Bipolar Electrochemical Etching Research Articles

Tuning the Optical Anisotropy in Gradient Porous Germanium on Si Substrate

AbstractPorous semiconductors have garnered significant attention owing to their distinctive physical and chemical properties. In this study, optical anisotropy is presented in porous germanium (PGe) on a Si (001) substrate. Both n‐ and p‐type PGe, achieved through bipolar electrochemical etching, exhibit optical anisotropy along the Ge <001> direction, as determined by spectroscopic ellipsometry. Birefringence and depolarization factors are controllable by adjusting the etching parameters and doping concentration of the epitaxial Ge layer. The gradient porosity and pore distribution in PGe can be well captured by the optical models. The findings of optical anisotropy in PGe‐on‐Si hold promise for applications in optical elements or sensors for gas or biomolecules.

Wafer-scale porous germanium bilayer structure formation by fast bipolar electrochemical etching

Recently, porous Germanium (PGe) structures have gained significant attention as a promising engineering material for a broad range of applications due to the versatility of its intrinsic physical and chemical properties. The Conventional Bipolar Electrochemical Etching (CBEE) technique favours a cost-effective and controlled synthesis of PGe layers, to tune the structural properties. However, some challenges still persist. Specifically, the etch rate of these layers remains low limited, resulting in extended process times. Achieving low porosity layers (< 40 %) remains unclear, and addressing the development of a non-sponge morphology is necessary for various fields, such as batteries. In this work, we investigate the impact of the anodic current density and electrolyte ratio on tubular single PGe layer (T-PGe) morphology with low porosities using a Fast Bipolar Electrochemical Etching (FBEE) process. We have demonstrated T-PGe structures with no parasitic sponge-like layer on top, providing insights to control their occurrence. We establish the feasibility of such layers as a template for the formation of tubular bilayers on the wafer's scale. This approach emphasizes the fast synthesis of promising T-PGe structures with low porosities (< 40 %).

Potential monitoring during Ge electrochemical etching: Towards tunable double porosity layers

Porous Germanium (PGe) has emerged as a promising material for applications such as substrate engineering, sensing, and energy storage, thanks to its uniquely versatile and tunable properties. The bipolar electrochemical etching (BEE) method is widely regarded as the go-to approach for producing high-quality PGe layers. However, the lack of profound understanding of BEE process limits its use to a few well-studied morphological structures, hindering the development of more complex multi-layered structures with variable morphologies. In this work, we introduce potentiometric measures of the system as a means of monitoring the BEE process. We establish a correlation between the potential response and the reactions occurring during the BEE process, as well as their evolution over time. This approach enables elucidating the importance of the passivation, and how to shape the double-layered PGe structures. These findings bring new opportunities for controllable BEE with process monitoring and fabrication of tunable PGe multilayers for advanced applications.

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.

The effect of passivation to etching duration ratio on bipolar electrochemical etching of porous layer stacks in germanium

The effect of the passivation to etching duration ratio on the bipolar electrochemical etching of highly p-doped germanium on its pore structures has been investigated. The final goal is the development of porous multi-layer stacks consisting of a closed growth template and separation layer for lift-off. In particular, changing the passivation duration allows different types of porous structures to be obtained. These range from a strong crystallographic orientation dependent dissolution to multi-layer structures consisting of “sponge” and “pine-tree” shapes. In this work, the degree of passivation was considered first and foremost since it is one way of controlling the formation of molecular hydrogen and surface atom protection. In addition, certain areas of the porous structure can be protected and, assuming a different space charge region width, selective etching is possible. Finally, by combining various successive processes, a porous multi-layer structure can be etched, including separation layers. In this work the most dominant reoccurring structures have been described using a model that contains the aforementioned assumptions.

Effect of Bipolar Electrochemical Process on Tunnel Etching Characteristics of Aluminum Foil

In this study, closed bipolar electrochemical etching was introduced to prepare etched tunnels on aluminum foils. The morphological evolution and tunnel etching behaviors of aluminum foils in the traditional electrochemical etching system and closed bipolar electrochemical etching system were thoroughly investigated using scanning electron microscopy and electrochemical measurements. Compared with the tunnels etched on the aluminum foil by traditional electrochemical etching, those by closed bipolar electrochemical etching had a larger diameter and a more uniform distribution of length. These characteristics are beneficial for improving the specific surface area of the etched aluminum foil. Furthermore, aluminum foils do not need direct electricity connection, thus offering a potential advantage in industrial production.

