# Morphological and chemical evolution of monocrystalline porous germanium over time in various storage environments

Valentin Daniel<sup>a,b,*</sup>, Jérémie Chretien<sup>a,b</sup>, Sonia Blais<sup>c</sup>, Jinyoun Cho<sup>d</sup>, Kristof Dessein<sup>d</sup>, Gwenaelle Hamon<sup>a,b</sup>, Abderraouf Boucherif<sup>a,b</sup>, Maxime Darnon<sup>a,b</sup>

## ARTICLE INFO

### Keywords:

- Electrochemical etching
- Mesoporous germanium
- Aging
- Oxidation
- Shelf life

## ABSTRACT

Mesoporous germanium (MP-Ge) emerges as a very appealing material for many applications such as anode material for Lithium-Ion batteries due to its high specific area and large void spaces or, in optoelectronics as sacrificial layer for III-V materials growth and detachment, allowing notably several uses of a single Ge substrate. These porous nanostructures are distinguished by a large specific surface area and are prone to degradation with time due to exposure to the environment. To understand and be able to reduce this effect, we studied the chemical and morphological evolution of porous germanium layers under various ambient storage conditions for 3 months to identify the main parameters responsible for material degradation. This study demonstrates that the ambient air environment leads to the growth of native oxide, leading to major morphology changes. Scanning electrons microscope (SEM) showed the formation of clusters and the enlargement of the pores after 90 days. These structural modifications are caused by the oxidation of Ge, and more specifically by the creation of $GeO_2$ matrices due to the synergy of dioxygen ($O_2$) and humidity ($H_2O_{(g)}$). The energy brought by light can exacerbate these phenomena and thus accelerate the degradation rate of the pore morphology. Based on these experimental results, we propose efficient solutions to limit the $GeO_2$ proportions and the clusters' appearance, by storing them under a dry neutral atmosphere (Ar) or by adding a hydrogen halide pre-treatment (10s 1% HBr solution).

## 1. Introduction

Germanium (Ge) is a group IV semiconductor that has applications for microelectronic devices [1], optoelectronic applications in the near-infrared region [2], and as a substrate for crystalline III-V compound semiconductor growth [3,4] Conserving its quality over time is thus a crucial parameter to improve the lifespan of technologies using this type of material and to facilitate its processing. Due to interesting characteristics such as its large spectral transparency range, its high specific surface, its tunable refractive index and its biocompatibility, the nanostructuring of germanium has been developed for various applications such as energy storage [5] or optoelectronic devices [6,7], using different techniques [8–12] to configure its optical [13], morphological, or mechanical properties. In the specific case of monocrystalline porous germanium made by bipolar electrochemical etching for epitaxy of

group IV and III-V materials [7,14,15], the preservation of the structural integrity (composition, porosity morphology, and surface roughness) of the material is an essential criterion for high-quality epitaxial growth. For this purpose, we propose in this study to observe the evolution of monocrystalline mesoporous germanium over time in ambient air (in a box in a clean room) and other specific conditions, to characterize the aging of the material, to analyze the origin of potential deteriorations, and to propose adequate storage conditions to maximize the shelf-life of a nanostructured germanium layer.

## 2. Methods

### 2.1. Porous germanium sample preparation

For our investigations, a mesoporous structure is created on a 4"

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

<footnote><sup>c</sup> Centre de Caractérisation des Matériaux (CCM), Université de Sherbrooke, 2500 Boul. Université, Sherbrooke J1K 2R1, Canada</footnote>

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

<footnote>* Corresponding author.
E-mail address: valentin.daniel@usherbrooke.ca (V. Daniel).</footnote>
monocrystalline p-type (100) oriented germanium wafer with 6° miscut toward (111) crystallographic direction by a bipolar-electrochemical etching. This nano-structuration technique uses alternately positives and negatives electrical pulses in an acid solution (HF). The additional advantage of this procedure is that it leaves the surface almost oxide-free due to the acid etch and the temporary passivation of the surface by F⁻. The porosification process used has been developed as a part of previous research [14] for detachable III-V germanium-based photovoltaic cells. The 250 nm-thick “sponge-like” porous layer (Ge<sub>porous</sub> layer) created is uniform and monocrystalline with a porosity of 51% ± 5% (estimated by image treatment). After porosification, the 4” Ge wafer is then cleaved into five 5 × 5 mm² samples to facilitate the storage and parallelize aging processes on originally similar samples.

