# Perovskite AMoO3 (A = Ca, Sr, Ba): A New Ambient-Stable and Water-Soluble Sacrificial Layer Family for Single-Crystal Self-Supporting Oxide Membrane Fabrication

Moussa Mebarki, Bruno Bérini, Vincent Polewczyk, Arnaud Fouchet, Valérie Demange, Yves Dumont

► To cite this version:

Moussa Mebarki, Bruno Bérini, Vincent Polewczyk, Arnaud Fouchet, Valérie Demange, et al. Perovskite AMoO3 (A = Ca, Sr, Ba): A New Ambient-Stable and Water-Soluble Sacrificial Layer Family for Single-Crystal Self-Supporting Oxide Membrane Fabrication. ACS Applied Materials & Interfaces, 2025, 17 (18), pp.27400-27407. 10.1021/acsami.5c01621. http://dx.doi.org/10.1021/acsami.5c01621

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<figure><img src="image_1.png" /></figure>
Perovskite $AMoO_3$ ($A$ = Ca, Sr, Ba): a new ambient- stable and
water-soluble sacrificial layer family for single-crystal self-
supporting oxide membranes fabrication

Moussa Mebarki¹*, Bruno Berini¹, Vincent Polewczyk¹, Arnaud Fouchet², Valérie Demange³ and Yves Dumont¹*

¹Groupe d'Étude de la Matière Condensée (GEMaC), Université Paris-Saclay, UVSQ, CNRS, 45 Avenue des États-Unis ; 78035 Versailles ; France

²Laboratoire de Cristallographie et Sciences des Matériaux (CRISMAT), Université de Caen Normandie CNRS-ENSICAEN, 6 Boulevard du Maréchal Juin, 14050 Caen, France

³Univ Rennes, CNRS, ISCR - UMR 6226, ScanMAT - UAR 2025, 263 Avenue du Général Leclerc, Rennes F-35042, France

*corresponding authors: moussa.mebarki@uvsq.fr, yves.dumont@uvsq.fr

**Keywords:** Perovskite oxides, sacrificial layers, freestanding membranes, molybdates, flexible electronics

**ABSTRACT:** Development of self-supporting oxide membranes fabrication using sacrificial layer method has attracted a great attention over the past decade, specifically for their promising integration as devices on silicon and flexible substrates due to their large panel of properties such as ferroelectricity, ferromagnetism, superconductivity, metal/insulator transition. Known sacrificial layer materials with perovskite related structure, such as $La_xSr_{1-x}MnO_3$ and $YBa_2Cu_3O_7$, are dissolved in acidic or basic solutions. To prevent the oxide membranes of attacks by the sacrificial layer dissolution solution, it is important to find perovskite water-soluble sacrificial buffers. In this work, we evidence the perovskite molybdates $AMoO_3$ ($A$ = Ca, Sr, Ba) as a new ambient stable and an efficient water-soluble sacrificial layer family for the fabrication of oxide membrane. We first show the stability in ambient environment of $CaMoO_3$, $SrMoO_3$, $BaMoO_3$ epitaxial films on (001)$SrTiO_3$ substrates and their water dissolution kinetics with membrane release times longer than the $SrVO_3$ material and the well-known $Sr_3Al_2O_6$ sacrificial layers material. We analyze the etching kinetics via an operando optical monitoring of the remaining sacrificial layer surface still attached to the initial template substrate. We found for molybdate family a first order reaction kinetics with a main exponential decay, with constants respectively of around 11h, 14h and 19h for $CaMoO_3$, $SrMoO_3$, $BaMoO_3$. We use this molybdate family to release an 80nm-thick $SrTiO_3$ membrane on a polydimethylsiloxane support with a smooth morphology observed by atomic force microscopy and a monocrystalline quality observed by high-resolution X-ray diffraction. These results open a range of pseudo-cubic lattice parameters of the ambient-stable and water-soluble sacrificial buffers with pseudo-cubic lattice parameters from 3.90 Å to 4.04 Å for self-supporting perovskite oxide monocrystalline membranes fabrication.

