Tuning the Luminescent Properties of Ytterbium Doped AlNxOy Thin Film Materials Libraries Using a Combinatorial Approach
Authors: Grieseler Rolf , Tucto Salinas Karem , Aponte Huamantinco Wilson , Guerra Torres Jorge Andres 
Published: February 2020

Rare-earth doped wide bandgap semiconductors give rise to many different luminescent applications. One main focus lies on electroluminescent devices due to the electronic behavior of the semiconductor host material. This kind of host materials allow the application in direct current as well as in alternating current electroluminescent devices with a generally low power consumption [1].
Using thin film devices makes it easy to design and tune the properties of electroluminescent devices. In the easiest form an electroluminescent device consists of a bottom electrode, the active material and a transparent top electrode. One candidate for the host material might be aluminum nitride AlN [2,3]. AlN is the semiconductor with the widest bandgap of 6.2 eV in its crystalline form. This leads to a good transparency but at the same time AlN still behaves like a semiconductor and is therefore widely used in electronic devices. Aluminum oxide, on the other hand, is an insulator with even higher transmittance and low refractive index. Both might be interesting candidates in light emitting devices. To obtain, for example, an infrared emitter, different rear earth materials can be used as dopant of the host material. Ytterbium is emitting with a wavelength of 980 nm, whereas erbium is emitting at 1535 nm [4].
One new approach to obtain and tune the properties of thin film devices is the deposition of material libraries. Using different approaches such as gradient co-deposition or mask based deposition opens up the possibility to produce a wide variety of different materials compositions on one single substrate. Adequate analytical screening methods allow the fast, easy and cheap evaluation of the interaction of different properties in a suitable range of interest [5].
In the presented work, the luminescent and optical properties of magnetron sputtered amorphous ytterbium doped aluminum oxynitride will be shown depending on the concentration of the ytterbium doping in a range of 0.9 to 4.2 at% as well as the ratio of oxygen to nitrogen of 0.5 to 3.5. To obtain a materials library the thin film was intentionally co-sputtered with a compositional gradient using two separate targets, a pure Yb and a pure AlN target with an unbalanced gas flux. Furthermore the activation of the rare-earth ions due to a temperature treatment in a range of 550 to 750°C as determined. It could be shown, that the emission is significantly higher at low oxygen concentrations meaning that the material closer to AlN is more favorable for the application in luminescent devices. This behavior could also be attributed to the activation energy of the rare-earth ions which was calculated from the intensity vs. concentration curves [6] of the materials libraries. Furthermore, the optical properties of the host material where investigated by means of the band gap and the refractive index.
The presented results will give a useful insight for the application of Yb:AlNxOy in electroluminescent devices and is furthermore a first step for the development of down-conversion thin films based on the co-doping of ytterbium and terbium [2] for different conversion applications.
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[2] K. Tucto, L. Flores, J. Guerra, J. Töfflinger, J. Dulanto, R. Grieseler, et al., Production and Characterization of Tb3+/Yb3+ Co-Activated AlON Thin Films for Down Conversion Applications in Photovoltaic Cells, MRS Adv. 2 (2017) 2989–2995.
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[6] J.A. Guerra, F. De Zela, K. Tucto, L. Montañez, J.A. Töfflinger, A. Winnacker, et al., Effect of thermal annealing treatments on the optical activation of Tb 3+ -doped amorphous SiC:H thin films, J. Phys. D. Appl. Phys. 49 (2016) 375104.

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