Thickness Of Porous Silicon Layer Research Articles

Tailoring Mesoporous Silicon Surface to Form a Versatile Template for Nanoparticle Deposition

Porous silicon (PS) can be used as a loading template in sensing or as a matrix to develop nanoparticle arrays. We present a comprehensive study of PS morphology and optical properties before and after the pore opening process, including the determination of thickness, pore size, and pore density of PS layers, its surface wettability, and reflectivity. The PS samples were fabricated by electrochemical anodization of monocrystalline silicon wafer in 5–20 wt.% hydrofluoric acid (HF) solution at a current density in the range of 20–200 mA/cm2. Anodization was followed by the pore opening process, i.e., the removal of a parasitic superficial layer with a “bottleneck” structure by reactive ion etching (RIE). The results illustrate that “bottleneck”-free PS allows to achieve a high pore density using a low HF concentration and a reduced current density. We established that this structure demonstrates higher hydrophobicity in comparison to the samples before RIE. The applicability of the developed “bottleneck”-free PS was tested via filling the pores with silver nanoparticles, indicating its potential use as a template for the fabrication of nanoparticle arrays.

Hydrosilylation of High Porosity Porous Silicon with 1-Hexene in Supercritical CO2 Fluid

Hydrosilylation of organic groups on hydrogen-terminated porous silicon enhances its stability and increasing the porosity of the porous silicon improves its luminescence efficiency. For as-formed thick porous silicon layers with high porosity, however, it is very difficult to preserve its fine structure during air-drying. Here, a process of both drying and hydrosilylation of highly porous silicon in ambient supercritical fluid was developed. 1-hexene was used for the hydrosilylation. The parameters of ambient temperature and processing time have been investigated for optimal process. The effects of the process were evaluated by means of Fourier transform infrared spectroscopy. Supercritical fluid hydrosilylation of porous silicon with thickness of 10 μm and porosity of 80% was achieved. Stable photoluminescence of processed porous silicon was also obtained.

Colorimetric analysis of optical reflection from thin porous silicon for detection of organic liquids

A single porous silicon (PS) layer sensor structure was applied to analyze the kinetics of adsorption/desorption processes of water, ethanol and ethanol-water solution using CIE1931 chromaticity coordinates based parameters such as dominant wavelength λdom, complementary wavelength λcom and excitation purity (EP). Matrix method was used to obtain reflectance spectra dependences on porosity (P), PS layer thickness (LPS), refractive index of analyte (nanalyte). Dependences of λdom, λcom, EP on P, LPS, nanalyte were computed on the base of computed reflectance spectra, and optimal parameters were determined for distinction of model analytes such as water and ethanol. Time dependences of λdom, λcom were experimentally obtained using CCD-based spectrometer for water, ethanol and ethanol-water solution. Advantage of characterisation of whole complex multiple peaks spectra using chromaticity parameters as opposed to sensor characterization with parameters of individual interference peaks as peak position was demonstrated.

Optical, morphological and electrical analysis of heterostructures PSi/c-Si and SiO2/MWCNTs/PSi/c-Si

Porous silicon (PSi) has been studied extensively due to its photo luminescence (PL) in the visible range at room temperature. PSi layers are useful for various optical and electrical applications among others. The PSi properties can be enhanced using different materials. In the present work, we have decorated PSi single layers with multiple wall carbon nanotubes (MWCNTs). The thickness of PSi layers was determined by the gravimetric analysis, the surface morphology by scanning electron microscopy (SEM), the optical properties were studied by measuring PL and reflectance spectra and the electrical behavior was analyzed from the measured I–V curves for which some theoretical transport mechanisms are used to make a study in depth. These materials and structures have potential applications in chemical sensors.

