# Wafer-scale Ge freestanding membranes for lightweight and flexible optoelectronics

Tadeáš Hanuš, Bouraoui Ilahi, Alexandre Chapotot, Hubert Pelletier, Jinyoun

Cho, Kristof Dessein, Abderraouf Boucherif

► To cite this version:

Tadeáš Hanuš, Bouraoui Ilahi, Alexandre Chapotot, Hubert Pelletier, Jinyoun Cho, et al. Wafer-scale Ge freestanding membranes for lightweight and flexible optoelectronics. Materials Today Advances, 2023, 18, pp.100373. 10.1016/j.mtadv.2023.100373. hal-04660423

HAL Id: hal-04660423

https://hal.science/hal-04660423v1

Submitted on 2 May 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Wafer-scale Ge freestanding membranes for lightweight and flexible optoelectronics

<figure><img src="image_2.png" /></figure>

Tadeáš Hanuš<sup>a, b, *</sup>, Bouraoui Ilahi<sup>a, b</sup>, Alexandre Chapotot<sup>a, b</sup>, Hubert Pelletier<sup>a, b</sup>, Jinyoun Cho<sup>c</sup>, Kristof Dessein<sup>c</sup>, Abderraouf Boucherif<sup>a, b</sup>

## ARTICLE INFO

**Article history:**

Received 2 March 2023

Received in revised form

18 April 2023

Accepted 22 April 2023

Available online 28 April 2023

**Keywords:**

Germanium

Porous substrate

Freestanding membranes

Epitaxial growth

Layer transfer

Substrate re-use

## ABSTRACT

Semiconductor-based freestanding membranes (FSM) have recently emerged as a highly promising area of advanced materials research. Their unique properties, such as lightweight and flexibility, make them attractive for a wide range of disruptive device applications. However, the production of high-quality, single-crystalline FSM, especially from elemental materials such as germanium (Ge), remains a significant challenge. In this work, we report on the formation of easily detachable wafer-scale Ge FSM on porous Ge (PGe) substrate. The proposed method relies on low-temperature Ge epitaxy, allowing to preserve the porous structure's integrity during the FSM formation, and an easy substrate preparation for multiple reuses. Analysis of the surface morphology as a function of the deposited Ge thickness reveals that the FSM formation occurs in two distinct regimes. During the initial epitaxial regime, the Ge growth is governed by 3D nucleation on the PGe top surface. The nanoscale islands size increase, and consequent coalescence are found to increase the surface roughness up to a critical thickness, allowing full coalescence of islands into a 2D epilayer. The analysis of the membrane's surface morphology for various thicknesses shows continuous improvement, achieving sub nanometer surface roughness. Moreover, we demonstrate that the FSM formation process is applicable regardless the PGe porosity and thickness, while offering facile and sustainable substrate reconditioning for multiple FSM generation from the same substrate. Our findings open new opportunities to produce lightweight and flexible, high-performance optoelectronics based on Ge FSM, while ensuring reduction of both cost and critical materials consumption.

© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license

(http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

In recent years, high-quality freestanding membranes (FSM) made of functional materials have become central to the rapidly expanding frontiers of nanoscience and technology [1–6]. Group IV, III-V and III-N semiconducting membranes in particular have high potential for applications such as stretchable on-skin electronic [7,8], vertically stacked LEDs [9], and flexible photodetectors [10]. Indeed, FSM offer an extra degree of freedom for

implementations that cannot be obtained by conventional techniques such as heterointegration of dissimilar materials with high lattice mismatch in crystalline structures [11]. In addition to being lightweight and flexible, FSM allow for various materials to be stacked on top of each other, enabling easy coupling of physical properties between dissimilar materials [12,13]. Furthermore, the use of FSM provides significant cost savings for the device production, especially for materials with orders of magnitude higher prices than that of silicon, when compared to bulky wafers. In this context, germanium (Ge) FSM particularly attract a lot of attention for their applications in high-performance optoelectronics and high-speed telecommunication devices such as wave guides [14,15], THz transmission [16], photodetectors [17–19] and lasers [20,21] as well as for their biocompatibility [22,23]. However, the fabrication of high-quality Ge FSM is still a challenging task.

<footnote><sup>a</sup> Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, 3000 Boulevard de l'Université, Sherbrooke, J1K 0A5, QC, Canada</footnote>

<footnote><sup>b</sup> Laboratoire Nanotechnologies Nanosystèmes (LN2), CNRS IRL-3463 Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, 3000 Boulevard de l'Université, Sherbrooke, J1K 0A5, QC, Canada</footnote>

<footnote><sup>c</sup> Umicore Electro-Optic Materials, Watertorenstraat 33, 2250, Olen, Belgium</footnote>

<footnote>* Corresponding author. Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, 3000 Boulevard de l'Université, Sherbrooke, J1K 0A5, QC, Canada.</footnote>