# 1. INTRODUCTION

Today's and tomorrow's information
technologies require more integrated, compact
and flexible electronics to meet the social demands of more and more data density and treatments, of easy portability and of lower energy consuming. In this perspective, complex transition-metal oxides, in particular $ABO_3$ perovskite oxides, are a promising and developing family of outstanding wide range of electronic properties including ferroelectricity,¹ multiferroicity,² metal-insulator transition,³,⁴ magnetism⁵ and superconductivity,⁶ naming only few.⁷ In addition, their characteristic nanoscale coupling lengths and epitaxial integration structures have enabled the generation of new interfacial properties such as two dimensional (2D) metallic systems at $LaAlO_3$/SrTiO₃ interfaces⁸ and coupled properties in superlattice heterostructures.⁹

Inspired by the development and studies of 2D van der Waals materials¹⁰,¹¹ and their intrinsic ability to exfoliate, a new field of fabrication, transfer, and fundamental physical properties of self-supporting oxide perovskite thin membranes has blossomed over the past decade¹². By applying concepts previously developed in semiconductors technology¹³ as photolithography techniques, the method of wet etching a sacrificial oxide buffer layer between the growth substrate and the upper membrane of interest demonstrates its efficiency thanks to the development of sacrificial layers with perovskite related structure (such as $La_xSr_{1-x}MnO_3$ and $YBa_2Cu_3O_7$) that dissolve in different acidic or basic solvents.¹⁴ Nevertheles, it is more suitable using water-soluble sacrificial layers such as $Sr_3Al_2O_6$ (SAO)¹⁵⁻¹⁷ to prevent any corrosion of the membrane of interest. The most used sacrificial layer is $Sr_3Al_2O_6$ (SAO) that possesses a cubic cell with lattice constant $a_{SAO}$ equal to 15.844 Å. Its oxygen sublattice partially matches (75%) four unit-cells of most perovskite oxides ($a_{SAO}/4 = a'_{SAO} = 3.961$ Å).¹⁸ Substitution of the $Sr^{2+}$ cation by other alkaline earth metals $Ca^{2+}$ or $Ba^{2+}$ in the (Sr, Ca, Ba)$_3Al_2O_6$ aluminate family enables the $a'/4$ lattice parameter to be tuned over a very wide range from 3.819 Å for $Ca_3Al_2O_6$ to 4.124 Å for $Ba_3Al_2O_6$.¹⁴,¹⁹,²⁰ This family of SL is completely dissolved in deionized (DI) water (within few minutes under certain conditions) and plays a dominant role in the release and transfer of perovskite oxide membrane of interest. Their easy solubility in water under ambient conditions - specially for the recently discovered $Sr_4Al_2O_7$ sacrificial layer²¹ faster to dissolve than SAO - requires integration of sequential *in situ* deposition or the use of a passivation or buffer barrier such as $SrTiO_3$ (STO).²²,²³ More recently, another water etched compound has been discovered by our group, this new sacrificial layer is the strontium vanadate perovskite $SrVO_3$ (cubic lattice constant $a_{SVO} = 3.841$ Å),²⁴ which is dissolved in deionized water more slowly than SAO, remains more stable and conserves its crystallinity through artificial aging.²⁵

In this context of the importance of finding water-soluble sacrificial buffer, the present work demonstrates both the stability in ambient environment of a new family of perovskite molybdates $AMoO_3$ (with A= Ca, Sr, Ba) and their ability to be dissolved in DI water in order to use them as epitaxial buffer and then as a sacrificial layer. We quantify the kinetics of water etching and compare the molybdate family to the other known sacrificial materials such as (Ca, Sr, Ba)$_3Al_2O_6$, and $SrVO_3$ phases. Finally, we demonstrate the use of this new family of sacrificial layer to release a freestanding membrane of STO on polydimethylsiloxane (PDMS) in a prototype case: 80 nm-SrTiO$_3$ epitaxially grown onto 001 oriented $SrTiO_3$ oriented single crystalline substrate by Pulsed Laser Deposition (PLD) with an ultra-thin $AMoO_3$ sacrificial layer.

## 2. EXPERIMENTAL SECTION

*Substrate preparation:* the samples were grown by PLD on single crystalline (001) $SrTiO_3$ (STO) substrate (dimensions: 10 × 10 × 0.5 mm³, precut into 5 × 5 mm²) (CODEX). Notches are first made on the back side with a diamond wire saw to be able, after growth, to separate them into 4 identical samples of 5 × 5 mm². Prior to PLD growth, the (001) STO substrates were chemically etched with a buffered hydrofluoric acid ($NH_4F$),²⁶ followed by a thermal annealing at 950°C in an oxygen atmosphere to obtain a step-terrace surface morphology.²⁷

