Porous Silicon Thin Films Research Articles

Nanoscale morphology, optical dynamics and gas sensor of porous silicon

We investigated the multifaceted gas sensing properties of porous silicon thin films electrodeposited onto (100) oriented P-type silicon wafers substrates. Our investigation delves into morphological, optical properties, and sensing capabilities, aiming to optimize their use as efficient gas sensors. Morphological analysis revealed the development of unique surfaces with distinct characteristics compared to untreated sample, yielding substantially rougher yet flat surfaces, corroborated by Minkowski Functionals analysis. Fractal mathematics exploration emphasized that despite increased roughness, HF/ethanol-treated surfaces exhibit flatter attributes compared to untreated Si sample. Optical approaches established a correlation between increased porosity and elevated localized states and defects, influencing the Urbach energy value. This contributed to a reduction in steepness values, attributed to heightened dislocations and structural disturbances, while the transconductance parameter decreases. Simultaneously, porosity enhances the strength of electron‒phonon interaction. The porous silicon thin films were further tested as effective gas sensors for CO2 and O2 vapors at room temperature, displaying notable changes in electrical resistance with varying concentrations. These findings bring a comprehensive exploration of some important characteristics of porous silicon surfaces and established their potential for advanced industrial applications.

Protein Identification and Quantification Using Porous Silicon Arrays, Optical Measurements, and Machine Learning.

We report a versatile platform based on an array of porous silicon (PSi) thin films that can identify analytes based on their physical and chemical properties without the use of specific capture agents. The ability of this system to reproducibly classify, quantify, and discriminate three proteins separately is demonstrated by probing the reflectance of PSi array elements with a unique combination of pore size and buffer pH, and by analyzing the optical signals using machine learning. Protein identification and discrimination are reported over a concentration range of two orders of magnitude. This work represents a significant first step towards a low-cost, simple, versatile, and robust sensor platform that is able to detect biomolecules without the added expense and limitations of using capture agents.

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.

Determination of thermal conductivity, thermal diffusivity and specific heat capacity of porous silicon thin films using the 3ω method

Here the thermal transport properties of low thermal conductivity porous silicon thin films attached to high thermal conductivity silicon substrates are studied using the 3ω method implemented over the 100 Hz to 33 kHz frequency range. The thermal conductivity and thermal diffusivity of the films are extracted using temperature impedance monitoring of electrical contacts deposited on films, combined with an extended-frequency, multi-layer thermal model. From the extracted thermal conductivity and diffusivity of the films, the heat capacity could be determined. Validation of the approach is performed using the known properties of thick substrate glass and SU-8 layers spun on silicon substrates, the latter ranging in thickness from 1.35 to 12.5 µm. The technique was then applied to porous silicon films with porosities ranging from 45% to 77%. The extracted thermal properties for as-fabricated films show a reduction of thermal conductivity and diffusivity from 1.7 to 0.15 W/mK and 1.9 to 0.2 mm2/s, respectively as the porosity increases. After passivation by annealing in nitrogen and at 600 °C, the same films exhibited higher values of thermal conductivity and diffusivity ranging from 2.7 to 0.7 W/mK and 2.5 to 0.65 mm2/s. The ability to extract both thermal conductivity and thermal diffusivity for these films removes the need to make assumptions around specific heat capacity, commonly made during analysis of porous media. These results show for the first time a monotonic increase in specific heat capacity of porous silicon films as a function of porosity.

Micromachined porous silicon Fabry-Pérot long wavelength infrared filters

Porous silicon Fabry-Pérot (PS-FP) filters operating in the long wavelength infrared (LWIR) range are demonstrated and studied. Porous silicon (PS) offers many advantages for such filters, including improved optical and mechanical properties compared to other LWIR filter materials, and the ability to modulate porosity to form multilayer structures that are compatible with conventional semiconductor technologies. This work discusses the optical and the mechanical characterizations of what are the first MEMS-based suspended PS-FP filters operating in the LWIR range. Methods developed to model and characterize PS-FP filters are discussed, with characterization a challenge for such porous structures considering their low refractive index and low Young’s modulus. The transmittance measurements show that introducing notches at the four corners of the top mirror minimizes the bowing of the top mirror significantly. The stress measurements of attached films show that the lattice mismatch at the PS/silicon interface is likely the key factor controlling the stress in multilayer PS thin films. The micromachining techniques developed to form a resonance cavity within PS-FP filters can also be tuned to reduce the stress mismatch and the absorption loss in the LWIR range. This study shows exciting promise for the use of micromachined PS-based filters for LWIR operation.