<footnote>E-mail addresses: tadeas.hanus@usherbrooke.ca (T. Hanuš), abderraouf.boucherif@usherbrooke.ca (A. Boucherif).</footnote>
For instance, remote epitaxy has shown tremendous potential for fabrication of III-N and III-V semiconductor compounds FSM [24–26], and for other materials such as complex oxides [27], perovskites [28], metals [29]. Nevertheless, this technique is based on ionic interaction between the epilayer and the underling substrate trough the graphene interface, prohibiting its application to nonpolar materials such as Ge. To this date, a variety of lift-off techniques allowing the production of single-crystalline Ge FSM and the reuse of wafers have already been reported, namely epitaxial lift-off (ELO) [30,31], mechanical spalling [32], smart cut method [33], growth on nanopatterned graphene [34], Germanium-on-nothing (GON) [35,36] or porous lift-off [37–39]. Despite achieving significant advancements, the widespread adoption of Ge FSM is still hindered by various obstacles including process complexity, high cost and substrates damage and/or contamination issues. Epitaxial growth on porous Ge (PGe) substrate has specially demonstrated high potential to produce lightweight solar cells [36,38]. The device detachment is achieved through the mechanically weak interface, formed by nano-to microscale-sized pillars [36,38,39]. After FSM detachment, the substrate surface contains broken pillars with various sizes whose reconditioning for multiple reuses requires either conventional chemical mechanical polishing (CMP) treatment [40] or wet chemical etching over several microns [41]. Despite the significant improvement compared to the use of conventional wafers, these techniques still leave room for further reduction of both cost and Ge consumption during the reconditioning process.

Indeed, PGe substrates offer a wide range of morphologies and physical properties [42–45], that can be directly used in the Ge FSM fabrication by epitaxial growth on an unreconstructed porous structure. Compared to the homoepitaxial growth on conventional substrate, where optimum 2D layer-by-layer growth can be easily reached, several challenges occur for nanostructured substrates. In fact, nanopores mediated epitaxial material diffusion [46] and porous reconstruction induced 3D micro-structuring [47] are among the encountered difficulties. These obstacles need to be overcome to ensure the epitaxial growth of FSM on PGe substrate. So far, successful epitaxy on PGe substrates is mainly based on high temperature annealing steps either before [35–38] or during the material deposition growth [39], triggering the thermal reconstruction of the porous layer. For instance, PGe with sponge like morphology show a strong temperature dependance [48], inducing the formation of large pillars [39] structures, while losing its intrinsic PGe properties and complicating the reconditioning process [41].

In this work, we demonstrate easily detachable wafer-scale Ge FSM formation on PGe substrate by low temperature growth. The method, based on the preservation of the porous structure's integrity, is applicable regardless the PGe porosity and thickness while ensuring easy substrate preparation for multiple reuses.

## 2. Results and discussions

The PGe substrates, used for epitaxial growth, are prepared by bipolar electrochemical etching (BEE) on P-type Ge substrate, in HF-based electrolyte. The process, which was reported in our previous work [49], enables fine tuning of the PGe thickness and porosity, providing on-demand properties while ensuring a low surface roughness and a substrate oriented crystalline nature, making them a viable option for epitaxial growth. The produced PGe thickness and porosity demonstrate an overall variation standing below 2% across the wafer. Fig. 1a depicts an optical image of typical 100 mm Ge substrate, with homogeneous porous layer on top, produced by BEE. To shed light on the initial stages of the Ge epitaxy on PGe nanostructures, we first consider 230 nm thick

uniform PGe layers (Fig. 1c) over 100 mm Ge wafer with intermediate porosity around 54%, calculated from critical angle, measured by X-ray reflectivity (XRR) [49] (Fig. 1b), and having a surface RMS (Root mean square) roughness below 2 nm (Fig. S1). All the investigated samples have been grown at 300 °C in Chemical beam epitaxy reactor (CBE) equipped with solid source Ge. The growth rate has been maintained constant throughout this study at 0.5 µm/h. Additionally, since our objective is to perform epitaxial growth of Ge on PGe structure, the growth temperature needs to be sufficiently low to avoid PGe reorganization as discussed further below.

To evaluate the stability of the porous structure at the growth temperature, the PGe substrate has been first in-situ annealed at 300 °C for 30 min and characterized by XRR, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Fig. 1b shows XRR measurement of the porous Ge layer before and after annealing, with a negligible shift of the PGe layer critical angle ($\theta_{PGe}$), from 0.414° to 0.410° (less than 1% increase of porosity), signifying that the porosity remains unchanged. Furthermore, Fig. 1c–f depict cross-sectional and top-view SEM images, showing identical in-plane and in-depth porous morphologies before and after the annealing step. The surface RMS roughness does not undergo any major changes either, with only slight increase from 1.1 nm to 1.6 nm (Fig. S1). Accordingly, Ge epitaxial growth can be performed on PGe substrate while preserving the porous structure integrity. For growth temperatures of 350 °C and above, morphological transformation of the PGe has been found to occur, making it unsuitable for the present work.