*Growth conditions of the samples:* $AMoO_3$ // (001)STO samples with (A= Ba, Ca, Sr) and STO/$AMoO_3$ // (001)STO heterostructures were grown by PLD. The substrate-target distance was maintained at 5.5 cm with a laser pulse frequency of 2Hz. The fluence was 1.85 J cm⁻², for both the sacrificial layer and STO growths. All samples were cooled under growth oxygen pressure for 2 hours. $AMoO_3$ ceramic targets were first pre-ablated using a KrF laser under 10⁻⁷ mbar ultra-high vacuum pressure. The first series of $BaMoO_3$ (BMO), $CaMoO_3$ (CMO) and $SrMoO_3$ (SMO) layers were deposited on non-treated (001) oriented STO single crystal substrates at around 650°C. 1000 laser pulses were used to obtain layers with a thickness of around 40-50 nm. Growth oxygen pressure $PO_2$ was 3×10⁻⁷ mbar, 6×10⁻⁷ mbar for SMO and BMO respectively, and 1.2×10⁻⁶ mbar for CMO, corresponding to optimal oxygen pressure to obtain a small roughness. The second series of STO(80nm) / $AMoO_3$ (100 pulses) // (001)STO heterostructures was also grown by PLD. The temperature was maintained for the deposition of the 80nm-STO top layer and the oxygen pressure was rapidly increased from $PO_2$ used for the SL to 10⁻⁵ mbar after few minutes. In our conditions, the thickness of the STO top layer is 80 nm and 4 nm for the $AMoO_3$ sacrificial layer.
**Optical measurement of the dissolution rate of the AMoO₃ sacrificial layers:** The dissolution was carried out in a DI water solution bath heated at 50°C. Each AMoO₃ (50nm)//(001)STO sample was placed upside down in the glass container filled with DI water to ensure a dissolution from the sides to the middle of the sacrificial layer. A calibrated Dino-Lite optical microscope was placed above the container to measure the sacrificial layer' remaining surface with an image recorded each 1 hour. The measurement method is schematized in DI water dissolution of AMoO₃ layers section (3.2).

**Self-supporting membrane release method:** A prepared rigid 1:10 polydimethylsiloxane (PDMS) polymer support was applied to the top surface of the STO film. The sample was introduced into DI water at a temperature of 50°C. After etching the AMoO₃ sacrificial layer, the 80nm thick SrTiO₃ membrane was released onto the PDMS support.

**Surface and structural characterizations:** Surface morphology characterization was carried out using a Dimension ICON (Bruker) atomic force microscopy (AFM) with PeakForce tapping mode using a Bruker ScanAsyst Air tip to control the surface morphology of the grown STO top layer and to control the surface of the obtained STO self-supporting membranes released on PDMS. The structural properties of the as-grown STO top layer and of the self-supporting membrane released on PDMS were characterized by in-plane and out-of-plane X-ray diffraction (XRD) analyses performed with a Rigaku SmartLab 5-circles diffractometer equipped with a Hypix detector. Out-of-plane scans were recorded with a Cu Kα1 radiation source (λ = 0.15406 nm) (2×Ge(220) monochromator). Rocking-curves, phi-scans and in-plane scans were recorded without the monochromator.

