Thermal Stabilization of Porous Silicon: A Key Step for High-Quality SiGe Epitaxy

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Algarni, A. K. Aladim, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7137558/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study focuses on the epitaxial growth of virtual silicon-germanium (SiGe) substrates on porous silicon (PSi). Epitaxy was performed on different types of PSi substrates, with or without prior thermal annealing. Morphological and structural investigations by atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) show that epitaxial SiGe films grown on double-layer PSi substrates, annealed at 1000°C, exhibit significantly higher crystalline quality than those grown on unannealed PSi substrates or PSi substrates annealed at temperatures lower than or equal to 900°C. This improvement is attributed to the beneficial effects of 1000°C annealing, which leads to stress relaxation, internal microstructure stabilization and significant improvement of PSi surface morphology. In contrast, direct growth of SiGe on unannealed PSi, even at moderate temperature (~ 400°C), induces structural degradation of the porous buffer, leading to a very high dislocation density in the epitaxially grown SiGe films. A well-optimized thermal treatment of PSi substrates promotes the growth of high-quality virtual SiGe substrates on PSi that is both efficient and economically viable for the development of SiGe-based photovoltaic cells. Porous silicon Molecular beam epitaxy Thin SiGe films Transmission electron microscopy Pre-annealing X-ray diffraction (XRD). Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Using silicon as a substrate for the growth of solar cells and electronic components based on SiGe films offers numerous advantages, including its widespread availability, low cost, and compatibility with the conventional CMOS industrial platform [ 1 – 4 ]. However, the difference in lattice parameters (4.2%) between silicon and germanium poses a major challenge. It generates mechanical stress in epitaxially grown SiGe films, causing either elastic relaxation via the formation of 3D nanostructures or plastic relaxation via the generation of crystal defects such as dislocations and cracks [ 5 – 7 ]. To overcome this obstacle, a conventional solution is to use a relaxed SiGe buffer layer, known as a virtual pseudo-substrate [ 8 – 10 ]. This concept, proposed by Lo et al. in 1991, is based on the use of a substrate capable of elastically or plastically deforming during growth, allowing the epitaxial layer to return to its unconstrained lattice parameter while minimizing dislocation density[ 11 ]. However, this approach requires the growth of very thick buffer layers (several microns), which results in high manufacturing costs, making them incompatible with large-scale industrial integration. To address this, several alternatives have been explored, including the use of porous silicon as a "compliant" buffer layer. The work of Calabrese et al. demonstrated the feasibility of directly growing germanium on porous silicon to compensate for the lattice mismatch. Ge deposition is performed using the LEPECVD (Low-Energy Plasma-Enhanced Chemical Vapor Deposition) technique, with a gradual increase in temperature from 500°C to 750°C [ 12 ]. The best results were obtained for a porosity of 22%, leading to a dislocation density of around 10 6 cm -2 for 5 µm of Ge on porous Si [ 13 ]. Boucherif and Lysenko proposed a strategy based on the controlled oxidation of porous silicon to adjust its lattice parameter [ 14 , 15 ]. This process causes a volume expansion of the substrate, generating a tensile stress in the thin epitaxial monocrystalline layer. The lattice parameter of the latter then increases and approaches that of germanium. By adjusting the porosity and/or the oxidation rate, this parameter can be precisely adapted to device requirements. This method achieved a dislocation density of around 6 × 10 6 cm -2 for thin (50 nm) layers of Si 0.72 Ge 0.28 [ 16 – 18 ]. This work aims to demonstrate, first, the technical feasibility and economic viability of manufacturing thermally treated porous SiGe/Si double-layer virtual substrates, free of crystal defects and using a reduced amount of germanium. Second, it proposes an optimized technological process for the growth of mismatched lattice buffer layers. This process aims to reduce the density of emerging dislocations, responsible for current leakage in electronic devices, thus improving their efficiency, breakdown voltage, and reliability critical parameters for their industrial competitiveness. The proposed method is applicable to large porous double-layer silicon wafers (Si (001) orientation) and can be easily integrated into existing industrial processes. It is based on a strategy inspired by phenomena already documented in the literature, aimed at limiting dislocation generation. It involves two key steps: i) the formation of porous silicon films with different porosities by electrochemical anodization; ii) high-temperature annealing to relieve stress and improve the crystalline quality of the epitaxial layer. Adopting this approach paves the way for efficient integration of multi-junction SiGe solar cells on silicon, as well as various microelectronics applications. Experimental 1) Preparation of porous silicon (PSi) substrates: In this study, two distinct types of porous silicon (PSi) substrates were prepared via controlled electrochemical etching. The first type consists of a single layer of porous silicon with a thickness of 270 nm, while the second type has double-layer architecture: a 400 nm top layer with low porosity and a 7 micron bottom layer with high porosity, optimized for the layer transfer process. For the first substrate, p-type silicon wafers with a (100) crystallographic orientation and a low resistivity of 0.01 Ω•cm were used. These wafers were anodized in a 35% hydrogen fluoride (HF) solution under a constant current of 10 mA/cm 2 . This method allows for the formation of a uniformly porous silicon layer with approximately 30% porosity, which is ideal for epitaxial growth. For the fabrication of the double-layer substrate, the same silicon wafers were used. The etching of the upper layer was carried out under identical conditions as for the first substrate, ensuring a similar porosity. However, for the formation of the lower layer, with a thickness of 7 microns, the current density was increased abruptly from 10 to 80 mA/cm² which led to a significantly higher porosity. This layered configuration is particularly suitable for film growth, especially for photovoltaic applications. The properties of the as-etched PSi substrate are indicated in the following Table 1 : Table 1 Characteristics of the two types of porous silicon substrates. Sample Thickness Porosity PSi 0.27 µm 20% PSi-DL LPL: 0.4 µm 20% HPL: 7 µm 35% 2) Heat treatment of porous silicon: The double-layer PSi substrates then underwent a rigorous heat treatment to refine their morphological and crystallographic properties, resulting in the formation of sintered porous silicon. This annealing process was performed in an ultra-high vacuum chemical vapor deposition (UHVCVD) reactor under a pure hydrogen (H 2 ) atmosphere at temperatures ranging from 900°C to 1100°C for 30 seconds. The objective of this heat treatment was to modify the structural and morphological properties of the porous layers, to close the surface pores, to minimize surface roughness, and to control mechanical stresses within the porous layers in order to improve the adhesion of future nanostructures for their integration into advanced optoelectronic devices. The heated porous silicon samples are listed in the following Table 2 : Table 2 Characteristics of the heated porous silicon samples substrates. Sample Heating temperature PSi-DL_900°C 900°C PSi-DL_1100°C 1100°C 3) Morphological and structural characterization: After substrate preparation, a thorough characterization was performed to assess the morphological and structural properties of the different porous layers. Surface observations were performed using a Lyra TESCAN scanning electron microscope (SEM), which provides nanometric images of the porous structures and measures the thickness of the layers and the dimensions of the pores. In parallel, atomic force microscopy (AFM) analyses were performed using a Parkway XE100 microscope, providing detailed information on the surface roughness. Structurally, the samples were examined using a JEOL 2010F transmission electron microscope (TEM), providing high-resolution images of the pores and porous layers, as well as the transition zones between the layers. X-ray diffraction (XRD) measurements were also performed to determine the crystalline quality, assess the mechanical stresses in the porous layers, and analyze their evolution after thermal treatments. 