To study the initial growth stages and understand the Ge nucleation on PGe substrates, several samples have been prepared with deposited Ge nominal thicknesses ranging from 5 nm to 1 µm. The morphological evolution of the epilayers was systematically evaluated as a function of the nominal thickness of Ge on the PGe structure and characterized via SEM and AFM. Typical cross-sectional and top-view SEM images taken from samples with 5, 30, 60 and 100 nm deposited Ge thickness are shown by Fig. 2a-d and Fig. 2e-h respectively (Additional data are provided in Fig. S2). The results reveal that the nucleation occurs on the top surface of the pore walls, forming nanoscale three-dimensional (3D) islands (Fig. 2: a and e). The combined low substrate temperature and small pores' size (below 10 nm) is likely to limit the adatoms diffusion into the porous structure [50] in favor of top surface nucleation. As more material is added, the 3D island's size increase, eventually coalescing to form a 2D Ge membrane (Fig. 2: b and f). Although, the densely packed nucleation enables continuous Ge membrane formation, the corresponding surface morphology remains rough as can be seen in Fig. 2 (c and g). The observed behavior is also confirmed by AFM morphological investigations (Fig. 3 and S3). Indeed, the analysis of the RMS roughness indicates that the roughness rises first, with increasing Ge thickness up to 60 nm and then drops rapidly towards surfaces with sub-nanometer roughness for membranes thicknesses beyond 750 nm.

Additionally, during the Ge nucleation on PGe structure, the first islanding regime starting with 3D nucleation, considerably impacts the surface morphology through islands size increase and consequent seed islands coalescence, leading to the observed RMS roughness increase. As shown by Fig. S3, the coalescence of the nanosized islands occurs for deposited Ge thicknesses higher than 10 nm giving rise to the appearance of pits. The islanding phase has been reported in case of low temperature growth of Ge on porous Si substrate [51]. However, in the latter case, the lattice mismatched strain and the differences in the thermal expansion coefficients have been shown to induce grain boundary formations that prohibits the full islands coalescence into homogeneously dense epitaxial layer. Moreover, similar behavior has already been reported at micrometer scale, where persistent separated microcrystals occur
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Evaluation of the PGe layer quality before and after the annealing at 300 °C during 30 min. (a) An optical image of a typical uniform PGe layer on 100 mm wafer. (b) XRR measures of PGe layer before and after annealing at 300 °C, the critical angles of both PGe layer(θ<sub>PGe</sub>) and Ge substrate(θ<sub>Ge</sub>) are indicated by dotted lines. (c)/(d) and (e)/(f) depict top view and cross-sectional SEM micrographs of PGe layer before (PGe) and after annealing at 300 °C, (APGe) respectively.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Initial growth stages of Ge on the PGe substrate a-d) cross-sectional and e-h) top view SEM micrograph of Ge layer at 5/30/60/100 nm of grown nominal thickness.</figcaption></figure>

for Ge growth on Si micropillars [50], while good quality suspended Ge layer can be achieved by high temperature epitaxy/annealing on Ge micropillars [35]. Furthermore, the formation of good quality

dense epilayer has also been recently reported by GaN nucleation on porous GaN buffer on sapphire substrate [52], suggesting that the homoepitaxial growth is a key for obtention of fully coalesced
layers on porous substrates at low temperature.

While the islands coalescence starts in an early nucleation stage, promoting initial pit formation, some deep pits crossing the membrane are still present up to a deposited Ge thickness of 50 nm (Fig. S2e). Also considering the measured RMS roughness peak around 60 nm of deposited Ge (Fig. 3g), this specific thickness appears as a critical one for the membrane formation process by homoepitaxial nucleation on PGe structure. Accordingly, we define the critical thickness (Tc), as the minimum thickness required to ensure the full islands' coalescence into continuously dense membrane, without deep pits crossing the entire membrane. Once Tc is reached, the fully densified membrane can be further thickened, while improving the surface morphology as shown in Fig. 3(g–i). The pits start merging (Fig. 3a–f) and their depth gets continuously reduced as more Ge material is deposited (Fig. 3h), while the surface pit density decreases until the obtention of perfectly flat surface seen in Fig. 3i. As the thicknesses rises beyond Tc, the growth becomes basically dominated by 2D layer by layer mode. Consequently, the RMS roughness decreases exponentially testifying an improved membrane's surface morphology. After the deposition of a 750 nm thick Ge layer, significant drop in the sub-micron scale pits' depth and increase of their diameter occur leading to their merging and flattening towards a smooth surface.

Consequently, the large pits turn into surface ripples-like morphology making them strongly anisotropic and hardly quantifiable (Fig. 3e and h). Despite the persistence of nanoscale sized pits residues (Fig. 3e), the surface roughness is already well below 1 nm, testifying an excellent morphology suitable for further epitaxial growth. Indeed, as shown by Fig. 3d, imperfection free completely smooth surface with RMS roughness of 0.3 nm is obtained for a 1 µm thick membrane. The critical thickness for fully coalesced layer may vary depending on the PGe layer's properties, as the high porosity structures obviously requires more material to ensure the transition from 3D nucleation to 2D growth mode. These results demonstrate that once the deposited thickness exceeds Tc, and the membrane is fully densified, its surface morphology improves with more deposited material. All corresponding growth stages on porous substrate are schematically illustrated in Fig. 4.

