# A-Site Cationic Variation to Expand the Sacrificial Layer AVO3 Family Dissolving in Water

Vincent Polewczyk, Bruno Bérini, Aïmane Cheikh, Deepak Dagur, Moussa Mezhoud, Oualyd El-Khaloufi, Moussa Merbaki, Luca Braglia, Simon Hurand, Arnaud Fouchet, et al.

► To cite this version:

Vincent Polewczyk, Bruno Bérini, Aïmane Cheikh, Deepak Dagur, Moussa Mezhoud, et al. A-Site Cationic Variation to Expand the Sacrificial Layer AVO3 Family Dissolving in Water. Advanced Materials Interfaces, 2025, 12 (12), pp.2500094. 10.1002/admi.202500094. hal-05033067

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# A-Site Cationic Variation to Expand the Sacrificial Layer $AVO_3$ Family Dissolving in Water

Vincent Polewczyk,* Bruno Bérini, Aïmane Cheikh, Deepak Dagur, Moussa Mezhoud, Oualyd El-Khaloufi, Moussa Merbaki, Luca Braglia, Simon Hurand, Arnaud Fouchet, Giovanni Vinai, Piero Torelli, Adrian David, Yves Dumont, and Ulrike Lüders

In the growing field of low-cost electronics, the epitaxy of complex oxide thin films on a Si substrate requires significant technical means. Therefore, a large attention is paid to the release of a freestanding oxide of interest from its deposition support which is then placed onto a low-cost substrate, via the etching of an intermediate sacrificial layer. The use of a sacrificial layer offers several advantages since the flexible polymer exploited for the transfer can also be fully utilized to design a flexible heterostructure. For green technology, more and more research investigations are being undertaken on these sacrificial layers etched by water. While $Sr_3Al_2O_6$ and $SrVO_3$ are archetypical examples, the need to find new materials with different lattice parameters and symmetry is critical to reach the epitaxy of numerous materials of interest. In this study, the possibilities of an A-site cationic variation in $AVO_3$ with $A = Sr$ and/or Ca thin films are highlighted to expand the water-soluble material's family reaching the smallest lattice parameter ever presented up to now. In addition to bring various compounds with different ageing properties to the literature, optical spectrophotometry *operando* characterizations to follow the chemical etching of the sacrificial layers in real-time are exploited.

## 1. Introduction

Transition metal oxides are widely studied due to their broad range of versatile functionalities arising from the complex interplay between charge, orbital, spin, and lattice degrees of freedom.[1-6] These are for instance highly transparent conductive oxide (TCO) materials and are integrated in a wide range of electronic devices, such as photodetectors, light-emitting diodes, transistors, sensors or photovoltaic devices.[7,8] Another very promising property is a certain propensity for solubility in

aqueous media, especially for acting as a sacrificial layer to achieve high-quality freestanding oxide thin films by wet- etching. In this process, a sacrificial layer with appropriate crystal structure, lattice parameters, and stability under subsequent epitaxial growth conditions is first deposited epitaxially on adapted substrates. On top, the material of interest is grown subsequently, usually with the aim of being in a monocrystalline state. This is also critical to reduce the lattice mismatch to decrease the apparition of cracks during the release of the freestanding film.[9] For these reasons, an appropriate choice of the underneath crystal structure parameters and symmetries is required. Being capable of spanning the entire lattice structure of the oxide family represents a huge challenge.

In order to release the material of interest from the growth substrate, multiple procedures[10,11] can be pursued. The

most known one consists of attaching a polymer, such as polydimethylsiloxane (PDMS), onto the surface of the heterostructure. The whole stack is then dipped in an etchant solution for the sacrificial layer to release the material of interest as a freestanding film. Before being transferred onto another substrate, usually a low-cost Si wafer, and released by a removal of the polymer with chemical or physical action,[9,10] it is important to stress that the material of interest can be also studied when attached to the flexible polymer, for example for *operando* deformation tests, which are of a great interest for flexible electronic technologies.