4) MBE Growth of SiGe thin films: The growth of SiGe on porous silicon occurs at the nanometric scale, where only a limited number of germanium and silicon atoms are available. This makes the crystalline quality of the SiGe structure highly susceptible to residual gas contamination. To ensure the necessary purity, we perform the growth in an ultra-high vacuum (UHV) environment within a molecular beam epitaxy (MBE) chamber, which provides optimal conditions for precise epitaxial growth. MBE reactors are particularly suited to this high-purity process due to their advanced dual-pumping systems. Initially, a turbomolecular pump reduces the pressure to approximately 10 − 8 torr, followed by an ionic pump that further reduces residual gases, achieving a pressure below 10 –11 torr. To enhance this ultra-clean environment, the reactor walls are cooled to approximately − 60°C using liquid nitrogen. This cooling causes residual molecules to condense on the walls, keeping them away from the sample and ensuring high structural purity of the deposited films. Prior to growth, the porous silicon (PSi) samples undergo rigorous cleaning to remove native silicon dioxide and contaminants and prepare a clean surface. Samples are first immersed in a Piranha solution, a 1:3 volume mixture of H₂O₂ (30%) and H₂SO₄ (96%) at 80°C for 10 minutes, to eliminate organic residues. After thorough rinsing with deionized (DI) water, a second cleaning with a H₂O₂ solution (1:10) is performed, followed by a final DI water rinse. The cleaned samples are then carefully introduced into a Riber 32-type ultra-high vacuum solid-source molecular beam epitaxy (UHV-SSMBE) chamber. This MBE reactor operates at extremely low residual pressures, maintaining a pristine environment ideal for high-purity growth [ 18 – 20 ]. The reactor's design enables precise control over the composition and thickness of silicon-germanium (SiGe) layers, which is critical for producing the high-quality films required in advanced electronic and photonic applications. The UHV-SSMBE system enables the fabrication of complex heteroepitaxial structures with nanometric precision—an essential requirement for optoelectronic devices such as semiconductor lasers and infrared detectors, where structural integrity critically influences performance [ 21 – 32 ]. Before growth, the samples undergo a degassing process at 550°C for 30 minutes, effectively desorbing any remaining surface gases and optimizing the surface for subsequent deposition. The SiGe thin films are then deposited at a carefully controlled temperature of 520°C, ensuring layer by layer growth mode of these advanced materials. Following growth, the epitaxial nanostructures on the various types of PSi substrates are characterized morphologically and structurally. Advanced scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques are employed to assess the growth quality, analyze the size and distribution of quantum dots, and verify interface integrity. X-ray diffraction (XRD) analyses are also conducted to evaluate crystal quality and identify any residual stresses in the epitaxial structures. Finding and discussion 1) Morphological and structural properties of as-etched PSi substrates Figures 1 (a) and 1 (b) illustrate, respectively, low and high-magnification AFM images acquired on the surface of the as-etched porous silicon (PSi) monolayer substrate. These images highlight the formation of a homogeneous and regular porous surface, free of anomalies, with extremely fine pores with an average diameter of about 40 nm. The top layer of the PSi double-layer has a surface porosity similar to that of the monolayer and comparable morphology and roughness. Figures 2 (a) and 2 (b) present TEM images of the porous silicon monolayer at low and high magnifications, respectively. These images reveal a single porous layer with uniform porosity, where the porous columns display a branched classical morphology characteristic of P-type porous silicon layer, as reported in the literature. This observation confirms the consistency and high quality of the etching process. The inset in Figure (2-a) shows a selected area electron diffraction (SAED) pattern of the silicon layer, confirming its crystallinity. Figures 2 (c) and 2 (d) provide TEM images of the porous silicon double-layer at low and high magnifications, respectively. These images clearly depict two distinct porous layers with different porosities. The upper layer, with a lower porosity (20%), was specifically designed to undergo thermal transformation into a continuous silicon film. In contrast, the lower layer, with a higher porosity (35%), was engineered to maintain its porous structure even under high-temperature heat treatments. The inset in Fig. 2 (c) presents selected area electron diffraction (SAED) pattern of the silicon layer, further verifying its crystalline nature. This intentional differentiation in porosity between the layers, combined with their crystalline properties, enables precise control over the mechanical and thermal characteristics of the final material. Such control is crucial for potential applications, particularly in the fabrication of solar cells using the layer transfer process (LTP). X-ray diffraction (XRD) is a fundamental technique for the characterization of porous substrates, providing detailed information on the crystal structure, lattice parameters, and internal stresses. It is essential to analyze the mechanical and structural properties of porous silicon (PSi) films, thus optimizing their performance in electronic and optoelectronic applications. XRD analysis provides an accurate assessment of the effects of porosity and manufacturing processes, which is crucial for the design of advanced devices with specified characteristics and improved performance. Figures 3 (a) and 3(b) respectively illustrate the X-ray diffraction (XRD) profiles of the as-etched porous silicon single-layer (PSi-ML) and double-layer (PSi-DL) substrates. These profiles reveal the typical features of porous silicon layers in contact with a crystalline silicon substrate, displaying two distinctive peaks. The XRD peak with the highest intensity, centered at 34.56°, corresponds to the monocrystalline silicon, while the lower intensity peak, centered at 34.54°, is attributed to the strained PSi layers. The shift of the XRD peak of the PSi layer to a slightly lower angle compared to the bulk silicon peak indicates the presence of an out-of-plane tensile strain. This shift suggests that the lattice parameter of the silicon nanocrystallites forming the porous layer is slightly higher than that of the bulk silicon. This increase in the lattice parameter is probably due to the presence of oxide formed during the electrochemical anodization process, which induces structural defects and slightly modifies the silicon crystal structure in the porous layer. As a result, a slight out-of-plane expansion, reflecting a tensile strain, is observed in the porous layer and confirmed by XRD mapping shown in Fig. 3 (c). This strain is even more pronounced in the PSi double-layer configuration due to the cumulative effect of the increased thickness of the successive porous layers. This amplification of internal stress makes the double-layer structure particularly sensitive to stress variations induced by structural or thermal modifications, thereby affecting its properties and performance in targeted applications. 