Engineering 1/f noise in porous silicon thin films for thermal sensing applications

The sensitivity of present-day thermal sensors operating in the long-wave infrared region, a critical range for night vision and other reconnaissance applications, is limited by the 1/ f noise in the thermistor material, while their widespread applicability is restricted due to microfabrication incompatibilities with the standard processing for silicon-based microelectronics. Porous silicon films have a high temperature coefficient of resistance ( TCR ~2–10%K −1 ) and are compatible with microfabrication processes, making them suitable candidates for use in thermal sensors. In this work, the 1 /f noise in porous silicon films in conjunction with TCR , both key parameters for high sensitivity thermal sensors are characterized. The inherent 1 /f noise in these as-fabricated films is very high and increases significantly after exposure to atmosphere (oxidation). However, surface passivation at 600 °C, stabilises the film against atmospheric oxidation and reduces the 1 /f noise level by orders of magnitude. Further, decreasing the porosity of passivated films from 90 to 40% decreased the 1 /f noise by an order of magnitude. High TCR ~6–7%K −1 along with low 1/ f noise constant, K~ 10 - 12 are obtainable with an optimised porosity~60–75% for films surface passivated at 600 °C. These results confirm great promise of porous silicon for low manufacturing cost, and higher sensitivity of the thermal sensors. • As-fabricated porous silicon films have very high 1/ f noise that further increases after exposure to atmosphere due to oxidation. • Surface passivation of porous silicon films reduces 1/ f noise by three orders of magnitude. • 1/ f noise can be further tuned by controlling the porosity: decreasing the porosity from 90 to 40% reduced the 1/ f noise constant by an order. • Higher temperature coefficient of resistance ( TCR ) and lower 1/ f noise constant ( K) important to realize high sensitivity based thermal sensors , can be simultaneously obtained by controlling the fabrication parameters and surface passivating the porous silicon films.

Porous Silicon-Based Aptasensors: Toward Cancer Protein Biomarker Detection.

The anterior gradient homologue-2 (AGR2) protein is an attractive biomarker for various types of cancer. In pancreatic cancer, it is secreted to the pancreatic juice by premalignant lesions, which would be an ideal stage for diagnosis. Thus, designing assays for the sensitive detection of AGR2 would be highly valuable for the potential early diagnosis of pancreatic and other types of cancer. Herein, we present a biosensor for label-free AGR2 detection and investigate approaches for enhancing the aptasensor sensitivity by accelerating the target mass transfer rate and reducing the system noise. The biosensor is based on a nanostructured porous silicon thin film that is decorated with anti-AGR2 aptamers, where real-time monitoring of the reflectance changes enables the detection and quantification of AGR2, as well as the study of the diffusion and target-aptamer binding kinetics. The aptasensor is highly selective for AGR2 and can detect the protein in simulated pancreatic juice, where its concentration is outnumbered by orders of magnitude by numerous proteins. The aptasensor’s analytical performance is characterized with a linear detection range of 0.05–2 mg mL–1, an apparent dissociation constant of 21 ± 1 μM, and a limit of detection of 9.2 μg mL–1 (0.2 μM), which is attributed to mass transfer limitations. To improve the latter, we applied different strategies to increase the diffusion flux to and within the nanostructure, such as the application of isotachophoresis for the preconcentration of AGR2 on the aptasensor, mixing, or integration with microchannels. By combining these approaches with a new signal processing technique that employs Morlet wavelet filtering and phase analysis, we achieve a limit of detection of 15 nM without compromising the biosensor’s selectivity and specificity.

