Large‐Scale Formation of Uniform Porous Ge Nanostructures with Tunable Physical Properties

Abstract

Porous germanium (PGe) nanostructures attract a lot of attention for various emerging applications due to their unique properties. Consequently, there is an increasing need for the development of low-cost synthesis routes that are compatible with large-scale production. Bipolar electrochemical etching (BEE) is a widely used solution for producing porous Ge layers. However, the lack of controllable production of large-scale uniform PGe layers is the limiting factor for mainstream applications. Large-scale homogeneous PGe layers formation is demonstrated by improving the BEE process. The PGe structures demonstrate excellent homogeneity in thickness and porosity, with a relative variation of below 2% across the 100 mm wafer. Furthermore, this process enables accurate tuning of the PGe's physical properties through variation of the etching parameters. PGe structures with porosity ranging from 40% to 80% and an adjustable thickness, while preserving low surface roughness are demonstrated, giving access to a large variety of PGe nanostructures for a wide range of applications. Ellipsometry and X-ray reflectivity are employed to measure the porosity and thickness of PGe layers, providing fast and non-destructive methods of characterization. These findings lay the groundwork for the large-scale production of high-quality PGe layers with on-demand characteristics.

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Large-Scale Formation of Uniform Porous Ge
Nanostructures with Tunable Physical Properties
Tadeáš Hanuš,* Javier Arias-Zapata, Bouraoui Ilahi, Philippe-Olivier Provost,
Jinyoun Cho, Kristof Dessein, and Abderraouf Boucherif*
DOI: 10.1002/admi.202202495
a potential in wide range of implementa-
tions such as energy storage systems,[5–10]
thermoelectric devices,[11] sensors,[12–14]
optoelectronics,[15,16] or synthesis of nano-
composite materials.[17–19] Moreover, PGe
has recently been demonstrated as an e-
cient virtual substrate for epitaxial growth
of detachable Ge membranes[20] and III-V
heterostructures with high crystalline
quality[21,22] paving the way to direct appli-
cation in the development of lightweight
and flexible photovoltaics and optoelec-
tronics.[23,24] Nevertheless, to bring these
applications to the real world, a large-
scale formation of homogeneous PGe
layers with on demand characteristics is
necessary.
The fabrication of PGe nanostruc-
tures was demonstrated using techniques
such as thermal reduction of GeO2 nano-
particles,[25] oxidative polymerization of
the deltahedral [Ge9]4− cluster,[26] spark
processing,[27] reduction–alloying–deal-
loying approach,[28] ion implantation,[29,30]
growth by Molecular Beam epitaxy,[15]
coupled plasma chemical vapor deposi-
tion,[31] metal-assisted chemical etching,[32] lithography and dry
etching,[23] and electrochemical etching.[33–35] Some of the major
challenges of the aforesaid techniques are the use of expensive
precursors, high investment in equipment and labor, low yields,
random crystallite orientation, low purity of PGe structures,
and intricate procedures making them nonviable for low-cost/
large-scale production. The electrochemical etching is a simple
Porous germanium (PGe) nanostructures attract a lot of attention for various
emerging applications due to their unique properties. Consequently, there is
an increasing need for the development of low-cost synthesis routes that are
compatible with large-scale production. Bipolar electrochemical etching (BEE)
is a widely used solution for producing porous Ge layers. However, the lack
of controllable production of large-scale uniform PGe layers is the limiting
factor for mainstream applications. Large-scale homogeneous PGe layers
formation is demonstrated by improving the BEE process. The PGe structures
demonstrate excellent homogeneity in thickness and porosity, with a relative
variation of below 2% across the 100 mm wafer. Furthermore, this process
enables accurate tuning of the PGe’s physical properties through variation
of the etching parameters. PGe structures with porosity ranging from 40%
to 80% and an adjustable thickness, while preserving low surface roughness
are demonstrated, giving access to a large variety of PGe nanostructures for
a wide range of applications. Ellipsometry and X-ray reflectivity are employed
to measure the porosity and thickness of PGe layers, providing fast and non-
destructive methods of characterization. These findings lay the groundwork
for the large-scale production of high-quality PGe layers with on-demand
characteristics.