## 2.2. Storage conditions

Each porous sample is stored individually for up to 90 days. In the first case, some of them are placed in semi-transparent boxes exposed to light under class 100 clean room ambient conditions (humidity = 43% ± 5%, temperature = 19.5 °C ± 2 °C). For experiments in controlled storage conditions, we developed a packaging process using hermetic metallic bags (Fig. 1) prepared in a glove box containing a neutral atmosphere: Argon (Ar). A preliminary study carried out with Nitrogen (N₂) demonstrated the same behaviour as Argon, indicating that both gases can be used as a neutral atmosphere for the storage. The samples are placed in individual boxes and sealed in hermetic bags either with wet tissue (dark moist Ar) or dry silicate beads (dark dry Ar). These storage containers can then be theoretically exposed to the ambient atmosphere with no contamination by the external environment. This encapsulation technique is validated by experiment results shown further (part 4.1).

For conditions with O₂, the samples are placed in non-hermetic black boxes exposed to the clean room air with dark dry O₂) or without silica beads (dark moist O₂). When the light impact is considered, the metallic bag is not used, and the sample is placed in an illuminated part of the cleanroom in a transparent box (light moist O₂). For all dry conditions (see Table 1), humidity testers are also placed in the storage area to prove the enclosure has remained dry throughout the entire study and we verified that the humidity never exceeded 10% in dry conditions.

## 2.3. Characterizations

Samples are analyzed after 2 h from porosification (initial condition – reference), 1, 7, 30, and 90 days of aging (one sample per aging duration for each condition). To characterize the evolutions in composition and the morphological transformation of the Ge<sub>porous</sub> layer, we used X-ray Photoelectron Spectroscopy (XPS) with an Al kα 15 kV (225 W, 15 mA) monochromatic excitation source as well as Scanning Electron Microscopy (SEM) at 20 kV with a 150,000× magnification and a working distance (WD) of 5.5 mm ± 1 mm, for cross-sectional and top views observations. For the XPS spectrum analysis, we set the C1s core level spectrum with sp³ (adventitious carbon) component peak at 284.8 eV (Thermo Fisher database). Then, on the Ge3d core level spectrum

<caption>**Fig. 1.** Schematic illustration of the sealing process used for controlled dark moist and dry Argon storage environment.</caption>

<table><caption>**Table 1**
Summarizes the storage conditions.</caption><thead><tr><th>Name</th><th>Objective</th><th>Storage condition</th></tr></thead><tbody><tr><td>Clean room</td><td>Normal storage (reference)</td><td>Semitransparent box in cleanroom environment (humidity = 43% ± 5%, temperature = 19.5 °C ± 2 °C).</td></tr><tr><td>Dark dry Ar</td><td>Inert storage</td><td>Metallic bag sealed in an Argon-filled glove box. Silica beads. Humidity tester below 10%</td></tr><tr><td>Dark dry O₂</td><td>Impact of O₂</td><td>Non-hermetic black box exposed to air with silica beads</td></tr><tr><td>Dark moist Ar</td><td>Impact of water vapor</td><td>Metallic bag sealed in an Argon-filled glove box with a wet tissue.</td></tr><tr><td>Dark moist O₂</td><td>Synergetic impact of water vapor and O₂</td><td>Unsealed metallic bag in a cleanroom environment (humidity = 43% ± 5%, temperature = 19.5 °C ± 2 °C).</td></tr><tr><td>Light moist O₂</td><td>Role of light</td><td>Transparent box cleanroom environment (humidity = 43% ± 5%, temperature = 19.5 °C ± 2 °C).</td></tr></tbody></table>

