Porous Silicon Layers Research Articles

Fabrication of Porous Silicon as a Gas Sensor: The Role of Porous Silicon Surface Morphology

Manufacture of an environmental polluting gas sensor with improved properties by controlling the preparation conditions of the photo-electrochemical etching technique (PECE). The amount of porosity, the diameter of the pores, and the thickness of the prepared layer of porous silicon (Psi) can be controlled by changing one or all of these conditions. In this paper, n-type Si with a crystalline orientation (100) was used, whereby PSi was prepared with the use of a red diode laser with a wavelength of 650 nm, using different radiation intensity, and with the constancy of etching time and current density. Through the results obtained, it was noted that: the porosity increases significantly up to 75% as well as the thickness of the PSi layer up to 1.45 µm with the increase in the intensity of the laser beam. Also, examining the morphology of the surface samples by field emission scanning electron microscope (FE-SEM) besides, the average pore diameters of the prepared samples were calculated. It is clear that the intensity of the laser beam used in the irradiation process is one of the important factors in determining the properties of the prepared PSi. PSi samples have been tested by FTIR to investigate chemical bonds on surfaces such as, (Si-Si, Si-H, Si-H2, Si-O-Si, Si-O-Si, Si-H, Si-O-Si). Samples tested as gas sensors and noticed that an increase in the sensing current to 5.3 µA has appeared with the increase of porosity value where methanol gas is used as background.

Reflection Coefficient Calculation of a Structure Including a Porous Silicon Layer with Transfer Matrix Method and FDTD

Porous silicon is an important material for a variety of application area such as anti-reflective coating for solar cells. Today, solar cell market is mostly dominated by silicon based solar cells. Porous silicon thin films are easy to fabricate and it is compatible with silicon technology. Designing porous silicon anti-reflective coating layers is a critical issue to enhance silicon based solar cell performance. There are several methods to calculate reflection coefficient of porous silicon thin layers. In this study, transfer matrix method and finite-difference time-domain method are used to calculate reflection coefficient of porous silicon thin layers. Because finite-difference time-domain method gives more accurate results, the results obtained with finite-difference time-domain method are used to control the results obtained with transfer matrix method. In transfer matrix method, refractive indices of the porous silicon layers are calculated with Bruggeman effective medium approximation.

Improved Difenoconazole pesticide detection limit via double-sided porous silicon layers' electrical sensor

In this novel work, high-performance laser synthesized double-sided parallel porous silicon layers (Ds-Psi) pesticide sensors were employed. The Ds-Psi layers were synthesized via double beam laser-assisted etching (D-LAE). An easy run, cost-effective and non-poisonous process was adopted to investigate the Ds-Psi sensor performance. This process depended on recording the resonance frequency shift when exposing the pesticides to the sensor's surface. Two configurations were adopted: single-sided Psi sensor of Al/Psi/n c-Si/Al and parallel connected Ds- Psi layer of Al/Psi/c-Si/Psi/Al were synthesized and studied. Under ultra-low concentrations of Difenoconazole pesticide at room temperature, the current-frequency properties of the sensors revealed an excellent response of the Ds-Psi sensor compared with the single-sided Psi sensor. The results of the Ds-Psi sensor showed a much higher sensitivity (82.9%) and a lower detection limit of detection 0.001 ppm. The principal reason for this high performance is effectively related to the high surface area and large capacitance of the parallel configuration of the Ds-Psi layers. The synthesized impedance matching of the parallel configuration is very suitable for quantitative sensing of pesticides.

Formation of SiC by Vacuum Carbidization on Porous Silicon

Planar-view TEM investigation revealed the formation of cubic silicon carbide layers on porous silicon by vacuum carbidization. The formation of cubic silicon layers in the form of a two-phase system was found. At the same time, the formed SiC layers on the mesoporous buffer layer are predominantly polycrystalline. Using the Rutherford backscattering method, it was found that the use of buffer layers of porous silicon makes it possible to obtain SiC layers of greater thickness than on a pure silicon substrate under similar conditions of vacuum carbidization. It is shown that an increase in the pore size in porous silicon layers leads to an increase in the thickness of the formed SiC layers. It has been shown by scanning electron microscopy that vacuum carbideization of porous silicon leads to formation of SiC grains in pores, partial overgrowth and sintering of pores. The dependence of the SiC grain size on the pore size was established.