V. Polewczyk, B. Bérini, M. Merbaki, Y. Dumont

UVSQ, CNRS, GEMaC

Université Paris-Saclay

Versailles 78000, France

E-mail: vincent.polewczyk@cnrs.fr

<figure><img src="image_3.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202500094

© 2025 The Author(s). Advanced Materials Interfaces published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/admi.202500094

A. Cheikh, M. Mezhoud, O. El-Khaloufi, A. Fouchet, A. David, U. Lüders

Université de Caen Normandie, ENSICAEN, CNRS UMR 6508, CRISMAT

Normandie Univ

Caen 14000, France

D. Dagur, L. Braglia, G. Vinai, P. Torelli

CNR - Istituto Officina dei Materiali (IOM)

Area Science Park, S.S.14, km 163.5, Trieste I-34149, Italy

L. Braglia

Area science park

Padriciano 99, Trieste I-34149, Italy

S. Hurand

Institut Pprime

CNRS - Université de Poitiers - ENSMA

11 Bd Marie et Pierre Curie, Chasseneuil-du-Poitou 86360, France
The goal is to propose new sacrificial layers for a large range of materials with interesting properties for electronics, spintronics and others.[12–21]

We note that etchants with high selectivity to the sacrificial layer reduce damage to the material of interest, the substrate and the polymer during the etching process and facilitate substrate reusability. While oxides are usually soluble in various liquids with different etching rates,[22] most often strong etchants such as an acid solution are necessary.[23] An alternative water solution, ecologically more respectful, can be used on specific sacrificial layers, which is the case of some transition metal oxide compounds. Among them, the most well-known are Sr<sub>3</sub>Al<sub>2</sub>O<sub>6</sub> (SAO)[24–27] and its recent sibling Sr<sub>4</sub>Al<sub>2</sub>O<sub>7</sub>.[28] As pointed out in,[29] the water-solubility is related to both the discretization of the Al-O clusters and the hydrotropic Sr-O networks. Indeed, it is well-known that alkaline earth metal oxides react strongly with water to form metal hydroxide salts, MO<sub>(s)</sub> + H<sub>2</sub>O<sub>(l)</sub> → M(OH)<sub>2(s)</sub> (M: alkaline earth metal). Alternative sacrificial layers are however searched intensively, especially to match with the multiple lattice parameters of the material of interest.[30] Our consortium revealed that SrVO<sub>3</sub> (SVO) with a V<sup>4+</sup> 3d<sup>1</sup> electronic configuration is an outstanding example.[31–33] This material possesses a cubic symmetry with a small lattice parameter (a = 3.84 Å)[34] with one V atom surrounded by six O atoms in an octahedral configuration, i.e., in a perovskite ABO<sub>3</sub> structure. This material suffers however from the risk of chemical degradation with either nanostructure formation depending on the amount of oxygen during growth,[33,35] or with ageing[36] due to a surface demixing in Sr-based oxides and V<sub>2</sub>O<sub>5</sub>,[37] Sr-segregation towards the surface[38] accompanied with an over-oxidization of V leading to the formation of Sr<sub>2</sub>V<sub>2</sub>O<sub>7</sub> or Sr-rich Sr<sub>3</sub>V<sub>2</sub>O<sub>8</sub> phases[35,37,39–42] occurring at least on the first atomic top layers. These surface layers are also soluble in water, even faster than their SVO parent compound.[31,33,43]

In the search for a new material with a different symmetry and lattice parameter, and especially with a smaller lattice parameter than the smallest present in the literature up to now (SAO with Ca-substitution and its 3.81 Å[24] that specifically demonstrates the importance of reducing the lattice mismatch between the layers to improve the overall quality of the freestanding film), here we explore possible cationic chemical variations of SrVO₃. By fixing the B site cation (V⁴⁺), we substitute the A site cation Sr²⁺ (1.44 Å) by Ca²⁺ (1.34 Å) in different solid solutions, i.e., Sr<sub>1-x</sub>Ca<sub>x</sub>VO₃ (0≤x≤1) (SCVO hereafter). For a full replacement of Sr, CaVO₃ (CVO) has an orthorhombic *Pnma* structure with a, b and c equal to 5.32, 5.34, and 7.54 Å respectively,[44] corresponding to the pseudo-cubic (pc) a<sub>pc</sub> of 3.77 Å, smaller than the one of SVO of 3.84 Å. While CVO is mainly studied for its TCO properties,[45,46] no research studies focused on its water solubility. Nevertheless, similar effects as those observed in SVO occur in this material such as a thermal decomposition into nonstoichiometric CaVO₃<sub>±δ</sub>, which has a mixed oxidation state V³⁺/V⁴⁺,[46,47] or the appearance of surface nanostructures such as Ca₃V₂O₈.[44] This ageing effect remains up to now, to our knowledge, not investigated, but constitutes an important factor in order to assess the possible layer solubility and the transfer process. Knowing all these similarities, one could expect that CVO can work as a new sacrificial layer.

