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: a) Operando XAS spectra of the V-L_2,3 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-L_2,3 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 Bassato^1,3, Arnaud Fouchet², Wilfrid Prellier², Mathieu Frégnaux², Damien Aureau⁴, Luca Braglia⁴, Giovanni Vinai¹, Piero Torelli¹, Ulrike Lüders^2
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

Contact person email:

Local contact person email: ,