© 2023 The Authors. Advanced Materials Interfaces published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution
and reproduction in any medium, provided the original work is properly
cited.
ReseaRch aRticle
1. Introduction
Porous semiconductor materials have received an increasing
interest for both fundamental research and advanced
applications owing to their unique mechanical and phys-
icochemical properties compared to their bulk material
counterparts.[1–4] Porous germanium (PGe) in particular shows
T. Hanuš, J. Arias-Zapata, B. Ilahi, P.-O. Provost, A. Boucherif
Institut Interdisciplinaire d’Innovation Technologique (3IT)
Université de Sherbrooke
3000 Boulevard de l’Université, Sherbrooke, Quebec J1K 0A5, Canada
E-mail: tadeas.hanus@usherbrooke.ca;
abderraouf.boucherif@usherbrooke.ca
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/admi.202202495.
T. Hanuš, J. Arias-Zapata, B. Ilahi, P.-O. Provost, A. Boucherif
Laboratoire Nanotechnologies Nanosystèmes (LN2) – CNRS IRL-3463
Institut Interdisciplinaire d’Innovation Technologique (3IT)
Université de Sherbrooke
3000 Boulevard Université, Sherbrooke, Québec J1K 0A5, Canada
J. Cho, K. Dessein
Umicore Electro-Optic Materials
Watertorenstraat 33, Olen 2250, Belgium
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and low-cost technique providing PGe layers with high material
purity and densely packed nanocrystals. However, its widespread
adoption remains strongly dependent on the controllable pro-
duction of homo geneous PGe layer on large surfaces.
In the last two decades, the electrochemical porosification of
germanium (Ge) experienced major advancements. The bipolar
electrochemical etching (BEE) was introduced[36,37] to overcome
the observed lateral dissolution of the PGe[34] when applying
a silicon-like unipolar anodic etching[38,39] on Ge substrate.
Accordingly, adding a cathodization step allows to passivate
the porous layer formed during the previous anodization step.
BEE produced PGe with various structures and morpholo-
gies has been reported by tuning BEE etching and passivation
para meters.[16,35,40] Recently, the Fast BEE was introduced,[33]
allowing higher etching rates for thicker PGe layers production.
Since then, this technique has been used to produce tubular and
columnar morphologies,[41] broadening the implementation of
PGe as anode material for Li-ion batteries.[42] Despite the pro-
gress made in the fundamental understanding of the electro-
chemical etching of Ge[36,37,43] and the demonstrated suitability
for various emerging applications, the formation of homo-
geneous PGe layers on large surfaces and the control of its
structural properties remain dicult to achieve because of the
number of interfering factors during BEE. Indeed, para meters
such as substrate characteristics, ratio etching/passivation
pulse duration, and current density or electrolyte composition
have strong impact on the final PGe properties (porosity, thick-
ness, and morphology).[35,40] Furthermore, PGe morphological
characteristics (thickness and porosity) are often extracted
using local and destructive techniques such as scanning elec-
tron microscopy (SEM), which are not suitable for large scale
assessment of the PGe homogeneity.
In this work, we demonstrate large-scale formation of homo-
geneous PGe layers by a modified BEE. The proposed method
allows fine tuning and control of the porous layer thickness and
porosity. Accordingly, widely tunable highly uniform PGe layer
across 100mm wafer has been demonstrated. Additionally, we
show that ellipsometry mapping and X-ray reflectivity (XRR) are
very accurate and nondestructive characterization techniques,
to measure the PGe layer thickness and porosity.