(Invited) Photo-Assisted Etching of Porous Silicon Nanostructures in Hydrofluoric Acid Using Monochromatic Light

Silicon can be made porous by electrochemical (anodization) etching in hydrofluoric acid (HF). This is the most controlled and used technique for the preparation of porous silicon (PSi). The porous structure depends on several factors, such as silicon resistivity and type, as well as HF concentration and anodization current. In the case of lightly-doped p-type silicon, uniform sponge-like nano-structures can be formed. This type of PSi layer is characterized in particular by its porosity and thickness. PSi is interesting for a variety of properties, some related to quantum confinement in its nanostructure, such as luminescence and porosity-dependent refractive index and surface area. It has potential applications in optoelectronics, photovoltaics, medicine and sensing. The post-anodization behavior of PSi in various environments is of interest for stability and degradation evaluation. The evolution of PSi is sometimes made intentional in order to further tailor the PSi properties after anodization. Typically, the effects of PSi oxidation (mainly for stabilization purposes), etching (to increase the porosity) or degradation in various environments (typically for biotechnology) are sometimes studied. Recently, we have made use of a particular behavior of the PSi photoconduction in HF: when PSi is illuminated in HF, only the charge carriers photogenerated in the Si substrate contribute to the measured photocurrent, and thus, the photocurrent is a measure of the optical transmission of PSi. This behavior allowed us to measure the PSi optical constants for various optical wavelengths and PSi porosities in situ and thus for the most pristine form of PSi and avoiding all the problems related to the drying and handling of high-porosity PSi free-standing layers1. We have also used the photoconduction to study the chemical etching of PSi in HF. We could monitor the etching as it proceeded and developed a model that allowed us to derive the HF-dependent etching rates and the absorption coefficient of PSi for a large range of porosities not explored so far, from as-prepared to close to 100% porosity (PSi fully dissolved)2. A small degree of tuning of the photoluminescence (PL) band of PSi may be achieved by adjusting anodization conditions, but fine tuning remains difficult. A few attempts aiming at controlling the PL or understanding PSi PL in HF have been attempted by using post-anodization photo-assisted etching in HF. The photoetching process itself was not studied in much detail, and were focusing on as-formed luminescent PSi. A better understanding of the photoetching process would be welcome to get better control and understanding of the effects. In this paper, we show how the photoetching of PSi (initially luminescent or not) can be monitored and its rate evaluated using the photoconduction technique that we have used previously in other studies1 , 2. The photocurrent was continuously measured during the illumination of PSi by monochromatic light, which was inducing photoetching. As PSi was etched, the PSi absorption was decreasing, more light was transmitted to the substrate and, as a result, the photocurrent was increasing. Figure 1 shows results for an initial porosity of 62%, an illumination wavelength of 450 nm, and different PSi thicknesses. A saturation state is reached when the PSi layer does not absorb anymore at the illumination wavelength. The waves observed for 1 μm and 2 μm-thick layers were attributed to thin film optical interference. The curve for 16 μm-thick PSi was affected by reproducibility problems because of instabilities in the diffusion of species and hydrogen bubbles inside the pores. For clean photoetching study, 4-μm thick PSi layers were used. Figure 2 shows the time evolution of the photocurrent for 4 μm-thick PSi layers, with different illumination powers at 450 nm. As expected, the photocurrent at saturation is proportional to the incident illumination power. Solid lines are fits obtained using a model that will be presented. Two regimes were characterized, one in which the photoetch rate is limited by the supply of photo-generated holes at the Si surface, and another one where it is limited by the photoetching reaction. The maximum photoetch rate was derived. The evolution of the in depth porosity profile can be obtained from the model as well. The effects of a range of parameters will be presented. 1 B. Gelloz, H. Fuwa, and L. H. Jin, Ecs Journal of Solid State Science and Technology 5 (3), P190 (2016). 2 B. Gelloz, K. Ichimura, H. Fuwa, E. Kondoh, and L. Jin, Ecs Journal of Solid State Science and Technology 6 (1), R1 (2017). Figure 1

Morphological and optical properties of n-type porous silicon: effect of etching current density

Morphological and optical properties of porous silicon (PS) layer fabricated on n-type silicon wafer have been reported in the present article. Method of PS fabrication is by photo-assisted electrochemical etching with different etching current densities (J). Porosity and PS layer thickness, obtained by the gravimetric method, increase with increasing J. Pore morphology observed by FESEM shows the presence of randomly distributed pores with mostly spherical shape. Calculated pore size is also seen to increase with increasing value of J. XRD gives the characteristic amorphous peak of PS along with some peaks corresponding to crystalline silicon (c-Si). Calculated crystallite size shows decreasing trend with increasing J value. The optical properties of these samples have been investigated by UV–visible reflectance, Raman spectroscopy and photoluminescence (PL) spectra. Reflectance measurement shows blue-shift of the spectrum with increased reflectivity for increasing J. Raman spectra show remarkable blue-shift with respect to the c-Si peak. PL spectra give the luminescence energy in the orange–red region of the visible spectrum and little change with variation of J.