To assess the feasibility of the proposed FSM growth process for different PGe thickness and porosity, 1 µm thick Ge membrane has been grown under the same conditions on 1 µm thick PGe layer with approximately 70% porosity. This represents an extreme case of thick and high porosity PGe substrate. Interestingly, fully densified Ge FSM has been successively fabricated at 100 nm wafer-scale (Fig. 5a), while the high porosity layer remains unreconstructed (as shown later in the inset of Fig. 7a). The FSM show

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (a)–(f) 5 × 5 µm² AFM scans of the Ge membrane for various Ge epilayers thicknesses. (g) Evolution of the surface RMS roughness during all the growth stages, where the Red dashed line indicates the complete coalescence of the layer and the blue one corresponds to the initial surface roughness of the PGe layer. (h) Surface pits' depth and size evolution as a function of the membrane thickness. (i) Pits surface density as a function of the membrane thickness.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Schematic illustration of initial growth steps starting with PGe substrate, 3D island growth, coalescence of islands into Ge membrane, and thickening of the Ge membrane by 2D layer-by-layer growth.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** (a) Optical image of ~1 µm thick Ge membrane grown on top of 1 µm thick PGe layer with high porosity. (b) AFM scan of Ge membrane's surface grown on high porosity substrate.</figcaption></figure>

RMS roughness of 1.2 nm, as illustrated by the AFM scan in Fig. 5. Indeed, the surface still show ripple-like morphology. Referring to the AFM analysis of the FSM thickness evolution for 54% porosity, such a surface state is likely to characterize fully coalesced pits with depth variation below 5 nm. This surface condition makes the layers comparable to membranes grown on 54% PGe substrate with thickness between 500 and 750 nm. This phenomenon is actually predictable as the higher porosity generates sparser nucleation sites and eventually necessitate more material to form a fully coalesced layer and annihilate all the pits on the surface. The surface morphology can be further improved by thickening of the membrane following the expected flattening trend.

To evaluate the crystalline quality of the membranes grown either on 70% or 54% PGe substrates, a comprehensive X-ray diffraction (XRD) analysis was conducted. The out-of-plane 2θ scans in Fig. 6a reveal unique well-defined Ge (400) and (200) [53] peaks that correspond to the (001) substrate orientation, indicating the monocrystalline nature of the FSM grown on both 54% and 70% PGe substrates. This is possible due to the substrate-oriented crystallites of the PGe layer [49], which transfer their orientation to the membrane during epitaxial growth. Furthermore, the in-plane configuration was used to ensure a low penetration depth of the beam, probing only the Ge membrane, thus discriminating its signal from that of the substrate. Interestingly, both in-plane pole figures of Ge (220), shown by Fig. 6b, depict 4 well defined sharp peaks, with fourfold symmetry, corresponding to the cubic crystal structure of Ge, undoubtedly confirming the single-crystal quality of the Ge FSMs independently of the PGe's porosity used as the substrate.

The presence of unreconstructed PGe structure underneath the Ge SFM constitutes a well-adapted separation layer with nano-structured interface allowing for membrane lift-off. Indeed, the cross-sectional SEM image (Fig. 7a) shows the easy fracture of the 70% porous interface between the Ge membrane and the bulk substrate. For demonstration purpose, we show a successful full 100 mm wafer Ge FSM release by simple adhesive polymer tape and transfer to a transparent, flexible plastic holder (Fig. 7 b).

Furthermore, the unreconstructed PGe separation layer also offers a unique opportunity for substrate reuse, as the bulk substrate material remains largely intact, with only PGe's residuals on the top surface as shown by Fig. 7c. Compared to sub-micron [38,41] to micron scale [35,36] pillars formed by high-temperature annealing methods, in our case, the PGe crystallites are only few nm in size [54] and have high specific surface compared to the volume of the Ge material (in respect to Ge bulk material or large pillars). This allows to completely oxidize the majority of the PGe structure in H₂O₂ solution, and then dissolve it in HF with minimal bulk Ge material etching, compared to wet etching methods. This treatment results in a clean surface with RMS roughness ~0.72 nm (Fig. 7d–e), which is suitable for additional cycles of porosification/epitaxial growth/membrane release, as previously demonstrated [41]. This further highlights the advantages of using unreconstructed porous interface. The estimated Ge consumption is approximately 1 µm per cycle, since the bulk material stays intact during the substrate cleaning. This process has the potential to produce multiple Ge membranes from a single substrate with minimal material loss. Accordingly, a 175 µm thick Ge wafer could be reused around 30 times before reaching the
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** (a) 2θ out-of-plane XRD scan of the Ge membrane grown on 54% and 70% PGe substrates and of the Ge bulk substrate as a reference, with logarithmic scale on y-axis. (b) Ge(220) In-Plane pole figure of the Ge membranes prepared on PGe substrates with 54% and 70% porosity.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** (a) Cross-sectional SEM micrograph of the Ge FSM on the weak porous interface illustrating the fracture of the nanostructured interface and the detachment of the membrane. The inset show a zoom on the unreconstructed high porosity layer underneath the membrane. (b) Optical image of 100 mm Ge membrane transferred to flexible substrate using adhesive tape. (c) Optical microscope image of the PGe remnants on the substrate after the detachment (d) and (e) Optical microscope image and AFM scan, respectively, of the Ge substrate after the cleaning.</figcaption></figure>

thickness of 145 µm (Thinnest commercially available 100 mm Ge wafers) and being recycled. Giving the rarity and cost of Ge [55], this method has a potential to significantly reduce the cost of Ge-based devices, while offering all the advantages of FSM.