Self-Supported Bragg Stacked Microcavities for DNA Sensing

Porous silicon (PSi) layers are prepared on the single-crystal silicon surface by electrochemical etching, and PSi membranes (PSiMs) are fabricated by separating the PSi layers from the single-crystal silicon substrate using high-current pulses. PSiMs have better adsorption and response time than nonseparated PSi layers, making them more suitable for sensing applications. PSi microcavity (PSM) structures are widely used for the detection of biomolecules in biosensors because of their high transmission resonance peaks with narrow full-width at half-maximum (FWHM) in the reflection spectra. However, when compared to PSiMs, the permeability of the PSM on the silicon substrate is low, resulting in a significant reduction in its detection sensitivity. In this study, an efficient self-supported Bragg stacked microcavity (SBSM) structure was prepared for the detection of DNA biomolecules by combining the advantages of PSM and PSiMs. The redshift of the reflectance spectra obtained in the concentration range of 0.1– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.5 ~\mu \text{M}$ </tex-math></inline-formula> shows a good linear relationship, with a detection limit of 2.2 nM. The detection sensitivity is improved by more than a factor of ten when compared to the PSM structure on the silicon substrate. The novel device allows DNA biomolecules to enter the porous layers more easily, especially the defective layer, thus improving its detection sensitivity.

High-performance photodetector of Au–MgO/PS nanostructure manufactured via pulsed laser ablation technique

This work focuses on photodetector Au:MgO nano-particles (NPs) have been prepared by laser ablation at various laser energies then deposited on porous-Si (PS). PS are produced with the use of the photo-electrochemical etching approach for n-type crystalline Si wafers of (100) orientation. XRD, scanning electron microscope, UV–visible spector-photometer and electrical characteristics that have been utilized for characterizations of products. The photo-detector measurements, the spectral responsivity curves inclusive three regions; the first peak was due to absorption of the ultraviolet light by Au:MgO NPs. The second peak corresponding to the absorption of the visible light with porous silicon layer and the third peak to absorption edge of the Si substrate. The higher responsivity of Au:MgO NPs/PS photo-detector were found to be 3.29 A/W for specimen prepared at laser energy 800 mJ/pulse.

Germanium electrodeposition into porous silicon for silicon-germanium alloying

A method of germanium electrodeposition from a GeO2-based aqueous solution into the pore channels of anodic mesoporous silicon formed on n-type highly-doped (100) silicon wafers is described. The effect of deposition time, pore channel shape and preconditioning of porous silicon layers in hydrofluoric acid is evaluated. Recommendations are given in regards to the optimal parameter combinations to ensure uniform pore channel filling with germanium. The possibility of producing silicon-germanium alloys by subsequent rapid heat treatment of the germanium-filled porous silicon layers is established.

Ball milling synthesis of robust sandwich-structured C/Si@SnO2 anode with porous silicon buffer layer for fast charging lithium-ion battery

Aiming at enhancing cyclic stability and fast charging performance of lithium-ion batteries (LIBs), a sandwich structured composite with a buffer layer of porous silicon (Si) between the carbon layer and SnO2 layer (C/Si@SnO2) was developed by using ball milling. The results showed that the Si layer with plenty of mesoporous and cracks was coated successfully and the introduction of Si increased the disordered structure of SnO2, including the expansion of interlayer spacing, the emergence of Sn and SiOx indicated the reaction between Si and SnO2 occurred during the ball milling process, while the generation of Si-O-Sn and Si-O-C bonds formed by Si integration connected the three components with intimate interfaces. Used as an anode for LIBs, the C/Si@SnO2 electrode performed a favorable discharge capacity (919.21 mAh·g−1 at 0.1 A·g−1), the ultra-long cyclability after 1000 and 2000 cycles, impressive rate performance (269.87 mAh·g−1 at 5 A·g−1), and fast charging capability accompanying discharge capacity of 805.57 mAh·g−1 at 0.2 A·g−1 and charge capacity of 400.55 mAh·g−1 at 2 A·g−1. In addition, the electrochemical impedance spectroscopy analysis showed that the graphitic carbon layer and porous Si layer improved the rapid Li+ diffusion and transportation, which was beneficial to fast charging in LIBs. Our work provided a facile and green ball milling method to construct porous sandwich structured metal oxide/C composites with the introduction of Si, which can effectively increase the disordered structure of composite and also open up a new scheme of Si materials as an anode in the field of fast charging for LIBs.

