Porous Silicon Structures Research Articles

Investigation of Different Colloidal Porous Silicon Solutions and Their Composite Solid Matrix Rods by Optical Techniques

Colloidal porous silicon (PSi) in different solvents was synthesized by simple chemical etching. Colloidal solutions were then prepared using different quantities of silicon wafer pieces (Pcs) and chloroplatinic (Pt) acid in catalyst solution. The effect on the properties of the colloidal solutions and composite rods were investigated using various optical characterization techniques. Absorption and photoluminescence (PL) intensity of the colloidal PSi solutions are observed to depend on the quantity of wafer Pcs, the Pt-solution, and the porosity formation on the wafer surface. The morphological structure of the PSi in a solvent and the solid–rod environments were studied using field-emission scanning electron microscopy (FE-SEM) and were observed to have different structures. A mono-oriented structure of PSi exists in tetrahydrofuran, which has stereo orientation in dioxane and dimethylsulfoxide (approximately 5–8 nm as confirmed using high resolution transmission electron microscopy). Subsequently, some colloidal PSi solutions were directly embedded in three types of sol–gel-based matrices, silica, ormosils (or organically modified silica) and polymer, which easily generated solid rods. Spontaneous emission (SE) of the PSi solutions and their composite rods were examined using a high power picosecond 355 nm laser source. The emitted PL and SE signals of the colloidal PSi solutions were dependent on the Pt volume, nature of the solvent, quantity of Si wafer piece, and pumping energy. The response of SE signals from the PSi composites rods is an interesting phenomenon, and such nanocomposites may be used for future research on light amplification.

Design Analysis and Electrical Characterisation of Porous Silicon Structure for Sensor Applications

Design Analysis and Electrical Characterisation of Porous Silicon Structure for Sensor Applications

Microwave plasma annealing of sol-gel deposited tantalum oxide and zinc oxide films

The sol-gel process allows the high throughput formation of transition metal oxide thin films. Microwave plasma annealing (MwPA) treatments have been performed on thin films of two different transition metal oxides, Ta2O5 and ZnO, selected as representatives of covalently and strongly ionic bonded oxides, respectively. Ta2O5 has been explored as a dielectric barrier for porous silicon structures. The main limitation of the sol-gel spin coating in this case is the surface roughness of the coating, which is highly improved upon Ar MwPA. The treatment leads additionally to a microstructural activation and interface development comparable to a 500 °C thermal annealing. The MwPA of ZnO is a quasi-equivalent process to a 200 °C thermal annealing, preventing grain growth and promoting nanocrystalline phases. This is suggested to have a direct impact on the optical and electronic properties of the ZnO films. The MwPA films show wider optical band gap than thermally annealed ones. An impedance analysis further shows that the MwPA ZnO films present lower equivalent resistance and higher equivalent capacitance than the thermal films. These results are promising for the development of new processing routes for widely demanded transition metal oxide thin films.

Silicon Structures with Variable Morphology of Pores Methods of Obtaining Physical and Optical Properties

This paper presents experimental results on the creation of porous silicon structures with variable pore morphology (GPS-var). At a pore depth of up to several hundred microns, the structures are characterized by the presence of a layer with nanoscale porous structures (hereinafter, nanoporous) on the surface. As the depth of etching increases, the pores smoothly transform into spongy, and then into a columnar structures with pore sizes of about a micron.

Morphology and luminescence of photo-electrochemically synthesized porous silicon: Influence of varying current density

Achieving high quality porous silicon (PSi) materials with desired porosity remains challenging. Three good qualities of PSi samples are prepared by Photo electro-chemically etching a piece of n-type Si inside the solution of 20 M HF, 10 M C2H5OH and 10 M H2O2 at fixed etching time duration (30 min) and varying current density (15 mA/cm2, 30 mA/cm2 and 45 mA/cm2). As-prepared sample morphologies are characterized via scanning electron microscopy (SEM) and atomic force microscopy (AFM). The gravimetric method is used to estimate the thickness and porosity of the prepared samples. Current density (etching time) dependent morphologies, electronic bandgap and room temperature photoluminescence (PL) properties of such PSi nanostructures are evaluated. These PSi structures revealed enhanced rectifying characteristics with increasing current density.