2) Thermal treatment effects on strain and morphological properties of PSi. To control and improve the morphological and structural properties of porous silicon for the epitaxial growth of semiconductor nanostructures, we implemented thermal treatments on double-layer silicon substrates, which are specially designed to withstand high temperatures, unlike single-layer porous silicon substrates, which are susceptible to damage under these conditions. The thermal treatments were carried out in a CVD reactor under a hydrogen atmosphere at temperatures ranging from 900 to 1100°C for 60 seconds. Figure 4 (a) presents an SEM image of the double-layer substrate annealed at 900°C, showing the closure of small pores on the surface and the formation of a continuous surface. This process is attributed to the migration of Si atoms on the surface, leading to microstructural changes in the porous layer, in accordance with the classical sintering theory described by Labunov et al [ 33 ]. Furthermore, the formation of a continuous Si film without pores is accompanied by the appearance of undulations of a few microns, much larger than the diameter of the initial pores. Figure 4 -b shows an SEM image of a substrate heated at 1100°C, where the undulations observed at 900°C have almost completely disappeared. This phenomenon was corroborated by AFM images taken on both samples, shown in Figs. 4 (c) and 4 (d). The evolution of the root mean square (RMS) roughness of the undulated surfaces measured by AFM and illustrated in Fig. 4 (e), shows that these undulations decrease with increasing annealing temperature. These undulations are attributed to a stress change in the porous layer caused by the sintering phenomenon. This hypothesis is reinforced by Omega-2 Theta XRD scans performed on the (004) crystallographic plane before and after annealing, which show a shift of the XRD peak associated with the porous layers from a lower angle to a higher angle relative to the peak of the silicon substrate, indicating a transition from tensile to compressive stress. The onset of this compressive stress leads to a shrinkage of the continuous Si film formed above the porous layer and the appearance of ripples. At 1100°C, the XRD peak approaches that of the silicon substrate, indicating a relaxation of the compressive stress, which reduces the surface ripples. These results demonstrate the ability to modulate the morphological and structural properties of porous silicon through thermal treatments, thereby optimizing the epitaxial growth of semiconductor nanostructures. 3) Growth temperature Effect on the strain of Porous silicon substrates After characterizing and optimizing the structural and morphological properties of porous silicon (PSi) substrates via thermal treatments, we undertook molecular beam epitaxy (MBE) experiments for the growth of silicon (Si) and germanium (Ge) thin films. The objective was to study the internal structural transformations of the porous layers due to the growth process and their impact on the mechanical stresses as well as on the quality of the epitaxial nanostructures. As-etched P-type PSi substrates, known for their high porosity and rough surface, were selected to evaluate their ability to relieve stress in epitaxial films, a phenomenon commonly exploited to reduce dislocations in optoelectronic devices [ 34 , 35 ]. However, the inherent structural instability of these porous layers, due to the disordered arrangement of pores and mechanical fragility, constitutes a challenge [ 36 – 40 ]. For this study, pure germanium (Ge) and silicon (Si) films of varying thicknesses were epitaxially grown by molecular beam epitaxy (MBE) at 400°C on well-cleaned, as-etched porous silicon (PSi) monolayer substrates. Figures 5 (a) and 5 (b) show the X-ray diffraction (XRD) profiles for the Ge/PSi and Si/PSi samples, respectively. After the growth of the silicon or germanium films, XRD revealed a shift in the peak corresponding to the porous layer to higher angles compared to that of relaxed Si. This shift indicates a transition from tensile to compressive stress during the growth of Ge or Si, attributed to the thermal destabilization of the porous microstructures at the growth temperature. The resulting compressive stress, further exacerbated by the initial roughness of the PSi surface, led to the formation of dense dislocation networks, primarily localized at the PSi-Ge and PSi-Si interfaces. This is confirmed by the transmission electron microscopy (TEM) images shown in Figs. 5 (c) and 5 (d). These dislocations, whose density is significantly higher than in epitaxial films grown on conventional substrates, propagate in the epitaxial films, altering the crystalline quality. Nevertheless, this high dislocation density, although generally undesirable, can be advantageous in some cases for rapid stress relaxation in thick SiGe layers by confining the dislocations near the interface and minimizing their propagation through the film on virtual substrates. 4) MBE growth of SiGe film on heated porous silicon (PSi) substrate: To overcome the constraints related to the instability of unheated porous layers, we opted for the use of porous silicon substrates heated to a high temperature of 1100°C. This approach aimed to stabilize the internal structure of the substrates and reduce their surface roughness. The heat treatment process carried out under high vacuum promotes the closure of pores at the surface of the material and results in the crystallization of the pore walls, which leads to an improvement in mechanical strength and a reduction in the defect density within the porous layer. Thus, these base materials are particularly well suited for the epitaxial growth of nanostructures intended for optoelectronic applications, such as high-efficiency solar cells produced by layer transfer process (LTP). In this context, SiGe films (30% Ge) were epitaxially deposited at a temperature of 500°C on these preheated substrates. The AFM analysis results shown in Figs. 6 (a) and 6(b) indicate that the surface roughness of the epitaxially grown SiGe film is less than 1 nm, which is a significant improvement over the epitaxially grown films on untreated etch substrates. The X-ray diffraction (XRD) spectrum shown in Fig. 6 (c) reveals no shift of the peak associated with the silicon of the substrate, indicating remarkable stability of the underlying porous layer during growth. The X-ray diffraction spectrum shows a germanium-related peak and a shape that suggests a uniform thickness of the epitaxially grown SiGe film on the annealed silicon double layer. This is consistent with the structural observations made by transmission electron microscopy (TEM), as shown in Fig. 6 (d). The high structural quality, combined with low dislocation density (Ds ~ 10 6 cm -2 ) [ 41 , 42 ] and low surface roughness, highlights the importance of pre-heat treatment for stress relief and stabilization of the internal microstructure of the porous silicon (PSi) substrate to ensure high-quality epitaxial growth of SiGe. . Conclusion This study highlights the fundamental role of optimizing the morphological and structural properties of porous silicon (PSi) substrates via thermal treatments for the growth of virtual silicon-germanium (SiGe) substrates. Using a combination of advanced characterization techniques such as transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD), we analyzed the influence of thermal treatments on PSi properties and their direct impact on the quality of SiGe films grown by molecular beam epitaxy (MBE). Our results show that, without thermal treatment of porous silicon substrates, even at moderate growth temperatures (around 400°C), significant microstructural transformations occur in the PSi layer, including stress changes and internal microstructural instability, which compromise the quality of epitaxial films. We have thus demonstrated that adequate pre-annealing treatments effectively stabilize the porous silicon substrate and release internal mechanical constraints, thus enabling efficient growth of high-quality monocrystalline SiGe layers, on a porous and low-cost support, opening promising prospects for the development of high-performance optoelectronic devices, particularly solar cells, where defect control and crystalline precision are essential criteria.. Declarations Conflicts of interest The authors declare that they have no competing interests Author Contribution The authors confirm contribution to the paper as follows: study conception and design: M.A.; M.I. Data collection: M.I.; M.A; S. A.; M. A.; K. S.; M. B.; I. B. Analysis and interpretation of results: : M.I.; M.A; S. A.; M. A.; K. S.; M. B.; I. B.; Draft manuscript preparation: M.A; M.I. All authors reviewed the results and approved the final version of the manuscript. 