# 3. RESULTS AND DISCUSSION

## 3.1 Growth and stability of new AMoO₃ sacrificial layers

**Figure 1** shows XRD patterns in Bragg-Brentano configuration of the as-grown 50 nm-thick CMO, SMO and BMO layers grown by PLD on (001)STO substrates (light black, red and blue respectively). The AMoO₃ layers show a (00l) orientation with no secondary phases present. In its bulk form, CMO is orthorhombic with $a_0 = 5.4499$ Å, $b_0 = 5.5811$ Å and $c_0 = 7.779$ Å (space group (SG): *Pbnm*), corresponding to a pseudo-cubic lattice with the following lattice parameters: $a_{pc} = 3.85$ Å, $b_{pc} = 3.94$ Å, and $c_{pc} = 3.88$ Å.²⁸ SMO and BMO compounds are both cubic (SG: *Pm-3m*) with $a = 3.976$ Å³¹ and $a = 4.04$ Å³², respectively. The out-of-plane lattice parameters of the CMO, SMO and BMO films were measured to be around $a_{pc} = 3.94$ Å, 4.014 Å and 4.088 Å, respectively, approaching the parameters of the bulk phases. More detailed results and scans (002 rocking-curves, asymmetric 103 reciprocal space maps and 110 phi-scans establishing the epitaxial relationships between AMoO₃ films and the substrate) are shown in **Table S1**, **Fig. S1** and **Fig. S2**. The Full Width at Half Maximum (FWHM) of the rocking-curves increases as the value of the AMoO₃ phase lattice parameter becomes more distant from that of the substrate (**Table S1**). The same samples were re-examined by XRD after 20 months aging in ambient air conditions. The 00l diffraction peaks of the AMoO₃ films show a slight shift of the 2θ position after aging with a maximum displacement of Δ(2θ) = 0.09° for the *BMO* film. It is worth to note that the *CMO* Bragg reflections are more intense after aging, but after calculating the integrated intensities ratio I(CMO)/I(STO) for the 002 reflection, we found 0.045 before aging and 0.018 after aging showing that the difference in intensities comes principally from the apparatus. In conclusion, the three AMoO₃ films retain their single-crystal feature without the appearance of secondary phases. This demonstrates the stability of this family of sacrificial layer to the ambient environment compared with the well-known SAO sacrificial layer which shows a loss of crystallinity when exposed to air within a few days even with a thin capping layer.²³
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Out-of-plane θ-2θ XRD patterns of the 50nm-thick CMO, SMO and BMO grown on (001)STO in the as-grown state (light black, red and blue) and after 20 months air aging (Black, red and blue) respectively. The curves are vertically shifted for sake of clarity. Asterisks (*) represent the 00l peak positions of the STO substrate.</figcaption></figure>

## 3.2 DI water dissolution of AMoO₃ layers

The 50 nm thick AMoO₃ sacrificial layers deposited on (001)STO (with no film of interest deposited on the sacrificial layer) was immersed upside down in 300 mL DI water, with the sacrificial layer side in contact with the beaker's bottom without stirring of the solution. This configuration is closer to the situation where a top film of interest is deposited on the AMoO₃/(001)STO, for which the dissolution occurs mostly on the perimeter of the sacrificial layer where water can only enter from the sides. **Figure 2.a** presents the scheme of the optical measurement to measure the remaining sacrificial surface during the dissolution. **Figure 2.b** shows the evolution of the normalized area of the remaining surface, Sremaining, of the three studied 50 nm thick AMoO₃ sacrificial layers as a function

of time. Sremaining were evaluated using ImageJ software,³¹ for each image collected with a time interval of 1 hour. It is interesting to note that each of the three molybdate's sacrificial layer shows two dissolution regimes. The etching rate is high at the start of dissolution for the 20% of the etched surface and then enters a slower regime after a few hours in solution for most of the remaining surface. **Figure 2.c** shows the optical images of the water-etching of the 50 nm CMO sacrificial layer. Total integrated etching times τe of the CMO, SMO and BMO were defined when the remaining surface is less than 1% and gave the respective values of 34 h, 37 h and 50 h, showing that the BMO SL is slowest to dissolve completely compared with the two others molybdate compounds.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** (a) Scheme of the optical measurement setup during the dissolution. (b) Time evolution of the normalized surface of CMO, SMO and BMO sacrificial layer remaining attached to the STO substrate without a top layer in DI water with a Y semi-logarithmic scale representation between 80% and 4% of the remained surface. Solid lines represent the fitted curves of dissolution kinetics (see text). (c) Optical pictures of the evolution of the etched surface of CMO layer, here in gray, without a top layer on a transparent 5×5 mm² STO substrate, for some selected dissolution times.</figcaption></figure>

To go further, an analysis of dissolution kinetics was carried out on the basis of these results. Assuming a simple chemical monomolecular dissolution reaction involving the total molecular AMoO₃ cell in abundant H₂O solvent, a first order kinetics with the following differential equation for remaining surface S<sub>remaining</sub>(t) is expected:

$$ \frac{dS_{remaining}(t)}{dt} = - \frac{S_{remaining}(t)}{\tau} \quad (1) $$

with τ the decay characteristic time, and consequently, exponential decay solutions in the form αᵢ exp(-t/τᵢ). For experimental data, for each AMoO₃ sacrificial layer, the best fit is with a linear combination of an early fast exponential decay for t < 2 hours, and a main exponential decay after, combined in the following equation:

$$ \frac{S_{remaining}(t)}{S_0} = \alpha \exp(-\frac{t}{\tau_{fast}}) + (1 - \alpha) \exp(-\frac{t}{\tau_{main}}) \quad (2) $$

with S₀ the total initial surface of the sacrificial layer, α the relative weight of the fast-initial etching τ<sub>fast</sub> time constant' decay, and (1-α) the relative one of the main τ<sub>main</sub> time constant.