2. Results and Discussions
Ge electrochemical porosification requires the use of BEE,
where the anodic pulses enable eective etching, and the
cathodic pulses protect the porous structure from dissolution
during the etching step.[36,37] This additional complexity, com-
pared to Si porosification, makes the formation of homog-
enous porous Ge layers by BEE very challenging. When
applying standard symmetrical BBE process on large surface,
the resulting layers exhibit inhomogeneous surface colors as
demonstrated by Figure S1, Supporting Information. Typical
cross-sectional SEM image of porous Ge structures obtained
by standard BEE process (Figure 1a) shows the lateral etching
induced damages that locally alters the PGe layer’s quality.
The origin of these inhomogeneities can be attributed to an
excessive formation of hydrogen gas during the BEE. The
mechanism of Ge passivation step has already been described
in previous works,[33,40] showing that the formation of hydrogen
terminations on the surface of the PGe structure, protects it
against dissolution during the subsequent etching step. As a
biproduct of this reaction, hydrogen gas is also formed inside
the structure. The growth and evolution of H2 bubbles generate
a pressure on the pore walls inducing physical damages to the
small and fragile PGe crystallites. Moreover, the large H2 bub-
bles stuck inside of the pores can isolate parts of the structure
from the system, locally causing etching in lateral direction,
which is detrimental for the porous layer homogeneity over the
large surface. Figure 1b schematically illustrates the accumu-
lation of the H2 bubbles inside the porous structure and the
consequent potential damage that may locally occur during the
porous layer formation.
To overcome these undesirable passivation eects, we intro-
duced some modifications to commonly used BEE condi-
tions. Indeed, to reduce the quantity of produced H2 gas, we
employed a low passivation current density of 1 mA cm−2.
Additionally, a rest time has been introduced at the end of each
etching/passivation cycle. The extra time after each cycle let
the system reach the equilibrium potential as well as it enables
the H2 gas to escape from the porous structure as shown in
Figure1b. Moreover, the proportion of ethanol in the electro-
lyte is increased by 20% to reduce the surface tension of the
solution.[44] This helps releasing the residual H2 bubbles, and
facilitating the electrolyte penetration inside the nanoscale
sized porous structure enabling an ecient diusion of the
ions toward the bottom of the pores. As a result, the introduced
modifications increase the overall stability of the process, inhib-
iting the damage of the PGe layers during the etching while
enabling homo geneous PGe structures formation over large
surfaces. Using this improved BEE recipe, well-defined PGe
structure with good lateral and in-depth uniformity has been
obtained as revealed by SEM micrograph (Figure1c).
To date conventional porosification cells for electrochemical
etching of semiconductors employ a clamping mechanism to
maintain the wafer inside the cell and to seal the reservoir for
the electrolyte.[34,45,46] Figure 2a shows typical homogenous PGe
layer produced in conventional 100 mm wafer porosification
cell. Since the edge of the wafer is isolated from the electrolyte,
it cannot be porosified leading to the formation of bulk material
rim surrounding the porous structure. Moreover, the clamping
mechanism causes additional defects/inhomogeneities in the
porous structure (as indicated by red arrows in Figure2a) near
the edges of the PGe layer. These combined eects reduce the
eective usable surface of the wafer by over 25%, which is sig-
nificant especially in case of rare and expensive material such
as Ge. Additionally, the interface between the bulk material and
the porous structure can cause formation of defect, accumula-
tion of materials and other problems for applications aiming
the use of full wafers such as epitaxial growth of heterostruc-
tures. To avoid these problems additional steps such as laser
cutting or mechanical grinding to remove the rim[47] need to
be undertaken, increasing the fabrication process’s cost and
complexity. Most industrial fabrication processes are developed
to work with the whole wafers to ensure the highest possible
eciency. Consequently, any unusable parts of the substrate
or additional steps will have a negative impact on process
throughput. To overcome this issue, we have developed custom
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design[48] of porosification cell enabling the porosification of the
entire wafer’s surface (edge included). Schematic illustration of
this design is shown in Figure 3b along with the produced uni-
form edge-to-edge PGe layer on full 100mm wafer (Figure3c).