AC Impedance Studies on Metal/Nanoporous Silicon/p-Silicon Structures

Alternating current (AC) impedance measurements have been performed on 10- to 15-μm thick porous silicon layers on a (100) p-type silicon (p(+)Si) substrate with the aluminium (Al) top electrode in a sandwich configuration in the range of 20 Hz–1 MHz and in the temperature ranging between 152 K and 292 K. The ac conductivity σac was found to increase with frequency f according to the universal power law: sigma_{text{ac}} , = ,,Af^{s} where the exponent s is a frequency and temperature-dependent quantity. A hopping process is found to be dominant at low temperatures and high frequencies, while a thermally activated free band process is responsible for conduction at higher temperatures. Capacitance is found to decrease with frequency but increase with temperature. Frequency dependence of the loss tangent is observed with a temperature-dependent minimum value.

(Invited) Porous Silicon Dissolution Monitoring and Optical Constants Measurement Using in Situ Photoconduction in HF

Porous silicon (PSi) is typically prepared by electrochemical etching of silicon substrates in hydrofluoric acid (HF). With lightly-doped silicon substrates, sponge-like nano-structures may be achieved and the PSi layer thickness and porosity can be rather well-controlled. Such silicon nanostructures have potential applications in particular in optoelectronics, photovoltaics, medicine and sensing. (1) Determination of PSi optical constants. For some applications such as optical sensing or photonics, the knowledge of PSi optical constants is often necessary. Most methods used to determine the optical constants of PSi use dried samples, sophisticated setups, and meticulous sample preparation, which may alter the PSi structure. We have shown that the optical constants of PSi can be measured very easily using in-situ photoconduction of PSi in HF during PSi formation. The liquid-PSi-Si substrate junction was illuminated from the liquid side and reverse-biased, allowing the observation of a photocurrent, resulting from the reduction of protons in solution. These charge exchanges do not affect PSi. We have confirmed that only the charge carriers photogenerated in the Si substrate contribute to the photocurrent. Thus, the measured photocurrent is proportional to the optical transmission through PSi. The absorption coefficient and the refractive index can be obtained by fitting photocurrent-PSi thickness curves. Figure 1 shows an example of determination of the absorption coefficient α of PSi of porosity 68% for three indicated illumination wavelengths. In this particular case, the photocurrent was studied for large thickness’s, so as to have a good precision of the absorption measurement. For the measurement of the refractive index, the photocurrent must be analyzed for much smaller thickness’s so as to observe interference patterns. This technique has several advantages over conventional ones. No particularly sophisticated sample preparation or handling is necessary, no high-precision optical instruments are necessary, while still preserving a very good structural state of PSi layers, even for high porosities and for arbitrary layer thicknesses. Moreover, only one sample, in a single experiment, is sufficient for the determination of the optical constants. [1] (2) Monitoring PSi dissolution in HF For some applications in medicine or sensing, pore engineering may be required. After PSi formation, pores can be further enlarged by using chemical dissolution, which can be achieved in some cases by dipping PSi in HF for a period of time. Dissolution of PSi is accompanied by an increase in PSi porosity. In order to be able to, for example, stop the dissolution when the desired porosity is achieved, a technique of monitoring of the dissolution would be welcome. Such monitoring of PSi dissolution has been difficult to achieve and is usually based on post-treatment inspection studies and calibration. Here, we show that it can be realized using the evolution of the PSi optical transmission in-situ in HF using the photoconduction technique described in (1). The liquid-PSi-Si substrate junction was illuminated from the liquid side with a monochromatic light probe and reverse-biased, and the photocurrent was measured during dissolution. Figure 2 shows an example using a 5 μm-thick PSi layer of porosity 68% in 5% HF and a 405 nm illumination probe. During dissolution, the photocurrent increases due to the decreasing optical absorption of PSi. When the whole PSi layer was dissolved, the photocurrent reached a plateau (here after about 2000 s), reflecting the fact that the PSi optical absorption has become zero. The results for a wide range of different HF concentrations, PSi porosities, and PSi thickness’s will be presented. In addition to monitoring, the analysis of the photocurrent as a function of time may be used to derive the optical constants of PSi as a function of porosity, for the wavelengths used as illumination probes. [1] B. Gelloz et al. ECS Journal of Solid State Science and Technology, 5(3) P190-P196 (2016) Figure 1