**3. Conclusion**

In summary, we demonstrate the growth of monocrystalline Ge membranes at 300 °C on PGe substrates, while leaving the porous structure of the substrate unchanged during the growth. The initial nucleation stages on porous structure have been experimentally

investigated showing two growth regimes. Initially the growth is dominated by 3D nucleation on top of the pores and their coalescence. Once the Ge membrane reaches the critical thickness of coalescence, a dense membrane is formed, and the growth becomes governed by 2D layer-by-layer growth regime. At this stage, the remaining pits at the surface are being annihilated during the thickening of the membrane and good surface quality, with an RMS roughness below 1 nm, can be reached. The XRD analysis demonstrate the monocrystalline quality of the grown Ge membranes for all samples independently of their porosity and thickness. Our results show that PGe layers can be used to fabricate detachable
wafer-scale Ge FSM. Moreover, the nanometric crystallite size and
high specific surface of the GeE remnants on the substrate surface,
allow an easy cleaning process by oxidation and reuse of the sub-
strate for production of multiple Ge FSM. Furthermore, our finding
paves also the way to the fabrication of wafer scale FSM from low
temperature grown small bandgap materials for mid-IR optoelec-
tronics such as Ge(Si)Sn.

4. Methods

4.1. Sample preparation

PGe layers were prepared by optimized BEE process [49] of Ga-
doped, 100 mm (100) Ge wafers with 6° off-axis miscut towards
(111) orientation and 8–30 mΩ cm in resistivity, provided by
Umicore. The BEE was performed in a custom made 100 mm
porosification cell [49], using SP-50 BioLogic generator. Prior to this
process, Ge wafers were treated with HF (49%) solution for 5 min to
dissolve any native oxides present on the surface, rinsed with
EtOH(99%) and dried under N₂ flow. Sample were then introduced
into the porosification cell with HF(49%):EtOH(99%) (4:1, V:V)
electrolyte and etching and passivation pulses with 1 s duration
and 1 s rest time at the end of each cycle were applied. The medium
porosity (~54%) layers were formed using 1 mA cm⁻² symmetric
etching/passivation current density. To produce high porosity
layers (~70%), the etching current density was increased to
2 mA cm⁻². At the end of BEE process substrates with PGe structure
were rinsed with EtOH (99%), dried under N₂ flow and introduced
into the loading chamber of the CBE reactor.

Ge growth was carried-out in VG Semicon VG90H CBE reactor, with a load-lock, transfer module maintained at ~6.10⁻⁹ Torr, and thermocouple as a mean of monitoring the temperature during the growth. The solid source of Ge, with a K-Cell temperature at 1250 °C, was used to growth Ge with nominal growth rate of 500 nm h⁻¹. Samples were introduced to the growth chambre directly at 300 °C. Then various nominal thickness of Ge (5–1000 nm) was deposited on PGe substrates at 300°C, and chambre pressure ~6.10⁻⁶ Torr.

After the detachment of the membrane with adhesive tape, the
retrieved substrate was immersed in concentrated H₂O₂(30%) so-
lution for 1 min to fully oxidize the remains of the PGe structure.
This is followed by deoxidation in concentrated HF (49%) prior to
the reporosification.

4.2. Characterization

The top-view and cross-section of PGe layers and Ge/PGe
structures were observed with a Zeiss LEO 1540 XB scanning
electron microscope at 4.3 mm of working distance and 20 keV of
acceleration voltage, to evaluate the thickness of deposited material
and any morphological changes of the structure. The surface
morphology of the membranes was evaluated using Veeco
Dimension 3100, atomic force microscopy system, in tapping mode
with SSS-NCHR silicon probe and scan resolution 512 × 512 pixel.
The collected AFM profile data on various wafers locations were
also used to evaluate the pits' size and depth evolution on the FSM's
surface for various thicknesses. The structural properties of PGe and
epitaxial layers were investigated using Rigaku smartlab HRXRD
system with Cu Kα X-ray source, Ge (220)x2 monochromator on the
incident beam, and HYPIX-3000 hybrid pixel array 2D detector. The
XRR was used to determine the critical angle of PGe layers, which is
directly linked to the porosity [49]. The out-of-plane and in-plane
XRD configurations were used to identify crystalline quality of
the Ge membranes.

Author contributions

The manuscript was written through contributions of all au-
thors. All authors have given approval to the final version of the
manuscript. Tadeás Hanus: Conceptualization, Methodology,
Investigation, Data curation, Original draft preparation, Review and
Editing, Visualization. Bouraoui Ilahi: Conceptualization, Method-
ology, Supervision, Validation, Review and Editing. Alexandre
Chapotot: Investigation, Review and Editing. Hubert Pelletier:
Investigation, Validation, Review and Editing. Jinyoun Cho: Vali-
dation, Review and Editing. Kristof Dessein: Validation, Review and
Editing. Abderraouf Boucherif: Supervision, Validation, Review and
Editing.

Declaration of competing interest

The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgement

We thank Thierno Mamoudou Diallo and Raphaël Dawant for
scientific discussions, Guillaume Bertrand, Philippe-Olivier Provost
and all the technical staff of 3IT for the technical support. We thank
Umicore, Saint-Augustin Canada Electric (Stace), Innovation en
energie électrique (InnovÉÉ), the Natural Sciences and Engineering
Research Council of Canada (NSERC), Fonds de recherche du Québec
(FRQNT), Mitacs, for the financial support. Abderraouf Boucherif is
grateful for a Discovery grant supporting this work.