The results show a rapid but minor decay for t < 2 hours and a slower and main decay between 3 hours and the end-of-dissolution phase, where surface measurement becomes difficult to measure accurately (when S<sub>remaining</sub>(t)/S₀ values are below 2%).

The fitting parameters of equation 3 for the three samples are summarized in **table 1**.
<table><caption>Table 1. Fitting parameters of equation (3): α, τ<sub>fast</sub> and τ<sub>main</sub>; with the total integrated etching time τ<sub>e</sub>.</caption><thead><tr><th>Sacrificial Layer</th><th>Relative weight α of the initial decay (%)</th><th>Fast initial decay time τ<sub>fast</sub> (hours)</th><th>Main decay time τ<sub>main</sub> (hours)</th><th>Total etching time τ<sub>e</sub> (hours)</th></tr></thead><tbody><tr><td>BMO</td><td>24</td><td>0.9 ± 0.1</td><td>18.7 ± 0.6</td><td>50 ± 1</td></tr><tr><td>SMO</td><td>32</td><td>1.7 ± 0.2</td><td>14.0 ± 1.1</td><td>37 ± 1</td></tr><tr><td>CMO</td><td>20</td><td>0.9 ± 0.1</td><td>11.1 ± 0.2</td><td>34 ± 1</td></tr></tbody></table>

The initial fast decay with the time constant τ<sub>fast</sub> is significant for t < 1-2 hours and represents only the 20-32% of the surface decay. More important, is the main etching between 2 hours and the total etching time τ<sub>e</sub>, with a weight of 68-80% of the surface decay. The corresponding characteristic time τ<sub>main</sub>, around 10 time longer than τ<sub>fast</sub>, give different values for each AMoO<sub>3</sub> materials: τ<sub>main</sub>(CMO) = 11.1 h < τ<sub>main</sub>(SMO) = 14.6 h < τ<sub>main</sub>(BMO) = 18.7 h. The time dependence of τ<sub>main</sub> are more distinct in the zoom inset of figure 2.b with a Y semi-logarithmic scale representation. And coherently, the same order between the different molybdate materials is found for the total integrated etching time τ<sub>e</sub> (table 1). This ordering is different from the (Sr,Ca,Ba)<sub>3</sub>Al<sub>2</sub>O<sub>6</sub> sacrificial layer's family where the etching rates is related to the ionic-like bonding of the Sr, Ca and Ba with oxygen <sup>18-20,32</sup>. Same as for SAO family, V. Polewczyk et al. reported that the (Ca,Sr)VO<sub>3</sub> sacrificial layer show the same etching rates tendency.<sup>33</sup> For AMoO<sub>3</sub> etching rates, the difference may be in part related to the molybdenum significant higher electronegativity

(2.16) in comparison to Al (1.61) and V(1.6) in Pauling scale.<sup>34</sup> This points out the specificity and complexity of each sacrificial family in microscopic dissolution mechanisms, where each A-O and B-O bounds in the perovskite ABO<sub>3</sub> structure are both involved, but with reactivities depending of the exact structure of each compound.

This study demonstrates the dissolution of the molybdate AMoO<sub>3</sub> perovskite oxide family in deionized water, with main etching kinetics of first order. In general, the decay time is of the order of 10 to 20 hours, slower for BaMoO<sub>3</sub>, than for SrMoO<sub>3</sub> and CaMoO<sub>3</sub>.

### 3.3 Study of the epitaxial STO membranes of interest grown on AMoO<sub>3</sub> sacrificial layers before release

We first analyze the structural properties and AFM surface morphology before and after deionized water etching and release.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a) Out-of-plane θ-2θ XRD patterns for as-grown STO<sub>film</sub>(80nm)/ AMoO<sub>3</sub> (4 nm)//(001)STO heterostructures with (b) a figure enlargement on the 002 reflection. Asterisks (*) corresponds to the single-crystal STO substrate reflections. (c, d, e) 4µm × 4µm AFM images of the upper surface of the as-grown STO on</figcaption></figure>
BMO//((001)STO, SMO//((001)STO and CMO//((001)STO heterostructures, respectively. White scale bars in bottom right of AFM images correspond to 800 nm.