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Micromachines. 11 (2), 199 (2020) Ramírez-González, F., García-Salgado, G., Rosendo, E., Díaz, T., Nieto-Caballero,F., Coyopol, A., … Gastellou, E. (2020). Porous silicon gas sensors: The role of the layer thickness and the silicon conductivity. Sensors, 20(17), 4942. G. Korotcenkov, B.K. Cho, Silicon porosification: state of the art. Crit. Rev. Solid State Mater. Sci. 35 (3), 153–260 (2010) V. Robbiano, G.M. Paternò, L. Mattina, A.A. Motti, S.G. Lanzani, G. Scotognella, F., G. Barillaro, Room-temperature low-threshold lasing from monolithically integrated nanostructured porous silicon hybrid microcavities. ACS nano. 12 (5), 4536–4544 (2018) M.J. Sailor, Porous silicon in practice: preparation, characterization and applications (Wiley, 2012) H.A. Santos (ed.), (2014). Porous silicon for biomedical applications Shrestha, N., Shahbazi, M. A., Araújo, F., Zhang, H., Mäkilä, E. M., Kauppila, J.,… Santos, H. A. (2014). Chitosan-modified porous silicon microparticles for enhanced permeability of insulin across intestinal cell monolayers. Biomaterials, 35(25), 7172–7179. Takane, H., Konishi, S., Hayasaka, Y., Ota, R., Wakamatsu, T., Isobe, Y., … Tanaka,K. (2024). Structural characterization of threading dislocation in α-Ga2O3 thin films on c-and m-plane sapphire substrates. Journal of Applied Physics,136(2). Ma, T. C., Chen, X. H., Kuang, Y., Li, L., Li, J., Kremer, F., … Ye, J. D. (2019).On the origin of dislocation generation and annihilation in α-Ga2O3 epilayers on sapphire. Applied Physics Letters, 115(18). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7137558","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":486545439,"identity":"a16166b8-147e-4c1c-bf74-69c8cf53dd4b","order_by":0,"name":"Mohammed Ibrahim","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2UlEQVRIiWNgGAWjYJCCw0CcwMbAfMAAyGBsIEELWwLxWphBWhgYeEA6iNCi296deLhwj10en9iZD8U8DDayGw6wP/yAT4vZmbMbDs94llzMJp27wZiHIc14wwEeYwm8Wm7kbjjMc4A5sQ2i5XAiUAsDMVrqgVpyHgC1/AdqYX/8gwgth0FaGIBaDgC1MJjhtwXslwPHgVrSDAznGCQbzzzMY2aBV8vx3s2fCw5UJ86fnfzM4E2FnWzf8fbHN/BpQQZsBgygqGEmVj1I7QMSFI+CUTAKRsEIAgDeyk/0ek7m9gAAAABJRU5ErkJggg==","orcid":"","institution":"Jouf University","correspondingAuthor":true,"prefix":"","firstName":"Mohammed","middleName":"","lastName":"Ibrahim","suffix":""},{"id":486545441,"identity":"e7560ac7-e927-4f40-9e26-3ac456591af9","order_by":1,"name":"Mansour Aouassa","email":"","orcid":"","institution":"Jouf University","correspondingAuthor":false,"prefix":"","firstName":"Mansour","middleName":"","lastName":"Aouassa","suffix":""},{"id":486545444,"identity":"dd59214d-3ace-45e1-b571-cfbc9e25c0bc","order_by":2,"name":"Saud A. Algarni","email":"","orcid":"","institution":"Taif University","correspondingAuthor":false,"prefix":"","firstName":"Saud","middleName":"A.","lastName":"Algarni","suffix":""},{"id":486545445,"identity":"df4b5180-53b1-413d-bc54-4ae525dcd0b5","order_by":3,"name":"A. K. Aladim","email":"","orcid":"","institution":"Jouf University","correspondingAuthor":false,"prefix":"","firstName":"A.","middleName":"K.","lastName":"Aladim","suffix":""},{"id":486545447,"identity":"ebb65aba-f4f7-4cc1-8f52-2a1d6916039f","order_by":4,"name":"Maha A. Alenizi","email":"","orcid":"","institution":"Northern Border University","correspondingAuthor":false,"prefix":"","firstName":"Maha","middleName":"A.","lastName":"Alenizi","suffix":""},{"id":486545448,"identity":"904a0f29-9ebb-42bf-9ca3-3fed65750023","order_by":5,"name":"K. M. A. Saron","email":"","orcid":"","institution":"Jouf University","correspondingAuthor":false,"prefix":"","firstName":"K.","middleName":"M. A.","lastName":"Saron","suffix":""},{"id":486545449,"identity":"d6939eca-fa1b-45e1-94ea-a0de4f83fc62","order_by":6,"name":"Mohammed Bouabdellaoui","email":"","orcid":"","institution":"CNRS","correspondingAuthor":false,"prefix":"","firstName":"Mohammed","middleName":"","lastName":"Bouabdellaoui","suffix":""},{"id":486545451,"identity":"44e2def3-0435-47f7-9a41-099a3e0b41d9","order_by":7,"name":"Isabelle Berbezier","email":"","orcid":"","institution":"CNRS","correspondingAuthor":false,"prefix":"","firstName":"Isabelle","middleName":"","lastName":"Berbezier","suffix":""}],"badges":[],"createdAt":"2025-07-16 08:23:27","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7137558/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7137558/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86905908,"identity":"8973163f-e829-4400-b560-877366552198","added_by":"auto","created_at":"2025-07-17 03:42:26","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":9151513,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) and (b) show respectively 2 D and 3 D AFM images of the porous silicon (PSi) layer showing consistent morphologies across all as-etched PSi samples.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7137558/v1/6f61cbf917fa532f153e9fec.png"},{"id":86906914,"identity":"c1378ee6-6842-4b30-bb05-091eddf2fa5b","added_by":"auto","created_at":"2025-07-17 04:06:26","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1199355,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) and (b) cross-sectional TEM images of PSi monolayer at low and high magnifications. (c) and (d) cross-sectional TEM images of PSi double layer at low and high magnifications. \u0026nbsp;respectively.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7137558/v1/af23e0f2dd74228fc1053669.jpeg"},{"id":86905912,"identity":"8494e23e-8ec0-4ac9-95d1-260c3f36134d","added_by":"auto","created_at":"2025-07-17 03:42:26","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":237733,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) and (b) show XRD profiles of the as-etched porous silicon (PSi) monolayer and double layer, respectively, with distinct peaks for the Si substrate and the PSi layer. (c) Presents the XRD map of the PSi double layer.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7137558/v1/d610f87fb8e932b48e7d1053.png"},{"id":86905917,"identity":"758d576f-a898-466d-a972-e924074a0b5e","added_by":"auto","created_at":"2025-07-17 03:42:26","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1509677,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) and (b) display AFM images illustrating the effect of heating on the morphology of porous silicon. (c) and (d) present SEM images showing the morphological changes in heated porous silicon. (e) depicts the RMS evolution of the surface roughness of heated porous silicon. (f) shows the XRD spectra of the porous silicon substrate, highlighting the impact of heating on the internal strain within the PSi layer.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7137558/v1/fd886801542f484694a709e9.jpeg"},{"id":86906697,"identity":"7dd17bea-fb29-456c-a3d6-08f5604f6f67","added_by":"auto","created_at":"2025-07-17 03:58:27","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":953399,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) XRD spectrum and (b) cross-sectional TEM image of Ge/PSi, highlighting the instability of the PSi layer and its impact on the grown Ge film. (c) XRD spectrum and (d) cross-sectional TEM image of Si/PSi, demonstrating the structural instability of the PSi layer and its effect on the epitaxially grown Si film\u003c/em\u003e.