LN2 is a joint International Research Laboratory (IRL 3463) funded and co-operated in Canada by Université de Sherbrooke (UdeS) and in France by CNRS as well as ECL, INSA Lyon, and Université Grenoble Alpes (UGA). It is also supported by the Fonds de Recherche du Québec Nature et Technologie (FRQNT).

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at
https://doi.org/10.1016/j.mtadv.2023.100373.

References

[1] K. Myny, Nat Electron 1 (2018) 30–39.

[2] Y. Lee, J.W. Chung, G.H. Lee, H. Kang, J.-Y. Kim, C. Bae, H. Yoo, S. Jeong, H. Cho, S.-G. Kang, J.Y. Jung, D.-W. Lee, S. Gam, S.G. Hahm, Y. Kuzumoto, S.J. Kim, Z. Bao, Y. Hong, Y. Yun, S. Kim, Sci. Adv. 7 (2021), eabg9180.

[3] P. Wang, M. Hu, H. Wang, Z. Chen, Y. Feng, J. Wang, W. Ling, Y. Huang, Adv. Sci. 7 (2020), 2001116.

[4] S. Choi, C. Kang, C.-W. Byun, H. Cho, B.-H. Kwon, J.-H. Han, J.-H. Yang, J.-W. Shin, C.-S. Hwang, N.S. Cho, K.M. Lee, H.-O. Kim, E. Kim, S. Yoo, H. Lee, Nat. Commun. 11 (2020) 2732.

[5] M.M. Shulaker, G. Hills, R.S. Park, R.T. Howe, K. Saraswat, H.-S.P. Wong, S. Mitra, Nature 547 (2017) 74–78.

[6] G. Qin, Y.H. Jung, H. Zhang, N. Jiang, P. Ma, S. Stetson, M. Racanelli, Z. Ma, Advanced Electronic Materials 8 (2022), 2101350.

[7] D.-H. Kim, J.-H. Ahn, W.M. Choi, H.-S. Kim, T.-H. Kim, J. Song, Y.Y. Huang, Z. Liu, C. Lu, J.A. Rogers, Science 320 (2008) 507–511.

[8] Y. Kim, J.M. Suh, J. Shin, Y. Liu, H. Yeon, K. Qiao, H.S. Kum, C. Kim, H.E. Lee, C. Choi, H. Kim, D. Lee, J. Lee, J.-H. Kang, B.-I. Park, S. Kang, J. Kim, S. Kim, J.A. Perozek, K. Wang, Y. Park, K. Kishen, L. Kong, T. Palacios, J. Park, M.-C. Park, H. Kim, Y.S. Lee, K. Lee, S.-H. Bae, W. Kong, J. Han, J. Kim, Science 377 (2022) 859–864.

[9] J. Shin, H. Kim, S. Sundaram, J. Jeong, B.-I. Park, C.S. Chang, J. Choi, T. Kim, M. Saravanapavanantham, K. Lu, S. Kim, J.M. Suh, K.S. Kim, M.-K. Song, Y. Liu, K. Qiao, J.H. Kim, Y. Kim, J.-H. Kang, J. Kim, D. Lee, J. Lee, J.S. Kim, H.E. Lee,
H. Yeon, H.S. Kum, S.-H. Bae, V. Bulovic, K.J. Yu, K. Lee, K. Chung, Y.J. Hong, A. Ougazzaden, J. Kim, Nature 614 (2023) 81–87.

[10] Y.-C. Tai, S. An, P.-R. Huang, Y.-T. Jheng, K.-C. Lee, H.-H. Cheng, M. Kim, G.-E. Chang, Nanoscale (2023), https://doi.org/10.1039/D2NR07107J.

[11] B. Kunert, Y. Mols, M. Baryshniskova, N. Waldron, A. Schulze, R. Langer, Semicond. Sci. Technol. 33 (2018), 093002.

[12] H. Jin, H. Jin, L. Chen, L. Chen, J. Li, J. Li, X. An, Y.P. Wu, L. Zhu, H. Yi, K.H. Li, K.H. Li, Opt. Lett., OL 45 (2020) 6671–6674.

[13] J. Chun, K.J. Lee, Y.-C. Leem, W.-M. Kang, T. Jeong, J.H. Baek, H.J. Lee, B.-J. Kim, S.-J. Park, ACS Appl. Mater. Interfaces 6 (2014) 19482–19487.

[14] J. Lim, J. Shim, D.-M. Geum, S. Kim, IEEE Photon. J. 14 (2022) 1–9.

[15] J. Kang, Z. Cheng, W. Zhou, T.-H. Xiao, K.-L. Gopalakrisna, M. Takenaka, H.K. Tsang, K. Goda, Opt. Lett., OL 42 (2017) 2094–2097.

[16] C. Liu, W. Ma, M. Chen, W. Ren, D. Sun, Nat. Commun. 10 (2019) 4873.

[17] M.R.M. Atalla, S. Assali, A. Attiaoui, C. Lemieux-Leduc, A. Kumar, S. Abdi, O. Moutanabbir, Adv. Funct. Mater. 31 (2021), 2006329.