Out-of-plane XRD patterns of the as-grown 80 nm thick STO<sub>film</sub> on SL//((001) STO heterostructures (Figure 3.a) show that all STO films are preferentially (001) oriented without the presence of secondary phases. The 002 rocking-curves and corresponding FWHM Δω are shown in Figure S4 and reveal that the film grown on BMO exhibits a higher mosaicity than the film grown on SMO, which in turn has a higher mosaicity than the film grown on CMO. These results are in line with the Δω values measured for the respective sacrificial layers alone (Table 1, Figure S1). Epitaxial cube-on-cube (for SMO and BMO) and pseudo-cube-in cube (for CMO) growth, reference of the initial (001)STO substrate, was confirmed by the presence of four peaks in the φ-scans (Figure S3). In Figure 3.a, the diffraction peaks of the STO<sub>film</sub> are shifted to a lower angular value, close to those of the substrate labeled with an asterisk indicating that the out-of-plane lattice parameters *c*<sub>film</sub> are higher for the STO films compared to the STO substrate (a = 3.905 Å) [28]. The values of the measured out-of-plane lattice *c* of STO<sub>film</sub>/CMO//((001)STO, STO<sub>film</sub>/SMO//((001)STO and STO<sub>film</sub>/BMO//((001)STO are quite similar with *c* = 3.956 Å, 3.945 Å and 3.950 Å, respectively (Table 2). This shift can be attributed growth at low oxygen pressure, which leads to an increase in the out-of-plane lattice parameter.<sup>35</sup> To confirm this oxygen-deficiency hypothesis, we deposited a 80 nm-thick reference STO film under the same growth conditions directly onto sacrificial layer-free strontium titanate substrate and measured by XRD the lattice parameters *c* = 3.955 Å (Figure S5), showing that our growth conditions lead to deposition of slightly under-oxidized STO films. Furthermore, a deposition of STO at lower oxygen pressure (see Figure S5) increases even more the out of plane parameter to 3.970 Å, showing that this parameter is very effective in order to tune the lattice parameter of STO. The in-plane lattice parameter of those films was characterized and shown in Figure S5-b. The in-plane lattice parameter *a* is equal to 3.902 Å, very closed to the bulk lattice parameter, showing that the films are fully strained-in-plane by the homo-compound substrate even for 80 nm STO thick films. The films are therefore showing a tetragonal distortion of *c*/*a* = 1.012.

The same measurements were done on films with sacrificial layer. The in-plane XRD of the STO membrane on CMO and SMO sacrificial layer, before and after release on PDMS are shown in the (Figure S6). The values are reported in the **table 2** for all the different heterostructures. Before the release, the in-plane lattice parameter is around 3.905 Å, showing that they are fully strained by the substrate, through the ultra-thin intermediate sacrificial layer of 4 nm for all the different AMoO₃ as we have seen previously for the STO(film)//STO(substrate). After release, the in-plane 200 Bragg reflection shows a shifting of the diffraction peaks to the lower 2θ values (see **figure S6-a** and **table 2**). The out-of-plane X-ray diffraction patterns around 002 Bragg reflection peaks of the self-supporting membranes are shown in **Figure 4-a**. The grey dashed line represents the bulk position of the 002-diffraction peak of the bulk strontium titanate. The black bars indicate the position of the 002 Bragg reflection of the as-grown STO top layer on the AMoO₃/STO<sub>substrate</sub> before dissolution of the sacrificial layer (from **Figure 3.a**). After releasing the STO membrane, the out-of-plane XRD pattern (Figure 4.a) shows that the 002 peak position shifted to higher values of 2θ compared to the as-grown 002 peak position. This shift indicates an epitaxial relaxation of the STO when released on PDMS after water etching the sacrificial layer.

Interestingly, two effects appear after the release. The decrease of the out-of-plane lattice parameter and the increase of the lattice parameter in-plane in order to obtain a *c*/*a* around 1 (see **table 2**). Therefore, the released STO membranes relax from a tetragonal to a quasi-cubic symmetry in the released state. The three STO freestanding films show approximatively the same value of the out-of-plane lattice parameter, independently of the intermediate grown 4 nm thick AMoO₃ layer, in coherence with the fact that the sacrificial buffer layer is too thin to induce an epitaxial strain to the membrane of interest of STO.