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7137558/v1/2127c10d8dc6c01881663b96.jpeg"},{"id":86905926,"identity":"7a1cd9c0-a01c-4891-a3b7-5d9dd496ff34","added_by":"auto","created_at":"2025-07-17 03:42:27","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":368864,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) AFM image of the SiGe film grown on a heated porous silicon (PSi) substrate. (b) Roughness profile along the green line in (a), confirming the low surface roughness of the SiGe film. (c) XRD spectrum of the SiGe film grown on a heated PSi substrate, showing no peak shift associated with the PSi layer and a clear peak for the SiGe \u0026nbsp;(30%) film. (d) Cross-sectional TEM image illustrating the high structural and morphological quality of the grown SiGe film.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7137558/v1/c53e85ea2374b264df7285bb.png"},{"id":87427311,"identity":"ae87df55-4955-4497-8c10-87794e0c6a02","added_by":"auto","created_at":"2025-07-23 16:46:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":12362605,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7137558/v1/875b2bd4-0996-4a1c-b4df-3840a71209ed.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Thermal Stabilization of Porous Silicon: A Key Step for High-Quality SiGe Epitaxy","fulltext":[{"header":"Introduction","content":"\u003cp\u003eUsing silicon as a substrate for the growth of solar cells and electronic components based on SiGe films offers numerous advantages, including its widespread availability, low cost, and compatibility with the conventional CMOS industrial platform [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. However, the difference in lattice parameters (4.2%) between silicon and germanium poses a major challenge. It generates mechanical stress in epitaxially grown SiGe films, causing either elastic relaxation via the formation of 3D nanostructures or plastic relaxation via the generation of crystal defects such as dislocations and cracks [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTo overcome this obstacle, a conventional solution is to use a relaxed SiGe buffer layer, known as a virtual pseudo-substrate [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. This concept, proposed by Lo et al. in 1991, is based on the use of a substrate capable of elastically or plastically deforming during growth, allowing the epitaxial layer to return to its unconstrained lattice parameter while minimizing dislocation density[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eHowever, this approach requires the growth of very thick buffer layers (several microns), which results in high manufacturing costs, making them incompatible with large-scale industrial integration. To address this, several alternatives have been explored, including the use of porous silicon as a \"compliant\" buffer layer.\u003c/p\u003e\u003cp\u003eThe work of Calabrese et al. demonstrated the feasibility of directly growing germanium on porous silicon to compensate for the lattice mismatch. Ge deposition is performed using the LEPECVD (Low-Energy Plasma-Enhanced Chemical Vapor Deposition) technique, with a gradual increase in temperature from 500\u0026deg;C to 750\u0026deg;C [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. The best results were obtained for a porosity of 22%, leading to a dislocation density of around 10\u003csup\u003e6\u003c/sup\u003e cm\u003csup\u003e-2\u003c/sup\u003e for 5 \u0026micro;m of Ge on porous Si [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eBoucherif and Lysenko proposed a strategy based on the controlled oxidation of porous silicon to adjust its lattice parameter [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. This process causes a volume expansion of the substrate, generating a tensile stress in the thin epitaxial monocrystalline layer. The lattice parameter of the latter then increases and approaches that of germanium. By adjusting the porosity and/or the oxidation rate, this parameter can be precisely adapted to device requirements. This method achieved a dislocation density of around 6 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e cm\u003csup\u003e-2\u003c/sup\u003e for thin (50 nm) layers of Si\u003csub\u003e0.72\u003c/sub\u003eGe\u003csub\u003e0.28\u003c/sub\u003e [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThis work aims to demonstrate, first, the technical feasibility and economic viability of manufacturing thermally treated porous SiGe/Si double-layer virtual substrates, free of crystal defects and using a reduced amount of germanium. Second, it proposes an optimized technological process for the growth of mismatched lattice buffer layers. This process aims to reduce the density of emerging dislocations, responsible for current leakage in electronic devices, thus improving their efficiency, breakdown voltage, and reliability critical parameters for their industrial competitiveness.\u003c/p\u003e\u003cp\u003eThe proposed method is applicable to large porous double-layer silicon wafers (Si (001) orientation) and can be easily integrated into existing industrial processes. It is based on a strategy inspired by phenomena already documented in the literature, aimed at limiting dislocation generation. It involves two key steps: i) the formation of porous silicon films with different porosities by electrochemical anodization; ii) high-temperature annealing to relieve stress and improve the crystalline quality of the epitaxial layer.\u003c/p\u003e\u003cp\u003eAdopting this approach paves the way for efficient integration of multi-junction SiGe solar cells on silicon, as well as various microelectronics applications.\u003c/p\u003e"},{"header":"Experimental","content":"\n\u003ch3\u003e1) Preparation of porous silicon (PSi) substrates:\u003c/h3\u003e\n\u003cp\u003eIn this study, two distinct types of porous silicon (PSi) substrates were prepared via controlled electrochemical etching. The first type consists of a single layer of porous silicon with a thickness of 270 nm, while the second type has double-layer architecture: a 400 nm top layer with low porosity and a 7 micron bottom layer with high porosity, optimized for the layer transfer process. For the first substrate, p-type silicon wafers with a (100) crystallographic orientation and a low resistivity of 0.01 Ω\u0026bull;cm were used. These wafers were anodized in a 35% hydrogen fluoride (HF) solution under a constant current of 10 mA/cm\u003csup\u003e2\u003c/sup\u003e. This method allows for the formation of a uniformly porous silicon layer with approximately 30% porosity, which is ideal for epitaxial growth. For the fabrication of the double-layer substrate, the same silicon wafers were used. The etching of the upper layer was carried out under identical conditions as for the first substrate, ensuring a similar porosity. However, for the formation of the lower layer, with a thickness of 7 microns, the current density was increased abruptly from 10 to 80 mA/cm\u0026sup2; which led to a significantly higher porosity. This layered configuration is particularly suitable for film growth, especially for photovoltaic applications. The properties of the as-etched PSi substrate are indicated in the following Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e:\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eCharacteristics of the two types of porous silicon substrates.