[18] N. Cho, M. Kim, Z. Ma, in: 10th International Conference on Group IV Photonics, 2013, pp. 77–78.

[19] S. Wu, H. Zhou, Q. Chen, L. Zhang, K.H. Lee, S. Bao, W. Fan, C.S. Tan, Appl. Phys. Lett. 119 (2021), 191106.

[20] D. Nam, A.M. Roy, K.C.Y. Huang, M.L. Brongersma, K.C. Saraswat, CLEO:2011– Laser Applications to Photonic Applications, 2011, Optica Publishing Group, 2011, p. JTu185. Paper JTu185.

[21] F.T. Armand Pilon, A. Lyasota, Y.-M. Niquet, V. Reboud, V. Calvo, N. Pauc, J. Widiez, C. Bonzon, J.M. Hartmann, A. Chelnokov, J. Faist, H. Sigg, Nat. Commun. 10 (2019) 2724.

[22] A.A. La Mattina, S. Mariani, G. Barillaro, Adv. Sci. 7 (2020), 1902872.

[23] H. Zhao, Z. Xue, X. Wu, Z. Wei, Q. Guo, M. Xu, C. Qu, C. You, Y. Mei, M. Zhang, Z. Di, Q. Guo, Npj Flex Electron 6 (2022) 1–10.

[24] Y. Kim, S.S. Cruz, K. Lee, B.O. Alawode, C. Choi, Y. Song, J.M. Johnson, C. Heidelberger, W. Kong, S. Choi, K. Qiao, I. Almansouri, E.A. Fitzgerald, J. Kong, A.M. Kolpak, J. Hwang, J. Kim, Nature 544 (2017) 340–343.

[25] W. Kong, H. Li, K. Qiao, Y. Kim, K. Lee, Y. Nie, D. Lee, T. Osadchy, R.J. Molnar, D.K. Gaskill, R.L. Myers-Ward, K.M. Daniels, Y. Zhang, S. Sundram, Y. Yu, S. Bae, S. Rajan, Y. Shao-Horn, K. Cho, A. Ougazzaden, J.C. Grossman, J. Kim, Nat. Mater. 17 (2018) 999–1004.

[26] S.-H. Bae, K. Lu, Y. Han, S. Kim, K. Qiao, C. Choi, Y. Nie, H. Kim, H.S. Kum, P. Chen, W. Kong, B.-S. Kang, C. Kim, J. Lee, Y. Baek, J. Shim, J. Park, M. Joo, D.A. Muller, K. Lee, J. Kim, Nat. Nanotechnol. 15 (2020) 272–276.

[27] H.S. Kum, H. Lee, S. Kim, S. Lindemann, W. Kong, K. Qiao, P. Chen, J. Irwin, J.H. Lee, S. Xie, S. Subramanian, J. Shim, S.-H. Bae, C. Choi, L. Ranno, S. Seo, S. Lee, J. Bauer, H. Li, K. Lee, J.A. Robinson, C.A. Ross, D.G. Schlom, M.S. Rzchowski, C.-B. Eom, J. Kim, Nature 578 (2020) 75–81.

[28] J. Jiang, X. Sun, X. Chen, B. Wang, Z. Chen, Y. Hu, Y. Guo, L. Zhang, Y. Ma, L. Gao, F. Zheng, L. Jin, M. Chen, Z. Ma, Y. Zhou, N.P. Padture, K. Beach, H. Terrones, Y. Shi, D. Gall, T.-M. Lu, E. Wertz, J. Feng, J. Shi, Nat. Commun. 10 (2019) 4145.

[29] Z. Lu, X. Sun, W. Xie, A. Littlejohn, G.-C. Wang, S. Zhang, M.A. Washington, T.-M. Lu, Nanotechnology 29 (2018), 445702.

[30] J. Yoon, S. Jo, I.S. Chun, I. Jung, H.-S. Kim, M. Meitl, E. Menard, X. Li, J.J. Coleman, U. Paik, J.A. Rogers, Nature 465 (2010) 329–333.

[31] T. Maeda, W.-H. Chang, T. Irisawa, H. Ishii, H. Hattori, V. Poborchii, Y. Kurashima, H. Takagi, N. Uchida, Appl. Phys. Lett. 109 (2016), 262104.

[32] J.S. Mangum, A.D. Rice, J. Chen, J. Chenenko, E.W.K. Wong, A.K. Braun, S. Johnston, H. Guthrey, J.F. Geisz, A.J. Ptak, C.E. Packard, Adv. Energy Mater. 12 (2022), 2201332.

[33] T. Akatsu, C. Deguet, L. Sanchez, F. Allibert, D. Rouchon, T. Signamarcheix, C. Richtarch, A. Boussagol, V. Loup, F. Mazen, J.-M. Hartmann, Y. Campidelli, L. Clavelier, F. Letertre, N. Kernevez, C. Mazure, Mater. Sci. Semicond. Process. 9 (2006) 444–448.