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.

The Effect of Passivation to Etching Duration Ratio on Bipolar Electrochemical Etching of Porous Layer Stacks in Germanium

The Effect of Passivation to Etching Duration Ratio on Bipolar Electrochemical Etching of Porous Layer Stacks in Germanium

RETRACTED: A correlation study between size and etching time in Ge mesoporous toward the incorporation of phonon dispersion effect in PCM

In this work, a new correlation study based on the phonon confinement model (PCM) and some advanced characterization tools was performed to extract the size of germanium (Ge) nanocrystals (NCs) fabricated by bipolar electrochemical etching. The PCM was established by incorporating simultaneously the size distribution and phonon dispersion effects. A correlation study of Raman spectroscopy with Atomic force microscopy (AFM), and High-resolution scanning transmission electron microscope (HRSTEM) was confirmed the presence of a spherical core with an average size of 8 nm and very small crystallites of 3 nm. This study confirms that the phonon dispersion has an excellent contribution to the broadness and the shifting of the Raman lines of Ge NCs. In addition, an observation of the breaking down and deviation of the PCM model due to the presence of crystallites in the range of [1–5 nm] is confirmed. • Strong contribution of phonon dispersion effect to quantum confinement effect in Ge nanocrystals. • AFM and Raman spectroscopy as powerful probing tools for tuning the synthesis parameters of Ge porous layers. • Limitations of phonon confinement model (PCM) is confirmed in the size range of [2–5 nm].

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.

Integration of 3D nanographene into mesoporous germanium.

Graphene is a key material of interest for the modification of physicochemical surface properties. However, its flat surface is a limitation for applications requiring a high specific surface area. This restriction may be overcome by integrating 2D materials in a 3D structure. Here, a strategy for the controlled synthesis of Graphene-Mesoporous Germanium (Gr-MP-Ge) nanomaterials is presented. Bipolar electrochemical etching and chemical vapor infiltration were employed, respectively, for the nanostructuration of Ge substrate and subsequent 3D nanographene coating. While Raman spectroscopy reveals a tunable domain size of nanographene with the treatment temperature, transmission electron microscopy data confirm that the crystallinity of Gr-MP-Ge is preserved. X-ray photoelectron spectroscopy indicates the non-covalent bonding of carbon to Ge for Gr-MP-Ge. State-of-the-art molecular dynamics modeling provides a deeper understanding of the synthesis process through the presence of radicals. The successful synthesis of these nanomaterials offers the integration of nanographene into a 3D structure with a high aspect ratio and light weight, thereby opening avenues to a variety of applications for this versatile nanomaterial.

Fast growth synthesis of mesoporous germanium films by high frequency bipolar electrochemical etching

Mesoporous germanium (MP-Ge) has been predicted to play an important role in a wide range of potential applications. These porous Ge networks are characterized by physical and chemical properties that contrast with their bulk counterpart. For more than two decades, the methods that were used to synthesize these materials were extremely slow and yield non-uniform layers, which limit their applications. Here we demonstrate the synthesis of thick MP-Ge films with high growth rate (up to 300 nm/min) by using high-current density and high-frequency bipolar electrochemical etching. An optimized nucleation procedure is also reported. It enables a much better lateral homogeneity of the MP-Ge films. Following these results a refined electrochemical Ge etching model is proposed. The model is confirmed by Fourier Transform Infrared Spectroscopy and Cyclic Voltammetry measurements. Reflectivity measurements were also used to demonstrate that the effective refractive index of MP-Ge can be adjusted as a function of layer porosity. It has been reduced down to 1.76 due to the presence of the mesopores. Finally, a basic interferometric porous Ge sensor is demonstrated, a detection sensitivity of 711 nm/RIU was measured upon exposure to glycerol-water solutions.