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eSample\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eThickness\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cem\u003ePorosity\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePSi\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.27 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e20%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003ePSi-DL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eLPL: 0.4 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e20%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHPL: 7 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e35%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\n\u003ch3\u003e2) Heat treatment of porous silicon:\u003c/h3\u003e\n\u003cp\u003eThe double-layer PSi substrates then underwent a rigorous heat treatment to refine their morphological and crystallographic properties, resulting in the formation of sintered porous silicon. This annealing process was performed in an ultra-high vacuum chemical vapor deposition (UHVCVD) reactor under a pure hydrogen (H\u003csub\u003e2\u003c/sub\u003e) atmosphere at temperatures ranging from 900\u0026deg;C to 1100\u0026deg;C for 30 seconds. The objective of this heat treatment was to modify the structural and morphological properties of the porous layers, to close the surface pores, to minimize surface roughness, and to control mechanical stresses within the porous layers in order to improve the adhesion of future nanostructures for their integration into advanced optoelectronic devices. The heated porous silicon samples are listed in the following Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e:\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eCharacteristics of the heated porous silicon samples substrates.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eSample\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eHeating temperature\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePSi-DL_900\u0026deg;C\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e900\u0026deg;C\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePSi-DL_1100\u0026deg;C\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1100\u0026deg;C\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\n\u003ch3\u003e3) Morphological and structural characterization:\u003c/h3\u003e\n\u003cp\u003eAfter substrate preparation, a thorough characterization was performed to assess the morphological and structural properties of the different porous layers. Surface observations were performed using a Lyra TESCAN scanning electron microscope (SEM), which provides nanometric images of the porous structures and measures the thickness of the layers and the dimensions of the pores. In parallel, atomic force microscopy (AFM) analyses were performed using a Parkway XE100 microscope, providing detailed information on the surface roughness.\u003c/p\u003e\u003cp\u003eStructurally, the samples were examined using a JEOL 2010F transmission electron microscope (TEM), providing high-resolution images of the pores and porous layers, as well as the transition zones between the layers. X-ray diffraction (XRD) measurements were also performed to determine the crystalline quality, assess the mechanical stresses in the porous layers, and analyze their evolution after thermal treatments.\u003c/p\u003e\n\u003ch3\u003e4) MBE Growth of SiGe thin films:\u003c/h3\u003e\n\u003cp\u003eThe growth of SiGe on porous silicon occurs at the nanometric scale, where only a limited number of germanium and silicon atoms are available. This makes the crystalline quality of the SiGe structure highly susceptible to residual gas contamination. To ensure the necessary purity, we perform the growth in an ultra-high vacuum (UHV) environment within a molecular beam epitaxy (MBE) chamber, which provides optimal conditions for precise epitaxial growth.\u003c/p\u003e\u003cp\u003eMBE reactors are particularly suited to this high-purity process due to their advanced dual-pumping systems. Initially, a turbomolecular pump reduces the pressure to approximately 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e torr, followed by an ionic pump that further reduces residual gases, achieving a pressure below 10\u003csup\u003e\u0026ndash;11\u003c/sup\u003e torr. To enhance this ultra-clean environment, the reactor walls are cooled to approximately \u0026minus;\u0026thinsp;60\u0026deg;C using liquid nitrogen. This cooling causes residual molecules to condense on the walls, keeping them away from the sample and ensuring high structural purity of the deposited films.\u003c/p\u003e\u003cp\u003ePrior to growth, the porous silicon (PSi) samples undergo rigorous cleaning to remove native silicon dioxide and contaminants and prepare a clean surface. Samples are first immersed in a Piranha solution, a 1:3 volume mixture of H₂O₂ (30%) and H₂SO₄ (96%) at 80\u0026deg;C for 10 minutes, to eliminate organic residues. After thorough rinsing with deionized (DI) water, a second cleaning with a H₂O₂ solution (1:10) is performed, followed by a final DI water rinse. The cleaned samples are then carefully introduced into a Riber 32-type ultra-high vacuum solid-source molecular beam epitaxy (UHV-SSMBE) chamber. This MBE reactor operates at extremely low residual pressures, maintaining a pristine environment ideal for high-purity growth [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The reactor's design enables precise control over the composition and thickness of silicon-germanium (SiGe) layers, which is critical for producing the high-quality films required in advanced electronic and photonic applications. The UHV-SSMBE system enables the fabrication of complex heteroepitaxial structures with nanometric precision\u0026mdash;an essential requirement for optoelectronic devices such as semiconductor lasers and infrared detectors, where structural integrity critically influences performance [\u003cspan additionalcitationids=\"CR22 CR23 CR24 CR25 CR26 CR27 CR28 CR29 CR30 CR31\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eBefore growth, the samples undergo a degassing process at 550\u0026deg;C for 30 minutes, effectively desorbing any remaining surface gases and optimizing the surface for subsequent deposition. The SiGe thin films are then deposited at a carefully controlled temperature of 520\u0026deg;C, ensuring layer by layer growth mode of these advanced materials.\u003c/p\u003e\u003cp\u003eFollowing growth, the epitaxial nanostructures on the various types of PSi substrates are characterized morphologically and structurally. Advanced scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques are employed to assess the growth quality, analyze the size and distribution of quantum dots, and verify interface integrity. X-ray diffraction (XRD) analyses are also conducted to evaluate crystal quality and identify any residual stresses in the epitaxial structures.\u003c/p\u003e"},{"header":"Finding and discussion","content":"\u003ch2\u003e1) Morphological and structural properties of as-etched PSi substrates\u003c/h2\u003e\u003cp\u003eFigures \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a) and 1 (b) illustrate, respectively, low and high-magnification AFM images acquired on the surface of the as-etched porous silicon (PSi) monolayer substrate. These images highlight the formation of a homogeneous and regular porous surface, free of anomalies, with extremely fine pores with an average diameter of about 40 nm. The top layer of the PSi double-layer has a surface porosity similar to that of the monolayer and comparable morphology and roughness. Figures\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a) and 2 (b) present TEM images of the porous silicon monolayer at low and high magnifications, respectively. These images reveal a single porous layer with uniform porosity, where the porous columns display a branched classical morphology characteristic of P-type porous silicon layer, as reported in the literature. This observation confirms the consistency and high quality of the etching process. The inset in Figure (2-a) shows a selected area electron diffraction (SAED) pattern of the silicon layer, confirming its crystallinity.