To study the impact of the etching parameters on the PGe
layer’s uniformity, the etching current density was varied
between 0.5 and 5.0 mA cm−2. Indeed, as shown in Figure3,
the improved stability of the proposed BEE recipe enables the
etching current density variation while preserving the PGe
layer’s uniformity. Indeed, Figure 3a shows a linear increase
of etching rate with etching current density. Compared to
previously reported data,[33] homogeneous PGe layers can be
produced even above 2.5 mA cm−2 eectively avoiding the lat-
eral dissolution at high etching current densities. This allows
to achieve high etching rates of above 40 nm min−1, being
previously reported only by Fast BEE.[33] Moreover, for a given
current density, the PGe layer thickness is found to increase lin-
early over time (Figure3b). This testifies that the etching rate
remains constant during the BEE process. This characteristic
enables time-based thickness modulation of PGe layers from
few nm up to 4µm. Furthermore, the porosity can be success-
fully tuned from 40% to 80% by varying the etching current
Figure 2. a) 100mm Ge wafer with homogenous PGe layer produced in conventional porosification cell. Red flashes indicate defects in PGe layer at
the edges. b) Schematic illustration of custom design porosification cell enabling edge-to-edge wafer porosification. c) Edge-to-edge homogenous PGe
layer with mirror finish produced in custom made 100mm cell.
Figure 1. a) Typical SEM micrograph of damaged PGe structure due to the lateral etching formed with 2 mA cm−2 etching current density. b) Schematic
illustration of H2 gas formation I) accumulation of H2 inside the porous structure and damage it causes due to the physical pressure and local insula-
tion inside of pores causing lateral etching. II) H2 release during the rest time step in between etching cycles, preventing damage to the PGe structure.
c) SEM micrograph of well-defined PGe structure etched with improved BEE process and with 2 mA cm−2 etching current density.
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density within the selected range, providing the formation of
adjustable medium to high porosity layers (Figure 3c). Two
main porosification regimes can be distinguished: I) From 0.5
to 2.0 mA cm−2, porosity exhibits high porosity layers to the
etching current density, enabling wide range of porosity varia-
tion between 40% and 70%. II) Meanwhile, for an etching cur-
rent density ranging from 2.0 to 5.0 mA cm−2, the porosity is
found to vary only between 70% and 80% allowing very fine
porosity modulation in this regime.
For all porosities the PGe structure is formed by inter-
connected mesopores, separated by Ge skeleton. Although
the pores are disordered at a short range, a certain degree of
ordering can be detected at a long range, particularly in the
case of high porosities (Figure3d–i). This shows that the PGe
layers maintain its sponge-like morphology, regardless of the
etching current density. This is possible thanks to the high
degree of passivation. Other morphologies such as “pine-tree”
and “fish-bone,” have been reported in the literature for lower
degrees of passivation.[40] While the porosity of the PGe layer
vary as a function of the etching current density, the pore size
seems to remain the same as shown by the cross-sectional
SEM images of PGe structures with porosities between 40%
and 80% in Figure 3d–i. The image processing reveals an
average pore diameter, around 5 ± 1nm across all the porosi-
ties. This value is consistent with the observations by trans-
mission electron microscopy (TEM) presented in Figure5a as
well as with the values indicated in literature.[33,40] The obtained
average pore size classifies at the lower end of the mesoporous
size domain. The provided flexibility in tuning of PGe struc-
ture’s physical properties enables the possibility of on-demand
porous layers formation, depending on the desired character-
istics. Many applications can take advantage of this kind of
versatility such as energy storage systems,[7] thermoelectric
devices,[11] or nanoengineered compliant substrates for epitaxial
growth.[21,23]
The demonstration of wafer-scale production and use of
porous Ge substrates for various applications, comes with a
crucial need to develop fast and nondestructive characteriza-
tion methods easily applicable for post-production PGe wafer’s
quality assessment. To date, PGe layers are mainly character-
ized by SEM. Accordingly, we have employed this method
on various locations along the PGe wafer’s diameter as refer-
ence data to assess the accuracy of the nondestructive charac-
terization techniques. Figure1c shows a well-defined interface
between the PGe layer with sponge-like morphology and bulk
Ge material. The Figure 4a shows that the PGe layer thick-
ness remains constant along the wafer’s diameter with a mean
value of 206 ± 4nm (Figure 4b). The SEM can also be used to
evaluate the porosity, using image treatment software. In the
present case the porosity value is estimated to be 67 ± 12%.
