Rare earth doped Zinc oxide thin films for optoelectronic applications

In the field of optoelectronic materials, Zinc oxide thin films are interesting due to their excellent electronic and optical properties. It is also inexpensive and environmentally safe. Zinc oxide, electrically excited, emits in the blue/green spectral region. By Stark effect, rare earths doped ZnO emits in the red (Eu3+) and the yellow spectrum (Tb3+). The result is a white light resulting from the recombination of these three emissions created in the ZnO matrix. The intended application is the manufacture of a light-emitting diode LED based on rare earth doped zinc oxide (RE : ZnO) emitting white light. In the present study, rare earth (Eu / Tb) doped ZnO thin films were grown by radiofrequency magnetron sputtering using a ZnO target. Doping is obtained by arranging calibrated europium oxide and/or terbium oxide (Eu2O3 / Tb4O7) pellets on the surface of the ZnO target. Depositions were performed at substrate temperature between room temperature RT and 500°C on Silicon (Si) and Sapphire (Al2O3) substrates, using different RF power and different target-substrate distances. Chemical analysis by EDX showed an atomic doping level between 1 to 5 % according to the experimental conditions during deposition. The structural properties of these films by X-ray diffraction XRD and transmission electron microscopy TEM show that the films are highly c-axis oriented but the increase of doping tends to deteriorate the crystalline orientation. The optical properties of the doped films are presented. The electrical properties are also explored as a function of the rare earth concentration.