Porous Polycrystalline Silicon Research Articles

Mesoporus Silicon through Magnesium Reduction of Polymer Templated Silica for High Power Li-Ion Batteries

This work describes the synthesis and electrochemical characterization of mesoporous silicon for high power Li-ion battery applications. Silicon is an attractive Li-ion battery anode material for its high energy density, high elemental abundance and non-toxicity. However, full lithiation of silicon is accompanied by a 300% volume expansion which pulverizes the particles leading to rapid material failure. We have addressed these issues with silicon by developing a highly tunable synthesis method to produce mesoporous silicon from ordered mesoporous silica. This synthesis method leads to micron sized grains consisting of a 3-D interconnected silicon network that should accommodate the electrochemical induced volume expansion while the nanocystalline pore walls provide short ion diffusion lengths for high rate applications. The ordered mesoporous silica starting material is synthesized by evaporation induced self-assembly of sol-gel type precursors and a di-block copolymer. The mesoporous silicon is produced through reduction of the silica precursor with magnesium metal at 650 °C followed by selective chemical etching. The particle morphology and the pore size of both the silica and the silicon can be tuned over a broad range, and have been characterized by small angle scattering, high angle X-ray diffraction, nitrogen porosimetry, and SEM imaging. This synthesis technique produces 1-10 µm silica particles with a cubic lattice of 15 – 20 nm pores and 10 - 15 nm thick pore walls. After magnesium reduction, the porous polycrystalline silicon retains the microscale morphology and the porous nanostructure of the parent material. The silicon pore walls consist of ~30 nm crystallites which are optimal for high power density electrochemical applications Magnesium reduction of silica is an effective method to reduce silica while preserving the nanostructured architecture, but little is understood about reaction mechanism. In an effort to control the nanoscale architecture of silicon we have investigated the reaction mechanism. We have performed in-situ X-ray diffraction during the reduction process combined with Raman spectroscopy to reveal a multi-step reaction pathway which leads to the mesoporous crystalline silicon final product. This study demonstrates that the reaction of mesoporous silica and magnesium metal first forms amorphous silicon that further reacts to form crystalline magnesium silicide. The remaining mesoporous silica is reduced by both the magnesium metal and the magnesium silicide to produce mesoporous crystalline silicon. The electrochemical properties of mesoporous silicon have been characterized by galvanostatic cycling and cyclic voltammetry. Galvanostatic measurements show capacities on the order of 1700 mAh/g while the nanocrystalline silicon pore walls and interconnected 3-D framework led to an extremely high rate capability. Over 40% of the capacity, or 720 mAh/g, can be accessed in 150 seconds. In light of the high capacity and highest reported power density for mesoporous silicon, this material is an exciting candidate for the next generation of high performance Li-ion batteries. Figure 1

Mesoporus Silicon through Magnesium Reduction of Polymer Templated Silica for High Power Li-Ion Batteries

This work describes the synthesis and electrochemical characterization of mesoporous silicon for high power Li-ion battery applications. Silicon is an attractive Li-ion battery anode material for its high energy density, high elemental abundance and non-toxicity. However, full lithiation of silicon is accompanied by a 300% volume expansion which pulverizes the particles leading to rapid material failure. We have addressed these issues with silicon by developing a highly tunable synthesis method to produce mesoporous silicon from ordered mesoporous silica. This synthesis method leads to micron sized grains consisting of a 3-D interconnected silicon network that should accommodate the electrochemical induced volume expansion while the nanocystalline pore walls provide short ion diffusion lengths for high rate applications.The ordered mesoporous silica starting material is synthesized by evaporation induced self-assembly of sol-gel type precursors and a di-block copolymer. The mesoporous silicon is produced through reduction of the silica precursor with magnesium metal at 650 °C followed by selective chemical etching. The particle morphology and the pore size of both the silica and the silicon can be tuned over a broad range, and have been characterized by small angle scattering, high angle X-ray diffraction, nitrogen porosimetry, and SEM imaging. This synthesis technique produces 1-10 µm silica particles with a cubic lattice of 15 – 20 nm pores and 10 - 15 nm thick pore walls. After magnesium reduction, the porous polycrystalline silicon retains the microscale morphology and the porous nanostructure of the parent material. The silicon pore walls consist of ~30 nm crystallites which are optimal for high power density electrochemical applications.Magnesium reduction of silica is an effective method to reduce silica while preserving the nanostructured architecture, but little is understood about reaction mechanism. In an effort to control the nanoscale architecture of silicon we have investigated the reaction mechanism. We have performed in-situ X-ray diffraction during the reduction process combined with Raman spectroscopy to reveal a multi-step reaction pathway which leads to the mesoporous crystalline silicon final product. This study demonstrates that the reaction of mesoporous silica and magnesium metal first forms amorphous silicon that further reacts to form crystalline magnesium silicide. The remaining mesoporous silica is reduced by both the magnesium metal and the magnesium silicide to produce mesoporous crystalline silicon.The electrochemical properties of mesoporous silicon have been characterized by galvanostatic cycling and cyclic voltammetry. Galvanostatic measurements show capacities on the order of 1700 mAh/g while the nanocrystalline silicon pore walls and interconnected 3-D framework led to an extremely high rate capability. Over 40% of the capacity, or 720 mAh/g, can be accessed in 150 seconds. In light of the high capacity and highest reported power density for mesoporous silicon, this material is an exciting candidate for the next generation of high performance Li-ion batteries.

