Porous Ge Research Articles

Sustainable Fabrication of Ge Freestanding Membranes and Substrate Reuse via Porosification Lift-Off

Recent advancements in flexible optoelectronics have spurred interest in developing nanoengineered substrates essential for the growth and fabrication of freestanding membranes (FSMs). The FSMs offer an extra degree of flexibility in applications previously unachievable through conventional techniques, such as the heterointegration of highly dissimilar materials. These membranes allow the stacking of various dissimilar materials, enabling the seamless coupling of their physical properties and integration onto a variety of substrates. Additionally, the use of FSMs instead of bulk materials significantly reduces costs and material usage during device production—a crucial advantage for non-silicon materials, which are considerably more expensive than silicon. Techniques such as substrate thinning, 2D-assisted epitaxy, and various substrate engineering approaches have demonstrated great potential for fabricating lightweight and flexible devices, underscoring the promise of FSM technology. However, the efficient production of FSMs based on germanium (Ge) and its alloys is still a challenging task.In this work, we present the porosification lift-off process, which involves the sequential formation of porous Ge (PGe) nanostructure formation, epitaxial growth of Ge membrane, detachment, substrate reconditioning, and subsequent reuse. We demonstrate edge-to-edge formation of PGe nanostructures across 100 mm Ge wafers with excellent uniformity (Fig. 1a) using bipolar electrochemical etching[1]. Moreover, the physical properties (Thickness and porosity) of these structures can be efficiently tailored to suit specific applications by adjusting the etching parameters. These PGe structures maintain their substrate-oriented crystalline nature and exhibit smooth surface morphology, making them ideal substrates for epitaxial growth. Thin Ge FSMs are grown atop these PGe structure (Fig. 1b) at low temperature, preventing the PGe transformation into large pillars[2]. The initial growth stages on nanostructured substrate are investigated, showing the membrane formation process starting with initial 3D nucleation atop of the pore walls, followed by the coalescence of nucleation seeds into dense membrane, and concluding with the surface flattening by final 2D growth mode. This process allows for the growth of even ultrathin membranes with thickness as low as 60 nm. The high-quality monocrystalline nature of these membranes is confirmed by a thorough XRD analysis. After membrane formation, the underlying PGe structure provides a weak interface that allows easy detachment and transfer of the Ge FSM (Fig. 1c). Furthermore, the remnants of the unreconstructed PGe structure on the parent substrate can be dissolved using slow chemical etching in H2O2 solution, enabling the recovery of an epiready surface and its reporosification to produce multiple Ge FSMs. These findings provide a groundwork for engineering of Ge substrates and their application in the sustainable fabrication of high-quality Ge FSMs for lightweight and flexible optoelectronic devices.[1] T. Hanuš et al., Adv. Mater. Interfaces (2023), 2202495 (https://doi.org/10.1002/admi.202202495)[2] T. Hanuš et al., Mater. Today Adv. 18 (2023), 100373 (https://doi.org/10.1016/j.mtadv.2023.100373)[3] T. Hanuš et al., Sustainability (2024), 16(4), 1444 (https://doi.org/10.3390/su16041444) Figure 1

Sustainable Production of Ultrathin Ge Freestanding Membranes

Germanium (Ge) is a critical material for applications in space solar cells, integrated photonics, infrared imaging, sensing, and photodetectors. However, the corresponding cost and limited availability hinder its potential for widespread applications. However, using Ge freestanding membranes (FSMs) allows for a significant reduction in the material consumption during device fabrication while offering additional advantages such as lightweight and flexible form factor for novel applications. In this work, we present the Ge FSM production process involving sequential porous Ge (PGe) structure formation, Ge membrane epitaxial growth, detachment, substrate cleaning, and subsequent reuse. This process enables the fabrication of multiple high-quality monocrystalline Ge FSMs from the same substrate through efficient substrate reuse at a 100 mm wafer scale by a simple and low-cost chemical cleaning process. A uniform, high-quality PGe layer is produced on the entire recovered substrate. By circumventing the use of conventional high-cost chemical–mechanical polishing or even substantial chemical wet-etching, and by using an optimized PGe structure with reduced thickness, the developed process allows for both cost and an environmental impact reduction in Ge FSMs production, lowering the amount of Ge used per membrane fabrication. Moreover, this process employs large-scale compatible techniques paving the way for the sustainable production of group IV FSMs for next-generation flexible optoelectronics.