region, the Ge⁰ components (Ge3d⁵/² and Ge3d³/²) are curve fit with the same full width at half maximum (FWHM), with a 2/3 intensity ratio and a shift of 0.58 eV [16,17]. These parameters allow determining the precise position of the germanium oxidation states 3d peaks, Ge¹⁺ (related to the Ge₂O suboxide), Ge²⁺ (related to the GeO suboxide), Ge³⁺ (related to the Ge₂O₃ suboxide), and Ge⁴⁺ (related to the GeO₂ oxide) shifted from the Ge⁰ position (calculated for the first case at 29.5 eV) by 0.8 eV, 1.8 eV, 2.75 eV and 3.4 eV, respectively [13,14]. One of their possible configurations is schematically represented in Fig. 2. The peaks are fitted by a Gaussian/Lorentzian function after subtracting a baseline using a Shirley function. The resulting fitting curve enables to extract of the proportion of each oxide species by considering the instrument transmission function. The surface roughness is eventually determined using tapping mode on 5 × 5 µm wide areas using atomic force microscopy (AFM).

## 3. Aging of porous Ge in clean room conditions

The SEM images comparison (Fig. 3 (a-d)) between the reference sample (as porosified after 2 h of aging) and the one aged during 3 months shows an important difference in their morphology (visible deterioration over time). The sample after 90 days has larger pores and seems to present clusters. The XPS survey spectrum (Fig. 3 (e)) shows major differences in the composition after 90 days, notably with the appearance of oxygen in the porous germanium structure observable via an increase of the Auger OKLL lines and O1s peaks and by modifications of the Ge-related peaks (GeLMM, Ge2p, and Ge3d). Coherently with the literature [16–20], we can identify on the Ge3d peak (Fig. 3 (f), the contribution from different germanium oxide and suboxide species: Ge₂O (Ge¹⁺), GeO (Ge²⁺), Ge₂O₃ (Ge³⁺), and GeO₂ (Ge⁴⁺).

When we look at the kinetic evolution of the Ge3d peak by XPS (Fig. 4), we observe initially the formation of Ge₂O quasi instantly after porosification (initial proportions around 17% after 2 h – reference). After 1 day of aging, Ge₂O increases up to ~18% and other suboxides such as the GeO and Ge₂O₃ appear, both with proportions around 10%. Ge₂O and GeO chemical contributions then decrease, while Ge₂O₃ continues to increase until one week of aging. At the same time, we begin to observe the creation of GeO₂ oxide (below 20% before the first 7 days of storage). All the suboxides then gradually give way to the formation of GeO₂ up to 60% after the first month of storage. This final oxide analogous to the Ge⁴⁺ germanium oxidation state keeps slightly increasing afterward. These observations indicate an evolution of the germanium oxidation state from Ge¹⁺ (Ge₂O) and Ge²⁺ (GeO) to Ge³⁺ (Ge₂O₃) and finally to Ge⁴⁺ (GeO₂). After 90 days, the GeO₂ oxide corresponds to approximately 65% of the Ge3d peak. The morphological modification of the porous germanium starts to be observed between 1 and 7 days of storage in clean room conditions (hatched zone in the
<figure><img src="image_1.png" /><figcaption>Fig. 2. Schematic representation of a possible configuration of Ge<sup>1+</sup>, Ge<sup>2+</sup>, Ge<sup>3+</sup> and Ge<sup>4+</sup>.</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>Fig. 3. a) Cross-section SEM, b) top-view SEM images of the "as porosified" germanium sample (initial conditions - reference) compared to c) Cross-section SEM and d) top-view SEM images of the same sample after 90 days of storage under clean room conditions. e) XPS survey spectrum with the main modified element peaks of Ge<sub>porous</sub> "as porosified" compared to the same sample after storage, and f) zoom on the Ge<sub>3d</sub> area detailing the different Ge (I-IV) oxidation states.</figcaption></figure>

graph Fig. 4).

Based on these first results, we can postulate germanium quickly ages
in clean room conditions (modification of the porosity and appearance
of cluster) and that the oxidation of the germanium porous structure is
responsible for its structural transformation. However, these pre-
liminary observations are not sufficient to determine the precise
mechanism of this degradation and to identify the contribution of each
environmental parameter to the morphological deterioration over time.