Sol–gel synthesized Titania nanoparticles deposited on porous polycrystalline silicon: Improved carbon dioxide sensor properties

Titania nanoparticles (TNPs) were synthesized by a sol–gel method in our laboratory using titanium tetrachloride as the precursor and isopropanol as the solvent. The particles׳ size distribution histogram was determined using ImageJ software and the size of TNPs was obtained in the range of 7.5–10.5nm. The nanoparticle with the average size of 8.5nm was calculated using Scherrer׳s formula. Homogeneous and spherical nanoparticles were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy (UV–vis). The X-ray powder diffraction analysis showed that the prepared sample (TNPs) has pure anatase phase. TNPs were deposited on porous polycrystalline silicon (PPS) substrate by electron beam evaporation. The TNPs thickness was 23±2nm at 10−5mbar pressure at room temperature. Porosity was performed by an anodization method. Since polycrystalline silicon wafers consist of different grains with different orientations, the pore size distribution in porous layer is non-uniform [1]. Therefore, the average diameter of pores can be reported in PPS layer analysis. Average diameter of pores was estimated in the range of 5μm which was characterized by FESEM. The nanostructured thin films devices (Al/Si/PPS/TNPs/Al and Al/Si/PPS/Al) were fabricated in the sandwich form by aluminum (Al) electrodes which were also deposited by electron beam evaporation. Electrical measurements (I–V curves) demonstrated the semiconducting behavior of thin film devices. The gas sensitivity was studied on exposure to 10% CO2 gas. As a result, conductivity of devices increased on exposure to CO2 gas. The device with TNPs thin film (Al/Si/PPS/TNPs/Al) was more sensitive and, had better response and reversibility in comparison with the device without TNPs thin film (Al/Si/PPS/Al).

Luminescence and photoconductivity properties of porous polycrystalline silicon

In recent years, the photoluminescence and the photoconductivity of porous silicon were comprehensively studied. But the photoluminescence and the photoconductivity of porous polycrystalline silicon have not been wholely studied. In this paper, the results showed that luminescent property of the samples prepared by poly-crystal silicon wafers may be related to the defects on Si complexes surface, which can be proved by microwave-detected photoconductivity decay measurements. Furthermore, the luminescence of samples was disappeared under the external illumination, which may be related to the elimination of luminescent-centers. In addition, the conductivities of the samples were dependent on etched time and current density, and the large porosity of samples led to isotropic photoconductivity, which may be related to the change of energy band structure of the devices.

The influence of microstructure on optical properties of porous silicon

The photoluminescence (PL) of as prepared porous silicon (PS) etched in different time were studied. The variation of PS blue emission was similar with that of Si complexes vibration mode on PS surface so blue emission may be related to the defect states of surface Si complexes and hydrogen atoms on PS surface can remove irradiative centers and result in increasing PL intensity. The peak centered at 570 nm may be associated to the emission of porous polycrystalline silicon in PS surface, which can be proved by X-ray powder diffraction (XRD) and electron diffraction (ED) methods.

