Understanding the Electronic Properties of Amorphous Al-doped ZnO and Niobium Oxide Thin Films

The growing use of oxide-based materials in diverse applications such as catalysis, electronics, and sensing has necessitated the importance of understanding their electronic structure and the ultrafast dynamics of electronic modifications. In this seminar, I will present recent findings on two such oxide systems: aluminum-doped zinc oxide (AZO) and niobium oxide (NbO_x).
AZO is a class of material commonly known as transparent conductive oxides (TCO) that combine high optical transparency and low electrical resistivity. TCO exhibits an interesting tunable plasmonic response in the near-infrared range, characterized by epsilon-near-zero behavior [1]. When conducting films are grown amorphous, they can overcome poor CMOS compatibility, discontinuity, and poor mechanical stability typical of noble metals. The intrinsic absence of grain boundaries allows for the growth of thin and smoother films with improved electronic and transport properties. Even though amorphous conductors are known, a theoretical and experimental understanding of the influence of structural disorder on the electronic and optical properties of the films is completely missing.
In this talk, we provide insights into the impact of structural disorder on the electronic and optical properties of AZO films. To understand the impact of reduced structural order, amorphous AZO films were grown by RF magnetron sputtering by increasing Ar pressure [2]. We correlate amorphization with electronic and plasmonic properties using HAXPES, Hall effect, XRD, HRTEM, and optical measurements. XRD analysis indicates a loss of long-range order with increased growth pressure, as confirmed by HRTEM. We have performed DFT calculations that predict electronic modifications with broadening of the valence and conduction edges due to amorphization and symmetry breaking [3] that also aligns with the HAXPES analysis. HAXPES reveals a significant increase in the density of states near the Fermi level in amorphous AZO compared to polycrystalline, along with notable valence band modifications, consistent with theoretical predictions by DFT calculations.
In the second part, we examine NbO_x, a material of interest for applications in photocatalysis, memristors, and photochromic devices. We developed a method to obtain various stoichiometric phases of niobium oxide (NbO, NbO₂, and Nb₂O₅) via physical vapor deposition followed by controlled reduction and oxidation [4]. Pump-probe X-ray absorption near-edge spectroscopy (XANES) at the Nb K-edge, performed at the European X-ray free-electron laser (EuXFEL) facility, provided information on the ultrafast dynamics of electronic modifications induced by photoexcitation in the most stable phase, Nb₂O₅. Pump-probe extended-range X-ray absorption fine structure (EXAFS) spectra have revealed that photoexcitation also induces detectable modifications in the local atomic structure around niobium atoms.

References [1] G. V. Naik, et al., Alternative Plasmonic Materials: Beyond Gold and Silver. Adv. Mater. 25, 3264 (2013);
[2] R. Magrin Maffei et al, Defectivity of Al:ZnO thin films with different crystalline order probed by Positron Annihilation Spectroscopy. Appl. Surf. Sci. 665, 160240 (2024);
[3] D. Mora-Fonz et al., Modeling of Intrinsic Electron and Hole Trapping in Crystalline and Amorphous ZnO. Adv. Elec. Mater. 6, 1900760 (2020);
[4] S. Pelatti et al., Niobium oxide films with variable stoichiometry: Structure, Morphology, and Ultrafast dynamics, J. Phys. Chem. C. 129, 17 8206, (2025).

Seminar realized in the framework of the funded projects: MAECI 2023 Italy – Germany Science and Technology Cooperation U-DYNAMEC and NextGeneration EU PRIN 2022 AMONIX – CUP B53D23004060006.