Infrared upconversion in radio frequency magnetron sputtered Er-doped zinc oxide thin films

Upconversion in radio frequency magnetron sputtered Er-doped zinc oxide thin films on Si substrate has been demonstrated using 1550 nm cw laser excitation. As-sputtered thin films did not show any upconversion emission, and annealing was required to optically activate the Er³⁺-ions. Emissions at 985, 809, and 665-675 nm were observed in annealed thin films, corresponding to transitions from ⁴I1111/, ⁴I₉11/, and ⁴F₉11/ to the ground state ⁴I₁₅11/, respectively. The emission from ⁴I₁₁11/ was the dominant one, whereas emission from ⁴I₉11/ was the weakest. The highest intensity at 985 nm was obtained with 2.4 at. % of Er by annealing the film at 700 C. Annealing at higher temperatures causes Er to diffuse and segregate to the Si-ZnO interface between the Si substrate and the ZnO film.