In this study, we have deposited epitaxial SCVO thin films on the whole composition range by pulsed laser deposition (PLD). After probing their structural quality with x-ray diffraction (XRD) measurements, we have performed spectroscopic measurements with x-ray photoemission (XPS) and x-ray absorption (XAS) techniques in ultra-high vacuum (UHV) conditions to gain insight into the chemistry of the film surfaces before and after water exposure. Having identified a surface stability improvement against the Sr segregation and the surface over-oxidization with the increase of the Ca content, we demonstrate the water etching of these new compounds for the first time. We have used operando optical spectrophotometry characterizations, a technique up to now unreported in this context and especially interesting, as it can be carried out in liquid water during the etching process. Finally, we have performed a transfer of a SrTiO₃ (STO) thin film as an archetypical example for a perovskite material of interest on a Si (001) wafer using CVO as a sacrificial layer. STO with a lattice parameter (3.905 Å) compatible with a large number of oxides has been chosen for its growth at low-O pressure avoiding the chemical degradation of the sacrificial layer, and because of its interest for various scientific communities.[48] Our study sheds light on the versatility of oxide properties and will stimulate several research studies on this sense into the future of flexible electronic technology.

## 2. Results and Discussion

### 2.1. Structural Characterizations

Despite the buckling of the V—O—V bond angle θ between SVO (180°) and CVO (160°),[49] resulting in a structural transition from the cubic SrVO₃ to an orthorhombic structure upon Ca doping, all structural parameters will be expressed in pseudo-cubic notation. After the growth of the sacrificial layer materials (see Experimental Section for details), we performed the first XRD diffraction measurements. **Figure 1a,b** show the lattice representation of the pseudo-cubic perovskite AVO₃ cells and their lattice parameters respectively, and **Figure 1c** the symmetric 0-2θ diffraction scans of the various solid solution thin films.

We observe in Figure 1c a high crystalline quality of the films, with intense XRD pseudo-cubic (002) diffraction peaks. No other phases with different stoichiometry, orientation or symmetry were recorded over a large angular scan (see Figure S1, Supporting Information). Since SVO exhibits a larger lattice parameter than CVO, due to the smaller Ca ions than the Sr, the SCVO solid solution peaks and shifts toward a higher 2θ value, i.e., lower lattice parameter, as the Ca concentration increases. The pseudo-cubic lattice parameters vary from 3.77 Å up to 3.84 Å by varying the amount of Sr/Ca in the AVO₃ perovskite, obeying Vegard's law, i.e., with a lattice parameter varying linearly with the Ca content x,[49] as shown in Figure S2 (Supporting Information). This linear evolution for films on the same substrate reflects a maintained strain without relaxation by dislocations, although the absence of Laue oscillations of the CVO film may be an indication of a loss of crystalline quality due to high strain. An additional reciprocal space map (RSM) on the S<sub>0.50</sub>C<sub>0.50</sub>VO in Figure S3 (Supporting Information) confirms this strain state. Throughout the compositional changes, the pseudo-cubic tensile epitaxial strain
<figure><img src="image_4.png" /><figcaption>**Figure 1.** a) AVO₃ perovskite crystal structures with blue, red and gray spheres corresponding to Sr/Ca, V and O atoms respectively. b) Sketch representing the various pseudo-cubic lattice parameters of the material in use. c) Specular θ/2θ measurements of SCVO samples covering the full solid solution range. The horizontal shift of the thin film diffraction peak indicates the change of the out-of-plane lattice parameter. All spectra are shifted vertically for clarity.</figcaption></figure>