Absorption and emission of porous silicon based on quantum dots models by TD-DFT: Experimental and theoretical approach

In the present work, the relationship between experimental and theoretical analysis of optical properties of Si-nanostructures is presented. Coral-like luminescent porous silicon (p-Si) thin films were synthesized, and the main photoluminescence peak was found at around 690 nm. Infrared spectroscopy revealed mainly the presence of Si–H and Si–O bonds. The structural characterization determined an average nanocrystal size of 5.4 nm. A model based on Si quantum dots (QDs) with sizes ranging from 1.0 to 2.2 nm by TD-DFT (PBE/6-31G) in the Gaussian-16 package was used to simulate p-Si nanostructures. Equations were proposed for QD-like morphology of p-Si to calculate absorption/emission energies that fit with experimental data with high accuracy. The present research will help to predict the optical properties of complex and heterogeneous Si nanostructures.

In Situ Photoacoustic Study of Optical Properties of P-Type (111) Porous Silicon Thin Films.

Porous silicon (PSi) on p-type (111) silicon substrate has been fabricated by electronically etching method in hydrofluoric acid (HF) media from 5 to 110 mA/cm of anodizing current density. The problem of determining the optical properties of (111) PSi is board through implementing a photoacoustic (PA) technique coupled to an electrochemical cell for real-time monitoring of the formation of porous silicon thin films. PA amplitude allows the calculation of the real part of the films refractive index and porosity using the reflectance self-modulation due to the interference effect between the PSi film and the substrate that produces a periodic PA amplitude. The optical properties are studied from specular reflectance measurements fitted through genetic algorithms, transfer matrix method (TMM), and the effective medium theory, where the Maxwell Garnett (MG), Bruggeman (BR), and Looyenga (LLL) models were tested to determine the most suitable for pore geometry and compared with the in situ PA method. It was found that (111) PSi exhibit a branched pore geometry producing optical anisotropy and high scattering films.

Mass Transfer Limitations of Porous Silicon-Based Biosensors for Protein Detection.

Porous silicon (PSi) thin films have been widely studied for biosensing applications, enabling label-free optical detection of numerous targets. The large surface area of these biosensors has been commonly recognized as one of the main advantages of the PSi nanostructure. However, in practice, without application of signal amplification strategies, PSi-based biosensors suffer from limited sensitivity, compared to planar counterparts. Using a theoretical model, which describes the complex mass transport phenomena and reaction kinetics in these porous nanomaterials, we reveal that the interrelated effect of bulk and hindered diffusion is the main limiting factor of PSi-based biosensors. Thus, without significantly accelerating the mass transport to and within the nanostructure, the target capture performance of these biosensors would be comparable, regardless of the nature of the capture probe–target pair. We use our model to investigate the effect of various structural and biosensor characteristics on the capture performance of such biosensors and suggest rules of thumb for their optimization.

High performance photodetectors using porous silicon-TiO2 heterostructure

In this study, a feasible approach for UV–vis photodetection using hybrid heterojunction by integrating porous silicon and TiO2 thin film is demonstrated. The photodetector demonstrates excellent photoresponse by three orders of magnitude enhancement and fast rise/decay time constants of 0.16/0.14 ms. Under small bias (1 V), the photodetector exhibits very high responsivity up to 40 A W−1 at 532 nm irradiation over a broadband wavelength range from 300–700 nm. The heterostructure geometry yields high-performance devices useful for many optoelectronic and biomedical applications.

Aptamers vs. antibodies as capture probes in optical porous silicon biosensors.

Over the past decade aptamers have emerged as a promising class of bioreceptors for biosensing applications with significant advantages over conventional antibodies. However, experimental studies comparing aptasensors and immunosensors, under equivalent conditions, are limited and the results are inconclusive, in terms of benefits and limitations of each bioreceptor type. In the present work, the performance of aptamer and antibody bioreceptors for the detection of a his-tagged protein, used as a model target, is compared. The bioreceptors are immobilized onto a nanostructured porous silicon (PSi) thin film, used as the optical transducer, and the target protein is detected in a real-time and label-free format by reflective interferometric Fourier transform spectroscopy. For the antibodies, random-oriented immobilization onto the PSi nanostructure results in a poor biosensing performance. Contrary, Fc-oriented immobilization of the antibodies shows a similar biosensing performance to that exhibited by the aptamer-based biosensor, in terms of binding rate, dynamic detection range, limit of detection and selectivity. The aptasensor outperforms in terms of its reusability and storability, while the immunosensor could not be regenerated for subsequent experiments.