Novel Polycrystalline SiC Films Containing Nanoscale Through-Pores by Selective APCVD

A selective atmospheric pressure chemical vapor deposition (APCVD) process has been developed to deposit porous polycrystalline silicon carbide (poly-SiC) thin films containing a high density of through-pores measuring 50 to 70 nm in diameter. The selective deposition process involves the formation of poly-SiC films on patterned SiO2/polysilicon thin film multilayers using a carbonization-based 3C-SiC growth process. This technique capitalizes on significant differences in the nucleation of poly-SiC on SiO2 and polysilicon surfaces in order to form mechanically-durable, chemically-stable, and well anchored porous structures, thus offering a simple and potentially more versatile alternative to direct electrochemical etching.

Study of the porous polycrystalline silicon formation process

Electrochemical dissolution (in hydrofluoric acid) of p-type polycrystalline silicon layers deposited on crystalline silicon substrates has been studied. Some properties of the porous silicon (PS) layers obtained are described. The dissolution occurs via two processes that take place simultaneously, at close rates. The first one is the formation of macropores, which results in the development of a columnar structure; the second is the transformation of the columns into a mesoporous medium. The PS layers obtained have a smooth surface, a sufficiently high porosity and a high etching rate in aqueous KOH solutions, which is important for the use of PS as sacrificial layers. Valuable information on the process is obtained by in situ capacitance measurements during electrochemical dissolution.

Key role of nanocrystalline feature in porous polycrystalline silicon diodes for efficient ballistic electron emission

The cold electron emission characteristics of nanocrystalline porous polysilicon (PPS) diodes have been investigated for two PPS diodes prepared under different conditions. The origin of the difference in the emission efficiency and electron energy distribution between the two samples is analyzed in relation to the respective photoluminescence properties. The photoluminescence spectrum in the efficient electron emitter consists of red and blue emission bands corresponding to nanocrystalline silicon and interfacial oxide, respectively, while that in the low-efficiency emitter consists only of a blue band. Too much oxidation results in a reduction of the emission efficiency due to increased electron scattering. It is evident that both the formation of nanocrystalline silicon and its appropriate surface oxidation are key issues for obtaining efficient ballistic emission. These results are consistent with the emission model that electrons are accelerated in the PPS layer by multiple tunneling through interfacial oxide barriers between interconnected silicon nanocrystallites.

P-30: The Effects of a Thermal Treated Au on the Porous Polycrystalline Silicon Nano-Structured Emitter

At present Porous polycrystalline silicon Nano-Structured PNS emitter shows the relatively high performance with lower turn-on voltage and higher emission current than the conventional field emitter. But it has still the problem to be solved. Since the Au as a top-electrode was thermally diffused to thin oxide during the vacuum packaging process, the electrical performance of PNS emitter was decreased by thermal activation energy between the thin firms. In this study, we analyze the changed property of Au using Auger Electron spectrometer AES and Scanning Electron Microscopy SEM. And the property of Au related with the thermal activation energy was discussed with the fundamental diffusion property. From this analysis, we can understand for electron emission property of PNS emitter and made the vacuum packaged PNS emitter.

Electron emission characteristics of the porous polycrystalline silicon diode

Abstract It is estimated that a porous polysilicon (PPS) diode with a structure of Au/PPS/n-type Si operates as an efficient stable surface emitting cold cathode. 2.0 μm of an non-doped polysilicon layer is formed on an heavily doped n-type silicon wafer and anodized in a solution of HF (50%):ethanol=1:1 under illumination by a 500 W tungsten lamp from a distance of 20 cm. The electron emission properties of the PPS diode were investigated as a function of anodizing condition such as anodizing current density. The electron emission trajectory was investigated, and it was also demonstrated their good uniformity in the emitting area.

Efficient and ballistic cold electron emission from porous polycrystalline silicon diodes with a porosity multilayer structure

Based on the previous investigations of the cold electron emission from porous silicon and porous polycrystalline silicon (PPS) diodes, we have fabricated PPS diodes with a porosity multilayer structure and evaluated the emission characteristics. The multilayer structure has functions of reforming the electric field distribution and improving the thermal conductivity of the PPS layer. It is demonstrated here that the multilayered sample exhibits high emission current density of ∼0.8 mA/cm2 and high efficiency of ∼1%, which are close to values required for practical use. The emission is quite stable even with a high emission current density. Ballistic electron emission mechanism is discussed in relation to the energy distribution of emitted electrons.