We characterized then, separately and combined, the impact of ambient
components presents during the storage of the samples and potentially
responsible for the Ge<sub>porous</sub> evolution: dioxygen (O<sub>2</sub>), humidity, and
light.
<figure><img src="image_3.png" /><figcaption>Fig. 6. a) XPS Ge3d spectrum, b) cross-section SEM images, c) Top SEM view and d) 5 × 5um surface roughness made by AFM of porous germanium stored in dry Ar after 3 months of storage. e) XPS Ge3d spectrum, f) cross-section SEM images, g) Top SEM view and h) 5 × 5um surface roughness made by AFM of porous germanium stored in dry O₂, after 3 months of storage.</figcaption></figure>

to identify the impact of humidity in the storage conditions and its
contribution to the structural modification of the porous germanium
layer over time.

4.3.1. Kinetic: germanium oxide and sub-oxides evolution

In the presence of humidity in a neutral atmosphere (dark moist Ar) as shown in Fig. 7 (a), we can distinguish the same oxides and suboxide evolutions as for the sample stored in a dry dioxygen environment (dark dry O2 Fig. 5 (b)), but slightly slower: initial formation of Ge₂O (Ge¹⁺) (proportions around 17%), followed by the germanium oxidation state Ge²⁺ and Ge³⁺ associated to the GeO and Ge₂O₃ formation respectively. We notice the formation of a few quantities of GeO₂ (related to the Ge⁴⁺ oxidation state) after several weeks of aging (1.4% after 90 days). When the humidity is combined with the O₂ in the storage conditions (dark moist O₂, Fig. 7 (b)), we can observe an oxidation synergy and the creation of a high quantity of GeO₂ after the first week of aging. This massive formation of Ge⁴⁺ oxidation states (45% after 90 days) is associated with a decrease in the suboxides proportion. With Fig. 7 (b), we can thus deduce that the germanium and its low oxidation states

evolve gradually into GeO₂ over time when O₂ is associated with
humidity.

4.3.2. Morphological evolution after 90 days of storage

When we focus on the morphological transformations of the Ge<sub>porous</sub>
layer stored with humidity in a neutral atmosphere (dark moist Ar, Fig. 8
(a-d)), we do not observe important structural changes (same
morphology compared to the reference), even if XPS reveals the pres-
ence of suboxides (GeO<sub>x</sub> with x < 2), similar to dark dry O<sub>2</sub> storage
conditions (Fig. 6 (e-h)).

In the case of the sample stored under an O₂ and H₂O environment (dark moist O₂) in Fig. 8 (e-h), we observe the degradation of Geporous structure with the creation of oxide matrices (clusters), an increase of the porosity (84% ± 5%) and a higher surface roughness (4.8 ± 0.5 nm). This transformation begins between 7 and 30 days of storage under these oxidative conditions. Combined with the XPS characterization, we can correlate the formation of a large amount of Ge⁴⁺ (GeO₂) with this appearance of clusters and the pore size expansion. This morphological evolution over time can be explained by the combination of the Ge

<figure><img src="image_4.png" /><figcaption>Fig. 7. Proportions of Ge oxides evolution during 90 days under a) dark moist Ar and b) dark moist O₂ atmosphere, during 3 months of storage. The gray background indicates a degraded morphology (observed by SEM) while the hatched background delimitates the period when the porous layer begins to morphologically evolve.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 8.** a) XPS Ge3d spectrum, b) cross-section SEM images, c) Top SEM view and d) 5 × 5um surface roughness made by AFM of porous germanium stored in Ar with humidity. E) XPS Ge3d spectrum, f) cross-section SEM images, g) Top SEM view and h) 5 × 5um surface roughness made by AFM of porous germanium stored in O₂ with humidity, after 3 months of storage.</figcaption></figure>

oxidation and the dissolution of Ge<sub>sub-oxides</sub>, as proposed in the mechanism illustrated in Fig. 9.