[34] H. Kim, S. Lee, J. Shin, M. Zhu, M. Akl, K. Lu, N.M. Han, Y. Baek, C.S. Chang, J.M. Suh, K.S. Kim, B.-I. Park, Y. Zhang, C. Choi, H. Shin, H. Yu, Y. Meng, S.-I. Kim, S. Seo, K. Lee, H.S. Kum, J.-H. Lee, J.-H. Ahn, S.-H. Bae, J. Hwang, Y. Shi, J. Kim, Nat. Nanotechnol. 17 (2022) 1054–1059.

[35] S. Park, J. Simon, K.L. Schulte, A.J. Ptak, J.-S. Wi, D.L. Young, J. Oh, Joule 3 (2019) 1782–1793.

[36] V. Depauw, C. Porret, M. Moelants, E. Vecchio, K. Kennes, H. Han, R. Loo, J. Cho, G. Courtois, R. Kurstjens, K. Dessein, V. Orejuela, C. Sanchez-Perez, I. Rey-Stolle, I. García, Prog Photovolt Res Appl. (2022) 1–14, https://doi.org/10.1002/pip.363414.

[37] A. Boucherif, G. Beaudin, V. Aimez, R. Arès, Appl. Phys. Lett. 102 (2013), 011915.

[38] E. Winter, W. Schreiber, P. Schygulla, P.L. Souza, S. Janz, D. Lackner, J. Ohlmann, J. Cryst. Growth 602 (2023), 126980.

[39] N. Paupy, Z.O. Elhmaidi, A. Chapotot, T. Hanus, J. Arias-Zapata, B. Ilahi, A. Heintz, A.B.P. Mbeunmi, R. Arvinte, M.R. Aziziyan, V. Daniel, G. Hamon, J. Chrétien, F. Zouaghi, A. Ayari, L. Mouchel, J. Henriques, L. Demoulin, T.M. Diallo, P.-O. Provost, H. Pelletier, M. Volatier, R. Kurstjens, J. Cho, G. Courtois, K. Dessein, S. Arcand, C. Dubuc, A. Jaouad, N. Quaegebeur, R. Gosselin, D. Machon, R. Arès, M. Darnon, A. Boucherif, Preprint (2022), 03144.

[40] J.S. Ward, T. Remo, K. Horowitz, M. Woodhouse, B. Sopori, K. VanSant, P. Basore, Prog. Photovoltaics Res. Appl. 24 (2016) 1284–1292.

[41] A. Chapotot, B. Ilahi, J. Arias-Zapata, T. Hanus, A. Ayari, G. Hamon, J. Cho, K. Dessein, M. Darnon, A. Boucherif, Surfaces and Interfaces (2023) (To Be Published) (n.d.).

[42] S. Tutashkonko, A. Boucherif, T. Nychyporuk, A. Kaminski-Cachopo, R. Arès, M. Lemiti, V. Aimez, Electrochim. Acta 88 (2013) 256–262.

[43] S. Tutashkonko, S. Alekseev, T. Nychyporuk, Electrochim. Acta 180 (2015) 545–554.

[44] A. Dupuy, M.R. Aziziyan, D. Machon, R. Arès, A. Boucherif, Electrochim. Acta 378 (2021), 137935.

[45] Y.A. Bioud, A. Boucherif, A. Belarouci, E. Paradis, S. Fafard, V. Aimez, D. Drouin, R. Arès, Electrochim. Acta 232 (2017) 422–430.

[46] A. Heintz, B. Ilahi, A. Pofelski, G. Botton, G. Patriarche, A. Barzaghi, S. Fafard, R. Arès, G. Isella, A. Boucherif, Nat. Commun. 13 (2022) 6624.

[47] A.B.P. Mbeunmi, M. El-Gahouchi, R. Arvinte, A. Jaouad, R. Cheriton, M. Wilkins, C.E. Valdivia, K. Hinzer, S. Fafard, V. Aimez, R. Arès, A. Boucherif, Sol. Energy Mater. Sol. Cell. 217 (2020), 110641.

[48] M.N. Beattie, Y.A. Bioud, D.G. Hobson, A. Boucherif, C.E. Valdivia, D. Drouin, R. Arès, K. Hinzer, Nanotechnology 29 (2018), 215701.

[49] T. Hanus, J. Arias-Zapata, B. Ilahi, P.-O. Provost, J. Cho, K. Dessein, A. Boucherif, Advanced Materials Interfaces (2023) 2202495.

[50] C.V. Falub, H. von Känel, F. Isa, R. Bergamaschini, A. Marzegalli, D. Chastina, G. Isella, E. Müller, P. Niedermann, L. Miglio, Science 335 (2012) 1330–1334.

[51] W.H. Thompson, Z. Yamani, H.M. Nayfeh, M.-A. Hasan, J.E. Greene, M.H. Nayfeh, MRS Online Proc. Libr. 452 (1996) 255–260.

[52] L. Liu, X. Zhang, S. Wang, G. Wang, J. Yu, X. Hu, Q. Xu, X. Xu, L. Zhang, CrystalEngComm 24 (2022) 1840–1848.

[53] P. Zaumseil, J. Appl. Crystallogr. 48 (2015) 528–532.

[54] A. Boucherif, A. Korinek, V. Aimez, R. Arès, AIP Adv. 4 (2014), 107128.

[55] G. Van Hoof, M. Schurmans, B. Robertz, J.-F. Ménard, K. Dessein, J. Sustain. Metall. 6 (2020) 333–343.