Fabrication of optically smooth surface on the cleavage of porous silicon by gas cluster ion irradiation

Electrochemical etching of Si is a promising method of fabricating multilayer photonic structures with hundreds of layers. Nevertheless, the natural cleavage of the porous silicon structure has roughness and then high optical scattering that embarrasses the usage of porous silicon in optical devices. In this study, gas cluster ion beam irradiation was suggested as a polishing technique. The bombardment results in surface smoothing and consequent enhancement of light reflection without significant difference in sputtering rate of layers with different porosity. Increasing irradiation dose results in different behavior of porous layers: densification of low-porosity structures and nano-rods formation in high-porosity layers.

Temperature dependence of photoluminescence peaks of porous silicon structures

Abstract Evaluation of photoluminescence spectra of porous silicon (PS) samples prepared by electrochemical etching is presented. The samples were measured at temperatures 30, 70 and 150 K. Peak parameters (energy, intensity and width) were calculated. The PL spectrum was approximated by a set of Gaussian peaks. Their parameters were fixed using fitting a procedure in which the optimal number of peeks included into the model was estimated using the residuum of the approximation. The weak thermal dependence of the spectra indicates the strong influence of active defects.

Effects of terminated atoms, porosity and drilling orientations on the band structure of porous silicon

We designed different models of nano porous silicon terminated by hydrogen or oxygen atoms, and analyzed their band gap structure and density of states by means of first-principles density functional theory calculation. For the hydrogen-atom-passivation porous silicon in the [100] direction, with the increase of porosity, the band gap increased. When the porosity is 64%, a direct band gap is obtained. The theoretical results were confirmed by our experiments. For the oxygen-atom-passivation porous silicon in the [100] direction, half-metallic behavior and ferromagnetism appear when the porosity is 64%, which is due to the Si 3p band affected by O 2p and H 1s orbitals. For the oxygen-atom-passivation porous silicon in the [110] direction, a direct band gap occurs when the porosity is 49%. Passivation atom, porosity and hollowed-out direction are key factors affecting the band gap structure and density of states of porous silicon. These results would have significant impacts on design of complex porous silicon structures for silicon band-gap engineering in the area of optoelectronics.

Tunable Visibly Transparent Optics Derived from Porous Silicon

Visibly transparent porous silicon dioxide (PSiO2) and PSiO2/titanium dioxide (TiO2) optical elements were fabricated by thermal oxidation, or a combination of thermal oxidation and atomic layer deposition infilling, of an electrochemically etched porous silicon (PSi) structure containing an electrochemically defined porosity profile. The thermally oxidized PSiO2 structures are transparent at visible wavelengths and can be designed to have refractive indices ranging from 1.1 to 1.4. The refractive index can be increased above 2.0 through TiO2 infilling of the pores. Applying this oxidation and TiO2 infilling methodology enabled tuning of a distributed Bragg reflector (DBR) formed from PSi across the visible spectrum. At the maximum filling, the DBR exhibited a transmission of 2% at 620 nm. Simulations match well with measured spectra. In addition to forming DBR filters, phase-shaping gradient refractive index (GRIN) elements were formed. As a demonstration, a 4 mm diameter radial GRIN PSiO2 element with a...

Double Etched Porous Silicon Films for Improved Optical Sensing of Bacteria

Combination of conventional electrochemical etching and subsequent metal-assisted chemical etching (MACE) was used to obtain double etched porous silicon (DEPSi) structures with good optical properties and high affinity to bacteria. Second etching step increased the roughness of the surface and created more cavities for entrapping of bacteria, while optical scattering on the surface was still insufficient and quality of infrared Fabry-Perot interferograms remained high. Obtained DEPSi structures were used for detection of bacteria (E. Coli). Fast Fourier transform of infrared spectra showed reversible broadening of the main band, what allows to detect bacteria in concentrations down to 104 CFU per mL.

Electrical Properties of Porous Silicon for N2 Gas Sensor

The application of porous silicon (PSi) for gas sensing devices has gained a considerable attention in the last decade. This work considers the electrical features of PSi layers prepared by electrochemical etching. We find that in order to get a better understanding of the absorption properties of PSi surface, it is necessary to know how the PSi morphology depends on the etching parameters. The physical structure of PSi, i.e., porosity, and pore size distribution can be controlled by changing the Hydrofluoric Acid concentration, current density, anodizing length and etching time in anodizing procedure. We describe our test system for gas sensors and investigate on the electrical behavior of PSi layers (p-type) in N2 gas for various fabrication conditions. The results show that the current density increases significantly as N2 gas is adsorbed. The measurements of the I-V characteristics were carried out at atmospheric pressure, room temperature, and with N2 gas as well.