increases linearly from 1.6% for SVO to achieve 3.4% for CVO, using the usual formula (a<sub>STO</sub>-a<sub>film</sub>)/a<sub>film</sub>*100, together with a cube-on-cube epitaxial relationship. Since the layers are strained and the out-of-plane lattice parameters of the films are close to the bulk values, it highlights the presence of off-stoichiometric defects, such as oxygen vacancies, which is a consequence of deposition conditions. These defects are even more present when films are strained.[42] The introduction of the oxygen vacancies leads to a reduction of the V<sup>4+</sup> (0.58 Å) to V<sup>3+</sup> (0.64 Å), which subsequently results in an expansion of the unit cell volume. The presence of V<sup>3+</sup> in our solid solutions will be further confirmed using XAS measurements (see below). An in-depth structural study of the structure of the films is reported elsewhere.[50] These materials lead to an extended window of possible sacrificial layers for the lattice-matched growth of various oxide materials of interest[51–53] and their subsequent transfer on a low-cost Si wafer.

## 2.2. Spectroscopic Characterizations

In order to determine the V oxidation states and therefore gain insights into any chemical effect of the Ca substitution, we have probed the surface of the sacrificial layers by XAS. No capping layer was deposited and the samples were air-exposed before the measurements. SVO is known to show a deteriorated surface after air exposition, which can be cleaned by a 1-min water immersion leading to a surface being representative of the film composition.[32] **Figure 2a** shows the XAS characterizations of the parent compounds SVO and CVO V-L edges before and after the water cleaning procedure. Both spectra sets are vertically shifted for clarity. Figure 2b presents the full set of V-L edges for all SCVO films before the water cleaning procedure.

In Figure 2a, the measured spectrum of SVO before the water cleaning procedure reveals a sharp V-L<sub>3</sub> edge, anticipated by multiple shoulder peaks, then followed by a broader V-L<sub>2</sub> edge. Performing a linear combination of the spectral features of simi-

lar reference AVOₓ spectra,[32,54–57] we obtain a pure V<sup>5+</sup> oxidation state in spite of the expected V<sup>4+</sup> of stoichiometric SrVO₃ (dashed fit of the blue line). Very similar results have been shown in our previous studies,[32,58] relating the presence of V<sup>5+</sup> to the air exposure of the film and therefore an aged surface. The pure V<sup>4+</sup> state is only recovered after the water cleaning procedure (green line). In this case, the V-L<sub>3</sub> edge shifts toward a lower photon energy by ≈0.2 eV. The V-L<sub>2</sub> edge shows a larger shift. Such a pure V<sup>4+</sup> state can also be obtained by protecting the film surface with a capping layer, as shown in Figure S4 (Supporting Information) with a 3 nm-thick Al₂O₃ layer on top of the SVO film.

On the other hand, the CVO sample shows a mix of V<sup>3+</sup> and V<sup>4+</sup> states on the top surface before water cleaning. The V<sup>3+</sup> character can be identified by a lower L-edge energy position, a small pre-edge ≈513 eV and a hump in between the L-edges. The fit of this data indicates a pronounced V<sup>3+</sup> content of 86%. Thus, a non-negligible portion of the film surface is heavily oxygen deficient. Ca-L edge spectra obtained before and after the cleaning procedure confirm that Ca retains its Ca<sup>2+</sup> valence state, as evidenced in Figure S5 (Supporting Information). The presence of V<sup>3+</sup> results both from oxygen vacancies usually introduced during deposition via PLD and from ageing effects, as observed in the same compound[47] or in similar oxides such as LaSrMnO₃ (LSMO), where the Mn<sup>2+</sup> contribution is usually observed in air-aged films.[59–61] After the water cleaning treatment, CVO recovers the stoichiometric state, i.e., a majority V<sup>4+</sup> state. A wider peak compared to the one of SVO after water etching is obtained, which can be related to the residual V<sup>3+</sup> surface state in the films. The linear combination fit specifies the presence of ≈60% of V<sup>4+</sup> at the CVO surface, while it is 95% in the case of SVO. Further studies have to be carried out in order to assess the surface quality of the sacrificial layers for a subsequent deposition of the material of interest, but the presented measurements show that the surfaces of both parent compounds avoid of any phases susceptible to block the water etching.
<figure><img src="image_5.png" /><figcaption>**Figure 2.** a) V-L edge XAS normalized spectra collected on SVO and CVO films before and after the water cleaning procedure. Reference spectra from[32,54–57] of the different V oxidation states are shown by the solid curves and were used to perform linear combination fits (dashed lines for before water, dotted lines for after water). b) V-L edge XAS normalized spectra collected on the SCVO sample series before the water cleaning procedure. Sample spectra are vertically shifted and dotted lines aligned in energy are presented here for clarity.</figcaption></figure>