Fabrication of Nanostructured Mesoporous Germanium for Application in Laser Desorption Ionization Mass Spectrometry.

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is a high-throughput analytical technique ideally suited for small-molecule detection from different bodily fluids (e.g., saliva, urine, and blood plasma). Many SALDI-MS substrates require complex fabrication processes and further surface modifications. Furthermore, some substrates show instability upon exposure to ambient conditions and need to be kept under special inert conditions. We have successfully optimized mesoporous germanium (meso-pGe) using bipolar electrochemical etching and efficiently applied meso-pGe as a SALDI-MS substrate for the detection of illicit drugs such as in the context of workplace, roadside, and antiaddictive drug compliance. Argon plasma treatment improved the meso-pGe efficiency as a SALDI-MS substrate and eliminated the need for surface functionalization. The resulting substrate showed a precise surface geometry tuning by altering the etching parameters, and an outstanding performance for illicit drug detection with a limit of detection in Milli-Q water of 1.7 ng/mL and in spiked saliva as low as 5.3 ng/mL for cocaine. The meso-pGe substrate had a demonstrated stability over 56 days stored in ambient conditions. This proof-of-principle study demonstrates that meso-pGe can be reproducibly fabricated and applied as an analytical SALDI-MS substrate which opens the door for further analytical and forensic high-throughput applications.

Nanoscale morphology tuning of mesoporous Ge: electrochemical mechanisms

The extended analysis of influence of the surface hydrogen passivation degree on morphology of mesoporous Ge (PGe) formed by Bipolar Electrochemical Etching technique is reported. It is shown that the high degree of the surface passivation results in formation of the isotropic sponge-like PGe layers, whereas at low degree of the surface passivation the crystallographic “pine-tree-like” pores are obtained. The intermediate passivation results in the “fish-bone” morphology. It should be noted that the last two morphologies were never obtained before and are unique to mesoporous Ge. The electrochemical reactions taking place on Ge during BEE process are identified by analyzing the evolution of the surface potential during the BEE process; the results were interpreted similarly to the cyclic voltametry data. Finally, a general electrochemical model describing the pore formation with different morphologies depending on the surface passivation degree is introduced.

Near-infrared emission from mesoporous crystalline germanium

Mesoporous crystalline germanium was fabricated by bipolar electrochemical etching of Ge wafer in HF-based electrolyte. It yields uniform mesoporous germanium layers composed of high density of crystallites with an average size 5-7 nm. Subsequent extended chemical etching allows tuning of crystallites size while preserving the same chemical composition. This highly controllable nanostructure exhibits photoluminescence emission above the bulk Ge bandgap, in the near-infrared range (1095-1360nm) with strong evidence of quantum confinement within the crystallites.

Mesoporous Germanium formed by bipolar electrochemical etching

The formation of mesoporous Ge layers with different morphologies by bipolar electrochemical etching is reported for the first time. A detailed analysis of the effect of anodic/cathodic pulse duration on the mesoporous Ge formation is performed. It is shown that together with the applied current density and anodization time this parameter has the most important influence on porous Ge morphology. Six different types of porous morphologies were identified with the nanopore diameter being in the range from 5 to 15nm. Their crystalline nature has been confirmed by Raman spectroscopy. The diameters of the nanocrystallites of the porous Ge layers were extracted from Raman spectra by using the phonon-confinement model. Depending of the layer morphology their values varied between 4 and 10nm. A conceptual analysis of several aspects of the formation mechanisms of mesoporous Germanium based on the available information has allowed us to predict and create a new complex morphology.

Structural and Morphological Study of Mesoporous Germanium Layers Formed by Bipolar Electrochemical Etching

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Preparation and functionalization of hydride terminated porous germanium

Porous germanium (PG) is prepared by a novel bipolar electrochemical etching (BEE) technique; scanning electron microscopy (SEM) clearly reveals formation of a porous layer up to a few microns thick that is Ge–Hx terminated as indicated by FTIR spectroscopy; the hydride terminated PG material is quite resistant to oxidation, even under thermal conditions, but can be induced to undergo hydrogermylation reactions with alkenes and alkynes.