Effect of surfactant and etching time on p-type porous silicon formation through potentiostatic anodization

Electrochemical anodization provides the scalability required for structuring porous silicon (PSi) layers for mass production; hence, new and feasible processes are highly sought-after. We investigate the effect of surfactant (additive) and etching time on the morphology of PSi matrix in a simplistic two-electrode anodization cell using aqueous HF electrolyte. Instead of the conventional galvanostatic mode (constant current density), we use the rarely reported technique of potentiostatic anodization (constant applied potential) for engineering PSi surface morphology. We demonstrate that under a constant applied potential, channel-like morphology, pyramids or well-ordered macropores are easily achieved through either increasing the processing time or adding a small amount of surfactant into the electrolyte. Our results provide better understanding of the mechanism underlying the formation of PSi and propose a practical solution for obtaining application-specific macrostructure of PSi.

Laser-ablated synthesis of Ag@TiO2 NPs on Si nanostructure as humidity sensor

ObjectiveThis work aimed to produce Ag@TiO2 nanoparticles/PS ultra-high sensitivity for humidity by laser ablation. Methodswe combine Ag/TiO2 NPs of 59.93 nm with porous silicon (PS) to create a nanocomposite system. On an n-type silicon wafer, the structural, morphological, and optical properties of a silicon bulk and PS layer are detailed in depth. ResultsThe PL regarding the sample PS and Ag/TiO2 NPs is measured in air at various relative humidity (RH) levels. PS’s PL band is associated with quantum confinement; PL for the Ag/TiO2 system has a broad peak. Even though the decline in intensity could be attributed to Ag/TiO2 absorption regarding the excitation and emission radiations, this shows the peak from PS. In the case when RH levels rise, the PL of the PS and Ag/TiO2/PS humidity sensors alter. The intensity of the PL lessens. This effect may be described as capillary condensation of water vapor within PS pores.

The role of various etching time in Si nanostructures for ultra-high sensitivity photodetector

The current study aimed to produce Si nanostructures ultra-high sensitivity photodetector by photo electrochemical method. The effect of changing some factors on the formation of porous silicon has been studied. The etching time has been changed using a photoelectrochemical procedure with a Halogen lamp. Relying on crystalline silicon n-type wafers (100) with resistivity of 0.1–100 Ω.cm, porous silicon layers were developed. Etching solution containing 16% HF was used at different etching times of (6,9,12,15,18, and 21) min. and 20 mA/cm2 current density. The properties of the (J-V) and (Jph-V) in both darkness and photocurrent density, were used in demonstrating PS. When the crystal size is around a nanometer, XRD broadening of PS nanostructures increased as the crystallite size dropped, with the peaks becoming more widened. The peak of the PL spectrum changes a little on the high-energy side, indicating the presence of quantum confinement effect in porous silicon. While analyzing SEM data, the pores have cylindrical shape is a consequence of increasing the morphological characteristics average diameter. When compared to bulk silicon reflectivity, the observed significantly – lower PS surface Reflectance, demonstrated the porous silicon layer's thinness. In responsivity, two peaks appeared. The first one reaction peak was absorbed in the depletion region, at 500 nm of the Al-PS. While the second peak absorption was in the PS-c-Si depletion area at 500–600 nm and on 21 min etching time, where a maximum quantum efficiency of 83% was obtained.

Preparation and Characterization of Porous Silicon for Photodetector Applications

Photoelectrochemical etching (PECE) was used to prepare porous silicon (PS) layers of polished surfaces of (100) n-type silicon wafers with a resistance of 0.1-100 μm and thickness of 600 ± 25 μm. The directed slices are to be catalyzed at different etching times (5, 15, 25 min) with a constant Hydrofluoric acid of 20% and with a fixed current density of 20 mA/cm². The porous silicon morphology was investigated using scanning electron microscopy (SEM). Samples were formed by different engraving times. It revealed that the silicon surface has a layer of sponge-like structure, with the average pore diameter (740±1 nm, 550±2 nm,460±3 nm) of the porous silicon increasing as the etching time increased. PS Al PS /Si /Al photodetectors were found to work as a photodetector over a wide wavelength responsivity.