Upon the first exposition of the sample to the H₂O(g) + O₂ in the dark (Fig. 9 (a)), pores walls are converted into native suboxides (Ge₂O followed by GeO and Ge₂O₃) over the entire depth of the porous structure [18] (Fig. 9 (b)). The suboxide layer is then progressively thickened by

the continuous oxidation of the germanium. In parallel, the ambient H₂O(g) and the H₂O(l) accumulated by capillary condensation in the cavities induce local dissolutions of oxides [21,22] by Ge—O bonds hydrolysis [20,23] (Fig. 9 (c)). This consumption (etching) of Ge and its oxides by water combined with constant O₂ germanium oxidation continuously forms high oxidation states (Ge<sup>4+</sup>), which at saturation, precipitate and form visible GeO₂ matrices (Fig. 9 (d) and Fig. 8 (f-g)).

<figure><img src="image_6.png" /><figcaption>**Fig. 9.** Schematic illustration of the Ge<sub>sub-oxides</sub> layers formation and GeO₂ matrices (clusters) growth mechanism steps in Ge<sub>porous</sub> structure, under O₂ and Humidity ambient conditions.</figcaption></figure>

## 4.4. Influence of light

All the previous characterizations and mechanism descriptions have been based on samples stored inside dark enclosures (dark boxes or hermetic metallic bags). But to clearly understand the role and impact of each ambient parameter, samples exposed to light need also to be characterized. The porous germanium layers in this part are stored in a transparent box with humidity and dioxygen (light moist O₂) (under clean room conditions).

### 4.4.1. Kinetic: germanium oxide and sub-oxides evolution

Fig. 10 shows that light exacerbates the oxidation mechanisms of porous germanium. Compared to the storage in similar conditions in the dark (dark moist O₂, Fig. 7 (b)), the presence of light influences the kinetic of germanium suboxides and oxide formation. The order of oxidation states appearance is similar but faster: after one week, the proportion of GeO₂ (Ge<sup>4+</sup>) already exceeds 60% and the remaining quantity of germanium Ge⁰ falls below 25%. After only 30 days, the quasi-totality of the porous layer has been transformed into GeO₂.

### 4.4.2. Morphological evolution after 90 days of storage

When we pay attention to the morphology of the structure at the end of the aging process (Fig. 11 (a-d)), where the whole germanium has completely evolved into oxide, we only see irregular, and continuous material. The GeO₂ clusters seem to have merged into a compact layer with a surface roughness of 16 ± 1.6 nm, which is not suitable for epitaxial growth. We can also deduce from the comparison with dark moist O₂ (Fig. 8(f-g)) and in clean room conditions (Fig. 3(c-d)) that the oxidation and morphological transformation are enhanced by the intensity of the light (increase of the energy density brought to the reactions). These SEM and AFM observations correlated to the XPS analysis demonstrate a photo-oxidation of germanium [24] which
<figure><img src="image_7.png" /><figcaption>**Fig. 10.** Proportions of Ge oxides evolution during 90 days under Argon and Oxygen atmosphere with humidity and light with transparent box. The gray background indicates a degraded morphology (observed by SEM) while the hatched background delimitates the period when the porous layer begins to morphologically evolve.</figcaption></figure>

accelerates the structural deterioration of the Ge<sub>porous</sub>.

## 4.5. Reducing porous germanium aging with post-porosification surface treatment

We have demonstrated that the porous germanium aging results from germanium oxidation, germanium sub-oxide dissolution, and GeO₂ precipitation, and that the suboxide formation seems to be limiting this mechanism in the presence of O₂ and humidity. To delay the aging, we can therefore tailor processes that further reduce the germanium oxidation rate. According to the literature, halogen acids can etch germanium sub-oxides and passivate the surface against oxidation [21,25,26]. B. Onsia et al. [21] even showed that HBr and HI remove all the oxides (including suboxides contrary to HF or HCl) and strongly functionalize the Ge surface by Br⁻ and I⁻, respectively, allowing limited passivation and preventing the oxidation over a time that depends on the concentration and the dipping duration in the acids.