Sensitivity and stability of porous polycrystalline silicon gas sensor

Porous silicon (PS) used as a sensing material for micro-sensors has attracted a lot of attention in recent years. Owing to the large surface area (∼200 m 2 cm −3) and high chemical activity, porous silicon would be a good choice for gas sensor applications. In this paper, a resistivity sensor using porous polycrystalline silicon (PPS) thin films was fabricated by anodization and standard lithographic technique. Analytical measurements on the sample reveal that these sensing porous poly-Si films consist of many pores with a diameter of ∼100 nm. A high sensitivity to ambient pressure and ethanol vapor was observed. The sensitivity and stability of PPS gas sensor have been characterized and analyzed as a function of gas concentration and storing duration. In particular, we will discuss the mechanism of the interaction between the surface state of PPS and ambient gases in detail. Suggestions to further improve the sensor stability will also be proposed.

Luminescent chemically-etched porous poly-crystalline silicon on insulating substrates

Polycrystalline silicon deposited on insulating substrates has been chemically-etched to form thin films of porous material exhibiting room temperature visible photoluminescence with emission wavelengths of around 650 nm. Material of 4000 A thickness was quickly converted to porous silicon within 15 s of etching, with an etch rate of 1–1.5 µm/h. In contrast to anodization, chemical-etching parameters have little effect on modulating the resultant peak wavelength. Peak photoluminescence intensity was achieved 8–12 s of etching in 1:3:5 parts HF:HNO3:H2O at room temperature with ambient lighting. The chemical etching process and its etch characteristics have been discussed in relation to its suitability for large area thin film devices.

Ballistic Electron Surface-Emitting Cold Cathode by Porous Polycrystalline Silicon Film Formed on Glass Substrate

AbstractIt is demonstrated that a porous polycrystalline silicon (PPS) film is useful as a ballistic electron emitter for excitation source of a flat panel display. A 1.5 µ m polysilicon layer is deposited on a silicon substrate by Low Pressure Chemical Vapour Deposition (LPCVD) technique and subsequently anodised in an ethanoic HF solution and oxidised in a Rapid Thermal Oxidation (RTO) furnace. A thin Au film is deposited onto the RTO-treated PPS layer and used as a top electrode. The electron emission current Ie and the diode current Ips are measured as a function of the bias voltage Vps. Electron emission of which onset voltage is about 8 V rapidly increases with increasing Vps. The Ie value reaches about 2 mA/cm2 for Vps= 20 V at which the emission efficiency defined as Ie/Ips is about 1 %. The emission mechanism has also been investigated in terms of the correlation between the emitted electron energy and the structure of PPS layer. The observed energy distribution curve of output electrons suggests that the PPS layer acts as a ballistic transport medium and the emission occurs based on multiple tunnelling through silicon nanocrystallites. The PPS layer is also formed on the polysilicon layer deposited on a glass substrate by Plasma Enhanced Chemical Vapour Deposition (PCVD) technique. In this case, the film is treated by an electrochemical oxidation (ECO) in an H2SO4 solution. Similar emission characteristics are observed, although the emission current is lower than that formed on silicon substrate. We also demonstrate the 2.6 inches diagonal 53×40 pixels multicolour flat panel display. We name it ballistic electron surface-emitting display device (BSD). BSD shows the possible application to the future flat panel display.

Porous polycrystalline silicon conductivity sensor

Porous polycrystalline silicon (PPS) based conductivity sensors were fabricated and characterized in this work. The PPS sensors show excellent sensitivity for detecting ambient pressures, gas species as well as temperature. Results show that the current increases significantly as the pressure decreases and an over two orders of magnitude change has been detected when the pressure was reduced to 10−2 atm. We also find that both ethanol and acetone vapors can significantly change the conduction in the sensors. In acetone vapor, the device even displays diode-like characteristics. Electrical conduction mechanisms of the sensors in vacuum and organic vapors are proposed. The fabrication process can be easily integrated with very large scale integrated technology.