The influence of oxidation properties on the electron emission characteristics of porous silicon

In order to investigate the influence of oxidation properties such as oxygen content and its distribution gradient on the electron emission characteristics of porous silicon (PS) emitters, emitters with PS thickness of 8μm, 5μm, and 3μm were prepared and then oxidized by electrochemical oxidation (ECO) and ECO-RTO (rapid thermal oxidation) to get different oxidation properties. The experimental results indicated that the emission current density, efficiency, and stability of the PS emitters are mainly determined by oxidation properties. The higher oxygen content and the smaller oxygen distribution gradient in the PS layer, the larger emission current density and efficiency we noted. The most favorable results occurred for the PS emitter with the smallest oxygen distribution gradient and the highest level of oxygen content, with an emission current density of 212.25μA/cm2 and efficiency of 59.21‰. Additionally, it also demonstrates that thick PS layer benefits to the emission stability due to its longer electron acceleration tunnel. The FN fitting plots indicated that the effective emission areas of PS emitters can be enlarged and electron emission thresholds is decreased because of the higher oxygen content and smaller distribution gradient, which were approved by the optical micrographs of top electrode of PS emitters before and after electron emission.

RESOURCE AND ENERGY SAVING TECHNOLOGIES BASED ON NANOSTRUCTURED SILICON

Porous silicon is advisable to get by electrochemical etching. The porosity of the obtained samples was about 70%. The thickness of the porous layer is (150–250) nm. The results of measurement of a reflectance of solar cells with different layer thicknesses of porous silicon show increased absorptive capacity of the porous silicon with increasing thickness, which directly indicates the possibility of using this material in solar cells. The porous silicon layer thickness from 70 microns has a greater dispersibility than the layers of lesser thickness. Thus, the use of porous silicon as a raw material for solar cells has excellent prospects. Such structures have a clear advantage over traditional and greatly improved technology to produce photovoltaic devices.

Fabrication of Gas Sensor Device for H2 and NO2 from Porous Silicon

A nanocrystalline porous silicon (PS) layer was prepared by electrochemical etching method of p–type silicon wafer in hydrofluoric acid (HF). The properties of porous silicon structure under various etching times (10–50 min), HF concentrations, and current density were studied. The study included photoluminescence (PL), morphology, x–ray, and gas sensor. The results (PL) spectra (peak wavelength) can be shifted from (505 to 625) nm. PL analysis indicated that the energy band gap can be tuned from (1.984–2.455) eV with respect to etched time. Atomic Force Microscopy (AFM) analysis showed that the PS layer had a sponge–like structure. Surface roughness and the pyramid–like hillocks on the entire surface play an important role in variation visible luminescence. The AFM images shows that the average diameter of the PS layer pore and thickness the Silicon decreased when etching time increase and the average diameter particleboard ranged from (80.8–35.4) nm. PS X–ray topography studies that the skeleton maintained its silicon crystalline structure after anodization. PS Sensors were fabricated and tested successfully on H 2 and No 2 gas.

Aligning Silicon Nanopillar Formed in Electroless HF/H2O2 Etching through Pre-Forming Porous Layer

For fabricating a controllable nanopillar array structure by utilizing the metal-assisted etching technique, we created a fine porous cladding layer on the surface of bulk silicon to settle down silver nanoparticles. Prior to metal-assisted etching process, a 10nm thick porous silicon layer was produced on the surface of a p-type, 1-10 Ohm-cm, (100) bulk silicon by HF based electrochemically etching and then rinsed by D.I. water. After spin-drying, the specimen and a same silicon specimen but without the porous layer were dipped in a solution (HF / AgNO3 / H2O) for 30 seconds for the nucleation of silver nanoparticles. The dipped specimens were then etched by HF/ H2O2/ H2O solution for forming nanopillar. Through the observation of Field Emission Scanning Electronic Microscopy (FE-SEM), we found the pores on the surface of the porous layer could provide priority nucleation sites for silver nanoparticles. Hence, we could achieve a designed silicon nanopillar array by controlling the formation of the porous layer. After etching, the silicon specimen with the pre-forming porous layer exhibits different characteristic from the one without the layer as shown in figure. Figure 1