To delay the first step of the porous Ge aging mechanism, we applied a wet treatment by dipping a fresh porous germanium sample during 10s in a 1% HBr solution (49% HBr: C₂H₆O (1:198)) before storage. We chose a short dipping time and a low concentration to limit the porous germanium etching that is enhanced by its very high specific surface compared to dense germanium. The sample was then stored in *dark moist* O₂ conditions and its surface composition and morphology evolution

with the storage time is compared to untreated samples stored in the same conditions.

### 4.5.1. Germanium oxides and sub-oxides evolution

As shown in Fig. 12 representing the surface composition of the treated porous samples aged during 90 days in *dark moist* O₂ conditions, the high oxidation state quantity has been significantly lowered compared to the untreated sample. During the storage, we observe the same oxidation mechanism with approximately the same proportions of suboxides compared to the samples with no HBr treatment, but at a lower rate. We noticed after 3 months that GeO₂ oxide formation has been limited to under 19% instead of 45% in the untreated sample. These results demonstrate a slowdown of oxidation and prove that the pore's surface passivation by Br can reduce or delay the final formation of Ge⁴⁺. This delay in porous germanium oxidation has also been observed (not shown here) for each other storage conditions considered here (*dark dry Ar*, *dark dry O₂*, *dark moist Ar*, and *light moist O₂*).

### 4.5.2. Morphological evolution after 90 days of storage

After 3 months of aging under air condition in the dark (*dark moist* O₂) with initial HBr treatment, the Ge<sub>porous</sub> layer presents slightly larger porosity (around 68% ± 5%) than the reference sample, a relatively low surface roughness (2.1 ± 0.2 nm) and no visible clusters as shown on Fig. 13 (b-d). These characterizations suggest a continuous etching of the pores during storage and that the creation of clusters is delayed by the initial deoxidation and passivation treatment.

With all the observations of this study (Fig. 14), we can deduce that the visible structural modification happens when the proportion of GeO₂ exceeds a value between 4% and 17% of the total amount of materials (valid with the specific germanium morphology, porosity, and thickness used for these experiments), except when an HBr solution has been applied on the samples before storage. In this condition with a hydrohalogenic acid pretreatment (Fig. 13), we do not observe any cluster appearance despite the presence of large proportions (19%) of GeO₂ oxide compared to the sample stored under a *clean room* environment (Fig. 4). The literature shows that the Ge surface functionalization by halogen atoms such as Br can be removed by water and transformed into HBr (Ge-Br + H₂O = Ge-OH + HBr) [22]. So, we can suppose that HBr is reformed during the dissolution of the walls of the pores suboxides by the H₂O(g) and its liquid condensation (H₂O(l)) inside the porous structure (mechanism illustrated in Fig. 9(c)), which then limit the local accumulation GeO₂ due to competition between oxidation O₂ + H₂O(l) and etching by HBr, and thus prevents or delay the clusters formation.

## 5. Conclusions

Porous germanium is prone to degradation if the storage conditions are inappropriate. Major morphological evolution of monocrystalline mesoporous germanium layers after 90 days of storage under normal clean room environment conditions was investigated by SEM, with an increase of porosity and the formation of clusters. Surface composition

<figure><img src="image_8.png" /><figcaption>**Fig. 11.** a) XPS Ge3d spectrum, b) cross-section SEM images, c) Top SEM view and d) 5 × 5µm surface roughness made by AFM of porous germanium stored in light moist O₂, after 3 months of storage.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 12. a) Proportions of Ge oxides evolution during 90 days under Argon and Oxygen atmosphere with humidity and light, with initial HBr treatment before storage (comparable to Fig. 7 (b)).</figcaption></figure>