Study of zinc oxide/porous silicon interface for optoelectronic devices

A study of the interface of the ZnO/PSi heterojunction with photoluminescense (PL), cathodoluminescense (CL) and Electron Beam Induced Current (EBIC) is presented. The fabrication process of the heterostructure consisted of three steps; First, it was formed the porous silicon (PSi) layer via electrochemical synthesis, it was followed by the growth of a ZnO thin film onto the Psi layer via RF-magnetron sputtering, Finally, a thermal annealing process at 700 °C was applied on the ZnO/PSi heterojunction device. The PL analysis revealed a widening of the luminescence spectrum (in a range of 350 to 850 nm) due to the application of a thermal annealing process which favored the crystallization of the ZnO/PSi structure. Monochromatic CL images were focused on the two main peaks of the CL spectra at 385 and 436 nm, showing the emission of the ZnO and the ZnO/PSi structure, respectively. EBIC characterization determined the electronic properties of the heterojunction such as deflection zone (50 nm) as well as the hole (Lp=55nm), and electron (Ln=60nm) diffusion lengths. Furthermore, EBIC characterization served to experimentally determined the density of donor impurity atoms in the ZnO film (Nd=3.75x1014cm−3).

High Energy Limit of the Size-Tunable Photoluminescence of Hydrogen-Terminated Porous Silicon Nanostructures in HF

The photoluminescence (PL) of various porous silicon (PSi) layers was studied during chemical dissolution in HF. The relative PL quantum efficiency of some layers was also monitored. Typically, the PL increased, reached a maximum and then dropped down to complete extinction, accompanied with a PL blueshift. During PL fall, both the PL intensity and layer quantum efficiency fell sharply, accompanied by a decrease in full width at half maximum and a slowing blueshift. In the final stage, the PL intensity decreased without any further blueshift, the saturated PL peak wavelength being ∼515 nm (∼2.4 eV) for most layers, identifying a high energy limit for the achievable PL of hydrogen-terminated Si nanostructures. Our results show that sudden catastrophic mechanical failure of nanostructure cannot explain the sharp PL drop and saturation of PL blueshift. Rather, they support the idea of a critical size (∼1.5–2 nm) below which the PL quantum efficiency vanishes. The possible reasons were discussed, privileging the emergence of structural non-radiative defects below a certain size, though the decreasing intrinsic quantum efficiency of Si nanocrystals with decreasing size could also play an important role. Maximum PL intensity was generally obtained for a peak wavelength of ∼565 nm (∼2.2 eV).

INVESTIGATION OF THE EFFECT OF ETCHING CURRENT ON TEXTURED SILICON SURFACE FOR FORMING POROUS SILICON LAYERS ACTING AS AN ANTI-REFLECTION COATING.

The porous silicon layers (PSLs) acting as an anti-reflection coating (ARC) formed with different surface porosities are prepared on n+ p textured crystalline CZ- silicon by electrochemical etching (ECE) in HF-basedelectrolyte using different current densities.The morphological properties of the PSLs are investigated by scanning electron microscopy (SEM). The results show that the porous silicon structure had a smaller pore size range, and the optical properties of the textured surfaces are studied using photoluminescence “PL” and reflectivity measurements. The band gap energy of the prepared PSLs increases to 1.904 eV. The reflectivity of the PSLs decreases to 0.75% in a wavelength range (350–750) nm. The current-voltage (I-V) characteristics of Al/ PS/n+ p/Al junction are investigated which reveal an increment in the resulting short-circuit current density and the open-circuit voltage up to 2.96 mA/cm2 and 0.385 V, respectively. These results show an improvement in the fill factor by 48.5%. The photosensitivity of PSLs solar cells showed a noticeable enhancement in quantum efficiency of the PS cells by increasing the porosity.

Comparative study of nanostructured ultra-thin AlGaN/GaN heterostructures grown on hybrid compliant SiC/porSi substrates by molecular beam epitaxy with plasma nitrogen activation

This study demonstrates the advantages of using a hybrid compliant substrate containing a porous silicon (porSi) layer, obtained through electrochemical etching of the original silicon substrate, and a silicon carbide (SiC) layer, formed on its surface through the Kukushkin method, to grow high-quality, ultra-thin nanostructured AlхGa1-xN/GaN heterostructures with nanocolumnar morphology by molecular beam epitaxy with plasma-activated nitrogen.Comparison of our experimental results with information from prior literature illustrates that the use of such a hybrid SiC/porSi substrate has a number of undeniable advantages for the growth of ultra-thin AlхGa1-xN/GaN nanoheterostructures without the use of thick AIIIN buffer layers. Direct growth on a hybrid compliant substrate and SiC/porSi leads to a substantial relaxation of elastic stresses between the epitaxial film, porous silicon, and silicon carbide; this consequently affects the structural quality and optical characteristics of AIIIN nitride-based transistor ultra-thin structures.The experimental and computational data obtained in our research are important for understanding the physics and technology of AlхGa1-xN/GaN nanoheterostructures, and they will contribute to their potential applications in optoelectronics.