The spectroscopic investigation evidently shows that SVO and CVO react in the opposite way to air exposure. While in SVO, the ageing leads to the pronounced V<sup>5+</sup> state, i.e., an over-oxidation of the Vanadium, in CVO the ageing is related to a strong presence of V<sup>3+</sup>, i.e., a reduction of the Vanadium. For the solid solutions (Figure 2b), the XAS shows that the A-cation substitution of Sr with 16% Ca (S<sub>0.84</sub>C<sub>0.16</sub>VO) is sufficient to avoid the formation of the V<sup>5+</sup> state, and a majority of V<sup>4+</sup> with an admixture of V<sup>3+</sup> is observed in the aged state. The linear combination fits can reproduce the overall tendencies with an increase of V<sup>3+</sup>/V<sup>4+</sup> ratio while increasing the Ca contribution, but a reliable quantification is complicated due to strong background dispersion from the O-K edge at a few eV higher (not shown here). When increasing the Ca concentration, the S<sub>0.50</sub>C<sub>0.50</sub>VO still shows a majority of V<sup>4+</sup> state, while the contribution of V<sup>3+</sup> increases strongly when exceeding 75% of the Ca substitution. Additional XPS measurements on the Sr-3d core levels of S<sub>0.50</sub>C<sub>0.50</sub>VO are shown in Figure S6 (Supporting Information), with just a minor contribution of Sr-3d doublet related to the Sr segregation and thus spurious surface phases, as evidenced for SVO.[32] This is true for all the SCVO samples, even for smaller amounts of Ca, i.e., Sr-rich compositions. Thus, despite the isovalent nature of Ca/Sr substitution, the A-site cation plays a strong role in the chemical effects of the Vanadates, with a stabilizing character of Ca on the V<sup>4+</sup> state even for small amounts. All the SCVO samples show a clear majority of V<sup>4+</sup> surface state after the water cleaning treatment (see Figure S7, Supporting Information). This ageing effect

is not yet fully understood and additional theoretical calculations should be performed, but remain beyond the scope of this study. Both electronegativity and ionic radius of Ca<sup>2+</sup> which is smaller than that of Sr<sup>2+</sup>, with a binding strength of the Ca-O bond becoming stronger, may play a role.

## 2.3. Operando Optical Spectrophotometry

In order to gain insights into the water etching process of the here proposed sacrificial layers, we have carried out optical spectrophotometry measurements during the immersion of the films in water. Figure 3 shows these characterizations a) for 80 nm-thick SVO and b) for 80 nm-thick CVO films. The film thickness was determined before water immersion by profilometry. From the optical properties, we have extracted the film thickness, plotted as a function of time in Figure 3c.

In the initial state of both materials, the transmittance measurements reveal mostly a smooth increase and large transmittance from 850 nm down to 500 nm with a drop from 500 to 400 nm for both samples. This pronounced decrease of transmittance below 500 nm is attributed to the threshold of interband transition O-2p and V-3d orbitals. It is important to note that SVO and CVO are transparent in the visible range and conducting[62] as is the SCVO solid solution.[50] We note also a transmittance cut-off occurring ≈390 nm, i.e., 3.2 eV, corresponding to the well-established bandgap of the STO substrate. The transmittance
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Variation of the optical transmittance and their associated fits in thin solid black lines during the water etching process ((a) and (b) for SVO and CVO respectively). The color bar and associated spectra refer to different etching times in minutes. In (a), the optical transmittance of the SVO//STO heterostructure after 1 day of water immersion (transparent gray line) is shown as an accurate reference for the substrate optical properties after the full etching of the film. (c) Estimation of both SVO and CVO layer thicknesses during the water-etching (symbols) with uncertainty values (solid transparent curves) through modelling.</figcaption></figure>

values of both layers recorded at a wavelength of 850 nm are sim-
ilar and in agreement with typical values of epitaxial films pre-
sented in literature,[45,63–65] especially for the used thicknesses.
With the immersion of the samples into the water, a gradual in-
crease of the transmittance is observed, until reaching a spectral
shape approaching that collected from reference STO substrate
(see Figure 3a with the STO used for SVO//STO). The transmit-
tance of the substrate is in agreement with the literature,[66] with a
small over-estimation due to the formation of air bubbles in water
leading to a higher diffusion of the light. These evolutions dur-
ing the water immersion constitute a ubiquitous proof of CVO
water solubility, demonstrating this material as a new candidate
as a sacrificial layer.