Effect of bismuth oxide nanoparticles on the physicochemical properties of porous silicon thin films

In this work, bismuth oxide nanoparticles were successfully deposited on porous silicon (PSi) in order to enhance the light absorption and reduce the optical losses. The obtained bismuth oxide $$(\hbox {Bi}_{2}\hbox {O}_{3})/\hbox {PSi}$$ samples were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS), atomic force microscopy (AFM), photoluminescence (PL), UV–visible absorption and reflection spectroscopy techniques. The XRD studies revealed the formation of the monoclinic $$\upalpha \hbox {-Bi}_{2}\hbox {O}_{3}$$ phase. The XPS analysis demonstrates the formation of highly pure $$\hbox {Bi}_{2}\hbox {O}_{3}$$ nanoparticles in accordance with XRD results. The SEM and AFM analyses confirmed that the bismuth oxide nanoparticles are well incorporated and uniformly distributed over the surface of PSi without changes in the arrangement and shape of the pores, resulting in an optimized microstructure. The $$\hbox {Bi}_{2}\hbox {O}_{3}/\hbox {PSi}$$ films showed better absorption than PSi layers as indicated by UV–Vis absorption technique. The reflection measurements confirmed a further reduction in reflectivity of PSi from 6.4 to 3.5% after the inclusion of $$\hbox {Bi}_{2}\hbox {O}_{3}$$ nanoparticles, which is of significant importance for solar cells application since it can enhance its conversion efficiency. The $$\hbox {Bi}_{2}\hbox {O}_{3}/\hbox {PSi}$$ films have a great promise to be used as efficient antireflection coatings in innovative concepts of higher efficiency and cost-effective solar cells.

Compensating porosity gradient to produce flat, micromachined porous silicon structures

Abstract Obtaining flat micromachined porous silicon structures is extremely challenging due to the myriad of factors affecting film stress in such structures. In this work, the relationships between the current control during anodization, average porosity, and residual stress within porous silicon thin films and micro-fabricated structures were investigated. Using a combination of electron microscopy, surface profilometry and reflectance spectroscopy, the optimum conditions to produce near zero residual stress in thin films and micro-fabricated structures were determined. The residual stress was adjusted by a continuous variation of the anodization current in order to achieve flat structures. The flattest released porous silicon microbeams were 2.3 μm thick with a peak to valley height variation of only 72 nm over a length of 150 μm. These were achieved by using an initial current density of 20 mA/cm2 that was reduced down to 8 mA/cm2 during anodization. By using this method of reducing the current during anodization, the inherent high porosity at the porous-silicon/silicon interface was reduced, which also enabled a 36% increase in the film thickness before film delamination compared with using a constant current. These results provide a pathway to fabricate thick, optically flat micromachined multi-layer filters using a single base material (silicon) for the structural and released layers.

Investigation of porous silicon thin films for electrochemical hydrogen storage

In this study, nanoporous silicon (PS) layers have been elaborated and used for hydrogen storage. The effect of the thickness, porosity and specific surface area of porous silicon on the amount of hydrogen chemically bound to the nanoporous silicon structures is studied by Infrared spectroscopy (FTIR), cyclic voltammetry (CV), contact angle and capacitance –voltage measurements. The electrochemical characterization and hydrogen storage were carried out in a three-electrode cell, using sulfuric acid 3 M H2SO4 as electrolyte by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge. The results indicate the presence of two oxidation peaks at 0.2 V and 0.4 V on the anodic side corresponding to hydrogen desorption and a reduction peak at −0.2 V on the cathodic side corresponding to the sorption of hydrogen. Moreover, the EIS studies performed on PS electrode in 3 M H2SO4 show that the hodograph contains a semicircle at high frequency region and a line in the lower frequency zone. An equivalent circuit has been proposed; the values of the equivalent circuit elements corresponding to the experimental impedance spectra have been determined and discussed. Finally, the highest hydrogen storage in PS was 86 mAh/g. This storage capacity decreases by only 7% of the initial capacity value, after 40 cycles.

Hyperspectral and Color Imaging of Solvent Vapor Sorption Into Porous Silicon.

