Radical enhanced atomic layer deposition (ALD) has been previously shown to enable the control of the Er³⁺ spatial distribution in Y₂O₃ thin films, thereby achieving an enhanced direct absorption cross section at 1540 nm and much improved photoluminescence with well-resolved Stark features.¹ Given the larger index of refraction of Y₂O₃ compared to that of SiO₂, this suggests that Er:Y₂O₃ thin films has the potential to achieve compact optical amplification. In this work, we report the utilization of radical enhanced atomic layer deposition to control the concentrations and spatial distances of Yb and Er in Yb³⁺ co-doped Er:Y₂O₃ thin films and the corresponding improvement in their optical characteristics.

The electronic energy level and a large absorption cross section of Yb³⁺ make it an effective sensitizer for Er³⁺. Thin films of approximately 10 nm are synthesized by sequential radical-enhanced ALD of Y₂O₃, Er₂O₃, and Yb₂O₃ at 350^oC. The composition, microstructure, cation distribution, local chemical bonding and optical properties of the as-synthesized thin films were determined by x-ray spectroscopy, electron microscopy and photoluminescent measurements. To optimize the effect of the sensitizer and minimize the concentration quenching, the concentration of Yb³⁺ and Er³⁺ were controlled by changing the global deposition cycle sequence. Extended x-ray absorption fine structure analysis verified the spatial control of Yb³⁺ and Er³⁺ in the Y₂O₃ host. Much improved effective absorption cross sections were estimated using the photoluminescence yield as a function of the pump power, as compared with measurements from thin films without Yb³⁺ sensitizers. From these measurements, an optimum concentration and spatial arrangement is chosen for the design of spiral ridge waveguide devices. These waveguides can achieve potential gains of two orders of magnitude over Er:SiO₂ and an order of magnitude over ion implanted Al₂O₃.

¹ Hoang, J., T. T. Van, et al. Journal of Applied Physics. 101(12), 123116 (2007).