Aluminum-doped zinc oxide (AZO) is a transparent conductive oxide (TCO) with high electrical conductivity and optical transparency. These properties added to its low cost and non-toxicity make it a good candidate for optoelectronic applications and sensing devices as an alternative to indium tin oxide (ITO) [1-3]. In this work, AZO thin films with different aluminum (Al) concentrations were deposited by rf-magnetron sputtering and were subsequently treated in an inert argon atmosphere at temperatures between 100°C and 700°C.

The optical and electrical properties of AZO were determined by variable angle spectroscopic ellipsometry measurements. The absorption spectrum of zinc oxide (ZnO) has an excitonic peak that remains visible when doped with small concentrations of Al. In this matter, the excitonic nature of AZO is not usually considered when estimating the optical bandgap from the light absorption spectra leading to unreliable values [3]. Additionally, ellipsometric data is typically fitted with the well known Tauc-Lorentz model, thus biassing even more the optical bandgap determination. In order to determine the optical bandgap and exciton binding energy, we used a recent model developed by our group that combines the Elliot formula [4] and the band-fluctuations approach [5] to describe the fundamental absorption near the band-edge.

The thickness, carrier density, charge mobility and electrical resistivity of the films were estimated using the Drude and Tauc-Lorentz models. We also evaluated the performance of more elaborated dispersion theories based on the energy loss method that describes infrared absorption of free charge carriers [6]. This analysis allowed us to examine the influence of Al concentration and annealing treatments on the optical and electrical properties of the AZO films. The optical bandgap is expected to increase with the concentration of Al due to the Burstein-Moss effect meanwhile the electrical resistivity is expected to decrease down to a minimum value and then rise again [2]. Accordingly, the results showed a consistent increase of the charge mobility with the annealing temperature. At the same time, the charge carrier density initially increased up to a maximum at 400°C, but decreased afterwards.

References

[1] H. Liu, V. Avrutin, N. Izyumskaya, Ü. Özgr, H. Morkoç Superlattices and Microstructures, 2010, 48(5), 458-484.

[2] S. Mridha, D. Basak Journal of Physics D: Applied Physics, 2007, 40(22), 6902–6907.

[3] F. Paraguay, M. Miki-Yoshida, J. Morales, J. Solis, W. Estrada Thin Solid Films, 2000, 373(1–2), 137–140.

[4] R.J. Elliot Phys. Rev., 1957 B 108, 1384–1389.

[5] J.A. Guerra, et al. J. Phys. D. Appl. Phys., 2019, 52, 105303.

[6] M. Piñeiro (Master Thesis). Pontificia Universidad Católica del Perú, 2022.

Acknowledgements

This research was supported by CONCYTEC-PROCIENCIA in the framework of the contest E074-2022-01 “Tesis y Pasantías en Ciencia, Tecnología e Innovación” grant no. PE501081866-2022-PROCIENCIA and the Office of Naval Research (ONR), grant no. N62909-21-1-2034. The authors acknowledge the Center for Materials Characterization (CAM) at the PUCP for supporting the development of this work.