Synthesis of Porous Silicon by Electrochemical Etching for Gas Sensor Application

In the present study, the layers of porous Silicon (PS) have been produced from the p-type Silicon with a (100) orientation using the approach of electrochemical etching. The samples were anodized in a solution of HF concentration 18% and 99% C2H2OH. Samples characteristics of PS were studied by etching time constant (15 min). In addition, the alteration of the current density value into (5, 10,15,20, and 25) mA/ cm2 was also studied. Samples were characterized by nanocrystalline porous Silicon via X-Ray Diffraction (XRD). The AFM (Atomic force microscope) analysis of PS shows the sponge-like structure. Also, a 39.76 nm average diameter was coordinated in the rod-like temperature variation, fabricated from prepared samples on the sensor's sensitivity, recovery time, and response time. The maximal level of the sensitivity has been approximately (20,11)% for porous Silicon of gas NO2 and NH3, respectively.

The Effects of a Few Formation Parameters on Porous Silicon Production in HF/HNO3 Using Ag-Assisted Etching and a Comparison with a Stain Etching Method

AbstractIn a solution of HF with HNO3 as an oxidizing agent, silver-catalyzed etching of p-type silicon is made easier. Before immersing in the etchant solution, silver (Ag) was electroless deposited on the p-Si (100) surface. By stain etching in HF/HNO3, a porous silicon layer (PSL) was also produced on p-Si. Electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), atomic force microscopy (AFM), and X-ray diffraction (XRD) were used to evaluate the properties of the produced PSL. According to the SEM, Ag+ ion at a concentration of 1 × 10−3 M is the optimal concentration for depositing on Si before chemical etching in HF/HNO3, resulting in PSL with uniformly distributed pores. The EIS data showed that coated Si dissolves faster in 22 M HF/0.5 M HNO3 than untreated Si, resulting in the formation of a homogenous PSL of regular round pores, as proven by SEM micrographs. An acceptable electrical circuit model with two-time constants was used to fit the experimental impedance values. Increased concentrations of the etchant HF or the oxidizer HNO3 aid in the dissolution of Si and the rapid development of PS. The AFM analysis revealed that when the etching time increases, the pore width and roughness of the Si surface increase. X-ray spectra diffraction was used to determine the crystallinity of the PSL after various etching times.

Correlation between microstructural and optoelectronic properties of porous Si/ SnO2 composites coated Al2O3

In this work, Al2O3 coating effect on morphology, structure and optoelectronic properties of Si/SnO2/Al2O3 porous matrix composites (PMCs) were investigated. A three-staked thin layers deposited on a 〈100〉 oriented silicon substrate made these composites. First, porous silicon layers were achieved by electrochemical etching method. Then Al2O3 and SnO2 layers were successively deposited by physical and chemical vapor deposition, respectively. Morphological and micro-structural properties of the as prepared composites were evaluated by Scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction. Results proved that Al2O3 concentration alters notably the porosity of the PMCs.Variable angle spectroscopic ellipsometry (SE) revealed a high correlation between the optical constants (n, k) and the PMC microstructure. Impedance spectroscopy revealed a semiconductor-metallic transition at high frequency in the temperature range between 340 and 410 °C.

Integrating porous silicon layer backing to capacitive micromachined ultrasonic transducers (CMUT)-based linear arrays for acoustic Lamb wave attenuation

Lamb waves propagating in the substrate of linear arrays integrated on a silicon (Si) chip may degrade the elementary performances of the imaging device. In fact, these waves are radiated in the imaging medium. Their superimposition with the relevant ultrasonic signals alters the image performances (i.e., lateral and axial resolutions). In this article, we investigate the interest of using a thin layer of porous silicon (PS) as an absorbing material, aiming to reduce the total device dimensions compared to more traditional backing materials and facilitate device integration with on-chip electronics. The proposed method was applied to Capacitive Micromachined Ultrasonic Transducers. To this purpose, a PS layer with a thickness of 60 μm and a porosity of 50% was etched on the rear side of a 256-elements linear array. The electroacoustic response of the elements integrated on the Si substrate was compared to those on the Si/PS substrate, showing no deterioration of the acoustic characteristics (i.e., center frequency and bandwidth) after PS layer fabrication. To assess the PS silicon layer influence on Lamb wave attenuation, acoustic cross-talks were measured for each array element. The radio-frequency dataset was used to determine the dispersion curves of Lamb waves in the substrate. The comparison between the two substrates showed a significant attenuation value (superior to 30 dB) of Lamb waves induced by the PS layer.