Even if the etching progression is probably not strictly respond-
ing to a layer-by-layer mode, we have modelled the operando ex-
periments to gain insights on the equivalent thickness reduction
occurring during the etching process for both layers. The fits, de-
tailed in the method section, present a good accuracy in describ-
ing the collected spectra throughout the etching process. The
used model was confirmed (Figure S8, Supporting Information)
with the SVO film in its initial state, in which the real and imagi-
nary parts of the dielectric functions (DF) are in good agreement
with the literature.[45,67] Different overall kinetics are observed for
CVO and SVO (see Figure 3c), reproducible on multiple samples.
With a two-step regime, the dissolution kinetics of the CVO sacri-
ficial layer is slower than that of SVO, especially during the initial

stage as highlighted with an asterisk on Figure 3b. This is ma-
jorly attributed to the lower solubility of CaO compared to SrO.[68]
as also observed in SAO with Ca substitution.[24] As expressed
in this reference, the higher the M²⁺ electronegativity (Ca²⁺ >
Sr²⁺), the lower its bonding ionicity with the non-bridging oxy-
gen of the structure, and thus it is more difficult to hydrolyze. It
may be also related to the surface deterioration of SVO related to
the ageing effect[69] (Figure 2a) with the appearance of the spuri-
ous Sr-rich V⁵⁺ phase, which is also soluble in water but faster,
as evidenced in previous studies.[31,32] Using a linear fit of the
data close to the end of the etching process to find the total etch-
ing time, (red and black lines in Figure 3c), values of 0.008 and
0.005 nm s⁻¹ can be estimated for SVO and CVO respectively, in
agreement with our previous work.[31] Thus, operando transmit-
tance spectrophotometry measurements represent an ideal and
easy-to-implement technique for studying etching dynamics in
thin films.

2.4. Transfer of an Oxide of Interest

As a last step, we have performed a transfer of a 5 × 5 mm
STO thin film onto a low-cost Si substrate using CVO as a sacri-
ficial layer. The sacrificial layer here is not aged since the ma-
terial of interest has been deposited onto it in situ. The full
process is described in **Figure 4a**. A thermal release tape poly-
<figure><img src="image_7.png" /><figcaption>**Figure 4.** a) Schematics of the transfer process steps of a STO/CVO//STO heterostructure using a TRT polymer. b) AFM characterization of the heterostructure as-grown. c) Optical image of the freestanding STO layer on the polymer. d) Specular XRD scan of the transferred STO layer on Si(001) and optical microscopy image of the surface in the inset.</figcaption></figure>

mer (TRT) was attached to the surface of the heterostructure, which possesses a smooth surface as observed with atomic force microscopy (AFM) (Figure 4b) with the typical terraces related to the STO substrate surface. The stack is then immersed in deionized water during 8 days, showing an important increase of the etching time due to the presence of the STO overlayer allowing the water to penetrate only from the sides of the heterostructure, and not any more from its top interface. With the etching of the sacrificial layer, the STO thin film is released from the growth substrate. An optical image of this step is presented in Figure 4c. Then, the released film is placed onto the Si substrate. With the help of an annealing at 90 °C, the polymer is being detached from the STO film. At the end of the whole process, XRD characterizations have been performed (see Figure 4d).

Although micro-cracks are observed from the optical image on the STO film attached to the polymer in Figure 4c, expected from the inherent wet-etching process and noticed in numerous research studies[9,22,24] including a similar heterostructure made of STO/SVO//STO,[31] the adhesion of the oxide of interest onto the polymer is successful. We note that the film remains flat on a local scale, as demonstrated by AFM in Figure S9 (Supporting Information). The successful transfer on the Si substrate was evidenced by the diffracting STO (002) peak after the last part of the process (Figure 4d). The process can evidently still be optimized, or adapted with an appropriate choice of oxide with a smaller lattice mismatch, but the possible use of CVO as a sacrificial layer is clearly evidenced.