\u003c/p\u003e\u003cp\u003eFigures \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (c) and 2 (d) provide TEM images of the porous silicon double-layer at low and high magnifications, respectively. These images clearly depict two distinct porous layers with different porosities. The upper layer, with a lower porosity (20%), was specifically designed to undergo thermal transformation into a continuous silicon film. In contrast, the lower layer, with a higher porosity (35%), was engineered to maintain its porous structure even under high-temperature heat treatments. The inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (c) presents selected area electron diffraction (SAED) pattern of the silicon layer, further verifying its crystalline nature. This intentional differentiation in porosity between the layers, combined with their crystalline properties, enables precise control over the mechanical and thermal characteristics of the final material. Such control is crucial for potential applications, particularly in the fabrication of solar cells using the layer transfer process (LTP).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eX-ray diffraction (XRD) is a fundamental technique for the characterization of porous substrates, providing detailed information on the crystal structure, lattice parameters, and internal stresses. It is essential to analyze the mechanical and structural properties of porous silicon (PSi) films, thus optimizing their performance in electronic and optoelectronic applications. XRD analysis provides an accurate assessment of the effects of porosity and manufacturing processes, which is crucial for the design of advanced devices with specified characteristics and improved performance. Figures\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) and 3(b) respectively illustrate the X-ray diffraction (XRD) profiles of the as-etched porous silicon single-layer (PSi-ML) and double-layer (PSi-DL) substrates. These profiles reveal the typical features of porous silicon layers in contact with a crystalline silicon substrate, displaying two distinctive peaks. The XRD peak with the highest intensity, centered at 34.56\u0026deg;, corresponds to the monocrystalline silicon, while the lower intensity peak, centered at 34.54\u0026deg;, is attributed to the strained PSi layers.\u003c/p\u003e\u003cp\u003eThe shift of the XRD peak of the PSi layer to a slightly lower angle compared to the bulk silicon peak indicates the presence of an out-of-plane tensile strain. This shift suggests that the lattice parameter of the silicon nanocrystallites forming the porous layer is slightly higher than that of the bulk silicon. This increase in the lattice parameter is probably due to the presence of oxide formed during the electrochemical anodization process, which induces structural defects and slightly modifies the silicon crystal structure in the porous layer.\u003c/p\u003e\u003cp\u003eAs a result, a slight out-of-plane expansion, reflecting a tensile strain, is observed in the porous layer and confirmed by XRD mapping shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (c). This strain is even more pronounced in the PSi double-layer configuration due to the cumulative effect of the increased thickness of the successive porous layers. This amplification of internal stress makes the double-layer structure particularly sensitive to stress variations induced by structural or thermal modifications, thereby affecting its properties and performance in targeted applications.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003e2) Thermal treatment effects on strain and morphological properties of PSi.\u003c/h3\u003e\n\u003cp\u003eTo control and improve the morphological and structural properties of porous silicon for the epitaxial growth of semiconductor nanostructures, we implemented thermal treatments on double-layer silicon substrates, which are specially designed to withstand high temperatures, unlike single-layer porous silicon substrates, which are susceptible to damage under these conditions. The thermal treatments were carried out in a CVD reactor under a hydrogen atmosphere at temperatures ranging from 900 to 1100\u0026deg;C for 60 seconds. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a) presents an SEM image of the double-layer substrate annealed at 900\u0026deg;C, showing the closure of small pores on the surface and the formation of a continuous surface. This process is attributed to the migration of Si atoms on the surface, leading to microstructural changes in the porous layer, in accordance with the classical sintering theory described by Labunov et al [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Furthermore, the formation of a continuous Si film without pores is accompanied by the appearance of undulations of a few microns, much larger than the diameter of the initial pores. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e-b shows an SEM image of a substrate heated at 1100\u0026deg;C, where the undulations observed at 900\u0026deg;C have almost completely disappeared. This phenomenon was corroborated by AFM images taken on both samples, shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (c) and 4 (d). The evolution of the root mean square (RMS) roughness of the undulated surfaces measured by AFM and illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (e), shows that these undulations decrease with increasing annealing temperature. These undulations are attributed to a stress change in the porous layer caused by the sintering phenomenon. This hypothesis is reinforced by Omega-2 Theta XRD scans performed on the (004) crystallographic plane before and after annealing, which show a shift of the XRD peak associated with the porous layers from a lower angle to a higher angle relative to the peak of the silicon substrate, indicating a transition from tensile to compressive stress. The onset of this compressive stress leads to a shrinkage of the continuous Si film formed above the porous layer and the appearance of ripples. At 1100\u0026deg;C, the XRD peak approaches that of the silicon substrate, indicating a relaxation of the compressive stress, which reduces the surface ripples. These results demonstrate the ability to modulate the morphological and structural properties of porous silicon through thermal treatments, thereby optimizing the epitaxial growth of semiconductor nanostructures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003e3) Growth temperature Effect on the strain of Porous silicon substrates\u003c/h3\u003e\n\u003cp\u003eAfter characterizing and optimizing the structural and morphological properties of porous silicon (PSi) substrates via thermal treatments, we undertook molecular beam epitaxy (MBE) experiments for the growth of silicon (Si) and germanium (Ge) thin films. The objective was to study the internal structural transformations of the porous layers due to the growth process and their impact on the mechanical stresses as well as on the quality of the epitaxial nanostructures.\u003c/p\u003e\u003cp\u003eAs-etched P-type PSi substrates, known for their high porosity and rough surface, were selected to evaluate their ability to relieve stress in epitaxial films, a phenomenon commonly exploited to reduce dislocations in optoelectronic devices [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. However, the inherent structural instability of these porous layers, due to the disordered arrangement of pores and mechanical fragility, constitutes a challenge [\u003cspan additionalcitationids=\"CR37 CR38 CR39\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. For this study, pure germanium (Ge) and silicon (Si) films of varying thicknesses were epitaxially grown by molecular beam epitaxy (MBE) at 400\u0026deg;C on well-cleaned, as-etched porous silicon (PSi) monolayer substrates. Figures\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a) and 5 (b) show the X-ray diffraction (XRD) profiles for the Ge/PSi and Si/PSi samples, respectively. After the growth of the silicon or germanium films, XRD revealed a shift in the peak corresponding to the porous layer to higher angles compared to that of relaxed Si. This shift indicates a transition from tensile to compressive stress during the growth of Ge or Si, attributed to the thermal destabilization of the porous microstructures at the growth temperature. The resulting compressive stress, further exacerbated by the initial roughness of the PSi surface, led to the formation of dense dislocation networks, primarily localized at the PSi-Ge and PSi-Si interfaces. This is confirmed by the transmission electron microscopy (TEM) images shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (c) and 5 (d). These dislocations, whose density is significantly higher than in epitaxial films grown on conventional substrates, propagate in the epitaxial films, altering the crystalline quality. Nevertheless, this high dislocation density, although generally undesirable, can be advantageous in some cases for rapid stress relaxation in thick SiGe layers by confining the dislocations near the interface and minimizing their propagation through the film on virtual substrates.