Influence of composite oxidation on electron emission characteristics of thick porous silicon

A new composite oxidation method combining rapid thermal oxidation (RTO) and electrochemical oxidation (ECO) is proposed to improve electron emission characteristics of electron emitters based on thick porous silicon (PS) layers. Four groups of electron emitters with 8 μm PS layers were oxidized by RTO, ECO, composite RTO–ECO, and composite ECO–RTO, respectively. The experimental results indicated greater emission current density and efficiency of electron emitters oxidized by composite oxidation versus ECO or RTO alone. The most favorable results occurred for an electron emitter oxidized by ECO–RTO, with an emission current density of 2.2 A/m2 and efficiency of 2.9%. The higher and more uniform the oxygen content in the PS layer after oxidation, the better the electron emission characteristics that the authors noted. Thick PS layers can be oxidized completely and uniformly by composite oxidation to achieve adequate oxygen content with uniform distribution. This subsequently leads to formation of a longer acceleration tunnel, which increases effective emission area and continuously speeds up electrons.

Thermal Elasticity Stresses Study in Composite System ‘Porous Silicon - Liquid’

In this report the study of the elasticity stresses arising in the composite system “porous silicon –etching solution” will be discussed. For this purpose, the modified etching cell with piezoelectric transducer was adopted It was shown that the thermally induced pressures of the liquid confined inside of pores influenced dramatically on the time-domain shape of voltage signal detected on electrodes of the piezoelectric transducer. Therefore, the significant role of these pressures on the formation of general elastic stresses in the system was stated. The dependence of their magnitude and time delay on the thickness of porous silicon layer was found. Thus, the obtained experimental results could be a background for the new technique for the porous silicon in-situ study.

Formation and structure of mesoporous silicon

This paper provides the results of studying the formation kinetics and the structure of mesoporous silicon layers obtained by electrochemical anodization in an electrolyte based on 12% aqueous hydrofluoric acid. The electrolyte consisted only of deionized water and hydrofluoric acid and contained no organic additives in order to prevent contamination of porous silicon with carbon during the anodization. All the experiments were carried out on whole silicon wafers 100 mm in diameter, rather than samples of a small size, which are often used to save silicon. As the initial substrates we used monocrystalline silicon wafers of brand KES-0.01, which were cut from the ingots produced by the Czochralski method. Dependences of the thickness of porous silicon layers, its growth rate, and the volume porosity on the anodic current density and the anodization time are determined. Scanning electron microscopy was used to study the structure of porous silicon layers and to define the size and density of the pore channels. The regimes of obtaining homogeneous porous silicon layers for their subsequent use as buffer layers in epitaxy are found.

Dynamics of fast pattern formation in porous silicon by laser interference

Patterns are fabricated on 290 nm thick nanostructured porous silicon layers by phase-mask laser interference using single pulses of an excimer laser (193 nm, 20 ns pulse duration). The dynamics of pattern formation is studied by measuring in real time the intensity of the diffraction orders 0 and 1 at 633 nm. The results show that a transient pattern is formed upon melting at intensity maxima sites within a time <30 ns leading to a permanent pattern in a time <100 ns upon solidification at these sites. This fast process is compared to the longer one (>1 μs) upon melting induced by homogeneous beam exposure and related to the different scenario for releasing the heat from hot regions. The diffraction efficiency of the pattern is finally controlled by a combination of laser fluence and initial thickness of the nanostructured porous silicon layer and the present results open perspectives on heat release management upon laser exposure as well as have potential for alternative routes for switching applications.