### 3. Conclusion

We highlight in this study how the A-cationic variation of the perovskite SVO compound to CVO leads to the discovery of new sacrificial layers. The spectroscopic characterizations show large differences between SVO and CVO surface reactivity with a higher robustness of the latter. In spite of this robustness with the absence of Sr segregation toward the surface and the overoxidization of Vanadium, we have evidenced an etching of these layers, with different water dissolution rates between the materials through operando spectrophotometry, shown in this configuration for the very first time. We eventually performed a transfer of an oxide of interest from the initial growth substrate to a Si wafer. Therefore, we demonstrate new compounds to the expanding water-soluble material's family, with both different lattice parameters and also crystal symmetries, reaching the smallest lattice units up to now. Our study helps to expand the library of sacrificial layers, opening as well to the flexible electronic technology.

### 4. Experimental Section

**Sample Fabrication:** Before being introduced in the deposition apparatus, STO substrates were either cleaned with acetone and ethanol in an ultrasonic bath, either chemically etched with a buffered hydrofluoric acid followed by a thermal annealing at 950 °C under oxygen atmosphere to achieve a step-terrace surface morphology. The thickness of the deposited Vanadate films varied in the 20–80 nm range, without changing
the overall tendencies highlighted in this study. The epitaxial growth of the two end-members (SVO and CVO), and Sr<sub>1-x</sub>Ca<sub>x</sub>VO<sub>3</sub> solid solution were performed using a KrF excimer laser of 248 nm wavelength at ≈600 °C with a laser repetition rate varying between 2 and 5 Hz, abla ting stable polycrystalline targets of Sr<sub>2</sub>V<sub>2</sub>O<sub>7</sub> and Ca<sub>2</sub>V<sub>2</sub>O<sub>7</sub> with a substrate-to-target distance of 45 nm. In order to prevent the over-oxidation of the SVO and CVO epitaxial layers and the formation of nanostructure phases at the surface, the deposition pressure was fixed at 1 × 10<sup>-6</sup> mbar. The solid solutions were synthesized by using the two end-member targets. This approach provides the flexibility to tune the content x. By calibrating the deposition rate and controlling the number of pulses on each target, the desired compositions are obtained. No capping layers were deposited on the top of the Vanadate films, so that the as-grown films, taken out of the growth chamber, are considered as aged for the further described characterizations.

**X-Ray Diffraction Characterizations:** Structural XRD specular measure-
ments were performed using a Bruker D8 Discover diffractometer, operat-
ing with a monochromatic Cu Kα₁ radiation (λ = 1.5406 Å). The different
diffractograms were obtained through a θ-2θ scan with an increment of
5 × 10⁻³(°) and a time interval of 1 s per step.

**Surface Morphology Characterizations:** Morphology characterizations were performed using an AFM setup in scan-asyst mode. As observed in Figure S10 (Supporting Information), flat surfaces were obtained on both parent compounds, with terraces coming from the substrate visible depending on the substrate preparation, and with a root mean square (RMS) roughness of 0.50 and 0.36 nm for SVO and CVO respectively. It confirms also the absence of spurious nanostructure phases, as in.[³³,³⁵,⁴⁴]

**Electrical Characterizations:** The sample resistivity was recorded using a four-point-resistance head in contact with the sample surface and using a Keithley 2450 sourcemeter (not shown here). The resistivity (ρ) values are close to hundreds of μΩ.cm at room temperature, in agreement with values for metallic SVO and CVO single crystal and polycrystalline samples,[⁴⁷,⁵⁸] reflecting again the high material quality.

**Synchrotron-Based Characterizations:** At the NFFA APE-HE beamline of the Elettra synchrotron radiation facility,[⁷⁰] XPS measurements were then performed in UHV with linearly polarized radiation of 900 eV photon energy, corresponding to a probing depth of ≈1 nm. The photoemission spectra were measured with the sample surface oriented at 45° with respect to the incident photon beam and normal to the analyzer axis. The binding energies of the photoemission peaks were calibrated with respect to the Fermi level of an Au reference sample. XAS measurements at Ca-L<sub>2,3</sub> and V-L<sub>2,3</sub> were recorded in total electron yield (TEY) mode, with the same incident angle of 45°, corresponding to a probing depth of ≈5 nm, and normalized to the incident photon flux. Both measurements were performed before and after the water cleaning procedures. For this, the samples were immerged and agitated by hand in deionized water for 1 min before being reintroduced in the measurement apparatus.