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003e4) MBE growth of SiGe film on heated porous silicon (PSi) substrate:\u003c/h3\u003e\n\u003cp\u003eTo overcome the constraints related to the instability of unheated porous layers, we opted for the use of porous silicon substrates heated to a high temperature of 1100\u0026deg;C. This approach aimed to stabilize the internal structure of the substrates and reduce their surface roughness. The heat treatment process carried out under high vacuum promotes the closure of pores at the surface of the material and results in the crystallization of the pore walls, which leads to an improvement in mechanical strength and a reduction in the defect density within the porous layer. Thus, these base materials are particularly well suited for the epitaxial growth of nanostructures intended for optoelectronic applications, such as high-efficiency solar cells produced by layer transfer process (LTP). In this context, SiGe films (30% Ge) were epitaxially deposited at a temperature of 500\u0026deg;C on these preheated substrates. The AFM analysis results shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) and 6(b) indicate that the surface roughness of the epitaxially grown SiGe film is less than 1 nm, which is a significant improvement over the epitaxially grown films on untreated etch substrates. The X-ray diffraction (XRD) spectrum shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c) reveals no shift of the peak associated with the silicon of the substrate, indicating remarkable stability of the underlying porous layer during growth. The X-ray diffraction spectrum shows a germanium-related peak and a shape that suggests a uniform thickness of the epitaxially grown SiGe film on the annealed silicon double layer. This is consistent with the structural observations made by transmission electron microscopy (TEM), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(d). The high structural quality, combined with low dislocation density (Ds\u0026thinsp;~\u0026thinsp;10\u003csup\u003e6\u003c/sup\u003e cm\u003csup\u003e-2\u003c/sup\u003e) [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e] and low surface roughness, highlights the importance of pre-heat treatment for stress relief and stabilization of the internal microstructure of the porous silicon (PSi) substrate to ensure high-quality epitaxial growth of SiGe.\u003c/p\u003e\u003cp\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study highlights the fundamental role of optimizing the morphological and structural properties of porous silicon (PSi) substrates via thermal treatments for the growth of virtual silicon-germanium (SiGe) substrates. Using a combination of advanced characterization techniques such as transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD), we analyzed the influence of thermal treatments on PSi properties and their direct impact on the quality of SiGe films grown by molecular beam epitaxy (MBE). Our results show that, without thermal treatment of porous silicon substrates, even at moderate growth temperatures (around 400\u0026deg;C), significant microstructural transformations occur in the PSi layer, including stress changes and internal microstructural instability, which compromise the quality of epitaxial films. We have thus demonstrated that adequate pre-annealing treatments effectively stabilize the porous silicon substrate and release internal mechanical constraints, thus enabling efficient growth of high-quality monocrystalline SiGe layers, on a porous and low-cost support, opening promising prospects for the development of high-performance optoelectronic devices, particularly solar cells, where defect control and crystalline precision are essential criteria..\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eConflicts of interest\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no competing interests\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eThe authors confirm contribution to the paper as follows: study conception and design: M.A.; M.I. Data collection: M.I.; M.A; S. A.; M. A.; K. S.; M. B.; I. B. Analysis and interpretation of results: : M.I.; M.A; S. A.; M. A.; K. S.; M. B.; I. B.; Draft manuscript preparation: M.A; M.I. All authors reviewed the results and approved the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was funded by the Deanship of Graduate Studies and Scientific Research at Jouf University under grant No. (DGSSR-2024-02-01213)\u0026rdquo;\u003c/p\u003e\u003ch2\u003eData availability statement\u003c/h2\u003e\u003cp\u003eData will be made available on reasonable request\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eM. Aouassa, M. Bouabdellaoui, W.B. Pessoa, A. Tsarev, M. Ibrahim, A.K. Aladim, I. Berbezier, SiGe Mie resonators grown on photoactive silicon nanodisks for high-performance photodetection. J. Mater. Sci.: Mater. Electron. \u003cb\u003e36\u003c/b\u003e(6), 1\u0026ndash;18 (2025)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eT. McJunkin, E.R. MacQuarrie, L. Tom, S.F. Neyens, J.P. Dodson, B. 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Applied Physics Letters, 115(18).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Porous silicon, Molecular beam epitaxy, Thin SiGe films, Transmission electron microscopy, Pre-annealing, X-ray diffraction (XRD).","lastPublishedDoi":"10.21203/rs.3.rs-7137558/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7137558/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study focuses on the epitaxial growth of virtual silicon-germanium (SiGe) substrates on porous silicon (PSi). Epitaxy was performed on different types of PSi substrates, with or without prior thermal annealing. Morphological and structural investigations by atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) show that epitaxial SiGe films grown on double-layer PSi substrates, annealed at 1000\u0026deg;C, exhibit significantly higher crystalline quality than those grown on unannealed PSi substrates or PSi substrates annealed at temperatures lower than or equal to 900\u0026deg;C. This improvement is attributed to the beneficial effects of 1000\u0026deg;C annealing, which leads to stress relaxation, internal microstructure stabilization and significant improvement of PSi surface morphology. In contrast, direct growth of SiGe on unannealed PSi, even at moderate temperature (~\u0026thinsp;400\u0026deg;C), induces structural degradation of the porous buffer, leading to a very high dislocation density in the epitaxially grown SiGe films. A well-optimized thermal treatment of PSi substrates promotes the growth of high-quality virtual SiGe substrates on PSi that is both efficient and economically viable for the development of SiGe-based photovoltaic cells.\u003c/p\u003e","manuscriptTitle":"Thermal Stabilization of Porous Silicon: A Key Step for High-Quality SiGe Epitaxy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-17 03:42:21","doi":"10.21203/rs.3.rs-7137558/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e6c398be-49af-4d80-a27f-cd03d9c0f792","owner":[],"postedDate":"July 17th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-07-23T16:38:28+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-17 03:42:21","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7137558","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7137558","identity":"rs-7137558","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}