Noncentrosymmetric two-dimensional Weyl semimetals in porous Si/Ge structures.

In this work we predict a family of noncentrosymmetric two-dimensional (2D) Weyl semimetals (WSMs) composed by porous Ge and SiGe structures. These systems are energetically stable graphenylene-like structures with a buckling, spontaneously breaking the inversion symmetry. The nontrivial topological phase for these 2D systems occurs just below the Fermi level, resulting in nonvanishing Berry curvature around the Weyl nodes. The emerged WSMs are protected byC3symmetry, presenting one-dimensional edge Fermi-arcs connecting Weyl points with opposite chiralities. Our findings complete the family of Weyl in condensed-matter physics, by predicting the first noncentrosymmetric class of 2D WSMs.

Honeycomb-like micro-/nano-hierarchical porous germanium for high-performance lithium-ion battery anodes

The synthesized honeycomb-like micro-/nano-hierarchical porous Ge (hp-Ge) anode demonstrates comparable capacity to that of pristine solid Ge microparticles but with an enhanced ICE, better rate capability and more excellent cyclability.

Achieving porous germanium from both p- and n-type epitaxial Ge-on-Si via bipolar potentiostatic etching

Due to the large surface specific area, tunable porosity and refractive index, porous germanium (Ge) has wide applications in optoelectronics, photovoltaics, anode materials in batteries. Electrochemical etching is the most used approach to fabricate porous Ge. However, previous work focused on bulk Ge wafer etching, which is not suitable for device integration on a Si substrate, and illumination had to be used for n-type porous Ge formation. In this paper, we show that by using bipolar potentiostatic etching, porous Ge with the thickness of more than 100 nm and the pore diameter in the range of 10 to 100 nm, can be obtained from both p- and n-type Ge-on-Si layers without illumination. The pore formation mechanism is interpreted by hydrogen evolution effect and Space Charge Region (SCR) model where pore nucleation, growth and dissolution are determined by the total etching time. Our work paves the way for porous Ge applications in Si based integrated photonics and microelectronics.

Wafer-scale Ge freestanding membranes for lightweight and flexible optoelectronics

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

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

AbstractPorous 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.

Wafer-scale detachable monocrystalline germanium nanomembranes for the growth of III-V materials and substrate reuse.

Germanium (Ge) is increasingly used as a substrate for high-performance optoelectronics, photovoltaics, and electronic devices. These devices are usually grown on thick and rigid Ge substrates manufactured by classical wafering techniques. Nanomembranes (NMs) provide an alternative to this approach while offering wafer-scale lateral dimensions, weight reduction, waste limitation, and cost effectiveness. Herein, we introduce the Porous germanium Efficient Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of wafer-scale detachable Ge NMs on porous Ge (PGe) and substrate reuse. We demonstrate the growth of Ge NMs with monocrystalline quality as revealed by high-resolution transmission electron microscopy (HRTEM) characterization. Together with the surface roughness below 1 nm, it makes the Ge NMs suitable for growth of III-V materials. Additionally, the embedded nanoengineered weak layer enables the detachment of the Ge NMs. Finally, we demonstrate the wet-etch-reconditioning process of the Ge substrate, allowing its reuse, to produce multiple free-standing NMs from a single parent wafer. The PEELER process significantly reduces the consumption of Ge in the fabrication process, paving the way for a new generation of low-cost flexible optoelectronic devices.

The local crystallization thresholds under laser irradiation of the Cu+‐ion implanted Ge layers with various thicknesses

AbstractPorous Ge layers were fabricated by implanting Cu+ ions of various acceleration energies into c‐Ge substrate. This made it possible to form amorphous porous Ge layers of thicknesses from 18.6 to 50.5 nm. The distribution depth profiles of implanted Cu+ ions in Ge were simulated by SRIM‐2013 program. The formed layers were studied by Raman spectroscopy. The excitation was carried out by a laser with a wavelength of 488 nm, which has a small penetration depth of ∼20 nm. In the Raman spectra, there is a possibility to distinguish the contribution from the amorphous Ge film, the crystalline c‐Ge substrate, and Ge nanocrystals, which appear under intense laser excitation due to local crystallization. The decomposition of the spectra into individual components makes it possible to subtract the contribution of the substrate and estimate the crystalline volume fraction in the layers. The results obtained confirm the hypothesis of low thermal conductivity of the porous layer, due to which local crystallization occurs.

