Highly erbium-doped zinc–oxide thin film prepared by laser ablation and its 1.54 μm emission dynamics

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Abstract
Erbium-doped ZnO (ZnO:Er) thin films were fabricated by the KrF excimer laser ablation technique, which is a useful and simple technique to dope Er atoms on the order of 1020 cm−3 into a host material. As-prepared ZnO:Er films showing strong c-axis orientation with a hexagonal crystalline structure indicate a low electrical resistivity of 6.4×10−3 Ω cm. The sharp and intense photoluminescence (PL) at 1.54 μm originating from the intra-4f transition in the Er3+ ions as well as PL in UV region from the ZnO host were observed even at room temperature. Significant distinction arising from the different Er emission centers responsible for the 1.54 μm emission cannot be found in the temperature dependence between the ZnO:Er and Si:Er film as a reference, except for the PL spectrum feature and main PL peak position. This result suggests the existence of Er emission centers in ZnO:Er and Si:Er films that are different from each other. The details of Er-related 1.54 μm emission dynamics of ZnO:Er films have been investigated for the different excitation conditions, where the Er3+ ions have been excited either through a carrier-mediated process in the ZnO host, or through direct pumping into the 4f energy level of the Er3+ ions. There is no change in the 1.54 μm PL spectrum feature in spite of the different excitation conditions, whereas a sensible change can be seen in the rise time of the 1.54 μm emission. The shorter rise time of the 1.54 μm emission observed for indirect excitation implies an excitation efficiency superior to the direct excitation of Er3+ ions. This result indicates that the ZnO:Er thin films are expected to be a promising infrared optoelectronic materials candidate for carrier injection devices because of the high electrical conductivity and high excitation efficiency of the Er3+ ions of an electron–hole-mediated process resulting from ZnO host excitation.