measurements of the p-type Si/ZnO:Eu³⁺ junctions showed a diode-like behaviour with turn on voltage of about 10 V.