To assess this estimation using nondestructive technique,
we first employed XRR measurement to precisely determine
the porosity. Indeed, it allows to measure the critical angle of
porous layer, which is directly linked to the material’s density
Figure 3. a) Linear growth of etching rate of PGe layer with increasing current density. b) Linear increase of PGe layer thickness over etching duration
for 1, 2, and 4 mA cm−2 etching current densities. c) Porosity of the PGe structure versus applied etching current density variation with I) corresponding
to domain with fast porosity increase from medium to high porosity and II) to slow progressive increase in high porosity domain. d–i) Cross-sectional
SEM micrographs of PGe structures etched with 0.50, 0.75, 1.00, 1.50, 2.00, and 5.00 mA cm−2 corresponding to 40%, 45%, 53%, 65%, 71%, and
80% porosity, respectively.
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and therefore its corresponding porosity. As the porosity of
the layer increases the value of critical angle decreases. This
enables the distinction between the critical angle of PGe layer
(θPGe) and of the bulk Ge (θGe) as shown in Figure 4b. The
porosity can therefore be calculated using Equation (1). Varia-
tion of PGe critical angle with porosity is shown in Figure S2,
Supporting Information. XRR enables non-destructive determi-
nation of porosity without relying on indirect image processing
algorithms. Figure 4c shows radial profile of porosity of full
wafer with an average value of 72 ± 2%. This result agrees with
the estimation made by SEM images treatment. These methods
give us good indication about the uniformity of the PGe layers,
but they are still, not suitable for the fast feedback loop neces-
sary for large-scale production.
Regardless of the precision and the accuracy of the provided
information (layer’s porosity, thickness, and homogeneity),
SEM remains local and destructive method, which is nonrep-
resentative of the whole PGe wafer’s surface and unsuitable
for quality control in production line. On the other hand, XRR
is nondestructive and production-line compatible, but it does
not oer reliable measurement of the PGe thickness. For this
reason, we employed more complete fast and non-destructive
ellipsometry measurements to access both thickness and
porosity at the same time. Mapping with over 100 measurement
points was performed to evaluate uniformity of the PGe over the
entire 100mm wafer as shown in Figure4d,e. The mean thick-
ness of the PGe layer is evaluated to be 207± 3nm (Figure4d).
In terms of porosity, the mean value is 72 ± 1% as shown in
Figure 4e. These results demonstrate excellent uniformity of
the PGe layer over the wafer’s surface with a standard deviation
below 2% for both thickness and porosity obtained by custom-
designed porosification cell and optimized BEE recipe (similar
results are obtained for medium porosity layers as shown in
Figure S3, Supporting Information). Moreover, the obtained
data are consistent with both SEM and XRR measurements,
making the ellipsometry mapping a fast, accurate, and nonde-
structive technique suitable for fast feedback characterization of
PGe layers.
For applications such as epitaxial growth, the crystalline
quality, and surface morphology are crucial characteristics
of the substrate. To further quality investigation of fabricated
PGe layers high resolution transmission electron microscopy
(HRTEM), X-ray diraction (XRD), and atomic force micro-
scopy (AFM) are used. The HRTEM image of a typical PGe
structure made by BEE (Figure 5a) shows Ge atoms oriented
in crystalline structure without any observable presence of
amorphous phase or oxides on the surface of the crystallites.