III-V material growth on electrochemically porosified Ge substrates

III-V semiconductor materials for high-efficiency multi-junction solar cells are often grown on germanium (Ge) substrates. However, apart from being considered as a rare element, Ge substrates are one of the major cost shares of a III-V multi-junction solar cell. To reduce costs and material consumption, we aim at re-usable porosified Ge substrates. Prior to the growth, the porous layers are subjected to an annealing procedure to close the wafer surface and to form a predetermined breaking area some microns below the surface. Later, the III-V epitaxial layers are mechanically lifted at the porous layer, so the substrate can be reused. Here, we demonstrate the III-V epitaxy material quality by growing Al0.5Ga0.49In0.01As/Ga0.99In0.01As double heterostructures on porous Ge substrates and characterize them in detail to understand how the porous layers affect the structural and opto-electronic properties of theIII-Vcompounds compared to a reference grown on germanium “epi-ready” substrates. We find no significant influence of the porous Ge substrate on the layer’s composition, thickness or roughness. However, cathodoluminescence measurements reveal a defect density of 4.5 × 105 cm−2 in comparison with 6.8 × 104 cm−2 for the reference case. Those defects were identified as threading dislocations by electron channeling contrast imaging. The lifetime of minority carriers measured by time resolved photoluminescence shows no difference in the low injection regime between both samples either, indicating a high quality opto-electronic material deposited on porous Ge. These first promising results indicate a path for both: reducing costs of III-V multi-junction solar cells and a reduced germanium consumption.

Mode- and space-resolved thermal transport of alloy nanostructures

Nanostructured semiconducting alloys obtain ultra-low thermal conductivity as a result of the scattering of phonons with a wide range of mean-free-paths (MFPs). In these materials, long-MFP phonons are scattered at the nanoscale boundaries whereas short-MFP high-frequency phonons are impeded by disordered point defects introduced by alloying. While this trend has been validated by simplified analytical and numerical methods, an ab-initio space-resolved approach remains elusive. To fill this gap, we calculate the thermal conductivity reduction in porous alloys by solving the mode-resolved Boltzmann transport equation for phonons using the finite-volume approach. We analyze different alloys, length scales, concentrations, and temperatures, obtaining a very large reduction in the thermal conductivity over the entire configuration space. For example, a ∼97% reduction is found for Al0.8In0.2As with 25% porosity. Furthermore, we employ these simulations to validate our recently introduced “Ballistic Correction Model” (BCM), an approach that estimates the effective thermal conductivity using the characteristic MFP of the bulk alloy and the length-scale of the material. The BCM is then used to provide guiding principles in designing alloy-based nanostructures. Notably, it elucidates how porous alloys such as SixGe1−x obtain larger thermal conductivity reduction compared to porous Si or Ge, while also explaining why we should not expect similar behavior in alloys such as AlxIn1−xAs. By taking into account the synergy from scattering at different scales, we provide a route for the design of materials with ultra-low thermal conductivity.

Laser‐induced heating of porous Ge layers implanted with Ag+ and Cu+ ions

AbstractPorous Ge layers consisting of nanowires were formed by low‐energy high‐dose implantation of monocrystalline c‐Ge substrate with Ag+ and Cu+ ions. The obtained layers were studied by Raman spectroscopy using solid‐state and He‐Ne exciting lasers with wavelength of 532 and 633 nm, respectively. The crystalline volume fraction and local temperature of porous layers at the surface sites of laser probing were determined. To interpret the change in the shape of the spectra under the action of laser radiation, deconvolution into amorphous and nanocrystalline components was carried out. Analysis of the spectral line shape made it possible to estimate the crystalline and amorphous volume fractions. The quantum confinement model for Ge nanocrystallites was taken into account for accurate estimations of volume fractions. These estimates showed that the implanted amorphous layers of all samples were locally crystallized with a solid‐state laser. He‐Ne laser excitation resulted in partial crystallization only for layers obtained by Cu+ ion implantation. The measured ratio of the Stokes to anti‐Stokes components demonstrated that the porous layers obtained by implanting Ag+ ion were not heated by a He‐Ne laser and according to the analysis of spectral lines, the crystalline volume fraction did not appear in these layers during probing. The results obtained are explained by the difference in the penetration depth for various samples of the exciting radiation.