**Optical Characterizations and Analysis/Simulations:** The optical properties of the layers were measured in the [350–850 nm] range using a Perkin Elmer spectrophotometer model Lambda 950 operating in the conventional specular transmittance mode, where light is transmitted through the sample and collected by a detector aligned on the incident wave-vector axis. For the *operando* process, the samples were immerged in a very large volume of fresh deionized water at room temperature and kept vertical using a homemade sample holder. The experimental conditions include a circular beam size of 1 cm diameter covering the sample surface. The transmittance measurements were corrected from the baseline offset, i.e., a measurement in the same conditions without a sample. Afterward, the data were fitted using the Complete Ease software of J.A. Woolam by means of an optical model that includes the different layers, their DFs and their thicknesses, using several fitting parameters in the effective medium approximation (EMA). Here the water all around the sample was neglected. A numerical regression using a Levenberg-Marquardt algorithm was performed by minimizing the mean-square error (MSE). For the substrate, a double-side polished was used with no background reflections. The optical indices have been simulated with different PSemicon oscillators. The surface roughness of the substrate was not taken into account as it was found to have no influence on the model. After having simulated and fixed the substrate properties from their reference spectra,

the thin films were implemented. Similarly to ref.,[⁴⁵,⁶⁷] for the sacrificial layers was implemented: 1) a Tauc-Lorentz oscillator to represent the direct bandgap absorption peak due to the interband transitions from the 2p to the t<sub>2g</sub> orbitals, 2) a Drude oscillator to represent the absorption due to the free carriers, i.e., the intraband transitions within the t<sub>2g</sub> band. A resistivity value of hundreds of µΩ.cm for both systems was used, 3) a Gaussian oscillator ≈5 eV to model the other UV interband transitions within the measurement range and 4) a Gaussian oscillator ≈1 eV to model the t<sub>2g</sub> to e<sub>g</sub> interband transitions. A constant value e<sub>∞</sub> was also added to the real part of the DFs in order to model the dispersion stemming from further interband transitions at even higher energy. Lastly, the possible roughness evolution while etching the layers was taken into account. For this, Brugge-man EMA consisting of a 0.5 layer and 0.5 void volume fractions was used, without a clear impact on the obtained results. For this reason, a reasonably small value of 2 up to 6 nm was kept, as an approximation since the etching of the layer is probably not layer-by-layer with the water dissolving from both the sides and the top of the films. In the end, different fitting approaches were performed, fixing the initial optical indexes or reducing the number of oscillators, for instance, to gain a relevant error bar range. It was stressed out that similar tendencies are obtained by plotting the intensity evolution of the transmittance at the highest wavelength and also the integral of the curves over the full wavelength range (not shown here).

**Transfer of the Oxide of Interest:** As a “proof-of-concept,” 80 nm-thick STO film of interest is deposited on the sacrificial 20nm-thick CVO layer without intermediate *ex situ* transfer in between layers. It is then released on a TRT polymer (Revalpha 3198MS (4M) from Semiconductor Equipment Corp.). Afterward, the STO film is transferred onto a Si (001) substrate. Optical microscopy images were collected together with XRD specular measurements.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from
the author.

**Acknowledgements**

This work has been partially performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MUR Italy Progetti Nazionali) facility (https://www.trieste.nffa.eu/), and by the Carnot ESP Institute through the project OpTCO project. M. M. acknowledges funding from the Normandy region through the BOSEED project. The authors acknowledge the French Agence Nationale de la Recherche (ANR) in the framework of the DisTCOvery project (ANR-23-CE08-0008) and the FLEXO project (ANR-21-CE09-0046) for financial support.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available from the cor-
responding author upon reasonable request.

**Keywords**

flexible electronics, low-cost electronics, sacrificial layer, thin film, water etching

Received: January 30, 2025

Revised: March 3, 2025

Published online:
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