Transparent conductive oxide water etching revealed by operando spectroscopy

Perovskite transition metal oxide thin films exhibit a wide range of functional properties that make them promising candidates for electronic devices. In their transparent conductive oxide (TCO) form, they possess a unique combination of high electrical conductivity and visible-range optical transparency. Among the TCOs, SrVO3 (SVO) has gained significant attention due to its electrical and optical properties comparable to the most commonly used indium-tin-oxide (ITO), which suffers from high costs due to the scarcity of indium. However, SVO is prone to surface chemical degradation over time, which poses a significant challenge. This degradation leads to the segregation of Sr towards the surface and the over-oxidization of V, resulting in the formation of a thin insulating layer at the TCO surface. A full understanding and control of these issues are needed to overcome its use in large-scale technology.

In our study we have used pulsed laser deposition (PLD) technique to deposit polycrystalline SVO film of few tens nanometers on top of a Si substrate, which is also suitable for further industrial transfer. To simulate the aging process, we subjected the SVO films to ex situ annealing treatments in air at 200°C, a well-established procedure Subsequently, we performed spectroscopic measurements using x-ray photoemission and x-ray absorption techniques under ultra-high vacuum conditions to characterize the samples. All spectroscopic characterizations were carried out at the APE-HE beamline of Elettra, partly in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MUR Italy Progetti Internazionali) facility.

Figure 1 of top story by V. Polewczyk et al, from Adv. Funct. Mater 2023

Figure 1: a) Operando XAS spectra of the V-L2,3 edges collected before (green) and during the exposure of the sample surface to water vapor in He fluxed at ambient pressure. b) Associated x-ray reflectivity measurements. c) Sketch of the mechanisms revealed by this study, i.e., time and/or annealing deteriorate TCO with the formation of a thin insulating top layer, which can be removed via solvation.

Our analysis revealed the presence of a thin Sr-rich surface layer resulting from the aging process, which was attributed to the formation of Sr2V2O7 and/or Sr3V2O8 compounds. To mitigate the surface degradation, we employed a cleaning process using water at ambient pressure, with water both in its vapor and liquid forms. As expected, we found that liquid water was significantly more effective and faster in removing the undesired layer compared to water vapor. To gain further insights into the cleaning dynamics, we performed operando x-ray absorption measurements at ambient pressure. In this way, we followed the V-L2,3 edges (Fig. 1a) evolutions upon exposure to a continuous stream of water vapor. The cleaning is here definitively associated to an etching of the 3 nm thick Sr-rich V5+ surface state, as attested by additional x-ray reflectivity measurements (Fig. 1b). Having achieved a clean and stoichiometric SVO surface, we investigated the effects of annealing treatments in different atmospheres, including reducing and oxidizing environments, to reproduce the initial state. By carefully controlling the annealing conditions, we gained a better understanding of the thermodynamic surface instabilities of perovskite TCOs and the factors influencing their degradation.

Overall, our study contributes to the advancement of knowledge in the field of perovskite TCOs, specifically focusing on SVO, and provides valuable insights for the future utilization of vanadium-based TCOs in large-scale technology. By addressing the challenges associated with surface degradation and establishing efficient cleaning methods, we bring closer the realization of low-cost oxide electronics and the widespread implementation of vanadium-based TCO materials.

This research was conducted by the following research team:

Vincent Polewczyk1, Moussa Mezhoud2, Martando Rath2, Oualyd El-Khaloufi2, Ferdinando Bassato1,3, Arnaud Fouchet2, Wilfrid Prellier2, Mathieu Frégnaux2, Damien Aureau4, Luca Braglia4, Giovanni Vinai1, Piero Torelli1, Ulrike Lüders2
1 Istituto Officina dei Materiali (IOM)–CNR, Laboratorio TASC, Area Science Park, Trieste, Italy
2 Normandie Univ, ENSICAEN, UNICAEN, CNRS, CRISMAT, Caen, France
3 Department of Physics, University of Trieste, Trieste, Italy
4 ILV, CNRS UMR 8180, Université de Versailles Saint-Quentin-en-Yvelines – Université Paris-Saclay, Versailles, France

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V. Polewczyk, M. Mezhoud, M. Rath, O. El-Khaloufi, F. Bassato, A. Fouchet, W. Prellier, M. Frégnaux, D. Aureau, L. Braglia, G. Vinai, P. Torelli and U. Lüders, “Formation and Etching of the Insulating Sr-Rich V5+ Phase at the Metallic SrVO3 Surface Revealed by operando XAS Spectroscopy Characterizations", Adv. Funct. Mater. 2301056 (2023); DOI:10.1002/adfm.202301056.

Last Updated on Thursday, 03 August 2023 22:59