Performance of III–V Solar Cells Grown on Reformed Mesoporous Ge Templates

We demonstrate a solar cell on reformed porous Ge with an efficiency of 7.7%. We generate mesopores in (100) Ge by bipolar electrochemical etching and anneal them at high temperature. The pores coalesce deep in the structure rather than at the surface as desired, although resulting in coarse superficial morphology unsuitable for device growth. To combat this issue, we developed a surface treatment involving an HBr dip, annealing at 415°C, and a postannealing ultrasonic De-ionized water dip to improve the surface structure, resulting in a smoother reformed surface on which we grow a GaInAs solar cell. The structure retains embedded pores after growth and the transitions between Ge and III–V layers are distinct. The solar cell fabricated using the improved coalescence has an efficiency of 4.5%. The efficiency improves to 7.7% by isolating the rest of the device from three limiting localized shunt areas. Protruding defects in the porous Ge and III–V layers still limit the performance, but this work establishes a step toward the technical viability of this exfoliation approach, showing decent efficiency if protruding defects can be removed or reduced.

Anisotropic mesoporous germanium nanostructures by fast bipolar electrochemical etching

Mesoporous germanium has received lately a growing interest in many fields. However, the lack of flexibility and knowledge concerning its electrochemical etching remains important. In this study, it is demonstrated the first synthesis of anisotropic porous morphologies using fast bipolar electrochemical etching (FBEE). The influence of the total porosification time, the etching density as well as the passivation time on the formation of tubular porous Ge (T-PGe) are investigated. It is also shown that is possible to modify T-PGe morphology by chemical etching to create Columnar Porous Germanium (C-PGe) that offers more opened porosity with lower specific surface area. Furthermore, we report the advantage of the electrochemical performances of C-PGe compared to T-PGe, when used as on-chip anodes for lithium-ion battery. Our findings demonstrate that by engineering the porous Ge morphology the cycle life of produced anodes could be extended by a factor 26 and 1.8 for high rate and high energy applications, respectively.

Anti-reflective porous Ge by open-circuit and lithography-free metal-assisted chemical etching

Porous Ge (PGe) layer is formed on single-crystalline Ge (c-Ge) as well as in a releasable form (e.g., free-standing PGe) by lithography-free metal-assisted chemical etching (MacEtch) at room temperature under open-circuit. A thin layer of Au is evaporated on the entire surface of c-Ge and Ge on insulator prior to immersion in an etching solution. It is found that an oxide-free interface between the surface and metal catalyst is vital to form uniform PGe layer. PGe layers with different morphologies and thicknesses are produced after various MacEtch times. In order to show the functionality of PGe, reflection spectra of c-Ge (i.e., before etching) and PGe layers are characterized at a wavelength range of 1000–1600 nm. The reflection of PGe is broadly reduced to 10%, which matches well with simulation results based on finite-difference-time-domain method. Among all the modeling factors, thickness of PGe layers is found to be the primary cause of the broadband reduction of the reflection. In addition, transfer-printable free-standing PGe layers are realized. The capability of the simple, clean, and lithography-free MacEtch to achieve PGe on rigid substrates as well as in a free-standing form holds significant potential in photonic and optoelectronic device applications.

Scalable synthesis of 3D porous germanium encapsulated in nitrogen-doped carbon matrix as an ultra-long-cycle life anode for lithium-ion batteries.

Germanium-based materials attract more interest as anodes for lithium-ion batteries, stemming from their physical and chemical advantages. However, these materials inevitably undergo capacity attenuation caused by significant volumetric variation in repeated electrochemical processes. Herein, we designed 3D porous Ge/N-doped carbon nanocomposites by the encapsulation of 3D porous Ge in a nitrogen-doped carbon matrix (denoted as 3D porous Ge/NC). The 3D porous structure can accommodate the volume change during alloying/dealloying processes and improve the penetration of the electrolyte. Furthermore, the doping of N in the carbon framework could introduce more defects and active sites, which can also contribute to electron transportation and lithium-ion diffusion. The half-cell test found that at a current density of 1 C (1 C = 1600 mA h g-1), the specific capacity stabilized at 917.9 mA h g-1 after 800 cycles; and the specific capacity remained at 542.4 mA h g-1 at 10 C. When assembled into a 3D porous Ge/NC//LiFePO4 full cell, the specific capacity was stabilized at 101.3 mA h g-1 for 100 cycles at a current density of 1 C (1 C = 170 mA h g-1), and the cycle specific capacity was maintained at 72.6 mA h g-1 at a high current density of 5 C. This work develops a low-cost, scalable and simple strategy to improve the electrochemical performance of these alloying type anode materials with huge volume change in the energy storage area.