A porous silicon thin film photonic crystal (rugate) sample with both a radial gradient in the rugate reflectance band wavelength and two spatially separated pore-wall surface chemistries (methylated and oxidized) was monitored by hyperspectral and color imaging while it was dosed with vapors of acetone, ethanol, heptane, 2-propanol, and toluene at concentrations ranging from 100 to 3,000 mg m−3. The shift in the wavelength of the rugate reflectance band maximum at each position along a transect across the two surface chemistries, as derived from the hyperspectral imaging, could discriminate between the different solvents and concentrations of solvents, while the change in hue derived from the color camera data along an analogous transect did not provide discrimination. The discrimination between solvents was mainly due to the two different surface chemistries, and the gradient associated with the change in the rugate reflectance band wavelength did not affect the selectivity significantly. There was spatial variability in the spectral and color responses along the transect independent of the overall rugate reflectance band wavelength gradient and pore-wall surface chemistries, and this was attributed to factors such as the presence of striations in the silicon wafer from which the porous silicon was prepared.

Porous silicon thin films as anodes for lithium ion batteries deposited by co-evaporation of silicon and zinc

Porous silicon thin films were fabricated by an innovative method using vacuum processing for the application as high capacity anode in lithium ion batteries. The deposition procedure comprises a co-evaporation of silicon and zinc, resulting in a deposition of a compound layer with deposition rates up to 100 nm/s and a subsequent thermal annealing. Due to its high vapor pressure, the zinc fraction is expelled and hence, a porous silicon matrix is formed. Herein, we introduce a novel and potentially scalable synthesis method for porous silicon films and show first analytical investigations concerning the layer morphology and the electrochemical properties. With the novel silicon anode excellent electrochemical performance, particularly high capacities of ≥3000 mAh/g, reasonable coulombic efficiencies of ≥90% in the initial cycle and comparably high cycle life >150 cycles can be demonstrated, which reveals their great potential for battery anode applications.

The Influence of Quantum Confinement on Third-Order Nonlinearities in Porous Silicon Thin Films

We present an experimental investigation into the third-order nonlinearity of conventional crystalline (c-Si) and porous (p-Si) silicon with Z-scan technique at 800-nm and 2.4- μ m wavelengths. The Gaussian decomposition method is applied to extract the nonlinear refractive index, n 2 , and the two-photon absorption (TPA) coefficient, β , from the experimental results. The nonlinear refractive index obtained for c-Si is 7 ± 2 × 10 − 6 cm 2 /GW and for p-Si is − 9 ± 3 × 10 − 5 cm 2 /GW. The TPA coefficient was found to be 2.9 ± 0.9 cm/GW and 1.0 ± 0.3 cm/GW for c-Si and p-Si, respectively. We show an enhancement of the nonlinear refraction and a suppression of TPA in p-Si in comparison to c-Si, and the enhancement gets stronger as the wavelength increases.

Porosity and roughness determination of porous silicon thin films by genetic algorithms

The problem of determining the porous silicon (PSi) optical constants, thickness, porosity, and surface quality using just reflectance data is board employing evolutionary algorithms. The reflectance measurements were carried out of PSi films over crystalline silicon (c-Si) substrate, and the fitting procedure was done by using a genetic algorithm (GA). The PSi is treated as a mixture of c-Si and air. Therefore, its effective optical constants can be correlated with the porosity through effective medium approximation (EMA). The results show that genetic fitting has a good match with the experimental measurements (near UV–vis reflectance) and the thickness obtained by scanning electron microscopy (SEM).

In situ study of porous silicon thin films thermal oxidation by pulsed laser photoacoustics

Passivation is of remarkable importance for porous silicon (PSi) applications to guarantee the chemical stability of its high surface area through time. Thermal oxidation is one of the most common methods to passivate the surface of PSi. In order to better understand the oxidation process, here we performed an in situ study by using pulsed laser photoacoustics. During the thermal oxidation, the photoacoustical signal was measured each 1 °C from 70 °C to 900 °C. The measurements were analyzed by standard correlation, and the significant signal changes were related to the presence of different surface species. We have found temperatures, where the photoacoustical signal changes drastically, that can be related to the absorption and desorption of chemical species in the surface. Species identification where made through Fourier-transform infrared spectroscopy-attenuated total reflection analysis of the PSi samples, at temperatures around where these notorious modifications on the surface were observed.