Relation between process parameters of ZnO host films and optical activation of doped Er3+ ions

• Photoluminescence at 1.54 μm occurs from metatable ZnO:Er films deposited at RT.
• Concentration quenching above 3 at.% is correlated with derivation from Vegard's law.
• The number of Er3+ substituting at Zn2+ sites is a function of annealing temperature.
• Precipitation of Er2O3 at temperatures above 900 °C is evidenced with XRD.
• Deposition above 530 °C intensifies emissions due to improved crystallinity.

We investigated how the crystallinity of ZnO host films deposited on Si(1 0 0) substrates by electron cyclotron resonance (ECR) plasma sputtering affects the optical activity of Er3+ ions doped in them under resonant excitation with 532-nm laser light. Photoluminescence from ZnO:Er films deposited at room temperature (RT) exhibited a main emission peak at 1538 nm with a sub-peak at 1553 nm. The emission intensity reached a steady state at Er content between 0.6 and 3 at.%, and attenuated above 3 at.%. This concentration quenching correlates with derivation of X-ray diffraction angles from the Vegard's law. Deposition at RT produced non-equilibrium state of ZnO:Er films possibly stabilized with vacancies to maintain charge neutrality. After post annealing of ZnO:Er films with Er content less than 1 at.%, the emission intensities were markedly reduced as the crystalline lattice was rearranged. For films in the concentration quenching range, however, post annealing increased the emission peak width while maintaining emission intensity, indicating that Er3+ ions migrated and populated various emission-active sites. In this case, the main peak wavelength blue shifted to 1536 nm and produced a new sub peak at 1546 nm. When the deposition temperature exceeded 500 °C, emissions from Er3+ ions were greatly enhanced. This will result from improvement in crystallinity and fixing Er3+ ions at six-fold coordinated C4v symmetry sites.