Electrochemical Fabrication of Silicon-Based Micro-Nano-Hybrid Porous Arrays for Hybrid-Lattice Photonic Crystal

Hybrid-lattice photonic crystals have received extensive attention in photonic device applications for their larger photonic band gaps. However, until now it is time-consuming and money-costing to realize large-area fabrication of hybrid photonic crystal structure at the nanoscale. This paper presents a flexible, effective and costless technique for the fabrication of 2D triangular-honeycomb hybrid-lattice photonic crystal with micro-nano-hybrid porous silicon structure based on the partly oxidized porous silicon (POPS) substrate. Self-positioned nanopore array surrounded with micro-sized macropores was achieved by photoelectrochemical etching on POPS substrate following several microelectrochemical system (MEMS) processing steps including chemical–mechanical polishing and KOH etching. A modified space-charge region (SCR) model was proposed to interpret the underlying mechanism of such self-positioned nanopores formation. Moreover, the dependency of the nanopore size on the morphology of the oxidized trenches with bottle-like widths was studied and a bottleneck effect was proposed. Our work suggests a new way of efficient realization of simple, effective and large-area fabrication of 2D hybrid photonic crystals, which is expected to foster the promotion in high performance photonic devices.

OPTIMIZATION PHOTODETECTORS FROM ZINC SULFIDE DEPOSITED ON POROUS SILICON WITH DIFFERENT DOPING METALS

In this work, the structural properties of the zinc sulfide (ZnS) films have been investigated using X-ray diffraction (XRD) analysis which show an enhancement in the crystallite degree after doping with copper (Cu). Good matching between the ZnS and porous silicon (PS) structure was noted from the atomic force microscope (AFM) results. The reflectivity gave a clear observation of anti-reflected coating improvement for PS layer and more enhancements after the ZnS deposition. The optical properties show a blue shift in the bandgap for the ZnS deposited with higher substrate temperature and a red shift after doped with different elements. For ZnS/PS heterojunction, the electrical resistivity has been increased after PS layer formed and changed with the variation of the pore size and it was much higher after ZnS deposited on the PS. However, use of ZnS:Cu/PS photodetector showed much higher output current at the ultraviolet (UV) region compared to ZnS/PS. The ZnS:Cu/PS photodetector showed higher output current value than that of the ZnS/PS leading to improvement in the quantum efficiency of 42%.

Room Temperature Crystallization of Hydroxyapatite in Porous Silicon Structures.

Porous silicon (PS) substrates, with different pore sizes and morphology, have been used to crystallize hydroxyapatite (HA) nano-fibers by an easy and economical procedure using a co-precipitation method at room temperature. In situ formation of HA nanoparticles, within the meso- and macroporous silicon structure, resulted in the formation of nanometer-sized hydroxyapatite crystals on/within the porous structure. The X-ray diffraction technique was used to determine the tetragonal structure of the crystals. Analysis/characterization demonstrates that under certain synthesis conditions, growth and crystallization of hydroxyapatite layer on/inside PS can be achieved at room temperature. Such composite structures expand the possibility of designing a new bio-composite material based on the hydroxyapatite and silicon synthesized at room temperature.

(Invited) Thin Porous Silicon Produced By Metal-Assisted Etching

Metal-assisted etching of silicon can produce porous silicon and silicon nanowires by simply immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias (1). Such etching proceeds by a local galvanic cell mechanism consisting of local anodic dissolution of silicon and local cathodic reduction of an oxidizing agent on metal. The structure of porous silicon is widely controlled from nm to sub-mm scale in pore size and from straight to an ants’ nest like in pore morphology by changing etching conditions (1-3). We have been studying the metal-assisted etching of silicon to produce porous structures for antireflection of solar cells (2), and for autocatalytic electroless formation of metal-nanorods and metal films on Si surfaces (4). In this paper, we present size-controlled thin porous silicon layers consisting of straight pores and those applications. Noble metal nanoparticles were deposited on crystalline silicon wafers by electroless displacement deposition immersing the wafers into a metal salt solution containing hydrofluoric acid. The metal-particle-deposited silicon wafers were immersed in a hydrofluoric acid solution including hydrogen peroxide as an oxidizing agent. Under suitable etching conditions, uniform porous silicon thin layers consisting of straight pores were formed on silicon wafers. The diameter of pores was consistent with the size of metal nanoparticles remained at the bottom of each pore. Thus, the diameter of pores, that is, the porosity of porous layers is easily controlled in the nm scale with the deposition time of nanoparticles. The thickness of porous layers, that is, the length of the straight pores is also easily controlled by the etching time. Optical properties of the porous layers thinner than 1 μm depends on those porosity and thickness (5). This is applicable for antireflection of solar cells. The strength of metal nanorods, which were produced by electrolessly filling the pores, increased with the diameter of pores from 10 to 25 nm. These nanorods work as nanoanchors of metal films on silicon wafers. Acknowledgements The present work was partly supported by JSPS KAKENHI (26289276). References 1) Z. Huang, N. Geyer, P. Werner, J. de Boor, U. Gösele, Adv. Mater., 23, 285 (2011). 2) S. Yae, H. Tanaka, T. Kobayashi, N. Fukumuro, H. Matsuda, Phys. Stat. Sol. (c), 2(9), 3476 (2005). 3) S. Yae, M. Tashiro, M. Abe, N. Fukumuro, H. Matsuda, J. Electrochem. Soc., 157, D90 (2010). 4) S. Yae, K. Sakabe, N. Fukumuro, S. Sakamoto, and H. Matsuda, J. Electrochem. Soc., 158, D573 (2011). 5) K. Yamakawa, S. Sakamoto, N. Fukumuro, S. Yae, ECS Trans., 69(2), 185 (2015).