<caption>**Table 2.** In-plane *a*, out-of-plane *c* and tetragonal distortion *c/a* of “as-grown” and “after release” 80 nm STO membrane of interest for each sacrificial layer.</caption>
<table><thead><tr><th>SL</th><th>As grown<br>a (Å)</th><th>As grown<br>c (Å)</th><th>As grown<br>c/a</th><th>As grown RMS<br>roughness<br>(nm)</th><th>Self-<br>supporting<br>g a (Å)</th><th>Self-<br>supporting<br>g c (Å)</th><th>Self-<br>supporting<br>c/a</th><th>Self-supporting<br>RMS roughness<br>(nm)</th></tr></thead><tbody><tr><td>CMO</td><td>3.906</td><td>3.956</td><td>1.0128</td><td>0.38</td><td>3.927</td><td>3.937</td><td>1.0025</td><td>0.56</td></tr><tr><td>SMO</td><td>3.904</td><td>3.945</td><td>1.0105</td><td>0.44</td><td>3.932</td><td>3.936</td><td>1.0010</td><td>0.61</td></tr><tr><td>BMO</td><td>3.906</td><td>3.950</td><td>1.0112</td><td>0.65</td><td>3.931</td><td>3.937</td><td>1.0015</td><td>1.01</td></tr></tbody></table>

At the nanometric scale, the morphology of the films before and after release are shown in **figure 3 (c, d, e)** and **figure 4 (b, c, d)** respectively. Before release, the films possess a smooth step-terrace morphology transferred from the treated growth STO substrates (see **experimental section**) with RMS roughness values of 0.38 nm, 0.44 nm and 0.65 nm for STO<sub>film</sub>/CMO, STO<sub>film</sub>/SMO and STO<sub>film</sub>/BMO, respectively. At this

point, we can assume that the sacrificial layer morphology is comparable to the STO morphology. The step-terraces are more visible in the STO/CMO//001STO sample which supposes a sharper interface in this heterostructure compared to the two others.

<figure><img src="image_5.png" /><figcaption>**Figure 4. (a)** Out-of-plane XRD around the 002 Bragg reflection of the released STO membranes on PDMS using AMoO<sub>3</sub> sacrificial layers. The vertical black bars indicate the 002 Bragg reflection position of the as-grown deposited STO film on AMoO<sub>3</sub>/STO. 4µm × 4µm AFM images of the down released surface of the STO membrane stuck to PDMS support after water etching respectively the BMO (b), SMO (c) and CMO layers (d) (the white scale bars on the AFM images correspond to 800nm).</figcaption></figure>
After their release on PDMS, the three released membranes of STO show a step-terrace morphology (as previously), showing a full sacrificial layer etching in DI water and a clean release of the STO membrane. The RMS roughness values are still very small with the values of 1.01 nm, 0.61 nm and 0.56 nm, (for BMO, SMO and CMO sacrificial layer respectively), slightly higher than before the release (Table 2).

On a macroscopic scale, important for applications transfer, such as electrical transport properties, we observe by optical microscopy that the STO membrane release induces mainly the appearance of cracks in the membrane after etching ¹⁸. For 5×5 mm² samples globally released, the PDMS supported membranes show macroscopic cracks with estimated surface densities of around 20 % (figure S7). A further analysis of the crack's statistics and their tailoring is under investigation, but out of the scope of this paper. Nevertheless, the typical dimension of the higher microscopic crystals is of around 50 µm × 50 µm, compatible with probe size analysis.

Going back to release times of the self-supporting PDMS/STO-80nm membranes epitaxially grown on $AMoO_3$-4 nm// (001)STO, they were longer as single $AMoO_3$-50nm deposited, as the geometry of water-etching process is more confined. Scale-laws under systematic investigation is out of order of this paper. For typical 4 nm-thick SL of any $AMoO_3$, we found almost around 6 days, twice longer than a $SrVO_3$ sacrificial layer with 3 days, and much longer than SAO sacrificial layer (around 1 h). ²³

