Electroluminescence from silicon-based light-emitting devices with erbium-doped ZnO films: enhancement effect of lithium codoping

We have realized the erbium (Er)-related visible and near-infrared (NIR) electroluminescence (EL) from the light-emitting device (LED) with an Au/Mg_0.4Zn_0.6O/ZnO:Er/n⁺-Si structure. Herein, ZnO:Er refers to the Er-doped ZnO film. In order to enhance the Er-related emissions from such a LED, we present a strategy of codoping lithium (Li) into the ZnO:Er film. Through the optimization of the Li-codoping content, the Er-related visible and NIR emission intensities can be enhanced by more than 8 and 2 times, respectively. Density functional theory calculations reveal that the Li-codoping results in more symmetrical crystal fields around the luminescent Er³⁺ ions, which is not favorable for the increase in the intra-4f transition probabilities of Er³⁺ ions. Nevertheless, it is found that the Li-codoping leads to the increase in the average size of ZnO grains from 26 to 47 nm, thus significantly reducing the segregation of Er³⁺ ions at grain boundaries. Moreover, the smaller ionic radius of Li⁺ ions (68 pm) with respect to that of Er³⁺ ions (88.1 pm) is believed to be energetically favorable for the incorporation of Er³⁺ ions into ZnO grains. Accordingly, the Li-codoping increases the number of optically active Er³⁺ ions in the ZnO:Er film, which is actually verified by the steady-state and transient photoluminescence characterizations. In brief, both the coarsened ZnO grains and the promoted accommodation of Er³⁺ ions into ZnO grains, resulted from the Li-codoping, are responsible for the significantly enhanced EL as mentioned above.