Effect of Rare-Earth Doping on Structural and Luminescent Properties of Screen-Printed ZnO Films

Over the past decades, zinc oxide has attracted considerable attention for its possible application in optoelectronics due to simultaneous observation of intense ultraviolet and visible emission offering the development of white light-emitting devices. However, in the most cases the native defects responsible for visible emission are not stable upon materials processing. Moreover, for white phosphors, efficient and controllable emission in specific spectral range is often required. This can be achieved in particular via materials doping with rare-earth (RE) ions.

In the present study the results on effect of doping of screen-printed ZnO films with Sm³⁺ and/or Ho³⁺ions are presented. Photoluminescence (PL) and Raman scattering spectra as well as the X-ray diffraction patterns of undoped and doped films are examined in details versus sintering conditions and doping level.

In the PL spectra of undoped ZnO films sintered at 400−700°C, the excitonic emission was observed only, whereas sintering at higher temperatures (up to 1200°C) resulted in the appearance of visible defect-related PL bands peaked at 540−600 nm. The most intensive defect-related emission was found in the films sintered at 1000°C and these latter were doped with rare-earth ions of different concentration in the range of 1·10¹⁹− 4·10²⁰ cm^-3.

In the PL spectra of the RE-doped films, the corresponding RE emission was observed at low temperatures, but not at room temperature. Since the defect-related PL band caused by intrinsic defects in ZnO overlapped essentially with corresponding Ho and Sm PL bands, the PL bands peaked at about 700 nm due to ⁴G_5/2→⁶H_11/2 transitions in Sm³⁺ ions were observed only. At the same time, simultaneous codoping with both Sm³⁺ and Ho³⁺ ions produced significant decrease in the intensity of the UV and visible defect-related PL bands as well as the increase in the intensity of Sm³⁺ and Ho³⁺ PL components. In the PL excitation spectra of the PL band peaked at about 720 nm in addition to excitation caused by ZnO band-to-band absorption and absorption involving intrinsic defects of zinc oxide, a new characteristic absorption band at about 410 nm appeared. The effect of RE doping on the PL and PL excitation spectra is discussed in terms of the formation of RE ions complexes as well as energy transfer from ZnO host to RE ions.