Stable co-emission of UV, green and red light in ZnO thin films with rapid annealing treatment

In this work, ZnO thin films were prepared by electron beam evaporation and rapidly annealed at different temperatures. The analysis of X-ray diffraction (XRD) showed that all the ZnO thin films were preferentially oriented along the c-axis. With the increase of annealing temperature, the degree of preferred orientation along the c-axis was enhanced and ZnO crystals also increased gradually. However, the transmittance and optical bandgap of the samples were almost not affected by the rapid annealing treatment. The photoluminescence spectra showed that all the samples exhibited strong UV and visible emissions (mainly green and red emissions). Although the strong UV emission along with strong green emission was often reported previously, the strong co-emission of UV, green and red light was rarely reported. If, as many researchers suggested, the green emission of ZnO was mainly related with oxygen vacancy defects and the red emission was associated with oxygen interstitial defects, thus the strong co-emission of green and red light was hard to happen in theory. The strong co-emission of green and red light or green/red emissions with different wavelengths at the same time implies that there are probably several sources for green or red emissions. Deeply and extensively investigating the luminescence behavior of ZnO materials is favorable for revealing visible emission mechanisms and developing ZnO-based light-emitting devices as possible as soon.