To investigate if there is any bending of the crystallites in the
Figure 4. a) Thickness variation of the PGe layer over diameter of 100mm wafer measured by SEM. b) Typical XRR measurement of the Ge bulk sub-
strate (Blue) and PGe layer (Red). c) Porosity variation of the PGe layer over the diameter of the 100mm wafer calculated from XRR measurements.
d,e) Mapping of the thickness and porosity of the PGe layer over the surface of 100mm wafer measured by ellipsometry.
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PGe structure, a PGe layer with ≈70% porosity and ≈1 µm
thickness is characterized by XRD, since the high porosity
and thickness make this type of structure most keen to crystal
bending and misorientation. Figure 5b shows 2θ scan with
only (400) and (200)[49] peaks of Ge, without signs of any other
orientations. Combined, the HRTEM and XRD studies dem-
onstrate the crystalline nature of the porous structure, main-
taining the substrate orientation without formation of any
amorphous phase or crystal bending. Furthermore, Figure5c
shows a typical AFM scan of the PGe layer’s surface topology
of the sponge-like structure showing a smooth surface and low
RMS (root mean square) roughness below 3 nm. AFM scans
for various PGe structures can be found in Figure S4, Sup-
porting Information. These surface characteristics were found
to be the same for all the produced porous structures inde-
pendently of their porosity as can be seen in Figure 5d. The
high single oriented crystallinity combined with low surface
roughness and good lateral uniformity make these PGe layers
an excellent candidate as virtual substrate for wafer-scale epi-
taxy. Recently, it has been demonstrated that, as porosified
PGe substrates allow epitaxial growth of monocrystalline Ge
membranes.[50] It has been also showed that native oxides,
that may be formed on PGe surface following a long storage
time and/or longer period of exposure to ambient atmosphere,
can be easily removed by diluted acidic solutions such as HBr.
Oxide-free PGe surface can be obtained, allowing monocrystal-
line Ge growth on top of it.[51]
3. Conclusion
We demonstrated the fabrication of edge-to-edge, 100 mm
wafer-scale, homogenous, and reproducible PGe layers. This
is possible, thanks to a finely optimized BEE process with an
additional rest time step to enable ecient evolution of H2
produced during the passivation step. The edge-to-edge layers
enabled by custom designed porosification cell increasing
the high-quality PGe surface by over 25% per 100 mm wafer
compared to conventional porosification cells. Moreover, we
show that the PGe structural properties such as thickness
and porosity can be accurately tuned by varying the etching
parameters to create PGe layers with on-demand characteris-
tics depending on the desired application. The produced PGe
layers’ properties are easily assessable by production line com-
patible, fast, and nondestructive techniques enabling the char-
acterization of entire surface. The resulting PGe layers present
excellent homogeneity with less than 2% variation for both
thickness and porosity. The HRTEM and XRD analysis shows
that the PGe structure maintains the substrate crystalline
nature, without any misorientation of the crystallites. More-
Figure 5. a) HRTEM micrograph of high porosity structure showing the crystalline nature of the porous structure. b) XRD 2θ scan of the thick, high
porosity PGe layer showing monocrystalline nature of the structure. c) AFM scan showing smooth surface morphology of the PGe layer. d) Constant
surface roughness of the PGe layers versus etching current density (porosity).
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over, the porous substrates show a good surface topology with
RMS roughness below 3nm over the entire range of accessible
porosities. This provides an opportunity for wafer-scale epitaxial
growth of detachable III–V heterostructures for optoelectronics
and photovoltaics applications. These results demonstrate the
viability of BEE for large-scale production of high purity, crys-
talline PGe layers with on demand characteristics and lay the
groundwork for various applications of PGe structures.