(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

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

The progress of the chemical dissolution of porous silicon (PSi), formed from lightly-doped p-type silicon, in HF was monitored by recording in situ the photocurrent from a monochromatic illumination. The photocurrent is a signature of the optical transmission of the illumination in the substrate underneath PSi. The time corresponding to complete PSi dissolution is easily identified by a plateau of photocurrent. The effects of HF concentration, PSi porosity and PSi thickness were studied. The PSi dissolution rate is discussed. Using a model, the relations between the dissolution time, the porosity, and the absorption coefficient are elucidated. The effects of both PSi structure topology and quantum confinement on the optical absorption of PSi are discussed. The methods shown here allow the study of very high porosities while still having a clean hydrogen-terminated PSi surface and very good structural integrity, since PSi is never dried or exposed to air.

Structure and Permeability of Porous Silicon Investigated by Self-Diffusion NMR Measurements of Ethanol and Heptane

The adsorption and phase transitions of confined fluids in nanoporous materials have been studied intensely because of both their fundamental interest and their crucial role in many technologies. Questions relating to the influence of the confinement of fluids, and the disorder or elastic deformation of porous solids on the liquid-gas phase transition are still under debate. Model systems are needed to understand the adsorption phenomenon. In this context, Porous Silicon (PoSi), which is a single crystal obtained by etching a (100) silicon wafer is an excellent candidate. Indeed, it consists of non-connected tubular pores running parallel to the [100] axis perpendicular to the wafer surface, with transverse sections with a polygonal shape of nanometric size whose areas are widely distributed. Once detached from the wafer, free PoSi membranes can be considered a nanoscale disordered honeycomb. Adsorption/desorption experiments have been performed to characterize the structure: they have shown that evaporation occurs collectively, an intriguing observation generally associated with a disordered pore structure with many interconnections through narrow necks. The characterization of fluid mobility inside the pores should give complementary information about the pore structure and topology. This paper focuses on the dynamics of a fluid confined inside the structure of porous silicon, and in particular the self-diffusion measurements (pulsed field gradient spin echo Nuclear Magnetic Resonance (NMR)). The results show a strong anisotropy of the self-diffusion tensor, as expected in this highly anisotropic structure. However, a non-zero self-diffusion in the directions perpendicular to the pore axis is observed. In order to interpret these puzzling results, molecular and Brownian dynamics calculations are underway.

WITHDRAWN: Investigations on SnO2:Pd/ Porous Silicon Structures for Solar Cell Application

WITHDRAWN: Investigations on SnO2:Pd/ Porous Silicon Structures for Solar Cell Application

Formation of the seed layers for layer-transfer process silicon solar cells by zone-heating recrystallization of porous silicon structures

Thin-film crystalline silicon is promising for photovoltaic application to reduce the cost of photovoltaic energy. Porous silicon structures have been intensively studied as a seed layer for epitaxial growth of thin Si film and layer-transfer process (LTP). In this article, another approach for LTP has been proposed. The seed layers for epitaxial silicon growth have been formed by zone-heating recrystallization of double-layer por-Si structures. The influence of annealing parameters on porous silicon structures was studied. The transformation of por-Si layer to crystalline Si was observed with the formation of smooth continuous surface with the roughness 0.3 nm, peak-to-valley distance around 3.5 nm, and reduced density of pores. The mechanism of the transformation of por-Si surface due to the action of hydrogen in the passivated pores with preventing surface oxidation was proposed.