Raman study of germanium nanowires formed by low energy Ag+ ion implantation

Porous Ge layers of nanowires were formed by low energy (30 keV) Ag+ ion implantation of crystalline c-Ge substrate. Different radiation doses resulted in the formation of nanowires with various mean diameters. The obtained layers were studied by Raman spectroscopy using helium-neon and argon exciting lasers at wavelength of 633 and 488 nm respectively. It was demonstrated that implanted amorphous layers were locally crystallized by the lasers. The crystallization thresholds were found to be 3 kW/cm2 for helium-neon exciting laser and 1 kW/cm2 for argon exciting laser. The variation of thresholds is suggested to be due to the different light penetration depths at these wavelengths in Ge. The crystalline volume fraction was determined and reached maximum value of 6–7%, the divergence of which can be explained by the difference in the distribution of Ge nanowire diameters.

Effect of sintering germanium epilayers on dislocation dynamics: From theory to experimental observation

High-quality germanium epilayers on Si with low threading-dislocation density were achieved by sintering of porous Ge/Si films. The process consists in the formation of porous Ge nanostructures by dislocation-selective electrochemical etching of Ge/Si films, followed by high-temperature treatment to generate a monocrystalline, voided Ge layer that intercepts dislocations and prevents them from propagating to the sample surface. In this work, we model the morphological changes that occur during the thermal treatment-induced surface diffusion of an axially symmetric hole. Simulations and experiments show individual large spherical voids, aligned along the dislocation core. The creation of voids could facilitate interactions between dislocations, enabling the dislocation network to change its connectivity in a way that facilitates the subsequent annihilation of dislocation segments. This confirms that thermally activated processes such as state diffusion of porous materials provide mechanisms whereby the defects are removed or arranged in configurations of lower energy. This demonstration paves the way to develop a virtual substrate on many other heterostructures for potential applications in integrated photonic and optoelectronic devices.

Effects of lithium on the electronic properties of porous Ge as anode material for batteries.

Recently, the need of improvement of energy storage has led to the development of Lithium batteries with porous materials as electrodes. Porous Germanium (pGe) has shown promise for the development of new generation Li-ion batteries due to its excellent electronic, and chemical properties, however, the effect of lithium in its properties has not been studied extensively. In this contribution, the effect of surface and interstitial Li on the electronic properties of pGe was studied using a first-principles density functional theory scheme. The porous structures were modeled by removing columns of atoms in the [001] direction and the surface dangling bonds were passivated with H atoms, and then replaced with Li atoms. Also, the effect of a single interstitial Li in the Ge was analyzed. The transition state and the diffusion barrier of the Li in the Ge structure were studied using a quadratic synchronous transit scheme.

Formation of porous germanium layers with various surface morphology in dependence on mass of implanted ions

The surface morphology of nanoporous Ge (PGe) layers formed by low-energy high-dose implantation with transition metal ions depending on their mass ( 52 Cr + , 55 Mn + , 56 Fe + , 59 Co + , 59 Ni + and 63 Cu + ) was studied by high-resolution scanning electron microscopy. Ion implantation of single-crystal c -Ge substrates was carried out using similar conditions with an energy of E = 40 keV and a dose of D = 5.0·10 16 ion/cm 2 . Additionally, c -Ge was also implanted with heavier ions 108 Ag + and 122 Sb + at E = 30 keV for comparative experiments on the surface PGe morphology. It was observed that the morphology of the PGe layers changed with increasing mass of the irradiated ion from a labyrinth, hole type, three-dimensional network to spongy structure. • Various surface morphology of porous Ge layer formed by implantation with metal ions depending on their mass. • Detected morphology PGe structures from a labyrinth, hole type, 3D network to spongy structure. • SEM image observed at angle 70° to the sample surface gives a false view for Co:PGe layer.