Mechanism of efficient and stable surface-emitting cold cathode based on porous polycrystalline silicon films

It is demonstrated that a porous polysilicon (PPS) diode with a structure of Au/PPS/n-type Si operates as an efficient stable surface-emitting cold cathode. 1.5 μm of an nondoped polysilicon layer is formed on an n-type (100) silicon wafer and anodized in a solution of HF (50%): ethanol=1:1 at a current density of 10 mA/cm2 for 30 s under illumination by a 500 W tungsten lamp from a distance of 20 cm. Subsequently, a PPS layer is oxidized in a rapid thermal oxidation furnace for 1 h at a temperature of 700 °C. A semitransparent thin Au film (about 10 nm thick) is deposited onto the PPS layer as a positive electrode and an ohmic contact is formed at the back side of the silicon wafer as a negative electrode. When a positive bias is applied to the Au electrode in vacuum, the diode uniformly emits electrons. No electron emission is observed in the negatively biased region. Emission current is about 10−4 A/cm2 at a 20 V bias. It is further demonstrated that electrons are quasiballistically emitted from a PPS diode due to a significantly reduced electron scattering in the PPS layer. As a result, the diode can emit fluctuation-free stable electron emission. The simplified model of emission and energy distribution of electrons are proposed and it can explain the experimental results.

Physical structure of light-emitting porous polycrystalline silicon thin films

Abstract This paper reports the surface electronic structure of light-emitting porous polycrystalline silicon (PPS) using X-ray photoelectron spectroscopy (XPS). We find that the PPS films with strong photoluminescence (PL) effect can only be observed in thin film with trace amount of silicon nanoclusters and the luminescence can be enhanced remarkably with proper passivation of the PPS surface. Incomplete oxidation of silicon (Si 3+ or Si 2+ ) does not lead to visible PL. We further estimate that the average size of silicon nanoclusters is in the range of 20–30 A in the sample having PL emission.

A Led Based on Porous Polycrystalline Silicon

AbstractPrevious studies on electroluminescence in porous silicon were based on crystalline wafers. In this paper, we shall report the characteristics of a LED based on porous effects in a cast polycrystalline silicon substrate. A layer of porous region was first formed on a cast polycrystalline silicon substrate by anodization, followed by the deposition of a semitransparent Au layer. Under forward bias, the LED emits stable yellowish white light (with the presence of bright spots) for currents above 20 mA/cm2. From the electroluminescence spectra measured, we suggest that the emission is due to the recombination of electron-hole pairs in a microplasma region. We propose a model where the microplasma is present in the depletion region of the heterojunction formed between the bulk polysilicon and the surface porous polysilicon. The defects and grain boundaries in a polycrystalline material facilitate the formation of such microplasma. The heterojunction model will also be used to explain the current characteristics of the LED. The effect on the LED characteristics due to indium coating on the porous substrate prior to Au deposition was studied, and the results agree with the heterojunction model. Our work shows that cast polycrystalline silicon substrates have potential for LED fabrication in cheap and large area applications.

Characterization of Light Emitting Porous Polycrystalline Silicon Films

AbstractPolycrystalline silicon (poly-Si) thin films (∼700nm) were deposited by LPCVD, doped with 950°C phosphorous diffusion, and rendered porous by anodization and stain etching. From x-ray photoelectron spectroscopy, poly-Si films have atomic concentration of C(ls):0(ls):Si(2p) = 6%:15%:79%. However, porous poly-Si (PPS) films with weak photoluminescence (PL) have C:O:Si of 20%:38%:42%. For PPS films with strong PL, C:O:Si is 11%:38%:51%. From micro-Raman, scattered spectra for 632nm laser source has peak at 735nm and full wave half maximum (FWHM) of 76nm, and is similar to the PL spectra excited by 400nm uv laser source. High resolution transmission electron microscopy (TEM) study shows that PPS film is of complex structure and composes of numerous Si nano-crystals (1∼10nm) surrounded by amorphous materials.

Effect of grain boundaries on the formation of luminescent porous silicon from polycrystalline silicon films

Luminescent porous silicon can be obtained by electrochemical etching under illumination from n-type polycrystalline silicon which has been fabricated by solid phase crystallization of in situ doped amorphous films deposited by low pressure chemical vapor deposition. The analyses of the nanostructures by microscopy show that the pore orientations mainly follow the current flow which is channeled by macropores located at grain boundaries. The luminescence results obtained from this porous polycrystalline silicon are comparable to those obtained from porous samples of single-crystals silicon.