Post-CMOS Integration of MEMS Inertial Sensors Using Oxidized Porous Silicon Technique

At present, MEMS inertial sensor technology has an exhaustive range of applications in the field of mass-produced commercial products, such as smart phones, PDAs, automobiles, and pedometers, which cause a strong demand for small, robust, low-cost and CMOS- integrated inertial sensors. DRIE process has been widely used in the post-CMOS approach to create high aspect ratio structures [1], but the high expense and long consuming time are usually unacceptable for the mass manufacturing. In addition, during the ion beam bombardment, CMOS structures inevitably suffers from a certain amount of damages which might reduce the device performance [2]. Here we demonstrate a low-cost and CMOS compatible production method to fabricate CMOS-MEMS inetial sensors. A thick oxidized prous silicon (OPS) layer is selectively grown on the silicon substate before the standard CMOS processes for dielectric isolation and thermal isolation. The OPS techique has a substantial cost advantage over DRIE techniqe and can produce very thick layers without severe stress problems. Lowly-doped p-type wafers (20-40 Ωcm, 500 μm-thick, [100] -oriented and polished) were used in our experiments. The whole process includes three main steps: formation of OPS layers, CMOS sturctures fabrication, and the MEMS structures fabrication. To create thick porous silicon layer, ohmic contact was achieved by sputtering an 800 nm-thick Al film on the backside of samples. The electrolytes used for anodization contained 48 wt.% aqueous HF and DMF. Galvanostatic current densities of 500 mAcm-2 were applied from an Agilent power source with a tolerance up to 60 V. Macropore arrays with depth of about 48 μm were obtained, followed by an optimized oxidation process to form OPS layers. After that, standard CMOS process steps were used to produce electronic components. Finally, the MEMS structures are completely built on top of the finished CMOS substrate by partely removing the OPS layers, leaving the CMOS layers unthouched. Figure 1 show SEM pictures of the cantilever beam-mass structure (Figure 1a) and comb structure (Figure 1b) released by removing the OPS layer. Figure 1c shows the MEMS inertial stuctures and CMOS electronic components obtained on the same chip. This represents an effective method to integrate MEMS sensors with electronic circuits.

Design, analysis and optimization of porous silicon microcavity based on silicon-on-insulator at optical communication wavelengths

With optimized parameters and the optimized sequence structure from simulations, proper porous silicon microcavity (PSM) based on silicon-on-insulator (SOI) wafer, constituted of a Fabry–Perot cavity between distributed Bragg reflectors (DBRs), has been achieved. Just as expected, the resonance wavelengths of these two oxidized PSMs are located separately at 1328nm and 1560nm, which are very close to the optical communication wavelengths of 1310nm and 1550nm. The effects of tuning resonance wavelength were shown by composition of Fabry–Perot cavity integrated on each thickness of porous silicon layer. Since the preparation technology of optical devices with these working wavelengths has been well developed, it is expected that such a design could be of great potential in realizing integrated optical devices. Compared with various optical devices on SOI wafer, PSM based on SOI by electrochemical anodization is simple and low-cost. Moreover, a sharp resonance in the reflectance spectrum means a high Q factor which predicts an interesting possibility in sensor application on SOI wafer.

Electrochemical preparation of microstructured porous silicon layers for drug delivery applications

Porous silicon (PSi) has emerged as a potential drug delivery vehicle because of its high surface area to volume ratio, convenient surface chemistry, biocompatibility and very low toxicity. PSi based drug delivery systems can be easily prepared by tuning its morphological characteristics (i.e. porosity, pore size, etc.). In the present study, several PSi single layers were prepared in order to study effect of current density and etching time on PSi morphology. Different characterization techniques including UV–VIS spectrometry and field emission gun scanning electron microscopy have been used to determine porosity, pore size and thickness of PSi layers. The results showed a higher porosity and larger pore size at high current density. Etching rate was also found to be more at higher current density. Thickness of PSi layer was increased with increase in etching time.

Porous Silicon in a Semiconductor Manufacturing Environment

In the Bosch-proprietary “advanced porous silicon membrane process”, porous silicon (PSi) is used for the first time in high-volume industrial production of microelectromechanical devices. Nanoporous silicon acts as an auxiliary layer during manufacturing of monolithically integrated pressure sensors in a mixed-signal IC process supplemented by microelectromechanical-systems-specific steps in the front end of line. In this paper, the technical design and performance of a fully automated production tool capable of high-volume fabrication of PSi under the specific constraints of a semiconductor manufacturing environment are discussed. The process requires stringent control on the PSi layer thickness, uniformity, porosity, and morphology. The impact of chamber and electrode geometry, the electrolyte flow, and the mode of current coupling into the wafer back side on the uniformity of the PSi layer is addressed. The need for a well-defined PSi morphology demands high reproducibility and stability of the electrolyte composition, particularly with respect to the hydrofluoric acid concentration.