analysis by XPS demonstrated that the changes in the porous structure are related to the presence of oxides during its aging. Each component of the storage environment was then studied independently and combined to identify precisely the mechanisms responsible for the degradation of the Ge<sub>porous</sub> layers. Chemical characterizations as a function of time showed that the order of appearance of the Ge suboxides and oxides is always sequenced in the same way: Ge<sub>2</sub>O, GeO, Ge<sub>2</sub>O<sub>3</sub>, and finally GeO<sub>2</sub>. The first exposition of the sample to the air induces instantly the formation of a native suboxide layer composed mostly of Ge<sub>2</sub>O. After only one day of aging, we also noticed the GeO and Ge<sub>2</sub>O<sub>3</sub> creation. Depending on the condition of storage, these suboxides then decrease to give way to GeO<sub>2</sub> oxide. Associated with SEM images of the porous germanium over time, these observations allowed us to conclude that this final oxide GeO<sub>2</sub> is responsible for the cluster formation and the enlargement of the pores by consumption of Ge and suboxides. Concerning the ambient parameters, we saw that ambient dioxygen (O<sub>2</sub>) and humidity (H<sub>2</sub>O<sub>(g)</sub>) are not individually responsible for GeO<sub>2</sub> production and the related morphological deterioration of the porous structure. This destructive oxidation solely appears when these two storage environmental components are combined. Moreover, when we add light to these aging conditions, we observe an increase in the oxidation effect and faster degradation of the Ge<sub>porous</sub> layers. This study also highlights that different options are possible to preserve the integrity of germanium samples during storage and increase the shelf life above several months. An encapsulation under a dry neutral atmosphere (Argon) with silicate

<figure><img src="image_10.png" /><figcaption>Fig. 13. a) XPS Ge3d spectrum, b) cross-section SEM images, c) Top SEM view and d) 5 × 5µm surface roughness made by AFM of porous germanium with initial post-porosification treatment stored in light moist O<sub>2</sub>, after 3 months of storage.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 14. Comparison of the proportions' evolution of GeO<sub>2</sub> oxides (Ge4+) over 90 days for the different conditions studied. The hatched area delimits the period when the porous layer begins to morphologically evolve. Curves with filled points represent the samples that show morphological evolution, while curves with empty points represent the samples that did not change.</figcaption></figure>
beads in hermetic metallic bags or an initial HBr wet treatment is very efficient in reducing or delaying the formation of oxides and thus the potential destruction of the pores. Thanks to these solutions, we can limit the GeO₂ local accumulation that seems to be responsible for the cluster's emergence. These techniques can thus be used to increase the shelf life of porous germanium.

## Copyright

For the purpose of Open Access, a CC-BY public copyright licence has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission.

## Acknowledgment and funding statement

The authors thank Bouraoui Ilahi for the scientific discussions, Alexandre Chapotot for the AFM characterizations, and the 3IT's clean room staff for technical support. The Natural Sciences and Engineering Research Council of Canada (NSERC), Innovation en énergie électrique (InnovÉÉ), Mitacs, and Umicore and Saint-Augustin Canada Electric (Stace) for financial support. LN2 is a joint International Research Laboratory (IRL 3463) funded and co-operated in Canada by Université de Sherbrooke 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). Abderraouf Boucherif is grateful for a Discovery grant supporting this work.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

Abderraouf Boucherif reports financial support was provided by Umicore. Abderraouf Boucherif reports financial support was provided by InnovÉÉ. Maxime Darnon reports financial support was provided by Natural Sciences and Engineering Research Council of Canada. Maxime Darnon reports financial support was provided by Mitacs Canada. Abderraouf Boucherif reports financial support was provided by Saint-Augustin Canada Electric (Stace). Maxime Darnon reports financial support was provided by Quebec Research Fund Nature and Technology. If there are other authors, they 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.

## References

[1] D.P. Brunco, B. De Jaeger, G. Eneman, A. Satta, V. Terzieva, L. Souriau, F.E. Leys, G. Pourtois, M. Houssa, K. Opsomer, G. Nicholas, M. Meuris, M. Heyns, ECS Trans 11 (2007) 479.

[2] R. Soref, Nat Photonics 4 (2010) 495.

[3] G. Brammertz, Y. Mols, S. Degroote, M. Leys, J. Van Steenbergen, G. Borghs, M. Caymax, J. Cryst Growth 297 (2006) 204.