4. Experimental Section
Bipolar Electrochemical Etching: Electrochemical etching of Ge was
performed in custom-designed 100mm electrochemical cell, composed
of Teflon body, solid copper working electrode, and Platinum wire
counter electrode. Gallium doped, p-type, (100) oriented Ge wafers
with 6° miscut toward (111) direction, and resistivity of 8–30 mΩ cm
were used as substrate. Prior to a galvanostatic BEE, Ge wafers were
deoxidized in the HF (49%) solution for 5min, rinsed with EtOH (99%,
Anhydrous), and dried under N2 flow. The BEE was carried out in HF
(49%):EtOH (99%, Anhydrous) (4:1, V:V) electrolyte using asymmetric
anodic (etching), cathodic (passivation) pulses, and 1 s rest time at
the end of each cycle. The passivation pulses were fixed at 1 mA cm−2
current density and 1 s pulse duration. The etching current density
varied between 0.5 and 5.0 mA cm−2 with pulse duration fixed at 1 s.
Prior to BEE, a direct current was applied to initiate the formation of
pores and to obtain their even distribution on the sample surface.[33] The
total duration of BEE varied between 2min and 1h.
Materials Characterization: Cross-sectional profile of samples was
observed by SEM using Zeiss LEO 1540 XB at 4.3mm of working distance
and 20 keV of acceleration voltage, to measure thickness of the layer.
The roughness measurements were performed, using the AFM Veeco
Dimension 3100 in tapping mode with SSS-NCHR silicon probe and with
a scan size of 5×5 and 1 × 1 µm2. The XRD and XRR measurements
were performed using Rigaku Smartlab HRXRD system with Cu Kα X-ray
source, Ge (220)×2 monochromator and HYPIX-3000 hybrid pixel array
2D detector. The Powder XRD configuration was used to investigate the
crystalline nature of the PGe layer. The XRR was used to measure the
critical angle of PGe layer. The porosity (P) was then calculated using
Equation(1) where θPGe and θGe correspond to the critical angle of the
PGe layer and of the bulk Ge, respectively.[52,53]
(1 )100
2
P
PGe
Ge
θθ
=−




× (1)
Fast feedback characterization of 100 mm wafers of PGe was
performed by ellipsometry using a J.A. Woollam Co. VASE instrument,
including mapping of the wafers. Spectral ranged from 500 to 900nm
and a model based on an eective material approximation (EMA) using
the Bruggerman analysis mode. This model used a mix of Ge and air
to represent the PGe layers on a Ge substrate,[54] allowing thickness
and porosity estimation. Ge material uses a Cody-Lorentz built-in
function to model the dielectric model of Ge as a wavelength-dependent
oscillator,[55,56] we used E0 and Eg as 6 and 1eV respectively.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors thank Mohammad Reza Azizian, Arthur Dupuy, Stéphanie
Sauze, Alexandre Heintz and Thierno Mamoudou Diallo for scientific
discussions, and providing many helpful comments and suggestions
throughout the project. The authors appreciate helpful advice from
Olivier Marconot and Hubert Pelletier concerning XRD, and are
grateful to René Labrecque, Julie Ménard and all the technical sta
of 3IT for the technical support. The authors thank Umicore, Saint-
Augustin Canada Electric (Stace), Innovation en énergie 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. The authors also thank Canadian
Centre for Electron Microscopy for TEM analysis. 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).
Conflict of Interest
The authors declare no conflict of interest.
Author Contributions
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript. T.H.:
Conceptualization, Methodology, Investigation, Data curation, Original
draft preparation, Review and Editing, Visualization. J.A.Z.: Investigation,
Supervision, Validation, Review and Editing. B.I.: Supervision, Validation,
Review and Editing. P.-O.P.: Conceptualization, Review and Editing. J.C.:
Validation, Review and Editing. K.D.: Validation, Review and Editing.
A.B.: Supervision, Validation, Review and Editing.
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Keywords
electrochemical etching, nanostructures, porous germanium, porous
substrate, wafer-scale
Received: December 20, 2022
Revised: February 21, 2023
Published online: April 13, 2023
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