## 4. CONCLUSIONS

In this work, we evidence the $AMoO_3$ ($A$ = Ca, Sr, Ba) family as DI water soluble efficient sacrificial layers. They exhibit two linked characteristics: first a larger stability in ambient conditions (room temperature and oxygen partial pressure) than the aluminates compounds ($Ca,Sr,Ba)_3Al_2O_6$, and comparable with $SrVO_3$, which will potentially extend the panel of growth techniques from in-situ under vacuum depositions; and then a slower water etching kinetics. This last point extends their use for ambient conditions transfer from these sacrificial buffer's realization to the membrane of interest deposition technique. Molybdate perovskite $AMoO_3$ ($A$= Ca, Sr, Ba) family, playing with "A" cation solid solution, opens a pseudo-cubic lattice parameter large range from 3.90 Å to 4.04 Å, with the respective bulk values of 3.90 Å, 3,972 Å, 4.04 Å for $A$ = Ca, Sr, Ba. Moreover, the etching process in DI water these new sacrificial layers similarly to $Sr_3Al_2O_6$ and $SrVO_3$ sacrificial layers prevents the membrane of interest materials from corrosion in more acidic or basic etching solutions needed to dissolve manganate $La_xSr_{1-x}MnO_3$, ³⁶,³⁷ and cuprate superconductor

$YBa_2Cu_3O_7$ ³⁸ In a more applicative perspective, we show how to analyze the etching kinetics via an operando optical monitoring of the remaining sacrificial layer surface still attached to the initial template substrate. We found for molybdate family a first order reaction kinetics with a main exponential decay associated to a dissolution of the total $AMoO_3$ molecular chemical. Important for real industrial transfer for flexible or silicon compatible of oxide perovskite electronic devices, this study can be completed by examining the effect of lattice mismatch between the sacrificial layer and the chosen membrane of interest, especially the crack densities.

## ASSOCIATED CONTENT

### SUPPORTING INFORMATION

Supporting Information is available from the American Chemical Society Online Library or from the author.

Crystallographic data for $AMoO_3$ films (Table S1); 002 rocking-curves and 103 reciprocal space maps of BMO film (Figure S1.a,d), SMO film (Figure S1.b,e), CMO film (Figure S1.c,f); Phi-scans of BMO film and STO substrate (Figure S2.a,b), SMO film and STO substrate (Figure S2.c,d) and CMO film and STO substrate (Figure S2.e,f); (Figure S3.a), STO/SMO (Figure S3.b), STO/BMO (Figure S3.c) on STO(001) substrates; XRD patterns out-of-plane 002 rocking-curve of STO films grown on BMO/STO (Figure S4.a), on SMO/STO (Figure S4.b), and on CMO/STO (Figure S4.c); Phi-scans of the (200) planes of the grown heterostructures STO/CMO (Figure S5.a) and in-plane (Figure S5.b) of the grown STO films on STO(001) substrates at different pressures with the out-of-plane and in-plane associated lattice parameters (Figure S5.c); in-plane XRD patterns around 200 reflection of the 80nm-thick STO film before and after release on PDMS with CMO-SL (Figure S6.a), SMO-SL (Figure S6.b) and BMO-SL (Figure S6.c); Optical micrographs of the released STO membranes on PDMS using CMO-SL (Figure S7.a), SMO-SL (Figure S7.b) and BMO-SL (Figure S7.c).

## AUTHOR INFORMATION

### Corresponding authors

Yves Dumont - Groupe d'Etude de la Matière Condensée (GEMaC), Université Paris-Saclay, UVSQ, CNRS, 45 Avenue des États-Unis ; 78035 Versailles ; France ; https://orcid.org/0000-0002-0739-428X ; yves.dumont@uvsq.fr

Moussa Mebarki - Groupe d'Etude de la Matière Condensée (GEMaC), Université Paris-Saclay, UVSQ, CNRS, 45 Avenue des États-Unis ; 78035 Versailles ; France ; moussa.mebarki@uvsq.fr

Author Contributions:
The manuscript was written through the
contributions of all authors. All authors have given
approval to the final version of the manuscript

**Notes:** The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors acknowledge the French “Agence Nationale de la Recherche – ANR” in the framework of the FLEXO project (ANR-21-CE09-0046) and DisTCOvery project (ANR-23-CE08-0008) for financial support. This work has benefited from the support of the ScanMAT Osirix platform (UAR 2025, University of Rennes -CNRS) for XRD.

ABBREVIATIONS

SL, sacrificial layer; CMO, CaMoO₃; SMO, SrMoO₃;
BMO, BaMoO₃; STO, SrTiO₃; PDMS,
Poly(dimethylsiloxane); XRD, X-ray diffraction; AFM,
atomic force microscopy; PLD, pulsed laser
deposition; RMS, root mean square.

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Graphical ToC:

<figure><img src="image_6.png" /></figure>