[4] S.P. Tobin, S.M. Vernon, C. Bajgar, V.E. Haven, L.M. Geoffroy, D.R. Lillington, IEEE Electr Dev Lett 9 (1988) 256.

[5] K. Mishra, X.-C. Liu, F.-S. Ke, X.-D. Zhou, Compos Part B Eng 163 (2019) 158.

[6] R. Zegadi, N. Lorrain, L. Bodiou, M. Guendouz, L. Ziet, J. Charrier, J Opt 23 (2021) 035102.

[7] N. Paupy, Z. Oulad Elhmaidi, A. Chapotot, T. Hanuš, J. Arias-Zapata, B. Ilahi, A. Heintz, A.B. Poungoué Mbeunni, 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, Nanoscale Adv 18 (2023) 4613.

[8] J.P. Toinin, A. Portavoce, K. Hoummada, M. Texier, M. Bertoglio, S. Bernardini, M. Abbarchi, L. Chow, Beilstein J Nanotechnol 6 (2015) 336.

[9] C. Jing, C. Zhang, X. Zang, W. Zhou, W. Bai, T. Lin, J. Chu, Sci Technol Adv Mater 10 (2009) 065001.

[10] H. Yin, W. Xiao, X. Mao, H. Zhu, D. Wang, J Mater Chem A 3 (2015) 1427.

[11] E. Garralaga Rojas, H. Plagwitz, B. Terheiden, J. Hensen, C. Baur, G. La Roche, G.F. X. Strobl, R. Brendel, J Electrochem Soc 156 (2009) D310.

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

[13] D. Guzmán, M. Cruz, C. Wang, Microelectron J 39 (2008) 523.

[14] T. Hanuš, J. Arias-Zapata, B. Ilahi, P. Provost, J. Cho, K. Dessein, A. Boucherif, Adv Mater Interfaces 10 (2023) 2202495.

[15] V. Daniel, J. Chrétien, G. Hamon, M. De Lafontaine, N. Paupy, Z.O. El Hmaidi, B. Ilahi, T. Hanus, M. Darnon, A. Boucherif, 2022 IEEE 49th Photovoltaics Specialists Conference (PVSC), Philadelphia, PA, USA, IEEE, 2022, pp. 0770–0772.

[16] X. Wang, Z. Zhao, J. Xiang, W. Wang, J. Zhang, C. Zhao, T. Ye, Appl Surf Sci 390 (2016) 472.

[17] J. Oh, J.C. Campbell, J Electron Mater 33 (2004) 364.

[18] A.J. Santos, B. Lacroix, F. Maudet, A. Corvisier, F. Paumier, C. Dupeyrat, T. Girardeau, R. García, F.M. Morales, Appl Surf Sci 493 (2019) 807.

[19] S.D. Wolter, T. Tyler, N.M. Jokerst, Thin Solid Films 522 (2012) 217.

[20] K. Prabhakaran, T. Ogino, Surf Sci (1995) 263.

[21] B. Onsia, T. Conard, S. De Gendt, M.M. Heyns, I. Hoflijk, P.W. Mertens, M. Meuris, G. Raskin, S. Sioncke, I. Teerlinck, A. Theuwis, J. Van Steenbergen, C. Vinckier, Solid State Phenom 103–104 (2005) 27.

[22] S. Sun, Y. Sun, Z. Liu, D.-I. Lee, P. Pianetta, Appl Phys Lett 89 (2006) 231925.

[23] M. Jin, O. Veselý, C.J. Heard, M. Kubů, P. Nachtigall, J. Čejka, L. Grajciar, J Phys Chem C 125 (2021) 23744.

[24] S. Sato, S. Nozaki, H. Morisaki, J Appl Phys 81 (1997) 1518.

[25] H. Jagannathan, J. Kim, M. Deal, M. Kelly, Y. Nishi, ECS Trans 3 (2006) 1175.

[26] A. Molina, J.R. Shallenberger, S.E. Mohney, J Vac Sci Technol Vac Surf Films 38 (2020) 023208.